Session C1 - Poster Session I.
POSTER session, Monday afternoon, March 03
Room Exhibit Hall 2/3, Austin Convention Center

[C1.001] Complex Structured Materials

[C1.002] Preparation and characterization of magnetic nanocomposite in polymer matrix by ion-implantation

Research interest in the field of nanocomposite materials is growing rapidly due to their size-dependent interesting electronic, magnetic, optical and mechanical properties that have many potential industrial applications. Ion implantation (IIM) is a versatile technique that can create nanocomposite materials with a wide range of interesting physical properties. Recently, Ag nanoparticles have been synthesized using (IIM) into polymer by A. Stepanov et al[1]. In the initial experiments we have prepared magnetic (Ni, Co and Fe) nanoparticles by IIM at 50 keV into polyether ether ether ketone (PEEK) at doses from 5x10^15 to 2x10^16 ion cm^-2. The microstructure of nanoparticles was studied by scanning electron microscope, transmission electron microscope, and atomic force microscope. The magnetic, electrical and mechanical properties have been investigated using SQUID magnetometer, magneto-transport and nano-indentation techniques. In this presentation many of the unique electrical, magnetic and mechanical properties of IIM nanocomposite will be discussed. 1. L. Stepanov et al., Phil. Mag. B, 80, 23 (2000).

[C1.003] First-principles Study of the Adsorption of Carbon Atoms on Copper Surfaces

The adsorption and diffusion of a carbon (C) atom on several low-index Cu surfaces have been investigated by the first-principles calculations. The methods are based on density-functional theory under plane wave formalism with ultrasoft pseudopotentials. The adsorption energies and diffusion barriers of a C atom on Cu surfaces are calculated. The interactions between a pair of C atoms at different separations on these surfaces are also investigated. These results are carefully compared with the results we obtained for nickel surfaces.

[C1.004] The effect of irradiation on the absorption and four-wave mixing properties in Eu3+-doped aluminosilicate glasses

The effect of irradiation on the formation of refractive-index grating in rare-doped alkali aluminosilicate glass has been studied using both absorption spectroscopy and non-degenerate four-wave mixing (FWM) techniques. The glass samples, which have different concentrations of modifying Eu3+ ions, were subjected to both electron and gamma irradiation. Changes in both the absorption and FWM signal will be presented and discussed in terms of radiation induced defects in these glasses.

[C1.005] Nanoscale Phase Separation of GeS2 Glass

Molecular structure of GexS100-x glasses in the 30 < x < 34 atomic percent range is examined by Raman scattering, 119Sn Mossbauer spectroscopy, and T-modulated DSC. The results show that the stoichiometric glass, x = 1/3 (Tg = 508 degrees centigrade) is chemically disordered. Raman scattering places mode strengths of corner-sharing Ge(S1/2)4 tetrahedra (A), ethanelike Ge2(S1/2)6 units (B), and the distorted rocksalt Ge(S1/6)6 units(C) at approximately 93.4 : 3.6 : 3.0 not normalized for mode cross-sections. Mossbauer spectroscopy places the concentration of these units A:B:C = 76.2 : 8.8 : 15. The drastic reduction in the slope of the glass transition temperature Tg with Ge content x, coincides with first appearance of these units once x exceeds 32.50 at. (to suggest that these units are demixed) and form separate nanophases to lower the global connectivity of the glass network.

[C1.006] Electrical characterization and morphological studies of conducting polymer nanofibers

Doped polyaniline blended with poly(ethylene oxide) has been electrospun in air to give fibers with diameters in the range 3 nm 200 nm. These fibers were captured on wafers of degenerately doped Si/SiO2 by placing the wafer in the path of the fiber jet formed during the electrospinning process. Individual fibers were contacted using shadow mask evaporation and were also captured on prepatterned wafers. Fibers having diameters greater than 100 nm show a slight increase in the conductivity as compared to the bulk film, while fibers with diameters less than 30 nm had lower conductivity than the bulk. Data on Scanning Conductance Microscopy along the length of individual fibers will be presented. For fibers where the diameter was not uniform, we found that below a certain diameter (\sim15 nm) the fiber was less conducting as compared to thicker diameter fibers. Dependence of the fiber conductivity on a gate bias is underway and these results will also be presented.

[C1.007] Temperature dependence of interface barrier height change as implicated by field emission studies of aligned-multiwall carbon nanotubes

We observed that the change of field emission currents of multiwalled nanotube (MWNT) is not sensitive but observable to temperature variation within the range from 300K to 20K. However, the characteristic curve can¡¦t be congruently fitted by Fowler-Nordheim (F-N) theory. We find that the field emission lineshape can be successfully portrayed by the semiconductor thermionic emission theory, with a proper selection of the electron affinity and interface height. Assuming a semiconducting cap inhibited to the tube body, the dominated field emission mechanism will be the tunneling effect caused by a high aspect ratio. It is believed that the thermionic tunneling plays a key role in the temperature dependence on field emission of MWNT. This analysis conclusively addresses that the electron affinity and the energy gap of the caps are not changed while the interface barrier height increases as temperature increases.

[C1.008] Properties of Individual Single-Walled Carbon Nanotubes Suspended in Various Surfactants

A series of anionic, cationic, and nonionic surfactants has been used to suspend individual single-walled carbon nanotubes. The resultant suspensions were characterized with raman, fluorescence, and UV-visible spectroscopy. Previously, it has been shown that nanotube spectroscopy is sensitive to the environment around the nanotubes. So in addition to studying effect of different surfactants, an extensive study of how salt, small molecules and pH affect the spectral properties in varying surfactant systems has been done. This work demonstrates the potential for nanotubes as chemical sensor.

[C1.009] Nanoscale Building Blocks and Nanoassembly of Structures

Electronics and photonics industries are highly interested in developing new methods for nanofabrication in order to be able to continue their long-term trend of building ever smaller, faster and less expensive devices. Conventional patterning strategies must be augmented by new techniques in order to truly take advantage of the quantum nature of novel nanoscale devices. In our research, we are developing a bottom-up approach to fabricate building blocks, which can be used to assemble nanostructures and devices. This involves the assembly of atom- and molecule-like nanostructures into functional 2-D and 3-D units. This will take advantage of the unique optical, electronic, and size-tunable properties of nanostructures and permit the use of these properties for “real” applications in a larger system (> 10 nm and < 1 um). Here, we demonstrate a novel technique for the fabrication of nano-assemblies of carbon nanotubes (CNT) and quantum dots (QD) (CNT-QD conjugates) for the first time using a zero length cross-linker. CNT’s are primarily functionalized with carboxylic end groups by oxidation in concentrated sulfuric acid. Thiol stabilized QD’s in aqueous solution with amino end groups were prepared in the laboratory. The ethylene carbodiimide coupling reaction was used to achieve the CNT-QD conjugation. Sulfo-N-Hydroxysuccinimide (sulfo-NHS) was used to enhance this coupling procedure. We present EDS and FTIR data for the chemical modification and SEM images of the first nano-building blocks. Current work includes the more complex 3-D assembly of QD’s and nanotubes on Anodized Aluminum Oxide (AAO) template for nanodevices. Potential future applications of our method include the fabrication of novel electronic and photonic devices, crystal displays and biosensors.

[C1.010] Insulating to metallic transition under pressure in SWNT

Resistance vs Temperature measurements, R(T), were performed in Single-Wall Carbon Nanotubes ropes (SWNT) at high quasi-hydrostatic pressure, from room temperature to about 1.6 K. At low pressures from about 0 to 0.5 GPa, we observed that the R(T) curves change from semi-metallic at room temperature to insulating at low temperatures. With increasing pressure, from 2 to 5 GPa, a continuous change occurs In R(T) to a quasi-metallic system. At low temperatures from about 10 to 1.7 K, the R(T) decreases precipitously. We believe that with further pressure increases, and low temperatures, SWNT will reach a superconducting state.

We acknowledge financial support from DGAPA-UNAM project IN102101, and CONACyT-México project G0017.

[C1.011] Charge Density Fluctuations and Magnetoplasmons for a Nanotube

The single-particle energy eigenstates are obtained for electrons on a cylindrical nanotube in the presence of a magnetic field applied in a direction perpendicular to the axis of the nanotube. The energy eigenvalues are analyzed as functions of magnetic field, nanotube radius and momentum along its axis by solving numerically the Schrodinger equation in a magnetic potential. We introduce the Coulomb interaction between electrons and determine the charge density fluctuations due to a weak external field. The plasmon dispersion relation is obtained and numerical results are presented.

[C1.012] Solubility and Phase Behavior of Chemically Oxidized SWNT in Super Acid Suspensions

The solubility of single-walled carbon nanotubes(SWNT) in super acids, such as sulfuric acid, chlorosulfonic acid and Trifluorsulfonic acid, has been demonstrated. Solutions with truly dissolved SWNT show liquid crystal behavior in super acids similar to conventional rigid rod polymeric system. The tubular interactions which determine the nature of phases can be controlled by the surface oxidation state of SWNT. Oxidation procedures, like nitric acid reflux and dichromate reflux, have also been performed on SWNT made by the HiPCo process. The phase behavior of the surface modified SWNT in super acids at various concentrations has been examined by optical microscopy and Polarized Raman Spectroscopy. Sediment obtained by quenching the above super acid suspensions was examined by Scanning Electron Microscopy for intrinsic ordering. The results will be presented in the context of the phase behavior of unmodified SWNT in super acids.

[C1.013] FIELD-EMISSION CHARACTERIZATION OF CARBON NANOTUBES GROWN ON METAL SUBSTRATES

Carbon nanotubes field emitters grown on various substrates were studied. The CNTs were grown on flat metal substrates using ferrocene / xylene synthesis in a flow-through CVD reactor. The CNT morphology varied dramatically with substrate material. A reliable method of large area field emitter’s characterization, useful to extrapolate emission characteristics beyond the range of measurements, was developed. Emission tests were performed in a vacuum chamber using a moving anode point probe. Voltages up to 800 V and gaps from 10 to 200 micron were used to obtain I-V and F-N characteristics. Low turn-on fields 2.6 V per micron and current densities up to 16 mA/sq.cm were obtained for some samples at field strength up to 5 V/micron. Stability of emission current was studied up to 40 hours. The developed test procedure helps to select deposition parameters by relating emission properties of CNTs to manufacturing conditions. The strong influence of the substrate material on emission properties was observed.

[C1.014] Light Scattering Study on SWNTs Solutions

SWNT/Oleum dispersions are being used to form film and fibers showing high electrical conductivity. PVP wrapped SWNTs have also been dissolved in aqueous medium. Light scattering studies have been performed in this study to understand the nanotube dispersion on both SWNT/Oleum as well as PVP/SWNT/surfactant/water dispersions. Preliminary studies on PVP wrapped SWNTs/water and SWNTs/oleum dispersion system show the existence of a critical concentration (c*) of SWNTs. Below c*, the detected scattering intensity increases with concentration; and above c*, the detected scattering intensity decreases with concentration. This observed phenomenon is attributed to the competition of scattering and strong absorption of SWNTs to visible light. Based on this study, level of SWNT dispersion is being studied. Results of these studies will be reported.

[C1.015] Polarized Raman Spectroscopy on the Intrinsic Alignment of Organically Modified SWNT Fibers Extruded from Super Acid Suspensions

Intrinsically aligned fibers have been produced from liquid crystalline-like suspensions of pristine and organically functionalized single wall carbon nanotubes (SWNT) in 100sulfuric acid. The tube-tube interactions of these rigid rod-like molecules in super acid solutions are influenced by side groups and determine the phase behavior of the suspension. Morphologies of the extruded fibers were examined by scanning electron microscopy. The intrinsic ordering of the SWNT in the extruded fibers has been determined by polarized Raman spectroscopy. Correlations were examined between aligned fractions of SWNT within the fiber and the nature of the side groups.

[C1.016] Ozone and Other Oxidant Reactions with Individually Dispersed Single Wall Carbon Nanotubes in Surfactant and Water Solutions

Ozone and other oxidants have been used to functionalize the sidewalls of individually dispersed single wall carbon nanotubes (SWNT) in anionic, cationic, and non-ionic surfactant micelles. Variations in the pH and durations of the reactions were examined. The degree of functionalization was monitored with Raman spectroscopy, UV-Vis-NIR absorption, AFM and STM. The functionalization rates in different surfactants were studied to assess the potential use as a control parameter for cutting SWNT.

[C1.017] Synthesis and characterization of Se nano-structures inside the one-dimensional channels of zeolite

Selenium species were introduced into the channels of aluminophosphate zeolite, and different attempts have been made to determine the nano-structures of the adsorbent. Thermal adsorption / desorption process of selenium species is studied by simultaneous thermogravimetry (TG) and differential scanning calorimetry (DSC). There exist two peaks in the TG and DSC curves, indicating that two different structures of Se species co-exist in the channels. In the X-ray diffuse scattering pattern, several parallel diffuse scattering layers are compatible with the existence of finite chain-like structures, while some weak scattering clouds correspond to disordered or localized structures. Polarized Raman peaks reveal the resonant features of the well-aligned one dimensional selenium helix, as well as the random distribution of the 8-member selenium rings. Polarized optical absorption spectra confirm that the helical chains are highly aligned along the channel direction, while the rings are randomly oriented in the channels.

[C1.018] Magnetoelectric Effects at Piezoresonance in Ferromagnetic-Ferroelectric Layered Composites

The coupling between electric and magnetic fields in composites consisting of magnetostrictive and piezoelectric phases is mediated by mechanical deformation. Most studies on magnetoelectric (ME) interactions in such composites deal with low frequency effects. Here we present a model for ME effects at piezoresonance. A bilayer of ferrite and lead zirconate titanate (PZT) is considered. Expressions for the magnetoelectric voltage coefficient have been obtained by solving elastodynamic and electrostatic equations for ferrite and PZT. Both longitudinal and transverse orientations for bias magnetic field and poling electric field are considered. The ME coefficient shows strong frequency dependence and has the highest value at the coincidence of mechanical resonance for both phases. Estimates of ME voltage coefficient are presented for cobalt ferrite-PZT samples. The resonant frequency is several MHz for composites with a layer thickness of several hundred microns and the ME coefficients are greater by two to three orders magnitude compared to values at low frequencies.

- work at Novgorod State University supported by the RFBR (Project No. 01-02-17579)

- work at Oakland University supported by the NSF (DMR-0072144)

[C1.019] Measurements of rock permeability and porosity by continuous-flow laser-polarized xenon NMR

We present a new method for the simultaneous measurement of permeability and effective porosity in porous geological rock samples using NMR of laser-polarized 129Xe gas.

[C1.020] Theory of Low-Frequency Magnetoelectric Effects in Ferromagnetic-Ferroelectric Layered Composites: Free and Clamped Samples

In a product-property composite of ferromagnetic-ferroelectric phases, the coupling between electric (E) and magnetic (H) fields is mediated by mechanical deformation. A theoretical model is presented for low frequency magnetoelectric (ME) effects in such bilayers[1]. A novel approach is proposed to take into account less-than-ideal boundary conditions. An averaging method is used to estimate effective material parameters. Both free and clamped samples are considered. Expressions for ME voltage coefficients have been obtained for (i) transverse (out-of-plane E perpendicular to in-plane H), (ii) longitudinal (E parallel to out-of-plane H) and (iii) both E and H in-plane and parallel to each other. The model predicts a 25-50transverse field orientation than for the longitudinal orientation in the case of unclamped samples. It is shown that ME coupling is maximum for the field orientation in (iii). The estimated ME coupling constants are compared with data for composites consisting of the following magnetic oxides: cobalt ferrite, nickel ferrite, lithium ferrite, and lanthanum strontium manganite, and piezoelectric lead zirconate titanate.

- work at Oakland University supported by the NSF (DMR-0072144)

1.M. I. Bichurin, V. M. Petrov and G. Srinivasan, in press, J. Appl. Phys. (2003)

[C1.021] Nano-structured Morphology of Electrospun Polymer-Carbon Nanotube Composite Fibers

The success of polymer-nanotube composites for any application requires good dispersion of nanotubes in the matrix and good interaction at the polymer-nanotube surface. Electrospun composite fibers of nanoscale dimensions facilitate a better understanding of these critical issues. Nanotube-polymer fibers are synthesized and the interface between the tubes and the polymer is studied. The alignment of nanotubes in the polymer matrix is also investigated. Both as-produced and functionalized nanotubes (to improve dispersion) are used. Potential applications are also identified

[C1.022] SWNT/PAN Composite Film based SuperCapacitors

Activated carbon powder and activated carbon fiber, due to their high specific surface area, porous characteristics, relatively high electrical conductivity and good chemical resistance, have been extensively investigated as the electrode materials for an electrochemical supercapacitor. In the present study, a core-shell structured SWNT/Polyacrylonitrile (PAN) composite film has been produced with ~30 nm in diameter SWNT bundles as the core and ~ 10 nm thick PAN polymers as the shell. SEM was used to investigate the evolution of the porous structure of SWNT/PAN composite film induced by heat-treatment and CO2 activation process. These composite films possess advantages of SWNT as well as that of the activated carbon for the supercapacitor electrode. The constant charging and discharging experiments show that these activated SWNT/PAN composite films are very promising electrode materials for high capacitance and power performance.

[C1.023] Influence of Defects on the Elastic Properties of Carbon Nanotubes

The mechanical properties of single-walled carbon nanotubes (SWNT) were studied for (5,5), (8,8), (6,0) and (10,0) models by means of molecular dynamics simulations. Elastic constants were measured for isolated nanotubes in vacuum (stiffness) and for a bundle of 4 infinite nanotubes in the cell with periodic boundary conditions in three directions (Young modulus, Poisson ratio). Changes in the modulus in the presence of various structural defects such as adjacent pentagon pairs were measured. The effect of temperature, nanotube diameter and various defect configurations on elastic properties was calculated. The inital calculations showed that the stiffnes of an isolated nanotube (of ~3 nm length) decreased by 5defect.

[C1.024] Frustration in Condensed Matter: Three-Dimensional Simulations.

We study frustration in condensed multi-body systems with three degrees of freedom. Our simulation model minimizes frustration, allowing particles to move in a given system. Frustration is defined in terms of individual frustrations, based upon relative distances between particles. Frustration is assumed to be linear with the energy of the system, and minimized using both energy and distance dependencies. Since frustration modeling mirrors the relaxation motions, our simulations appear to minimize more efficiently than using energy critirion. This modeling in condensed matter has possible applications to vortex states and spin glasses, which attempt to relax to an ordered state. In such systems, random magnetic interactions compete. While energy is not used as a criterion for relaxation, the problem of disorder is simplified, when relative distances (or discrete spin orientations) are dominant in relaxation processes. The relaxation behavior suggests that frustration minimizes with a pseudo-exponential time dependency, substantially different from power law or exponential behavior. Ref: I.M. Suarez, Modeling Frustration for Physical Systems, SJSU Thesis (1990) and references therein. C. Boekema, I.M. Suarez et al Hyperfine Interactions 64 (1990) 467. Research is supported by NSF-REU and Physics@SJSU.

[C1.025] The Effects of Cs on the Raman Modes of Single Wall Carbon Nanotubes

We present a theoretical study of the Raman modes in single-wall carbon nanotubes (SWCNTs), and in particular the effects of Cs doping. It was previously shown that alkali-metal doping results in low-frequency modes involving both the radial motion of the tube and alkali-atoms vibrations (N. Bendiab, A. Righi, E. Anglaret, J. L. Suavajol, L. Duclaux, F. Beguin, Chem. Phys. Lett., 339, 305 (2001). Applying a first principles density functional theory approach, we report results for the radial breathing modes of pristine SWCNTs in crystalline-rope and isolated tube forms, validating the method for large diameter tubes, and then discuss the modes for tubes intercalated with Cs at various sites.

[C1.026] Characterization of traps in SOI wafers by transconductance characteristics of MOSFETs

We characterized trap states in bonded SOI wafers and SIMOX wafers. By the analogy of Terman method using high frequency C-V characteristics, we estimated the distribution of density of trap states from DC transconductance of SOI MOSFETs in detail. As a result, we found that SIMOX wafers have accepter-like electron traps near the conduction band edge and donor-like hole traps near the valence band edge in the forbidden band of Si at SOI/BOX interfaces with the density of more than 10E12 (cm-2·eV-1). Our DC transconductance technique is practical to characterize trap states in thin SOI layers and to design ultra thin SOI MOSFETs, because it enables us to characterize energy distributions of trap states at FOX/SOI interface and at SOI/BOX interface of MOSFETs separately without any additional process or any special test structure. We also simulated Id-Vd characteristics of SOI MOSFETs with various carrier lifetimes and measured Id-Vd characteristics of MOSFETs fabricated on bonded SOI wafers and SIMOX wafers with various substrate bias conditions. Consequently we concluded that high-density trap states at SOI/BOX interface are effective to suppress kink effect in SOI MOSFETs. This work was partially supported by the Asahi Glass Foundation.

[C1.027] Defect Structures in Zinc/Tin Codoped Indium Oxide Transparent Conductors

Tin-doped indium oxide (ITO) and Zinc/Tin codoped indium oxide (ZITO) are transparent n-type conducting oxides (TCOs), widely used in such devices as solar panels, electro-active windows, flat panel and liquid crystal displays. The conducting properties of both materials are critically dependent on the defect chemistry. In contrast to ITO, comparatively little is known about the defect chemistry in ZITO. Here we report the results of extensive atomistic model calculations to illuminate how Zn_In, Sn_In, V_O, and O_i point defects aggregate into defect clusters. We identify low-energy defect clusters and use these to rationalize the observed conduction properties as a function of Sn/Zn doping level.

