Session D40 - Poster Session I.
POSTER session, Monday afternoon, March 12
Exhibit Hall, Washington State Convention Center

[D40.001] Complex Structured Materials

This abstract not available.

[D40.002] Interpreting the Raman spectra of GeSe-I network glasses

We have used a first-principles computational scheme to investigate the Raman spectra of the Ge_xS_(1-x-y)I_y family of glasses. The scheme involves forming cluster models of atomic arrangements expected to be important in the glasses. Density-functional theory calculations are carried out on these models to optimize the bond lengths and bond angles, and to compute the vibrational normal modes. To make contact with experiment, the Raman scattering strength of each mode is also calculated, using a new, first-principles method (see Jackson et al., Phys. Rev. B 60, R14 985 (1999)). The results show clear features related to I atoms replacing S atoms in GeX_4 tetrahedra. These features are in good agreement with recent measurements of Boolchand et al. (unpublished).

[D40.003] Probing Nanoscale Cooperativity at the Glass Transition

Using non-contact atomic force microscope techniques, we measure low frequency dielectric noise in a glassy polymer, polyvinyl-acetate (PVAc) near the glass transition (290K-310K) on a 50 nm length scale. Using a capacitance -measuring scheme ,with a PVAc film acting as the dielectric, we measured time dependent changes in dielectric polarization of the sample. The time series of PVAc polarization fluctuations, showed random telegraph switching (RTS) .We further observed that RTS has two to four states, associated with distinct molecular cluster configurations. By changing the applied electric field, we measured reproducible changes in RTS state occupancy probabilities, which was related to changes in cluster dipole moment in the z direction. We analyzed the RTS kinetics, by observing the RTS state occupancy time distribution. The distributions were fitted with a stretched exponential with 'bulk-like' stretching exponents.

[D40.004] The Equations of State for Metal Nanocluster by a New Constant-Pressure Molecular Dynamics Simulation

Recently, we present a new constant-pressure molecular dynamics method for finite system. This method is specially suitable for studying the properties of nonperiodic system under pressure. In this paper, by using the new constant-pressure molecular dynamics method, We study the properties of metal nonaclusters under pressure, the equations of state for these nonaclusters at different temperature are obtained. We found that the bulk moduli increases linearly with the inverse radii of nanocluster and reduces linearly with the increase of the temperature.

[D40.005] First-principles Investigations of the Structures and Hyperfine Properties of Na _7, K_7, Cu_7 and Ag_7 Clusters

First principles investigations of the structures and hyperfine properties of Na_7, K_7, Cu_7 and Ag_7 clusters are carried out within the framework of the density functional theory. The lowest energy isomer for Cu_7 and Ag_7 is a pentagonal bipyramid structure with a C_5v symmetry, while the C_3v trigonal structure emerges as the higher energy isomer. In the case of Na_7 and K_7 the energy difference between the two isomers obtained in theoretical calculations is small enough that for all practical purposes the two isomers could be considered degenerate. Experiments measuring the hyperfine properties of these clusters indicate that the structures of these clusters is very likely the pentagonal bipyramid. The calculated hyperfine properties of the pentagonal bipyramid clusters are in excellent agreement with those measured and are used to conclusively identify the structure of these clusters.

[D40.006] A tight-binding/density functional search for the structures of Ge clusters

We have used a combined tight-binding/density functional theory approach to search for the optimal structures of small and intermediate-sized Ge_n clusters, with n \le 30. The approach uses a computationally fast tight-binding method to survey the cluster energy surface for candidate structures. The search employs a novel single-parent genetic algorithm (SPGA) that successfully located the global minima for Si clusters in the same size range (Rata et al., Phys. Rev. Lett. 85, 546 (2000)). Gradient-corrected density functional theory (DFT) calculations are then performed to study the candidate structures in more detail. We show that our tight-binding model and the DFT yield consistent global minima for clusters up to n=10. The initial results of the SPGA searches for larger clusters show that prolate Ge_n clusters remain more stable than compact structures beyond n=26, the size at which Si clusters are known to undergo a prolate to compact transition.

[D40.007] Friction force microscopy investigation of nanostructured carbon films

Frictional properties of nanostructured carbon films,obtained by deposition of supersonic carbon clusters, have been investigated by friction force microscopy under ambient conditions. The experiment was performed at low loads to avoid plastic deformation and wear. Films were scanned on nanometric scale toreduce the topographical contribution to lateral forces. By analysing the load-dependent measurements acquired on samples with diferent composition, we observed that the Herzian-plus-offset model can take into account the frictional behaviour of these materials. A strong dependence of adhesive forces on the specific surface location was observed. A quantitative comparison among these films and others carbon compounds is presented.

[D40.008] Measurement of Fullerene Polarizabilities

We will report on a method for measuring cluster polarizabilities in the gas phase. In this method a beam of clusters traverse a standing wave formed by a laser beam. By measuring the downstream spatial distribution with and without the laser, the polarizability can be determined. To date, the optical polarizability of C_60 at the fundamental wavelength of a Nd:YAG laser (1.064 \mum) has been measured(A. Ballard, K. Bonin, and J. Louderback, J. Chem. Phys. 113), 5732-5735(2000).. We also hope to report on a measurement of the polarizability of C_70 using the same technique.(This work was supported by the National Science Foundation under contract CHE-9420441 and by the Dept. of Energy under contract LLNL/DOE B278815.)

[D40.009] Persistent Currents in One-dimensonal Thue-Morse Rings

In the framework of tight-binding model, we present the behavior of persistent currents in a one-dimensional mesoscopic ring with a magnetic flux threading through it. The ring was constructed by a Thue-Morse sequence, which contains two units A and B, and can be obtained by the substitution rules A--AB and B--BA . Using a transfer-matrix method, we have calculated the energy spectra and persistent currents of the system. It is shown that the flux-dependent energies gradually form ''band '' structure as the degree of disorder increases in the system. Actually the disorder and the filling-up number are two important elements which influence the overall current evidently. Increment of the disorder in the system will lead to dramatic suppression of the currents. Meanwhile when the highest-occupied energy level is on the top of the band, the overall current is very limited and the system exhibit insulted in the whole. Otherwise, the ring becomes metallic. A metal-insulator transition can be easily observed in the vicinity of band gaps.

*to whom correspondence should be addressed. Electronic address:superlat@netra.nju.edu.cn

[D40.010] Multiscaling in Inelastic Collisions

We study relaxation properties of two-body inelastic collisions processes on the mean-field level. We show that this process exhibits multiscaling asymptotic behavior as the underlying distribution is characterized by an infinite set of nontrivial exponents. These nonequilibrium relaxation time scales are found to be closely related to steady state cumulants of the velocity distribution in the presence of noise. This behavior can be viewed as generalized fluctuation-dissipation relations.

[D40.011] Shape-Dependence of Ordering for Granular Materials in Two Dimensions

In three dimensions, granular systems settle into random structures, while in two dimensions grains can often be annealed into dense crystals. We study the shape dependence of ordering by welding ball bearings together to make clusters of spheres. We restrict our studies to two-dimensional systems by confining the shapes in a single plane. We show several measures of how well the different shapes order under a series of taps. Generally, shapes with higher rotational symmetry form more nearly perfect lattices. In most cases the nature of the disorder present is qualitatively different from that found in three dimensional random packings of spheres, with sharp boundaries dividing perfectly ordered crystallites. Only arrangements with the lowest observed densities have crystallite size comparable to the cluster size and appear random. Our data suggest a transition between random and partially crystalline arrangements at a filling factor of 84%, near the density for a similar transition found earlier for circles in two dimensions. Supported by NSF under DMR-9733898.

[D40.012] Effect of Substrate Friction in a Two-Dimensional Granular Couette Shearing Cell

An investigation of the effect of substrate friction on the kinematics of rigid granular material in a two-dimensional granular Couette shearing cell was conducted. Cylindrical disks resting on a substrate were packed between a stationary outer ring and a rotating inner wheel. Previous work reports the velocity and particle rotation rates as a function of packing fraction and shearing rates [1]. The authors report the existence of a stick-slip condition of the disks in contact with the shearing wheel. The focus of our study is to investigate the impact of the substrate friction on the stick-slip condition as well as the kinematics of the system in general.

[1] C.T. Veje, Daniel W. Howell, and R.P Behringer, Phys. Rev. E 59, 739 (1999).

This research was partially supported by the Copeland Fund, administered by The College of Wooster. C.T. received support from NASA GRC LERCIP internship program.

[D40.013] Rotational dynamics of a driven two dimensional granular array

Video imaging is used to study rotation, and translation in one and two dimensional arrays of cylinders. The cylinders are confined by a block, and pushed along a level track. The position, force and slip events are recorded. In this simple granular system we observe distinct phases of motion depending on the contact forces. This behavior is predicted for the one dimensional case, where the equations of motion can be solved analytically. We discuss how these features relate to the bulk properties of granular media.

[D40.014] The Role of Local Icosahedral Symmetry in the Compaction of Granular Beds

The role of local icosahedral symmetry of bond orientations in the random dense packings of monodispersive spheres has been discussed for several decades. The two key features of this topic are, first, the strong tendency for local tetrahedral packings, and second, the geometrical frustration inherent in clusters of tetrahedral packings. We investigate reducing the geometric frustration by using spheres of two different sizes, where the diameter of the smaller sphere is approximately 0.9 times the diameter of the larger sphere. At this ratio, a frustration-free icosahedron can be formed by tightly packing twelve of the larger spheres around a single smaller sphere. Starting with various relative compositions and with random initial conditions, we have measured the subsequent compaction on vertical shaking of these bidispersive granular beds. We will discuss these results in terms of the decrease in complexity of the energy landscape caused by the possibility of frustration-free local icosahedral packing, and comparison will be made with simulations.

[D40.015] Surfaces, Interfaces and Thin Films I

This abstract not available.

[D40.016] Ordered alloy films of Ni and Mn grown on Ni(111)/W(110)

The preparation and study of ultrathin films of metal-metal antiferromagnetics is important both for fundamental studies of surface-driven magnetic phenomena, and for the understanding of exchange bias. Ultrathin alloy films of Ni and Mn have been grown by depositing Mn on 4 ML Ni(111)/W(110), and annealing to 550 K. Directional Auger electron spectroscopy (DAES) shows that the films have short range structural order that is at most slightly distorted from fcc (111). LEED reveals that the films have long range order due to chemical ordering of the Ni and Mn. Depending on the Ni:Mn ratio, the experiments are consistent with structures similar to bulk Ni_3Mn, bulk NiMn, and a subsurface structure similar to the (\sqrt3 \times \sqrt3)R30, 2:1 surface alloy observed for Mn/Cu(111). The magnetic properties of these ultrathin film alloys are currently being investigated.

[D40.017] Durability of Polymeric Coatings: UV-Irradiation

Durability of polymeric coatings is investigated by using positron annihilation spectrscopy and gloss measurements in a variety of commercial coatings and model polyurethane under UV irradiation of both accelerated and natural weathering conditions. Significant variations of sub-nanometer defect parameters determined from PAS results are observed as a function of depth, of exposure time, and of weathering conditions. The loss of durability at the early stage of UV irradiation is interpreted in terms of photo-oxidation mechanism which involves the change of surface roughness and chemical structures. This is correlated with results obtained using glossiness and cross-linking dennsity measurements.

* Supported by NSF-CMS-9812717; AFOSR:F49629-97-0162,F49629-98-1-0309

[D40.018] Preparation of Various TiO2 Buffer Layers for YBa2Cu3O7 Thin Films

Titanium dioxide (TiO_2) thin films have been deposited on Si, MgO and SrTiO_3 (STO) substrates by pulsed laser deposition (PLD) and dc sputtering technique. For the case of PLD, pure anatase(001) or rutile(110) TiO_2 films with smooth surfaces have been obtained either by the oxidation of titanium nitride (TiN) films or by the deposition of TiO_2 films directly at different temperatures and oxygen pressures. The YBa_2Cu_3O_7(YBCO) thin films then deposited sequentially on the TiO_2 layers by PLD. The zero-resistance temperature (Tco) for the YBCO films deposited on TiO_2/Si(100) and TiO_2/STO(100) were 85 K and 89 K, respectively. Comparative studies of depositing YBCO films directly on to a dc sputtered TiO_2/STO(100) template commonly used in selective epitaxial growth (SEG) process have, however, resulted in non-superconducting YBCO top layer. The characteristics of the resultant TiO_2 layers obtained from various processes were analyzed to delineate the apparent discrepancies. Experiments of resolving the possible interface layer formation such as Auger electron spectroscopy (AES) depth profile analyses and the interfacial x-ray absorption spectroscopy (XAS) are expected to give more insight on this matter.

[D40.019] Characterisation of Pt and PtSi thin films at 3.39 \mu m using a low temperature prism coupler

PtSi films on Si (and precursor samples of Pt on Si) have been characterised using a novel, variable temperature (ambient to 85K), in-vacuo prism coupler - this couples IR radiation (\lambda = 3.39 \mu m here) to surface plasmons on the sample via an evanescent field across a uniform, remotely adjustable gap between prism and sample. The resulting attenuated total reflection (ATR) curves for Pt films decrease in half-width with decreasing temperature, as anticipated due to decreased electron-phonon scattering. The formation of PtSi by thermal annealing of the Pt films is revealed by XRD and AFM to be partial and less uniform than commonly supposed. However, even for PtSi films of uniform composition and smooth morphology on p-Si substrates, the shape of the ATR curves remain defiantly constant between ambient temperature and 85K. Consideration of increased carrier scattering due to the preclusion of thermionic emission over the low Schottky barrier with reduced temperature is set against decreased electron-phonon scattering to explain the observation.

[D40.020] Theoretical Investigation of Ge adsorption on H/Si(001) surface

Ge_1-xSi_x/Si heterostructure system has been intensively investigated because of its importance both for the materials science and for the technological application. The Ge growth on a clean Si(001) surface shows undesired features such as a rough surface due to the Stranski-Krastanow (SK) growth and a large interface mixing. Surface hydrogen is known to act as a surfactant for this system. It can change the growth mode from the SK mode to a layer-by-layer growth and improve the interface mixing. The origin of such H effects is, however, still unclear. To unveil the mechanism of the H surfactant effects, it is indispensable to know the Ge adatom behaviors on the H-terminated Si(001) surface. In this paper, we theoretically investigate the adsorption and diffusion of Ge atoms on the H/Si(001)-(2x1) surface by using first-principles calculations. The Ge atom deposited on the surface forms a Ge di-hydride at the dimer center, as a Si adatom does. Then, it exchange the site with a substrate Si atom with an activation barrier of as small as 0.75 eV to form a substrate Ge-Si dimer with an ad-Si dihydride. Although the Ge atom forms substrate Ge-Si dimer, it can diffuse along the dimer row by hopping from a backbond of a dimer to another.

[D40.021] Dynamics of electron confined to random dendritic structures

Motivated in part by the suggestion that high T_c superconductivity via the BCS mechanism may relate to branched domain structures, we have computed spectral and dynamical properties of a tight-binding electron in several types of random dendrites. We form a structure ensemble and compute dynamical properties by renormalization. Structures include random-alloy Bethe lattices, Vicsek-related random trees, and other related structures. Properties of interest are the densities of states and Kubo-Greenwood conductance at the Fermi level, as well as related vibrational properties.

[D40.022] Investigation of Surface Structure and Dynamics of H_2, p-H_2, HD and D_2 Molecules Absorbed on in situ Cleaved MgO(001) by Helium Atom Scattering

Helium atom scattering has been used to probe the surface structure and dynamical character of H_2, p-H_2, HD and D_2 molecules grown by isothermal adsorption at partial pressures in the 10^-7 - 10^-5 mbar range onto an in situ cleaved MgO(100) substrate held at temperatures between 7-12 K. The background pressure was in the mid 10^-11 mbar range. A first and second adsorbed layer of hydrogen were observed. In the first, the sequence c(2x2), c(4x2) and c(6x2), as mentioned in the literature, was measured while in the second layer, the structure was not so clear. The surface dynamics for the c(4x2) showed a single mode at 9-10 meV in the [100] direction, but in the [110] direction two Einstein modes at about 10.5 and 8.5 meV were observed and a third mode (with dispersion) which ranged from 2 to 6 meV was also measured. The c(6x2) surface dynamics was similar to c(4x2). The second layer had a single Einstein surface mode very near 5 meV. The dispersive mode is believed associated with parallel oscillations along the long edge of the cell and the dispersionless modes from parallel and perpendicular oscillations along the short length of the elementary cell.

[D40.023] X-ray Absorption and Emission Spectra at the Nitrogen K-Edge in Dilute GaNxAs1-x

Nitrogen K-edge absorption and emission spectra of the dilute nitride alloy, GaNxAs1-x, were measured at the Advanced Light Source, with N-concentrations between 0.5 and 2at 500ºC with an RF plasma source for the nitrogen, with variable As overpressure. The absorption spectra show a strong excitonic type peak just above the bottom of the conduction band. We attribute this peak to a nitrogen related resonant state in the conduction band. Annealing the samples at 750ºC had no detectable effect on the soft x-ray absorption or emission spectra. Emission spectra were measured as a function of excitation energy through the absorption threshold and showed two peaks spaced by 8 eV but no resonant features as a function of the excitation energy. Results of calculations of the emission spectra based on the local density approximation will be presented.

[D40.024] Structural and Magnetic Ordering of the Interface of Fe/Tb Multilayers

The interface of Fe/Tb multilayers was investigated using x-ray diffraction and Mossbauer spectroscopy to study structural and magnetic ordering of the interface and its dynamics of growth as a function of Fe and Tb layer thicknesses. Two groups of samples, one with fixed Tb layer thickness and one with fixed Fe layer thickness, were prepared by planar magnetron sputtering. Two major structures were identified with x-ray, and two minor structures were identified from Mossbauer measurement. The composition and magnetic ordering of the interface critically depends on the thickness of both Fe and Tb layers. Results of Mossbauer measurements implies that an amorphous Tb-Fe component at the interface with an average composition close to that of the intermetallic compound TbFe-sub-2 may likely be the origin of perpendicular magnetic anisotropy (PMA) of Fe/Tb multilayers. It is also suggested that PMA in Fe/Tb multilayers and amorphous Tb-Fe films may have the same origin.

