Molecular dynamics simulation of local chain motion in bulk amorphous polymers. II. Dynamics at glass transition

Abstract

The local chain dynamics of bulk amorphous polymers has been studied by means of molecular dynamics simulation in the vicinity of the glass transition temperature Tg. Two models of polymers are used, one mimicking a polyethylene chain and the other a hypothetical freely-rotating chain, both of infinite chain length. The structural relaxations are examined by means of the time-correlation function of vectors embedded in the polymer backbone and of the distribution of the angles by which these vectors reorient after a time interval t. Some degree of chain mobility is seen to persist even as the temperature is lowered to Tg and below. In addition to the rotational diffusion process that takes place as a result of a series of small step motions, some large-angle jump motions are also seen to occur in both models. The jump motions, which are obscured by prevalent faster modes of motions at high temperatures, become clearly revealed in the reorientation angle distributions as the temperature is lowered. In the polyethylene model, the large-angle jump is directly associated with conformational transitions, while in the freely-rotating chain model, in which no torsional barrier exists, the jump probably arises because of the local potential minima created by the surrounding chains. The conformational transitions in the polyethylene model are highly cooperative among the segments neighboring along the chain, especially so as the temperature is lowered through Tg.