Experiments have shown that flow drastically accelerates the nucleation rate in semicrystalline polymer materials; however due to the fact that nucleation occurs beyond the spatio-temporal detection limits, there is a lack of quantitative results on how the specifics of the flow field control the kinetics of nucleation. Using molecular simulation, we are able to observe nucleation at the small length scale at which it occurs, and under well-defined flow conditions. We performed nucleation studies using nonequilibrium molecular dynamics to impose steady-state shear and uniaxial extension on monodisperse alkane melts containing short C20 chains and long C150 chains. The results of these studies were mapped to a stochastic model, allowing us to quantify the dependence of important kinetic quantities on the flow conditions, including the free-energy barrier, critical nucleus size, nucleation rate, and a characteristic growth rate. As the strain rate was increased, the nucleation rate increased as expected, and the free-energy barrier and critical nucleus size decreased in accordance with a classical mechanism in which flow increases the free-energy driving force due to molecular orientation and stretching of the pre-crystallized melt. Additionally, we found uniaxial extension to be more effective than shear in promoting nucleation, and observed flow-induced effects to begin at a critical strain rate approximately equal to the inverse of the intrinsic relaxation time of the alkane chain.

Molecular simulation of flow-enhanced nucleation in alkane melts

251st National Meeting (2016)

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