Atomistic Details Of Disordering Processes in Superheated Polymethylene Crystals II. Effects of Surface Constraints

Abstract

ABSTRACTAtomistic details of disordering in superheated polymethylene crystals have been studied using full molecular dynamics simulations of crystals containing 9600 CH2-groups. The crystal size was about 227 nm3 Simulations were carried out for up to 100 ps, starting at temperatures about 100 K above the melting temperature. Typically 1.5 h of CPU time on a Cray X-MP were necessary per ps simulation. Superheating causes a quick development of large-scale disorder throughout the crystal, including reorientation, translation, and the destruction of crystal symmetry. This is followed ultimately by surface Melting. Crystallization centers with hexagonal packing are found in superheated, unconstrained crystals. On cooling during the simulation, recrystallization processes compete with the disordering, resulting in a reorientation of the molecular chains and reorganization of the crystal. Neither the fully amorphous phase nor the ordered crystal are reached during these short-time simulations using an instantaneous temperature increase to above the melting temperature, followed by a slow cooling into the crystallization temperature region.