Low-temperature gelling phenomenon for a class of slow-curing phthalonitrile-benzoxazine resin

Bifunctional phthalonitrile-benzoxazine (PNBZ) usually suffers from slow-curing characteristics. Recently, we unexpectedly observed a waste liquid containing PNBZ solution and excess ethylenediamine (EDA) had turned into a gel at room temperature. Since the report on the PNBZ gelling phenomenon below 150 degrees C was rare, this accidental finding manifested the possible formation of PNBZ gel at low temperatures. By varying the EDA molar ratio, PNBZ gels with different microstructures were successfully realized at the low temperature of 50 degrees C. Through the differential scanning calorimetry, thermogravimetric analyse (TGA), and Fourier transform infrared technique, the driving force behind the gel formation was found to be the low-temperature copolymerization between the amino group of EDA and the BZ ring of PNBZ, resulting in the amine-containing intermediate. PNBZ gel was thermally unstable, but was mechanically robust. The current method was also universal to realize polymer or composite gels for a class of slow-curing PNBZ resin (i.e., allyl-containing PNBZ gel and its carbon nanotube-filled composite gel). This study not only expands our understanding of the low-temperature polymerization behavior of PNBZ but also provides a versatile strategy for low-temperature preparation of PNBZ-based gels. Making the most of current approach, it is believed that multifunctional PNBZ-based gels will be developed for various applications. The typical low-temperature formation process of PNBZ gel.image