RIS_polybutadiene_JPhysChemA_102_1998_Smith&Paul - J....

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United Atom Force Field for Molecular Dynamics Simulations of 1,4-Polybutadiene Based on Quantum Chemistry Calculations on Model Molecules Grant D. Smith* Department of Materials Science and Engineering and Department of Chemical and Fuels Engineering, Uni V ersity of Utah, Salt Lake City, Utah 84112 Wolfgang Paul Institut fu ¨ r Physik, Johannes Gutenberg Uni V ersita ¨ t, D-55099 Mainz, Germany Recei V ed: September 22, 1997; In Final Form: No V ember 26, 1997 We present a united atom force field for simulations of 1,4-polybutadiene based on ab initio quantum chemistry calculations on model molecules. The geometries and energies of conformers and rotational energy barriers in model alkenes and dienes have been determined from high-level quantum chemistry calculations. A rotational isomeric state (RIS) model for 1,4-polybutadiene based on the conformer geometries and energies of the model molecules has been derived. The characteristic ratio and its temperature dependence for cis - 1,4-polybutadiene and trans -1,4-polybutadiene, and the characteristic ratio of a random copolymer of cis and trans units, as predicted by the RIS model, are in good agreement with experimental values, thereby supporting the accuracy of the quantum chemistry calculations. Torsional potentials for the united atom force field have been parametrized to reproduce the quantum chemistry conformer energies and rotational energy barriers for rotations about the C(sp 2 ) - C(sp 2 ), C(sp 2 ) - C(sp 3 ), and C(sp 3 ) - C(sp 3 ) dihedrals for the model compounds. The CH 2 - CH 2 united atom nonbonded potential has been taken from previous work on polyethylene melts, while the CH - CH united atom nonbonded potential has been parametrized so as to reproduce the energies of those conformers of the model molecules involving conformation-dependent second-order interactions. Finally, NPT molecular dynamics simulations have been performed on a melt of 1,4-poly( cis 0.5 - r - trans 0.5 - butadiene), and the CH 2 - CH nonbonded potential has been adjusted so that the experimental melt density of the polymer as a function of temperature is accurately reproduced. Introduction The dynamics and relaxational behavior of 1,4-polybutadiene have been the subject of extensive experimental study in recent years. Polybutadiene is a good glass former, and its simple chemical structure, narrow molecular weight distribution, and wide variety of available microstructures, which can be con- trolled through adjustment of solvent, polar modifiers, and temperature during anionically initiated polymerization, 1 make it ideal for investigations of the glass transition as well as subglass and high-temperature relaxations. Dielectric spectroscopy, 2,3 NMR spin - lattice relaxation, 4,5 and neutron scattering methods 3,6 - 12 have been applied in the study of polybutadiene glasses and melts. These dynamic spectroscopic techniques are the fundamental experimental probes of molecular motions because they either directly measure microscopic correlations or measure macroscopic
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This note was uploaded on 07/20/2011 for the course EMA 6165 taught by Professor Brennan during the Spring '08 term at University of Florida.

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RIS_polybutadiene_JPhysChemA_102_1998_Smith&Paul - J....

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