Abstract :
Diffusion limitations have an impact on epoxy cure reactions near vitrification. The effects of diffusion control on the rate of reaction can be described by the evolution of a mobility factor, which is directly based on the experimental heat capacity evolution measured with differential (scanning) temperature modulated calorimetry (D(S)TMC) as diffusion and mobility factor coincide for the epoxy resins studied. A model based on the free volume theory and the Rabinowitch activated complex theory is presented here. It relates the apparent rate constant to the difference between the reaction temperature and Tg. Very good correlations are derived between model calculations and the experimental normalized mobility factor measured at 1/60 Hz. A further extension of the model to allow for experimental mobility factors measured at different modulation frequencies is also proposed. The model successfully predicts the deceleration of reactions due to diffusion limitations, which demonstrates the applicability of D(S)TMC to estimate the effect of the reaction-induced vitrification on the polymerization and to model the overall cure kinetics.