[D40.025] BULK SUPERCONDUCTIVITY OF INDIUM-DOPED TIN-LEAD-TELLURIDES

We report results on a new class of superconducting (SC) materials, In doped Sn1-xPbxTe solid solutions. The interest in this system is stimulated by the unique properties of indium that creates a resonance band in the valence band of Sn1-xPbxTe. The high density of states in the resonance band stabilizes the Fermi level, which is responsible for spatial homogeneity. These materials exhibit SC transition with abnormally high critical temperature Tc for semiconductors. We carried out the magnetic susceptibility (Maxwell bridge) and resistivity (four-point probe) measurements from room temperature down to 1.5 K, on polycrystalline samples with an average grain size of 200mm. The simultaneous measurements show the bulk character of the SC transition with the resistivity transition leading the transition in magnetic susceptibility. In spite of the high homogeneity of these materials, the SC transition in resistivity can happen along the crystallites’ surfaces. The correlation in the temperature dependence of resistivity and magnetic susceptibility with almost zero slope at low temperatures above Tc, shows the dominance of impurity scattering into the resonance band, which is known to be important for occurrence SC transition in these materials.

[D40.026] Superconducting and structural properties of pure and Fe-doped YBCO.

Pure and Fe-doped (upto 25%) YBCO samples were prepared and studied. An annealing technique[1] was devised for control and determination of O-stoichiometry. Using this technique, multiple chain oxygen desorption processes in pure YBCO were discovered[2]. Superconducting critical temperature was determined using magnetization measurements, and lattice constants were computed from x-ray diffraction studies. Details will be presented.[1]R. K. Siddique, Physica C228, 365 (1994). [2]R. K. Siddique, Zeitschrift fur Physik, B93, 411 (1994).

[D40.027] Toulouse strings and off-diagonal disorder in 2D

A tight-binding model H with random nearest-neighbor elements H_ij=\pm 1 is sometimes used in connection with Anderson localization. It is not trivial to say how much disorder this Hamiltonian represents. For example, in 1D, there is no disorder at all, as all elements +1 can be gotten by unitary transformation. We note the connection between characterizing the disorder implied by H on a 2D square lattice and the Ising spin-glass ground-state problem treated by Toulouse. Thus a normal form of H with minimal number of -1 entries is related to the problem of connecting a given set of points on the lattice a minimum total length of string. Several reduction algorithms are discussed. Local densities of states are computed near the ends of strings for the case of low string concentration, hence of low irreducible disorder.

[D40.028] Critical Dynamics and Diffusion on W(001) Surface

A clean W(001) surface exhibits a temperature-driven order-disorder reconstruction to a c(2 \times 2) phase [4,6]. This structural phase transition has been the subject of numerous studies: it has attracted both experimental and theoretical groups [2-6]. We present results of simulation studies of the critical dynamics of the W(001) model system, concentrating on dynamical scaling [2] and anomalous behavior of diffusion [1].

[1] T. Ala-Nissila, W. K. Han, and S.-C. Ying, Phys. Rev. Lett. 68, 1866 (1992).

[2] M. Baldan, E. Granato, and S.-C. Ying, Phys. Rev. B 62, 2146 (2000).

[3] H.-J. Ernst, E. Hulpke, and J. P. Toennies, Phys. Rev. B 46, 16081 (1992).

[4] W. K. Han, S.-C. Ying, and D. Sahu, Phys. Rev. B 41, 4403 (1990).

[5] W. K. Han and S.-C. Ying, Phys. Rev. B 48, 14524 (1993).

[6] C. Z. Wang, A. Fasolino, and E. Tosatti, Phys. Rev. B 37, 2116 (1988).

[D40.029] The thermal diffusivity of Mn_xCd_1-xTe using the interferometric Mirage effect

Recently, N. Munoz-Aguirre et. al (N. Munoz-Aguirre, G. Gonzalez de la Cruz, Yu. G. Gurevich, G.N. Logvinov, and M.N. Kasyanchuk, Phys. Stat. Sol. (b), vol. 220, p. 781 (2000))reported a theoretical treatment of the heat diffusion in two-layer structures for application to photoacoustic experiments. They showed that the influence of the interface thermal contact between the layers plays an important role on the effective thermal parameters. In this work, the theory has been extended to the case of the Mirage effect detected by means of a Michelson-Morley interferometer, and applied to the system air-Mn_xCd_1-xTe, with x in the range 0.05 - 0.60.

[D40.030] Surface Acidity by Positronium Spectroscopy

The application of the technique of angular correlation of the annihilation radiation (ACAR) from positron-electron pairs for investigating Bronsted acid sites in zeolytic catalysts was initially introduced by the authors with remarkable success. As incident positrons enter polycrystalline zeolites, a portion of the incident positrons would diffuse through the bulk and emerge on the internal surfaces and form with electrons there hydrogen-like positronium atoms (Ps). The Ps atoms will eventually annihilate also and generate an ACAR spectrum that is distinctively narrower than the one from the bulk.

In the presence of Bronsted acid sites, the Ps atom population would be reduced through oxidation by protons, resulting in alteration of the ACAR spectrum which is gauged by using a lineshape parameter S. In this report, results from ACAR measurements for based-adsorbed NaHY zeolites will be presented and discussed in light of acidic stregth and effect of molecular size.

Acknowledgement: This project was partially supported by grants from NSF and Taiwan National Science Council.

[D40.031] potential profile on clean and oxygen adsorbed Si(111) surfaces measured with non-contact atomic force microscope

Electronic states and trapped charges at the interface of silicon oxide layer and silicon substrate is one of the most important issues in the MOS device technology. In order to understand their properties and spatial distribution in an atomic scale, we have developed a technique, based on non-contact atomic force microscopy, to visualize potential profile over the surfaces that enable us to probe a potential variation due to single charge. Using the method, we have studied potential profile over clean and oxygen-adsorbed Si(111)7x7 surfaces, dangling-bond free hydrogen-terminated Si(111)1x1 surface, and silicon oxide / silicon interface. We discuss the results in terms of electrical state density and screening length of the substrates.

[D40.032] A Simple Automated Valve

We have developed a simple computer controlled valve based on the popular Nupro B-4HK bellows valve. The valve is controlled by a stepping motor (Donovan Micro-Tek Inc.) using a Motorola MC3479 driver. TTL level input signals to the driver actuate the motor. With this stepping motor/driver combination it is possible to independently set the CW and CCW torque of the motor, something needed to avoid having the valve stick in the closed position. In order to get reproducible performance in setting a leak rate through the valve it was necessary to replace the supplied threaded handle used to open and close the valve with a fixed housing through which a threaded rod passes that operates the valve. The rod is moved in or out by rotating with the stepping motor. Performance data on the flow rate through the valve as a function of the opening of the valve will be presented. Three of these valves are in use in the gas handling system for an automated adsorption isotherm machine. They have each been cycled open/close over 1000 times with no difficulties.

[D40.033] Fluids

This abstract not available.

[D40.034] Collapse Dynamics and Resonance Behavior of Axisymmetric Slender Liquid Bridges

The evolution of axisymetric liquid bridges subjected to static and oscillatory forces have been studied numerically. When the liquid bridge is subject to constant axial gravity, the collapse time is found to be largely independent of the length of the bridge when other parameters are held constant. For the case of dynamic oscillations and a given forcing amplitude, the frequency of the first resonance peak is found to be maximum when the static body force is zero and decreases with increasing total body force or length of the bridge.

[D40.035] Modeling of Small Moving and Reacting Drops

Moving and steering fluids on a very small length scale, ranging from nanons to microns, is an important aspect of small devices, including MEMS, NEMS and (bio)chem-labs on chips. On large length scales fluids are typically moved by pumping or gravitational action. On small length scales, fluids can be transported by exploiting another driving force: spatial surface free energy gradients. A fluid droplet will experience such a gradient when the surface with which it is in contact exhibits a spatial variation in wettability ("hydrophilicity"). We have recently embarked on the modeling of drops on hetereogenous surfaces with a powerful lattice-gas modeling approach that combines both equilibrium and non-equilibrium techniques (i.e. Monte Carlo and density functional theory) that can include chemical reactions and be applied in both open and closed ensembles. We present modeling results on the unbalanced Young equation using droplets equilibriated on gradient surfaces in the presence of gravity and we will discuss the effects of contact angle hysteresis. We will also report on calculations involving moving droplets on gradient surfaces and, in addition, we will investigate the effects of having present a component that reacts with the surface to vary the wettability.

[D40.036] Self-assembly in Surfactant Oligomers: A Coarse Grained Description through Molecular Dynamics

We have developed a simple microscopic model of surfactant oligomers. Surfactant oligomers are made up of x (\ge 2) single chain surfactants connected at the level of, or close to, the headgroups by spacer groups(s). We have studied the formation and morphologies of supramolecular aggregates of these model surfactant oligomers in aqueous media by molecular dynamics (MD) simulation. In particular, we have investigated the effect of degree of oligomerization x on the self-assembly and diffusivity in bulk. For dimeric surfactants (x = 2), simulations exhibit a transition from spherical micelles to cylindrical micelles with an increase in surfactant concentration. With further increase in concentration these cylindrical micelles transform into extremely long `worm-like' or `thread-like' micelles. These findings are in excellent agrement with experimental results. For trimeric surfactants (x = 3), our simulation results give direct evidence for the formation of closed-loop micelles at intermediate concentration. The self-diffusion coefficients of both dimeric and trimeric surfactant solutions show remarkably similar behavior, indicating some universality in the behavior of such worm-like micellar solutions.

[D40.037] Optical Properties of Microlenses Containing DNA Fragments

An investigation of evaporated droplets containing DNA fragments in a buffer solution is reported. During the evaporation, the sample concentration increases, resulting in phase transitions. The resulting crystalline structure acts like a lens, giving rise to characteristic optical properties. The focus of our study is to understand the formation of the lenses and to be able to control the process. Based on the morphology of the droplet, as well as the optical properties, a model for the molecular organization of the DNA is being developed.

This research was partially supported by the Copeland Fund, administered by The College of Wooster.

[D40.038] Statistics of Nematic Ordering in Nanodroplets

In studies of nematic liquid crystals, the order parameter S describes the tendency to alignment of an assembly of rod-shaped molecules. It takes on its maximum value of 1 when the long axes of the molecules are all perfectly parallel, and vanishes in an infinitely large ensemble in which the molecular orientation is completely random. In a finite ensemble of randomly oriented molecules, however, S will always be greater than zero, as it is defined as the largest eigenvalue of a traceless matrix. We present a calculation based on random matrix theory of the probable magnitude of S in small systems in the absence of any nematic interactions.

[D40.039] Dynamics of optical signals in a nematic waveguide

We study the modes in a nonlinear nematic waveguide above the Frederickz transition and calculate each of the thresholds associated with different optical and orientational modes. Then, we exhibit the presence of kink-like solutions for the orientational equation under the action of optical fields and study its propagation. Finally, we analyse the dynamics of optical signal in the presence of orientational kinks for different modes and type of signals.

[D40.040] Multiple light scattering study of the Nematic to Smectic A transition

Anisotropy in the spatial dependence of multiply scattered transmitted light intensity in an oriented nematic phase as it is cooled towards the smectic A is measured. As shown by previous authors(H. Stark,M.H. Kao, K.A. Jester, A.G. Yodh, and P.J. Collings, J. Opt. Soc. Am. A 14 156 (1997).) these measurements allow the determination of the light diffusion constants parallel and perpendicular to the director. It has been suggested \footnote H. Starl and T.C. Lubensky Phys. Rev. E 55 514 (1996). that the transition to the weakly scattering smectic phase may be explored in a similar way. The diffusion constant depends on the ratio of elastic constants, of which the twist and bend constants diverge. We present data on the temperature dependence of the diffusion constant ratio and compare divergence of the elastic constants to standard models and methods.

[D40.041] Effect of Dynamics Surface Tension on the Motion of Foam Film in Constricted Tube

Foam flow in porous media exhibits striking properties that have been found desirable in many practical undertakings. These properties have been identified as enhanced gas viscosity by virtue of liquid film rheology, and reduced relative gas permeability by virtue of gas trapping. Many previous works treated foam in porous media as bubble trains flowing in a bundle of constricted capillary tubes. Prediction of macroscopic foam rheology can thus be upscaled from liquid film behavior in capillaries. In particular, the modified viscosity of gas in foam was expressed as intrinsic gas viscosity plus additional viscosity imposed mainly by the motion of liquid films through the constrictions of flow channels. The latter was proved (for instance ref. 1) to be directly proportional to pressure drop induced by liquid film curvature. The Young-Laplace relation was then employed to describe the pressure drop over the curved films. The use of this relation results in a maximum pressure drop at the constrictions, explaining the existence of additional viscosity. In this present work, we first develop a simple experimental setup to measure the pressure drop profile induced by one lamella moving through divergent-convergent tubes of varying aspect ratio. The results are surprising because the observed pressure drop profiles are opposite to prediction based on Young-Laplace relation: pressure drop increases when the film is moving downstream from constriction, and falls down when the film motion comes to a halt. Having a better understanding of this phenomenon, we develop a simple mechanical model of lamella motion in constricted tube, taking into account dynamic surface tension and viscous drag at the sidewall of the tube. The simulation results, that show the same trend as observations, address the significant effect of Marangoni surface elasticity and viscosity on dynamic gas viscosity though dynamic lamella tension. On basis of the experimental and theoretical studies, one of our conclusions is that the Young-Laplace relation is inadequate to describe the dynamic viscosity of gas in foam. Rather, it should be used as a static criterion for the onset of lamella mobilization. The overall results of this work are also used as physical background to explain partially the mechanisms of foam resistance and blockage to flow in porous media.

Relevant references: 1. Falls, A. H. Musters, J. J., and Ratulowski, J., SPE Reservoir Eng., 6, 37 (1991) 2. Xu, Q. and Rossen, W. R., 2000 EuroConference on Foams, Emulsions and Applications 3. Rossen, W. R., J. Colloid Interface Sci., 136, 1, 17, 38 (1990) 4. Hirasaki, G. J., and Lawson, J. B., SPE Journal 25, 176 (1985)

[D40.042] Diffusion and structure of a quasi-one-dimensional hard-sphere fluid

We report the results of an experimental study of a quasi-one-dimensional hard-sphere fluid. The system consists of uncharged Si colloidal spheres confined in long, uncorrelated 1D-channels whose narrow width forbids mutual passage of spheres along the channel. By tracking the trajectories of the spheres using digital video microscopy, we studied the diffusion and structure of the system as a function of the density of the fluid. Our results show that the behavior of the spheres in self-diffusion is changed gradually from Fickian to non-Fickian near the onset of the collision between the spheres, indicating the correlation between the collision of the hard-spheres and the change in diffusion mechanism. At high density, the self-part of the van Hove function of the system is no longer a Gaussian distribution but a Poisson distribution which can be interpreted using a hydrodynamic analysis for effective wall-drag effect. The pair distribution function of the system can be explained by an analytical expression for a 1D hard-sphere fluid [1]. [1] Y. Rosenfeld, M. Schmidt, H. Lowen and P. Tarazona, Phys. Rev. E 55, 4245 (1997).

[D40.043] Effects of a random pinning potential on the structure and driven dynamics of two-dimensional colloidal crystals.

A model system for studying the effects of quenched disorder on the properties of a two-dimensional elastic system is introduced. The system is a mono-layer colloidal crystal of charged polystyrene micro-spheres confined between two silica plates. One of the plates is prepared with the appropriate amount of roughness to act as a random, un-correlated pinning potential, whose strength is controlled by the separation of the two plates. At weak disorder we observe formation of ordered regions up to a few hundred lattice constants, with a low density of dislocations. At increasing disorder, we observe a transition to a disordered, glassy phase. Upon application of an electric field, the system can be set into motion. Different regimes are observed as a function of disorder and driving force, ranging from plastic flow, constrained into fluid channels around pinned regions, to elastic, were the whole crystal is sliding uniformly, without tearing.

[D40.044] Template-Directed Convective Assembly of Colloidal Crystals

We investigate the use of geometirc surface templates to enhance the convective assembly of colloidal crystals[1,2]. In our experiment, we immerse on- and two- dimensional grating templates as substrates into ~5 volume percent monodisperse colloidal suspension at 50 degree. Assembly oxxurs as the solvent evaporates[2]. SEM characterization revealed ~50 micron single domain size, [100]-oriented fcc crystals. Additional experiments are underway to investigate the effect of the added depletents on the assembly and crystal quality.

Fundied by NSF (DMR99-71226), MRSEC (DMR00-79909), NASA (NAG3-2172).

[1] K.H. Lin etal, Phys. Rev. Lett, 85, 1770(2000). [2] P. Jiang etal, Chem. Mater., Vol.11, No.8, 2132(1999)

[D40.045] Dark soliton creation and interactions in Bose-Einstein condensates

It is demonstrated that stable, standing dark solitons can be created in current dilute--gas Bose-Einstein condensate experiments by the proper combination of phase and density engineering [1]. Other combinations result in a widely controllable range of grey solitons. The phonon contribution is small and is calculated precisely. The interaction between grey solitons, manifested by a negative shift in their positions compared to a freely moving soliton, is also studied. Experimental parameters and a trap geometry which would allow for direct, in situ observation of dark solitons and soliton--soliton interactions are provided.

PACS numbers: 03.75.Fi, 05.30.Jp, 05.45.Yv

[1] e-print cond-mat/0011397 (2000)

[D40.046] Wave Chaos and Speckle in Bose Einstein Condensates

In agreement with the theory of phase rigidity in superfluids, simulations of repulsive Bose Einstein condensates (BECs) in one and two dimensions show that shocking a condensate causes it to break up into domains of constant phase. The size of the phase domains is inversely proportional to the strength of the initial shock, so that for strong shocks structure forms on a scale smaller than a healing length. In a two dimensional rectangular billiard a strong shock at an oblique angle results in a chaotic wave function with a density pattern similar to laser speckle. Speckle patterns do not normally occur in two dimensions, since they result from a spread in wave vectors due to the projection of three dimensional monochromatic light onto a two dimensional screen. In the two dimensional BEC dispersion due to the non-linearity mimicks this effect, giving rise to chaos even in an integrable billiard. Using a chaotic billiard enhances the effect.

