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 T(d5)s > 515 degrees C and T(d10)s > 520 degrees C in air and char yields > 50% (1000 degrees 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.

Automated summary

High-temperature polymers, such as phenylethynyl-terminated ortho-hydroxy imide oligomers (oHIOs), exhibit excellent thermal stability and solubility, making them suitable for aerospace applications. Through thermal rearrangements, oHIOs can be converted into polybenzoxazole networks with high thermal stability. This work highlights the importance of preparing imide oligomer networks with outstanding thermal stability and processability for carbon/carbon composites.