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 (unpublished).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 in