[C1.028]

This abstract not available.

[C1.029] Carbon nano-fiber growth on the anode during hydrogen DC arc-discharge

A carbon nano-fiber with a diameter of 25-100 nm and 98.4 percent purity was produced on the heated anode surface in hydrogen DC arc-discharge [1]. Hydrogen arc plasma was generated between the graphite cathode and the carbon/metal composite anode (Fe/Co/Ni/FeS). X-ray diffraction analysis revealed that the carbon nano-fiber had a turbostratic structure with a (002) interlayer spacing of 0.346nm. Three types of nano-structures were observed using transmission electron microscopy, (1) those with a bamboo structure, (2) with a hollow core, and (3) without a hollow core. The formation of the nano-fiber was initiated by arc-generated metal particles with a diameter of 5-75 nm, and carbon for further growth was supplied by the decomposition of polycyclic aromatic hydrocarbons that were created by interaction between arc-generated carbon clusters and hydrogen atoms. The nano-structure of the fiber is thought to depend on the size and morphology of the catalytic metal particles. [1] H. Kajiura et al., Carbon 40(2002)2423.

[C1.030] Magneto Electronic and Optical Properties of Carbon Nanotubes

Magneto electronic and optical properties of carbon nanotubes are, respectively, studied within the sp^3 tight-binding model and the gradient approximation. They strongly depend on the magnitude and the direction of the magnetic field, the nanotube geometry (radius and chiral angle), and the Zeeman splitting. The magnetic field would lead to the change of energy gap, the destruction of state degeneracy, and the coupling of different angular momenta. Hence there are magnetic-field-dependent absorption frequencies and more absorption peaks. The types of carbon nanotubes predominate in the band structure and thus the range of absorption frequencies and the number of absorption peaks. The Zeeman splitting makes the semiconductor-metal transition occur at lower magnetic flux. It metalizes armchair carbon nanotubes in the presence of the perpendicular magnetic field. However, it does not affect the optical excitations except for metallic carbon nanotubes.

[C1.031] Obtaining Isolated SWNTs from Bundles without the Use of a Surfactant

Wet chemical methods involving ultrasound were used to separate large bundles containing hundreds of SWNTs into individual nanotubes supported on Si substrates. Atomic Force Microscopy (AFM) was used to investigate the resulting distribution of tubes/bundle on the substrate and Raman Scattering was used to investigate potential damage from acids (e.g. HNO_3, HCl) used in the process. From AFM we found that approximately 50% (6N HCl) and 90% (3N HNO_3) of the filaments on the substrates could be identified with individual nanotubes, where the acid in ( ) is used in the removal of catalyst. Although HNO_3 seems more suitable for obtaining isolated SWNTs, Raman Scattering spectra showed that this acid results in significant wall damage.

[C1.032] Magneto Electronic Excitations in Single-Walled Carbon Nanotubes

The low-frequency single-particle and collective excitations of the single-walled carbon nanotubes are studied in the presence of magnetic field. They strongly depend on the magnitude and the direction of magnetic field, the transferred momentum, the temperature, the nanotube geometry, and the Zeeman splitting. A narrow-gap nonarmchair carbon nanotube exhibits two interband magnetoplasmons, while a metallic nonarmchair carbon nanotube exhibits one interband magnetoplasmon and one interband and intraband magnetoplasmon, or two interband magnetoplasmons and one intraband magnetoplasmon. The differences among these plasmons are relatively obvious, when the magnetic field is oriented closer to the nanotube axis. The transferred momentum determines the plasmon frequency and the existence of plasmons. The temperature can induce a intraband magnetoplasmon, or change a interband magnetoplasmon into a intraband and interband magnetoplasmon.

[C1.033] Raman modes of carbon nanotubes under pressure

High pressure Raman scattering experiments were performed at room temperature in a diamond anvil cell using green (514.5 nm) and red (632.8 nm) laser excitations on HiPCO-SWNTs. This study enabled us to probe the pressure dependence of the radial modes due to different diameter tubes in the range of 0.7 to 1.4 nm. We find that with increasing pressure, the radial modes due to large diameter tubes lose their intensity at a lower pressure than those due to small diameter tubes. Furthermore, the logarithmic pressure derivative of the radial mode frequency decreases with decreasing tube diameter, in sharp contrast to the theoretical predictions for isolated tubes. We suggest that this difference is due to the faceting of the tubes in the bundles under pressure. During initial compression, a small discontinuity in the slope of frequency change with pressure is observed between 2 and 3 GPa. However, the decompression and subsequent compression data do not exhibit any discontinuity. We attribute this behavior to the alignment of the tubes within the bundles and to the pressure-induced deformations of the tube cross-section.

[C1.034] Magneto-band of the stacked Nanographite Ribbons

Magnetoband structures of AA- and AB-stacked nanographite ribbons are studied by the tight-binding model. The magnetic field changes band width, energy space, and energy dispersions (the produce of Landau subbands and Landau levels). Electronic properties are strongly dependent on the structure geometry of ribbons, such as ribbon width, edge structure and stacking sequence. Especially, magnetic filed causes many zero energy points in the band dispersions of AB-stacked zigzag ribbons. Such points and corresponding localized states are analyzed. The differences between localized states and edge states are clarified. Oscillation period of Landau subbands are determined by these points. Moreover, the interribbon interactions also affect magnetoband structures, such as band width, energy space, energy dispersions, oscillation period of Landau subbands, and Hofstadter butterflies. In addition, the density of states of armchair and zigzag ribbons under the certain magnetic field are the same each other. Cyclotron radius less than the geometrical size is the main reason.

[C1.035] Electronic transport in monolayers of multiwall carbon nanotubes

We introduce the method of assembly of functionalized multiwall carbon nanotubes (MWCNT) into monolayers (dense arrays). In contrast to the standard morphologies of the samples of arrays of nanotubes involving definitions of bundles (ropes), mats, networks, etc., based on hardly controlled deposition from organic solvent dispersions of pristine nanotubes, we use ?simulation? of the Langmuir-Blodgett (LB) technique. The method we propose offers a radical departure from the existing methodology due to the possibility to cover large surfaces with dense thin films of carbon nanotubes. To our knowledge, up to now, there is no experimental data on electrical and magneto-transport properties characterization of dense monolayers manufactured using LB assembling of functionalized nanotubes. The obtained layers (dense arrays) of MWCNTs are expected to be used for new applications in chemical and biosensors, controlled by the electrical transport. We also have tested the samples of MWCNT, deposited on the substrates from a suspension in organic solvents for comparison. Due to the low concentration of the MWNT in the solution, the coverage of the electrodes was found to be ?island??like. The nanotubes samples on the electrodes with ?finger-shape? geometry have shown low resistance(<1kW ?at room temperature) and a ?weak? power law dependence of the resistance on temperature in the range T=4.2 ?300 K. The temperature dependences of the resistance represents a power-law in the temperature range lower than ~100K with the exponents?0.22. While such a power law can fit the temperature dependence of the resistance in MWCNT, it is suggested the formation of a Luttinger liquid. A negative magnetoresistance, as a characteristic of the weak localization state was observed. In addition to the high field negative magnetoresistance for the functionalized nanotubes we observed a positve magnetoresistance at low fields as a signature of weak antilocalization.

[C1.036] Phase Transitions in Single-wall Carbon Nanotube Bundles under Hydrostatic Pressure

We have performed Parrinello-Rahman constant pressure molecular-dynamics simulation on the lattice of crystaline carbon nanotubes. The interaction between carbon atoms are simulated by an empirical Tersoff-Brenner many-body potential, which has been extensively used to model graphite and carbon nanotubes. The intratube and intertube van der Waals interactions are modelled by Lennard-Jones potential with a cutoff of 15 ÅWe find a hard-soft transition under external pressure, with a shape change from circular-like to elliptical-like. For a (3n, 3n) tube, with n being an integer, it can maintain hexagonal symmetry before collapsing into an ellips-like shape, while other tubes undergo deformation into low symmetry structures long before the hard-soft transition.

[C1.037] Insight Into Carbon Nanotube - Polymer Nanocomposite Stabilization From Simplified Models

A key impediment to the widespread application of carbon nanotubes as reinforcing fillers in nanocomposite applications is the lack of a general method for uniformly dispersing individual tubes in the polymer matrix and stabilizing the resulting composite. In an effort to address the latter challenge, this contribution examines the stability of the complexes formed by a number of small molecules with single wall carbon nanotubes using density functional theory calculations and molecular dynamics simulations. The trends in binding strength are compared with trends derived from experimental adhesion measurements made using atomic force microscopy. Future work will incorporate the most promising candidates from this work as functional groups on high performance polymers, hopefully yielding outstanding materials for future structural applications.

[C1.038] Fluids

[C1.039] Phase separation in a multi-component driven fluid mixture:a lattice gas computer simulation approach

An interacting lattice gas model is used to study phase separation and density profiles in a multi-component fluid mixture described by constituents A and B in a host matrix (S). Initially, particles (A,B) are distributed randomly on a cubic lattice. Nearest neighbor interactions (I) among particles (i.e, I_AA = I_BB = -I_AB, immiscible) are considered along with excluded volume. Bottom of the lattice (x=1) is connected to a source of particles while the top end (x=L) is open. Dissilimar mass of A and B (M_B = 3 M_A) is used for gravitational sedimentation. Apart from the concentration gradient, a hydrostatic pressure bias (H) is used to drive particles from the source. Metropolis algorithm is used to move particles with a periodic boundary condition along the transverse (Y,Z) direction and open boundary along the flow (X). As simulation proceeds, flux rate of particles approaches an steady-state. Density and velocity profiles are studied in detail as a function of hydrostatic bias.

[C1.040] Complex Structure Formation in a Ternary Mixture Driven Through the Patterned Microchannel

Through the computer simulations, we probe the behavior of a A/B/C ternary mixture in which two immiscible components, A and B, undergo the interfacial chemical reaction and produce third component, C. The reverse chemical reaction, namely consumption of the A and B species from the C components, is also possible. This mixture is driven by the imposed pressure gradient (Poiseuille flow) through the three dimensional microchannel. The microchannel is decorated with chemically distinct patches that display preferential wetting interactions for the A or B components. Namely, A(B)-like patches are placed in the way of B(A)-rich fluid stream. At the beginning of the simulation two streams of fluid, A and B, are flowing parallel each other. As system evolves to it’s steady state, the C component is formed in the A/B interfacial area. Simultaneously, A(B) reach fluid diffuses to A(B) –like patches, so that two stream flow distorts and additional interfacial regions, where chemical reaction may occur, are created. Interplay between the advection of all three components by imposed flow and all the diffusion processes in the presence of spatially non-uniform interactions with the substrate, lead to the formation of complex structures, periodic in space and time.

[C1.041] A molecular dynamics simulation study of the switching dynamics of a nematic liquid crystal under an applied electrical field

To understand the microscopic mechanism of nematic switching in many liquid crystal devices, we have performed a molecular dynamics simulation study of the switching dynamics of nematics with positive polarizability anisotropy under an applied electrical field. Both pretilted nematics (PNs) and nonpretilted nematics (NPNs) under different field strengths are studied to investigate the effects of pretilt and field strength on the switching dynamics. Nematic molecules were modeled as rigid rods which experienced electrical torque, in a mean field approximation, imposed by uniform electrical fields. Our measured switching dynamics agree qualitatively with experiment by exhibiting initiation, fast reorientation, and slow relaxation stages. Coherence lengths under applied fields were estimated from the elastic constants calculated from our simulations. For all systems where the coherence length was larger than the simulation cell size (weak fields), unidirectional switching was observed. For field strengths yielding a coherence length smaller than the simulation cell size (strong fields), NPNs exhibited bidirectional switching. For the PNs, the reorientation of the global nematic director in response to the applied field was well described by a simple Leslie¨CEricksen equation with the rate of reorientation being closely related to the torque due to the external field. For NPNs in the strong field regime, the local director within each unidirectionally switching domain exhibited reorientational dynamics similar to that of the PNs.

[C1.042] Liquid crystal orientation transition induced by microtextured substrates

We have fabricated microtextured substrates with alternating horizontal (x) and vertical (y) sub-micron corrugations. As the period of alternation, \lambda, was decreased from 12 \mum to 0.4 \mum, the liquid crystal (LC) undergoes an orientational transition near \lambda = 0.8 mm, from an inhomogeneous in-plane (pre-tilt angle, \theta = 0^o) configuration, with the LC copying the substrate corrugations, to one that is homogeneous with a large pre-tilt angle (\sim 35^o). This transition, predicted previsouly, is pertinent to a frustrated boundary condition wherein a lowering in the LC elastic energy (due to the spatial variation in the LC director) compromises an increase in the surface anchoring energy. Our result demonstrates the viability of using microfabrication for control of LC alignment.

[C1.043] Investigating the Elastic Properties of N4 Through Freedericksz Transitions

Utilizing a combination of geometries and external fields, we were able to measure the splay elastic constant (K_1) and the diamagnetic anisotropy (\chi _a) for the liquid crystal mixture N4. Data were collected over various temperatures for these measurements. Splay magnetic Freedericksz transitions enabled us to obtain the ratio K_1/\chi _a. Currently, we are in the process of collecting more data for splay electrical Freedericksz transitions, from which we will obtain the splay elastic constant K_1 which will allow us to indirectly measure \chi_a. This research was supported by NSF DMR 9987850.

[C1.044] Physicochemical Properties of Novel Polymer Nanoparticles

The physical and chemical properties of novel methacrylate-based polymer nanoparticles (PNPs) were characterized. AFM and TEM showed that the PNPs in the dry state were nearly spherical nanoparticles with sized of 2-5 nm. FT-IR results showed the appearance of new vibrational modes after solubilization in basic solution. Hydrodynamic diameters and zeta potential as functions of pH, ionic strength, and temperature were examined in order to determine the colloidal stability of PNPs using photon correlation spectroscopy (PCS) and electrophoretic light scattering (ELS). PCS showed that PNPs can be aggregated or swelled depending on the concentration and the pH of the solution. Zeta potential measurements indicated that PNP solutions were stable against sedimentation at low concentrations and high pHs.

[C1.045] Structure and Phase Transitions in Confined Binary Colloid Mixtures

We have studied the packing structures and transitions between phases in quasi-two-dimensional binary mixtures of large and small colloids. The experiments sample the parameter spaces of particle diameter ratio, s, large particle density and small particle density. The depletion potential between the large particles, induced by the presence of the small particles, affects the density at which the large particles undergo a liquid-to-solid transition. For systems with s equal to 4.6, the addition of small particles increases the large particle freezing transition density, a seemingly counterintuitive result given that the depletion potential is purely attractive at low small particle density. When s is equal to 8.8, the large particle freezing transition density is much less affected, but the behavior of other system properties show the same trends as for the system with s equal to 4.6. Phase separation is observed for binary mixtures with s as large as 40. Our results imply that the depletion interaction is strongly dependent on the degree of confinement of the system.

[C1.046] Surface Chemistry Effects for Microrheology of Biological Materials

Microrheology techniques are sensitive probes of the mechanics and microstructure of soft materials. In one application, small colloidal particles are embedded into a complex material, and the Brownian motion of the particles is used to measure local mechanical response. These motions are often interpreted in terms of the local viscoelastic modulus; however, this interpretation depends sensitively on the particle/material interactions. We demonstrate this effect by using colloids with three different surface chemistries to probe the mechanical properties of biopolymer networks. We graft short methoxy-terminated poly(ethylene glycol) (mPEG) chains to the particle surface, and compare the mPEG-coated particles to particles coated with bovine serum albumin, and bare carboxylate-modified latex spheres. We demonstrate that these particles adsorb differing amounts of protein, and using microrheology techniques, show that they measure different physical properties of the biomaterials they probe.

[C1.047] Absorption Optics of Aqueous Foams

The absorption of photons by aqueous foams is studied using diffuse transmission spectroscopy (DTS). The liquid phase of the foam is made absorbing by adding rhodamine dye to the surfactant solution. It is found that for a range of foam liquid volume fractions, the absorption lengths calculated from DTS experiments are smaller than the estimates given by the respective volumes of the two phases. We conclude that multiply scattered photons can get channeled into the liquid network. Computer simulations also show this phenomenon. Our results encourage us to consider novel mechanisms of transport of photons, such as their total internal reflection inside the Plateau borders.

[C1.048] Discrete analogs of two-dimensional flow

Two-dimensional flow patterns on the surface of a torus arising from the Navier-Stokes equation and other related equations are studied in discrete analog. In the topological calculus, gradient, divergence and curl correspond to the boundary \partial and coboundary \partial^ and the cross product, dot product and scalar multiply are the cup ^\cup and cap ^\cap. Incompressible flow on the torus, described by the discrete Navier-Stokes \[ \rho\,\fracddt\,v\,+\frac12\,\partial^\dagger\,(v^\cap v)-\rho\,v^\cap (\partial^\dagger v)\,=\,f-\mu\,\partial \partial^\dagger\,v, \] is driven by an applied shear f. We observe transitions as a function of Re.

[C1.049] Surfaces, Interfaces and Thin Films

[C1.050] Positron induced Auger spectroscopy of Ge/Si (001)

Ge/Si(001) self-assembled nanoscale islands have attracted a lot of attention because of its promising future for optical devices. These islands structures are grown by Stranski-Krastanov model. Prototype samples were produced in the homemade MBE system successfully in The University of Texas at Arlington. The intermixing of Ge and Si during growth and following annealing process is critical for the optical properties of islands structure. Raman spectroscopy analysis found the content of Ge in islands is above 80% for a specific growth procedure. The use of Positron induced Auger spectroscopy using Time of Flight (TOF PAES) technique analysis to study surface composition variation of islands structure will be discussed. TOF PAES measurement could help to confirm the Raman spectroscopy results and better control of the quality of those nano-structures. Depth profile of islands composition will also be studied using combination of PAES and Electron induced Auger Spectroscopy (EAES).

[C1.051] STUDY OF GROWTH AND MORPHOLOGY OF Ge/Si (001) NANOSTRUCTURES

Self-assembled, coherent, dislocation-free Ge quantum dots in Si constitute a novel class of materials for Si-compatible optically active structure and are candidate materials for future Near Infrared optoelectronic devices. Control of the quantum dots’ elemental contents permits manipulation of their optical properties. In the research reported here, Ge quantum dots were grown on Si (001)using a custom built solid-source MBE system. The Ge coverage, Si temperature and the growth rates were varied in order to study the influence of these parameters on islands morphology and size. Prototype samples with different growth conditions were produced and characterized using Atomic Force and Transition Electron Microscopes. The results were used to determine the dependence of the Ge nanostructures size, shape and elemental concentration distribution on growth conditions.

[C1.052] Gamma spectra resulting from the annihilation of positrons with elecrons in single core levels of Cu, Ag and Au

The gamma spectrum associated with the annihilation of positrons with individual core levels (Cu 3p, Ag 4p,and Au 5p) are presented. The spectra were obtained by measuring the energy of gamma-rays time-coincident with Auger electrons emitted as a result of positrons annihilating with a selected core level. Relativistic calculations show good agreement with experiment over a limited range of momenta. However, statistically significant differences indicate that the measurements can provide an impetus to new calculations of many body effects in positron-core electrons annihilation.

[C1.053] A Surface X-ray Diffraction Study of Hydrogen on Ni(111)

Surface X-ray diffraction has been used to study the 2x2 reconstruction of Hydrogen on Nickel (111) at low temperature. In agreement with the previous LEED study of Hammer et al. we observed a small buckling of Ni atoms. This experiment shows that X-ray diffraction is as sensitive as LEED for surface crystallography. Experiments were also carried out at higher temperature on Ni(111) with hydrogen pressures from 10^-6mbar to 1 bar. Under these conditions, changes to the surface probably due to hydrogen dissolution are observed.

[C1.054] Anomaly of the height correlation in a surface with a globally constrained dynamics

We consider the scaling behavior of a one-dimensional model whose local motion is stochastically controlled by a global surpression of the extremal heights. The model is related to the stochastic even visiting random walk problem. Using Monte Carlo simulation, the root-mean-square height of the surfaces are analyzed and the roughness exponents \alpha = 1/3 and the dynamics exponents z=3/2 are reproduced. However, our analysis of the height correlation function gives rise to the wandering exponent \alpha'=1/2 and its dynamic exponent z'=9/4 which are different from \alpha and z. These results indicate that the saturated correlation length is not the system size L but is only L^\delta with \delta=2/3.

[C1.055] Time Dependence of Step Fluctuations on Vicinals of Cu(100)

We have examined thermally induced time dependent changes in the step edge profiles of vicinals of fcc(100), for a range of surface temperatures, using kinetic Monte Carlo simulations and activation energy barriers calculated from reliable, semi-empirical interaction potentials. Equilibrium fluctuations of steps on Cu (1,1,13) and Cu (1,1,19) surfaces show a power law time dependence with exponent 0.25. Theoretically, such a power law behavior arises when the dominant process is that of atoms hopping along the step edges. Analysis of our KMC simulation indeed reveals the dominance of processes related to motion of atoms along the step edge, in the temperature range 320K to 400K. We also present simulations of terrace width distribution on Cu(1,1,13) as a function of temperature. Using a mean field model, the strength of the step-step interaction is determined from the width of a Gaussian fit. Our calculated step - step interaction strength is of the order of 1 meV which is smaller than those inferred from experiments [1].

