From curing pathways to divergent properties in phenylethynyl-terminated polyimides: a multiscale DFT/MD study of network structure–property relationships

Abstract

The properties of phenylethynyl-terminated polyimide resins, which are critical high-performance matrices for aerospace composites, are fundamentally determined by their cured network structure. Clarifying this relationship has been challenging due to the complexity of the curing process, which involves multiple competing reaction pathways. This work employed a combined computational approach—utilising density functional theory and all-atom molecular dynamics simulations—to systematically investigate how different curing pathways influence the resulting microstructure and properties. The simulations suggest that diradical-initiated polymerization and cyclotrimerization are plausible and likely important among the pathways considered, whereas monoradical-initiated polymerization and the dehydro-Diels–Alder reaction appear less favorable under the studied DFT conditions. Networks formed via the diradical-initiated pathway endow the model with a high glass transition temperature, and high Young's and shear moduli. In contrast, structures produced by the cyclotrimerization pathway confer a high coefficient of thermal expansion. The distinct thermal and mechanical properties arising from the two curing pathways were elucidated by analysing chain flexibility, free volume, and radius of gyration. By comparing these two idealised, pure-pathway network archetypes, this work suggests a link between curing pathways, microstructure, and macroscopic performance. Although real curing involves mixed pathways, these limiting cases provide a rational, principle-guided framework for designing high-performance phenylethynyl-terminated polyimide resins.