Molecular and thermodynamics descriptions of flow-induced crystallization in semi-crystalline polymers

The flow-induced crystallization (FIC) is commonly encountered in the polymer industry as more than 70% of commercial polymers are crystalline, which needs to be processed before the final application. The complicated external flow field, i.e., shear or extensional flow, results in a great challenge in understanding the FIC phenomenon from both general thermodynamics and detailed molecular level aspects. The current tutorial first describes the general phenomenon of FIC from the aspect of different morphologies and enhanced kinetics induced by the flow. Second, characterization methods for monitoring FIC are introduced. Here, the in situ synchrotron x-ray scattering and non-equilibrium molecular dynamics simulation are selected as typical examples. Then, the theoretical descriptions of FIC are summarized from the aspects of molecular origin and thermodynamics. The coil–stretch transition theory and later developed stretched network theory are highlighted, where the former mainly accounts for polymer dilute solution and the latter for highly entangled cases. Also, multi-step features for the formation of various intermediate states during flow-induced nucleation are depicted. Despite non-equilibrium nature, the FIC can still be treated by thermodynamics, especially under weak flow conditions. The classic entropy reduction model is introduced together with later modifications. In all, understanding the fundamental mechanism of FIC is crucial for optimizing external processing parameters and internal molecular characteristics, and useful to guide current or further applied techniques.