Gelation kinetics of PVDF/GVL thermoreversible gels: A rheological and thermal analytical overview

Poly(vinylidene fluoride) (PVDF) is a commodity polymer that shows a variety of excellent properties such as piezoelectricity, pyroelectricity, hydrophobicity, and biocompatibility. PVDF is either processed as a melt or a solution to fabricate materials for different applications as films or fibers. Extremely hazardous solvents like dimethylacetamide (DMAc), dimethylformamide (DMF), and N-methyl-2-pyrrolidone (NMP) are commonly used to process PVDF solutions. As an alternative solvent, we are investigating γ-valerolactone (GVL) which is a biorenewable, biodegradable, and water-soluble solvent derived from hydrolysis of levulinic acid. Several studies have shown that PVDF dissolves in GVL quite well above a temperature of 60 - 65 °C. However, at lower temperatures, PVDF/GVL solutions form a thermoreversible gel. In this study, we investigated the rheological and thermal properties of the solutions during the sol-to-gel and gel-to-sol transition range to understand their gelation kinetics. We prepared thin films of PVDF using the PVDF/GVL solutions and characterized their crystalline structure using wide angle X-ray scattering and Fourier-transform infrared spectroscopy. These studies show that PVDF crystallizes into its polar phases (the electroactive β- and γ-phases) from GVL solutions.