Critical Insights into the Effect of Shear, Shear History, and the Concentration of a Diluent on the Polymorphism in Poly(vinylidene fluoride)

The effect of shear, shear history, and addition of poly(methyl methacrylate) (PMMA) in inducing the β-polymorph in poly(vinylidene fluoride) (PVDF) was investigated systematically for melt-mixed PVDF/PMMA blends. Various techniques such as polarized optical microscopy (POM) coupled to a hot stage and a shear cell, Fourier transform infrared spectroscopy (FTIR), differential thermal analysis, melt-rheology, and dielectric relaxation spectroscopy were used to gain a mechanistic insight for the observed polymorphism in PVDF. Different PVDF rich blends were prepared by melt-mixing and were subjected to different shear histories on a rheometer, and the induction time was monitored with respect to PMMA content and the applied shear flow in the blends. The rheological measurements revealed that the induction time was significantly lower for blends with a higher PVDF content (≥80 wt %), which was ascribed to the diluent effect of PMMA that restricts the chain mobility of PVDF and a longer time is required to start the crystallization process. The crystalline morphology observed from POM demonstrates that the growth rate of spherulites was greatly reduced with increasing PMMA content in the blends. FTIR results were used to determine the amount of β phase in the blends before and after the shear history. The blends that were sheared at high temperature (220 °C) showed more β phase than the blends that were sheared around the crystallization temperature. This study clearly demonstrates the fact that shear, shear history, and the content of PMMA significantly influence the conformational change that results in phase transformation in PVDF. Further, this study will help guide researchers working on various aspects of polymer processing where the effect of blending and shear on polymorphism is very important.