(291c) Molecular Simulation of Flow-Enhanced Nucleation in Monodisperse and Bidisperse Alkane Melts

The high molecular weight tail has been widely observed to have a dramatic impact on the kinetics of polymer crystallization under flow. Experiments have shown that the crystallization rate drastically increases when the strain rate is large enough to stretch and orient the longest chains present in the melt. The mechanism by which these long chains accelerate crystallization at the nucleation stage, however, is not well-understood due the fact that experiments lack the spatiotemporal resolution to observe a nucleation event. Molecular simulation, on the other hand, has proven to be a useful tool for observing the nucleation stage in polymer melt systems. Using non-equilibrium molecular dynamics, we studied flow-enhanced nucleation in a bimodal mixture of 150 carbon (C150) and 20 carbon (C20) n-alkanes in order to observe and quantify the effect of presence of long chains. The mixtures studied ranged from 3-9 wt% C150. We found three regimes of nucleation associated with the relative magnitude of the applied strain rate, and longest relaxation rates of the chains in the melt. At strain rates below the inverse relaxation time of C150 and above the inverse relaxation time of C20, we did not observe a dramatic difference between the rates of nucleation in the bidisperse and monodisperse C20 melts. However, when the strain rate was larger than the inverse of the relaxation time for C150, but smaller than the inverse relaxation time of C20, we observed a templating effect in which the extended C150 chains served as templates for the nucleation of C20 crystals. This templating effect resulted in a dramatic increase in the nucleation rate as compared to nucleation under similar conditions for monodisperse C20 melts.