Dielectric relaxation spectroscopy during isothermal curing of semi-interpenetrating polymer networks

Dielectric relaxation spectroscopy measurements have been performed during isothermal curing of semi-interpenetrating polymer networks (semi-IPNs) and the related pure networks at frequencies between 3 kHz and 3 MHz and curing temperatures between 313 and 393 K. The pure networks consist of diglycidyl ether of bisphenol A (DGEBA) cross-linked with diaminodiphenylmethane (DDM) and the semi-IPNs contain in addition 10 or 20 wt% of polysulfone (PSn) as the linear component. Temperature dependent dielectric measurements have been performed on the fully cured samples in the temperature range from 130 to 550 K for the same frequencies. For the pure networks, the imaginary part of the complex permittivity epsilon('') shows a decrease with curing time t(cure) followed by a peak. For the semi-IPNs this peak is much broader for all cases and can be resolved into two maxima for several curing conditions. The decrease in epsilon('') is connected to the decrease in the de-conductivity due to gelation, whereas the peak is related to the relaxation of dipoles. The existence of two maxima in the time dependence of epsilon('') is an indication for two different relaxations in a phase separated structure, This is supported by temperature dependent measurements on the fully cured samples. The characteristic relaxation times tau, which are extracted from the maxima of epsilon('') (t(cure)) for different frequencies f using the relation omega tau = 1 (omega = 2 pi f), increase during curing by several orders of magnitude for both the pure networks and the semi-IPNs. In order to extract the characteristic times for gelation t(g) and the times t(0) where the de-conductivity sigma(dc) approaches a singularity, sigma(dc) was fitted by a power law, sigma(dc) = sigma(0)[(t(g) - t)/t(g)](p), and an exponential function, sigma(dc) = A exp [B/(t - t(0))], respectively. In the error limits of the experiments both procedures lead to similar results. The t(g) and t(0) values are in good agreement with those measured mechanically and no significant differences between pure networks and semi-IPNs cured at the same temperature were found.