An assessment of models for flow-enhanced nucleation in an n-alkane melt by molecular simulation

Flow-enhanced nucleation of the crystal phase under shear and uniaxial extension for a monodisperse melt of n-pentacontahectane (C150H302 or C150) chains was studied by nonequilibrium molecular dynamics simulation. The resulting acceleration in the crystal nucleation rate was correlated with macroscopically measurable properties of the flow field and with microscopic conformational statistics. Based on the fidelity of the observed correlations, several empirical models reported in the literature were evaluated for their abilities to account for the observed enhancement of the nucleation rate due to flow, and new models are proposed for data that do not comport with existing models. In agreement with prior reports, the nucleation rate was found to correlate well with first-normal stress difference, the second invariant of the deviatoric conformation tensor, and the stretch ratio, albeit with some differences from the existing models. New models based on conformational invariants for Kuhn segments are proposed and shown to describe the simulation data more accurately than those based on conformational behavior of entire chains. Within the applicability of the stress-optical rule, related models are proposed based on invariants of the extra stress tensor