Linear polysilsesquioxane hybrid thermosetting polyimide as antioxidant coating for organic composite to improve long-term thermo-oxidative stability

Abstract

Organic thermosetting polyimide (TSPI) is prone to degrade with long-term exposure in oxidative high-temperature environment. To address this issue, linear polysilsesquioxane (LPSQ) hybrid TSPI was prepared and selectively applied on the surface of organic TSPI composite to achieve in situ formation of the antioxidant coating at high temperature. The LPSQ with both 3-[(4-phenylethynyl)phthalimide]propyl groups and phenyl groups bonded to silicon atoms (LPSQ-PE) was synthesized, and used as a reactive polymeric modifier to prepare hybrid TSPI resin. The physical interaction between imide groups and crosslinking linkages generated from phenylethynyl groups in LPSQ-PE and TSPI led high T_gs of hybrid resin, exceeding 450 °C. The hybrid rein with LPSQ-PE content of 50 wt% gave a mass retention twice higher than TSPI after isothermal heating at 500 °C for 2 h in air, due to formation of continuous layer containing silicon and oxygen-rich inorganic compounds on the surface. Because reactive group content in LPSQ-PE was precisely adjusted to be consistent with the reactive endcapper content in TSPI oligomer, the hybrid resins exhibited good mechanical performance. The composite with designed distribution and content of LPSQ in the direction of thickness was fabricated. After isothermal heating at 400 °C for 100 h in air, the fabricated composite gave a flexural strength 39.61 % higher than that of organic TSPI composite. This work would offer useful insight for fabricating structural composites worked in oxidative high-temperature environment for long times.