[D40.047] Flow Response and Density Profile in a Driven System: A Monte Carlo Simulation

Driven flow response in an open system is studied using a Monte Carlo simulation. A lattice of size L_x \times L \times L is considered with the bottom (x=1) connected to a source of fluid while the top end (x=L_x) is open for fluid to escape. The host matrix of porosity p_s is prepared by distributing sediment barriers at a fraction p_b = 1-p_s of lattice sites. Fluid constituents are represented by mobile particles with a particle-particle repulsive and particle-pore attractive interaction. Metropolis algorithm is used to move particles. Depending on the case under study, one may consider uniform temperature (T), temperature gradient (\Delta T), and pressure gradient (\Delta P) in a homogeneous matrix (p_s=1) or a highly ramified porous medium with porosity near the percolation threshold (p_s=0.32) Further, the effect of a fault line/plane can also be incorporated. As fluid particles enter from the bottom, the fluid density evolves and equilibrates to a steady-state density gradient which drive the particles even with a uniform temperature. Density profile and flow response, therefore, depend on the temperature and other parameters. Linear and non-linear flow response in some of the parameters regimes will be presented.

[D40.048] Phase Transitions and Strongly Correlated Systems

This abstract not available.

[D40.049] TEMPERATURE DEPENDENT Mn K-EDGE NEXAFS STUDY OF La_1-xNa_xMnO_3

Temperature dependent (300 K amp; 70 K) Mn K-edge NEXAFS on La_1-xNa_xMnO_3 (0.05

[D40.050] On the Interdependence of Disorder, Localization and the Lanczos Method

The Anderson model has been widely studied to elucidate the localization properties of electronic states in simple disordered systems. In recent years, the Lanczos method has been employed in a number of numerical investigations. The Lanczos method is computationally efficient as it requires only two basis vectors to be stored at a time. Yet, it suffers from an instability due to the finite precision of computer arithmetic that manifests itself in a loss of orthogonality in the Lanczos basis set. In our present work, we have uncovered a surprising dependence of the severity of this instability, which leads to spurious eigenvalues, so-called ``ghosts,'' and the strength of disorder in the sample. We show that qualitatively incorrect conclusions for the localization of states result if the number of ghosts generated is not taken into account properly in the analysis.

[D40.051] A problem of solid state: "Relation between structure and electrical properties of different ferroelectric ceramic systems"

We present three different ferroelectric ceramic systems of recent research La_0.03Sr_0.255Ba_0.7Nb_2-yTi_yO_6-y/2 (LSBNT), Sr_0.3Ba_0.7Ti_1-5/4yNb_yO_3 (SBNT), xPbTiO_3-(1-x)SrTiO_3 (PST). These systems permit us to study interesting phenomena related with the solid state as nature of ferroelectric-paraelectric phase transitions, iconmmenurated structures, antiferroelectricity, coalescence of crystalline phases and ferroelectric nucleation. Nature of the ferroelectric-paraelectric phase transition is studied by Thermoelectric Analysis for each system for different concentrations of doping or mixing. It was observed that the Curie temperature varies linearly with composition. This value reflects the cation distribution within each unit cell and the type of crystalline structure. The temperature dependence of the dielectrical permittivity presents strongly broadened curves, which suggest a non Curie-Weiss behavior near the transition temperature in each crystalline system . The diffuse phase transition coefficient was also determined and its value leads to the conclusion that the degree of disorder depends with the presence of the dopant. Transmission Electron Microscopy evidenced the existence of a superstructure associated to incommensurability in the crystal structure of the LSBNT ceramic. In the electron diffraction patterns it was possible to measure the location of the incommensurate superlattice spots to determine the structural variations. The possibility of an antiferroelectric state in LSBNT is analyzed based on dielectric hysteresis. Antiferroelectricity seemed to be stronger for higher titanium concentrations. In SBNT the displacement toward higher temperatures shown by T_max corresponding to the t/o and o/r transitions for the samples doped with y=0.01 and the fact that the maxima corresponding to those transitions are not present in samples with y=0.0 5 and 0.1 lead us to think in a coalescence of the three phase transitions. In PST we study the nucleation of the ferroelectric phase and its influence on the electrical properties.

[D40.052] Piezoelectric and Ferromagnetic Properties of PMN-PFN Ferroelectromagnetic Ceramics

The induced piezoelectricity and magnetic properties of the (1-x)Pb(Mg_1/3Nb_2/3)O_3-xPb(Fe_1/2Nb_1/2)O_3, ((1-x)PMN-xPFN, x= 0.1, 0.2, 0.3, 0.4, 1) ferroelectromagnetic ceramics have been investigated. The effect of the variation of Fe content on the electromechanical properties of the ceramic is analyzed observing that the electromechanical efficiency decreases when either the applied external electric field is increased or the concentration of the ferroactive Fe cation is decreased. The dependence of the magnetic characteristics such as the specific magnetization (\sigma) and magnetic susceptibility (\chi) were investigated using thermo-magnetic analysis. The magnetic behavior obeys a weak ferromagnetism law. A regular decrease of the \sigma and \chi values as the amount of Fe in the (1-x)PMN-xPFN is decreased is observed. A study of the influence of the Fe cations on the dielectric and magnetic properties is used to analyze the coupling between the ferroelectric and the antiferromagnetic order in the ceramic.

[D40.053] Simulation of phase transitions in RbNO_3

The structural phase transitions in RbNO_3 have been studied by using parameter-free Gordon-Kim potentials. With the interionic potentials calculated from ab initio electron charge densities of the ions, we performed first-principle static structural relaxation and supercell molecular-dynamics simulation.We found that the phase IV\rightarrowIII transition in RbNO_3 is initiated by the in-plane and out-of-plane rotations of the NO_3 ions, and the phase III\rightarrowII\rightarrowI transitions are due to dilatation along a trigonal axis of phase III giving phase II a rhombohedral structure which is similar to the disorded phase of NaNO_3.

[D40.054] Kinetics of phase transformations in FCC alloys with L1_2 and L1_0 type orderings

We outline the earlier-developed master equation approach to configurational kinetics of non-equilibrium alloys and the kinetic tetrahedron cluster field method (KTCFM) and use them to study the kinetics of microstructural evolution under L1_2 and L1_0 type orderings in FCC alloys. We present the results of extensive simulations of A1\toL1_2 and A1\toL1_0 phase transformations after a quench of an alloy from the disordered A1 phase to the single-phase L1_2 or L1_0 state for a number of alloy models with both short-range and long-range interactions, including the cases of a significant deformational interaction due to the tetragonal distortions accompanying the L1_0 type ordering. The simulations reveal a number of peculiar features both in the transient microstructures and in the transformation kinetics, many of them agreeing well with experimental observations. We also present a Ginzburg-Landau-type theory describing the structure and orientations of antiphase boundaries in characteristic polytwin structures of alloys with L1_0 type ordering and compare the results with the experiment.

[D40.055] Combined Raman and Dielectric Study of Structural Phase Transitions in BaZr_xTi-1-xO_3 ceramics

BaZr_xTi-1-xO_3 (BZT) ceramics are of great interest because of a different character of dielectric response. In this work, combined micro-Raman scattering and dielectric studies were carried out to investigate the phase transition behavior of BZT ceramics and crystals prepared using a conventional powder processing and laser heated pedestal growth techniques, respectively. The single-phase perovskite structure of the ceramics was identified by the X-ray diffraction technique. The basic phase transition temperatures in these compositions were studied in the temperature range 70 –575 K. The tetragonal to cubic transition temperature was found to decrease with increasing Zr-content. The orthorhombic to tetragonal transition temperature that increases with an initial increase in Zr-content merges with tetragonal-cubic transition at 15% Zr substitution. An excellent agreement between crystallographic transition temperatures obtained by both the techniques suggested that Zr substituted octahedra were uniformly distributed in the material. The presence of any mechanical stress or unwanted phases could not be detected within the sensitivity of Raman micro-probe..

This work was supported in parts by DEPSCoR (# DAAG55-98-1-0012), NSF-DMR (#NSF-DMR9801759), and NASA (#NASA-NCC5-518) grants.

[D40.056] Roughness influence on surface melting

The influence of surface roughness on surface melting phase transition is discussed within the molecular field theory. The roughness is characterized by the surface order parameter average over all the density fluctuations whose description corresponds to the discrete Gaussian solid-on-solid model. The potential governing the transition between the rough surface and the surface melting is considered using the modified Van der Waals equation of state. Its effective shape represents two intersecting parabolas with non equal curvatures for the solid and liquid phases. The phases diagram shows the coexistence.

[D40.057] Complete solution of dynamical system associated with Ashkin-Teller lattice model

Discrete dynamical \nobreaksystems of Cremona maps in n variables are well studied in connection with solvable lattice models, e.g. by Maillard and others in search of symmetries of the Yang-Baxter equations. Here we give an explicit solution to the dynamics of a Cremona map associated with the Ashkin-Teller model. Starting from the matrix of Boltzmann weights w,\,x, and y, of the Ashkin-Teller model, \[ m\,=\,\left[ \beginarraycccc w & x & y & x \cr x & w & x & y \cr y & x & w & x \cr x & y & x & w \cr \endarray \right] \] Bellon and Maillard derive a dynamical system for the map I \circ J, with I a matrix inversion and J taking the reciprocal of each matrix entry. These recursions admit dilation, and there is an additional conserved quantity, resulting in a complete linearization of the map. We give an explicit solution of this dynamical system for w,\,x and y as functions of the number n of iterations.

[D40.058] Spin Gap and Spin-Spin Correlations in 2, 3 Leg t-U-J Ladders in the Presence of Magnetic Impurities

White's Numerical Density Matrix Renormalization Group method is used to study 1/2 filled t-U-J Hubbard chains coupled together to form 2, 3 leg ladders in the presence of magnetic impurities. t-U-J Hamiltonian was proposed by S. Daul et al. Spin gap and spin-spin correlations are calculated for NX2, NX3 ladders with N = 16, 32 for 2 leg ladder and N = 10, 20 for 3 leg ladder. Impurity concentration of 1 magnetic atom per chain is assumed. Two impurity configurations are studied for 2 leg ladder : impurities on a single rung at the center of the ladder, impurities on different rungs. For 3 leg ladder impurity configurations are : impurities on two collinear rungs in the center of the ladder, two impurities on a rung and the third on a non collinear rung, impurities on non collinear rungs.

[D40.059] HIGH-MAGNETIC-FIELD AND NEUTRON-DIFFRACTION STUDIES ON STRONGLY-CORRELATED-ELECTRON SYSTEMS UNDER MULTI-EXTREME CONDITIONS

Driving thermodynamical parameters, like temperature, external pressure and applied magnetic field, to extreme values can induce unusual magnetic properties in systems with strong electron correlations. At the Pulse Field Facility, NHMFL, Los Alamos, and at the Los Alamos Neutron Science Center, we are developing a complementary program for doing high-magnetic-field and neutron-scattering studies under multi-extreme sample-environment conditions, which will provide further insight into the nature of magnetism in strongly-correlated-electron systems. For example, the configuration of magnetic moments can be perturbed by the application of sufficiently high pressures or high magnetic fields, where different dependencies allows separating direct and indirect exchange interactions as well as contributions due to coherence, Zeeman and crystal-field terms. I will discuss some of the technological challenges and the scientific opportunities for studying such systems under multi-extreme conditions.

[D40.060] Specific Heat of Antiferromagnetic/Superconducting CeRhIn_5: Effect of Pressure and Magnetic Field

Resistivity measurements(H. Hegger et al.), Phys. Rev. Lett. 84, 4986 (2000). on CeRhIn_5 have shown a change in the ground state from an antiferromagnet (AF) at low pressure (P) to a superconductor (SC) for P \geq 1.4 GPa. Measurements of the specific heat (C) to P = 2.1 GPa, including some in magnetic field (H), are reported for 0.4 \leq T \leq 12 K. They show that the superconductivity is a bulk property and provide new insight into the AF/SC transition. With increasing P the sharp anomaly in C, associated with the AF order at P = 0, broadens and is attenuated. Although the anomaly maximum is shifted only weakly, first to higher than to lower T, the low--T spin--wave contribution to C monotonically increases, implying a weakning of the AF interaction. In parallel, the low--T electeron density of states (EDOS) increases. The coefficient (\gamma) of the normal--state EDOS contribution to C is \gamma \sim 380 mJ K^-2 mol^-1 at P = 2.1 GPa, a value characteristic of heavy--fermion behavior, and more than an order of magnitude greater than at P = 0. Superconductivity appears for P \geq 1.6 GPa, but the critical temperature (T_c) is only weakly dependent on P. The H dependence of parameters characteristic of the superconductivity suggests an upper critical field H_c2 \sim 16 T.

[D40.061] Polymers I

This abstract not available.

[D40.062] A study of Exciton relax process in a polymeric molecule

As the polymer chain is a quasi-one-dimensional system, the property of the polymeric molecule is that its energy spectrum and bond distortion depend on the excitation. The bond structure is easily distorted in this quasi-one-dimensional system. In our calculation, the Hamiltonian of exciton includes three parts, the electron-lattice interaction, electron-electron interaction and the interaction of the electrons with the electric field. We have obtained the total energy E in the Hartree-Fock approximation and the force of every lattice. Then we can get the new displacement order parameter of distorted lattice. At last, the electron's energy spectrum, wave function and the functions of lattice distortion can be obtained by self-consistently solving the combined eigen-equation and force equations. We found that the energy levels of the highest in valence band and the lowest in conducting band appear as periodic vibration during about 100 femtosecends, and that the two electronic bound states created near the center of the energy gap and caused self-trapping exciton.

[D40.063] ESR and PAS Studies of Photo-Degradation of a Polyurethane Coating

Polyurethane film samples were exposed to different accelerated aging environments, including narrow-band irradiation from 340 and 313 nm UV sources, broadband irradiation from a xenon arc lamp, and elevated temperature without irradiation. The effect of oxygen and the influence of titania on free-radical formation were examined. Early stage radicals were detected by generating them in the ESR cavity at 77 K. A correlation between ESR and positron annihilation spectroscopy (PAS) data was observed, which indicates that free-radical-type chemical defects may be the cause of sub-nanometer physical defects. (Support by AFOSR and NSF is gratefully acknowledged.)

[D40.064] Energy dispersive electron beam irradiation of vinylidene fluoride-trifluoroethylene copolymer [P(VDF-TrFE)]

Backscaterring of a 1 MeV electron beam from lead or aluminum plates was used to vary the energy-dispersive content of the electron irradiation applied to P(VDF-TrFE) samples with 50 or 65 wt. were found on the Curie and crystalline melting transitions. The dosage range was 10 Mrads to 100 Mrads. The atomic number dependence of the stopping power and U-shaped energy dependence showing a rapid increase as the energy decreases below 0.3 MeV result in much higher electron density in this region of higher stopping power for lead than for aluminum. Correspondingly more defects and disorder are produced by the lead backscattering as indicated by a 6 degree greater depression of the melting temperature for both copolymers with a more broadened melting transition as determined by modulated differential scanning calorimetry. The Curie temperature is also reduced by 2 to 6 degrees more by the lead than by the aluminum with no significant difference in transition energy. These trends correlate with lattice spacing changes revealed by wide angle x-ray scattering and indicate that the conversion from the ferroelectric to the highly electrostrictive paraelectric phase induced by the iradiation is sensitive to lower electron beam energies in the region of higher stopping power. (sponsored by ONR)

[D40.065] Properties of Polymer-Infiltrated Carbon Foams

There is considerable interest in extending the use-temperatures of both commodity and high performance polymers. There is also interest in improving the mechanical strength of carbon foams. Composites prepared by infiltrating carbon foam with polymers may offer significant improvements in both, the polymer helping to rigidize the foam and the foam providing thermal protection by virtue of its high thermal conductivity. The mechanical properties and thermal stability of carbon foams of various densities infiltrated with polyurethane have been studied. When used with a heat sink, the composite is able to maintain a substantial thermal gradient which provides stability of the polymer nominally above its decomposition temperature. The composite also has much improved strength properties without sacrificing tensile modulus. The composites may be very well suited for thermal management applications.

[D40.066] Crystallization in Multi-layered Crystalline Block Copolymer Thin Films

The evolution of the morphology of a crystalline/amorphous diblock copolymer poly(ethylene oxide -b- 1,4 butadiene) (P(EO-b-BD)) upon crystallization in thin films was studied via interference optical microscopy. Two-dimensional crystallization confined within the PEO lamellar layers was observed with retention of the microphase separated lamellar morphology formed in the melt-state. The morphology was further characterized by TEM and electron diffraction which showed it to consist of alternating layers of PEO and PBD with PEO crystalline chains oriented perpendicular to the lamellar layers of the microphase separated structure. Multiple parallel layers of crystalline PEO were found by electron diffraction to be in crystallographic registry even though they were separated by approximately 10 nm thick layers of amorphous PBD. This behavior can only occur if the PEO crystals in adjacent layers originate from the same nucleus and there is an interconnection between layers. The strong immiscibility of PEO and PBD restricts layer interconnection to defect structures such as edge and screw dislocations that may provide connections for crystallites among different layers. Furthermore, the lamellar spacing and melting temperature increases as the degree of undercooling decreases as indicated by interference optical microscopy and atomic force microscopy. In contrast to the integral chain folding observed in PEO homopolymer, the increase in lamellar thickness with decreasing undercooling is continuous. This indicates that the PEO crystalline chains in P(EO-b-BD) were non-integral chain folded.

[D40.067] In-situ X-ray studies of Molecular Orientation Enhancement in Polyhedral Oligomeric silsesquioxanes(POSS)/iPP nanocomposites

The effect of molecular orientation enhancement in isotactic-polypropylene(iPP)/polyhedral oligomeric silsesquioxanes (POSS) nanocomposite at different step shear conditions was investigated by means of In-situ SAXS and WAXD techniques. The WAXD results showed that under the same step shear conditions, the resultant \alpha-phase crystals were more oriented than those produced from pure i-PP. In addition, the \beta-phase growth was considerably reduced in iPP/POSS nanocomposite. Overall, the oriented fraction of iPP crystallites, calculated from the SAXS data, showed higher orientation in iPP/POSS nanocomposite than in pure i-PP. Both WAXD and SAXS results suggest that the presence of POSS nanostructured molecules increases the molecular orientation of iPP chains and reduces the formation of the \beta-phase crystals subjected to the step shear.