1. M.Giesen, Surface Science 370 (1997) 55.

[C1.056] Scattering of Electromagnetic Waves From a Rough Dielectric Film on a Planar Metallic Substrate

We present the scattering of electromagnetic waves from a rough surface of a dielectric film deposited on a planar metallic substrate. The diffuse light (the incoherent part of the mean differential reflection coefficient) is computed by using a numerical method and perturbation theory, for both p- and s- polarization of the incident light. In abscence of roughness the dielectric film supports two guided modes, which wave numbers are q1 and q2, for wavelength l. For this system, the angular distribution of the diffuse light display two satellite peaks besides the backscattering peak, owing to the coherent interference from incident light and guided modes. We present theoretical results for air/ZnS/Au and air/MgF2/Ag using the West-O’Donnell and Gaussian spectra.

[C1.057] Self Assembly of Paired Plate Precipitate Structures in Pb0.91La0.09Zr0.65Ti0.35O3 PLZT Thin Films

We report on the phase transformation and formation of paired plate precipitates in Pb0.91La0.09Zr0.65Ti0.35O3 (9/65/35) PLZT films. Growth of the PLZT films was conducted using magnetron sputtering on sapphire substrates. A random equiaxed polycrystalline grain morphology was observed after furnae annealing or rapid thermal processing. The as-deposited films had predominantly the pyrochlore phase. Paired plate precipitate formation was observed after furnace annealing at 700C in films with deposition temperatures in excess of 490C and with a perovskite structure. Average precipiate dimensions were 150 angstroms in length and 30 angstroms ini width. This phase transformation behavior could be related to internal stresses in the films due to the lattice mismatch and the thermal expansion mismatch between the film and the substrate.

[C1.058] Electrical Transport Properties of InAs Epilayer Grown on GaAs (001) Substrate By MBE

We have developed two-step method to obtain high quality InAs epilayer on GaAs substrates. Firstly, InAs epilayer is grown under In-rich conditions as prelayer; secondly, growth of InAs layer continues on such a prelayer under As-rich conditions. The two-dimensional growth mode is maintained through the entire growth process of InAs under In-rich conditions and As-rich conditions. The hall measurement shows that the thickness of undoping layer of InAs epilayer grown under As-rich conditions affect electron mobility and electron concentration. When the thickness of InAs epilayer grown under As-rich conditions is up to 1500nm, the electron mobility and electron concentrations reach saturation. By doping Si into the part of InAs epilayer, which is on undoping 1500 thick InAs epilayer grown under As-rich conditions, InAs epilayer with high electron mobility are obtained.

[C1.059] Determination of the activation energies based on the surface morphology of YBa2Cu3O7-x thin films

Activation energies in complex oxides has been investigated taking as an example high quality YBCO thin films grown by sputtering. Atomic force microscopy gave good statistical information on average terrace width and island density. The above measurements and its temperature dependence provide a very accurate technique to determine the surface activation energies. The data fits were made with the expression obtained from a two components model. A comparison between experiment results and data published by other groups allowed us to determine the values of the activation energies which are relevant for the surface diffusion of adatoms as well as for the evaporation of adatoms from the surface to the vapour. The activation energies has been obtained from upper and lower limits for diffusion length and diffusion time. Until now only upper limits have been used.

[C1.060] Quasi-Equilibrium in Epitaxial Growth

We have produced a square geometry solid-on-solid (SOS) kinetic Monte Carlo model to investigate the presence of a low coverage quasi-equilibrium regime during epitaxial growth. The SOS model is a restricted pair-bond model wherein atoms on the surface with one in-plane nearest neighbor are allowed to detach from island edge positions with a rate, r1. Thus, the simulation is applicable to homoepitaxial (100) metallic surfaces with a critical nucleus size of 3 (the critical nucleus is defined as one less than the number of atoms required to form a stable island). In the quasi-equilibrium regime the number density of islands of size less than or equal to the critical nucleus is predicted to scale with the number density of monomers according to the so-called Walton relation. We have studied the scaling of the number of unstable dimers and trimers with the number of monomers during growth to low coverages to determine the presence of the equilibrium distribution as evidenced by the verification of the Walton relation. We show that this relation does not always hold during epitaxial growth and that its presence depends strongly on the ratio of the single atom diffusion rate D to the deposition flux F, and the ratio r1/D. We report on the ranges of these ratios over which equilibrium is observed.

[C1.061] Local atomic structure of partially ordered NiMn films investigated by EXAFS spectroscopy

The local atomic structure of NiMn in NiMn/NiFe exchange coupled films was investigated using Mn K-edge x-ray absorption fine-structure measurements. Different stages of L1_0 chemical ordering in the NiMn pinning layer were produced by changing the substrate growth temperature from 3 to 200^oC. A short range order (SRO) parameter was evaluated and compared to a long-range order (LRO) parameter obtained from conventional diffraction measurements. The SRO is notably larger than the LRO parameter, indicating a local phase separation in the films. The SRO parameter increases with samples growth temperature, in agreement with the observed ferromagnetic exchange coupling, providing an evidence of nanometer-scale ordered clusters at the beginning stages of macroscopic L1_0 phase formation. This work is supported by Materials Science and Technology Division of LANL.

[C1.062] Heats of Segregation of BCC Metals Using Ab Initio and Quantum Approximate Methods

Many multicomponent alloys exhibit surface segregation, in which the composition at or near a surface may be substantially different from that of the bulk. A number of phenomenological explanations for this tendency have been suggested, involving, among other things, differences among the components' surface energies, molar volumes, and heats of solution. From a theoretical standpoint, the complexity of the problem has precluded a simple, unified explanation, thus preventing the development of computational tools that would enable the identification of the driving mechanisms for segregation. In that context, we investigate the problem of surface segregation in a variety of bcc metal alloys by computing dilute-limit heats of segregation using both the quantum-approximate energy method of Bozzolo, Ferrante and Smith (BFS), and all-electron density functional theory. In addition, the composition dependence of the heats of segregation is investigated using a BFS-based Monte Carlo procedure, and, for selected cases of interest, density functional calculations. Results are discussed in the context of a simple picture that describes segregation behavior as the result of a competition between size mismatch and alloying effects

[C1.063] Energy spectrum of Bloch electrons under periodic electric and magnetic modulations

We study the energy band structures of Bloch electrons under periodically modulated electric and magnetic fields. The electric field and the magnetic field are assumed to have the same translational symmetry. Two types of modulation are considered. The first type is a one-dimensional stripe modulation. The second type is a two-dimensional checker-board modulation. The calculation is based on a tight-binding model with nearest neighbor and next-nearest neighbor hoppings. The energy spectra for different relative strengths between the electric and magnetic modulations are presented. This calculation, under an appropriate Chern-Simons transformation, can be related to the study of the magnetization process of antiferromagnetic spin-1/2 J_1-J_2 Heisenberg model.

[C1.064] Investigation of thermal vibration correlation of [110] silicon lattice atoms by ion scattering

When a beam of fast ions hits an atom, a shadow cone is formed behind the atom. By varying the ion energy, the radius of the shadow cone can be changed and also the interaction probability between ions and lattice atoms. At strong vibration correlations, neighboring atoms are located almost inside the shadow cone and this acts to reduce the interaction probability. The Si-KLL Auger electron emission was used to detect the reduction of the interaction between ions and lattice atoms, which is a measure for the vibration correlation. The yield of the Si-KLL Auger electrons was measured as a function of the incidence angle about the [110] axis with helium ions and protons. The correlation coefficient was determined by comparison of the minimum Auger electron yield with the results of computer simulations. The correlation coefficient of the normal displacement of nearest-neighbor silicon atoms along [110] at room temperature is determined to be 0.90.

[C1.065] First-principles calculations of Pt surface phonon dispersion curves

We have calculated dispersion curves for surface phonons of Pt(100), Pt(110) and Pt(111), using first-principles, total energy calculations based on a mixed-basis set and norm-conserving pseudopotentials. Linear response theory and the harmonic approximation of lattice dynamics are also invoked. For bulk Pt our calculations show the experimentally observed anomaly along (110) direction, and the calculated relaxations on the three surfaces are also in agreement with previous results. The dispersion of the Rayleigh wave and the resonance modes on Pt(111) are in good agreement with data from He scattering measurements. Our results of phonon dispersion for Pt(100) and Pt(110) are for their unreconstructed surfaces. The richness in the phonon disersion curves for the three surfaces will be compared and conclusions presented about the changes in the surface force constants from the bulk values. The propensity of two of the surfaces to reconstruct will also be discussed.

[C1.066] The electronic and atomic structure of Ag/Cu(110) and Ag/Cu(100)

The electronic structure of Ag on Cu(110) and Cu(100) has been studied with angle-resolved photoemission spectroscopy (ARPES). Previous STM results reveal that as a function of coverage, Ag undergoes an alloy-to-dealloy transition as a function of increasing coverage below a monolayer coverage. In the case of Cu(110), Ag forms a substitutional, surface confined alloy below ~0.4 ML and ARPES reveals bulk-like dispersion of Ag-3d bands. However, at higher coverages (~ 0.8 ML), Ag “de-alloys” and forms 1-D atomic chain structures along [001]; ARPES reveals a quasi-1D electronic structure of the Ag d-bands parallel to the chains. In the case of Cu(100), Ag also forms a surface confined alloy at low coverages and the Ag electronic structure is 3-D like, due to hybridization with Cu. At higher coverages (> 0.2ML), Ag dealloys, forming a 2D overlayer structure whose d-bands disperse only in-plane. ARPES results will be compared to STM data and theoretical band structure calculations.

[C1.067] A Nonlocal Image Formulation of van der Waals Atom-Surface Attraction in a Magnetic Field

The theory of van der Waals (vdW) atom-surface attraction (to second order vdW energy) is explicitly exhibited as a correlation/self-energy of atomic electrons generated by a dynamic, nonlocal image potential due to polarization of the electrons of the bounded metal-semiconductor surface system in the electrostatic limit. This formulation is applied to a metal/semiconductor plasma in a magnetic field perpendicular to its bounding surface. The dependence of atom-surface vdW energy on magnetic field strength provides an adjustable parameterization of the underlying zero-point photon energy (represented in terms of the nonretarded longitudinal plasmon-photons of the Coulomb interaction), opening the possibility of analyzing the concomitant fundamental quantum phenomenology in detail with material parameters that can be examined experimentally. The determination of the image potential, including its nonlocal and dynamic magnetic field effects, involves the construction of a ``surface dielectric function'', which is carried out using a Green's function joining procedure for nonlocal, dynamic electrostatics. In this aspect of our second order vdW energy calculation, we take account of the role of the magnetic field by means of a hydrodynamic model of magnetoplasma nonlocality in dynamic longitudinal dielectric response. Both local and nonlocal magnetic field effects in vdW energy are analyzed within the framework of a multipole expansion, and are also discussed, respectively, in expansions in powers of ømega_c^2 (ømega_c is the cyclotron frequency). Furthermore, we determine the role of Landau quantization magnetic field effects in the skewing of the surface electron charge distribution from its uniform positive background, exhibiting de-Haas-van Alphen oscillatory (and ``staircase'') behavior arising in connection with first order vdW energy.

[C1.068] Enhanced optical transmission through subwavelength holes randomly distributed in a thin gold film

Randomly distributed small holes of various subwavelength sizes were fabricated in a thin gold film. We study the optical near-field transmission of the film. In the wavelength spectrum from 350nm to 650nm, a number of strongly enhanced transmission peaks were observed. These transmission peaks can only been observed in the near field. We attribute the new phenomenon to the surface plasmon coupling inside the holes and between the surfaces on the two sides of the thin film.

[C1.069] A Method for Suppressing Superconductivity of Thin Films

We have developed a method for suppressing superconductivity of thin films. Thin stripes of cobalt grown by e-gun evaporation and patterned by e-beam lithography were placed in the vicinity of aluminium thin film structures. The cobalt stripes were magnetized at 4.2 K with a superconducting coil and the remanence suppressed superconductivity of the Al stripe at temperatures down to 50 mK at least. The magnetization remained in thermal cycling and in a longer storage at room temperature. Motivation for this work is the Coulomb Blockade Thermometer(CBT)^1 which has to be in a normal state to operate. The CBT sensor contains aluminium which is superconducting at temperatures below 1.4 K. An external magnetic field is not always available or acceptable in cryostats. A small grain of permanent magnet mounted to the sensor is another solution, but suspicious if the sensor is put in strong magnetic fields or if "zero field" environment is required. We have shown that suitably patterned and magnetized Co stripes in the vicinity of tunnel junctions of the CBT can solve this problem. The amount of magnetic material in the sensor, as well as the stray field, is very small. This technique may be useful in other low temperature thin film devices also.

1) Product of Nanoway Ltd.

[C1.070] The Role of Landau Quantization in Atom-Surface van der Waals Interaction

The role of magnetic Landau quantization effects in second-order van der Waals (vdW) atom-surface energy are analyzed here, for the case in which a magnetic field is applied perpendicular to the surface. Focusing on the electrostatic nonretarded limit, we employ a random phase approximation (RPA) description of the dynamic, nonlocal polarizability of the mobile semi-infinite Landau quantized plasma behind the semiconductor surface. Our initial examination is carried out using a general low wavenumber approximation of the bounded plasma dielectric function both parallel and perpendicular to the surface, to expeditiously determine quantum magnetic field effects in the second-order vdW energy. Since the formulation calls for an integration over all wavenumbers perpendicular to the surface, p_z, we subsequently eliminate the (reasonable) approximation of the RPA polarizability in powers of p_z to obtain more accurate results in this respect. In this study, we note that quantum magnetic field effects are most prominent in semiconductors which have relatively low density, and correspondingly small polarizability, so we expand the vdW energy as a linear functional of the RPA polarizability. Explicit analytic results for quantum magnetic field effects in vdW energy due to a neutral atom in Coulombic interaction with a semi-infinite nonlocal, dynamic semiconductor plasma are obtained and presented.

[C1.071] The Role of 2D Magnetoplasmons in van der Waals Interaction of a Quantum Dot Atom with a 2D Quantum Well Plasma

The van der Waals (vdW) interaction of a quantum dot artificial atom with a two- dimensional (2D) quantum well plasma in a normal magnetic field is addressed here in the electrostatic limit to dipole-dipole terms. In this analysis I employ a dynamic, nonlocal image potential due to polarization of the electrons of the 2D quantum well induced by electrons of the neutral atom. The dependence of the atom-dot vdW energy on magnetic field strength provides an adjustable parameterization of the underlying zero-point (longitudinal) photon energy. An explicit analytic result involving the principal local 2D magnetoplasmon alone is obtained and seen to have a \vert Z \vert^-4 dependence (\vert Z \vert is the dot-2D well seperation). Furthermore, I determine nonlocal corrections which include the role of the 2D Bernstein mode (n=2) branch of the magnetoplasmon spectrum, as well as nonlocal corrections to the contribution of the local magnetoplasmon. These nonlocal terms produce a \vert Z \vert^-6 dependence in the vdW atom-well interaction energy.

[C1.072] Mossbauer Study of Magnetic Ordering of Fe/Gd Multilayers

Fe/Gd multilayers of varying Fe and Gd layer thickness were fabricated at room temperature on polyester substrate by planar magnetron sputtering to study the magnetic ordering of their interface. Two major magnetic structures were identified with x-ray, and, in addition to these, two more nonmagnetic structures were identified from Mossbauer measurement. The composition of the interface structure changes with Gd as well as Fe layer thickness. For an iron layer thickness larger than some critical thickness the perpendicular uniaxial anisotropy(PMA) is very small, and it increases with decreasing Fe layer thickness. The PMA of Fe/Gd multilayers has the same characteristic as that of Fe/Tb multilayers, and suggests that the PMA of these two groups of multilayers may have a common source of origin.

[C1.073] Normal Metal-Superconducting Proximity Stacks

Much recent work has examined transport properties of mesoscale normal metal-superconducting (N-SC) proximity structures. Relatively little attention has been placed on magnetic properties of such systems. We present preliminary SQUID measurements of magnetization of lithographically defined mesoscale N-SC stacks made from Au and Pb. Size and temperature dependence are discussed.

[C1.074] Magnetic Resonance Force Microscopy Investigation of Yttrium Iron Garnet Films

We present the design and operation of a Magnetic Resonance Force Microscope (MRFM), and data from the investigation of ferrimagnetic Yttrium Iron Garnet (YIG) films. Two types of probe tips are used in this investigation. The first probe is a conventional Magnetic Force Microscope (MFM) tip. This probe tip shifts the resonance frequency and magnitude of the YIG’s resonant absorption, which is frequency and field-dependent. The second probe uses cantilever with a micro-fabricated conducting loop on its tip. The loop is used both as a high frequency receiver and transmitter antenna, in separate experiments, allowing for localized probing of spins in the YIG sample. This approach to MRFM has not been reported previously. Images and MRFM spectra from YIG films are presented.

[C1.075] Biological Physics

[C1.076] Dielectric properties of gel collected from shark electrosensors

To investigate the physical mechanism of the electric sense, we present an initial characterization of the dielectric properties of the glycoprotein gel that fills the electrosensitive organs of marine elasmobranches (sharks, skates, and rays). To ascertain the properties of the gel, low-frequency impedance spectroscopy is used. The impedance data collected from a dialyzed sample show large values of static permittivity and a loss peak corresponding to a long relaxation time (about 1 ms). Impedance measurements of the native (nondialyzed) gel reliable to 0.1 Hz will be presented and compared to the dialyzed gel. Ramifications of the gel’s dielectric properties for the electric sense will be explored.

[C1.077] Numerical Investigation of Macroscopic Cardiac Mechanics.

In order to gain insight into the complex interactions between electrical excitation of the myocardial tissue, the mechanical contraction of the heart muscles and cardiac fluid dynamics, three computational techniques are successfully coupled. A Gerhardt-Schuster-Tyson Cellular Automata algorithm enables the excitation kinetics of myocardial tissue to be simulated in a computationally efficient manner. The cardiac excitation spreading is then coupled with a dynamic Born Lattice Spring Model which enables the contraction of the heart muscles and their subsequent relaxation to be modelled. The velocities at the inner surfaces of the heart can then be transferred to a Lattice Boltzmann simulation of blood flow within the cardiac chambers. The interactions (and complex feedback mechanisms) between electrical excitation, mechanical deformation, and fluid flow in the heart are explored through these three-dimensional models and the regular functionality of the whole heart is visualised.

[C1.078] Measurement of electro osmotic flow (EOF) in a DNA surface electrophoresis cell

Determining the influence of electro osmotic flow ( EOF) on DNA motion in elctrophoresis is essential in understanding the mechanism of electrophoretic mobility of DNA. Inorder to optimize and control the conditions for DNA separation by electrophoresis on flat surfaces we measured the EOF as a function of concentration, geometry of the cell, and field strength. Uncharged Polystyrene beads of diameter 1 mm with fluorescent labels(Molec Probes) were used to measure flow patterns. The Confocal microscopy was used to measure EOF profiles for various distances from the Si surface. Results were compared to Poisson-Boltzmann/ Fluid dynamical calculations and qualitative agreement was found for the trends in buffer concentration.

Support from NSF ( MRSEC) is gratefully acknowledged.

[C1.079] On a modified directed percolation model

Using extensive numerical simulations, two variants of directed polymer/directed percolation problem in 1+1 dimensions are studied in details. Due to their similarity to biosequence alignment, we called the two variants Local and Global, corresponding to the local and global versions of sequence alignment. For both variants, we have verified and characterized both the ground state energy and cluster size distribution. Similar to the traditional directed polymer(DP) problem, each path transversing the disorder poteintial landscape carries an energy given by the sum of disorder potential energy and the elastic (or gap) penalty. Unlike the traditional DP problem, path length is not a fixed number in a given ensemble. By varying the probability of getting a low energy bond, we observed the system undergoing a percolation transition. For Local alignment, we observed the free energy diverging with the critical exponent 1/2 near the phase transition, as expected from KPZ universality. For Global alignment, however, the free energy (cluster size) decrease (increase) exponentially, different from predictions of renormalization group calculation on the usual percolation systems. In addition to characterizing the finite size effect of both variants, we also verified and characterized both the free energy and cluster size distribution functions above, right at, and below the percolation transition.

[C1.080] The effect of denaturant on protein stability: a Monte Carlo lattice simulation

Denaturants are the reagents that decrease protein stability by interacting with both nonpolar and polar surfaces of protein when added to the aqueous solvent. However, the physical nature of these interactions has not been clearly understood. It is not easy to elucidate the nature of denaturant theoretically or experimentally. Even in computer simulation, the denaturant atoms are unable to be dealt explicitly due to computationally enormous costs. We have used a lattice model of protein and denaturant. By varying concentration of denaturant and interaction energy between protein and denaturant, we have measured the change of stability of the protein. This simple model reflects the experimental observation that the free energy of unfolding is a linear function of denaturant concentration in the transition range. We have also performed a simulation under isotropic perturbation. In this case, denaturant molecules are not included and a biasing potential is introduced in order to increase the radius of gyration of protein, which incorporates the effect of denaturant implicitly. The calculated free energy landscape and conformational ensembles sampled under this condition is very close to those of simulation using denaturant molecules interacting with protein. We have applied this simple approach for simulating the effect of denaturant to real proteins.

