Free surfaces overcome superheating in simulated melting of isotactic polypropylene*

The equilibrium melting point ($T_m$) is a challenging experimental benchmark for molecular dynamics simulation of polymer melting and crystallization. $T_m$ obtained from melting simulation of $\alpha$ phase isotactic polypropylene (iPP) can exhibit superheating of over 100$^{\circ}$C. Superheating has been attributed to the use of periodic boundary conditions and ultrafast simulated heating rates, both of which inhibit melting. We have developed a simple method to overcome 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 nucleation of the melt phase. Above $T_m$, we observe that the melting front advances into the crystal with a velocity proportional to $T-T_m$. This correspond to a quadratic rise in the system energy versus temperature, at constant heating rate. We obtain $T_m$ as the onset of this quadratic rise in energy, which give values consistent with experimental melting points for iPP oligomers. The same simulations allow reasonable estimates of the crystal-vacuum interfacial free energy, from the energy difference between crystalline slabs and periodic crystals.