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

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 alpha -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(hkl)degrees /I(Il0)degrees) 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=alpha*,) of the blends increase with an increase in PMA concentration. In terms of PVF2 concentration the C-t=alpha* values are also higher for the blends than that of the pure PVF2 at a given gelation temperature. The (t(gel)(-1)) has been analyzed from the equation t(gel)(-1) infinity f(C)f(T). At a constant temperature analysis gelation rate (t(gel)-1 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 --> TGTG conformer --> fibrillar crystallization (gelation). The free energy of activation (DeltaF) of the conformational ordering increases, and the free energy of formation of the critical size nucleus (DeltaG*) decreases with increasing PMA concentration of the blend. A possible explanation for this difference has been offered. A comparison of DeltaF and DeltaG*: of the two processes indicates that transformation of the coil --> TGTG 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 DeltaF value.