A molecular dynamics simulation study of the α- and β-relaxation processes in 1,4-polybutadiene

We have conducted molecular dynamics simulation studies of melts of 1,4-polybutadiene (PBD) using a quantum chemistry-based force field where the rotational energy barriers between conformational states have been reduced (LB-PBD model, where LB indicates lowered barrier). Segmental relaxation in the LB-PBD melts was investigated over a wide range of temperature by monitoring the decay of the torsional autocorrelation function (TACF). The decay of the TACF could not be well-represented by a single stretched (Kohlrausch-William-Watts) exponential indicating the presence of resolvable alpha- and beta-relaxation processes even at the highest temperatures investigated. We found that all or nearly all dihedrals undergo conformational transitions on the time scale of the beta-relaxation process. The beta-relaxation process was observed to weaken with decreasing temperature due to an increasingly heterogeneous population of conformational states with decreasing temperature on time scales longer than the beta-relaxation time but shorter than the alpha-relaxation time. The heterogeneity in conformational populations is imposed by the matrix which biases the conformational states of individual dihedrals (but not the average over all dihedrals) on time scales shorter than the a-relaxation time. Complete segmental relaxation (alpha-relaxation) occurred on longer time scales over which all dihedrals are able to populate each conformational state with near-equilibrium probability, a process that requires cooperative motion of the matrix. The beta-relaxation process as monitored by the TACF was also found to broaden with decreasing temperature, consistent with an increasingly broad distribution of times required for individual dihedrals to visit each conformational state. (c) 2006 Elsevier B.V. All rights reserved.