[D40.068] Effect of supercooling on crystalline morphology in blends of syndiotactic polypropylene and poly(octene-ethylene) copolymer

A phase diagram has been established in blends of syndiotactic polypropylene and poly(octene-ethylene) copolymer has been investigated on the basis of differential scanning calorimetry. The phase diagram consists of liquid-liquid and crystal-liquid coexistence region. Morphology development during phase separation and crystallization has been determined as a function of composition and supercooling based on optical microscopy and scanning probe microscopy. Of particular importance is that the supercooling has resulted not only in the change in length scale, but also in the emergence of a variety of structures. That is to say faceted single crystals can be grown in a rectangular-shape at high crystallization temperatures (low supercooling) from the melt, but with decreasing crystallization temperature, the crystalline lamellae branch out to emerge to dendrites and eventually spherulites.

[D40.069] Morphological Evolution of Semi-crystalline Poly(ethylene terephthalate) During Large Scale Simple Shear Deformation

In this study, the morphological evolution of semi-crystalline poly(ethylene terephthalate) (PET) under large scale simple shear is investigated. The equal channel angular extrusion (ECAE) process is used to induce the simple shear deformation. The deformation of semi-crystalline PET at different length scales is studied. At the spherulite scale, optical microscopy (OM) and scanning electron microscopy (SEM) are used. Lamellar scale information is obtained by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Molecular chains in the crystalline lamellae are obtained by wide-angle X-ray scattering (WAXS), and the molecular chains in the amorphous phase are studied by annealing the sample at temperatures above glass transition but below melting point. Structural characterization shows that PET spherulites are highly elongated into macrofibrils after ECAE. Within the macrofibrils, a "V-type" of crystalline lamellar orientation is induced. Molecular chains in the crystalline lamellae are tilted to the lamellar surface, whereas the molecular chains in the amorphous phase are highly stretched.

[D40.070] OBSERVATIONS OF A FLATTENED HELICAL BACKBONE CONFORMATION IN REGULARLY TWISTED POLY(m-PHENYLENE DIISOPHTHALAMIDE) (MPDI) FIBERS

Poly(m-phenylene diisophthalamide) (MPDI) is a polyaramide used to manufacture thermally stable high strength fibers (Nomex® by DuPont). When MPDI dissolved in N,N-dimethylacetamide (DMAc) is crystallized over several weeks by exposure to a non-solvent (H2O), the polymer precipitates into uniform, regularly twisted crystalline bundles. Low dose HREM of the 0.38 nm lattice fringes perpendicular to the fiber axis are visible everywhere in the fiber whereas the lattice planes parallel to the fiber axis are only visible over a distance of 10-20 nm. The different sets of fringes appear periodically in agreement with regularly twisted fibers. The lattice spacings can be explained well by a hexagonal unit cell with a = b = 1.65 nm, c=0.38 nm. Molecular modeling showed that a flattened helical molecular structure explained the intensity distribution of the electron diffraction pattern as well as the high-resolution data. This helical molecular conformation is similar that seen in the topologically related meta-phenylene ethynylenes. The three-dimensional translational symmetry of a perfect crystal is geometrically incompatible with uniform twisting. The high-resolution images reveal that the twisted crystals accommodate this distortion by lateral shift-disorder of the helices parallel to the twist axis. This results in an overall symmetry similar to an ordered hexagonal columnar liquid crystal, resolving the geometrical incompatibility.

[D40.071] Crystallization of Polyethylene and its Octene Copolymers over a Wide Range of Supercoolings

Crystallization studies of linear polyethylene and its copolymers with small amounts of octene have been studied as a function of supercooling using conventional hot stage microscopy and the Ding Spruiell method of rapid cooling, generating an unprecedented range of supercoolings. The homopolymer shows the well-known Regimes I amp; II, and a 3rd slope at high supercoolings, which was previously identified as RegimeIII. A copolymer with 4 hexyls per 1000C atoms shows three distinct regimes and a fourth region at high supercooling, which coincides with the 3rd region of the homopolymer. Other copolymers show only Regimes II amp; III together with a low temperature region, coinciding with that of the two aforementioned polymers. Analysis of the Regime III behavior of the copolymers using the Andrews theory indicates a critical nucleus size of three to four stems, in conflict with the Hoffman Lauritzen assumption of a single stem. Additionally, the high supercooling behavior of a constant rate of crystallization, regardless of comonomer content, indicates a significant change of crystallization mechanism. This new mechanism begins in the homopolymer at what has previously been identified as the Regime II-III transition and this transition should not be so described. The behavior requires a mechanism which is independent of comonomer content. Possibilities include a switch to a single stem nucleus, as was previously suggested for the crosslinked homopolymer, the interference of a mesophase or the hexagonal phase as an intermediate, or a switch to a surface roughening mode of growth.

[D40.072] Analysis of Displacement Fields Near Dislocation Cores in Ordered Polymers

We have significantly improved a method to characterize the displacement fields near edge dislocations in ordered polymers. Our extended analysis now makes it possible to predict and explain the variation in tilt of different lattice planes in the vicinity of dislocations in isotropic solids, anisotropic crystals and liquid crystals in terms of their elasticity constants. Direct images of the dislocation cores were obtained in three different polymer systems using bright field transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). A b[010]=63 nm edge dislocation was imaged in the ABC triblock copolymer polystyrene-block-poly(ethylene-co-butylene)-block-poly(methylmethacrylate) (SEBM). Fitting the theoretical isotropic displacement fields to the displacements measured from the image, an estimate of the elastic constant anisotropy was obtained. For this material the ratio of the bulk modulus to the shear modulus, K/G, was equal to 0.8 +/- 0.2. A similar analysis using anisotropic dislocation theory was applied to a 3 chain-end, b[200]=2.4 nm edge dislocation in the crystalline polymer [1,6-di(N-carbazolyl)-2,4-hexadiyne] (DCHD). Information about the anisotropy of DCHDs stiffness matrix, C_ij, was obtained. An anisotropy parameter W_2, defined as (C_11+C_33)/(2C_55), was found to be 3.0 +/- 0.1. A b=2.6 nm dislocation in a smectic polymalonate was analyzed using liquid crystalline dislocation theory, and an estimate of \lambda, the material's characteristic deformation length, was determined to be 1.0 +/- 0.1 nm.

[D40.073] Influence of Neutralization Method and Cation on the Morphology of Styrenic Ionomers

Previous studies of ionomers using scanning transmission electron microscopy (STEM) indicate a difference in the shape, size and spatial distribution of ionic aggregates depending on the chemistry and processing. For example, melt neutralized ethylene-methacrylic acid ionomers neutralized with Zn exhibit randomly dispersed solid spherical aggregates ~ 2 nm in diameter. However, solution neutralized styrene-methacrylic acid ionomers neutralized with Cs and sulfonated polystyrene neutralized with Zn show randomly dispersed vesicular aggregates greater than 10 nm in diameter with shell wall thicknesses of ~ 3-4 nm. The current study seeks to resolve the nature of some of these differences, specifically with respect to neutralization method and neutralizing cation. Styrenic ionomers will be neutralized using both melt and solution methods as well as different neutralizing agents. Characterization of the microstructures with STEM, small-angle x-ray scattering (SAXS) and infrared (FTIR) spectroscopy will be conducted to determine the morphology of these materials.

[D40.074] From Aliphatic Polyurethanes to Linear Polyethylene: Influence of Hydrogen Bonding on the Thermal and Morphological Features of Semi-Crystalline Polymers

A series of polyurethanes derived from long-chain aliphatic diols and much shorter diisocyanates were synthesized by melt polyadditions and characterized. Their solubility in m-cresol at slightly elevated temperatures indicated the absence of crosslinking. By diluting the amount of carbamate esters, polyethylene-like polyurethanes were synthesized. As the aliphatic segments were increased and the hydrogen bonding densities decreased, the polyurethanes displayed physical and thermal characteristics, including melting point, long period, and solubility, typical of polyethylene. However, crystallization studies showed that hydrogen bonding still controlled the crystallization process of these long-chain aliphatic polyurethanes resulting in the analogous crystal structure and morphology as polyamides and polyurethanes of higher hydrogen bonding densities.

[D40.075] Application of Parallel Tempering to Molecular Dynamics Simulations of Polymer Chains with Strongly Attracting End-Groups

Strongly attracting sites of telechelic polymer chains aggregate to form three-dimensional nanoscale domains that play a central role in determining mechanical and transport properties of the nanostructured material. Depending on the degree of attraction between associating groups, the rough energy landscape inherent in such systems can prevent traditional simulation methods from achieving phase space sampling adequate to predict equilibrium properties. In the present work, molecular dynamics simulations are aided by a parallel tempering algorithm to explore broad regions of phase space not accessible in a reasonable time frame by ordinary MD. We elucidate the equilibrium properties of a solution of telechelic polymer chains that have end-group attractions on the order of 10 k_BT. The features predicted by this approach are compared to results of MD simulations performed without parallel tempering and also with other results from the literature.

[D40.076] Morphology development upon melting of ultrahigh molecular weight polyethylene formed at high pressure by Ultra SAXS and DSC

The morphology development on the melting of ultrahigh molecular weight polyethylenes (UHMWPE), formed from melt crystallization at high different pressures, was studied by ultra small-angle X-ray scattering (USAXS)and DSC. The heating rate used was 5C/min. At 41M psi, UHMWPEs show dominant chain-extended lamellae (CEL) at Mw 4MM, 5MM and 6MM. At 30M psi, UHMWPEs show dominant chain-folding lamellae (CFL) at Mw 4MM, 5MM and 6MM. At 35M psi, UHMWPEs show both CEL and CFL at Mw 5MM and 6MM and a dominant CFL at Mw 4MM. With dominant CFL, USAXS shows a measurable long spacing, which increases with temperature. With dominant CEL, USAXS shows an immeasurable long spacing and relatively strong ultra-small angle scattering intensity. The integrated scattering intensity shows an exponential increase with temperature. The thickest CEL and CFL melt at the end of the DSC endotherm, where the USAXS (corrected for melt scattering) shows a diffuse profile only. This CEL or CFL thickness was estimated using an approach based on the single lamella structure factor. The results show a largest thickness about 100nm for CEL and about 50nm for CFL.

Acknowledgement: This work was supported by a NSF grant (DMR 9732653).

[D40.077] Polymer Melting and SAXS: A Modeling Study

Semicrystalline polymers melt over a temperature interval of ca. 10-40 ^oC, an effect attributed to crystals of increasing stability (thickness) melting at higher temperatures. Small-angle X-ray scattering (SAXS) has long been used to follow this melting process. We model the scattering I(\its) of an isotropic assembly of lamellar stacks in which the crystals are melted sequentially according to thickness. Average crystal thickness, average amorphous layer thickness and the average long period all increase as melting proceeds. As expected, the maximum of the SAXS pattern first shifts to smaller \its, then converts to single particle scattering from the unmelted, thickest crystals in the initial distribution. The transition from discrete scattering from spatially correlated lamellae to continuous scattering from isolated lamellae occurs when the crystalline fraction has been reduced to ca. 0.1. Correlation function g(\itr) and interface distribution function \gamma (r) are analyzed to provide average phase dimensions and apparent crystalline fraction during the melting process. Model results are compared to experiments

[D40.078] Confining Polymer Crystallization in One, Two, and Three Dimensions

The crystallized structures of a series of polyethylene-polyvinylcyclohexane, E/VCH, diblock copolymers of varying microdomain morphology were examined. Because the VCH block is glassy during E crystallization, the microdomains prescribed by microphase separation in the melt are always retained on cooling. Using TEM, we could resolve the individual crystals within these microdomains, allowing us to quantify the crystal size, the number of crystals, and the orientation of the individual crystals. We also examined the dynamics of crystallization using time-resolved SAXS/WAXS. The E-poor sphere- (S), cylinder- (C) and gyroid-forming (G) materials exhibit simple crystallization kinetics; the first-order kinetics observed in S and C materials suggest crystallization of isolated E microdomain via homogeneous nucleation while the sigmoidal kinetics in the G material reflects connectivity between crystallizable channels. A lamellar-forming sample, however, exhibits complex two-step crystallization kinetics.

[D40.079] A Major Intermediate Component in Drawn High-Density Polyethylene Identified by Solid-State NMR

In a commercial polyethylene (HDPE) highly drawn at 295 K, a distinct morphological component intermediate to the crystalline and the almost isotropic amorphous phases has been identified by solid-state nuclear magnetic resonance (NMR). This intermediate component accounts for nearly 25% of the material bulk, exceeding the amorphous fraction at the highest draw ratios. In the neat isotropic material examined for reference, the NMR-derived composition shows excellent agreement with other techniques. 13C NMR isotropic chemical shifts of the intermediate component, whose signal was selected using an “inverse T1,C filter”, prove chains of nearly all-trans conformations; the line width indicates significant disorder. Reduction of dipolar couplings and the chemical-shift anisotropy show fast rotations of 30 – 50 deg. amplitude around the chain axes. The degree of orientation of the chain axes is high. Spin diffusion experiments suggest that the intermediate component consists mostly of extended chain bundles closely associated with the amorphous phase (tie-molecule bundles ?).

[D40.080] Structure of the Molten Stereo-regular Polyolefins with Different Side Chain Size from X-ray Diffraction and PRISM Theory

The melt structures of linear polyethylene and the isotactic vinyl polymers - polypropylene, poly(1-butene) and poly(4-methyl-1-pentene) - with the corresponding methyl, ethyl and isobutyl side chains, were recently studied with wide-angle X-ray diffraction. As the size of the side-branch increases from zero (PE) to methyl, ethyl and isobutyl, 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. We present Polymer Reference Interaction Site Model (PRISM) calculations of a melt of vinyl chains as a function of the side chain size. The origin of the pre-peak for vinyl polymers based on experiment and theory will be discussed.

[D40.081] In-situ Structural Studies during PBO Fiber Spinning by Synchrotron WAXD/SAXS

A spinning unit was adapted to the Advanced Polymers Beamline at X27C of the National Synchrotron Light Source (NSLS), Brookhaven National Lab (BNL) to perform in-situ fiber spinning studies. WAXD patterns indicated that the structure before coagulation had a lyotropic liquid-crystalline order that cannot be simply nematic. After the fiber had passed through the coagulation water bath, ranging from 25-60 C, a significant ordering had taken place. The meridian peaks showed streaks. Two models, a Pi-interacting sanidic lyotropic solution and a crystal with translational disorder, could be used to explain the structures before and after coagulation. SAXS patterns of the PBO fiber after coagulation showed equatorial streaks.

[D40.082] Caged Polymer Crystallization in Perforated Layers*

Confined polymer crystallization is studied in a poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymer. The number-average molecular weights for the PEO and PS blocks are 11k g/mol and 17k g/mol, respectively. After planar oscillatory shear, a perforated layer phase is obtained. This perforated layer phase is found by two-dimensional (2D) small angle X-ray scattering (SAXS) to have two commensurate crystalline structures. One is a trigonal phase, and the other is a hexagonal phase. The crystal orientation within the confined perforated layers is studied by simultaneous SAXS and wide angle X-ray scattering. The PEO crystal orientations with respect to the layers have been found to be dependent on the crystallization temperatures (Tc). At low Tcs, the PEO crystal c-axis preferentially orients parallel to the layers. At high Tcs, the crystal c-axis orients inclined to the layers. During high temperature crystallization in the perforated layers, the PEO lamellar crystals grow specifically along the (100) planes of the hexagonal lattice within the layers. Specific crystal orientation is found in an early stage of crystallization as studied by simultaneous time-resolved 2D SAXS and WAXS.

[D40.083] Effect of Preparation Conditions on Microphase Separation in Poly(urethane urea) Block Copolymers.

Poly(urethane urea) segmented block copolymers were synthesized from MDI, ethylene diamine and poly(tetramethylene oxide) [MW = 2000], with hard segment contents varying from 14 to 47 wt percent. Samples of six of these copolymers were cast from dimethylacetamide at different temperatures and pressures to control the rate of solvent evaporation. Microphase separation behavior is being investigated utilizing FTIR, atomic force microscopy and small-angle x-ray scattering experiments. Slower evaporation rates generally result in more efficient hard and soft segment phase separation. At relatively low hard segment contents, the effects of preparation conditions are modest or negligible, but become more pronounced at higher hard segment concentrations. The origin of these changes will be discussed.

[D40.084] Crystallization and Structure Formation of Polyethylene oxide Fractions and Their Blends

Time-resolved simultaneous wide- and small-angle x-ray scattering experiments were used to investigate crystallization and microstructure development of two poly(ethylene oxide) molecular weight fractions and their mixtures, at several crystallization temperatures and compositions. In addition, crystallization of melt-miscible blends of PEO with narrow molecular weight fractions of PMMA and a 50/50 styrene-hydroxystyrene random copolymer was also investigated. During isothermal crystallization, secondary crystallization was observed for both the low (Mw = 50K, PDI = 1.07) and high (Mw = 334.5K, PDI = 1.14) molecular weight PEO fractions. The decrease in SAXS long periods observed at early crystallization times (2-3 nm) was similar to that noted previously for polydisperse PEO [Macromolecules 33, 4842 (2000)] and lamellar insertion appears to be the likely origin of this behavior. The origin of the behavior observed during crystallization of the PEO mixtures, as well as that of the weakly- and strongly-interacting PMMA and SHS blends, will be discussed.

[D40.085] Morphology of Crystalline Block Copolymers

The morphology and crystallization behavior of a symmetric crystalline / crystalline block copolymer: poly(ethylene-b- ethylene oxide) (P(E-b-EO)) under different thermal conditions was studied. The diblock copolymer was found to have an alternating lamellar morphology in both molten and solid state. Both polyethylene and poly(ethylene oxide) crystalline chains were found to orient normal to the interface of microphase separated domains by TEM and electron diffraction. Crystallization of polyethylene block resulted in a significant increase (~4nm) of lamellar spacing while the subsequent crystallization of poly(ethylene oxide) block caused little variation. Although such a dramatic change in lamellar spacing were observed upon crystallization, the block copolymer lamellar grain structure was maintained as observed by TEM and optical microscopy indicating that the pre-existing microphase separated morphology acted as a template for crystallization. The crystallization characteristics of poly(ethylene oxide) inside the hardened polyethylene domain of P(E-b-EO) were compared with its unhydrogenated counterparts: poly(butadiene-b-(ethylene oxide)) where the other block is amorphous and flexible.

