Chemorheology relations for epoxy‐amine crosslinking

The goal of this study is to elucidate the relationship between the mechanism of bond formation, resulting structure, and the viscosity during the step polymerization of a thermoset. The system chosen for this work consists of a bifunctional epoxy, diglycidyl ether of bisphenol A, cured with a tetrafunctional amine, diaminodiphenyl sulfone. Steady shear and dynamic viscosity are measured as a function of extent of reaction throughout the epoxy‐amine cure. Branching theory is used to relate the extent of reaction (i.e., conversion of epoxy groups) to certain structural parameters, including the weight‐average molecular weight. The combination of the viscosity‐conversion data with the branching theory enables the correlation between changes in rheological properties and network structure during polymerization. The steady shear viscosity rise is modeled as the product of a structure‐dependent friction factor and weight‐average molecular weight. The structural significance of the point in cure when the dynamic storage and loss modulus are equal (i.e., tan δ = 1) is also explored.