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.

A series of 4-phenylethnylphthalic anhydride (PEPA)-terminated oligoimides were prepared by co-oligomerizing isomeric dianhydrides, i.e., 2,3,3′,4′-biphenyltetracarboxylic dianhydride (a-BPDA), 2,3,3′,4′-benzophenonetetracarboxylic dianhydride (a-BTDA) or 2,3,3′,4′-diphenylethertetracarboxylic dianhydride (a-ODPA), with diamines mixture of bis(4-aminophenoxy)dimethyl silane (APDS) and 2,2′-bis(trifluoromethyl) benzidine (TFDB). The effects of siloxane content and dianhydride structure on the rheological properties of these oligoimides and thermal stability of the corresponding cured polyimide resins were investigated. The results indicated that the introduction of the siloxane structure improved the melt processability of the oligoimides, while the thermal stability of the cured polyimide resins reduced. The oligoimide derived from a-ODPA revealed better melt processability and melt stability due to the existence of a flexible dianhydride structure. The oligoimide PIS-O10 derived from a-ODPA gave the lowest minimum melt viscosity of 0.09 Pa·s at 333 °C and showed the excellent melt stability at 260 °C for 2 h with the melt viscosity in the range of 0.69–1.63 Pa·s. It is also noted that the thermal stability of these resins can be further enhanced by postcuring at 400–450 °C, which is attributed to the almost complete chemical crosslinking of the phenyethynyl combined with oxidative crosslinking of siloxane. The PIS-T10 and PIS-O10 resins that were based on a-BTDA and a-ODPA, respectively, even showed a glass transition temperature over 550 °C after postcuring at 450 °C for 1 h.