Development of Tg-less epoxy thermosets by introducing crosslinking points into rigid mesogenic moiety via Schiff base-derived self-polymerization

Novel glass-transition-temperature-less (T-g-less) epoxy thermosets were prepared by a Schiff base (CH=N)-derived self-polymerization of mesogenic epoxy. The self-polymerization mechanism was verified by differential scanning calorimetry, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The epoxy-CH=N addition reaction formed a new tertiary amine, introducing a crosslinking point into the rigid mesogenic moiety. Dynamic mechanical analysis of the cured system showed a slight alpha-relaxation in tan delta, and a high storage modulus was maintained at > 250 C. Also, no drastic increase in the coefficient of thermal expansion related to the glass-transition behavior was observed. The results suggested that the crosslinking point formed in the rigid mesogenic moiety effectively suppressed the thermal motion of networked chains, resulting in T-g-less behavior. An imidazole catalyst was used to enhance the self-polymerization reactivity. The imidazole content effect on the temperature-dependency of the mean free volume in the network structure was investigated by positron annihilation lifetime spectrometry.