Thermal Rearrangement Conversion of Cross-Linked ortho-Hydroxy Polyimide Networks

High-temperature polymers provide an important platform for lighter, yet stronger, next-generation aerospace materials for civilian and military applications. Polyimide networks have exhibited great thermal stability and high char-yield properties due to their aromaticity, making them prime candidates for carbon/carbon composite applications, such as hypersonic aircraft. However, the interrelation between thermal stability and high aromaticity content has created a challenging structure/processing relationship paradigm. Here, it was demonstrated that phenylethynyl-terminated ortho-hydroxy imide oligomers (oHIOs) provide unique processing benefits, exhibiting high solubilities in common alcohols (50% solids) while maintaining thermal stabilities via thermal rearrangements (TRs) from polyimide networks to polybenzoxazole (PBO) networks. Theoretical and experimental TR conversion elucidations were accomplished using thermal gravimetric-mass spectrometry (TGA-MS) and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) characterization studies, resulting in TR-PBO networks capable of achieving 35–75% TR conversions, displaying Td5s > 515 °C and Td10s > 520 °C in air and char yields > 50% (1000 °C). This work highlights the preparation of imide oligomer networks that maintain outstanding thermal stabilities with facile processabilities, promoting such networks as sacrificial polymer matrices for application in carbon/carbon composites.