Free Surfaces Overcome Superheating in Simulated Melting of Isotactic Polypropylene
The equilibrium melting temperature Tm is a challenging experimental benchmark for molecular dynamics (MD) simulations of polymer melting and crystallization. Tm obtained from MD heating scans of a phase isotactic polypropylene (aiPP) can exhibit superheating of over 100 degC. This superheating has been attributed to the combined effects of periodic boundary conditions and ultrafast heating rates, both of which inhibit nucleation of the melt. We have developed a simple method to minimize this superheating; we replace the periodic crystal structure with a periodic array of finite thickness slabs, separated by vacuum gaps. Thermal disorder at the slab surface promotes melting by reducing the melt nucleation barrier. For experimental comparison, we synthesized and measured the melting temperatures of a series of low molecular weight iPP oligomers. In simulations of slab systems, melting initiates at the free surface; as the temperature rises above Tm, the interface advances into the crystal, with a velocity pr.