Several glass forming systems have been studied by in situ scanning calorimetry and by solution calorimetry in Molten 2PbO.B2O3 at 973 K. In systems which show macroscopic glass-glass phase separation on annealing, namely K2O-La2O3-SiO2 and Li2O-SiO2, the enthalpies of solution of optically clear quenched glasses and milky annealed glasses are almost identical. This behavior, coupled with a linear variation of heat of solution with composition along joins which have a large change in overall degree of polymerization, support the contention from spectroscopy that short range clustering dominates even in nominally single phase Materials, with exsolution observed as the initially present heterogeneous regions grow in size. Alkali titanosilicate melts show Cp anomalies consistent with breakdown of alkali titanate complexes or changes in Ti coordination. Calorimetry is a sensitive quantitative probe of crystallization as a function of time in both ceramic and geologic systems.

The technique of high-performance liquid chromatography (HPLC) provides unique detailed information regarding the structure of partially disordered or amorphous phosphate solids. Applications of the experimental technique of HPLC to phosphate solids are reviewed, and examples of the type of information that can be obtained with HPLC are presented.

Single crystals of Pb2P2O7 (beam stable under 200 kV TEM electrons to a fluence > 1027 e/M2 and an ionizing dose > 1014 Gy) were rendered Metamict (amorphized) with ion-implantation (100 kV P+ ions with several fluences between 5 × 1017/M2 - 2 × 1020/M2). Pb2P2O7 and PbO-P2O5 glasses were also ion implanted at identical fluences. Radial distribution functions for metamict and glassy phosphates generated using energy filtered electron diffraction (EFED) data collected on 100 kV field emission STEM (VG HB-5) indicate significant alterations in the medium range order.

Dynamic Raman spectra of sodium borosilicate glasses containing large amounts of boron oxide have been studied as functions of temperature and holding time. Such characterization enabled measuring the changes in the nature and states of the molecular species that were present in the glasses and their Melts. The data indicate that boroxol rings involving one or more four-coordinated boron ions in the sodium oxide-containing glasses decreased in concentration as they were heated through melt temperatures as was noted earlier for boroxol groups involving three-coordinated boron ions in pure borosilicate compositions.

Radial distribution functions (RDFs) for vitreous silica (V-SiO2) have been obtained from energy-filtered electron diffraction data obtained in the HB5 scanning transmission electron Microscope. Results have been compared with those obtained from high-resolution neutron diffraction experiments, and are in good agreement within experimental errors. It was found to be impractical to obtain partial RDFs for this material from combined neutron, X-ray and electron diffraction data, because the similarities in characteristics of X-ray and electron scattering cause indeter-Minacies. A criterion equation has been given to determine feasibility.

Positron-annihilation-lifetime (PAL) spectroscopy has been utilized to investigate the free-volume properties of two types of polymer blends, a miscible blend of bisphenol-A polycarbonate (PC) and tetramethyl bisphenol-A polycarbonate (TMPC), and an immiscible blend of PC and polystyrene (PS). In the miscible blend, the free-volume hole size and its fraction follow a linear relationship with respect to the weight fraction while in the immiscible blend, the relationship is not linearly additive. The free-volume hole distributions in the immiscible blend are found to be significantly broader than those in the pure polymers. The difference is thought to be a result of the free volume formed and associated with the conformation and interchain packing between the dissimilar chains in incompatible polymers.

The nonequilibrium behavior of symmetrical tetra-n-alkylammonium halides is described from a study of the transition behavior using thermal analysis and hot-stage optical Microscopy. Results are discussed in terms of reversibility, supercooling, sharpness of transitions on thermal cycling, and glass transitions. The transitions of plastic to rigid crystals have the largest supercoolings. Gradual disordering on heating and incomplete ordering on cooling are found for some of the salts and represent interesting examples of order/disorder and glass transitions with reduced cooperativity.

We report initial results of an x-ray diffraction study of the pressure-dependence of the interdiffusion rate in amorphous Si/Ge Multilayers. Anneals were performed in a diamond anvil cell at 700 K for various pressures and durations. Interdiffusion was measured by Monitoring the rate of decay of the artificial Bragg peaks associated with the multilayer periodicity. A consistent increase in diffusivity was seen with pressure, characterized by an activation volume of -25±11 percent of the atomic volume of Si. An atomistic mechanism that Might account for such behavior is discussed.

