Relaxations in thermosets. IX. Ionic conductivity and gelation of DGEBA-based thermosets cured with pure and mixed amines

The results obtained during the isothermal curing of diglycidyl ether of bisphenol-A-based thermosets cross-linked with pure diaminodiphenyl methane and pure diaminodiphenyl sulfone and with their mixtures have been analyzed to determine how the dc conductivity changes with time during the conversion of its liquid to a gel. The complex permittivity data are first analyzed to show that ac measurements can be used to obtain the ionic conductivity over a considerable period of the curing process. The procedure allows one to obtain the dc conductivity without having data as a function of frequency. The shape of the complex plane plots of the electrical modulus are semicircles, but with small deviations that appear at long times during the curing process. The dielectric consequences of the chemical changes with time during the cross-linking of the thermoset are analogous to the frequency dependence of the complex permittivity of a liquid. The analysis shows that the dc conductivity σ_o of a thermoset during its cure follows a power law, σ_o∝ (t_g—t)^x, where t is the curing time (t < t_g). The results can also be described equally well by a new equation, σ_o ∝ exp—B/(t_o—t)], where x, t_g, B, and t_o are empirical constants all of which vary with the temperature of the cure. t_g is close to the time for gelation known from independent studies and t_o is close to but longer than the time for vitrification. These conclusions are discussed in terms of scaling concepts for the gelation phenomenon.