Enthalpy recovery and its relation to shear response in an amine cured DGEBA epoxy

A description of enthalpy recovery for an amine-cured, digylcidyl ether of bisphenol A (DGEBA) epoxy (Epon 828/Jeffamine T403) was acquired through a series of experiments targeting the effects of the individual parameters: cooling rate, heating rate, aging temperature, and aging time. From these experiments, the peak temperature, T_p, and fictive temperature, T_f, were tracked as measures of the extent of structural relaxation. The KAHR formalism provides a standard for comparison to similar epoxies and was parametrized using the peak shift method. The resulting parameterization was then used to assess the relation between volumetric and shear relaxation. In particular, the KAHR model was used to predict volumetric relaxation time-temperature shift factors that are directly compared to measured time-temperature shift factors in shear. This unique methodology to assess equivalence between volumetric and shear relaxation shift factors further establishes the basis for this key assumption of continuum constitutive models based on rheological simplicity.