CONFORMATIONAL DYNAMICS AND RELAXATION IN BULK POLYBUTADIENES - A MOLECULAR-DYNAMICS SIMULATION STUDY

Conformational dynamics and relaxation of the torsional angle autocorr elation (ACF) function in bulk polybutadienes (PBD) have been studied over a wide temperature range via molecular dynamics simulations. All trans, all cis and 70 mol % trans were the configurations of the syste ms studied. Companion studies of these systems in the phantom state we re also carried out to illuminate the effects of packing in the bulk. Conformational transitions at the -CH2-CH= bond alpha to the double bo nd and at the -CH2-CH2- beta bond were monitored. The activation energ ies for the overall transition rates for the ct bond and for the beta bond were found to correspond in both bulk and phantom states to one b arrier height in their respective torsional potentials. A considerable degree of correlation was found between second neighbor bonds. At the alpha bond in tran PBD correlated conformational changes across the d ouble bond predominated but changes across the beta bond when in the t rans conformation were common also. In cis-PBD, due to steric hindranc e at the alpha bond when in the cis conformation, correlated changes a cross the beta bond were relatively more common although exchanges acr oss the double bond were common also. In all of the above correlations , the direction of rotation in the two bonds are of opposite sign. In cis-PBD a correlated transition with same sign rotation was also found . The activation energies for relaxation times for the torsional ACF i n phantom PBD were also found to correspond to single torsional barrie r heights. In bulk PBD, however, the activation energies were signific antly higher than single barrier. This behavior is the result of the c onformational transitions becoming more heterogeneously distributed sp atially over the bonds in the bulk as temperature is lowered. This phe nomenon has also been found recently in simulations on polyethylene.