Macromolecules, Vol.28, No.7, 2371-2376, 1995
On the Gelation Rates of Thermoreversible Poly(Vinylidene Fluoride) Gels
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 (phi)(n); phi is the reduced overlapping concentration of the polymer chains. phi 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 phi, and it lies between 0.45 and 0.6 which is closer to percolation exponent beta (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=alpha*) measured from gelation kinetics by an extrapolation procedure increases with temperature in each case. Thermodynamic analysis of the variation of C-t=alpha* 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.