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

Abstract

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 ≥ 0.1) and visually for very low PVF₂ concentration (W_PVF2 ≤ 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.