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

Abstract

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.

Mechanisms of Flow-Induced Multistep Structure Evolution in Semicrystalline Polymers Revealed by Molecular Simulations

“…One of the fascinating aspects of studying polymer nucleation induced by flow is its multiscale and multistep processes, as demonstrated in Figure and supported by previous studies. − ,,,,,,,,− ,,,,− The corresponding multiscale structural changes range from conformation of local segments to deformation of entire chains, and the multistage changes include conformation orders, orientation orders and crystal orders proposed by Li’s group. − , In other words, the shish and kebab formation processes are continuous ones, which contain different steps: conformational transition/segmental orientation/chain extension, aggregation of oriented segments, formation of precursors, formation of oriented crystal nuclei or shish, and formation of kababs. − ,,,,,,,,− ,,,,− First, under the action of flow, chain segments will gradually undergo orientation and conformational changes, while entire chains will be extended. − ,− ,…”

“…In a quiescent state, it is theorized that polymer chains crystallize via intramolecular chain-folding nucleation model, − ,− yielding folded-chain lamellar crystals within systems. − ,− However, when subjected to shear or stretching forces, various other crystalline morphologies emerge, such as oriented crystals, shish-kebab crystals, or extended-chain crystals. − ,,,,,,,,,− ,− ,− The formation of these distinct crystalline structures is likely governed by different nucleation modes. − ,− , For instance, Hu’s group analyzed polymer nucleation modes during stretching at different temperatures using dynamic MC simulations. − ,,,,,, In Figure a, adjacent chain-folding probability in small crystallites under different temperatures remains almost unchanged, but decrease after some critical strains . At low strains, polymer chains crystallize predominantly via the nucleation mode of intramolecular chain-folding but change to the nucleation mode of intermolecular fringed-micelle above critical strains.…”

“…Generally, two-thirds of polymer materials produced each year can crystallize, which have wide applications in various fields of the national economy. The exceptional versatility of semicrystalline polymers stems from the complexity of semicrystalline structure, containing both crystalline and amorphous regions. − The crystalline component imparts sufficient mechanical properties to the polymer materials, while the amorphous part allows for a certain degree of deformability. − ,,,, Studies have further shown that semicrystalline polymers’ properties are influenced by crystal structure characteristics, including crystallinity, crystal size, crystal orientation, and crystalline morphology. − …”

“…Moreover, it has been found that crystal structures of polymers could be tuned by adjusting the processing conditions of semicrystalline polymers. ,− ,− During the polymer processing, polymer melts experience external flow, which affects the subsequent crystallization process. ,− ,− For example, polymer melts undergo shear forces in extruders or injection molding machines, leading to chain segment orientation and elongation of the entire chains. − ,,,,,− ,,,− These oriented polymer melts exhibit significantly different crystallization behaviors compared to the melts under quiescent state, including faster crystallization rates, higher orientation degree of crystals, and appearance of new crystalline morphologies (e.g., shish-kebabs). − ,,,…”

“…In light of this, establishing the relationship between polymer processing conditions and polymer crystal structure characteristics is essential for effectively controlling the mechanical properties of polymers. ,,− ,,,,,,,,, Consequently, numerous scientists have dedicated themselves to elucidating the mechanisms through which polymer processing conditions influence polymer crystallization, yielding fruitful research results. ,,− ,,,,,,,,, Although the mechanisms of polymer crystallization induced by flow, particularly the nucleation mechanism, have not been fully elucidated, progress is being made toward revealing the ultimate “truth”. − An increasing body of experimental and simulation results displays that multistep and multiscale processes exist in flow-induced polymer crystallization. − ,,,− Specifically, flow-induced polymer crystallization encompasses processes such as conformational transition/segmental orientation/chain extension, aggregation of oriented segments, formation of precursors, formation of oriented crystal nuclei or shish, and formation of kababs (crystal growth). − ,,,− …”