Spherulitic nucleation and growth rates in a sheared polypropylene melt

Abstract

In common polymer processing operations such as injection molding, film blowing, and fiber spinning, the molten polymer is subjected to intense shear and/or elongational flow fields and crystallizes during or after the application of flow. The semicrystalline morphology that develops in the final product is typically very different from what is observed during quiescent crystallization of the same polymer, and the properties change accordingly. The possibility of controlling the final morphology and the resulting mechanical and functional properties of semicrystalline polymers based on the study of polymer melt crystallization stimulated by flow is highly intriguing. This work starts from the experimental evidence that there exists qualitatively three regimes of crystallization under shear: (a) very low shear rates, in which there is no effect on kinetics; (b) higher shear rates, in which orientational effects enhance just the nucleation and growth rates, and spherulitic crystallization is observed; and (c) high shear rates, in which molecular stretching occurs giving rise to a fibrillar morphology development under very fast kinetics. The first two regimes are explored and analyzed by means of experimental protocols developed on purpose. In particular: -spherulitic nucleation and growth rates under continuous shear rates were carefully measured and related to molecular strain -the condition below which crystallization turns out to be essentially quiescent was evidenced.

Effect of the application of low shear rates on the crystallization kinetics of PLA

“…Considering the error bars, the nucleation density is well described by a linear correlation with shearing time, which is consistent with a constant nucleation rate under constant shear rate, in agreement with literature indications. [ 31 ]…”

Polymer crystallization under external flow

“…In addition to nucleation rate, anisotropic growth of precursors which would generate nuclei of varying sizes and shapes should be considered, especially for the strong flow condition. Moreover, the crystal growth is of central importance [245,246], which together with nucleation kinetics to control the final microstructure of materials [247,248]. Several works have been contributed to connect all these steps and model the FIC during real processing, i.e.…”

Image Analytical Determination of the Spherulite Growth in Polypropylene Composites

“…The measurement is however not a local one and moreover requires a high calibration effort. Even more recently, spherulite radia are still measured manually from image data (De Santis et al, 2014;Nomai et al, 2015). However, this procedure is extremely time consuming.…”