[D40.086] Twin-like Superstructure in a Ferroelectric LC Polyester

We have reported right-handed helical single crystals grown from PET(R*-9). PET(R*-10), which has one more methylene unit in the liquid crystal soft spacer, shows three LC phases, namely TGBA*, SA*, and SC* phases similar to PET(R*-9). However, the handedness of PET(R-10) helical single crystals exhibits a left-hand feature, which is complete opposite with respect to PET(R*-9). Isothermal crystallization at 130 °C leads to flat elongated lamellae which is similar to PET(R*-9). However, the PET(R*-10) lamellae show a interesting twin-type superstructure. The basic unit cell is orthorhombic with a = 1.43, b = 2.57, and c =3.04 and the twin axis is (110). By mechanical shearing, lamellae having pure basic unit cell have been obtained.

[D40.087] Crystal Structure of a Model Spider Silk Peptide

Crystallization study on a novel model silk peptide has been carried out using optical microscopy, AFM, TEM and electron diffraction. The sequence of the peptide, (E)5(GDVGGAGATGGS)2(E)5, is based on the GXYGGZ motif in the less repetitive amorphous blocks of Nephila clavipes spider dragline silk. When the peptide was crystallized out of aqueous solution, spherulites as well as dendritic crystals on the order of several to tens of microns in diameter were observed under polarizing optical microscope, depending on drying speed, volume of the droplet and concentration. The same crystals were collected and sonicated in methanol, a non-solvent, to yield individual crystals that were later examined in the electron microscope. Regular-shaped lamellar crystals of micron size were observed in the TEM. The lamellar thickness as determined by Pt/Pd shadowing and AFM is ~50 Å. Selected area electron diffraction showed single crystal diffraction patterns indicating a possible orthorhombic unit cell of 9.91 x 5.57 x 20.40 Å.

[D40.088] Mass Transport within Block Copolymers: The Relationship with Morphology and Grain Structure

The fundamental relationship of polymer morphology and grain structure on mass transport properties has been examined employing diblock copolymers and graft copolymers with unique molecular architectures. A custom-built gravimetric sorption apparatus has been implemented to measure the permeation and diffusion coefficients of small molecule gases in polymer films. The role of grain structure has been ignored in the literature due to lack of reliable characterization techniques. In the present study, the grain structure of the block copolymers was determined by digital analysis of images acquired by transmission electron microscopy. Diffusion and permeation coefficients have been correlated to total component volume fractions, strength of segregation, and grain structure.

[D40.089] Side Chain Liquid Crystalline Poly(silylenemethylene)s

The structures of side chain liquid crystalline poly(silylenemethylene)s (-(SiCH_3R-CH_2)- : R=O(CH_2)_NO-Ph-Ph-R, Ph; phenyl) ((PSM-N; N=3,6,8,11, R=H) and (CN-11; N=11, R=CN)) have been studied by X-ray diffraction, DSC, polarized optical microscopy and transmission electron microscopy. All these PSM-Ns have multiple transitions which are dependent on side chain length. All PSM-Ns studied and CN-11 show a series of sharp equatorial reflections in the X-ray fiber pattern at room temperature characteristic of well-ordered smectic phases. The layer thicknesses of PSM-Ns correspond to a single-layer structure. For PSM-3 and PSM-6, the mesogens pack in a two-dimensional orthorhombic cell characteristic of a smectic E phase. For PSM-8 and PSM-11, the mesogens pack in a hexagonal smectic B cell at room temperature. During room temperature annealing, PSM-8 and PSM-11 crystallize into orthorhombic cells. Upon heating, the crystalline structures of PSM-11 and PSM-8 change to smectic A before becoming isotropic. For CN-11, the mesogens pack in a monoclinic unit cell with parameters a=16.8Åb=7.42Åc=43.6Åand b=102.1 deg. (b: fiber direction), representing a smectic G structure with layer thickness of ~43 ÅUpon heating, the structure changes into a less ordered smectic G phase, maintaining monoclinic symmetry but with loss of order in the smectic layers. The observed layer thickness, ~43Åis about ~1.5 times the most extended chain length, indicating a double-layer structure with the tilted or interdigitated side chains.

[D40.090] Characterization of elastomers by micellar formation at a Lower Critical Solution Temperature

Polymer solutions, made in volatile solvents, phase-separate at a Lower Critical Solution Temperature (LCST),Ti ,due to the difference in solvent and polymer expansion coefficients. As Ti is molecular weight (MW)-dependant ,the MW distribution of a sample can be obtain through a thermogram .This is a set of hi (Ti) where ,hi, is the turbidity for a fraction phase-separating a at Ti, in a step by step T-increase.The range of Ti is 70 K lower for polyethylene (PE) than for polypropylene (PP) .Random copolymers of ethylene (E) and propylene (P) such as synthetic rubbers are expected to phase-separate, as do homopolymers , on a continuous range of Ti such as Ti (PE)< (Ti (copolymers) < Ti(PP) as is indeed found. In the present work, a sonication is used to avoid raising T for the dissolution of rubber samples in pentane.In these conditions,two distributions are observed,separated by 30-40 K.The range of Ti suggests that micelles with E segments and inverse micelles with P segments at the molecular surface are stable in solution and probably reflect the morphology of the solid.The thermograms of samples with the same average E/P content are found catalyst-dependent. They can be tracers of the length of E and P sequences in the sample.

[D40.091] Computational and Experimentally Obtained Crystal Modulus of Silk

Mechanical properties of polymers, such as silk, are influenced by the fundamental properties: crystallinity, orientation, and crystal modulus. X-ray diffraction with the assumption of uniform stress has been used frequently to determine the crystal modulus. The values for silk fibers have been previously reported to be about an order of magnitude lower than those obtained by computational modeling. This difference is outside the limit of experimental and computational error and probably reflects some conceptual error. The moduli of silk have been reexamined, both experimentally and computationally, in an improved manner. For the X-ray method, this includes making measurements of samples with various degrees of crystallinity and orientation obtained along the cocoon. The measurements yielded crystal moduli in the range of 20-28 GPa, depending on crystallinity and orientation. For the computation, molecular dynamics are used with periodic boundary conditions. The three-dimensional fluctuations analysis performed on the data yields a modulus of approximately 16 GPa. The calculated and experimentally obtained moduli, agree reasonably with those of polymers having similar conformations.

[D40.092] Aspects of the Morphology of Silk Fibroin Membranes

Although silk has been used for more than 5,000 years, interest in membranes has grown only in recent decades. Silk membranes have considerable potential as materials for photovoltaic and photochromic devices as well as for controlled release, growing cells, etc. Despite the scientific and technological potential of these materials, not much work has been done regarding the fine scale morphology. Membranes of Bombyx mori have been cast with liquid silk from the middle part of the gland, as well as from solution made from the fibroin in fibers. They were annealed over a range of temperatures and examined by AFM. The initial observation revealed a broad range of morphologies. These include individual particles, densely packed grains, nanofibrils and irregular structure. The effect of casting temperatures on morphological features will be shown.

[D40.093] AFM Studies of Fracture Surfaces Of Composition B Energetic Materials

Start your abstract by replacing this line with your text. The size of TNT (trinitrotoluene) crystals remains a subject of investigation in melt-cast TNT formulations that are filled with a large percentage of RDX. One such formulation is Composition B (59cyclotrimethylenetrinitramine (RDX), 40can be used to examine the size of TNT crystals is to analyze the surface structure that is exhibited after mechanical failure of the energetic material. The mechanical failure of melt-cast Composition B has been obtained by subjecting the material to high acceleration in an ultracentrifuge. Failure occurs when the shear or tensile strength of the Composition B sample is exceeded. Scanning electron microscopy (SEM) studies of the Composition B as-cast surface and fracture surface reveals that the RDX crystals range in size from 100 mm to 1,000 mm, as specified. More sensitive studies of the topography of the Composition B fracture surface by atomic force microscopy (AFM) reveals transgranular failure in the TNT between the RDX crystals in the Composition B. The transgranular failure consists of long thin features that appear to be cleaving and tearing across the TNT grains. The width of the long thin features ranges in size from 0.5 mm to 3.0 mm. The height of the long thin features ranges in size from 50 nm to 300 nm.

[D40.094] Light Extinction Spectra of Crazes in Polymers

Light extinction properties of crazed polymeric samples are studied both theoretically and experimentally. The dependence of the apparent absorption spectra of the sample on the light incidence angle, crack/craze depth, and crack/craze surface density is analyzed. A theoretical model of light scattering and diffraction by cracks and crazes in transparent and semitransparent polymers is developed. Good quantitative agreement is obtained between theoretical and experimental results.

[D40.095] Synchronized stress-PALS test on a series of polyester copolymer glasses

Previous work in our group suggests that, in craze nucleation, only when a nanovoid grows to a critical size can it serve as a nucleus for stable craze growth. However, what factors influence this nanovoid growth process remains unclear. A new technique, synchronized stress-PALS test, has been established to study the change in size and intensity of nanovoids when polymer glasses are under stress. A series of glassy copolymers based on poly (ethylene terephthalate) (PET) and poly (1,4-cyclohexylenedimethylene terephthalate) (PCT) were investigated by synchronized stress-PALS at different stress levels. The results suggest that both stress level and chain mobility of the polymers influence nanovoid growth. Based on these experimental results, a craze nucleation model has been proposed. This model, based on thermodynamic and kinetic analysis, show that the craze nucleation process is determined by the modulus, surface energy and chain mobility of polymer glasses.

[D40.096] Effect of Thermal History on the Deformation and Fracture of a Semicrystalline-Glassy Block Copolymer*

We investigate the influence of thermal history on the deformation and fracture of a poly(vinyl cyclohexane)-poly(ethylene)-poly(vinyl cyclohexane) (PCHE-PE-PCHE) CEC triblock copolymer (M=107,000 g/mol; wt(PE)=0.29). Ordered thin films of CEC are bonded to ductile copper grids, deformed in tension and then examined by transmission electron and atomic force microscopy. We find that the deformation and fracture mechanisms of CEC depend significantly on the thermal history. The CEC thin films undergo crazing and the crazes break down to form cracks at low strains when the films are prepared by slow cooling (- 0.5 C/min) from 190 C, where both PCHE and PE domains are rubbery. On the contrary, after being quenched to room temperature from 190 C, the CEC films become ductile with a change in deformation mechanism to competing shear deformation zones and crazing. Both physical aging of the PCHE domains and different semi-crystalline microstructures of the PE domains may play a role in determining these mechanisms and the resultant ductility or brittleness.

*Supported in part by the NSF-DMR-MRSEC Program under the UCSB MRL.

[D40.097] Light Scattering as a Tool to Study Cavitation in Rubber-Toughened Polymers

Being able to characterize the cavitation mechanism in rubber-toughened polymers is a key factor for the understanding and improvement of the impact properties of such materials. In order to follow the evolution of cavitation in initially transparent materials subjected to a tensile stress, we developed an experimental method based on light scattering measurements. Since the refractive index of void is different from that of the polymer, cavities appearing during damage scatter the light and the material progressively becomes opaque. The scattering mean free path L, which is related to the number and the morphology of the scatterers, is a relevant quantity to define the opacity of a material. By measuring the intensity of 35 separate wavelengths of a white light beam transmitted through a sample, we determined the evolution of the scattering mean free path, and its wavelength dependence, during the first stages of the cavitation process. The analysis for various grades of RT-PMMA shows that cavitation occurs with an increasing number of scatterers with strain whose size is closely related to that of the rubber particles.

[D40.098] Dynamic Shear Modulus of Polymers from Molecular Dynamics Simulations

In this work we describe the methodology for using equilibrium molecular dynamics simulations (MD) simulations to obtain the viscoelastic properties of polymers in the glassy regime. Specifically we show how the time dependent shear stress modulus and frequency dependent complex shear modulus in the high-frequency regime can be determined from the off-diagonal terms of the stress-tensor autocorrelation function obtained from MD trajectories using the Green-Kubo method and appropriate Fourier transforms. In order to test the methodology we have performed MD simulations of a low-molecular-weight polybutadiene system using quantum chemistry based potential functions. Values of the glassy modulus and the maximum loss frequency were found to be in good agreement with experimental data for polybutadiene at 298 K.

[D40.099] CRYSTALLOGRAPHIC TEXTURE EVOLUTION IN HIGH DENSITY POLYETHYLENE DURING UNIAXIAL TENSION

The crystallographic texture evolution of a high-density polyethylene(HDPE) is analyzed for samples subject to different strains in uniaxial tension. The present studies differ from prior reported data in three important aspects: (1) The texture of the undeformed sample is quite strong. (2) Present texture analyses include complete recalculated pole figures based on spherical harmonics. (3) The WAXS measurements were obtained while the samples were kept deformed. The results shed new insights into the complex texture development of HDPE. Three distinct preferred orientations were identified: a component with (001)aligned along the extension axis, a component with (011) aligned close to the extension axis, and a component with (010) aligned along the extension axis. Only the first component has been reported to be stable at high strains in previous studies. The rate of texture evolution in the present study is significantly lower than that previously reported. The natural relaxation of strain following the tensile loading had a significant impact on the texture in the sample. The relaxation process mitigated or eliminated the second and third preferred texture components, while strengthening the first. The evolution of the lamellar morphology is followed by AFM.

[D40.100] Probing polymer networks using pulse propagation and Brillouin light scattering measurements

The microscopic structure of polydimethylsiloxane (PDMS) networks was probed using two tools --- pulse propagation and acoustic phonons. Pulse propagation in polymer networks is strongly influenced by the microscopic structure of the network. An experimental technique based on these pulse propagation measurements was used to measure structural features of networks at a molecular level. Combining these measurements with molecular models for networks allowed the estimation of structural parameters such as the degree of cross-linking and the molecular weight between entanglements. At much higher frequencies, these networks were probed using inelastic light scattering (Brillouin scattering) from phonons. The dependence of sound velocity on the cross-link density and strain are shown to be very small in comparison with the pulse propagation measurements. Differences in mechanical response of the networks observed in the low and high frequency regimes using these two techniques will be discussed.

Supported by NSF DMR-9422223

[D40.101] Contribution of edge recoiling of diffusion to anomalous transient photo-currents in amorphous materials

We demonstrate that the edge recoiling of diffusion may cause anomaly in transient photo-currents measured by the time-of-flight technique for homogeneous amorphous materials. The universality of the anomalous transports is successfully revealed in this new macroscopic explanation.

The anomaly in the transient currents is directly related to the degree of the diffusion and the drift. In the previously established continuous-time-random-walk theory, similar anomalous transports were attributed to some unusual microscopic process of retarded trapping-releasing and hopping. We show that the same effects may present in homogeneous materials with normal diffusion process.

[D40.102] Dielectric Normal Mode Relaxation of Polyisoprene-Polybutadiene Diblock Copolymers

Dielectric normal mode relaxation of type-A polymers such as polyisoprene (PI) reflects the fluctuation dynamics of the end-to-end vector. Studies concerning the normal mode process have been made extensively in the last two decades for non-entangled and entangled PIs aiming to test theoretical models, e.g., bead spring models, tube model (Doi-Edwards model) and some modified versions of the original tube model. In this study, we investigated the dielectric normal mode relaxation on polyisoprene-polybutadiene (type-A/non-type-A) diblock copolymers (IB) with several I/B compositions. Through the dielectric measurements on BI, we can get information on the dynamics of the subchain at the chain end. Only the PI block is labeled with the type-A dipole and is dielectrically active. At the same time, we carried out viscoelastic measurements on the same samples to know the behavior of whole chain motion, especially the longest relaxation times. Using these viscoelastic and dielectric data, we can directly compare the relaxation spectra of subchains with those predicted by the tube model. From such a direct comparison, we have found that the end part of the chain relaxes faster than the theoretical prediction. This suggests that there exist extra relaxation mechanisms other than reptation in the dynamics of chain ends such as the contour length fluctuation.

[D40.103] Conformational Order and Chain Dynamics in Molten Poly(n-alkyl methacrylates) Revealed by Solid-State NMR Spectroscopy

The current knowledge of structure and complex dynamics of amorphous polymers is still insufficient for understanding important polymer properties, such as the glass transition temperatures, on a fundamental level. In this respect, poly(n-alkyl methacrylates) PnAMAs are of special use, since these macromolecules exhibit highly anisotropic motional processes in the molten state. Here, the combination of classic and recently developed solid-state NMR spectroscopy methods provides a powerful new probe for investigating the segmental and translational chain dynamics of molten PnAMA samples (n-alkyl = methyl, ethyl, butyl, hexyl). Depending on the alkyl side-chain, at 20 to 50 degrees above the glass transition, a lineshape corresponding to an axially symmetric chemical shift tensor is observed in 1D ^13C spectra for the carboxyl carbon. This occurs on account of the local \beta-process becoming fast on the NMR time scale. Further line narrowing with increasing temperature is attributed to a randomisation due to translational chain motion. Assuming a rotational random jump model, line shape analysis leads to correlation times for the chain motion over nearly three orders of magnitude. The obtained correlation times are in good agreement with those determined by Hahn-Echo and proton double-quantum filtered NMR measurements. For longer time scales, the translational motion of the polymer chain in the melt leads to a complete loss of correlation and therefore yields, even for moderate temperatures, an isotropic 2D exchange pattern. A typical length scale for the local ordering in PEMA can be estimated by small-angle X-ray measurements.

[D40.104] Characterization of Distributive Mixing in Polymer Processing Equipment

Mixing is an important component of practically every polymer processing operation. Material processability and product properties are highly influenced by mixing quality. Applying the findings of fundamental research on mixing mechanisms to processes taking place inside industrial mixing devices constitutes an important and challenging endeavor. The characterization of the mixture requires detailed information regarding the size, shape, orientation and spatial position of every material element. The development of advanced computing resources has enabled the use of numerical modeling in studying equipment performance in terms of mixing efficiency. This poster focuses on the analysis of distributive mixing in a twin-flight single screw extruder by virtue of using particle tracking as a method of capturing the dynamics of the mixing process. We assess the degree of distributive mixing (system homogeneity) by calculating the evolution of Renyi entropies and dimensions for the minor component at various cross sections along the extruder length. We determine the fractal, the information and the correlation dimensions. Comparison of the minor component dimensions to the ideal case of a statistically random distribution provides a quantitative mixing index function.

