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

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 o f the PVF2/diethyl oxalate (DEO) gel is spheroidal, but the morphology of P VF2-diethyl malonate (DEM) gel is a mixture of both spheroidal and fibrilla r. The PVF2/diethyl succinate (DES), PVF2/diethyl gluterate (DEG), PVF2/die thyl pimelate (DEP), and PVF2/diethyl azelate (DEAZ) gels are "fibrillar-li ke" as evidenced from scanning electron microscopy (SEMI and transmission e lectron microscopy (TEM). The X-ray and solvent subtracted FT-IR spectra in dicate the presence of a-polymorph PVF2 in all the gels. The enthalpy of ge l formation and the enthalpy of gel fusion, measured from differential scan ning calorimetry (DSC), show linear plot with PVF2 concentration for PVF2-D EO gels but others exhibit positive deviation from linearity. From the devi ation vs PVF2 weight fraction (W-PVF2) plot, the compositions of the polyme r solvent complexes are found to be 1:3, 1:2, 1:4, 1:4, and 1:3 in the mola r 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 p oint while those of the PVF2-DEG and PVF2-DEAZ gels indicate compound forma tion with an incongruent melting point. The polymer solvent compound format ion is also studied by molecular mechanics calculations using MMX program. The pairs of alpha-PVF2 and diester molecules with appropriate conformation to match the composition of the complex are energetically minimized. The d istances between the >CF2 group and the carbonyl oxygen are lower than the summation of their van der Waals radii for all the diesters. The discrepanc y between molecular modeling and morphology of the PVF2-DEO gels and the bo rderline morphology of PVF2-DEM gels have been explained from molecular mob ility of the solvent and enthalpy of complexation (Delta H-c). The gel melt ing temperature and gelation temperature increases with increase in intermi ttent length (n) for a particular PVF2 concentration. Also, Delta H-c incre ases with "n", and this indicates that the intermittent length of diesters has both enthalpic and entropic contribution on gel behavior of PVF2.