The glass transition temperature (Tg) as an index of chemical conversion for a high-Tg amine/epoxy system: Chemical and diffusion-controlled reaction kinetics



The glass transition temperature (Tg) is used as a parameter to monitor the isothermal cure of a tetrafunctional aromatic diamine and a difunctional aromatic epoxy system. There is a one-to-one relationship between Tg and conversion that is independent of cure temperature, Tcure. Prior to vitrification (Tg = Tcure), the reaction is only kinetically controlled; after vitrification, the reaction becomes diffusion-controlled. Time-temperature shifts of Tg vs. In (time) data at different cure temperatures to form a master curve for the kinetically controlled reaction at an arbitrary reference temperature yield a single Arrhenius activation energy (15.2 kcal/mole). The master curve and the reaction activation energy are used in calculating iso-Tg contours prior to vitrification and also the vitrification contour in the time-temperature-transformation (TTT) isothermal cure diagram for the system. The chemical kinetics of the reaction is satisfactorily described by an autocatalyzed reaction mechanism. The overall rate constant of the reaction in both kinetically and diffusion-controlled regimes is modeled by a combination, in parallel, of the chemical rate constant and the diffusion rate constant. The chemical rate constant has the usual Arrhenius form, whereas the diffusion rate constant is assumed to be given by a modified form of the WLF equation. Results suggest that both primary and secondary amino hydrogens are equally reactive. A theoretical model for calculating Tg as a function of conversion is presented for a network-forming system with one bond-forming reaction.