Thermoreversible gelation of poly(vinylidene fluoride) in diesters: influence of intermittent length on morphology and thermodynamics of gelation

Abstract

Poly(vinylidene fluoride) (PVF₂) produces thermoreversible gels in diesters. By variation of the number of intermittent carbon atoms (n = 0-7) of the diesters, the physical properties of the gels are studied. The morphology of the PVF₂/diethyl oxalate (DEO) gel is spheroidal, but the morphology of PVF₂-diethyl malonate (DEM) gel is a mixture of both spheroidal and fibrillar. The PVF₂/diethyl succinate (DES), PVF₂/diethyl gluterate (DEG), PVF₂/diethyl pimelate (DEP), and PVF₂/diethyl azelate (DEAZ) gels are "fibrillar-like" as evidenced from scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The X-ray and solvent subtracted FT-IR spectra indicate the presence of α-polymorph PVF₂ in all the gels. The enthalpy of gel formation and the enthalpy of gel fusion, measured from differential scanning calorimetry (DSC), show linear plot with PVF₂concentration for PVF₂-DEO gels but others exhibit positive deviation from linearity. From the deviation vs PVF₂ weight fraction (W_PVF2) plot, the compositions of the polymer solvent complexes are found to be 1:3, 1:2, 1:4, 1:4, and 1:3 in the molar ratio of the diester and PVF₂ repeating unit, for gels in DEM, DES, DEG, DEP, and DEAZ, respectively. The phase diagrams of PVF₂-DEM, PVF₂-DES, and PVF₂-DEP gels indicate polymer-solvent compound formation with a singular point while those of the PVF₂-DEG and PVF₂-DEAZ gels indicate compound formation with an incongruent melting point. The polymer solvent compound formation is also studied by molecular mechanics calculations using MMX program. The pairs of α-PVF₂ and diester molecules with appropriate conformation to match the composition of the complex are energetically minimized. The distances between the >CF₂ group and the carbonyl oxygen are lower than the summation of their van der Waals radii for all the diesters. The discrepancy between molecular modeling and morphology of the PVF₂-DEO gels and the borderline morphology of PVF₂-DEM gels have been explained from molecular mobility of the solvent and enthalpy of complexation (ΔH_c). The gel melting temperature and gelation temperature increases with increase in intermittent length (n) for a particular PVF₂concentration. Also, ΔH_c increases with "n", and this indicates that the intermittent length of diesters has both enthalpic and entropic contribution on gel behavior of PVF₂.