[D40.105] Separation of polymeric topoisomers in a microchannel device: a Monte Carlo study

Polymer molecules come in a variety of topological flavors, from simple linear chains to branched, ring, or even more intricate knotted or catenane structures. This aspect of polymer chains is increasingly recognized as a key factor in biological processes, in particular those associated with DNA manipulation; specialized enzymes regularly induce DNA topological changes in the course of replication, recombination and packaging. Separating molecules of different topologies is important to assess the activity of those enzymes, and it has previously been achieved via gel electrophoresis. However, in the wake of the current trend towards lab-on-a-chip technologies, we present simulation results for the separation of topoisomers in an entropic trap array, a microdevice etched on a silicon wafer recently used to separate DNA by size(J. Han, S. W. Turner and H. G. Craighead, Phys. Rev. Lett. \bf83), 1688 (1999). We show that topology-based separation for linear, ring and branched polymers is indeed possible in such a device.

[D40.106] Stress Relaxation of Model Polymer Networks with Pendant Chains

We present a theoretical and experimental study for the stress relaxation of model poly(dimethylsiloxane) networks with pendant chains and nearly constant density of cross-linking. The fitting of experimental values to the classic Chasset-Thirion equation shows a strong dependence on the exponent of the equation with the molecular weight and molecular weight distribution of the pendant chains. A new model for the terminal relaxation of elastomers is proposed taking into account these molecular parameters. The dynamic of polymer networks in the terminal relaxation region is modeled with the constrain-release scheme. This model adequately describes the behavior of networks synthesized by end-linking of chains with terminal functional groups. It arrives to similar scaling relationships than those obtained by Curro and Pincus for the Thirion and Chasset exponent for networks obtained by random polymerization.

[D40.107] Strength of Reaction Runaway in Free Radical Polymerization: Competition between Diffusion-Controlled and Chemically-Controlled Reactions

Free radical polymerization (FRP) exhibits autoacceleration or reaction runaway, the magnitude of which depends on polymer molecular weight (MW) and species. Autoacceleration in methyl methacrylate (MMA) FRP may be understood from the fact that the concentration dependence of the termination rate parameter scales with the concentration dependence of the diffusion coefficient of the "average short chain" participating in the polymer-polymer termination reaction. However, FRP of other monomers exhibits a weak runaway indicative of a competition between diffusion-controlled termination and chemically-controlled chain transfer. Analysis of this competition in styrene and other FRP systems (and the lack thereof in MMA) is made from studies of polymer self-diffusion via PFG-NMR, diffusion-limited interaction rates via phosphorescence quenching, and overall reaction rates. Differences in polydispersity and MW expected from termination by combination versus dead polymer production from chain transfer will be tested by cumulative/differential analysis of MW distributions.

[D40.108] An Exact Solution to the First-Passage Time in Low-Functionality Star Polymer Melts

We present an extension to the Milner and McLeish (MM) theory for stress relaxation in star polymer melts. In the MM model [Macromolecules, 30, 2159 (1997)] the dynamic and relaxation moduli were determined through an approximation to the first-passage time (FPT). In this work we calculate exactly the FPT, which is related to the time required for each arm to escape from its original tube, and determine the range of applicability of the MM approximations, as well as different viscoelastic properties: the terminal relaxation time, the steady-state recoverable compliance, and the zero-shear viscosity, among others. In addition, we show that several viscoelastic properties obtained through our exact expression for the FPT compare very well with experimental data.

[D40.109] Flow Studies of Highly Filled Fluids in Small Gaps

Liquid flow through small-confined geometries has been studied extensively. In the 1920's, Washburn and Rideal developed a formalism describing capillarity that dominates the fluid driving force for these situations. Although detailed studies of polymer liquid in narrow geometries has been studied extensively, the flow becomes difficult to understand when the fluid contains particles that are within the order of magnitude of the gap height. Flow rates between parallel plates were determined as a function of gap spacing, temperature, and particle concentration. We investigated the limitations of the Washburn-Rideal equation and found it work for well except for spacings approaching the average particle size. We will discuss our preliminary results on the transition from simple capillary flow. This work is supported by U.S. DOE Contract DE-AC04-94AL85000.

[D40.110] Glass Transition near Polymeric Surfaces Studied by Positron Annihilation

Glass transition as a function of depth in polymers can be studied by using positron annihilation spectroscopy and by controlling positron incident energy. Early positron studies showed that glass transition temperatures could be largely reduced near the surface in films prepared by slovent evaporation method.1 This paper will report more recent results of S parameters and positron lifetimes in polymeric films after different thermal and chemical treatments. 1. Y.C. Jean at al, Phys. Rev. B, 56, R8459 (1997). * Supported by NSF-CMS-9812717; AFOSR:F49629-97-0162,F49629-98-1-0309

[D40.111] Characterizing nanoscopic heterogeneity in polymers near Tg: simulation and single-molecule spectroscopy

Recent advances in single-molecule fluorescence spectroscopy can be exploited to investigate heterogeneity on nanoscopic length scales in polymers near Tg. Translational diffusion of single-molecule fluorophores in polymers may be observed via near-field scanning optical microscopy (NSOM) and wide-field optical microscopy (WFOM). Depending on resolution achieved in determining diffusive displacements, it may be possible to visualize the broad distribution of cooperative relaxation environments inferred from previous ensemble measurements of relaxation. Here we show how individual probe diffusion can be quantified, how the diverse response of many probes relates to an ensemble-average diffusion coefficient, and how the diffusion response may reveal details of the distribution of relaxation environments. A computational model of 2-D probe diffusion in a heterogeneous medium will be presented. The model has yielded a set of expectations that are currently being critically tested through NSOM and WFOM studies.

[D40.112] Glass Transition Temperature for PMMA from Molecular Dynamics Simulations

The glass transition temperature, T_g, in syndiotactic PMMA has been studied through atomistic molecular dynamics simulations of short duration. The simulations were performed at temperatures in the range 296 K to 683 K. The mean squared deviations calculated for the case of atoms, monomers and molecules from their initial positions show a characteristic time and temperature dependence. A glass transition temperature of about 405 K was found, which is in good agreement with experimental observations of 397 K. Evaluating the self part of the van Hove correlation function for the case of monomers shows no indication of structural arrest or hopping processes. However, evaluating it for the case of atoms indicates that there is indeed a hopping process near and below the glass transition temperature and structural arrest at the lowest temperature studied.

[D40.113] FT-IR studies of Hydrogen Bonding in a Blend containing an N-methylated Liquid Crystalline Polyurethane

Our previous FT-IR studies of blends containing a liquid crystalline polyurethane (LCPU) [rod-like] and copolymers of styrene and 4-vinylphenol (PS-co-VPh) [coil-like] indicate that a miscible blend can be formed by optimizing the amount of intermolecular H-bonding between these two polymers. Miscibility of the blends showing the optimum amount of intermolecular H-bonding was verified using DSC and optical microscopy. Currently, we are examining methods to further improve the extent of intermolecular H-bonding and thus the miscibility of LCP/coil blends. We will discuss results of systems containing an N-methylated LCPU (LCPUM) and PS-co-VPh copolymers. We expect that the absence of intramolecular H-bonding among LCPUM chains will further improve the amount of intermolecular H-bonding

[D40.114] Splat: A non-equilibrium morphology of PPE-epoxy blends

We have studied effect of the addition of a polystyrene-polymethyl methacrylate (PS-PMMA) diblock copolymer on the morphology of a mixture of polyphenylene ether (PPE) and an amine-cured polyepoxide. The PPE and the diblock were soluble in the mixture of epoxy and amine precursors, and as the epoxy cured the system phase separated to form PPE rich domains in a polyepoxide matrix. The PMMA block of the diblock is miscible in the polyepoxide at all states of cure whilst the PS block becomes immiscible during cure and so the diblock can form micelles in the polyepoxide. The PS block, but not the PMMA block, is miscible in the PPE. The strong attraction between the PS and the PPE permits the diblock to drive the interfacial tension between the PPE and polyepoxide phases negative as the diblock micelles dissolve PPE. The morphology 'freezes' as the epoxy cures leaving a morphology of approximately spherical PPE droplets with a very convoluted interface (splat) within the polyepoxide matrix.

[D40.115] Ordered Multilayers Obtained by Electrostatic Self-Assembly.

The formation of ultrathin polymer films by successive adsorption of polyelectrolytes of opposite charge has been extensively studied from 1992 on. In all reported cases where organic polyelectrolytes have been used, the profuse intermixing which exists between subsequent layers results in virtually structureless films. However, we have recently discovered a possibility to obtain internally-structured films, by using polyelectrolytes of reduced linear charge density, which contain either long hydrophobic sequences or mesogenic moeities. In the present communication, we report on the principles governing growth and internal organization of such multilayers obtained from a series of ionenes of systematically varied chemical structure. We also compare the structure of these fully organic multilayers with the one of self-assembled hybrid systems, where the polyanion is replaced by negatively charged clay platelets.

[D40.116] Self-assembled nanostructures of amide containing dendrimers in lyotropic and thermotropic conditions

A material having ordered structure in nanometer scale has drawn much attention in recent years because the material exhibits unique physical and chemical properties. One special approach to fabricate such nanostructured materials is based on self-assembling nature of the large molecules such as linear polymers or dendrimers. Particularly perfect monodisperse dendrimers and their self-assembling process have emerged as one of the main research area in such efforts. To elucidate self-assembling process of dendritic molecules, we systematically studied the assembling behaviors of the amide dendrons synthesized up to third generations. Main structural elements for the amide dendrons to organize spontaneously are amide and carboxyl groups to provide intermolecular hydrogen bonds and the balancing alkyl tails. In this presentation, we will discuss the morphology of self-assembled nanostructures and their assembling behaviors in both thermotropic and lyotropic conditions.

[D40.117] Electrospinning from Molten Polymers in Vacuum

An electric field was used to produce fibers with diameters in the range from nanometers to microns. Polypropylene, polyethylene and poly(ethylene naphthalate) were electrospun from polymer melts into fibers in a vacuum. Pellets of each polymer were melted with a radiant heat source. An electric field was applied between the molten polymer and an electrically conducting plate. Video images showed that a droplet of molten polymer, trailed by a jet that became thin and soon broke, was pulled out when the electric field strength reached about 4 kV/cm. When the electric field strength was increased, a steady charged jet moved toward the conducting plate where it was collected. Higher electric field strength produced a bending instability (1). Jets solidified either in flight or after reaching the collector, as processing parameters were varied. The diameters of fibers were in the range of 300 nanometers to 10 microns. Flat fibers, coiled fibers, sinusoidal fibers and helical fibers were observed. (1) D. H. Reneker, A. Yarin, H. Fong and S. Koombhongse, J. Appl. Phys., 87, 9, 4531 (2000)

[D40.118] Flat, Branched and Split Electrospun Fibers

The electrospinning process uses electrical force to overcome the force from surface tension. As the electric field increases, the surface of a droplet becomes nearly conical and a charged jet flows from the vertex. The charged jet moves along a straight line for some distance and then begins a spiraling path, which is triggered by a bending instability.[1] The charged jet solidifies as it dries and electrospun nanofibers are collected.

The electrospinning process normally produces cylindrical fibers, but sometimes the fibers are flat, branched or split. Flat fibers were electrospun from polystyrene (PS) and poly(2-hydroxyethyl methacrylate) (HEMA) solution. Flat fibers were formed by the collapse of a tube.

Branched fibers of HEMA, PS and poly(vinylidene fluoride) were observed. The thinner branch was usually perpendicular to the axis of the primary jet. Branched fibers are formed by a smaller secondary jet ejected from the surface of the primary jet.

The charged jet can split apart into two smaller jets to reduce the charge per unit surface area. Split fibers of HEMA, in which two smaller jets run parallel to the axis of the primary jet were observed.

1. D.H. Reneker, A.L. Yarin, H. Fong, and S. Koombhongse, J. Appl. Phys. 87, 4531 (2000).

[D40.119] Controlling phase and alignment in thin films of block copolymer templated mesostructured metal oxides

Mesostructured inorganic-organic hybrid thin films have been generated using nonionic triblock copolymers as templating agents. By systematically varying the polymer content, different structures can be obtained, including lamellar, hexagonal and cubic phases. Various inorganic materials have been templated by this process including silica and titania. These mesoporous oxide films are potentially useful as optical and electronic materials, membranes, sensors, photovoltaic devices and in catalysis. For many applications it is desirable to control the orientation of mesostructured domains within the films. A strategy is being developed to direct the alignment of the templating material. In particular we focus on the preparation of 2D hexagonally structured films with cylindrical domains oriented perpendicular to the film plane.

[D40.120] Solvent Effect on ABA and ABABA Block Copolymer Morphology

We report the effect of casting solvent on the microdomain morphology of ABA triblock and ABABA pentablock copolymers. Specifically, low molecular weight hydrogenated poly(styrene-butadiene-styrene) (H-SBS) triblock or H-SBSBS pentablock copolymers with either cylindrical or lamellar morphology are studied. Solvents either non-preferential or preferential to one of the blocks were used in the solution casting process. Small angle x-ray scattering and electron microscopy were used to characterize the resulting morphologies. Mechanical toughness was measured for a variety of solvent cast samples and correlated to the block copolymer morphology.

[D40.121] Nanoscopic Posts via Block Copolymer Templates

Free-standing nanoscopic cylinders of silicon oxide were generated using ordered, nanoporous block copolymer templates. The templates were prepared by spin casting solutions of asymmetric diblock copolymers of polystyrene and poly(methyl methacrylate) onto a nonpreferential surface, which was made by end grafting poly(styrene-r-methyl methacrylate) copolymer to silicon substrates having a styrene fraction of 0.58. UV exposure of the copolymer films, followed by solvent rinsing produced an ordered, nanoporous film. Silicon oxide was grown inside of the holes by the reaction between SiCl4 vapor and substrate hydroxyl groups. A well defined array of silicon posts was made by removal of the polymer matrix via reactive ion etching. Controlling the height and lateral size of the silicon posts will be addressed along with the applications.

[D40.122] Fabrication of Rod-Coil Nanocomposites via Emulsion Technique

There is increasing technological interest in polymers reinforced by nanoparticles because of their potential to provide enhanced mechanical properties, decreased permeability and flammability, as well as increased conductivity. Emulsion polymerization offers a viable, flexible route for nanocomposite fabrication from nanoscale spheres, rods, and plates. Combining emulsion generated poly(methyl methacrylate) (PMMA) particles that are ionically stabilized in aqueous solution with a dispersion of nanoparticles of opposite sense results in an interfacial exchange reaction and co-precipitation. The rod-coil nanocomposites are composed of fully conjugated sulfonated phenylene benzobisimidazole polymer (sPBI) with pendant PMMA chains. Synthesis and characterization of sPBI-PMMA nanocomposites with increasing nanoparticle content using reactive and non-reactive emulsifiers are discussed, and related according to their inherent physical properties.

[D40.123] Unimolecular amphipolar nanocylinders via a "grafting from" process using a ATRP

Core-shell cylindrical polymer brushes were synthesized by a “grafting from” procedure using Atom Transfer Radical polymerization (ATRP) from functionalized poly(2-hydroxyethyl methacrylate). We obtained well-defined brushes with polystyrene (PS), poly(tert-butyl acrylate) (PtBA), PS-b-PtBA, or PtBA-b-PS as side chains. AFM images of these brushes on silicon and mica surfaces reveals unique structural patterns. The hydrolysis of the PtBA blocks of brushes with diblock side chains afforded amphipolar brushes, serving as unimolecular micelles. These polymers show interesting structural response to solvent quality.

[D40.124] Synthesis and Aggregation Behavior of Janus micelles

A novel strategy to synthesize amphiphilic, surface-compartmentalized nanoparticles based on linear ABC triblock copolymers is presented. These so-called Janus micelles consist of a crosslinked core and a corona with a “northern” and a “southern” hemisphere. Selectively crosslinking spherical domains of the polybutadiene middle block in a well-ordered bulk morphology of a polystyrene-block-polybutadiene-block-poly(methyl methacrylate) triblock copolymer (SBM) leads to the conservation of the compartmentalization of the outer blocks after dissolution of the material. Multi-angle laser light scattering GPC, fluorescence correlation spectroscopy, small-angle neutron scattering, static and dynamic light scattering, as well as scanning force microscopy indicate an equilibrium between molecularly dissolved Janus micelles (unimers) and aggregates (multimers), so-called supermicelles.

[D40.125] The Orientation of Cylindical Microdomain in polystyrene-block-poly(n-butyl methacrylate) thin film.

The orientation of cylindrical microdomain in polystyrene-block-poly(n-butyl methacrylate) (P(S-b-nBMA)) thin films was studied by atomic force and an optical microscopy. P(S-b-nBMA) with two different weight fractions of the PS block (one is 30 wt % showing PS cylinders, and the other is 70 wt % showing PnBMA cylinders) were prepared by anionic polymerization. Unlike P(S-b-PMMA) thin films that exhibited a vertical orientation of cylinders on a neutral surface, P(S-b-nBMA) thin film did not exhibit this behavior. This is attributed to the larger difference in the surface tensions (\sim 8 mN/m) between PS and PnBMA compared with that (< 1 mN/m) between PS and PMMA. In the case of P(S-b-nBMA), the entropic penalty arising from the mismatch between the film thickness and the domain spacing, which is a strong driving force to induce vertical HEX cylinders in PS-PMMA, is much less than the enthalpic penalty resulting from the segregation of the PS block at the free surface required for a vertical orientation of the cylindrical domains. Electric fields are currently being studied as a role in overcoming this enthalpic barrier.

[D40.126] Side-chain Effect on the Properties of Oxadiazole-containing PPVs

Oxadiazole-containing PPVs are potential candidates for single-layer light-emitting diodes (LEDs) because of their more balanced charge mobilities, resulting from the introduction of oxadiazole into the PPV backbones. By varying the chemical structures of the alkoxy side-chains and their substitution pattern, we are looking into the effects of the side-chains on the properties (UV-Vis absorption, photoluminescence, electroluminescence, etc.) of these polymers. Polymers with asymmetric substitution show very different external electroluminescence efficiencies (about one order lower) from the symmetrically substituted polymers. Photoluminescence results do not follow the same trend. Taken together, the PL and EL data provide insight to the balance between photoluminescence efficiency and carrier mobility in these polymers.

