Analysis of crosslinking in amine‐cured epoxy systems: The one‐to‐one relationship between Tg and conversion

Network formation, in a system such as a tetrafunctional aromatic diamine cured with a difunctional epoxy, is considered on the basis of only interconnected trifunctional crosslinking sites. The number‐average molecular weight, the crosslinking density, and the gel point, and also the vitrification contours of the time—temperature—transformation (TTT) isothermal cure diagram, are related to the ratio of the reaction rate constants, k2/k1, of the competing reactions of epoxy with secondary amine group (k1) and with primary amine group (k1). It is considered that the glass transition temperature, Tg, is related uniquely to conversion on the basis that the number‐average molecular weight and crosslinking density are insensitive to the ratio, k2/k1. Experimental data support the theoretical result. The potential influences of chemical structure and physical aging on Tg are discussed.