UBC Theses and Dissertations
Molecular dynamics simulation of polymer crystallization process Triandafilidi, Vasilii
Large scale molecular dynamics simulations were carried out to study the kinetics of polymer melt crystallization. A coarse-grained model CG-PVA developed by Meyer and Muller-Plathe is applied. A new algorithm for analyzing crystallization is proposed. It is based on the alignment of individual chains which speeds up previous similar calculation by a factor of ten. Moreover, it is found to be more suitable for investigating chain crystallinity in polydisperse systems. Different thermodynamic protocols of polymer crystallization were studied: deep quench, shallow quench and cooling with various rates, as well as polymer pre-stretching and consequent cooling and quenching. Cooling with the slowest rate was shown to generate the highest terminal crystallinity values. Resulting curves were fitted using the Avrami equation that showed good agreement at the early stages of crystallization. As a result shorter chains were found to exhibit higher terminal crystallinity value than the longer ones. Pre-stretching and subsequent quenching was found to have a minor effect on thefinal crystallinity, whereas pre-stretching followed by an intermediate rate cooling was found to increase the terminal crystallinity. The effect of polydispersity was modeled via two bidisperse melts comprising of different proportion of short and long chains. Due to the presence of two relaxation times in the melt, initial stages of bidisperse polymers crystallization were found to be dominated by the short chains, whereas the final stages were dominated by the long ones. Further investigation concluded that the behavior of bidisperse melts is governed by the proportion of short and long chains in the melt. When a critical fraction of the long chains was reached, they appeared to act as baby nuclei for the short chains to attach themselves onto resulting in bundle-like fringed micelle structures. Otherwise, they acted as "molecular traps" hindering crystallization of the short chains. When a critical fraction of the short chains was reached, they were found to assist crystallization of the long chains at the initial stages of crystallization but impede crystallization dynamics at the final stages.
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Attribution 2.5 Canada