화학공학소재연구정보센터
Macromolecules, Vol.33, No.7, 2616-2625, 2000
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 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 (SEMI and transmission electron microscopy (TEM). The X-ray and solvent subtracted FT-IR spectra indicate the presence of a-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 PVF2 concentration 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 alpha-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 (Delta H-c). The gel melting temperature and gelation temperature increases with increase in intermittent length (n) for a particular PVF2 concentration. Also, Delta H-c increases with "n", and this indicates that the intermittent length of diesters has both enthalpic and entropic contribution on gel behavior of PVF2.