| Crystallization that occurs during or after a strong shear is important in polymer processing due to its strong effect on product properties and production rate. This study addresses several major gaps in our knowledge of this phenomenon by investigating shear-induced crystallization of nucleated isotactic polypropylenes by simultaneous measurement of light intensity and rheology using a sliding plate rheometer at high, uniform shear rates. An optical fiber probe was devised for the light intensity measurement. For the conditions studied, both measurements were found equally able to detect nascent crystalline structures, but light intensity is more suitable for monitoring changes during the early stages, whereas rheology is more useful for tracking the late kinetics. The relative influence of a melt-insensitive nucleating agent, molecular weight, shear rate, and strain were studied under isothermal, low-supercooling conditions following brief shearing. In contrast to that in quiescent crystallization, the nucleation pathway of nucleated polymers after strong shear was found to be governed not by the nucleating agent but by the molecular weight. The primary effect of shear was confirmed to be in inducing nucleation and is much weaker in changing growth kinetics. Shear and nucleating agent both shorten the induction time, but the effects were found not to be additive. With increasing shear rate or strain, crystallization was found to first accelerate as a result of the increase in the number of point-like nuclei, then saturate due to either slip or consumption of high-molecular weight components for the creation of nuclei, and finally accelerate again at the onset of the transition from spherulitic to highly oriented morphology. Both a critical shear rate and a critical strain are necessary to initiate this morphological transition. The product of shear rate and strain was found useful for describing the acceleration in crystallization associated with the increase in point-like nuclei. Specific work and the Weissenberg number are inadequate for characterizing kinetic and morphological changes. The complex interplay between nucleating agent, shearing conditions and molecular weight can lead to segregation of crystalline structures, resulting in non-uniformity in the bulk samples. |