[D40.127] Scanning Electroluminescence Microscopy and Combinatorial Characterization of OLEDs

A new method for local investigation of charge transport and electroluminescence in OLEDs has been developed. We modified the Scanning Near-field Optical Microscope TwinSNOM (Omicron) such that a platinum tip could be used as scanning probe similar to conventional STMs. To provide additional information on the topography we use shear force detection. At each point of the sample, we scan the tip/sample distance while detecting shear force, current flow and electroluminescence. Measurements of single layer Alq3 devices are presented. We find that electroluminescence typically starts only when the tip is indented several nanometers into the sample and increases on further indentation. The onset of electroluminescence shows strong local variation. As a second field of research we present the fabrication and testing of multi-layer OLEDs with a systematic thickness variation of different layers. An array of 8x8 diodes is produced on a square ITO-glass substrate under high vacuum conditions. Two gradients of two different materials (e.g. Alq3, TPD) are evaporated onto the substrate. Evaporation conditions for all different layers including the 64 electrodes are kept constant, thus granting comparability of the different diodes. The diodes are simultaneously tested for long-term stability: the development of the electroluminescence- and IV-curves are recorded and optimum film thickness combinations can be determined.

[D40.128] Observations of phase deformation of monomeric systems in electric fields and subsequent polymerization

When a droplet from one phase of a phase-separated solution of two immiscible homopolymers is injected in the matrix of the other phase and a homogeneous electric field is applied, the droplet can deform either parallel or perpendicular to the electric field direction. In this work one or the other phase consisted of a monomer, which could be polymerized after being distorted in an electric field. DGEBA and Epoxidized Linseed Oil (ELO) were used as monomers along with deionized water and various silicone oils. For example, using DGEBA as the matrix and ELO as the dispersed phase, polymerization of the DGEBA was achieved using a cationic UV-sensitive initiator, that was dissolved only in the DGEBA. In addition, the deformation cycle in 0.1 Hz of deionized water in ELO was observed and compared with theoretical predictions. (Supported by NSF, DMR-9521265)

[D40.129] Electronic Structure Calculations of Two-Photon Absorbing Materials: A Time-Dependent Density Functional Theory Study

Although two-photon absorbing organic materials are increasingly recognized as important for a variety of technological applications, it is difficult to obtain predictive molecular structure-to-property relationships for these compounds. In this study we investigate the potential of time-dependent density functional theory as an accurate and computationally feasible method for the prediction of two-photon absorption spectra. We show that the calculated excited-state energies for the smaller conjugated polyenes and their phenylated analogs are in good agreement with experiment, as well as with the highest level of theoretical predictions to date. Calculated two-photon absorption cross-sections are reported for these compounds, and compared with previously calculated values and with available experimental data. Our results indicate that these moelcular systems may have a larger potential as two-photon absorbing materials than has been previously estimated.

[D40.130] Stress-induced birefringence associated with latent image development in UV-photopatterned chemically amplified polymeric resists

Orientation of polymer chains is important for diffusion processes in chemically amplified photoresist. For the photoresist system of poly(t-butyl methacrylate) (pTBMA) with TPS-SbF_6 as photoacid generator birefringence is observed at the interface between the unmodified polymer (pTBMA) and the UV-irradiated, post baked product poly(methacrylateacid) (pMAA). Previously unoriented polymer chains become aligned due to material losses in the UV-irradiated polymer. This local shrinkage leads to orientation of the polymer chains with a preferred direction perpendicular to the lithographic patterning. We discuss the rate of anisotropic diffusion in the transfer of the initial pattern during the chemical amplification step. A confocal microscope is used in reflection mode to spatially resolve lithographically written line edges in all three dimensions. We observe that a fraction of linearly polarized light locally turns into elliptically polarized light representing a measure for the amount of orientation of the polymer at the interface. The investigation is done quantitatively by utilizing theory of stress-induced birefringence in aligned polymers.

[D40.131] Two-Dimensional, Optical Ellipsometric Studies of Polymer Orientation

Crystalline or liquid crystalline polymers exhibit optical birefringence as a result of formation of superstructures, such as spherulites, axialites, dendrites or liquid crystalline phases. Our method of choice for optical characterization is a variation of ellipsometry based on Stokes analysis. We use ellipsometry to measure the retardation and azimuthal angle of optically anisotropic polymeric materials. In addition, the embodiment of the method provides two-dimensional, i.e., spatially resolved, information about the optical parameters across the field of view. Monochromatic, incoherent light is polarized through states of differing ellipticity using liquid crystal variable retarders as universal compensator, as suggested by the method of Oldenbourg and Mei. After transmission through the polymer sample, the state of ellipticity of the polymer is quantitatively determined at every pixel in the two-dimensional optical image. To date we have investigated zone drawn polyethylene tapes, electrically activated liquid crystal display cells, and several thermotropic liquid crystalline polymers including Vectra. We aim to provide fundamental information about the formation of optically anisotropic structure, including measurement of phase transformation kinetics and development of textures.

[D40.132] Morphology of immiscible polymer blends made by polymerizing one component in an electric field

Immiscible binary polymer blends were made by polymerizing a monomer in which a different polymer was dissolved in the presence of DC or AC electric field. Scanning electron microscopy (SEM) was used to study the morphology of the resulting polymer mixture. A typical system started with 10 –- 40 wt % polystyrene dissolved in cyclohexene oxide; the cyclohexene oxide was then cationically polymerized using UV-sensitive initiator under room temperature. Electric fields used during the polymerizations ranged from 600 v/cm to 10,000 v/cm. The dispersed phase had dimensions in the range 0.1 –- 10 \mum; it was spherical in the absence of an electric field but consisted of ellipsoidal phases with the long axis aligned in the field direction. Both the distortion direction of the dispersed phase and evidence from the SEM studies indicated that the dispersed phase was the poly (cyclohexene oxie) even when this phase comprised as much as 80 wt % of the sample. These results and data on other systems will be discussed. (supported by NSF, DMR-9521265)

[D40.133] Photoluminescence characteristics of phenylated siloxanes

The unique interaction between conjugated \pi -electron systems and the Si atom in phenyl-substituted silanes and siloxanes gives rise to chromophore behavior of these substances. Phenylsilanes and phenylsiloxanediols as well as variously substituted cyclotrisiloxanes and cyclotetrasiloxanes were synthesized, and complete excitation-emission maps were obtained in the ultraviolet-to-visible spectral range. The position of the emission maxima was analyzed from intensity contour plots. Two different types of behavior were identified for compounds such as the cyclosiloxanes: with hexaphenylcyclotrisiloxane, the position of the main emission maximum at 357 nm did not depend on the excitation wavelength, while for octaphenylcyclotetrasiloxane the spectral position of the maximum increased with the excitation wavelength. A model for this phenomenon and evidence for the effects of methyl vs phenyl substituents will be discussed.

[D40.134] Self-assembled Block Copolymers as Two-Dimensional Photonic Band Gap Materials

To form photonic band gaps in the visible regime, we synthesized monodisperse, high molecular weight poly(styrene-co-isoprene) and poly(a-methylstyrene-co-isoprene) using anionic living polymerization. By controlling the percentage of the two different blocks, we obtain self-assembled 2-D cylindrical photonic crystals. The polyisoprene block in poly(styrene-co-isoprene) can be decomposed by ozone irradiation. By heating poly(a-methylstyrene-co-isoprene) above 170 C, the poly(a-methylstyrene) block depolymerizes and the polyisoprene block cross-links. Thus, after removing the cylindrical phase in both block copolymers, air filled 2-D photonic structures are formed. Reflectivity versus wavelength results are compared with theoretical models.

[D40.135] Orientational phase transitions in block copolymer melts under shear flow

We discuss orientational phase transitions in block copolymer melts under shear flow. We demonstrate that the phase diagrams predicted by theories of orientational phase transitions in sheared block copolymer melts (G. Fredrickson, J. Rheol.v.38, 1045 (1994) and A.N. Morozov et al., Phys. Rev. E v.61, 4125 (2000)) depend on the architecture of molecules. According to theory, in diblock copolymer melts the parallel phase is stable at low shear, and the perpendicular phase is stable at high shear. We show that this does not necessarily hold for other molecular architectures. We find that some systems exhibit the reverse phase behaviour: the perpendicular and parallel phases exchange their positions in the phase diagram. The application of the reorientational transition theory to each particular system should be accompanied by calculation of the architecture-dependent parameters(angle-dependence of the fourth order vertex function).

[D40.136] Gyroid Single Crystal Diffraction

A “single crystal” of a block copolymer / homopolymer blend with the Gyroid morphology was obtained by slow solvent casting. The block copolymer was an I2S2 star shaped block copolymer, which has two polystyrene arms and two polyisoprene arms connected at a single junction point. The homopolymer was low MW polyisoprene. The sample was studied by SAXS and TEM. SAXS of this sample produced single crystal diffraction patterns of several different zones with as many as 128 diffraction peaks in a single diffraction pattern, and 148 total unique diffraction peaks. Analysis of this data provides the most unambiguous proof to date of the Ia3d symmetry and Gyroid structure.

[D40.137] Four-Colour Mesoscale Morphologies in External Fields

The fascinating mesoscale morphologies one finds in copolymer materials (bulk or thin films) are usually prepared by some quenching process, involving external fields conditions such as solvent evaporation, and/or applied shear, confinement (surfaces), and sometimes electric fields. By means of the dynamic mean-field density functional method we have calculated the 3D dynamical morphology diagram of 4-colour star-block copolymers, in the presence of external shear and electric fields. The number and type of micro-phases one can get with four colour polymers is already staggering, let alone the combination with shear and electrostatics. In the absence of external fields, quenching leads to a peculiar 4-colour checkerboard in 3D. Due to the symmetry of the morphology, shear hardly affects the dynamical phase diagram, which is in very good agreement with theoretical predictions (presentation A.N. Morozov at this conference).

[D40.138] Temperature-Induced Solid-Solid Phase Transitons of HMX and TATB*

The chemical activation energy barrier of metastable anisotropic solid phase decomposition reactions of energetic materials depends, in part, on the crystal symmetry. A detailed understanding of the solid-solid phase transitions is critical for accurate modeling of decomposition. It is known that delta-HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) is more sensitive to shock initiated chemical reactions than beta-HMX polymorph. In our laboratory, we have focused on quantifying the isothermal kinetic phase diagram of energetic materials and their corresponding thermodynamic stability fields. We have developed and employed several techniques, namely, a second harmonic generation (SHG) optical diagnostic, to monitor reaction progress and real time x-ray diffraction utilizing synchrotron radiation. These experiments will provide a unique set of data for the understanding of material stability of the decomposition reactions of beta and delta-HMX. Initial results for HMX indicate incremental conversion from beta- to delta-HMX at approximately 160 C. With continuing heating to 165 C, an amorphous component coexists with a small component of a somewhat ordered structure. The present talk will focus on the energy dispersive x-ray diffraction results carried out at X17C beam line at NSLS on HMX and TATB (1,3,5-triamino-2,4,6 trinitrobenzene) as a function of temperature. In our second abstract, we will focus on simultaneous SHG/XRD results recently performed at SSRL.

[D40.139] Brownian Dynamics Investigations of Dendrimer Structures.

Dendrimers possess regularly branched, tree-like structures and intrinsic viscosities that increase with molecular weight up to a critical point where they peak and decrease. This is in distinct contrast to the behaviour of linear polymers, earmarking dendrimers as possible solution viscosity modifiers. Brownian dynamics computer simulations with excluded volume and hydrodynamic interactions have been performed for a variety of AB2 dendrimer structures that differ slightly in their branching pattern. The impact of these methodical structural alterations on dendrimer rheology is observed through comparisons of the simulated intrinsic viscosities. Additional insight will be offered into the simulated intrinsic viscosities of various hyperbranched polymers which are often viewed as being the cost-effective alternative to dendrimers.

[D40.140] AN NPT MOLECULAR DYNAMICS SIMULATION OF THE RESPONSE OF THE LOCAL SEGMENTAL DYNAMICS OF MELT POLYETHYLENE TO PRESSURE AS A FUNCTION OF TEMPERATURE.

NPT molecular dynamics simulations of a melt of linear unentangled united atom polyethylene chains have been performed well above Tg. The major aim of this study is to observe how increasing pressure affects local segmental motion. Transition rates, torsional angle coupling, geometric autocorrelation functions and distributions of relaxation times will be shown at several pressures and temperatures. At high pressure and low temperature, a dynamic process between the timescale of conformational motion and motion of the end-to-end vector

[D40.141] Semiconductors I: Growth and Structure

This abstract not available.

[D40.142] Measurements of the Relative Backscattering Yields of Positrons and Electrons from GaAs (100), Ge (100), Si(100)

The backscattering yield of positrons and electrons were measured from clean and PMMA coated semiconductor surfaces to determine if the unique contrast mechanisms available using positron beams can be exploited in developing improved alignment methods for the semiconductor industry. The experimental results (obtained using the electrostatically-guided positron beam system at the University of Texas at Arlington) shows that the contrast from backscattered positrons is higher than the contrast from backscattered electrons for the GaAs (100) / Ge (100) and 5mm PMMA coated on Si (100) / Si (100) samples. Our results suggest that the enhanced contrast possible using positron beams could be useful in the development of new alignment tools for use in IC fabrication.

[D40.143] Heating Effect on Silicon Containing Magnesium and Oxygen

It is well known that magnesium, when diffused into silicon, behaves like an interstitial donor impurity. For silicon containing oxygen, the interstitial magnesium is able to pair with dispersed oxygen in the crystal to form magnesium- oxygen complex impurity, which is also an interstitial donor. Further study has found that, after high-temperature diffusion, most magnesium impurities introduced into silicon are isolated donors while only very few magnesium-oxygen pairs can be created. In order to form enough magnesium- oxygen complex donor impurities to study their behavior in silicon, experimental results from spectroscopic measurements will be presented clearly demonstrating that their concentration can be enormously increased by a simple heating process.

[D40.144] Growth and Characterization of Fe Doped InSb Films

Samples of InSb doped with Fe have been grown on sapphire 0001 substrates using RF magnetron sputtering. Powders of 5N purity in atomic percents of: 47.5, 47.5, 5, : In, Sb, Fe were pressed into a sputtering target. Samples were grown at substrate temperatures between 240 and 360 C producing films with a smooth metallic appearance. STM images showed rounded features with a 20 nm variation in height. Electron microprobe data on a single sample found concentrations of 55% Sb, 44% In, and 2mobilities of 400 - 800 cm^2/Vs and carrier concentrations of 5x10^17cm^-3. No large magneto-resistance effects were observed. Magnetic measurements on a thick film sample as well as measurements on samples with higher Fe concentration will also be presented.

[D40.145] Structural study of a commensurate phase at Co/Si(111) interface using in-situ surface x-ray scattering

Interfacial structure and reaction stoichiometry of the Co-adsorbed Si(111) surface at room temperature have been studied by in-situ surface x-ray scattering using synchrotron photons. The intensity oscillation at the anti-Bragg position during deposition indicates that a layerwise consumption of silicon substrate occurs for the first four Co layers deposited. Our data suggests that the Co-Si interface formed at the initial stage of growth is a silicide layer with atomic stoichiometry of Co2Si. This silicide layer was found to be grown epitaxially in a commensurate phase of pseudohexagonal Co2Si, which shows a long-range order imposed by the Si substrate with significant local atomic displacements. The Co2Si was completely transformed into a B-type CoSi2 upon annealing at 500¡É.

[D40.146] Synthesis and properties of CuInS_2-based magnetic semiconductors

We have prepared rare-earth doped CuInS_2 bulk single crystals, and have studied their optical properties. Photoluminescence lines due to Yb^3+ ions have been observed in CuInS_2:Yb at low temperatures. The Yb ions are considered to be doped in crystallographically one site from the temperature dependence of the line-width and the magnetic-field variation of the spectra. We have also investigated the magnetic properties of Mn-doped CuInS_2 systems on polycrystalline samples. We have not observed magnetic ordering above 2K even in the highly-doped CuIn_0.8Mn_0.2S_2-\delta system.

[D40.147] Step Meandering Induced by Ce Silide Formation on Si(111)

Rare earth Ce silicide on Si(111) can be formed from the adsorption of Ce atoms on Si(111) above 500 ^ \ circC. We report the formation and diffusion process of the Ce silicide formed by the initial (less than 4.0 ML) adsorption of Ce atoms on Si(111) followed by annealing to 500 ^ \ circC using variable temperature Scanning Tunneling Microscopy (STM). Below 2.0 ML Ce coverage, we find Ce atoms which form homogeneous overlayer on the terrace of Si(111) move to the step edge of Si(111) and diffuse into the step edges by forming Ce silicide at 500 ^ \ circC. The meandering of Si step structure take place as the result of this Ce silicide formation. Step meandering due to bulk diffusion of Ce silicide can be also observed upon 3.3 ML Ce adsorption followed by annealing to 750^\circC.

[D40.148] Deposition of Nickel and Rhenium onto 6H-SiC and 3C-SiC

Quantum mechanical, CASTEP software, calculations were performed using nickel and rhenium atom deposition onto cleaved 6H-SiC and 3C-SiC surfaces. Minimizations of silicon and carbon cleaved surfaces (active, hydrogenated, and oxygenated) were performed without metal atoms and with metal atoms at selected positions in the unit cell (origin, a-axis, b-axis). Binding energies for each of the metal atoms (nickel and rhenium) were calculated. Additionally, calculated energy bands with associated density-of-states will be presented. Partial density-of states calculations provide a means to study the bonding of each metal atom (s,p,d,and f) helps explain the experimental finding, silicon surfaces accept the rhenium atoms bonded only to the silicon atoms. Nickel atom deposition bonded to the silicon surface atoms and also to the underlying carbon atoms. Experimentally this was verfied by conductivity measurements. Future simulations will consider the deposition at elevated temperatures to more closely match experimental conditions. All presented simulation results are for absolute zero.

[D40.149] A Lattice Approach to Elastic Energy, Equations, and Boundary Conditions

We derive bulk elastic equations and boundary conditions from a discrete form of the elastic energy. Our method provides boundary conditions for sharp corners as well as smooth edges. Our method is flexible enough to allow the discrete points where the strain is calculated to be interpreted either as the points of a computational grid for continuum elasticity, or as the positions in an atomic lattice. It thus provides a connection between continuum elasticity and atomistics. In fact, our model may be put in explicit correspondence with a Keating type of model. Computational results for crystal growth on vicinal surfaces will be presented, showing the effect of epilayer thickness on step-bunching instabilities.

