Several glass forming systems have been studied by in situ scanningcalorimetry and by solution calorimetry in Molten 2PbO.B2O3 at 973 K. In systems which showmacroscopic glass-glass phase separation on annealing, namely K2O-La2O3-SiO2 and Li2O-SiO2, the enthalpies of solution of opticallyclear quenched glasses and milky annealed glasses are almost identical. Thisbehavior, coupled with a linear variation of heat of solution withcomposition along joins which have a large change in overall degree ofpolymerization, support the contention from spectroscopy that short rangeclustering dominates even in nominally single phase Materials, withexsolution observed as the initially present heterogeneous regions grow insize. Alkali titanosilicate melts show Cp anomalies consistentwith breakdown of alkali titanate complexes or changes in Ti coordination.Calorimetry is a sensitive quantitative probe of crystallization as afunction of time in both ceramic and geologic systems.

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

Single crystals of Pb2P2O7 (beam stableunder 200 kV TEM electrons to a fluence > 1027 e/M2and an ionizing dose > 1014 Gy) were rendered Metamict(amorphized) with ion-implantation (100 kV P+ ions with severalfluences between 5 × 1017/M2 - 2 × 1020/M2). Pb2P2O7and PbO-P2O5 glasses were also ion implanted atidentical fluences. Radial distribution functions for metamict and glassyphosphates generated using energy filtered electron diffraction (EFED) datacollected on 100 kV field emission STEM (VG HB-5) indicate significantalterations in the medium range order.

Dynamic Raman spectra of sodium borosilicate glasses containing largeamounts of boron oxide have been studied as functions of temperature andholding time. Such characterization enabled measuring the changes in thenature and states of the molecular species that were present in the glassesand their Melts. The data indicate that boroxol rings involving one or morefour-coordinated boron ions in the sodium oxide-containing glasses decreasedin concentration as they were heated through melt temperatures as was notedearlier for boroxol groups involving three-coordinated boron ions in pureborosilicate compositions.

Radial distribution functions (RDFs) for vitreous silica (V-SiO2)have been obtained from energy-filtered electron diffraction data obtainedin the HB5 scanning transmission electron Microscope. Results have beencompared with those obtained from high-resolution neutron diffractionexperiments, and are in good agreement within experimental errors. It wasfound to be impractical to obtain partial RDFs for this material fromcombined neutron, X-ray and electron diffraction data, because thesimilarities in characteristics of X-ray and electron scattering causeindeter-Minacies. A criterion equation has been given to determinefeasibility.

Positron-annihilation-lifetime (PAL) spectroscopy has been utilized toinvestigate the free-volume properties of two types of polymer blends, amiscible blend of bisphenol-A polycarbonate (PC) and tetramethyl bisphenol-Apolycarbonate (TMPC), and an immiscible blend of PC and polystyrene (PS). Inthe miscible blend, the free-volume hole size and its fraction follow alinear relationship with respect to the weight fraction while in theimmiscible blend, the relationship is not linearly additive. The free-volumehole distributions in the immiscible blend are found to be significantlybroader than those in the pure polymers. The difference is thought to be aresult of the free volume formed and associated with the conformation andinterchain packing between the dissimilar chains in incompatiblepolymers.

The nonequilibrium behavior of symmetrical tetra-n-alkylammonium halides isdescribed from a study of the transition behavior using thermal analysis andhot-stage optical Microscopy. Results are discussed in terms ofreversibility, supercooling, sharpness of transitions on thermal cycling,and glass transitions. The transitions of plastic to rigidcrystals have the largest supercoolings. Gradual disordering on heating andincomplete ordering on cooling are found for some of the salts and representinteresting examples of order/disorder and glass transitions with reducedcooperativity.

We report initial results of an x-ray diffraction study of thepressure-dependence of the interdiffusion rate in amorphous Si/GeMultilayers. Anneals were performed in a diamond anvil cell at 700 K forvarious pressures and durations. Interdiffusion was measured by Monitoringthe rate of decay of the artificial Bragg peaks associated with themultilayer periodicity. A consistent increase in diffusivity was seen withpressure, characterized by an activation volume of -25±11 percent of theatomic volume of Si. An atomistic mechanism that Might account for suchbehavior 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 significantchange in the atomic dynamics between 960 and 1200 K. Above this range,normal liquid-like behavior is found; at lower temperatures, we find aresidual diffusivity and cooperative atomic Motion. Atomic jumps areprocesses smeared out in time and space over continuous rather than discretescales.

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

The sorption of organic penetrants is found to be sensitive to thermalannealing conditions in a series of glassy, nematic, thermotropic, randomcopolyesters. Controlled thermal annealing of two polymers in this seriespermitted a systematic variation of chain packing and, presumably, higherorder molecular suprastructure, ranging from a disordered amorphousmorphology to More ordered nematic liquid crystalline and semi-crystallinemorphologies. The development of liquid crystalline order appears to reduceor preclude small molecule solubility in nematically ordered forms of thesepolymers.