[C1.081] Denaturation and Amyloid Fibril Formation of Insulin at Model Lipid-Water Interfaces

The role played by surfaces in the denaturation and subsequent amyloid fibril formation of certain proteins is unclear. In this study we consider the effects of confining the model protein, bovine insulin, at different charged lipid surfaces. The aim is to determine the effects that localisation at an interface, has upon the rate of unfolding and the changes in the \beta-sheet content of the protein when it is placed under denaturing conditions (low pH and elevated temperature). Adsorption, unfolding and changes in the \beta-sheet content of the protein are monitored using FTIR ATR spectroscopy and are compared to the behaviour of the bulk protein solution. We show that localisation of the proteins at these surfaces results in a more rapid unfolding than is observed in the bulk solution and that the \beta-sheet content of the insulin molecules is also reduced at the surfaces. The resulting external aggregate/fibril morphologies are then compared using a series of atomic force microscopy experiments (AFM) performed on samples taken from both of the surfaces and the bulk protein solution. The AFM images show that the increased level of disorder in the protein molecules at the surface, (arising from a greater extent of the unfolding and reduction in \beta-sheet content) affects the morphology of high \beta-sheet content structures like fibrils. An attempt is made to explain the observed differences in the behaviour of the adsorbed protein molecules, in terms of the balance of enthalpic and entropic mechanisms involved in protein adsorption.

[C1.082] Crystallographic and Computational Studies of a Class II MHC Complex with a Nonconforming Peptide: HLA-DRA/DRB3*0101

The stable binding of processed foreign peptide to a class II major histocompatibility (MHC) molecule and subsequent presentation to a T cell receptor is a central event in immune recognition and regulation. Polymorphic residues on the floor of the peptide binding site form pockets that anchor peptide side chains. These and other residues in the helical wall of the groove determine the specificity of each allele and define a motif. Allele specific motifs allow the prediction of epitopes from the sequence of pathogens. There are, however, known epitopes that do not satisfy these motifs: anchor motifs are not adequate for predicting epitopes as there are apparently major and minor motifs. We present crystallographic studies into the nature of the interactions that govern the binding of these so called nonconforming peptides. We would like to understand the role of the P10 pocket and find out whether the peptides that do not obey the consensus anchor motif bind in the canonical conformation observed in in prior structures of class II MHC-peptide complexes. HLA-DRB3*0101 complexed with peptide crystallized in unit cell 92.10 x 92.10 x 248.30 (90, 90, 90), P41212, and the diffraction data is reliable to 2.2ÅWe are complementing our studies with dynamical long time simulations to answer these questions, particularly the interplay of the anchor motifs in peptide binding, the range of protein and ligand conformations, and water hydration structures.

[C1.083] Effects of the Heme Side Chains on Ligand Rebinding-Kinetics of CO binding to porphine reconstituted myoglobin

We use porphine reconstituted myoglobin to study the effects of heme side chains on CO rebinding kinetics. We measured CO rebinding kinetics of porphine reconstituted myoglobin by laser flash photolysis and characterized samples of the deoxy and CO bound states using Raman spectroscopy. Comparison of the CO rebinding kinetics of porphine reconstituted myoglobin with native myoglobin shows that the geminate rebinding and ligand escape are almost the same, but the ligand entry rate in porphine reconstituted myoglobin is 9 times faster. The Raman spectra of the deoxy and CO complexes of porphine reconstituted myoglobin show significant structural changes due to the removal of the heme side chains.

[C1.084] Raman Study of Wet-Spun Films of Lithium Hyaluronate as a Function of Relative Humidity

Raman scattering experiments have been performed on wet-spun films of lithium hyaluronate as a function of relative humidity (RH). Vibrational data have been recorded between 600 and 1700 cm^-1. The frequency of the observed modes are essentially independent of RH. However, changes in the relative intensities of the Raman bands are observed as the RH is varied.

[C1.085] THEORETICAL CHARACTERIZATION OF THE LONG-RANGE ATTRACTION BETWEEN G-ACTIN MOLECULES THROUGH THE EXCLUDED VOLUME EFFECT.

One of the interactions between macromolecules is the attractive force through the excluded volume effect. We studied the attraction between the molecules of muscle protein, actin, in the two points by using the extended scaled particle theory (XSPT). I) we verified the basic assumption used in the XSPT that topological elements which determine the analytical expression of the excluded volume are almost unchanged through the scaling down of the solute molecule in the thought experiment. Results of the computational geometry method (alpha-shape method) showed that this assumption is valid even in the case of the actin molecule. II) we calculated the attraction between actin monomer molecules, G-actin. Calculated differences of the values of the attraction potential of two macromolecules between at contact and at one macromolecule apart by the XSPT is almost the same as those by the Asakura-Oosawa theory.

[C1.086] BULK SPECTROSCOPY AND SINGLE-MOLECULE STUDIES ON eqFP611, A NOVEL RED FLUORESCENT PROTEIN.

Fluorescent Proteins (FPs) have become extremely popular in life science research as protein labels, markers of gene expression and reporters of environmental conditions in living cells. The fluorescent protein eqFP611, cloned from the sea anemone Entacmaea quadricolor, has the largest Stokes shift (52 nm) and the most red-shifted fluorescence emission (611 nm) of all non-modified FPs that are currently available in recombinant form. Other properties make it even more advantageous for biological applications: pH independent fluorescence emission in the pH range 4 – 11, fast and complete maturation of the fluorophore and reduced oligomerization tendency. To examine the photophysical properties of eqFP611, we have measured absorption, excitation, and emission spectra using bulk spectroscopy in a wide temperature range (12 – 350 K). From these data, the temperature dependencies of the quantum yield, electron-vibronic coupling factors, and oscillator strength of the electronic transition have been determined. Moreover, we have studied the fluorescence emission using fluorescence correlation spectroscopy (FCS) and single-molecule studies. We present a quantitative model of the light-driven dynamics.

[C1.087] DNA Amplification Using Rayleigh-Bénard Convection

We demonstrate a novel device employing the circulatory flow field established by Rayleigh-Bénard convection to perform amplification of DNA using the polymerase chain reaction (PCR) inside a 35 microliter cylindrical cavity. This Rayleigh-Bénard PCR (RB-PCR) cell eliminates the need for dynamic external temperature control required in conventional thermocyclers that repeatedly heat and cool static sample volumes to denaturation, annealing, and extension temperatures (approximately 95, 55, and 72 °C respectively). Instead, a convective flow field is harnessed to perform temperature cycling through thermal equilibration of fluid packets with their surroundings as the flow continually shuttles fluid packets vertically through the temperature zones associated with denaturation, annealing, and extension. Although we found it necessary to employ high aspect ratio cavities in order to generate a suitable flow field to drive the PCR reaction, a variety of flow patterns ranging from steady convective motion to multiple rolls to turbulence can be established to suit the requirements of other reaction systems. Using this RB-PCR technique, we are able to successfully amplify a 295 base target region from a human genomic DNA template.

[C1.088] AM1 study of N-2-acetylaminofluorene bonded to deoxyguanosine at the minor adduct site

We have computed the total energy as a function of six important torsion angles of the carcinogen N-2-acetylaminofluorene (AAF) bonded to the nitrogen N2 of deoxyguanosine using the semiempirical quantum mechanical method AM1. One global minimum and one local minimum are found separated by a modest barrier. We have computed the normal-mode frequencies of the relevant torsional motions and have determined the rate of conversion between the two minima.

[C1.089] Experimental and Theoretical Study of the Vibrational Spectra of Cytidine and Deoxycytidine

Raman and infrared spectra have been recorded for crystalline cytidine and deoxycytidine samples at 300, 200, 100 and 10 K and compared to the calculated eigenfrequencies for these systems under a harmonic approximation including a standard valence force field, van der Waals and Coulombic interactions. Data were taken over the spectral ranges from 20 to 4000 cm-1. The wavenumbers of most vibrational modes were found to increase as the temperature is lowered. A number of peaks split into multiplets at low temperature. A new mode, observed at 375 cm-1, is a marker band for 2'-deoxyribose in cytosine-containing nucleic acids. Evidence is presented supporting the assignment of marker bands associated with the C3'-endo and C2'-endo puckers of deoxyribose.

[C1.090] Experimental and Theoretical Study of the Vibrational Spectra of Adenosine and Deoxyadenosine

Raman and infrared spectra have been recorded for crystalline adenosine and deoxyadenosine samples at 300, 200, 100 and 10 K and compared to the calculated eigenfrequencies for these systems under a harmonic approximation including a standard valence force field, van der Waals and Coulombic interactions. Data were taken over the spectral ranges from 20 to 4000 cm-1. The wavenumbers of most vibrational modes were found to increase as the temperature is lowered. A number of peaks split into multiplets at low temperature.

[C1.091] Dynamics of DNA Chains on Flat and Patterned Surfaces

The electrophoresis of DNA chains on flat silicon and patterned surfaces was studied by Confocal Fluorescence Microscopy and Atomic Force Microscopy. Solutions of lambda DNA of 48,502 bp and Schizosaccharomyces pombe (S. pombe) of 3~6 Mb were deposited on different surfaces. The surfaces were chemically modified to be hydrophilic or SAM-covered and the patterns were produced over length scales from nano to micro size in the form of gratings or square arrays. The interaction with the surface and mobility of DNA chains depended on the surface chemistry, topography and ion concentration of buffer. The motion of individual chains in the electric field was analyzed both in terms of the dimensions and orientation of the pattern structure.

Supported by NSF-MRSEC program (DMR-9632525)

[C1.092] Studies of DNA Droplet Drying Kinetics on Flat/Structured Gold Surfaces

Previous studies have shown that on OTS surfaces the evaporation kinetics of droplets containing DNA is a function of DNA molecular weight, DNA concentration, and buffer concentration. Surface properties also affect DNA droplet drying and its morphology on the surface. In this study, DNA droplets were loaded on flat gold and structured gold-silicon micro-grating surfaces. The drying kinetics were observed by measuring time dependence of contact angles and drops were imaged by confocal microscopy. The DNA was labeled with ethidium bromide. On gold surfaces, the drying kinetics were nearly independent of DNA concentration. The drying kinetics will be correlated to the structure of the DNA chains on the patterned surfaces.

Supported by NSF-MRSEC program (DMR-9632525)

[C1.093] Elastic Response of Single DNA Molecules in a Reentrant Collapsing Transition

The elastic response of single DNA molecules in a transition between elongated and collapsed states was observed by dual trap optical tweezers. We monitored DNA extension during stretching and relaxation at various concentration of the trivalent cation, spermidine. When the DNA was collapsed by the addition of spermidine, the force-extension curves showed plateaus and stick-release patterns depending on the concentration. The stick-release response reveals a periodicity with a certain characteristic length, which indicates abrupt release of toroidal structure. Under high concentration of spermidine, the reelongation of a collapsed DNA was observed at single molecule level, implying reentrant transition as was predicted by theory. A simple phenomenological model reproduces the force plateaus and stick-release patterns in the force-extension curves.

[C1.094] High Pressure Infrared Study of Deoxycytidine

Infrared (IR) absorption experiments have been performed on solid deoxycytidine as a function of pressure up to 12.5 GPa at room temperature. A piston-cylinder diamond anvil cell fitted with type IIa diamonds was used to generate the high pressures. KBr was used as the pressure-transmitting medium and to dilute the amount of sample in the IR beam. Vibrational data were recorded between 400 and 1600 cm^-1. The frequencies of the observed modes were found to increase with pressure.

[C1.095] Computational Analysis of DNA Sequences

The availability of complete (or nearly complete) genetic sequences for a growing number of organisms presents an opportunity to extract information regarding physical, chemical, and biological processes from these sequences. Quantities such as gene number density, gene linear density, and single nucleotide and dinucleotide frequencies for exons, exons plus introns, and for all regions have been calculated for the human genome and for other organisms. Among the conclusions drawn is the observation that much of the distinctiveness of the gene regions is attributable to the actual coding regions. Further, while nucleotide frequencies vary between coding and non-coding regions, the dinucleotide frequencies relative to the background seem to be independent of the coding or non-coding character of the sequence. To understand the origin of nucleotide frequency variation, we have performed a cross-species investigation of DNA mutational drift.

[C1.096] Direct evidence for very different excited-state lifetimes in two adenine tautomers

Adenine, like the other DNA bases, can exist in a variety of tautomeric forms in which hydrogen atoms are attached to different heteroatoms. For many of the bases, different prototropic tautomers have significantly different energies and only a single tautomer is present in room-temperature solution. However, for adenine it was argued many years ago that two tautomers co-exist at room temperature, 7H-adenine and 9H-adenine. Using 7-methyladenine and 9-methyladenine to represent the 7H- and 9H-tautomers, we have been able to definitively show that adenine is present as both these tautomeric forms in aqueous room temperature solutionby measuring the respective excited-state lifetimes. The transient absorption signal of electronically excited adenine shows clear biexponential decay due to the 7H- (lifetime ~10 ps) and the 9H- (lifetime ~ 250 fs) tautomers. The replacement of H with a methyl group does not lead to a significant change in the observed lifetimes. Solvent effects on the excited-state dynamics can be rationalized in terms of the different dipole moments of the two tautomers.

[C1.097] Probing active electron transfer branch in photosystem I reaction center.

Complimentary point mutations were introduced at the primary electron acceptor sites in A and B branches of the photosystem I (PS I) reaction center (RC) from Synechocystis sp. PCC 6803 and their effect on the kinetics of the electron transfer process was studied by means of ultrafast pump-probe spectroscopy. The results indicate that in these species the electron transfer occurs primarily along the A-branch. Previous optical experiments on PS I complexes from Chlorella sorokiniana demonstrated that both branches of RC are equally active. That suggests that the directionality of electron transfer in PS I is species dependent.

[C1.098] A 240 GHz high-field transient EPR study of the primary donor triplet state g-tensor in photosynthetic reaction centers of Rhodobacter sphaeroides R-26.

We report time-resolved 240 GHz EPR spectra of the primary donor triplet state ^3P from photosynthetic reaction centers of Rhodobacter sphaeroides R26.1 as a function of temperature in the range 10-230K. The data allow the determination of the principal g-tensor values and the principal axes directions of the ^3P g-tensor with respect to its zero-field axes. The g-tensor measured at 240 GHz differs appreciably from previous measurements of ^3P at lower frequencies and also differs from that of the cation radical state P^+, which has previously been characterized at high frequencies. In contrast to P^+, the ^3P state exhibits significant temperature dependence in its g-tensor, particularly in the direction of the principal axes. The ^3P yield anisotropy first observed by Boxer and coworkers at high field using photoselection methods is also evident in the high-field EPR spectrum as a significant variation of intensity across the spectrum. This variation is analyzed in terms of a radical pair and a yield ratio model.

[C1.099] Investigation of Dendrimer-Membrane Interactions

Modified Polyamidoamine (PAMAM) dendrimers show great promise as targeted drug transport agents. Current research efforts point to the possibility of dramatic improvements to conventional chemotherapy by selectively delivering a therapeutic to antigen bearing tumor cells. In order to better understand the uptake mechanism of such devices into cells we are investigating dendrimer-surface adsorption and dendrimer-membrane interactions using atomic force microscopy, light scattering and computer simulations. Model systems consisting of supported DMPC lipid bilayers have shown interesting results suggesting the shape and architecture of nano-devices play an important role for their biologic activity. We are also investigating the effect of targeted drug vehicles on cells in vitro.

[C1.100] Phospholipid Langmuir layers mixed with novel lipophilic-fullerene-derivative molecules

A lipophilic C_60 derivative, having three lipid-like tails chemically bonded on one olefinic moiety of the C_60 cage has been synthesized, largely motivated by its potential in the biological application. With its lipid-like tails simulating largely the molecular structure of a phospholipid (DPPC), the lipophilic-C_60 molecule is expected to self-incorporate into the phospholipid membranes. Previous X-ray scattering studies showed that the lipophilic-C_60 molecule molecules could intercalate into the DPPC monolayers and modified bending and compression modulus of the host DPPC membranes significantly. In order to reveal the detailed information of their monolayer structures, combined in-situ studies of neutron reflectivity and Brewster angle microscopy have been performed at the air/water interface. The results established that the phase and ordering properties of the mixed monolayer are strongly influenced by an addition of the fullerene bearing lipid molecules. Detailed domain structures and temperature effect will be discussed.

[C1.101] LT-STM study of self-organisation of b-carotene molecular layers on Cu (111)

All-trans b-carotene molecules have been deposited at room temperature on Cu (111) and subsequently studied by STM at liquid nitrogen. They form a self-organized multilayer structure. The STM images allow to unambiguously identify single molecules and to resolve important aspects of their internal structure like its curved backbone polyene chain, its attached methyl groups and the b-ionone rings. Mechanical properties of the carotene film is further tested by using STM lateral manipulation procedures. We suggest that the molecules lose their center of symmetry, being the molecule flexible enough to allow the formation of large three-dimensional periodic extensions.

[C1.102] Development of an open, human-scale low field MRI system

NMR of laser-polarized noble gas (129Xe and 3He) has great utility as a probe of a wide variety of physical and biomedical problems. We report recent progress in the development of an open access, human-scale MRI system that operates at very low applied magnetic fields and enables imaging of lung ventilation with freedom of orientation of subjects within the applied field.

[C1.103] Sound Transduction: A Complex System

We describe analytical and computational studies of the function of the mammalian cochlea (inner ear). Both two dimensional and three dimensional simulations show the pathways, mechanical and biochemical, for energy transduction. We also explain why hearing can be aproached as the prototypical complex system.

[C1.104] PHASE DEVELOPMENT AND MAGNETISM IN FERROMAGNETIC BIOGLASS CERAMICS

The major phases that develop in ferromagnetic glass-ceramics are Fe_3O_4, Ca_3(PO_4)_2 and CaSiO_3. The biocompatibility properties of the systems are associated with the calcium phosphate that forms apatite in a physiological environment. The magnetite phase in a bioglass ceramic makes it a possible candidate for hyperthermic treatment of animal bone cancer. The qualitative and quantitative development of the phases in systems with various starting concentrations and heat treatment parameters was studied by powder x-ray diffraction and Rietveld refinement methods. It was found that the magnetic properties of each system are strongly correlated with the corresponding structural properties.

[C1.105] Investigating the local mechanical properties of cells using microinjected colloidal particles

The mechanical properties of tissues are dominated by polymer networks that are organized into a variety of structures over a range of spatial scales. Within individual cells, the cytoskeleton determines the deformation in response to an applied load and the resulting mechanochemical signal transduction. Structural heterogeneities of the cytoskeleton play an important role in regulating this response. To study these local mechanical domains we use a particle tracking technique to measure the Brownian motion of colloidal particles that have been microinjected into the cell. From the motion of these particles we can extract the local viscoelastic properties. Using this technique we investigate changes in the local mechanical properties in response to pharmacological disruption of the cytoskeleton. We also study the response to forces applied using a magnetic tweezer device.

[C1.106] Continuum models of electrostatic interactions in proteins and protein-protein complexes

The conformations of proteins observed in nature must be low in free energy relative to alternative (not observed) conformations, and it is plausible that components of the electrostatic free energy are also low relative to alternative conformations. Therefore, we evaluate continuum models of electrostatics in proteins based on the size of the free energy gap between native and non-native conformations. It is observed that the total electrostatic free energy computed using the Poisson-Boltzmann equation or the Generalized Born model exhibits free energy gaps that are comparable to, or smaller than the free energy gaps resulting from Coulomb interactions alone. Sizable free energy gaps obtained using simple distance-dependent dielectric models suggest their usefulness in approximating the attenuation of long range Coulomb interactions by induced polarization effects. Hydrogen bonding interactions appear to be better modeled with an explicitly orientation-dependent hydrogen bonding potential than with any of the purely electrostatic models of hydrogen bonds. Finally, a combined electrostatics-hydrogen bonding potential captures the free energy differences between native, native-like and non-native structures better than electrostatic or hydrogen bonding models alone.

[C1.107] Force of an actin spring

The acrosomal process of the horseshoe crab sperm is a novel mechanochemical molecular spring that converts its elastic stain energy to mechanical work upon the chemical activation by Ca2+. Twisted and bent, the initial state of the acrosomal bundle features a high degree of complexity in its structure and the energy is believed to be stored in the highly strained actin filaments as an elastic potential energy. When activated, the bundle relaxes from the coil of the highly twisted and bent filaments to its straight conformation at a mean velocity of 15um/s. The mean extension velocity increases dramatically from 3um/s to 27um/s when temperature of the medium is changed from 9.6C to 32C (respective viscosities of 1.25-0.75cp), yet it exhibits a very weak dependence on changes in the medium viscosity (1cp-33cp). These experiments suggest that the uncoiling of the actin spring should be limited not by the viscosity of the medium but by the unlatching events of involved proteins at a molecular level. Unlike the viscosity-limited processes, where force is directly related to the rate of the reaction, a direct measurement is required to obtain the spring force of the acrosomal process. The extending acrosomal bundle is forced to push against a barrier and its elastic buckling response is analyzed to measure the force generated during the uncoiling.

[C1.108] Dynamics of an actin spring

The acrosome of the sperm of the horseshoe crab (Limulus Polyphemus) is an unusual actin based system that shows a spectacular dynamical transition in the presence of Ca++ that is present in abundance in the neighborhood of the egg. During this process, the bundle, which is initially bent and twisted uncoils and becomes straight in a matter of a few seconds. Based on microstructural data, we propose a model for the dynamics of uncoiling that is best represented by a triple-well potential corresponding to the different structural arrangements of the supertwisted filaments. Each of the false, true and coiled states corresponds to a local minimum of the energy, with the true state being the one with the lowest energy. Using an evolution equation derived by balancing torques, we investigate the nucleation and propagation of the phase transition and compare the results with those of experiments. Our model quantifies the hypothesis that the acrosomal bundle behaves like a mechano-chemical spring.

