Relative Contributions of Coupled Rotations and Small-Amplitude Torsions to Conformational Relaxation in Polymers

Local dynamics of polyethylene and 1,4-trans-polybutadiene chains are analyzed by the cooperative kinematics approach. The method includes the effects of environmental friction and the conformational energy changes involved in bond rotational motions. A given bond is rotated by 120° and the response of the surrounding bonds along the chain, which are necessary for localizing the motion, are analyzed. The conformational responses are classified into two sets, one exhibiting a coupled rotation of another bond, i.e. involving a rotational barrier crossing, and the other accommodated by collective small-amplitude torsions (librational) of several bonds in the neighborhood of the rotating bond. Coupled rotations are shown to increase significantly with increasing environmental frictional resistance. Conversely, librational motions are dominant for localization in less viscous environments. An important finding is that, for polyethylene, the subset of transitions in which the relaxation is achieved by torsional...