Molecular dynamics simulation of polymer liquid and glass. I. Glass transition

Abstract

Molecular dynamics simulation of bulk liquid and glass of long‐chain molecules has been performed. The system consists of linear chains of up to 50 spherical segments, each subject to forces due to bond stretching, bending, and torsion, and to nonbonded interaction, according to a truncated Lennard‐Jones potential, between segments in neighboring chains and between segments separated by more than three bonds along the chain. The parameters are chosen to mimic polymethylene, the segment representing a CH₂ unit. Behaviors suggestive of liquid‐to‐glass transition were exhibited by (i) cessation of trans–gauche conformational transitions, (ii) changes in the temperature coefficients of the density and internal energy, and (iii) effective vanishing of the segmental self‐diffusion coefficient. The ‘‘freezing in’’ of these properties occurs at decreasing temperatures in the order given above, indicating the decreasing size of domains of cooperative motion required. The dependence of the transition temperature on the chain length and on the flexibility of the chain (effected by switching of the torsional potential off) obtained is in accord with experimental observations. Below the transition temperature the system behavior depends on the path through which the state was reached, suggesting that simulation of relaxation effects could be achieved in longer runs.