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RESEARCH

INDEX: ABSTACTS:

I. The Continuous Configurational Boltzmann Biased Direct Monte Carlo Method for Polymer Chains

    To predict the properties of polymers it is necessary to determine an ensemble of conformations highly populated at the temperature and pressure of interest. The most efficient method for predicting these conformations is by Monte Carlo (MC) sampling. However, for polymers with molecular weights of interest in polymer applications, such MC is too slow. The CCBB, an improved Monte Carlo method, was developed to solve this difficulty. CCBB combines continuous configurational biased (CCB) direct sampling method with Boltzmann factor biased (BFB) enrichment. CCBB is 300,000 times faster than simple sampling Monte Carlo in generating the free energy properties of polymer chains [J. Chem. Phys., 106, 6722-6729, (1997)]. CCBB has been implemented in a program compatible with the MPSim MD software. It is being applied to various polymers including branched and star-like polymers. The partition function from CCBB will be used to calculate thermodynamic quantities including Helmholtz free energy.

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II. Molecular Dynamics Simulations of Stimuli-Responsive Macromolecule in Various Solvents

    A novel amphiphilic hybrid macromolecule has been built by Jean M. J. Frechet et al. [J. Am. Chem. Soc., 118, 3785-3786, (1996)]. This macromolecule has hydrophobic dendritic groups at the periphery of a hydrophilic polyethylene glycol (PEG) star. Light scattering experiments suggest that changing the solvent from THF (tetrahydrofuran) to methanol leads to large changes in structure. To study the response of these macromolecules to such variations in environment, we used molecular dynamics (MD) to predict structures and properties for the macromolecule in methanol and THF. These calculations used explicit solvent and periodic boundary conditions. The Frechet macromolecule has 2761 atoms and we included also 26,892 methanol molecules or 13,140 THF molecules. We used the MSC MD program (MPSim) to carry out 700ps of NVT dynamic simulations at 300K. These results show that that in THF the Frechet macromolecule has a somewhat compact PEG core with the dendrimer extending outward into the solvent while in methanol the PEG tends to wrap around the dendrimer to bury it away from the solvent. We calculated scattering intensities expected from Small Angle Neutron Scattering (SANS) for comparisons to ongoing experiments by Diallo et al. These results validate the interpretations by Frechet et al.

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III. Molecular Dynamics Simulations of a<111> Screw Dislocation in Ta

     Using a new, first principles based, Embedded-Atom-Method (EAM) potential for tantalum (Ta), we have carried out atomistic simulations to investigate the core structure, core energy and Peierls energy barrier and stress for a<111> screw dislocation. Equilibrated core structures were obtained by relaxation of dislocation quadrupoles with periodic boundaries. We found that the equilibrium dislocation core has three-fold symmetry and spreads out in three <112> directions on {110} planes. Core energy per Burgers vector b was determined to be 1.36 eV/b. We studied dislocation motion and annihilation via Molecular Dynamics simulations of a periodic dislocation dipole cell, with <112> or <110> dipole orientation. In both cases the dislocations move in zig-zag on primary {110} planes. Atoms forming the dislocation cores are distinguished based on their atomic energy. In this way we can accurately define the core energy and its position not only for equilibrium configurations but also during dislocation movement. Peierls energy barrier was computed to be ~ 0.08 eV/b with a Peierls stress of ~ 0.03 u , where u is the bulk shear modulus of perfect crystal. The preferred slipping system at low temperature is <112> directions and {110} planes.

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