[C1.109] Polymer Physics I

[C1.110] Real Space Structure of Associating Polymer with Selective Solvents

Microscopic polymer integral equation theory is applied to numerically investigate thermally-driven self-assembly, clustering, microdomain formation, and interchain pair correlations for telechelic and multiblock associating polymer solutions in solvents of varying selectivity for the polymer sticky group. The monomer-sized solvent experiences solvent-solvent attractions in addition to solvent-sticky group attractions. These attractions increase the self-assembly temperature, but at the same time lead to real space structure which differs vastly from athermal solvents and can include vesicle-like structures. Local polymer clustering can be adjusted by tuning the solvent selectivity. With increasing dilution, enhanced polymer-solvent macrophase fluctuations emerge, though no true macrophase separation is found.

[C1.111] A Light Scattering Investigation of a Sol-gel/melt Transition: the Poly(ethylene oxide) (PEO)/methanol/LiClO4 System

The structure and dynamics of 50K PEO methanol solutions and PEO melts, with and without LiClO_4, were studied using static light scattering (SLS) and photon correlation spectroscopy (PCS). Scattering light intensity autocorrelation functions revealed that with increasing PEO concentration, fast PEO concentration fluctuation relaxations gradually disappeared even as slow mode relaxations emerged. The fast relaxation was diffusive, exhibiting q^2 dependence. Associated dynamic screening lengths, \xi_H, scaled as the -0.73 power of polymer concentration, as expected, in the semidilute solution regime. Significantly, the slow mode relaxation was also diffusive, both in solution and in PEO melts, behavior interpreted in both cases as due to the relaxation of a transient PEO network. Static measurements revealed power law behavior in PEO melts, I(q)\simq^-2.0 in agreement with percolation theory despite the absence of an abrupt sol-gel transition. Moreover, for a given polymer concentration, the degree of PEO chain entanglement varied depending on the salt concentration.

[C1.112] Slow Dynamics and the Glass Transition in Colloidal Suspensions and Polymer Melts

A microscopic theory of single particle dynamics and vitrification in colloidal suspensions is formulated using mode-coupling, density functional and activated process ideas. Entropic barriers, characteristic length scales, activated hopping rates, and transport coefficients have been calculated for hard sphere suspensions. No adjustable parameter quantitative comparisons with experiment have been performed. Strong connections between an effective free energy, quasilocalized state properties, thermodynamics, and long time relaxation are predicted. The approach is generalized to treat free energy barrier formation and segment activated hopping in polymer melts. Applications to calculate the glass transition temperature, dynamic fragility, and the slow relaxation time associated with barrier crossing will be presented.

[C1.113] Study of Carrageenan Conformation Using Time-Dependent Light Scattering and Viscometry

Polysaccharides of the carrageenan family are believed to undergo a conformational transformation when varying either or both of temperature and concentration of added salt. Specifically, increasing ionic strength at fixed temperature or decreasing temperature at fixed ionic strength results in a transition from random coil to a helix. It is a point of contention as to whether this helix is formed from one coil wrapping upon itself or two coils wrapping around one another. We present the results of a study of this conformation change using single capillary viscometry and multi-angle laser light scattering (MALLS). We present measurements of reduced viscosity and angle dependent light scattering, both time-dependent and batch, as a function of added salt. As we increase added NaCl, we see a clear increase in both molecular weight and radius of gyration for iota-carrageenan, which is commonly taken as evidence of the conformation change. On the other hand, estimates of persistence length (~ R_g ^2 / M_W) calculated using MALLS data show no strong variation when increasing added salt. Furthermore, time-dependent results show a slow, continuous increase in M_W. These results suggest a loose aggregation of polymer molecules that preserves the overall structure rather than a significant change in polymer architecture. Triple-detector gel phase chromatography data and results for lambda-carrageenan, which is not believed to change conformation with added NaCl, will be presented and discussed.

[C1.114] The structure factor of poly(1-butene) and poly(4-methyl-1-pentene) from wide angle X-ray scattering, molecular dynamics and PRISM.

The melt structures of the isotactic vinyl polymers poly(1-butene) and poly(4-methyl-1-pentene), with corresponding ethyl and isobutyl side chains, were recently studied with wide-angle X-ray diffraction. For these polymers, a "pre-peak" appears below the main diffraction peak in the carbon-carbon structure factor. The pre-peak becomes stronger and shifts to lower scattering vectors with increasing bulkiness of the side chain. These features are predicted on the basis of molecular dynamics simulations and Polymer Reference Interaction Site Model (PRISM) calculations. Good agreement is found for the structure factor determined with simulation and theory and the X-ray scattering experiments. The origin of the pre-peak for vinyl polymers based on experiment and theory will be discussed.

[C1.115] The photochemical properties of dye rotaxane

A rotaxane is a supramolecular structure of a dumbbell-like molecules trapped within the cavity of macrocycles. Dye rotaxane has drawn an intense interest since several articles have been published recently about its synthetic process, but not much has been known about its properties. Here we synthesize some rotaxanated dyes and are going to present some of its characteristic photochemical properties in terms of absorption, emission and fluorescent spectra. Its photochemical stability will also be covered, along with its promising further applications.

[C1.116] Phase Separation, Structure and Gelation in Polymer-Particle Suspensions

Equilibrium and nonequilibrium phase behavior, structure and thermodynamics of suspensions of colloids, nanoparticles and nonadsorbing polymer depletants have been experimentally and theoretically studied. The role of polymer-particle size asymmetry (Rg/R), solvent quality and particle volume fraction are systematically explored. PRISM theory successfully predicts the very strong influence of solvent quality and size asymmetry on phase separation of silica – polystyrene – solvent (toluene, decalin) mixtures. Small angle x-ray scattering studies over a wide range of wavevectors and polymer concentrations also quantitatively agree with theoretical predictions in the fluid phase. For strongly asymmetric (Rg/R=0.06) mixtures at high colloid volume fractions, both theory and experiment find a nonmonotonic variation with polymer concentration of the local cage order. Comparison of the theoretical and experimental scattering profiles in the gel state allow conclusions to be drawn concerning the length scale dependent loss of ergodicity.

[C1.117] Obervation of E-relaxation Process in Polybutadiene

We report the observation of an unusual relaxation process in light scattering spectra of polybutadiene (PBD). It shows up in the GHz-frequency range, has relatively mild temperature dependence and is similar to a secondary relaxation process. The most surprising observation is that the process exists even at high temperatures and does not merge with the segmental relaxation up to Temperature of 350K. Most of the polymers do not demonstrate any secondary relaxation (i.e. a relaxation process between the fast and the segmental relaxations) at such a high temperature T~2Tg [1]. Possible mechanisms of this particular relaxation in PBD are discussed. In particular, similarity with the so-called E-process [2] is stressed. Existence of the E-process has been proposed from analysis of neutron scattering spectra and the process has been ascribed to a conformational transition represented by a jump motion between rotational isomeric states. According to this mechanism, the E-process should be the “elementary” relaxation process for the polymers that have different rotational isomeric states. We stress, however, that this process seems to be specific for PBD only and has not been observed in the light scattering spectra of polyisobutylene or polystyrene [1].

1. A.Kisliuk, R.T.Mathers, A.P.Sokolov, J.Pol.Sci. Phys.38, 2785-2790 (2000). 2. T. Kanaya and K. Kaji, Advances in Polymer Science 154, 87-141 (2001).

[C1.118] Nanocrystal Formation under Confinement in a Rubbery Polymer: Large Melting Point Depression in Telechelic, Pyrene-End-Labeled PDMS as a Function of PDMS Molecular Weight

The associative behavior of telechelic, pyrene-end-labeled PDMS (Py-PDMS-Py) has been studied by excimer fluorescence and thermal analysis. In 24k g/mol Py-PDMS-PY, there is significant agglomeration of pyrenyl labels as evidenced by a high excimer to monomer intensity ratio, the presence of ground-state dimers or aggregates observed via excitation spectra, and substantial modificiation of rheology. In 5k g/mol Py-PDMS-Py there is a higher aggomeration level that evolves substantially with time in films prepared from solution, eventually yielding pyrenyl nanocrystals with a melting temperature (Tm) of ~ 40C, representing a 90C depression of Tm relative to that of bulk pyrenyl dye. Cooling the 24k g/mol Py-PDMS-Py to -70C for an extended period yields a Tm of -40C, indicative of much reduced nanocrystal size relative to that in the 5k g/mol Py-PDMS-Py. Progress in understanding the relationships of molecular weight and confinement in ultrathin films to nanocrystal vs. agglomerate/aggregate formation as well as the level of nanocrystal vs. agglomerate formation to gelation will be discussed.

[C1.119] Orientational Order and Mechanical Response in Strained Polymer Liquids and Networks

A new liquid state theory has been developed to predict the influence of interchain repulsions on strain-induced orientational order and mechanical properties of polymer solutions, melts and networks. Within the assumption of affine deformation, the strain-induced nematic order parameter is predicted to scale as the inverse square root of the degree of polymerization and increase in a supra-linear manner with segment concentration. Two nonclassical contributions to the stress arise from the influence of anisotropic packing correlations on the excess free energy. The theory is in good agreement with NMR and mechanical experiments and recent simulations, although systematic deviations exist for stress-strain behavior due to “trapped entanglements” which are not explicitly taken into account. Significant enhancement (softening) of the strain-induced orientational order (modulus) is found as the isotropic-nematic phase transition is approached.

[C1.120] Adsorption of Polyelectrolytes onto Like-Charged and Oppositely Charged Surfaces

We study the adsorption of strongly charged polyelectrolytes onto like-charged surfaces via multivalent ions. At some multivalent salt concentrations and surface charge densities, polyelectrolytes form a pancake structure on like-charged surfaces. We determine the ratio of multivalent salt concentration to polymer concentration for adsorption. We also analyze the adsorption of rigid and flexible strongly charged molecules at weakly oppositely charged surfaces in salt solution. Polyelectrolytes can form periodic structures on the surface. The distance between them strongly depends on the charge density of the surface and is related to the salt and polyelctrolyte concentration in solution. We determine the number of condensed counterions along the polyelectrolytes after the chains are adsorbed as a function of surface charge density.

[C1.121] Polyelectrolyte Dynamics Studied by Neutron Spin-Echo Spectroscopy

Polyelectrolyte Dynamics Studied by Neutron Spin-Echo Spectroscopy

We have measured the dynamic structure factor using neutron spin-echo spectroscopy (NSE) and static structure factor using small-angle neutron scattering (SANS) to probe the effect of sodium versus magnesium counter-ions for aqueous solutions of poly(styrene sulfonate) within semidilute solutions. The neutron spin-echo spectrometer at the NIST Center for Neutron Research measures the intermediate scattering function over a momentum transfer range of 0.01 to 1.6 Åand Fourier times covering 45 ps to 80 ns. These NSE experiments provide length scale and time scale information complementing techniques such as NMR and dynamic light scattering. We compare our results to similar charged systems and recent theories and simulations which study the nanosecond dynamics in polyelectrolyte solutions.

[C1.122] Modeling Electrostatic Forces Between Opposing Polyelectrolyte Brushes: Interdigitation versus Compression

In this study, we employed a Poisson-Boltzmann continuum approach in order to investigate how nonuniform molecular level charge distributions affect the nanoscale electrostatic forces between opposing oppositely charged polyelectrolyte brushes. The time-averaged space taken up by each macromolecule was represented as a finite-length cylindrical rod of uniform volume charge density. Finite difference methods were used to solve for the potential, ion concentrations, and interaction force in a variety of solution conditions (i.e. ionic strength, pH, brush height, and brush density). Depending on whether the brush layers compress (exclude each other) or whether they can interdigitate, there are significant differences in the magnitude and form of the predicted force, even when the total charge remains the same. The compressed model is equivalent to doubling the compressive force of one brush while in the interdigitated model there are additional brush-brush repulsions as the rods from the different opposing brushes are interspersed and closer to each other.

[C1.123] Dissolved charged linear macromolecules of finite volume: counterion distribution and effective forces

We describe counterion distributions and fluctuations in an electrolyte containing charged linear macromolecules, which are modelled as cylinders of finite radius. A path integral representation of the systems free energy, which allows for condensation,too, reproduces a Poisson Boltzmann theory at its saddle point. We investigate the effect of Gaussian fluctuations on the counterion distribution and condensation. Furthermore we show that these fluctuations sometimes have considerable influence on the effective forces between charged linear macromolecules. The importance of these results for cell-biological problems, e.g. DNA - membrane interaction, is emphazised.

[C1.124] Effect of Solution Conditions on Nanoscale Interactions Between Opposing Glycosaminoglycan Brushes

Repulsive electrostatic interactions between negatively charged glycosaminoglycans (GAGs) are a major determinant of compressive strength of cartilage. To probe the molecular origin of cartilage biomechanical properties, the interactions between two opposing end-grafted GAG brushes were measured using the technique of high-resolution force spectroscopy. A nanosized probe tip was functionalized with GAG using electric field chemisorption and the parking density was determined using a "reverse calibration" curve obtained from HRFS experiments on GAG-functionalized planar substrates of various parking densities (3.6-10 nm intermolecular separation distance) measured by DMMB assay (dimethylmethylene blue). The GAG-GAG interaction forces were measured as a function of ionic strength, 0.0001M-1M NaCl at pH\sim5.6, and pH=3 amp; 7 at 0.015 M NaCl, and compared to the predictions of a newly developed Poisson-Boltzmann model that represents the GAGs as rods of a uniform charge density, thus more accurately describing the molecular morphology and non-uniform charge distributions of the GAG brush.

[C1.125] Molecular Modeling of Nanostructure and Water Transport in Nafion

Hydrated Nafion membranes are used in fuel cell because of their high proton conductivity as well as dimensional and electrochemical stability. Nafion is a polyelectrolyte consisting of a rigid hydrophobic backbone and hydrophilic side chains with ionizable sulfonate end groups. There is a general consensus that water and the polymer nano-segregate. Despite numerous studies on hydrated Nafion, the details of the structure and its influence on water and proton transport at the molecular level is not yet fully understood due to the limitation of experimental resolution to investigate the spatial nano-heterogeneity of the structure. Atomistic simulations complement experiments in this particular length scale. We studied the structure of Nafion with different monomeric sequence, ionization, and water content using molecular dynamics simulations. We discuss the effect of these factors on the structure (particularly, water/polymer interface) and dynamics of water and hydronium. We found that thr sulfonate groups are exposed to water for both neutral and ionized states of the hydrated polymer. However, we find that a significant fraction of the interface is still between water and the hydrophobic backbone. We analyze local structure and transport of water in this ionized and confined state.

[C1.126] Phase behavior of solutions of flexible oppositely charged polyelectrolytes

Phase behaviour of a symmetric solution of flexible oppositely charged polyelectrolyte chains is considered theoretically in both dilute and semidilute regimes. The free energy of the weakly charged complexes is obtained in the dilute regime by using the linear reponse theory. We describe the interactions of the complexes with each other in the solution in order to analyze their stability against multichain aggregation. The weak segregation theory is employed for description of the semidilute phases. The effects of the length of the chains, their charge density and concentration, as well as the concentration of added salt on the phase diagram are discussed.

[C1.127] Control of Surface Properties Using Fluorinated Polymer Brushes

Surface-grafted polystyrene-based homopolymer and diblock copolymer brushes bearing semifluorinated alkyl side groups were grown from flat silicon oxide surfaces by nitroxide-mediated controlled radical polymerization. Angle resolved XPS studies show that for the diblock copolymer brushes, the block from the last monomer to be added always covers the polymer-air interface, regardless of its surface energy. This lack of surface reconstruction, together with evidence that the macromolecular chains are significantly stretched away from the surface, suggests that the brush is dense. Near-edge X-ray absorption fine structure analysis shows that the surface orientation of the semifluorinated side chains depends on the brush thickness, with thicker brushes having the -CF_2- helices of the side chains oriented more nearly parallel to the surface normal, giving a larger orientational order parameter S. Increasing S of the brush improved the surface stability, leading to a surface resistant to reconstruction upon prolonged exposure to water.

[C1.128] Remarkable Differences Between the Flow Orientation and Mechanical Properties of Pentablock and Triblock Copolymers by Solution Extrusion

The effects of molecular architecture on the flow orientation and mechanical properties of block copolymers were examined with polystyrene (S) - polybutadiene (B) pentablock (SBSBS) and triblock (SBS) copolymers. Fully hydrogenated polycyclohexylethylene (C) and polyethylene (E) versions of these materials were also used. A novel solution extrusion technique was employed as the processing method. The most remarkable observation was the generation at high shear rates of transverse lamellae (i.e., the lamellar normal parallel to the extrusion direction) for SBSBS but not SBS. At low shear rates both copolymers showed a transition from parallel to perpendicular orientations. Since the formation of transverse lamellae was observed only for SBSBS, it is clear that the unique architecture of the pentablock copolymer (i.e., the existence of both bridging and looping blocks and an absence of slip planes) plays an important role. The molecular architecture also affects the mechanical properties; for example, a significant improvement of toughness was observed for pentablock copolymers.

[C1.129] An improved algorithm for spectral self-consistent field theory

We present an improved algorithm for solving the self consistent field (SCF) equations for a block copolymer melt in the spectral representation of Matsen and Schick. We solve the SCF equations by Newton-Raphson iteration using an approximate Jacobian obtained by perturbation theory. The algorithm is stable and significantly faster than previously proposed methods. We apply the method to predict the gyroid phase boundaries at high chi N.

[C1.130] Phase structure and liquid crystal orientation in a series of rod coil block copolymers

A series of rod-coil molecules have been synthesized with mesogenic jacketed liquid crystalline polymers (MJLCP) as the ¡°rod¡± and polystyrene (PS) as the ¡°coil¡± segments. The block copolymer microphase separate into ordered structures at temperatures below ca. 250¡ãC. Differential Scanning Calorimetry experiments show that, compared to the PS segments, the MJLCP components possess high glass transition temperatures (TODT > Tg MJLCP > TgPS). This unique transition temperature relationships allow us to study the MJLCP phase structure and liquid crystalline orientation within the soft, nano confined PS environments. LC orientation has been studied using Wideangle X-ray diffraction techniques while block copolymer microphase structures have been monitored by small angle X-ray scattering techniques. Both lamellar and HPL-like structures have been observed. Detailed molecular orientations will be reported.

[C1.131] Size Effect on Crystal Orientation Changes in Nano-Confined Lamellae of PEO-b-PS Block Copolymer

A series of symmetric poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymers with different molecular weights (Mw) was used to construct nano-lamellar morphology, for studying PEO crystal orientation changes in these lamellae whileh varying spacings and crystallization temperature (TODT > TgPS > Tc). Using combined synchrotron 2D-SAXS and WAXS techniques, it has been found that the crystal orientation changed with respect to the lamellar surface from random to perpendicular, then to inclined and finally to parallel with increasing TC. With increasing the spacing of the lamellae, the PEO crystals can grow in a larger space, and therefore, the PS glassy walls have less confinement effect on the crystallization of PEO. This leads to a systematic role that the PEO crystal orientation varies with changes in the Mws of thePEO-b-PS.

[C1.132] A continuosuly branched model for the dendrimer-melt brush

Working off the topic of highly branched dendrimer melts, a model for a continuously branched brush, with initial unbranched monomer fixed to a flat surface, is developed. In this model the free energy for a single chain in the brush is independent of its free end location. As the degree of branching gets large the flat distribution of free ends, creates a brush much like a filled-core dendrimer.

[C1.133] EUV Sensitive Chemically Amplified Imaging Layers for use with Diblock Copolymer Thin Films

Block copolymers thin films are extremely promising materials for patterning at sub 50 nm length scales because they can self assemble into a variety of different morphologies. To achieve long range order in films of block copolymers with morphology perpendicular to the substrate we propose patterning imaging layers and altering surface chemistry at the sub 50 nm length scale. Standard organic materials pose one problem however in that they are not chemically sensitive to most forms of ionizing radiation such as x-ray, e-beam and EUV. To address this issue we form sensitive imaging layers, containing carbonate linkages, attached to the silicon substrate which are modified using the principles of chemical amplification in which this acid labile group is cleaved by a photo generated acid in a catalyzed reaction. This cleavage upon exposure gives contrast in the surface chemistry in exposed vs. unexposed regions. We report on imaging layers which upon exposure show chemical functionality changes as evidenced by contact angle measurements, ellipsometry and block copolymer wetting behavior.

[C1.134] Achieving long range order in diblock copolymer thin films by the use of thermal gradients

We have studied the annealing of monolayer films of cylindrical polystyrene-b-poly(ethylene-alt-propylene) (PS-PEP) diblock copolymers at fixed temperature, in temperature gradients and in swept temperature gradients. For our PS-PEP system conventional annealing yielded correlation lengths on the order of about 3 \mu m, sweeping the gradients through the order-disorder transition at a rate of 1 \mu m/s yielded correlation lengths on the order of 20 microns and repeated sweeps (from an AC modulation of the temperature) yielded correlation lengths greater than 50 \mu m

[C1.135] Imaging the morphology of block copolymer films using scanning electron microscopy

The most frequently used techniques for imaging the domain structure in block copolymer films are atomic force microscopy (AFM) and transmission electron microscopy (TEM), and both techniques offer excellent resolution. With AFM, however, scanning times are long and it is difficult to survey large areas. With TEM, sample preparation can be difficult, and often the block copolymer samples must be stained. In this study we detail the condition under which high resolution imaging of poly(styrene-b-methyl methacrylate) films can be achieved using scanning electron microscopy (SEM). SEM requires little sample preparation and allows imaging and surveying of large areas. The resolution of SEM images of block copolymer films is compared as a function of the electron energy of the beam (1 to 5 kV). The origin of the contrast between polystyrene and poly methyl methacrylate domains is deduced to be a combination of beam-damage induced topography and differences in electron charging of the two components in this energy range.

