Free Energy Analysis of Crystal Nucleation of Semiflexible Polymers

Despite the prevalence of semicrystalline polymers and their long manufacturing history, there are still many fundamental questions surrounding the process of crystallization in a polymer melt. Specifically, recent simulations and experiments have called into question the validity of classical theories of the nucleation process. Emerging competing theories postulate the existence of stable or metastable intermediates, including nematically aligned states. We look more closely at the crystallization of polymers in a melt using advanced Monte Carlo methods such as Wang-Landau sampling and expanded ensemble density of states. Specifically, we explore the nucleation behavior of a family of models of semiflexible oligomers and use these methods to compute phase diagrams, free energy landscapes, and minimum free energy paths. Consistent with emerging theories, we find that models with continuous potentials have a universal phase diagram that contains an intermediate nematic phase that mediates nucleation. However, models with discontinuous potentials have a cooperative phase transition that does not have an intermediate nematic phase. These results further our understanding of crystal nucleation of polymers and improve our ability to compare molecular simulations to experiment.