On water absorption and its impact on the dielectric spectra of epoxy network with different stoichiometries

The effect of changing the resin/hardener stoichiometry on moisture absorption of an epoxy/amine network was investigated. The results indicate that decreasing the hardener content below the stoichiometric ratio leads to lower water uptake levels, whereas increasing it beyond the stoichiometric ratio has the reverse impact. Hence, in terms of water absorption, using epoxy-rich formulations might be beneficial. Dielectric spectroscopy measurements imply that the absorbed water molecules interact with the polar components of the matrix, which lowers the frequencies at which these water molecules relax. Analysis of the obtained spectra showed that a significant part of the absorbed water molecules relaxes in phase with the existing β relaxation, which implies that these water molecules are bounded to the polar moieties that are responsible for the β relaxation. The other part of the absorbed water relaxes at higher frequencies (> 1 MHz). Therefore, this part may be bound to other polar moieties that relax at frequencies higher than 1 MHz and/or may represent unbounded or ‘free’ water clustered within the free volume inside the epoxy matrix and, thus, exhibit a relaxation that is more similar to the relaxation of bulk water.