Synthesis, characterization, and thermal pyrolysis mechanism of high temperature resistant phenolphthalein-based poly (arylene ether nitrile)

Phenolphthalein (PP) is usually used as a functional monomer to design novel high performance polymers with adjustable thermal, mechanical, and fluorescent properties. In this study, a soluble, amorphous, high temperature resistant PP-based poly (arylene ether nitrile) (PAEN) was synthesized and its corresponding thermal decomposition mechanism was reported via TGA/DTG, TGA/FTIR and TGA/MS analysis. PP-based PAEN showed a Tg of 250 °C a Tdmax of 503 °C and a char residue of 59.2% at 800 °C, respectively. According to Kissinger and Flynn–Wall–Ozawa model, the activation energy of thermal pyrolysis was calculated as 225±4 and 284±17 kJ/mol respectively, higher than those of other poly arylene ethers. PP-based PAEN showed a three-stage thermal pyrolysis mechanism. Its initial degradation happened before 400 °C and liberated the inherent moisture. The latter backbone decomposition between 400 and 600 °C involved multiple routes mainly from the C-O and C-C cleavages of lactone ring, resulting in a Ph2-CH-Ph-O-Ph intermediate by releasing HCHO, HCOOH, CO2, H2O, CH4, C6H6, and C6H5OH. The third carbonization stage over 600 °C included the transformation from biphenyl linkages to aromatic graphite by dehydrogenation. This study not only gives a comprehensive understanding of the thermal pyrolysis behavior of PP-based PAEN, but also provides valuable references for the thermal behaviors of other PP-containing polymers.