The equilibrium melting temperature Tm is a challenging experimental benchmark for molecular dynamics (MD) simulations of polymer melting. MD heating scans of α-phase isotactic polypropylene (α iPP) can exhibit superheating of over 100C. This superheating has been attributed to periodic boundary conditions and ultrafast heating, both of which impede nucleation of the melt. We have developed a simple method to minimize superheating: we replace the usual periodic crystal structure with a periodic array of finite thickness slabs, separated by vacuum gaps. In our 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 proportional to T-Tm. At a constant heating rate, this results in a quadratic rise in the system energy versus temperature. We obtain Tm as the onset of this quadratic rise in system energy, which corresponds well to the experimental melting points for iPP oligomers. The same simulations give reasonable values for the crystal-vacuum interfacial free energy, from the energy difference between crystalline slabs and periodic crystals. As an application of our method, we investigate a hypothetical structure for oligomeric iPP rotator phases (a hexagonal packing of helices of random handedness), by melting the phases. Comparing Tm and heat capacity values for rotator and crystal phases, we can assess how nearly stable the rotator phases are.

MD simulations of crystal and rotator phase melting in oligomeric iPP

251st National Meeting (2016)

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