Flow-Induced Polymer Crystallization under Pressure and Its Engineering Application in "Structuring" Polymer Processing

Polymer processing is an essential step in achieving the desired shape and properties of polymer parts that are being broadly used in areas from packaging to aerospace. However, the design of the processing for new products is a matter of trial and error in practice. This is partly due to a limited understanding of polymer crystallization under realistic processing conditions, where strong shear flow and elevated pressure are coupled in an inhomogeneous manner. Unlike the well-established research topics that focus on polymer crystallization under the sole effect of pressure or shear flow, the subject of this Perspective is crystallization under processing conditions which include both, i.e., flow-induced crystallization under pressure (FICP). Here we focus particularly on recent advances in this field. Starting with a brief introduction of the special experimental devices for studying FICP, we discuss the remarkable interplay between flow and pressure and its effect on crystallization kinetics, crystal structure, and morphology. Accelerated crystallization, multiphase diagrams, and newfound crystalline morphologies may enrich the theory of polymer crystallization and require its further development. We then summarize the in situ studies of polymer crystallization under realistic processing conditions based on state-of-the-art characterization systems. This is followed by an overview of "structuring" polymer processing methods based on the theoretical understanding of FICP. The paper concludes with an assessment of the prospective future research directions.