Free energy analysis of polymer crystal nucleation indicates cooperative crystallization and nematic alignment

Despite the prevalence of semicrystalline polymers and their long manufacturing history, the fundamentals of crystallizing a polymer melt are still debated. In particular, recent observations have called into question the validity of classical models of primary nucleation. Several competing newly-proposed models aim to explain these observations by incorporating more polymer features. These near-equilibrium theories behave in accordance with a postulated free energy landscape (FEL). To qualify their applicability, we recently performed calculations of the FEL of oligomer crystal nucleation using advanced Monte Carlo methods. 2D FELs of crystallizing polymers show two basins (equilibrium states), one at low crystallinity and nematic alignment (liquid) and the other at high values of both (crystal). Extracted minimum free energy paths (MFEP) indicate that crystallinity and nematic alignment increase cooperatively during nucleation. We further used the MFEP to extract the nucleation barrier height, which enables calculation of nucleation rates and comparison to experiment. These methods demonstrate quantitative methods for probing nucleation behavior of polymer systems more rigorously, which will aid in validation of future proposed theories and their reconciliation with experiment.