A free‐volume‐based approach to modeling thermoset cure behavior

The free‐volume theory for the temperature dependence of transport properties of glass‐forming polymers is extended to obtain their relationship to the extent of cure. This treatment centers on the unifying role of molecular mobility and yields a model which connects extent of reaction, viscosity, diffusivity, ionic conductivity and dipole relaxation time. The temporal dependence of these properties is expressed by coupling the extended free‐volume model with a relationship for the rate of cure, which included diffusional limitations. Analyses based on this model are applied to the observed behavior of a model epoxy‐amine resin system. The intrinsic kinetics of this model system are shown to be first order. It is shown that diffusional limitations strongly affected the progress of the reaction in the final stages of cure. The diffusion‐modified rate expression predictions agree with extent of reaction versus time data over the range of experimental temperatures. The temporal dependence of viscous behavior of the curing resin is measured. The extended free‐volume model accurately describes the evolution of resin viscosity during cure. The dielectric behavior is similarly characterized and is in close agreement with the predictions of the general free‐volume expression. The results of this study indicate that the free‐volume theory modified to account for molecular weight effects allows prediction of resin properties with a two‐parameter model. The results show that a power‐law relationship exists between viscosity and ionic conductivity. This result suggests that electrical properties may be used for on‐line measurement of resin viscosity during cure.