Hierarchical curing mechanism in epoxy/bismaleimide composites: Enhancing mechanical properties without compromising thermal stabilities

It remains a considerable challenge to enhance mechanical properties without sacrifice of thermal properties in epoxy-based materials. In this study, epoxy/bismaleimide (EP/BMI) composites were synthesized via a melting blend technique using 4,4 '-diaminodiphenylsulfone as the curing agent. It is found that there is a unique hierarchical curing mechanism in EP/BMI systems, including (i) respective curing of EP and BMI monomer, (ii) copolymerization between EP prepolymer and BMI monomer, (iii) homopolymerization of EP prepolymer and BMI oligomer. Via the hierarchical copolymerization, EP and BMI are compatible without phase separation phenomenon. The copolymerization concurrently improved the composites' resistance to deformation under stress and restricted the thermal mobility of the polymer chains upon heat. Compared to pure EP resin, EP/BMI composite (4:1) shows a flexural strength of 133.6 MPa, tensile strength of 106.4 MPa and toughness of 3.6 MJ/ m3 respectively, increased by 20.7 %, 36.4 % and 45.0 %. Meanwhile, T g and Y c were also improved by 10.3 degrees C and 12.3 % in EP/BMI composite. Furthermore, the mechanical and thermal properties of the EP/BMI composites can be efficiently tailored by curing conditions and EP:BMI molar ratios. This discovery provides a significant benchmark for the design of composite material and to expand the application horizons for both EP and BMI materials.