Synthesis of novel hydrazide-substituted phthalonitrile resin: Effect of keto-enol tautomerism on structure–property relationship

Hydrazide-substituted phthalonitrile resin (1e) has been synthesized via nucleophilic substitution reaction of hydrazide-substituted bisphenol (1d) with 4-nitrophthalonitrile to introduce a flexible linking group between rings for low-melting material. The resin was molecularly designed with two flexible hinges of hydrazide functionality with an arm and a low melting point resin system. The resin monomer in Differential scanning calorimetry (DSC) analysis, instead of showing a low melting point, showed a relatively high melting peak (T_m) at 266 °C, due to the formation of a more stable enol form on account of an extended conjugation, which was confirmed by Nuclear Magnetic Resonance (NMR), Fourier Transform Infrared (FTIR), and Ultraviolet (UV) spectra. The monomer was assessed for its thermal, rheological, and curing characteristics. The DSC analysis of uncured 1e showed the onset of curing (T_ons) at 269 °C along with curing window of 69 °C. The processing window was found to be _~40 °C by rheological tests. The enthalpy of curing for 1e was found to be 151.5 Jg⁻¹. Thermogravimetric analysis (TGA) of the fully cured polymer (1f) revealed the onset of thermal degradation (T_do) of 450 °C along with a char yield of 66 wt.% at 800 °C in N₂ atmosphere. In air atmosphere multistep degradation was obtained and nil ash yield was found at 600 °C. The limiting oxygen index (LOI) value of the cured polymer (1f) was found to be ̴ 43.9 (estimated by Krevelen’s equation).