Process mediated polymorphism, crystallographic texture and structure-property correlation in crystalline/amorphous blends

Abstract

Specific interactions between the blend components (especially in crystalline/amorphous blends) induce miscibility under certain conditions However, depending on the processing history and the composition, the crystalline component crystallizes in different polymorphs. Herein, the effects of the amorphous content and various processing conditions on the phase transformation and the resulting texture were systematically assessed in blends of polyvinylidene fluoride and poly(methyl methacrylate). To this end, various blends were prepared by melt mixing and were subjected to different processing conditions like rolling, poling, and rolling followed by poling. To assess the different polymorphs of the crystalline component (polyvinylidene fluoride) on account of the processing history, spectroscopic, X-ray diffraction, thermal, mechanical and segmental transitions were systematically investigated and an attempt was made to correlate the observations with the resulting texture in the blends. The crystallographic texture was evaluated using X-ray diffraction and the mechanical properties as a result of the texture were studied by dynamic mechanical analyzer. The spectroscopic techniques revealed predominantly α phase in as-pressed samples, irrespective of the amorphous content However, interestingly, after rolling, the blends showed predominantly β phase. More interestingly, the highest β phase content (>95%) was obtained for blends that were rich in the crystalline phase which were deformed up to a strain of 80%. It was observed that higher the strain, lower the relaxation leading to stronger texture and facilitated the formation of β polymorph. This phase transformation after rolling was accompanied by an increase in the degree of crystallinity and storage modulus. The dielectric response revealed that the samples that were initially rolled followed by poled showed maximum dielectric constant and low dielectric loss as compared to the rolled or compression molded samples. This study evidently demonstrates that various processing conditions and the content of the amorphous phase significantly influenced α to β transition, crystallographic texture and mechanical properties and the dielectric response of the blends. Moreover, the mechanical relaxations suggest that relaxations originating from crystalline defects shifts to higher temperatures upon rolling correlating well with the observations that were made using calorimetric transitions.