Dynamics of carbonyl-modified-epoxy/amine networks by FTIR and dielectric relaxation spectroscopy

An investigation was conducted to determine how the molecular architecture of prepolymers affects the dynamics of epoxy/amine networks at various stages of cross-linking. An epoxy prepolymer with a carbonyl group attached to the glycidyl ether moiety was utilized, in anticipation that the presence of the carbonyl group would have a specific affect on the interactions and dynamics in these networks. Experimental results were obtained in real time by simultaneous broad-band dielectric relaxation spectroscopy and near-infrared Fourier transform spectroscopy. An unusual increase in the relaxed (limiting low-frequency) dielectric constant was observed, and an explanation was offered in terms of an intramolecular hydrogen-bonded complex that co-involves carbonyl and hydroxyl groups. A pronounced change was also noted in the IR absorption of epoxy and carbonyl groups near the gel point and was assigned to specific dipolar interactions in the growing network. Frequency sweeps of reactive networks reveal initially the alpha beta process at high frequency (peak at ca. 2 GHz). With the advancement of reactions, however, alpha and beta relaxations begin to separate out; alpha relaxation shifts gradually to lower frequency as the glass transition of the growing network increases, while beta relaxation, because of its localized origin, is little affected by the network growth. An unusual form of thermodielectric complexity is observed in the relaxation spectrum for the alpha process. Finally, building upon an earlier study by Fournier et al. (Macromolecules 1996, 29, 7097), a correlation between molecular dynamics and chemical kinetics during network formation is reported.