(724e) Molecular Modeling of Nucleation Under Shear and Extension in Short- and Long-Alkane Melts

During melt polymer processing, macromolecules become perturbed from their equilibrium state due to flow-induced orientation and stretching. In semicrystalline polymers, this results in a dramatic increase in the rate of crystal nucleation as compared to the quiescent case. The change in nucleation rate has the potential to change the resultant morphology and, in turn, alter the properties of the material. Since flow-enhanced nucleation is tied to the conformational response of chains under flow, it is dependent on the specifics of the flow field and the material itself. In order to understand better this effect, a computational study was undertaken to observe and quantify the effects of both shear and extensional flow on the kinetics of nucleation in systems containing either short C20 or long C150 chains using non-equilibrium molecular dynamics. Shear simulations were conducted using the traditional Lagrangian-Rhomboid method, and recently-developed boundary conditions were used to study uniaxial extension. Faster nucleation was observed as the strain rate was increased, and extensional flow was found to result in a more drastic increase at a given rate.  In both the short- and long-chain systems, the flow-enhanced nucleation effect was found to emerge when the strain rate approached the intrinsic relaxation rate of a chain. Additionally, the nucleation events were interpreted in terms of a stochastic birth-death process, yielding quantitative information regarding the effect of flow on the nucleation rate and the critical kinetic parameters of the process, namely the critical nucleus size and free-energy barrier. Both critical parameters were observed to decrease with increasing strain rate, in accordance with a mechanism in which the free-energy barrier is lowered by the increase of flow-induced entropy in the melt prior to nucleation.