Accurate Tg Characterization and Enthalpy Relaxation Mapping of Aerospace Epoxies via Modulated DSC (MDSC)

Thermosetting epoxy resins used as matrices for high performance aerospace carbon fiber reinforced polymers undergo physical aging and enthalpy relaxation depending on their thermal history. During conventional DSC characterization, this structural relaxation induces a large endothermic peak that overlaps with the glass transition temperature step, making it challenging to isolate the true glass transition onset. This study investigates a benchmark system of Diglycidyl ether of bisphenol A and a latent aromatic diamine, Diethyltoluene diamine. To overcome the resolution limits of conventional DSC, TA Instruments Modulated DSC technique, which superimposes a sinusoidal temperature oscillation onto a linear heating ramp, was utilized. The experimental results successfully decoupled the overlapping thermal events from the Total Heat Flow into Reversing and Non reversing Heat Flow components. In the Reversing curve, kinetic aging artifacts were eliminated, enabling the precise determination of the true physical glass transition governed solely by reversible heat capacity changes. Simultaneously, the Non reversing curve independently isolated and quantified the pure enthalpy relaxation energy and the subsequent high temperature crosslinking exothermic peak. This study demonstrates that Modulated DSC serves as a vital analytical solution to ensure data reliability for the high fidelity characterization of polymers with complex thermal histories.