Role of fluorine polyetherimide molecular weight in regulating toughness, phase separation and thermal stability of bismaleimide resins

Due to its great mechanical properties and high-temperature resistance, bismaleimide (BMI) has important applications in the aerospace field. The incorporation of thermoplastic polyimide (PI) into thermosetting resins is an effective strategy for enhancing toughness. The phase separation mechanisms of PI within the resin matrix have been extensively studied. However, the majority of studies have concentrated on epoxy, and research on BMI is limited. Moreover, compared with factors such as the content and structural composition of the modifier PI, the molecular weight is often overlooked. In this work, a tailored series of fluorine polyetherimides (abbreviated as PI-X) with weight-average molecular weights (M_w) ranging from 1.5 to 3.6 × 10⁴ were synthesized using an in situ prepolymerization method. In this method, the basic modifier of BMI, ortho-diallyl bisphenol A (DABPA) was used as a solvent to synthesize a PI-X solution; here, the process was simplified and met the requirement of green chemistry with no pollutant emissions. The novel PI-X contains fluorine elements, ether bonds and asymmetric structures; these bonds and structures are beneficial for synergistically improving toughness and high-temperature resistance. By maintaining a constant concentration of 4 wt% DABPA, the effects of the PI-X molecular weight on the phase separation and performance of the PI-X/BMI/DABPA system (BDP) were investigated in detail. Notably, the impact strength and flexural strength of the BDP resins showed significant improvements, reaching 30.1 kJ/m² and 157.7 MPa, respectively, which were 67.2% and 16.1% greater than those of pure BDP. Additionally, the thermal stability was enhanced, with a 9.8 °C increase in the T_g when the molecular weight of PI-X was approximately 2.5 × 10⁴. Based on Flory lattice model, the influence of BMI curing degree on PI-X compatibility and phase separation mechanism was explored. As the molecular weight increased, the compatibility between PI-X and BMI gradually deteriorated. This led to pronounced phase separation and the formation of spherical PI-X particles, which impacted the overall performance.