On the Dynamics of Moisture Absorption and Its Impact on Dielectric Properties of Epoxy Networks under DC and AC Voltages

The effect of stoichiometry on the dynamics of moisture absorption in epoxy networks is reported. The results show that formulations containing an excess of epoxide groups are characterized by reduced water absorption. For all the considered formulations, water absorption is a reversible process. Water absorption reduces the glass transition temperature, Tg, with each 1 wt% of absorbed water reducing Tg by 10 °C. We attribute this effect to water molecules masking network polar sites and, thereby, inhibiting the formation of intra- or inter-segmental hydrogen bonds. Dielectric spectroscopy reveals that a significant fraction of the absorbed water exhibits relaxation processes similar to the relaxation of the network polar groups and, thus, results in strengthening the existing relaxation peaks within the system. This phenomenon may be employed to magnify the presence of material polar groups, particularly when these groups have low concentration or cannot produce a detectable effect on the dielectric spectra. The presence of distributed absorbed water enhances charge transport, an effect reflected in a proportional reduction in DC breakdown strength. Conversely, AC breakdown strength is not critically affected for the levels of water absorption seen here.