Effect of chain stiffness on the dynamics and microstructure of crystallizable bead-spring polymer melts

摘要

We constrast the dynamics in model unentangled polymer melts of chains ofthree different stiffnesses: flexible, intermediate, and rodlike. Flexible androdlike chains, which readily solidify into close-packed crystals (respectivelywith randomly oriented and nematically aligned chains), display simple meltdynamics with Arrhenius temperature dependence and a discontinuous change uponsolidification. Intermediate-stiffness chains, however, are fragileglass-formers displaying Vogel-Fulcher dynamical arrest, despite the fact thatthey also possess a nematic-close-packed crystalline ground state. To connectthis difference in dynamics to the differing microstructure of the melts, weexamine how various measures of structure, including cluster-level metricsrecently introduced in studies of colloidal systems, vary with chain stiffnessand temperature. No clear static-structural cause of the dynamical arrest isfound. However, we find that the intermediate-stiffness chains displayqualitatively different dynamical heterogeneity. Specifically, their stringlikecorrelated motion (cooperative rearrangement) is correlated along chainbackbones in a way not found for either flexible or rodlike chains. Thisactivated "crawling" motion seems to be the cause of the dynamical arrestobserved for these systems, and shows that the factors controlling thecrystallization vs.\ glass formation competition in polymers likely dependnonmonotonically on chain stiffness.