Identifying the Factors Governing Flow-Enhanced Nucleation in n-Alkanes with Molecular Simulation

The application of a flow field is known to drastically accelerate the rate of crystal nucleation in semicrystalline polymer materials. Using non-equilibrium molecular dynamics (NEMD), nucleation studies are performed under shear and uniaxial extension for monodisperse melts of short (C20) and long (C150) alkanes, as well as for bimodal mixtures composed of both short and long chains. These studies reveal how the acceleration in the nucleation rate correlates with macroscopic measureable quantities and conformational statistics of the flowing melt. The observed correlations are used to evaluate of the capacity of various models for flow-induced nucleation to describe the NEMD data. It is observed that the monodisperse nucleation rate for C150 shows excellent agreement with a literature model based on the stretching of the end-to-end vector across the range of strain rates and flow fields studied. A similar correlation is found for the bimodal mixtures, however it is the amount of local stretching on the Kuhn segment length scale that correlates with the nucleation rate across the range of flow conditions and melt compositions. Based on this result, a mechanism in which crystalline clusters are formed from locally stretched chain segments is proposed.