Thermoreversible Gelation of Poly(vinylidene fluoride) in Diesters:  Influence of Intermittent Length on Morphology and Thermodynamics of Gelation

Poly(vinylidene fluoride) (PVF 2 ) 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 PVF 2 /diethyl oxalate (DEO) gel is spheroidal, but the morphology of PVF 2 −diethyl malonate (DEM) gel is a mixture of both spheroidal and fibrillar. The PVF 2 /diethyl succinate (DES), PVF 2 /diethyl gluterate (DEG), PVF 2 /diethyl pimelate (DEP), and PVF 2 /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 PVF 2 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 PVF 2 concentration for PVF 2 −DEO gels but others exhibit positive deviation from linearity. From the deviation vs PVF 2 weight fraction ( W PVF 2 ) 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 PVF 2 repeating unit, for gels in DEM, DES, DEG, DEP, and DEAZ, respectively. The phase diagrams of PVF 2 −DEM, PVF 2 −DES, and PVF 2 −DEP gels indicate polymer−solvent compound formation with a singular point while those of the PVF 2 −DEG and PVF 2 −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-PVF 2 and diester molecules with appropriate conformation to match the composition of the complex are energetically minimized. The distances between the >CF 2 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 PVF 2 −DEO gels and the borderline morphology of PVF 2 −DEM gels have been explained from molecular mobility of the solvent and enthalpy of complexation (Δ H c ). The gel melting temperature and gelation temperature increases with increase in intermittent length ( n ) for a particular PVF 2 concentration. Also, Δ H c increases with “ n ”, and this indicates that the intermittent length of diesters has both enthalpic and entropic contribution on gel behavior of PVF 2 .