New Physical Insights into Shear History Dependent Polymorphism in Poly(vinylidene fluoride)

Abstract

Poly­(vinylidene fluoride) (PVDF) is a semicrystalline polymer that exists in four crystalline phases (α, β, γ, δ). Among these, the β-phase has received tremendous techno-commercial importance due to its higher dipole moment as compared to the other phases, and thus many strategies have been explored in the recent past to obtain the β-polymorph of PVDF. In this study, the effect of shear history on crystalline morphology and behavior of PVDF has been investigated systematically by polarized optical microscopy coupled to a hot stage, Fourier transform infrared spectroscopy, differential thermal analysis, rheometry, and dielectric relaxation spectroscopy. Thin films of PVDF (120–150 μm) were sheared at different temperatures ranging from 155 to 220 °C and were allowed to isothermally crystallize at 155 °C. When the samples were isothermally crystallized at 155 °C, a remarkable increase in β-phase content was observed. More interestingly, this phenomenon was observed to be shear history dependent. For instance, the samples which were sheared at high temperature (220 °C) reflected in higher β fraction as compared to samples which were sheared at lower temperature (155 °C). It is envisaged that the distance between T shear (temperature at which the samples were sheared) and T cry (crystallization temperature) significantly influences the content of the β-phase in PVDF. This study clearly demonstrates the fact that both shear history and the depth from T c influence the conformational changes in PVDF.

Friction‐induced electroactive β polymorph of poly(vinylidene fluoride)

“…Apparently, the fractions of β form at PS domains are higher than that at the MF domains. Besides, fractions of β phase at both PS and MF domains simultaneously decrease with time, attributed to the increasing mean surface temperatures …”

“…Under this situation, the dominant formation mechanism of β crystal has been shear‐induced crystallization, rather than PS. However, the nucleation and growth of α phase is also accelerated to depress crystallization of β polymorph, arising from high shear rate (10 5 –10 8 s) . As a consequence, fraction of β polymorph obviously decreases in comparison with that under PS.…”

“…On the other hand, the shear history exhibited a noticeable influence on direct formation of β phase by crystallization from melt . Under the quiescent condition, β ‐ crystal content was only ∼38% by crystallization.…”

Impact of vanadium‐, sulfur‐, and dysprosium‐doped zinc oxide nanoparticles on various properties of PVDF/functionalized‐PMMA blend nanocomposites: Structural, optical, and morphological studies

“…It found in five crystalline polymorphs (α, β, γ, δ, and ε) based on crystalline dipoles and each owing characteristic nature. The nonpolar α polymorph is most stable with trans‐gauche‐trans‐gauche (TGTG) conformation while polar β polymorph is most remarkable due to its piezoelectric and pyroelectric properties with crystal TTTT (trans zigzag) conformation and δ (TGTG conformation) and γ (T3GT3G conformation) . Apart from these merits, there are some demerits also associated with PVDF, that is, poor matrix for the dispersion of immiscible particles, high melt viscosity, and low transparency due to semicrystalline nature .…”

Capturing the Role of Temperature and Entropy in Crystal Polymorphs Through Molecular Simulations

“…In this instance, polarity of the solvent kinetically biases the molecule in to one specific molecular packing. Other environmental factors that can influence the synthesized crystal structure include temperature [26,29,44,75], pressure [76][77][78][79][80][81], humidity [47,56,82,83], impurities [84][85][86], impeller speed, and other shear forces [27,52,75,85,87].…”