Flow-Induced Crystallization of Self-Nucleated Poly(ε-caprolactone) with Molecular Hydrogen-Bonding Interactions

In this study, polar poly(ε-caprolactone) (PCL) was utilized to generate a self-nucleated melt with persistent hydrogen bonding. The mutual effects of flow and molecular interactions on the crystallization behavior of a self-nucleated melt were explored. Notably, the strength of self-nucleation could be precisely controlled through temperature adjustment (T_SN). The effective self-nucleation effect not only enhanced crystallization kinetics but also increased melt viscosity. These results indicate that the residual clusters with molecular hydrogen-bonding interactions, in addition to the topological entanglement network, act as the temporary structure to facilitate crystallization. Under continuous flow, a self-nucleated melt initiates and develops sufficient crystallization, leading to an improvement in rheological properties. Furthermore, the step-shear flow was also applied, and the following isothermal process was monitored to understand the influence of molecular interactions on crystallization. As flow duration extended, both the completely relaxed melts and self-nucleated melts experienced accelerated crystallization kinetics, with the latter consistently exhibiting a shorter half-time (t_1/2). Interestingly, the crystallite orientation of the self-nucleated melt initially increased and then gradually decreased with an extending flow duration, different from the monotonous decrease of t_1/2. Based on the above results, a hypothetical mechanism was proposed that some of the residual clusters comprising closely packed chain segments could be disturbed under severe flow conditions. The observed nonmonotonic correlation between crystallite orientation and flow duration suggests that the residual clusters of self-nucleated PCL melt or their corresponding developed nuclei are disturbed by the applied flow, offering insights into the temporary feature of flow-induced self-nucleation with molecular hydrogen-bonding interactions and physical network formation.