[C1.136] Structure Formation in Crystallization of Crystalline-Crystalline Block Copolymers

Recently we have reported that Poly(e-caprolactone)-poly(ethylene oxide)-poly(e-caprolactone)(PCL-PEO-PCL) triblock copolymers show unique concentric double spherulites while the diblok ones form ordinary single spherulites in crystallization. In this study, small- and wide-angle X-ray scattering techniques(SAXS and WAXD) and diffrential scanning calorymetory(DSC) were performed to compare the crystallization behavior from thermal and structual aspects. The diblock copolymers as well as the triblock show WAXD peaks suggesting that both components crystallize even in the single spherulites. In the SAXS profiles, only one peak was observed for diblock copolymers, while the SAXS profiles of triblock have two peaks. Furthermore, in both the crystallization and melting processes of the diblock copolymers, the long spacings changed discretely. These results suggest that one component crystallizes first and then the other one crystallizes between the former crystal lamellae while the components of triblock copolymers crystallize in the diffrent regions to form concentric double spherulites.

[C1.137] An effective \chi parameter for block copolymer melts with finite compressibility

An effective Flory-type interaction parameter \chi_cRPA is extracted from a compressible random-phase approximation (cRPA) theory to analyze microphase separation behavior of compressible block copolymer systems. It is found that N \chi_cRPA, where N is the total chain size, replaces its incompressible cousin, N \chi, in Leibler's theory as a relevant parameter for microphase sepration in the compressible situation, where not only microphase separation upon cooling but also microphase separation upon heating are observed in some selected copolymers. Through this newly defined N \chi_cRPA, the feasibility of immiscibility loop phase behavior in block copolymers either with only nonpolar interactions or with certain specific interactions between dissimilar monomers is investigated. The effects of finite chain size and pressure on the copolymer phase behavior are additionally discussed.

[C1.138] Semi-invariants and Landau free energy

Landau free energy approach serves a basis for the theory of weak crystallization of multicomponent polymer systems and plays a fundamental part in numerous practical applications. Unfortunately, existing approaches do not discriminate total density and composition modes, which play very different physical role. In order to take this polymer specific into account, we developed general formulation of the Landau free energy in terms of such modes. Elements of this diagram expansion are shown to be ``semi-invariant'' correlation functions, which remain finite in the limit of infinite chain lengths. In this limit our formulation can be thought as the result of numerous cancellations of diverging terms of the standard approach. Explicit expressions are given for semi-invariants in terms of structure correlation functions of the polydisperse polymer system.

[C1.139] Nano Structure of Liquid Crystalline Poly(benzyl L-glutamate) – X Diblock Copolymer Cast Films

Poly(benzyl L-glutamate) (PBLG) forms a rigid helical rod in organic solvents. Cholsteric liquid crystalline ordering of these rods has been observed in PBLG solutions and cast films. In this research, two series of block copolymers were created with PBLG in an attempt to alter the classic PBLG cholesteric phase behavior. First, peptide blocks with inherent secondary structures, random coil or rigid rod, were attached to PBLG molecules. The phase behavior was controlled by altering the block’s secondary structure and length relative to the PBLG block. Second, a series of hybrid block copolymers with synthetic polymers of polystyrene and polybutadiene were synthesized. The liquid crystalline/self-assembly ordering of these two molecule series in organic solvent was probed with transmission electron microscopy (TEM), polarizing optical microscopy (POM), and small-angle x-ray scattering (SAXS). In pure PBLG and PBLG blocks with relatively small additional blocks, cholesteric liquid crystalline ordering was observed. However, when the relative block size of the coil block was increased significant changes in the phase behavior was observed. This control over molecular self-assembly and phase behavior reveals the ability to pattern materials with peptidic functionalities by taking advantage of molecular architecture and polymer secondary structure.

[C1.140] Microphase-Separation of Cyclic Block Copolymers of Styrene and Butadiene and of Their Corresponding Linear Triblock Copolymers

A series of five cyclic block copolymers of styrene and butadiene, having essentially the same molecular weight (52 ± 5 Kg/mol) and PS volume fraction varying from 11 to 70%, were synthesized by cyclization of á,ù-dilithium polystyrene-polybutadiene-polystyrene triblock copolymers with bis(dimethylchlorosilyl)ethane. The cyclic block copolymers thus obtained have practically the same molecular weight and composition as their corresponding linear triblock copolymers. All materials were investigated via transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) techniques. In three cases where the cyclic and the corresponding linear block copolymer had the same morphology, the domain spacings of the cyclic block copolymers are found to be 84%-89% of those of their respective linear triblock copolymers. In the other two cases different morphologies are found in the cyclic and its corresponding triblock copolymer. Compared to the linear triblocks, there is an increased tendency for the interfaces to curve away from the connected end blocks.

[C1.141] Measurement of Grain Growth Rates for Order-Order Transitions in Block Copolymers

The kinetics of grain growth during thermally induced transitions between two ordered phases have been determined by polarized optical microscopy (POM). Measurements were made on poly(styrene-b-isoprene) copolymers, f_PS = 0.60 and 0.23, in solutions with either dibutyl phthalate (DBP) or diethyl phthalate (DEP) as solvent, at a 70% polymer volume fraction. These solutions exhibit transitions of the metastable hexagonally perforated layer (HPL) phase to the gyroid (G) phase, and cylinder (C) phase to G. The HPL to G and C to G transitions occur through classical nucleation, with radially symmetric grain growth, which allowed the growth front velocities of individual G grains to be measured with POM. Temperature jump experiments were used to determine the effect of quench depth on the growth front velocities as well as the overall transition rates. Growth front velocities are compared quantitatively to the theoretical predictions of Goveas and Milner, with good agreement observed.

[C1.142] Molecular Weight Effect of End-Functionalized Polymers on Reaction at the Interface between Immiscible Polymers

Diblock copolymers were formed at the interface between immiscible polymers, polystyrene (PS) and poly(2-vinylpyridine) (PVP), by the reaction between amine end-functionalized deuterated PS (dPS-NH_2) and anhydride end-functionalized PVP. Dynamic secondary ion mass spectrometry has been used to measure the normalized interfacial excess (z^*/ R_g) of dPS-NH_2 with different molecular weights (Mn = 3.5, 6.5, 11.5, and 29 kg/mol) as a function of reaction time t. As the molecular weight of dPS-NH_2 increases, the z^*/R_g for a given t decreases significantly. As the z^*/R_g increases, the interfacial tension decreases toward zero and the interface eventually becomes unstable for all but the 29 kg/mol dPS-NH_2, leading to the formation of droplets emulsified by block copolymers near the interface. These droplets diffuse from the interface to either the PS surface or the interface between PVP and SiO_x. The diffusion rate and diffusion direction are dependent on the molecular weight of diblock copolymers and the ratio of dPS to PVP in the diblock copolymers, respectively.

[C1.143] Deformation and Fracture of Lamellar PCHE-PE Block Copolymers: Effect of Chain Architecture

We investigate the influence of chain architecture on the deformation and fracture properties of poly(cyclohexylethylene)-poly(ethylene) (PCHE-PE) lamellar block copolymers. The micromechanical deformation behavior is investigated using a “fragility test” in which thin polymer films, bonded to ductile copper grids, are strained in tension. Optical microscopy, TEM and SFM are used to study the domain structure and examine the details of deformation and fracture. We find that the final fracture properties of PCHE-PE lamellar block copolymers depend significantly on their chain architecture. Triblock copolymer (CEC) with M_W = 45 kg/mol and pentablock copolymers (CECEC) with M_W = 66 kg/mol, both containing a PE fraction f_PE = 0.48, do not fail under strains of up to 25%. However, a 50/50 blend of diblock (CE, M_W = 22 kg/mol, f_PE = 0.48) and triblock copolymers (CEC, M_W = 45 kg/mol, f_PE = 0.48) annealed at 200^°C shows a median strain to failure of about 7%. A 50/50 blend of the same diblock and pentablock (CECEC, M_W = 66 kg/mol, f_PE = 0.48) copolymers annealed at 200^°C, on the other hand, does not show any failure at strains up to 25%.

[C1.144] Evolution of Surface Morphology in Thin Block Copolymer Films

In this study we investigate the pathways spin coated diblock co-polymer thin films approaches their thermodynamic equilibrium. Theses films assume a configuration that minimizes interaction between its incompatible components and accommodates confinement effects. Several groups have shown that following annealing, symmetric diblock copolymer thin films adopt a lamellar structure and exhibits a hierarchy of patterns, determined by the commensurability between the local film thickness and the interlamellar spacing. This study addresses the morphology of symmetric polystyrene-b-polyisoprene (PS-PI) (Mw.7k-7k) thin films by a combination of Atomic Force Microscopy, x-ray reflectivity and small angle neutron scattering as films of different thickness approach equilibrium. In contrast to previous studies, this polymer spontaneously forms lamellar structure at room temperature. The films exhibit a variety of surface features with the increase of thickness (bicontinuous, islands, and holes). The interlamellar spacing was determined by the x-ray reflectivity. The height of each surface topological feature was found to be roughly equal to one-half of the interlamellar spacing. Studies of temperature effects and the size of the blocks on the evolution of surface morphology are currently underway.

[C1.145] Clustering and Continuum Percolation in Macromolecular Systems

We investigate geometrical percolation in (i) one-component systems of linear macromolecules with different degrees of flexibility, as well as (ii) athermal mixtures of semiflexible and rod-like polymers. The volume fraction at the percolation threshold depends strongly on macromolecular architecture and size. Attractive inter-segmental interactions are included in our treatment of flexible polymers, and are shown to have a pronounced effect on the threshold volume fraction in the vicinity of the theta temperature. The percolation threshold for rod-like particles dispersed in a medium of flexible polymers is investigated as a function of the particle aspect ratio. The dependence of critical volume fraction on rod aspect ratio is strikingly similar to that found for the analogous one component model. The primary effect of explicitly including the flexible matrix polymer molecules is a reduction of the critical volume fraction by a factor which depends only weakly on the particle aspect ratio.

[C1.146] Enhanced Diffusion in a Polymer-Silica Nanocomposite Viewed by Pulse Field Gradient NMR

Enhanced Diffusion in a Polymer-Silica Nanocomposite Viewed by Pulse Field Gradient NMR

Junyan Zhong, Wen-Yang Wen and Alan A. Jones Carlson School of Chemistry and Biochemistry, Clark University, Worcester MA 01610

The addition of fumed silica to the high permeability random copolymer of tetrafluoroethylene (TFE) and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (PDD) increases diffusion by up to an order of magnitude. The self-diffusion constant of pentane was measured using pulse field gradient NMR and non-Fickian diffusion is observed in cast films of the pure polymer and nanocomposites containing 10, 20 and 30 wtNMR experiment, diffusion can be monitored for different periods of time with an apparent diffusion constant determined for each period. At all compositions, the apparent diffusion constant appears to be faster when observed over shorter times and decreases towards a plateau value at longer times. This result is indicative of tortuous diffusion. The addition of fumed silica not only increases the apparent self-diffusion constants but also changes the dependence of the apparent self-diffusion constants on the time over which diffusion occurs. The self-diffusion constants decrease more slowly as a function of observation time indicating better connectivity of the more permeable domains. The apparent diffusion constants also increase as a function of time after the introduction of pentane. This phenomenon is observed in the pure polymer as well and is described as conditioning of the membrane.

[C1.147] The Application of Two-dimensional Correlation Infrared Spectroscopy to the Study of Polymer Blends with Relatively Weak Intermolecular Interactions.

Following work reported in the literature, we first applied two-dimensional (2D) correlation infrared spectroscopy to a study of immiscible blends of polystyrene (PS) and poly(methyl methacrylate) (PMMA). Asynchronous spectra should not be obtained from such mixtures, but usually are. This can be a consequence of concentration errors, but most of the effects can be attributed to small differences in bandwidth or/and peak shift that are a result of sample preparation problems. Asynchronous spectra, when associated with mean normalization pretreatment of the data, appear to be very sensitive to these effects. As for the miscible blends with relatively weak intermolecular interactions, such as blends of polystyrene (PS) with poly(vinyl methyl ether) (PVME) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), a rotated cloverleaf-like pattern was obtained in many regions of the spectrum, characteristic of bandwidth changes or overall spectral profile changes as a function of concentration. Frequency shifts also give a characteristic pattern, a band splitting into two along the diagonal, that can be easily misinterpreted. These features have previously been interpreted in terms of the detection of hidden bands, specific interactions and conformational changes. It¡¯s shown here that these features correspond to lobes or points of inflection in the difference spectra used to generate the 2D plots. So, great care must be taken in the interpretation of features in these spectra.

[C1.148] Shear coalescence studies of compatibilized polymer blends

The suppression or retardation of droplet coalescence during shear flow has become one way to assess the effectiveness of compatibilization in polymer blends. Such studies have typically been performed on immiscible blends to which pre-made block copolymer is added as a compatibilizing agent. In this work, we explore immiscible blends that have been compatibilized using solid-state shear pulverization. Recent research in the Torkelson lab at Northwestern has demonstrated that chain scission and recombination during pulverization can lead to in situ formation of block copolymers. In this work, we study immiscible blends of polystyrene and polypropylene. Chain scission during pulverization of individual homopolymers leads to reductions in viscosity and elasticity. We compare the coalescence behavior in blends that have been co-pulverized to those which have been simply physically mixed.

[C1.149] Transport controlled segregation in a single-phase regime of mixtures of Liquid crystal and Polymer

In a liquid crystal/polymer system, phase separation driven by pattern-photopolymerization has been known to be of thermodynamic origin. An interesting question is what happens if photo-patterning were to take place in a single phase region in the post reaction. We extended the studies on pattern photopolymerization to generate predetermined structures in the one-phase region. The movement of phase diagram during the course of photopolymerization has been calculated based on the Flory-Huggins free energy of isotropic mixing. The pattern forming process was simulated in the context of the time-dependent Ginzburg Landau (TDGL) equation (Model B) coupled with the photopolymerization reaction kinetics. It turned out that pattern formation in the single phase is essentially governed by transport. For the purpose of comparison, the dynamics of photopolymerization induced phase separation was undertaken in the two-phase region. Supported by WPAF/AFOSR through Collaborative Center for Polymer Photonics, NSF-DMR 02-09272 and Ohio Board of Regents.

[C1.150] POSS-PDMS Blends at the Air/Water Interface

Hybrid organic-inorganic nanocomposites are of interest for numerous applications including coatings, stronger adhesives, and lightweight materials for the extreme environments encountered in aerospace applications. Polyhedral oligomeric silsesquioxane (POSS) molecules serve as model nanofillers with rigid inorganic cores and flexible organic coronae. The fact that POSS can also be surface active at the air/water interface makes monolayers convenient model systems for studying POSS-polymer interactions with surface active polymers like poly(dimethylsiloxane) (PDMS) and poly(t-butyl acrylate) that represent adhesive analogs. Results for Brewster angle microscopy and isotherm studies will be presented to show how PDMS alters the aggregation of amphiphilic trisilanolisobutyl-POSS and non-amphiphilic octaisobutyl-POSS.

[C1.151] Thickness of Spin-Cast Polystyrene/Clay Nanocomposite Films

Solutions of monodisperse (MW 280K) polystyrene (PS) and Cloisite 6A clay in toluene were spin-cast in room ambient conditions onto silicon wafers. The clay is known to exfoliate into platelet-shaped particles within the resultant films. Film thicknesses were examined as a function of concentration, spin rate, time, and radial position. Thickness is determined by a competition between viscous flow and evaporation. Solution viscosities were determined as a function of shear rate in the cone-plate geometry. Evaporation rates in still air were determined as a function of concentration. The results were incorporated into a simple numerical model of spin-casting. The model provides a reasonable description of thickness with no clay present, but underestimates the thickness of clay-containing films. In the latter case, better fits to data are obtained when the evaporation rate is assumed to be substantially reduced, as expected, by the oriented and densely packed clay platelets.

[C1.152] Ultrathin POSS-Polymer Blends

Polyhedral oligomeric silsesquioxane (POSS) derivatives serving as nanofillers in polymer blends have potential aerospace engineering applications such as space-survivable coatings, ablative insulation in solid rocket motor casings, and lightweight polymer composites to replace metal components. Understanding how POSS structure affects dispersion within polymeric matrices provides a challenging scientific problem for developing heat-resistant coatings. Several strategies exist for dispersing POSS in a polymer matrix including direct blending, POSS-co-polymers, and the blending of POSS-co-polymers with another polymer. Model systems of trisilanol-POSS derivatives and poly(t-butyl acrylate) have been used to study POSS-polymer blends at the air/water interface and as Langmuir-Blodgett films on solid surfaces. Brewster angle microscopy and atomic force microscopy studies characterizing these systems will be discussed.

[C1.153] Improving the Properties of Polymer Blends with Supercritical Carbon Dioxide

Improving the properties of polymer blends is of technological interest because of the commercial and industrial benefits involved. Ethylene vinyl acrylate (EVA) and polystyrene (PS) were met blended in Brabender twin-extruder with different ratio and then exposed to Supercritical CO2. Dynamic mechanical analyzer (DMA), Differential Scanning Calorimeter (DSC), Scanning Electron Microscopy (SEM) and Instron tensile test were use to measure the properties of the samples before and after exposure. On the molecular level, the compatibility between EVA and PS was improved after supercritical CO2, which was confirmed by the experiment data. Such improved compatibility served to improve the strength and elasticity of larger bulk samples, as supported by the Instron data.

[C1.154] Near Field Spectroscopic Investigation of Fluorescence Quenching by Charge Carriers in Pentacene-Doped Tetracene

Fluorescence quenching by charge carriers in pentacene-doped single crystals of the organic semiconductor tetracene is studied using a near field optical probe as both a microelectrode and an illumination source. Pronounced fluorescence quenching by positively charged carriers (holes) is observed. A comparison of the near field fluorescence spectra with and without an applied field indicates that primarily the pentacene fluorescence is quenched by carriers, indicating trapping of holes at pentacene sites. A large quenching volume for pentacene cations is determined. The results have implications for organic electroluminescent devices based on doped materials or polycrystalline materials operated at high charge densities.

[C1.155] A Broadband Dielectric Investigation of the Dynamics of Miscible Polymer Blends with Intermolecular Hydrogen Bonding

The dynamics of a variety of miscible polymer blends exhibiting intermolecular hydrogen bonding have been studied using broadband dielectric spectroscopy in the frequency domain. These include blends of poly(4-vinylphenol) with poly(ethyl methacrylate), polyvinylacetate, an ethylene-co-vinylacetate random copolymer, and poly(vinyl ethyl ether). Our results show that strong intermolecular hydrogen bonds are capable of suppressing concentration fluctuations, and coupling the component segmental relaxations. The local relaxation was found to be unaffected by the presence of the hydrogen bonding. The influence of the Tg difference between the component polymers and hydrogen bond strength/fraction on segmental dynamics and fragility will be discussed in detail.

[C1.156] Broadband Dielectric Investigation of Semi-Crystalline Poly(ethylene oxide) - Poly(styrene-co-hydroxystyrene) Blends

Melt-miscible blends of poly(ethylene oxide) (PEO) and a poly(styrene-co-hydroxystyrene) random copolymer (SHS) were studied using broadband dielectric spectroscopy, FTIR spectroscopy and differential scanning calorimetry. Blends with less than 50wtcrystallinity (normalized to PEO content) changing slowly up to 30hydrogen-bonded –OH groups are observed in FTIR spectra of all blends, and a band due to free –OH is evident at SHS contents above 70wtsegmental relaxation process, and this behavior will be compared to observations on other blends that exhibit strong intermolecular hydrogen bonding. The fragility of the blends was found to increase with increasing SHS content, consistent with greater cooperativity. Differences in the details of the relaxation processes as a function of crystallinity and composition will be discussed, as will the origins of the observed local relaxations.

[C1.157] The behavior of PS-PMMA block copolymer blends in thin films on rough substrates.

Rough substrates have been shown to induce perpendicular orientation of symmetric poly(styrene-block-methyl methacrylate) block copolymers (SMMA). We have used this phenomenon to observe the mixing behavior of blends of symmetric SMMA systems in thin films using cross sectional TEM and AFM. In this way we established the size and nature of the mixed SMMA domains using binary combinations of the pure SMMA systems up to ten times mutually disparate in size. We will present data comparing the behavior of the mixed systems to the pure block copolymer analogue with a similar number averaged molecular weight and discuss the variation of domain spacing with film thickness.

[C1.158] Organoclays Effect on Crystallization of Semicrystalline Poly(L-lactic acid)

Poly(L-lactic acid) (PLLA) is of interest as a neat and composite matrix material due to its biocompatibility, biodegradability via hydrolytic/enzymatic degradation, and high extent of crystallization. We’re studying the possibility of making fully exfoliated layered silicate nanocomposites out of PLLA. In the process we’ve explored the crystallization behavior of PLLA both from solution and the isothermal melt and both with and without organically modified, exfoliated silicate layers. Polarized light microscopy images reveal that in spite of silicate layers acting as crystallization nucleation sites, spherulite sizes become extensively large by clay incorporation into the matrix. Kinetics of spherulite growth has been captured by hotstage-polarized light microscopy and shows highly increased kinetics/radial growth rate due to the interaction with clay platelets. Probing Si, Al and Mg elements, what are all constituents of silicate layers, via Energy Dispersive Spectroscopy (EDS), shows that silicate layers are randomly distributed and not excluded from growing spherulites. Differential Scanning Calorimetry (DSC) shows a decrease in crystallinity of the PLLA from the bulk, which is commensurate with a decrease in intensity of Wide Angle X-ray Scattering (WAXS) peaks corresponding to the crystallites. Single crystals of pure PLLA were also grown from toluene dilute solutions and their lattice parameters were compared with the crystals grown from solution in the presence of silicate layers.

