High strain rate mechanical properties of a cross-linked epoxy across the glass transition

Molecular dynamics simulations were used to study the high strain rate mechanical properties of a cross-linked epoxy system comprised of diglycidyl ether of bisphenol A (DGEBA) that is cross-linked by a poly(oxypropylene) diamine with three propylene oxide moieties per diamine. Atomistic network structures were characterized using volume-temperature behavior and their response to mechanical deformation. The Young's modulus was determined as a function of temperature across strain rates spanning three decades in magnitude, and collapsed onto a single “master curve” using the time–temperature superposition principle (TTSP). The master curve obtained from molecular dynamics simulation data shows good agreement with a similar master curve of the reduced storage modulus as a function of frequency, which was obtained using experiments. At higher strain rates, the simulation master curve deviated from the experimental master curve. This deviation could be attributed to the lack of occurrence of sub-T_g motions on the time scale of simulations due to the use of higher strain rates in simulations compared to experiments. Our work demonstrates the utility of TTSP in connecting the thermo-mechanical behavior of polymers at high strain rates and high temperatures to experiments performed at much different conditions. To the best of our knowledge, the use of the time–temperature superposition to compare mechanical properties determined from molecular simulation and experiments is the first reported effort of its kind.