Length Dependent Crystallization of Linear Polymers under Different Cooling Rates: Molecular Dynamics Simulations

The crystallization behavior of polymers is significantly influenced by molecular chain length and the dispersion of varying chain lengths. The complexity of studying crystallization arises from the dispersity of polymer materials and the typically slow cooling rates. Recent advancements in fast cooling techniques have rendered the investigation of polymer crystallization at varying cooling rates an attractive area of research; however, a systematic quantitative framework for this process is still lacking. We employ a coarse-grained model for polyvinyl alcohol (CG-PVA) in molecular dynamics simulations to study the crystallization of linear polymers with varying chain lengths under variable cooling rates. Monodisperse, bidisperse and polydisperse samples are simulated. We propose two formulae based on a two-phase assumption to fit the exothermal curves obtained during cooling. Based on these formulae, better estimations of crystallization temperatures are obtained and the effects of chain lengths and cooling rates are studied. It is found that the crystallization temperature increases with chain length, similar to the Gibbs-Thomson relation for melting temperature, indicating a strong relation between fast crystallization and glass formation in linear polymers. Extrapolation to the infinitely slow cooling rate provides an easy way in simulations to estimate the equilibrium crystallization temperature. The effective chain lengths of polydisperse and bidisperse samples are found to be the number-averaged chain lengths compared to the weight-averaged ones. The chain length-dependent crystallization exhibits crossover behavior near the entanglement length, indicating the effects of entanglements under fast cooling conditions. The effect of chain length dispersity on crystallization becomes more obvious under fast cooling conditions.