A thermodynamic study on the thermoreversible poly(vinylidene fluoride) gels in acetophenone, ethyl benzoate, and glyceryl tributyrate

The gel melting temperatures of poly(vinylidene fluoride) (PVF2) gels in acetophenone (ACTP), ethyl benzoate (EB), and glyceryl tributyrate (GTB) are measured by differential scanning calorimetry (DSC) at high PVF2 concentration (W-PVF2 greater than or equal to 0.1) and visually for very low PVF2 concentration (W-PVF2 less than or equal to 0.1) The gelation temperatures are also measured in DSC by dynamic cooling method. The enthalpy of gelation and enthalpy of gel fusion vs weight fraction of PVF2 (W-PVF2) plots exhibit different nature in the three solvents. It is linear for the PVF2/ACTP gels but the PVF2 gels in the other two solvents exhibit positive deviation from linearity. A thermodynamic analysis of the enthalpy values in PVF2/EB and PVF2/GTB systems indicates the polymer-solvent complex formation in both the systems. The stoichiometry of the PVF2-EB compound is 1:1 molar ratio and that of PVF2-GTB compound is 3:1 molar ratio with respect to PVF2 monomeric unit and the solvent molecule, respectively. The phase diagram of the PVF2/ACTP system is linear, that of PVF2/EB system exhibits compound formation with incongruent melting point, and that of PVF2/GTB system exhibits compound formation with singular point. The spheroidal morphology of the PVF2/ACTP gels has been attributed to the chain folding process, whereas the fibrillar morphology of PVF2/GTB gels is due to polymer-solvent complex formation. The reason for the mixed morphology of PVF2/EB gels is not clear and is probably due to the comparable rates of the polymer-solvent compound formation and the chain-folding processes.