Kinetic study of the multistep thermo-oxidative degradation of thermosetting siloxane-containing polyimide and unmodified polyimide

In this study, the thermo-oxidative degradation kinetics of thermosetting siloxane-containing polyimide (SPI) and unmodified polyimide (PI) were comparatively studied to reveal effect of siloxane on the thermo-oxidative stability, and to understand how they behave under elevated temperature. Results of thermogravimetirc analysis complemented by structural and morphological observations indicate two individual processes compose thermo-oxidative degradation of SPI and PI. Therefore, different kinetic triplets are employed to describe the two processes. It is found that the degradation activation energy (E) of the first process for SPI is lower than that for PI, while in the second process, the E value of SPI goes beyond that of PI. The difference of structural changes between SPI and PI during thermo-oxidative degradation offers rational explanation. The oxidation of siloxane in the first process results in weight loss of SPI at relatively lower temperature, while the formed silica-type structures retard the second degradation process, making SPI exhibited a slower degradation rate than PI. In addition, the two constituent processes for thermo-oxidative degradation of SPI and PI obey the Avrami-Erofeev models. Based on the obtained kinetic parameters, simulations of thermo-oxidative degradation for both SPI and PI were successfully realized, which gives a precise mathematical description of their degradation behaviors.