Interlude of metastability in the melting of polymer crystals

We have studied the process of melting of polymer crystals using Langevin dynamics simulations with a coarse-grained united atom model. We have considered two ideal situations: one in which a single crystal melts and the other in which a multichain crystal melts. We show that the melting of the single crystal proceeds through a globular metastable state, which is followed by expansion to a more random coil-like state. Similarly, the melting of the multichain crystal reveals a special mechanism comprising two steps: one in which a long-lived partially molten metastable state is formed, followed by a second step in which the chains peel off from the crystalline core to a free state. We elucidate the nature of the metastable state close to the equilibrium melting temperature and show that the multichain crystals equilibrate to states of intermediate order, with the extent of ordering decreasing as we increase the melting temperature. We quantify the kinetics of melting by estimating a free energy landscape using parallel tempering Langevin dynamics simulations. These simulations reveal a metastable state in the single molecule systems, allowing us to estimate the free energy barriers. Additionally, the melting of the multichain crystals reveals the existence of two barriers, with the preference for the intermediate state reducing with increasing temperature. We compare our findings to the existing experimental evidence and find qualitative agreements.