Free Surfaces Overcome Superheating in Simulated Melting of Isotactic Polypropylene

The equilibrium melting temperature T_m is a challenging experimental benchmark for molecular dynamics (MD) simulations of polymer melting and crystallization. T_m obtained from MD heating scans of α phase isotactic polypropylene (αiPP) can exhibit superheating of over 100 °C. 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 T_m, the interface advances into the crystal, with a velocity proportional to T - T_m. At a constant heating rate, this results in a quadratic rise in the system energy versus temperature. We obtained T_m as the onset of this quadratic rise in system energy, which corresponds well to the experimental melting points. The same simulations give reasonable values for the crystal-vacuum interfacial free energy, from the energy difference between crystalline slabs and periodic crystals.