| Abstract
| - We have made detailed comparison of the local and chain dynamics of a melt of1,4-polybutadiene (PBD) as determined from experiment and molecular dynamics simulation at 353 K.The PBD was found to have a random microstructure consisting of 40% cis, 50% trans, and 10% 1,2-vinyl units with a number-average degree of polymerization 〈Xn〉 = 25.4. Local (conformational) dynamicswere studied via measurements of the 13C NMR spin−lattice relaxation time T1 and the nuclearOverhauser enhancement (NOE) at a proton resonance of 300 MHz for 12 distinguishable nuclei. Chaindynamics were studied on time scales up to 22 ns via neutron spin−echo (NSE) spectroscopy withmomentum transfers ranging from q = 0.05 to 0.30 Å-1. Molecular dynamics simulations of a 100 carbon(Xn = 25) PBD random copolymer of 50% trans and 50% cis units employing a quantum chemistry-basedunited atom potential function were performed at 353 K. The T1 and NOE values obtained from simulation,as well as the center of mass diffusion coefficient and dynamic structure factor, were found to be inqualitative agreement with experiment. However, comparison of T1 and NOE values for the variousdistinguishable resonances revealed that the local dynamics of the simulated chains were systematicallytoo fast, whereas comparison with the center of mass diffusion coefficient revealed a similar trend in thechain dynamics. To improve agreement with experiment, (1) the chain length was increased to matchthe experimental Mz, (2) vinyl units groups were included in the chain microstructure, and (3) rotationalenergy barriers were increased by 0.4 kcal/mol in order to reduce the rate of conformational transitions.With these changes, dynamic properties from simulation were found to differ 20−30% or less fromexperiment, comparable to the agreement seen in previous simulations of polyethylene using a quantumchemistry-based united atom potential.
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