Morphology, structure, rheology, and thermodynamics of piezoelectric poly(vinylidene fluoride)-ethylene carbonate thermoreversible gel

Abstract

Poly(vinylidene fluoride) (PVF₂) produces a thermoreversible gel with the important solvent, ethylene carbonate, which is used as a dispersing medium in a lithium ion battery. This is evidenced from the fibrillar network morphology and reversible first-order phase transition of the gels. The invariance of the storage and loss modulus with frequency at lower temperatures (≤110 °C) and the existence of a crossover point between them at higher temperatures further supports thermoreversible gel formation in the system. Solvent subtracted FTIR spectra of the gels indicate formation of a β polymorph, which is retained even after drying using cyclohexane as a guest solvent. The wide-angle x-ray scattering (WAXS) diffractogram of the dried gel also indicates formation of the piezoelectric β polymorph, giving a first time report of thermoreversible piezoelectric gel formation. The temperature-concentration phase diagram in conjunction with Tamman's plot indicates the presence of a polymer-solvent molecular compound whose stoichiometry is around 1.2 solvent molecules/monomer unit. The presence of eutectic and metatectic transitions in the PVF₂- ethylene carbonate (EC) system is also evidenced from the phase diagram. Time-resolved X-ray diffraction patterns also show a new peak at q =13.15 nm^-1 which is assigned to the molecular compound on the basis of a possible crystalline lattice. Molecular modeling using molecular mechanics calculations with the MMX program supports polymer-solvent complex formation. It also indicates that electrostatic and H-bonding forces are operating side by side on the carbon skeleton fixing the all trans chain conformation of β PVF₂ in the gel.