| As amorphous polymers are cooled, a temperature (Tg) is reached where molecular rearrangements are no longer possible on the time scale set by the rate of temperature change and hence, chain segments become "kinetically trapped". This results in the formation of a non-equilibrium glassy structure that can be defined in terms of excess thermodynamic quantities, such as volume and enthalpy. The evolution of these quantities results from gradual structural rearrangements (relaxation), or physical aging, that take place as the polymer tries to approach its thermodynamic equilibrium state. Samples of a polyetheramine (Jeffamine T-403, Huntsman)-cured digylcidyl ether of bisphenol A (DGEBA) resin (EPON 828, Hexion) that exhibits a Tg of 90 °C were aged isothermally between 1 and 8000 hours to characterize changes due to structural relaxation. Samples were aged at temperatures near, but below, Tg so that the effect of physical aging could be observed on a practical timescale. The effects of structural relaxation were tracked through 1) uniaxial compression tests to determine changes in the stress-strain response (e.g., the yield stress evolution) and 2) differential scanning calorimetry to observe the evolution of the peak height in the heat capacity under constant pressure. The compression tests were performed both at the temperature the samples were aged at and a common temperature for all aging temperatures. The later enables direct comparison of changes occurring during aging at all temperatures. The compressive yield stress substantially increased with aging time. This yield stress evolution ceased after 250 and 1000 hours for samples aged at 83 °C and 76 °C, respectively. Furthermore, the magnitude of the peak in heat capacity and the temperature at which the peak occurred increased with aging time, indicating that substantial molecular reconfigurations took place, most significantly at the 76 °C aging condition. The enthalpy relaxation was tracked from the heat capacity curves to observe the enthalpic approach to equilibrium; however, equilibrium was not reached on the timescale of the experiments. It is important to understand these changes and the implications of the changes on the lifetime of engineering devices that utilize this epoxy system. |
