Flow-Induced Crystallization Starting from Self-Nucleated Precursors

Abstract

Flow-induced crystallization is a multistep process, in which precursors are often generated as a crucial ordering state intermediate between the initial amorphous state and the ultimate crystallite. In this work, the self-nucleation effect of isotactic polybutene-1 was employed to obtain the ordering precursors and to study flow-induced crystallization that started from both the self-nucleated precursors and the completely relaxed amorphous phase. This could answer the long-standing question whether the additional self-nucleated precursors behave as the flow-induced ordering structures to facilitate crystallization or are erased back to the completely relaxed melt under the action of flow. The results showed that for quiescent crystallization, the self-nucleated precursors effectively accelerated kinetics and exhibited a monotonously increasing dependence with decreasing self-nucleation temperatures over a broad range of 117−180 °C. Interestingly, with flow, the self-nucleated precursors were able to accelerate crystallization significantly by facilitating the initial step of generating local ordering precursors. In addition, it was unexpected to find a type of unique precursor through self-nucleation at 137 and 138 °C. Such self-nucleated precursors remained unavailable for quiescent crystallization but could be activated by flow, referred to as dormant self-nucleated precursors in this work. To the best of our knowledge, the dormant self-nucleated precursors, which have nonfunctionality under the quiescent condition but can be visualized by flow, were reported for the first time. Moreover, the flow-induced evolution of various self-nucleated precursors was investigated by controlling the shearing duration. It was found that the self-nucleated precursors can develop into oriented nuclei, accelerating crystallization kinetics and increasing the crystallite orientation.

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. − ,− ,…”

“…While experimental findings have demonstrated the emergence of precursors in flow field, their structural details remain a puzzle to researchers. − ,,,,,,,,− ,,,,,, Both Kanaya’s group ,, and Matsuba’s group , observed that precursors can contain a small number of crystallites in melts subjected to relatively high shear rates. Molecular simulations also offer insights into the microstructure details of the precursors.…”

“…That is, reducing stretching rate is beneficial for the crystallization process of polymer network systems. In free-chain systems, there is a competition between stretching rate and relaxation ability of polymer chains. ,,,,,,, In general, the systems with higher stretching rates can exhibit higher degree of segmental orientation (the stretching rate should be higher than the reciprocal of Rouse relaxation time; that is, the stretching effect overcomes the relaxation effect) and, thus, can have stronger nucleation/crystallization ability. ,,,, Namely, increasing the stretching rate facilitates the crystallization process in free-chain systems.…”

“…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). − ,,,…”

Flow-Induced Crystallization Coupled with Strong Memory Effect: Revealing Melt Heterogeneity

“…Interestingly, at relatively weak flow, recovery from the heterogeneous melt to the ideal homogeneous melt was enhanced through the accelerated re-entanglement process, and consequently, the flow-induced crystallization kinetics of the heterogeneous melt were unexpectedly reduced compared to those of the homogeneous melt. Our group exploited the enhanced phase transition from the tetragonal to trigonal phases of polybutene-1 to study the self-nucleated precursors and the coupled effect with flow . The results indicated that residual self-nucleated precursors starting from the heterogeneous melt act as crucial ordering structures to facilitate crystallization rather than destruction back to the completely relaxed melt, leading to much faster crystallization kinetics and higher orientation than those in the homogeneous melt with complete relaxation.…”