Conformational dynamics and relaxation in bulk polybutadienes: A molecular dynamics simulation study

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Abstract
Conformational dynamics and relaxation of the torsional angle autocorrelation (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 systems studied. Companion studies of these systems in the phantom state were also carried out to illuminate the effects of packing in the bulk. Conformational transitions at the –CH2–CH= bond alpha to the double bond and at the –CH2–CH2– beta bond were monitored. The activation energies for the overall transition rates for the α bond and for the β bond were found to correspond in both bulk and phantom states to one barrier height in their respective torsional potentials. A considerable degree of correlation was found between second neighbor bonds. At the α bond in trans-PBD correlated conformational changes across the double bond predominated but changes across the β bond when in the trans conformation were common also. In cis-PBD, due to steric hindrance at the α bond when in the cis conformation, correlated changes across the β bond were relatively more common although exchanges across 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 in phantom PBD were also found to correspond to single torsional barrier heights. In bulk PBD, however, the activation energies were significantly higher than single barrier. This behavior is the result of the conformational transitions becoming more heterogeneously distributed spatially over the bonds in the bulk as temperature is lowered. This phenomenon has also been found recently in simulations on polyethylene.