Molecular simulation of the structural formation of mono- and bidisperse polyethylene upon cooling from the melts

Abstract

Monte Carlo (MC) simulation of the coarse-grained polyethylene (PE) model on the high coordination lattice was employed to investigate the structural formation of mono- and bidisperse PE chains upon stepwise cooling from the melts. The evolution of the ordered structure was analyzed in terms of the fraction of trans state, the length of trans sequence, the orientation correlation function at bond and molecular scale, the intermolecular pair correlation function, and the anisotropic change of chain dimension. Simulation data suggest that the rate of structure formation and the degree of crystallinity were decreased for the bidisperse PE mixture. In general, polymers in the mixture form a less-ordered structure, exhibit slower crystallization rates and have lower melting temperatures. For the monodisperse systems, the shorter chains exhibit an increase of the local bond properties including a fraction of trans conformation, intra- and intermolecular order parameters, but show a decrease in molecular properties such as chain orientation and chain packing. For mixed molecular weight systems, short-chain components are perturbed by the long chains resulting in a lower fraction of trans conformation, less dense structures, and a significant decrease in the degree of the bond orientation. Both short and long polymers can co-crystallize initiated at the same simulation time and finally reach the same magnitude of the chain order parameter.