Exploring the Impact of Processing Techniques on the Structural and Mechanical Properties of Multifunctional Phthalonitrile‐Modified Bismaleimide Resins

Processing methodology critically dictates the crosslinking architecture and macroscopic properties of maleimide copolymer resins. In this work, an allyl‐functional benzoxazine–phthalonitrile resin (DABA‐ph) bearing both oxazine‐ring and phthalonitrile functionalities was employed to modify bismaleimide (BMI) resin via two distinct processing routes: a sequential route (ph‐BD), in which DABA‐ph is grafted predominantly at the chain ends of BMI/DABA prepolymer chains, and a simultaneous route (ph‐B/D), in which DABA‐ph is incorporated in the middle sections of the chains. Systematic characterization by FTIR, DSC, rheology, DMA, TMA, TGA, and mechanical testing reveals that the processing protocol significantly tailors the network microstructure, engendering divergent thermal, mechanical, and thermomechanical performances. Chain‐end grafting (ph‐BD) affords an exceptionally broad low‐viscosity processing window (130°C–210°C) and high flexural strength at low DABA‐ph contents, whereas mid‐chain incorporation (ph‐B/D) enables multipoint crosslinking, delivering higher crosslinking density, high‐temperature modulus, compressive strength, and dimensional stability. Both systems exhibit excellent thermal stability (char yields above 60% at 800°C) and high glass transition temperatures (up to 402°C), surpassing most conventional allyl‐modified BMI resins. This work offers novel insights into processing–structure–property relationships and high‐performance modification strategies for maleimide‐based copolymer resins.