Effects of Substituent Groups on Curing Reaction and Thermal Stability of Nitrile Functional Benzoxazine Resins

Three nitrile functional benzoxazine monomers (PH-bn, mPH-bn, aPH-bn) were synthesized based on phenols containing different substituents and m-aminobenzonitrile, and their cured products (poly(PH-bn)、 poly(mPH-bn)、poly(aPH-bn)) were obtained. The structures and compositions of benzoxazine monomers were characterized by Fourier Transform Infrared spectroscopy (FT-IR) , Nuclear Magnetic Resonance spectroscopy (¹H-NMR) and Gel Permeation Chromatography (GPC). The effect of the substituents on curing behaviors was compared by differential scanning calorimetry (DSC), and the curing reaction mechanism was further discussed. In addition, thermogravimetric analysis (TGA) and TGA-FTIR were used to compare the thermal stability and pyrolysis mechanism of the three cured products. The results showed that the crosslinking reactions of the oxazine rings and a part of nitrile groups occurred simultaneously during the curing process. Due to the presence of the electric-donating methyl group, the peak temperature of the curing reaction increased and the thermal stability decreased. However, the electric-absorbing aldehyde group made the curing peak temperature decrease. Moreover, the thermal stability significantly increased because of the crosslinking reaction of the aldehyde groups. The 5% mass loss temperature of poly(aPH-bn) was 380 ℃, and its char yield was as high as 67.3% in N₂ atmosphere at 800 ℃.