[D40.150] Pascal's Triangle and its related high-tec

Some self similarity properties of two new numerical series derived from the Pascal's triangle are presented. One of them was reported earlier^1. The other one shows a parallelism with the Fibonacci series. The n-th term is written as: L_n.=E_n./2^n., where E_n. is given by the recursion relation: E_n.=E_n-1.+2E_n-2.. The E_n. series also shows self similarity. One of the high-tec applications of Pascal's triangle is as one of the best cell phone antennas design^2.. In this work it will be shown some other applications of another numerical series which appear in the simulation of the growth of metal clusters of a few atoms^3..

1. E. López-Cruz and GH Cocoletzi, Bull. Amer. Phys. Soc. vol. 43, 483 (1998) 2. George Musser, Sci. Amer. Vol. 281, 38 july (1999) 3. S.M. Reimann et al Physical Review B vol. 56, 12147 (1997)

[D40.151] THE NATURE OF THE PRE-SURFACE LOW-OHMED LAYER

The diffusion distribution of cobalt, nickel, manganese elements in silicon is well known, the concentration of which in pre-surface area increases their solubility in the silicon volume. Although various explanation of these anomalous distributions of the concentration in silicon are assumed, there is no unequivocal description in literature. The high values of concentrations in the pre-surface area of Si, their significantly low values in the volume part, which are described by the function of errors, indicate the processes difference which takes place in the volume and pre-surface area of silicon at manganese diffusion. The purpose of this article is to investigate the nature of the pre-surface low-ohmed layer formation in silicon compensated by manganese. On the strength of the results of complex investigation the model of the structure of the pre-surface area of silicon compensated by manganese is proposed.

[D40.152] Electronic band structure of defect chalcopyrites

The defect chalcopyrites of chemical composition II-III-VI_4 in which II, III and VI mean group-II elements such as Cd or Hg, group-III elements such as Al and Ga and group-VI elements such as S, Se, Te, form an interesting family of semiconductor compounds with potential nonlinear optical applications. They can be thought of as derived from the regular I-III-VI_2 chalcopyrites by doubling the formula unit and replacing the group I element, for example, Ag by the group-II element and a vacancy in an ordered manner. The chalcopyrites themselves are derived from II-VI compounds by replacing the group-II by a group I and a group-III element. In this contribution we present electronic band structure calculations of some of these compounds, calculated using the linear muffin-tin orbital method combined with the local density functional approximation. We discuss the relation of the band structures of the corresponding zincblende, chalcopyrite and defect chalcopyrite compounds. In particular, the role of the group I or group II d-band energy will be shown to be important. The trends with chemical substutions and the effects of structural distortions c/a and internal parameters accompanying the chemical distortion will be discussed.

[D40.153] Electronic structure of magnetic ternary alkali metal manganese hydrides

Magnetic ternary alkali metal?manganese-based hydrides have recently been discovered. These hydrides show an antiferromagnetic order transition at low temperatures. We have investigated the electronic structure, chemical bonding, and magnetic moment of such materials, by means of the full-potential linear-augmented-plane- wave method. The corresponding isostructural manganese halide has been also considered. We found that these materials are all semiconductors that show common features in terms of chemical bonding. The estimate of the magnetic moment is consistent with the experimental observations, and indicates that the manganese compounds have a high-spin ground state.

[D40.154] Self-Consistent Calculation for the Interface Between the Semiconductor and Vacuum

We apply the density-functional theory to calculate the junction characteristics between the semiconductor and vacuum. We obtain the self-consistent solution of the charge distribution around the interface. The zero-temperature form of the exchange-correlation potential which corrected by Sham for semiconductor is used, but full account is taken through the Fermi distribution function of the finite-temperature occupancy effects. We also apply the effective mass approximation to the Schroedinger equation in our method. The electronic structure and potential for the interface are reported. Our calculations for the semiconductor with the conduction electron densities r=24, 36, and 48 a.u..

[D40.155] Non-equilibrium phase transition of an electron gas

We study a nonequilibrium phase transition-like behavior of a photogenerated electron gas in semiconductors. The kinetics of the electron gas is given by a set of nonlinear rate equations. For low temperatures we show that they have three steady state solutions when the photoexcitation energy is in a certain interval which depends on the electron-electron interaction. Two of them are stable and the other is unstable. By defining a generalized free potential we obtain the Maxwell construction that determines the order parameter, namely the difference of the electron population in the bottom of the conduction band of the two steady stable states. Hence, this phase transition is a nonequilibrium first order phase transition.

[D40.156] Artificially Structured Materials

This abstract not available.

[D40.157] Conductivity with the higher order approximation

Kubo formula including all higher-order terms has been derived. As an application, the second-order corrections for both the conductivity response function and the conductance coefficient have been deduced using a subdynamic-based formulation. This formula can be used to describe a mesoscopic system influenced by an external field. The influence of the second-order corrections on the current is related to temperature that is consistent with the original Laudauer-Büttiker formula. The further work on the concrete calculations will be done in the future. Here we have discussed only the case without localized electrons in the device. If there exist localized electrons, maybe some formulae can be deduced by using this method.

[D40.158] Stacked Billiards: Examining the Effect of Soft-wall Potential Profile on Fractal Conductance Fluctuations

We demonstrate the relationship between the "soft-wall" potential profile found in surface-gate semiconductor billiards and fractal conductance fluctuations using "stacked billiards". In this device two billiards are formed at different depths beneath the surface-gates that are used to define them, allowing these billiards to nominally differ only in potential profile. We devise a technique to experimentally demonstrate the presence of a soft-wall potential profile in a billiard, and combine numerical modeling of the potential profiles with fractal analysis to show that changes in the "softness" of the potential profile act only to modify rather than suppress the fractal behavior in the conductance fluctuations.

[D40.159] The Evolution of Fractal Patterns during a Classical-Quantum Transition

The mixed (stable/chaotic) phase-space found in "soft-wall" semiconductor billiards has been predicted, through a semi-classical process, to generate fractal conductance fluctuations (FCF) that have subsequently been observed experimentally. This fractal generation process can be suppressed using controlled transitions towards either fully classical or fully quantum mechanical transport through the billiard. Investigating a range of billiards, we show that these transitions induce a smooth evolution rather than deterioration in the fractal scaling properties of the conductance fluctuations. Furthermore, we find a remarkable behavior where the fractal dimension of the FCF depends solely on an empirical parameter that quantifies the average discreteness of the billiard energy levels.

[D40.160] Interacting electrons in parabolic quantum dots

The problem of interacting electrons in a parabolic confinement has attracted considerable attention recently since experiments on parabolic quantum dots have revealed peculiar properties.

We investigate this phenomenon numerically. To this end we diagonalize the Hamiltonian for Coulomb as well as for short range interactions using the Hartree-Fock based exact diagonalization method. In addition to the ground state energy we calculate the spatial charge distribution and the density-density correlations and compare the results to those of the classical calculation. We find that a screened Coulomb interaction produces energy level bunching for classical as well as for quantum-mechanical dots. The bunching in the quantum-mechanical system occurs due to an interplay of kinetic and interaction energy, moreover, it is observed well before reaching the limit of a Wigner crystal. We also find that the shell structure of classical and quantum mechanical charge distribution is quite similar.

[D40.161] Collective Oscillations of Quantum Wires in a Bounded Semiconductor

We have analyzed the collective oscillation spectrum for a system of N-parallel quantum wires, which are uniformly spaced in a plane with the first wire parallel to the bounding surface of the plasma-like host semiconductor and at a distance z_0 from it. The N-wire plasmons couple to the bulk and surface plasmons of the host medium. These coupled electron plasma oscillation frequencies are examined for N values up to N=6 as functions of z_0 and as functions of wavenumber along the direction of the wires.

[D40.162] Paramagnetic and Optical Properties of Organic Capped ZnO Nanocrystals

Nearly monodisperse ZnO nanocrystals capped with trioctylphosphine oxide are prepared by high-temperature organometallic synthesis. Changes in the paramagnetic and optical properties of ZnO nanocrystals caused by UV-excitation and electron transfer are investigated by ESR, IR, UV-VIS, and Photoluminescence studies. ESR and mid-IR absorption studies confirm the presence of extremely long-lived (days) electrons in the quantum confined states of the conduction band. The similarities and differences of UV and electron transfer induced properties are presented.

[D40.163] Magneto-transport in InAs/AlGaSb open quantum dot structures

We report on the magneto-transport properties in InAs/AlGaSb open quantum dot structures, in which electron interference should be an important process. The open dot structures such as the square or circle cavities with sub-micron scale have been fabricated by electron beam lithography. Since the device size is small enough to be less than the elastic mean free path, the electron transport is expected to be predominantly ballistic in nature. Oscillations or fluctuations of magnetoresistance have been observed from low magnetic fields below 1 T, which were not seen in the conventional devices. The periodic fluctuations indicate the important effect of electric wave interference in the quantum dot structures. The large and simple structures of magnetoresistance fluctuations can be analyzed from the semi-classical scattering trajectory depending on the geometrical feature of the devices. In addition to these large-period magneto-oscillations, the short-period magnetoresistance fluctuations have been also observed even at 4.2K [1]. These fluctuations may result from the complex modes of the interference electron waves possibly characterized by highly characteristic wave function scattering [2]. From the analyses of correlation functions of these oscillations, interference of electron waves in the InAs/AlGaSb open quantum dot structures will be discussed. [1] T. Maemoto, M. Ichiu, A. Ohya, S. Sasa, M. Inoue, K. Ishibashi, Y. Aoyagi, Physica B 272, 110 (1999). [2] R. Akis, D. K. Ferry, and J. P. Bird, Phys. Rev. Lett. 79, 123 (1997).

[D40.164] Two-electron states for a cylindrical quantum dot

The cylindrical quantum dot two-electron states with Coulomb interaction have been constructed based on the one-electron wavefunctions obtained when confining one electron in a cylindrical box. Numerical results and energy-level diagrams will be presented as the length and the radius of the cylinder change. On average, the energy levesl change 10when the Coulomb interaction is taken into account.

[D40.165] Crystalline structure of sulfur nanowires.

Sulfur nanowires have been synthesized by a nanoporous alumina template approach. Two types of wires were obtained, some of them straight and very long but the most of them curly. The diameter was 15nm, typically more than 1000nm of length and the longest of these wires seems to be almost monocrystalline.A first sight on them by electron microscopy showed differences, on the crystalline structure, compared to the most stable bulk allotrope. Studying carefully the wires' structure by X-ray diffraction on the confined wires, and by high resolution electron microscopy and electron diffraction, on the released ones, we found that the cell parameters are near the ones for \alpha bulk sulfur.

[D40.166] Vertical and Lateral Manipulation of Adatom on Surfaces

We have carried out calculations of vertical and lateral manipulations of adatoms on flat, stepped and kinked (111) surfaces of several fcc metals to understand the dependence of the process on the cohesive energy, local atomic coordination and tip geometry. Vertical manipulation was found to be most effective on flat surfaces, while on stepped surface a "floating" region was found at which the adatom was equally attracted to the tip and the surface (for a blunt Cu(111) tip on (100)-microfacetted step edge at a height of 3.6Å). With a Cu(100) sharp tip on the same stepped surface, for heights ranging from 2.75Å-3.5Å the total energy of the system is found to be minimum when the adatom is very close to the surface. As the adatom is pulled the total energy of the system first rises and then reaches another minimum when the adatom touches the tip. Thus to pull an adatom vertically, the tip should be brought close to the adatom and then quickly pulled to a higher height. In the case of lateral manipulation it was found that Au had the lowest barrier to diffuse from one hollow site to the next (37 meV) at a tip height of 2.75Å and a lateral separation of 2.5Å. In this systematic study on six metal surfaces the effect of kinks and local step edge geometry is examined and results are rationalized on the basis of bond-length/bond-order correlation.

[D40.167] Polystyrene Containing Self-Assembled Nanoribbons

Polystyrene Containing Self-Assembled Nanoribbons

Leiming Li^a, John C. Stendahl^a, Eugene R. Zoubarev^a, Eli D. Sone^b, Martin U. Pralle^a, and Samuel I. Stupp^a,b,c

^aDepartment of Materials Science and Engineering, ^bDepartment of Chemistry, ^cMedical School, Northwestern University, Evanston, Illinois 60208

Small amounts, less than 1 weight synthesized in our laboratory and known as dendron rodcoils (DRCs) self-assemble into nanoribbons which in turn form gel-forming networks in organic solvents. The gels induced by self-assembly of DRCs are birefringent and the solvents include monomers such as styrene and 2-ethylhexyl methacrylate. The gels contain supramolecular ribbons that are approximately 10 nm wide, 2 nm thick, and 1 micron long. After polymerization of the gels, birefringent solids form which reveal enhanced chain orientation relative to pure polymer when samples are mechanically drawn under identical conditions. This difference was observed by both x-ray diffraction and optical measurements in samples containing polystyrene. Furthermore, small percentages of fluorescent dyes such as tetramethylrhodamine lead to drawn samples with enhanced photoluminescence anisotropy. The scaffolding of polystyrene or other polymers by the self-assembled nanoribbons and their networks could lead to enhancement of physical properties.

[D40.168] Nonlinear oscillations in a semiconductor superlattice under intensive terahertz irradiation

We have examined nonlinear oscillations of the internal field and current in semiconductor superlattices excited by terahertz laser radiation using a self-consistent multifrequency internal field approach. The oscillatory character of the nonlinear susceptibilities and dissipative and parametric instabilities in superlattices is seen to lead to multivaluedness and hysteresis in the dependence of spectral harmonics on the external field amplitude. The underlying dynamics of this behavior in the spectrum are the spontaneous generation of a static field and parametric creation and amplification of the external field harmonics and subharmonics. Our results clearly show that the field inside the superlattice cannot be described properly within a single frequency approach, especially for a high electron density SL.

[D40.169] Formation of electric field domains in a one-dimensional superlattice

We have analyzed the dynamics of the temporal formation of high electric field domains in a one-dimensional superlattice miniband subject to inelastic phonon scattering. Our formulation is based on the derivation of equations of motion for generating functions of electron momentum fluctuations, which take account of electron scattering from acoustic phonons, facilitating the determination of explicit analytical expressions for both the electron drift velocity and the diffusion coefficient as functions of the applied bias voltage. While our considerations here are limited to a one-dimensional superlattice, they can also be applied to transverse degrees of freedom for a three-dimensional superlattice. Our results provide a clear determination of the domain growth rate and its dependence on temperature and carrier concentration as well as bias field.

[D40.170] Wave Packet Dynamics in a Superlattice

We have examined the dynamics of an electron wave packet in a superlattice miniband in the presence of a uniform electric field. Using the Heisenberg equations of motion we determined the time-dependence of the electron probability density in the superlattice starting with a Gaussian wave packet at initial time. Our results provide an analytical expression for the dynamics of the electron wave packet which describes all experimentally observed phenomena such as, for example, Bloch oscillations and breathing motion. Furthermore, we have shown that, in the case of a narrow initial wave packet, the breathing motion represents a redistribution of the wave packet among various wells. On the other hand, for a relatively wide packet the breathing motion corresponds to oscillations of the packet width. Our results can be easy generalized to take account of dissipative electron-phonon scattering.

[D40.171] Image Potential States - A Probe of the Surface and Electronic Structure of Self-Assembled Monolayers

Electrons in image potential states (IPSs) are easily observed on clean and coated metal surfaces. An IPS model has been successfully applied to spectra produced via surface-enhanced electronic Raman scattering (SEERS) from a roughened metal substate coated with a self-assembled alkanethiol monolayer.[1,2] The IPS model and SEERS excitation spectra are used to locate the position of the electron in the monolayer, establish the presence of a partially populated energy band between the Fermi level and the vacuum level, and detect variations in the image plane distance that are related to the location and strength of bonds between the monolayer and the substrate. [1] B. K. Clark, B. W. Gregory, A. Avila, T. M. Cotton, and J. M. Standard, J. Phys. Chem. B 103, 8201, 1999. [2] B. K. Clark, B. W. Gregory, and J. M. Standard, Phys. Rev. B, (accepted, Fall 2000 publication).

[D40.172] Scanning Tunneling Microscopy Study of Carbon Tetrachloride Adsorption and Degradation on a Natural a-Fe2O3(0001) Surface in Ultrahigh Vacuum

Scanning tunneling microscopy and low energy electron diffraction have been used to study a natural a-Fe2O3(0001) surface and the adsorption and degradation of carbon tetrachloride on the reduced Fe3O4(111) terminated surface. A natural a-Fe2O3 (0001) surface was prepared by repeated cycles of Ar+ ion sputtering and annealing in vacuum or in O2 at 850 K. STM images and a LEED pattern indicate that an Fe3O4(111) terminated surface and a bi-phase can be formed depending on annealing conditions. The Fe3O4(111) terminated surface was dosed with CCl4 at room temperature, and flashed up to 590 K and 850 K. STM images show adsorbates on the surface at room temperature and the degradation products of CCl4 are isolated on the surface as the flashing temperature increases up to 850 K. Results from a companion temperature programmed desorption investigation are used in conjunction with the STM images to propose site specific reactions of CCl4 on the Fe3O4(111) terminated surface.

[D40.173] Cylinder Microdomain Orientation of polystyrene-block-poly(n-butyl methacrylate) thin film

The orientation of cylindrical microdomain in polystyrene-block-poly(n-butyl methacrylate) (P(S-b-nBMA)) thin films was studied by atomic force and an optical microscopy. P(S-b-nBMA) with two different weight fractions of the PS block (one is 30 wt /the other is 70 wt /by anionic polymerization. Unlike P(S-b-PMMA) thin films that exhibited a vertical orientation of cylinders on a neutral surface, P(S-b-nBMA) thin film did not exhibit this behavior. This is attributed to the larger difference in the surface tensions (~ 8 mN/m) between PS and PnBMA compared with that (< 1 mN/m) between PS and PMMA. In the case of P(S-b-nBMA), the entropic penalty arising from the mismatch between the film thickness and the domain spacing, which is a strong driving force to induce vertical HEX cylinders in PS-PMMA, is much less than the enthalpic penalty resulting from the segregation of the PS block at the free surface required for a vertical orientation of the cylindrical domains. Electric fields are currently being studied as a role in overcoming this enthalpic barrier.

Part D of program listing