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

Atomistic details of disordering in superheated polymethylene crystals havebeen studied using full molecular dynamics simulations of crystalscontaining 9600 CH2-groups. The crystal size was about 227 nm3 Simulations were carried out for up to 100 ps, starting attemperatures about 100 K above the melting temperature. Typically 1.5 h ofCPU time on a Cray X-MP were necessary per ps simulation. Superheatingcauses a quick development of large-scale disorder throughout the crystal,including reorientation, translation, and the destruction of crystalsymmetry. This is followed ultimately by surface Melting. Crystallizationcenters with hexagonal packing are found in superheated, unconstrainedcrystals. On cooling during the simulation, recrystallization processescompete with the disordering, resulting in a reorientation of the molecularchains and reorganization of the crystal. Neither the fully amorphous phasenor the ordered crystal are reached during these short-time simulationsusing an instantaneous temperature increase to above the meltingtemperature, followed by a slow cooling into the crystallization temperatureregion.

There exist large amounts of experimental evidence on stress relaxation formetals and their alloys, synthetic and natural polymers, glasses and frozennon-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. Allcurves consist of three regions: initial, nearly horizontal, starting at σ0; central, descending approximately linearly; and final,corresponding to the internal stress σi = σ(>). We discussbriefly the experimental evidence as well as the main features of the cooperative theory which does not involve specificfeatures of different classes of Materials. The bulk of the paper deals withcomputer simulations. Simulation results obtained with the method ofmolecular dynamics are reported for ideal metal lattices, Metal latticeswith defects, and for polymeric systems. In agreement with both experimentsand the cooperative theory, the simulated σ (log t) curves exhibit the samethree regions. In agreement with the theory, the slope of the simulatedcentral part is proportional to the initial effective stress σ0* = σ0 - σi. The time range taken by thecentral part is strongly dependent on the defect concentration: the lowerthe defect concentration, the shorter the range. IMposition in the beginningof a high strain ε destroys largely the resistance of a material todeformation, resulting in low values of the internal stress σo.Since the cooperative theory assumes for particles (atoms, polymer chainsegments) the existence of two states, unrelaxed and relaxed, and has aformal connection to the Bose-Einstein (B-E) distribution, we first simulateB-E systems, recording the formation of relaxed clusters of particles ofdifferent sizes. Differences in cluster sizes predicted from a B-E Model andthose obtained from the simulations are recorded and analyzed. On the jointbasis of experimental, theoretical

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

Results from As K-edge XAFS studies on polysulfone films implanted with 50KeV As+ in the dose range of 1015 to 1016 ions/cm2 indicate that As reacts chemically with O atoms toform As- (3) 0 based molecular structures having As-0 bond lengths equal to1.81±0.02 Å. In comparison to samples implanted in the dose range 101S to 1016 ions/cm2, the molecularenvironment surrounding As in polysulfone implanted at 1017 ions/cm2 has additional structure beyond the nearest-neighborO atoms.

Iridium oxide films produced by reactive sputtering (SIROFs) haveconsiderably higher densities than those made by other techniques such asthe anodic reaction at metal surfaces which contain pores and Microvoids. Ithas been previously reported that SIROFs deposited with substratetemperatures of 300K are amorphous. Here we report x-ray and electrondiffraction 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 qualitativelythe main features of the diffraction pattern by convolving the crystallinerutile powder pattern with a Lorentzian profile. The width of this profileis compatible with that determined from the Scherrer equation althoughsignificant peak shifts and texture are observed. This analysis reveals thatas-deposited SIROFs are crystalline with particle sizes in the range 25À to35Å rather than amorphous (for which no crystalline model would beappropriate). The electron microdiffraction data are also consistent with acrystallite size of a few nm and high resolution TEM reveals lattice fringesfrom crystallites few nm in size. The peak shifts, however, remain to beexplained.

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

Molecular dynamics simulations of the structure of crystalline and glassytitanium and zirconium wadeites have been undertaken to study themicroscopic origin of the striking difference between these two systems interms of nucleation and surface crystallization. The results of simulationare in accord with Raman and EXAFS spectroscopy as well as neutronscattering and DTA scans on the same structures.

We investigate the short-range structural properties of a-GaAs as obtainedin a computer experiment based on a tight-binding molecular dynamicssimulation. The amorphous configuration is obtained by quenching a liquidsample well equilibrated at T=1600 K. A detailed characterization of thetopology and defect distribution of the amorphous network is presented anddiscussed. The electronic structure of our sample is calculated as well.Finally, we discuss the reliability and transferability of the presentcomputational scheme for large-scale simulations of compound semiconductormaterials by comparing our results to first-principles calculations.