Characterization of the cure-state of DGEBA-DDS epoxy using ultrasonic, dynamic mechanical, and thermal probes

Three experimental techniques were used to characterize the cure state of an epoxy resin system of DGEBA epoxide and DDS diamine curing agent. Samples were prepared from non-stoichiometric monomer mixtures designed so as to simulate various stages of cure of a stoichiometrically prepared epoxy. Such an approach allows for variable temperature characterization of specimens without concern for ongoing chemical reactions that would cloud interpretation of results. Additional experiments were performed on stoichiometric samples that were isothermally cured. Differential scanning calorimetry (DSC) was used to measure the heat of reaction and glass transition temperature. Dynamic mechanical analysis (DMA) was used to measure complex modulus, while ultrasonic cure monitoring (UCM) was used to measure longitudinal velocity throughout cure. DSC analysis was found to be insensitive to changes occurring at the latter stage of polymer network development, especially after vitrification. DMA characterization, however, was found to be quite sensitive to the rubbery modulus (and as such, the cure state), but is limited to cure states above gelation. Only the UCM technique was robust enough to accommodate all cure states while providing highly sensitive measurements of mechanical property development.