On the gelation rates of thermoreversible poly(vinylidene fluoride) gels

Abstract

The gelation rates of poly(vinylidene fluoride) (PVF₂) gels have been measured for three commercial samples each in two different solvents at different polymer concentrations and also at different temperatures. The rate has been expressed as a combination of temperature-dependent function f(T) and concentration-dependent function (φ)ⁿ ; φ is the reduced overlapping concentration of the polymer chains. φ has a resemblance with the P - P_c term of percolation theory, P being the conversion factor and P_c its critical value. Exponent n has been measured from double logarithmic plots of t_gel^-1 and φ, and it lies between 0.45 and 0.6 which is closer to percolation exponent β (0.45) of a three-dimensional lattice. This indicates that three-dimensional percolation is a suitable model for this thermoreversible gelation process. The temperature-dependent function of the gelation rate has been found to be analogous to the temperature-dependent function of the nucleation rate of crystalline polymers in dilute solutions. There is some dependency of the gelation rate on the amount of H-H defect (-CF₂CF₂-) structures present in the polymers and also on the molecular weight of the samples. Solvents also have a significant influence on the gelation rate of the polymer : the poorer the solvent, the faster is the gelation rate. The critical gelation concentration (C_t=α^∗ ) measured from gelation kinetics by an extrapolation procedure increases with temperature in each case. Thermodynamic analysis of the variation of C_t=α^∗ with temperature reveals that about three to six crystallites (depending on the solvent and sample) are involved to produce a single crosslink in the gel.