[C1.159] Carbon Nano Tube Composites with Chemically Functionalized Plant Oils

Carbon Nano Tube Composites with Chemically Functionalized Plant Oil Wim Thielemans, R., P. Wool, V. Barron and W. Blau

Multi-Wall Carbon Nano Tubes (MWCNT) made by the Kratchmer-Huffman CCVD process were found to interact and solubilize by slow mechanical stirring, with chemically functionalized plant oils, such as acrylated, epoxidized and maleinated triglycerides (TG) derived from plant oils. The chemical functionality on the TG imparted amphiphilic properties to the oils which allows them to self-assemble on the nanotubes, promoting both dissolution and the ability to make nanocomposites with unusual properties. Once in solution, the MWCT can be processed in a variety of methods, in particular to make composites with enhanced mechanical, fracture and thermal properties. Since the tensile modulus of MWs is about 1 TPa and a vector percolation analysis indicated tensile strengths of 50-100 GPa, we obtain significantly improved properties with even small amounts (1-3the glass transition temperature of the composite by about 20 oC, and the tensile modulus by about 11significant effects on the fracture stress can be obtained due to the both the influence of the strength and length of the MWNT at the crack tip. The ability of the oils to self-assemble on the carbon nanotube surfaces also makes them ideal candidates for self-healing materials. The properties with different functionalized oils will be reported. Supported by EPA, DoE and ISF

[C1.160] Synthesis And Single Molecule Force Spectroscopy Of Poly(hydroxyethyl methacrylate-g-ethylene glycol)

With the advent of nanotechnology, miniaturized devices will soon need nanoscale springs with well-controlled nanomechanical properties such as shock absorbers, or to control the adhesive interactions between two components. In order to understand, manipulate, and control single macromolecule nanomechanical properties, mono(thiol)-terminated poly(hydroxyethyl methacrylate-g-ethylene glycol) has been synthesized via atom transfer radical polymerization. End-functionalization, chemical structure, molecular weight, side-chain graft density, radius of gyration, and polydispersity were characterized by 1H nuclear magnetic resonance, static light scattering, and gel permeation chromatography. The polymer chains were attached to Au-coated Si wafers via chemisorption to prepare well-separated "mushrooms", as verified by atomic force microscopy. Single molecule force spectroscopy was then used to measure the extensional elastic properties, i.e. force (nN) versus end-to-end separation distance (nm), of the individual chains by tethering to a Si3N4 probe tip via nonspecific, physisorption interactions.

[C1.161] Defect–Mediated Plastic Deformation Near Scratches and Indentations in Thermally Evaporated Pentacene Thin Films

Pentacene is a material under active development for use in organic thin film electronic devices. We have investigated the plastic deformation and resulting molecular alignment of scratched and rubbed polycrystalline pentacene thin films using optical and electron microscopy and Electron Diffraction (ED). Polycrystalline films of pentacene were produced by vacuum sublimation onto amorphous carbon coated substrates in a glass tube, and the films had grain sizes of 500 – 5000 nm. Before deformation the films were textured with the (001) planes parallel to the substrate and the long axis of the molecules nearly perpendicular to the substrate. Defects such as low angle grain boundaries in which the tilt across the boundary was mediated by partial edge dislocations were imaged using High Resolution Electron Microscopy (HREM). Inside the plastically deformed zone near the scratch, the molecules were tilted over with their long axis nominally parallel to the substrate, and they had a preferred orientation in the scratch direction. We used ED and HREM to quantify the extent of alignment in the deformed region and to directly image defects visible after alignment. Nanoindentation was used to investigate the mechanical response of the films and to quantify the amount of plastic deformation at a given indentation load and loading rate.

[C1.162] NEW EXPERIMENTS WITH (COLLOIDAL PROBE-) LABELD ACTIN FILAMENTS AND MOTOR PROTEINS; 3D MOTILITY ASSAY, THERMODYNAMIC ANALYSIS AND INDUCED REPTATION.

We developed a method to attach small, fluorescently labelled polystyrole beads to actin filaments. The position of these beads can be measured accurately by digital microscopy to determine the longitudinal and transversal movement of the filaments. With this technique various statistic properties can be obtained, such as the end-to-end distance fluctuations versus time. To combine this method with our Magnetic Tweezers setup, magnetic beads with diameter smaller than the average network mesh size are attached to the ends of the filaments. By tracking the position of these beads under a well-defined external force one can measure local properties of single actin filaments in the network such as frictional forces, persistence lengths or binding forces. The introduction of motor proteins into the actin network revealed completely new and unexpected effects on a microscopic scale such as 3D transports of actin filaments with non processive, single motor proteins or local torsion of the gelating network. In this context evidence for dynamic transitions in those networks is provided.

[C1.163] Percolation clusters in sheared polymer nanocomposites

We use Molecular Dynamics simulations to determine the role that percolating clusters play in the rheology of nanofilled polymer materials. We model the filler particles as spherical filler particles in a matrix of unentangled polymer chains. We observe the effect of shear on the ability of the percolating clusters to affect chain orientation. Our results indicate that at low shears, the presence of these clusters leads to increased chain orientation. We discuss the effect of filler size, filler polymer interaction and filler volume fraction on rheology of the polymer melt.

[C1.164] Noncircular Pores on the Surface of Asymmetric Polymer Membranes: Evidence of Pore Formation via Spinodal Demixing

To investigate the origin of noncircular pores in the selective layer of asymmetric poly(vinylidene fluoride), PVDF, membranes cast by immersion precipitation, the mass transfer kinetics were controlled by casting membranes at different temperatures and casting solution concentrations. Interconnected surface pore morphologies were observed by scanning electron microscopy (SEM) for membranes cast at high concentration and/or temperature, supporting a mechanism of liquid-liquid demixing at the membrane surface by spinodal decomposition. The coarsening of this highly porous, interconnected morphology leads first to chain-like formations of discreet, noncircular pores and ultimately to scattered pores of circular shape and large size dispersity. Similar results for poly(sulfone), PSF, membranes, which solidify via vitrification rather than crystallization, suggest a degree of universality in the mechanism of morphological development of asymmetric membrane surfaces. This new understanding of pore formation kinetics opens the possibility of tailoring membrane surfaces to achieve a high degree of porosity and pore size uniformity.

[C1.165] On the formation of multi-generation buckles in elastomeric films

We report on the formation of multi-generation buckles in elastrometric films and their utilization in material assembly and surface chemical patterning. In order to create the buckles-containing surface, we stretch uniaxially a piece of poly(dimethyl siloxane) network film by \Deltax and expose it to ultraviolet/ozone (UVO) treatment for t minutes. Upon releasing the stress from the stretched PDMS sheet, we observe the formation of buckles perpendicular to the stretching direction. The height, h, and width, w, of the buckles are in the range of tens of micrometers. Both h and w can be tuned by varying the molecular weight of PDMS, \Deltax, and t. We observe that the buckle surfaces are not smooth they are decorated with smaller buckles (widths \sim micrometers) and those are covered with yet another generation of buckles (width \sim tens of nanometers). We discuss a model explaining the formation of such buckled structures. We also show how these surfaces can be used in microfabrication, including material assembly and printing.

[C1.166] Surface Grafted Copolymer Assemblies with Gradient in Molecular Weight and Composition

We present a simple technique to modify silicon surfaces with short polymer brushes with precisely and systematically varying polymer molecular weight and copolymer composition along the surface. An initiator layer is first chemisorbed onto the silica-covered surface and an Atom Transfer Radical Polymerization (ATRP) solution containing the monomer, metal halide, ligand, and solvent is placed into a custom designed chamber. The treated wafer is then partially immersed into the solution. During the polymerization, the solution is steadily removed from the chamber by a pump, thereby lowering the solution level. This causes different parts on the wafer surface to remain in contact with the polymerization media for different amounts of time. As a result, a surface-anchored polymer with a gradient in molecular weight is formed which maps out the entire surface polymerization process from start to finish. With this technique, we can control the polymer thickness vs. wafer length slope along the surface by controlling pump speed and synthesize copolymers with gradually varying composition and/or molecular weight. We will demonstrate that surface properties of large sets of copolymer compositions and molecular weights may be studied on one sample and data may be generated much more quickly.

[C1.167] Effect of Solution Conditions on the Nanoscale Intermolecular Interactions Between Human Serum Albumin and Low Grafting Density Surfaces of Poly(ethylene oxide)

The first step in the biological rejection response to an implanted blood-contacting biomaterial is the non-covalent adsorption of proteins onto the surface, which triggers a cascade reaction ultimately resulting in thrombus formation. Using the technique of high resolution force spectroscopy, we have quantified the nonspecific intermolecular forces between fatty acid-complexed human serum albumin (HSA) covalently attached to a cantilever probe tip and individual end-grafted poly(ethylene oxide) mushrooms. In order to help elucidate the molecular origins of the constituent forces (e.g. steric, electrostatic, van der Waals), experiments were performed varying both the solution environmental conditions (e.g. ionic strength, removal of the bound fatty acids, and the addition of the antihydrophobic agent isopropanol), and the probe deflection rate.

[C1.168] Direct Imaging of Nanoparticle Embedding into Thin PS Films

Non-contact Atomic Force Microscopy (AFM) was used to study the embedding of gold nano-particles into the surface of polystyrene films. The rate of embedding was determined by measuring the apparent nanosphere height as a function of annealing time. This was performed at a number of temperatures near (both above and below) the bulk glass transition value, T_g. Above the bulk glass transition, particle embedding occurs as expected. Changes in rates of engulfment suggested the embedding was driven by two different processes; a faster process occurring for approximately the first half of the particle diameter, and a subsequent slower process. Relative height measurements of the particles were also made at temperatures below the bulk T_g value where no embedding is expected to occur. Measurements on various sized particles (10, 20, and 50nm) indicated that the particles did embed 3-4 nm into the polymer at least seven degrees below the bulk glass transition. Both the extent and time frame for engulfment appears to be independent of the particle diameter. The results may suggest a more mobile surface region in the order of 3-4 nm with a lower glass transition temperature than the bulk.

[C1.169] Chain end effect on the surface glass transition temperature

While the Flory Fox relationship has successfully predicted the bulk glass transition temperature polymers, there has been much controversy as to how this effect manifests itself at the surface of a polymeric material. where other relaxation mechanisms have been proposed. In the absence of specific end groups, the enhancement of the chain end density at a free surface is too small to be observed and hence it has not been possible to probe these assertions quantitatively. We show that by controlling the free end density at the polymer film surface using graft copolymers, we can rigorously prove that the Flory postulate holds even at the surface of polymer films. This work is supported by a grant from the NSF-MRSEC program.

[C1.170] Influence of Chain End Groups on Surface Segregation in Miscible Blends of Polystyrene and Poly (vinyl methyl ether)

Surface and interfacial composition in symmetric miscible blends of monodisperse deuterated polystyrene (dPS) and poly (vinyl methyl ether) (PVME) was revisited on the basis of X-ray photoelectron spectroscopy (XPS) in conjunction with neutron reflectivity (NR). PVME and dPS were enriched at the surface and the silicon substrate interface, respectively, and the concentration profile near the surface was consistent with the mean-field prediction. Also, surface composition in symmetric blends of PS with fluoroalkyl chain ends (\alpha,ømega-PS(R_f)_2) and PVME was examined by XPS. In this case, the surface enrichment of PVME was suppressed by virtue of the surface localization of the R_f chain end groups. Combining the chain end effect with molecular weight disparity, PS component could be preferentially partitioned to the surface.

[C1.171] ULTRAMICROINDENTATION AT SILK MEMBRANES SURFACES

Indentation with a sharp indenter, involving deformation on a submicron scale is an easy way to measure the mechanical properties of materials. The ultramicrohardness technique has been used to evaluate the plastic and elastic properties of polymers[1,2]. It has been used to measure the plastic, elastic and flow properties of silk membranes, cast at different temperatures. The hardness H was determined from the load-penetration depth curves after a holding time of 6 s at maximum load. The elastic modulus E, was derived using the method of Oliver and Pharr fitting a power law to the unloading data. Results reveal that conversion from the a-structure of the membrane cast at 20ºC to the b-pleated structure of the sample cast at 80ºC, showing larger and densely packed nano-fibrils, gives rise to a huge hardness increase from 64 to 212 MPa. The data also show that elastic recovery of the silk membranes is an increasing function of the maximum load applied. References: 1. A.Flores, F.J. Baltá Calleja, Phil. Mag. A78, 1283 (1998). 2. A.Flores, F.J:Baltá Calleja, T. Asano, J. Appl. Phys. 90, 6006 (2001).

[C1.172] Topographically Tuning Polymer Adhesion

Nature often uses geometry on micro and nano length scales to systematically tailor performance in multivariable environments. A great example, which has received much attention recently, is the foot of a gecko. The gecko's foot is covered with hundreds of thousands of "hair"-like protrusions which dictate a gecko's precise control of adhesion through van der Waals forces.(1) In our research, we fabricate controlled structures ranging from the nano to micro length scales on elastomeric surfaces. Our initial results are based on the topography of spherical caps and high-aspect ratio posts that decorate the surface of polydimethylsiloxane layers. Based on initial calculations, we demonstrate how the aspect ratio and inter-feature spacing greatly affects the near-surface compliance, thus impacting the processes of interface formation. The density and shape of the features are also shown to enhance the prevention of interfacial failure. These results are relevant for the refinement of the soft lithography processing technique, the development of smart adhesives, and the fabrication of bonding sites for biological implants. (1) Autumn, K.; Liang, Y.A.; Hsieh, S.T.; Zesch, W.; Chan, W.P.; Kenny,T.W.; Fearing, R.; Full, R.J. Nature 2000, 405, 681-685.

[C1.173] Isotherm Studies of Telechelic POSS-PEO Polymers

Telechelic polymers provide an alternative polymer architecture to normal amphiphilic block copolymers for producing novel solution structures through self-assembly. High molecular weight poly(ethylene oxide) (PEO) is a well-known water-soluble polymer that also forms stable monolayers at the air/water interface. In contrast, oligomeric PEO does not form stable monolayers. Due to their relatively large size and hydrophobic character, polyhedral oligomeric silsesquioxane (POSS) end groups enhance the amphiphilic nature of the oligomeric PEO. Surface pressure-area per monomer isotherm and Brewster angle microscopy studies comparing telechelic-POSS with high molecular weight PEO, amphiphilic trisilanolcyclohexyl-POSS, and non-amphiphilic octacyclohexyl-POSS will be presented.

[C1.174] The Mobility of Polymer Chains Confined at a Free Surface

Dynamic Secondary Ion Mass Spectrometry (DSIMS) was used to investigate the chain mobility of polystyrene (Mw ranging from 4.3 kg/mole to 957 kg/mol) at the free surface. The data show that the diffusion coefficient was reduced relative to the bulk value within a distance, d \leq 4R_g, from the surface and scaled as 1/N^2.5 at fixed d. These results are in excellent agreement with SCF calculation of the surface segmental distribution and provide the first direct confirmation of various theoretical models that predict asymmetric segmental fluctuation which arise from surface induced orientation of polymer chains.

[C1.175] Modification of Gold Surfaces with Ultrathin Polyacrylonitrile Films

Ultrathin films of polyacrylonitrile (PAN) were grafted onto gold substrates from self-assembled initiator monolayers via the “grafting from” technique. The polymerization was initiated at room temperature by irradiating the dithiol azobisisobutyronitrile-type initiator at 300 nm. The films were characterized by x-ray photoelectron spectroscopy, reflection absorption infrared spectroscopy, ellipsometry and contact angle measurements. PAN films up to a thickness of ~ 46 nm with a root mean square roughness of 0.17 nm could be formed and the thickness of the films varied as a function of monomer concentration. The contact angle was 55° and the film thickness increased by 12105°C for 3 days. The results of these studies will be presented.

[C1.176] Generation and characterization of three-dimensional nanoparticle assemblies on molecular weight gradients of surface-anchored macromolecules

We describe preparation of three-dimensional (3-D) nanoparticle assemblies on substrate-anchored macromolecules having a gradient in chain length (i.e. molecular weight, MW). Such a gradient is created by first attaching a silane-based polymerization initiator homogeneously to a silica substrate and then exposing various parts of the substrate to the polymerization reaction mixture for varying amounts of time. Immersion of a model polyacrylamide MW gradient prepared by aqueous atom transfer radical polymerization in a colloidal gold solution results in the formation of 3-D number density gradient of gold nanoparticles. Characterization of this 3-D gradient by atomic force microscopy, spectroscopic ellipsometry and ultraviolet visible light spectrocopy show that number density of attached particles increases as MW of anchored polymer chains increases along the gradient. Further analysis by x-ray photoelectron spectroscopy (XPS) indicate that for a given polymer brush gradient, larger particles (\sim15nm diameter) tend to reside near the free end of chains (near air interface) whereas smaller particles (\sim3.5nm diameter) are distributed more homogeneously.

[C1.177] Crystallization of Ultra-thin Film of Polyethylene and Its Copolymers

Semi-crystalline thin films of low density polyethylene and poly(vinyl acetate) with thicknesses ranging from 0.4um to 7703 were prepared by spin coating. Morphology and lamellae orientation were measured by atomic force microscopy(AFM) and transmission electron microscopy(TEM). Melting temperature was measured by shear modulus force microscopy(SMFM).The degree of crystallinity was tested by polarized attenuated total reflection(ATR) fourier transform infrared spectroscopy(FTIR). With decreasing film thickness, the morphology was found to change from typical 2D spherulite with perpendicularly oriented lamellae to flat-on orientated lamellae. A densely branched structure was observed within the lamellae by TEM. For the ultra-thin films, shish-kebab crystals prevail all over the film in between the big lamellae. The melting temperature decreases drastically with the film thickness after a critical thickness of around 30003. A decrease of Tm of about 60K from the bulk material was observed for films 10003 thick. ATR-FTIR results showed no obvious decrease in crystallinity with decreasing film thickness with large crystalline peaks(CH2 rocking band at 730cm-1) being observed in films as thin as 10003.

[C1.178] Parallel and Perpendicular Diffusion of Polymer into Its Confined Matrix

When a polymer film is much thinner than twice its radius of gyration(Rg), its chain conformation parallel or perpendicular to the surface changes relative to the bulk configuration. As a result, the chain entanglements may be modified by the confinement effect and the diffusion rates may differ from the bulk. A blend of deuterated polystyrene(dPS) and hydrogenated polystyrene(hPS) was allowed to diffuse into either a confined or bulk like matrix. In this study, Secondary Ion Mass Spectrometry (SIMS) was used to investigate the diffusion perpendicular and parallel to the surface.

[C1.179] Control of Surface Morphology in PS-b-PMMA/PPO Blend Thin Films

The structure of symmetric diblock copolymer thin films is governed by boundary conditions, such as commensurability and interfacial interactions. If the initial film thickness is not commensurate with the quantization of characteristic long period of the block copolymer, either holes or islands of height Lo are formed. We have previously shown the abnormal dual morphology of interconnected islands and fractal holes in polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymer thin films with an initial thickness of 1.5Lo < to < 2.0Lo. The development of fractal holes is initiated by the lateral strain induced by the post evaporation of a small amount of residual high boiling solvent after the fast formation of parallel lamellae with interconnected islands. In present study, we report unusual hierarchical structures of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) thin films with various initial thicknesses. Surface morphology and domain spacing were investigated using small angle x-ray scattering (SAXS), optical microscopy (OM) and atomic force microscopy (AFM). PS-b-PMMA/PPO (poly (2,6-dimethyl 1,4-phenylene oxide)) binary blend thin films were also prepared in order to study how the addition of PPO homopolymer can have an effect on hole formation. By adding PPO homopolymer, which is very miscible with PS, to the PS-b-PMMA block copolymer, Lo in bulk state was increased and the hierarchical structure of the thin films was effectively suppressed.

[C1.180] Late Stage of Dewetting of Inverted PVP/PS Bilayer Films

We report the behavior of dewetting and layer inversion process of inverted poly(2-vinlypyridine)/polystyrene (PVP/PS) bilayer thin films, which were prepared by spin coating a PVP layer on top of the PS layer bearing silicon wafer, using optical microscopy (OM) and atomic force microscopy (AFM). In comparison with previous data on the bilayer film with similar thicknesses and molecular weights, two types of inverted bilayer films were prepared in this study as follows: one has a thicker PS layer than the PVP layer with similar molecular weight and the other has a higher molecular weight (i.e., more viscous) PS layer at the bottom layer with similar thickness. When the inverted bilayer is annealed well above the glass transition temperatures of both PVP and PS, the upper PVP layer first dewets on the lower PS layer (i.e., liquid/liquid dewetting) at the early stage of annealing due to the strong immiscibility between the PVP and the PS. Prolonged annealing, however, leads to the layer inversion where the dewetted upper PVP layer segregates toward the silicon wafer due to its stronger affinity toward the substrate while the PS layer wraps around the PVP domain due to its lower surface energy compared with that of the PVP. The final dewetted morphology was found to be significantly different depending on the layer thickness and molecular weight since the layer inversion starts at different dewetting stage.