Molecular dynamics simulations of glassy Ni81B19, starting with a configuration obtained by the Reverse Monte Carlo Method, indicate a calorimetrie glass transition at 960 K and point to a significant change in the atomic dynamics between 960 and 1200 K. Above this range, normal liquid-like behavior is found; at lower temperatures, we find a residual diffusivity and cooperative atomic Motion. Atomic jumps are processes smeared out in time and space over continuous rather than discrete scales.

X-ray diffraction and computer molecular modeling Methods are being used to investigate the ordering of random sequence copolyesters when cooled for the nematic Melt. Data will be presented for the copolymers prepared from p-hydroxybenzoic acid (HBA) and 6-hydroxy-2-naphthoic acid. The non-periodic diffraction patterns arising from the random monomer sequences are also indicative of the presence of limited three dimensional order. For both polymer systems, we can generate the qualitative feature of the x-ray data using models in which short non-identical segments of 10–12 Monomers on adjacent chains are approximately in register at their center. Molecular Mechanics calculations show that mese non-identical sequences can be packed on the observed hexagonal lattice with only small energy differences when compared to analogous homopolymer structures. Data will also be presented showing how x-ray diffraction can be used to follow transesterification of copoly (HBA/HNA) in the nematic MelL

The sorption of organic penetrants is found to be sensitive to thermal annealing conditions in a series of glassy, nematic, thermotropic, random copolyesters. Controlled thermal annealing of two polymers in this series permitted a systematic variation of chain packing and, presumably, higher order molecular suprastructure, ranging from a disordered amorphous morphology to More ordered nematic liquid crystalline and semi-crystalline morphologies. The development of liquid crystalline order appears to reduce or preclude small molecule solubility in nematically ordered forms of these polymers.

A series of Main-chain liquid crystalline polymers with long and polydisperse soft segments has been synthesized. The thermal properties of these polymers have been characterized by differential scanning calorimetry, optical microscopy and wide angle X-ray diffraction. Unexpectedly, these polymers have a strong tendency to form purely smectic Mesophases. The degree of ordering of these polymeric mesophases is Much higher than that of the mesophase formed by a low molecular weight Model compound with the same Mesogen. The soft segment length effects on thermal behavior of such segmented copolymers have been elucidated. A semi-quantitative theory has been formulated to interpret these observations. A phase diagram relating thermal properties to soft segment length has been proposed.

Atomistic details of disordering in superheated polymethylene crystals have been studied using full molecular dynamics simulations of crystals containing 9600 CH2-groups. The crystal size was about 227 nm3 Simulations were carried out for up to 100 ps, starting at temperatures about 100 K above the melting temperature. Typically 1.5 h of CPU time on a Cray X-MP were necessary per ps simulation. Superheating causes a quick development of large-scale disorder throughout the crystal, including reorientation, translation, and the destruction of crystal symmetry. This is followed ultimately by surface Melting. Crystallization centers with hexagonal packing are found in superheated, unconstrained crystals. On cooling during the simulation, recrystallization processes compete with the disordering, resulting in a reorientation of the molecular chains and reorganization of the crystal. Neither the fully amorphous phase nor the ordered crystal are reached during these short-time simulations using an instantaneous temperature increase to above the melting temperature, followed by a slow cooling into the crystallization temperature region.

There exist large amounts of experimental evidence on stress relaxation for metals and their alloys, synthetic and natural polymers, glasses and frozen non-polymeric organic liquids. The results, typically presented as curves a (log t) of relaxation of stress aas a function of logarithmic time t, exhibit common features, apparently independent of the type of Material. All curves consist of three regions: initial, nearly horizontal, starting at σ0; central, descending approximately linearly; and final, corresponding to the internal stress σi = σ(>). We discuss briefly the experimental evidence as well as the main features of the cooperative theory which does not involve specific features of different classes of Materials. The bulk of the paper deals with computer simulations. Simulation results obtained with the method of molecular dynamics are reported for ideal metal lattices, Metal lattices with defects, and for polymeric systems. In agreement with both experiments and the cooperative theory, the simulated σ (log t) curves exhibit the same three regions. In agreement with the theory, the slope of the simulated central part is proportional to the initial effective stress σ0* = σ0 - σi. The time range taken by the central part is strongly dependent on the defect concentration: the lower the defect concentration, the shorter the range. IMposition in the beginning of a high strain ε destroys largely the resistance of a material to deformation, resulting in low values of the internal stress σo. Since the cooperative theory assumes for particles (atoms, polymer chain segments) the existence of two states, unrelaxed and relaxed, and has a formal connection to the Bose-Einstein (B-E) distribution, we first simulate B-E systems, recording the formation of relaxed clusters of particles of different sizes. Differences in cluster sizes predicted from a B-E Model and those obtained from the simulations are recorded and analyzed. On the joint basis of experimental, theoretical

