Gelation mechanism of thermoreversible gels of poly(vinylidene fluoride) and its blends with poly(methyl acrylate) in diethyl azelate

Abstract

Poly(vinylidene fluoride) (PVF₂) and its blends with poly(methyl acrylate) (PMA) produce thermoreversible gels in diethyl azelate. SEM studies indicate a fibrillar network structure, and WAXS/FTIR studies indicate the presence of α-polymorphic crystals of PVF₂ in the gels. Some newer X-ray diffraction peaks are observed in the dried gels than those in the melt-crystallized PVF₂ samples, and the intensity ratios (I^°_hki/I^°_IIO) of the diffraction peaks of the dried gels are also different from those of the melt-crystallized sample. The gelation rates of these systems are measured by the test tube tilting method. At a particular isothermal temperature the gelation rates of the blends decrease and the critical gelation concentrations ( C_{t = α}^∗ ) of the blends increase with an increase in PMA concentration. In terms of PVF₂ concentration the C_{t = α}^∗ values are also higher for the blends than that of the pure PVF₂ at a given gelation temperature. The gelation rate ( t^-1_gel) has been analyzed from the equation t^-1_gel ∞ f(C) f(T). At a constant temperature analysis of the concentration function f(C) indicates three-dimensional percolation is a suitable model for gelation of both PVF₂ and its blends, supporting that blending does not alter the macroscopic mechanism. The microscopic mechanism, determined from f(T), is however affected due to blending. The gelation process is considered as a two-step process: coil → TGT conformer → fibrillar crystallization (gelation). The free energy of activation (ΔF) of the conformational ordering increases, and the free energy of formation of the critical size nucleus (ΔG^∗) decreases with increasing PMA concentration of the blend. A possible explanation for this difference has been offered. A comparison of ΔF and ΔG^∗ of the two processes indicates that transformation of the coil → TGT conformer is the rate-determining step of the gelation process for all the samples. The lower gelation rate of the blends may be due to its increased ΔF value.