Effect of crosslink density on physical ageing of epoxy networks

Physical ageing of polypropylene oxide/DGEBA networks with different crosslink densities was investigated using the small-strain stress relaxation technique in simple extension. The effects of crosslink density on the glass transition temperature, T_g, and the change in specific heat ΔC_p at T_g were measured using a differential scanning calorimeter (d.s.c.) in heating. Although we observed an increase in T_g as crosslink density increased, contrary to other studies of crosslinked polymers, the ΔC_p did not change as the crosslink density changed. Ageing was studied at several temperatures below T_g after quenching from above T_g. It was possible to superimpose the stress relaxation curves at different ageing times, temperatures and crosslink densities to form a single master curve, demonstrating the applicability of a time-ageing time-temperature-crosslink density superposition principle to this type of network. Under all ageing conditions, the double logarithmic shift rate was found to decrease with increasing crosslink density while being independent of temperature for a given network. Furthermore, at temperatures of 10 and 5°C below T_g, we were able to age the network glasses into structural equilibrium, thus obtaining t*, the time required to reach structural equilibrium. At a constant temperature ΔT below T_g, we observed increases in t* as the crosslink density increased.