Molecular dynamics study of the rate of melting of a crystalline polyethylene molecule: Effect of chain folding

Molecular dynamics is used to study the melting on the surface of a polyethylene-like crystal. The rate constant for melting of a crystalline molecule without and with one to four folds is determined at several different temperatures and molecular lengths. The results show a strong dependence of the transition rate on the number of folds. For a constant lamellar thickness, the transition rate decreases with increasing number of folds for temperatures near the equilibrium melting temperature, as expected from analogy with experimental melting temperatures. In contrast, the transition rate increases with increasing number of folds for temperatures that exceed the equilibrium melting temperature by more than 100 K. Two melting paths are suggested to explain the simulation data. One pathway involves a competition between melting and crystallization. This pathway leads to a decreasing transition rate as a function of increasing folding. The second pathway exhibits dominating melting. In this case, the rate of transition tends to increase with increasing number of folds. Diffusion coefficients of segments at different locations along the chain show that motion of the ends of a polymer chain or of the folds is faster than in the center of the stem. The overall effect of increasing temperature is to increase the diffusion coefficients.