[C1.181] Effect of Ge-overlayers on the resistivity of ultra thin metal films

Ag- and Cu-films about 3 nm thick deposited at room temperature in a high vacuum system on Ge-substrates show unusually low resistance values. The resistance was of the order of 10 kOhm/square for films 1 nm thick [1]. Even lower values were found for Fe-films prepared in the same manner. The effect of the germanium-metal interface on the the resistance was studied by depositing Ge-overlayers on metal films. The overlayers produced a resistance change of less than 6an resistance increase of 20nm thick Ge-overlayer on silver. The resistance did not change if more germanium was added. This suggests that only the first two Ge-monolayers on the metal influence the electron transport process at the germanium-metal interface. The Ge-substrate influences the growth mechanism, leading to a layer by layer growth of the metal film.

10 K. Schroder and J. Hollander, submitted for publication

[C1.182] Nano-Calorimetric Studies of Polymeric Phase Transitions in Confined Geometries

Despite the immense technological significance of phase transitions and thermodynamic properties of polymer thin films and other nano-structures, several fundamental issues remain unresolved. This is partly due to insufficient sensitivity and high thermal time constants of conventional thermal characterization techniques, e.g. differential scanning calorimetry (DSC). On the other hand, the sensitivity of nano-calorimetric devices (.1microW-1nW) has been demonstrated to be two-three orders of magnitude better than conventional DSC systems. Moreover, internal time constants on the order of 1 ms are readily achievable. In this work, MEMS fabrication technology is used to develop ultra-sensitive thin film differential scanning nano-calorimeters. Design optimization of the nano-calorimeter is achieved by numerical finite element simulation of heat transfer across its various constituents. Glass transition and melting behavior of polystyrene and polyethylene oxide thin films are studied using the nano-calorimeter.

[C1.183] The effect of nanoscale fillers on the tracer diffusion in polymers

The effects of nanoscale fillers on the tracer diffusion coefficient were studied using neutron reflectivity and secondary ion mass spectrometry. The results indicate that the diffusion is increase when the size of the fillers is large compared to the polymer radius of gyration. The zero shear viscosity extracted from the diffusion coefficient will be compared to the shear strain measured by other techniques, and theoretical prediction. Support from the NSF MRSEC is gratefully acknowledged.

[C1.184] Influence of mobile nanoparticles on phase separation dynamics in thin film polymer blends

Nanocomposite thin films, composed of a binary polymer mixture and mobile nanoparticles, undergo a unique phase evolution mechanisms incorporating wetting and phase separation. According to recent simulations, the addition of hard mobile particles may significantly impact both the dynamics and mechanism of phase separation (Ginzburg et al., PRL, 1998). Despite current technical and theoretical interests, polymer blend thin films containing nanoparticles have received little experimental interest because reliable sample preparation, such as particle dispersion, is difficult and the need for multiple techniques for following phase separation, wetting and particle partitioning. Using our prior understanding of phase evolution in PMMA/SAN thin film blends as a foundation (Wang et al., J.Chem Phys., 2000), we next investigate how the addition of spherical silica nanoparticles impacts phase dynamics. Because of the complexity of the problem, various characterization tools are utilized in this study, including optical microscopy, AFM, RBS, FReS, SEM, and TEM. The addition of particles is found to distort the phase evolution mechanism and, in agreement with simulations, retard the growth kinetics. Wetting layer growth was also influenced by the nanoparticles.

[C1.185] Synthesis and characterization of polymer brushes that can be cleaved from the substrate by photo-generated acid

An established method for synthesizing monodisperse polymer brushes is to deposit a monolayer of "living" free radical initiator molecules and to polymerize the brushes from the substrate. We have developed a method for patterning these brushes by incorporating an acid-labile linkage into the tether that allows the brushes to be cleaved from the surface by exposing them to Extreme Ultra-Violet radiation in the presence of a photo-acid generator. A number of acid-labile linkages were studied, including a wide range of reactivities to acid. The reaction of the acid labile linkage to acid was monitored using Polarization Modulation Infrared Reflection Absorption Spectroscopy (PM-IRRAS). Only the most reactive linkage, a menthyl carbonate linkage, could be cleaved by the photo-acid. The polymer brushes containing the acid labile linkage were studied as imaging layers in terms of resolution and sensitivity.

[C1.186] Dynamic Separation of Polymer Thin Blend Film on Au/Si

We have investigated dynamic phase separation of two immiscible polymers on Au/Si using AFM. The domain structures are dependent on molecular weight and the film thickness. The dynamic domains are reversed after annealing. We analyze intermediate structures using AFM and dynamic SIMS (secondary ion mass spectroscopy). Furthermore, we present the effect of supercritical CO2 on those phase separations.

[C1.187] Contrasting viscoelasticity with atomically rough and smooth surfaces in molecularly-thin fluid films

The spectacularly different viscoelastic properties of fluids when the degree of confinement approaches molecular dimensions between smooth surfaces is well known. Here we used a surface forces apparatus modified to measure film thickness using capacitance to study viscoelastic responses of fluids confined between two mica surfaces with imposed oscillatory shear or hydrodynamic drainage. Depending on the method used to cleave the mica, these nanorheological properties differ significantly.

[C1.188] The effect of nano-confinement on the mechanical properties of thin polymer films

The behavior of polymeric materials on the nanometer scale is of increasing interest in the areas of microelectronics, optoelectronics and nanocomposites. We describe here a novel technique for studying the mechanical properties of thin films constrained at the sub nanometer range. The experimental set-up is similar to the equi-biaxial bubble inflation technique but scaled to study films of nanometer thickness. A thin polymer film of thickness in the range of a few hundred nanometers is cast on to a wafer containing an array holes of the order of one micron in diameter. The membrane/wafer is loaded into a temperature controlled pressure cell capable of pressures up to 100 psi and temperatures to approximately 100 ^oC. Application of air pressure to the lower side of the wafer causes the free standing polymer membrane to expand and form a bubble over each hole. The shapes of the bubbles are then imaged using an atomic force microscope and the bubble growth followed as a function of time. From a knowledge of the bubble profile, hole diameter and air pressure applied, the stress and time dependent strain can be calculated and hence the compliance determined as a function of time. Results will be presented on the time dependent compliance at these length scales and compared to those of the bulk material. In addition the applicability of time-temperature superposition concepts at these length scales will be discussed.

[C1.189] Radial thickness profiles of spincoated polymer wedge films

We have used spincoating to deposit wedge-shaped polymer films in which the film thickness varies in the radial direction. The radial dependence of the film thickness of the polymer wedge films was measured using focused ellipsometry and atomic force microscopy. By considering radial flow of a Newtonian fluid and requiring that the film thickness decrease to zero at a nonzero radial position, we have derived an analytical expression for the radial dependence of the film thickness that contains only two adjustable parameters. Using this expression, we have performed nonlinear least squares fits of the measured radial film thickness profiles for a variety of polymer wedge films prepared under different deposition conditions.

[C1.190] Experimental and theoretical investigation of hole growth in freely-standing polymer films

Measurements of hole growth in high molecular weight freely-standing polymer films have revealed a transition from linear to exponential growth of the hole radius with time at temperatures close to the bulk glass transition temperature T_g. We have modeled the hole growth process in freely-standing polymer films and these results will be discussed within the context of recent work published on the dewetting of polymer films near T_g. In addition, the shape of the edge of the holes, measured using atomic force microscopy in both the linear and exponential hole growth regimes, will be compared to those predicted theoretically.

[C1.191] Effect of Annealing of Polystyrene Films in the Freely-Standing State

Spincoating of dilute polymer solutions is a powerful technique to prepare thin, uniform polymer films. Spincoated films on substrates are typically annealed under vacuum at temperatures above T_g and it is supposed that the interaction between the substrate and the polymer is small enough to allow equilibration of the film. We have removed the films from the substrate after annealing and then annealed the films in the freely-standing state to temperatures low enough that hole formation was not observed. Using ellipsometry, optical and atomic force microscopy, we have observed irreversible changes in the indices of refraction parallel and perpendicular to the plane of the film and in the radial thickness profiles of the freely-standing portions of the films. The measured values for the freely-standing portions of the films approach constant values with time, and they are compared to those measured for the supported portions of the films. Based on these observations, we suggest a mechanism for the observed changes to the optical anisotropy and film thickness and discuss the implications of these results on previous studies.

[C1.192] Moisture absorption in thin polymer films

We explore the kinetics and dynamics of moisture transport in a series of poly(hydroxystyrene), poly(tert-butoxycarboxystyrene), and epoxy films as a function of film thickness. Specular X-ray reflectivity is used to monitor the equilibrium swelling in films exposed to water vapor while a quartz crystal microbalance is used to track the kinetics of the absorption process. For relatively thick films we find that the equilibrium swelling is nominally consistent with the bulk moisture absorption properties of the polymer. However, when the film thickness drops below 100 nm, enhanced swelling occurs and the extent of the enhancement increases with resin polarity. Below the same thickness threshold of 100 nm, we also observe a strong retardation in the uptake kinetics, i.e., a decrease in the effective diffusion coefficient of the water. The results are discussed in detail with respect to confinement of the polymer films.

[C1.193] Coil to brush-like transition of polymer thin films in supercritical CO2

We have performed X-ray and in-situ neutron reflectivity on liquid polymer films and analyzed the spectra as a function of film thickness and molecular weight. The results show that polymer thin films on Si are swollen due to density fluctuation of scCO2. The films whose thickness are less than three times of the radius of gyration (Rg) were swollen to become maximum double in thickness. After release of CO2, the swollen chains are collapsed to form island structures which are also confirmed by AFM. This indicates that scCO2 behaves like a good solvent and polymer chain behaves like brushes.

[C1.194] Micro-shearing of Block Copolymer Thin Film

Micro-patterned crosslinked polymers are used as shear plates, and mechanical shearing is achieved using dynamic mechanical analyzer with precise control of strain amplitude, frequency, and temperature. Initial orientation of block copolymer cylinders perpendicular to the film surface was achieved by either controlled solvent casting or annealing. After application of shear, we investigated local parallel alignment of block copolymer cylinders using AFM. If necessary for the AFM characterization, wetting layer of block copolymer films was removed using XPS.

[C1.195] Ultrastructural and Nanomechanical Studies of Fresh Bovine and Human Cortical Bone

Cortical bone is a classic example of a complex hierarchical biocomposite with structural features ranging from the mm to the nm scale. Nanoscale ultrastructural and mechanical studies can provide important insights into molecular mechanisms of fracture, resorption, disease, and aging. Fresh adult compact bovine and human bone from the tibia metaphysis and diaphysis were polished down to a 0.05 mm grit size (AlO), partially demineralized with a 42.5etch, followed by 24 hrs of rehydration in phosphate buffered saline (IS=0.15M, pH7.4). Tapping mode atomic force microscopy in air was employed to directly visualize the detailed morphology of osteons, lacunae, canaliculi, cement lines, Haversian canals, type I collagen fibrils, and apatite crystals. Ongoing nanomechanical experiments include high-resolution chemical force spectroscopy with nanosized probe tips functionalized with self-assembled monolayers containing charged end groups (e.g. NH3+, COO-) and nanoindentation.

[C1.196] Adsorption of myoglobin to metal-chelating lipid monolayers by neutron and X-ray reflectivity and GIXD

In an effort to devise simple and robust systems that can reproduce in synthetic membranes important features of biological targeting and surface assembly, a versatile system for targeting proteins to lipid membranes has been developed.1 This system utilizes metal-chelating iminodiacetate lipids (DSIDA) loaded with divalent metal ions (Cu+2 or Ni+2) to target adsorption of specific residues in proteins. In this work we used neutron and X-ray reflection and grazing incidence X-ray diffraction (GID) to study the adsorption of myoglobin to monolayers of DSIDA at the air-water interface. The combination of these techniques allows characterization of the thickness and concentration of the adsorbed protein layer as well as the changes in the packing of the lipids upon adsorption of protein. We find that the protein adsorbs to form a dense packed monolayer underneath the DSIDA monolayer, with thickness and segment density that are consistent with crystallographic data for myoglobin. No evidence for multilayer adsorption was observed. The final state was achieved after roughly 12 hours. We observed that the response of the lipid layer to protein adsorption is a strong function of surface pressure in the range of 35 mN/m to 45 mN/m.

1K. Ng, D. W. Pack, D. Sasaki, F. H. Arnold, Langmuir 1995, 11, 4048.

[C1.197] Self-Assembling Diblock Polypeptide Hydrogels: Effects of Salt and Cell-Growth Media on the Self-assembly Process and Material Properties

Self-assembling peptide based hydrogels having a unique nano- and microscopic morphology are being studied for potential use as tissue engineering scaffolds. Low molecular weight (~20 kg/mol), amphiphilic, diblock polypeptides of hydrophilic, polyelectrolyte cationic lysine (K) or anionic glutamic acid (E) and hydrophobic leucine (L) or valine (V) form hydrogels in aqueous solution at neutral pH and at very low volume fraction of polymer (vol. fraction polypeptide less than 0.5 wtbeen characterized using laser confocal microscopy (LCM), ultra-small angle neutron scattering (SANS), and cryogenic transmission electron microscopy (cryoTEM) imaging. Studies of the self-assembly process with and without significant ionic solution strength (i.e. in the presence of salt and cell growth medium) will be discussed. Interactions of the hydrogels with bacterial and mammalian cells reveal that these materials are non-cytotoxic and biocompatible. Hence, the chemistry of the assembled diblock polypeptides allows for cellular proliferation whereas the same chemistry in the homopolymeric form is cytotoxic. Proper molecular design for optimal cell viability and gel integrity in the presence of high ionic strength aqueous solution will be discussed.

[C1.198] Mechanic properties of freely suspended quasi two-dimesional model actin cortex on microfabricated three-dimensional pillar substrates

Conical micropillars with diameters of 1µm and more and variable interpillar distances were microfabricated on silicon or polymer substrates by photolithography and anisotropic reactive ion etching. By in vitro polymerisation of actin oligomers, grafted onto the surface of the pillar substrates, actin filaments were selectively bound to the top of the pillars. The addition of the actin crosslinking protein filamin led to the formation of a square mesh network of actin bundles and the confinement of the entire network to the plane of the pillar tops. This strategy allowed us to measure elastic properties of freely suspended actin filaments, bundles and network structures under controlled boundary conditions by means of flicker spectroscopy and magnetic bead micromanipulation. This quasi two-dimensional actin network also served as model substrate to study the transport behavior of the processive molecular motor myosin V, mimicking the transport at the interface between the actin cortex and the cytoplasm.

[C1.199] Magnetic Nanoparticle-Phospholipid Interactions in Monolayer Films

Magnetic nanoparticles (MNPs) have potential applications in drug delivery and as anti-cancer agents through hyperthermia, which is induced by hysteric magnetic heating. In order to determine the potential value of the MNPs in these applications, their interactions with cell membranes and phospholipid vesicles must be understood. As the primary structure of the cell membrane is a phospholipid bilayer, a phospholipid monolayer can be used as a biomimetic model for MNP-phospholipid interactions. Monolayer studies have been conducted using 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and varying concentrations of MNPs. The MNPs are magnetite nanoparticles stabilized by triblock copolymers. These copolymer coatings are comprised of poly(ethylene oxide) (PEO) tail blocks with a short carboxylic acid-functionalized urethane central block. Isotherm studies and Brewster angle microscopy (BAM) are used to examine the phase behavior within the monolayer.

[C1.200] Structure of Individual Cartilage Aggrecan Macromolecules and Their Constituent Glycosaminoglycan Chains Visualized via Atomic Force Microscopy

Aggrecan, a member of the hyalectan family of large aggregating proteoglycans is composed of a core protein, which may be substituted with \sim100 chondroitin sulfate (CS) chains and \sim25 keratan sulfate (KS) chains. This tissue-bound polyanionic structure provides >50equilibrium compressive modulus in cartilage. To visualize this structure, bovine aggrecan from fetal epiphyseal and mature nasal cartilages was adsorbed to mica that had been previously functionalized with 3-amino-propyltriethoxysilane to produce a monolayer and then imaged using tapping mode atomic force microscopy (AFM.) Individual aggrecan molecules were clearly visualized, as well as the N-terminal globular (G1) domain and individual CS-GAG chains. The core protein contour length, CS-GAG length, and height were recorded and analyzed statistically, and found to complement that obtained by electron microscopy and biochemical assays (e.g. Fluorophore Assisted Carbohydrate Electrophoresis (FACE), Western Blots). The high resolution obtained via AFM should provide new insights into conformational aspects of hyalectans at the single molecule level.

[C1.201] X-ray scattering and optical ellipsometric studies of collagen-model peptides

We report results of optical ellipsometry and X-ray scattering studies of structure in collagen-model peptides. Proline and hydroxyproline residues stabilize the triple-helical conformation of collagen proteins in the collagen consensus sequence. Regular modifications have been introduced into the collagen consensus sequence, forming model systems for the study of bio-macromolecular organization. The model systems are oligomers with hexapeptide sequences of the form: (Glu)5(Gly-Ala-Pro-Gly-Pro-Pro)6(Glu)5, or (Glu)5(Gly-Pro-Ala-Gly-Pro-Pro)6(Glu)5. The glutamic acid capping the ends of the hexapeptide sequences imparts solubility in water. Depending upon concentration and temperature, the peptides form lyotropic liquid crystalline structures, and maintain their order when dried to powders or films suitable for X-ray and optical studies. Through the use of the high intensity source of X-radiation at the Brookhaven National Synchrotron Light Source, phase transformation kinetics and structure development are studied in-situ, providing time-resolved characterization of these peptides. Two-dimensional optical ellipsometry provides direct measure of the optical anisotropy and retardance of the structures. The goal of our research is to evaluate the ability of these model peptides for self-assembly into liquid crystalline and true three-dimensional crystalline phases and to assess the temperature stability of resultant higher order structures.

[C1.202] CHARACTERIZATION OF HYDROGELS FORMED VIA INTRAMOLECULAR FOLDING AND CONSEQUENT SELF-ASSEMBLY OF Beta-HAIRPIN PEPTIDES

Stimuli-responsive and biocompatible networks formed via self-assembly serve great opportunities in tissue engineering and drug delivery applications. In this work we present the formation of hydrogels via the intramolecular folding and consequent self-assembly of 20 aminoacid long beta-Hairpin peptide molecules. These hairpin molecules are amphiphilic in nature with an alternating sequence of hydrophobic valine and hydrophilic lysine amino acids. These molecules are found to form hydrogels with a unique microstructure and nanostructure at different physical condition at low peptide concentrations (\sim 1 wt %). However, gelation is observed at high pH values (\sim9) and at high salt concentrations (\sim150 mM) where beta-sheet secondary structure due to hairpin folding is observed. The intimate relationship between beta-Hairpin molecule turn sequence and the consequent materials properties will be discussed. LSCM data reveals that hydrogel structure is heterogeneous at the microscale with water channels in the order of 10 \mum. Cryo-TEM technique shows that the network is consists of interconnected fibrillar/tubular networks. The viscoelastic properties of the hydrogels, as measured by rheology. Importantly, the gel is also reversible with pH, returning to a viscosity of water with a drop in pH that unfolds, and disassembles, the hairpin molecules.

[C1.203] Phase behavior of Crosslinked Diblock Copolymers

We investigate the microphase separation transition in crosslinked di-block copolymer melts using a coarse grained free-energy and the Random Phase Approximation (RPA). The quenched distribution of crosslinks is averaged using the Replica method. The recent small angle X-ray scattering (SAXS) experiments by N. P. Balsara et. al on randomly crosslinked polyisoprene chains of polystyrene-polyisoprene block copolymers reveal decrease in the order to disorder transition temperature T_ODT and an increase in the width of the primary peak \sigma of the structure factor as the number of crosslinks N_c per polymer chain increases. We find for a crosslink density greater than N_c^* the monomers are localized within a region of size \xi\sim 1/(N_c-N_c^*)^1/2. The crosslinks strongly oppose the ordering in the system as xi becomes comparable to the radius of gyration R_G\propto \sqrtN, where N is the number of monomers in a polymer chain. This results in a non-monotonic dependence of T_ODT on the crosslink density; T_ODT increases and then decreases with the crosslink density.

[C1.204] Block Copolymer Thin Films and the Double Gyroid Motif - A Combinatorial Study

The cubic symmetry and dual-phase bicontinuous network structure of the double gyroid (DG) morphology make it unique among the motifs exhibited by diblock copolymers DBCs). While the technological potential of DG structures to photonic and membrane applications has been recognized, little is known about the behavior of this system when in thin film form. The increased influence of confinement (film thickness) and surface energetics in thin films could shift the morphology of bulk-gyroid systems towards neighboring motifs (i.e. columns or lamella) or previously unknown and possibly useful forms. In order to determine the conditions under which the gyroid structure persists in thin films, and investigate novel forms that may be exhibited via others, we have recently initiated a combinatorial study designed to comprehensively map the morphology of (bulk) DG-forming DBCs as a function of film thickness and substrate surface energy. Our experimental approach uses novel specimen fabrication techniques, developed at the NIST Combinatorial Methods Center, to create a surface energy gradient on the substrate and an orthogonal thickness gradient in the overlying film. The film morphology across this specimen library is probed through automated AFM analysis. In this presentation, we will detail our experimental techniques and discuss our findings.

Part C of program listing