Differential scanning calorimetry (DSC) was used to study blends of polystyrene (PS) and poly (p-Methylstyrene) (PpmS). The presence of two glass transitions on heating after quenching was interpreted as evidence of phase separation at the temperature of the liquid before quenching. The small difference between the glass transitions of the homopolymers in the PS/PpmS system of “13 K limits the reliable detection of double glass transitions for blends to concentrations between 30 and 70%. The results of the DSC technique are supported by comparison with small angle neutron scattering (SANS) data.

Results from As K-edge XAFS studies on polysulfone films implanted with 50 KeV As+ in the dose range of 1015 to 1016 ions/cm2 indicate that As reacts chemically with O atoms to form As- (3) 0 based molecular structures having As-0 bond lengths equal to 1.81±0.02 Å. In comparison to samples implanted in the dose range 101S to 1016 ions/cm2, the molecular environment surrounding As in polysulfone implanted at 1017 ions/cm2 has additional structure beyond the nearest-neighbor O atoms.

Iridium oxide films produced by reactive sputtering (SIROFs) have considerably higher densities than those made by other techniques such as the anodic reaction at metal surfaces which contain pores and Microvoids. It has been previously reported that SIROFs deposited with substrate temperatures of 300K are amorphous. Here we report x-ray and electron diffraction measurements on SIROFs deposited on Al2O3, Si, NaCI and MgO substrates at 40°C. The x-ray diffraction patterns, I(Q), show “diffuse” structure that extends to beyond Q(=4εsinθ/) =10Â−1. We show that it is possible to reproduce qualitatively the main features of the diffraction pattern by convolving the crystalline rutile powder pattern with a Lorentzian profile. The width of this profile is compatible with that determined from the Scherrer equation although significant peak shifts and texture are observed. This analysis reveals that as-deposited SIROFs are crystalline with particle sizes in the range 25À to 35Å rather than amorphous (for which no crystalline model would be appropriate). The electron microdiffraction data are also consistent with a crystallite size of a few nm and high resolution TEM reveals lattice fringes from crystallites few nm in size. The peak shifts, however, remain to be explained.

It previously has been difficult to probe the localized coordination environment of aluminum atoms in amorphous alumina samples by solid state NMR spectroscopy. Such an in situ technique has benefits in probing the structure of the material and monitoring the evolution of the microstructural development during thermal processing. The recent developments of more sophisticated NMR techniques by Pines, et al., specifically devised for treatment of problems unique to the solid state spectroscopy of quadrupolar nuclei, have permitted a glimpse into these Materials. As a result of applying these experimental techniques to new materials systems, a better understanding of the solid state structure of amorphous aluminas derived from polymer pyrolysis has emerged. Correlations between the onset and completion of crystallization and the localized structure may now be possible.

Molecular dynamics simulations of the structure of crystalline and glassy titanium and zirconium wadeites have been undertaken to study the microscopic origin of the striking difference between these two systems in terms of nucleation and surface crystallization. The results of simulation are in accord with Raman and EXAFS spectroscopy as well as neutron scattering and DTA scans on the same structures.

We investigate the short-range structural properties of a-GaAs as obtained in a computer experiment based on a tight-binding molecular dynamics simulation. The amorphous configuration is obtained by quenching a liquid sample well equilibrated at T=1600 K. A detailed characterization of the topology and defect distribution of the amorphous network is presented and discussed. The electronic structure of our sample is calculated as well. Finally, we discuss the reliability and transferability of the present computational scheme for large-scale simulations of compound semiconductor materials by comparing our results to first-principles calculations.