Partially Bio‐Based Reactive Poly(Arylene Ether Ketone)s With Tunable Carboxyl Contents for Toughening Epoxy Resin

Abstract

Epoxy thermosets combine high strength, thermal stability, and chemical resistance, but their densely crosslinked networks also make them susceptible to brittle fracture. Here, partially bio‐based reactive poly(arylene ether ketone) copolymers (PEK‐DA) containing different amounts of pendant carboxyl groups were synthesized by nucleophilic aromatic substitution polycondensation from biomass‐derived diphenolic acid and phenolphthalein. The polymers exhibited reduced viscosities of 0.38–0.41 dL g −1 in N,N‐dimethylformamide, glass‐transition temperatures above 190°C, and 5% mass‐loss temperatures above 410°C. When incorporated into an E51/methyltetrahydrophthalic anhydride system, PEK‐DA produced rougher fracture surfaces without resolvable thermoplastic‐rich domains at the SEM magnification examined. At 5 phr, PEK‐DA5 increased the notched Izod impact strength to 17.58 kJ m −2 , representing increases of 70% over the unmodified epoxy and 30% over the PEK‐C control, while maintaining T g above 140°C and T d,5% above 360°C. Molecular‐dynamics simulations gave PEK‐DA5 a free‐volume fraction of 7.04%. The results indicate that free volume, improved compatibility, and possible reactive interfacial coupling collectively contribute to toughening with limited loss of thermal performance.