Atomistic 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.