Flow Induced Crystallization: Insights from Molecular Simulation

Crystallization is an essential step in the processing of most polymers. It takes place under conditions of rapid cooling and high strain rate, and is strongly accelerated relative to quiescent conditions. The initial step in this process is flow-enhanced nucleation (FEN), which occurs on time and length scales that are hard to capture experimentally. To resolve this problem, we use nonequilibrium molecular dynamics simulations to characterize FEN from a melt of linear polyethylene-like chains. Both short and long (entangled) chains are simulated. First, methods are described for identifying the critical nucleation event using mean first-passage times (MPFT). Fitting of the data to a master equation is used to extract important thermodynamic and kinetic quantities. Results for nucleation kinetics accelerated under different modes of deformation, e.g. simple shear or uniaxial extension, and rates of strain are used to assess several of the existing models in the literature for flow-enhanced nucleation, based on their abilities to describe the data accurately, and new models based on the orientational ordering of Kuhn segments induced by flow are proposed. Evidence is presented for a breakdown of classical nucleation theory for entangled polymer melts at high strain rates, which in turn is traced to the flow-induced formation of nematic domains in the melt. The appearance of such domains suggests a different perspective on the underlying physics of flow-enhanced nucleation of long chain molecules.