Hydroxyl-functionalized block co-polyimide enables simultaneously improved toughness and strength of tetrafunctional epoxy resin

Tetrafunctional epoxy resins are widely used as matrix of advanced composite materials due to their excellent high temperature performance. However, the highly crosslinking density of tetrafunctional epoxy resins results in an inherent brittleness, which limits its further application. In this work, hydroxyl-functionalized block co-polyimide (BPADA-ODA/HAB) is first synthesized by the condensation of 3,3′-Dihydroxybenzidine and 4,4′-Diaminodiphenyl ether with 4,4'-(4,4′-isopropylidenediphenoxy) di-phthalic anhydride, and used to toughen tetrafunctional epoxy resins. The co-polyimide molecules provide reactive blocks that participate in the curing reaction of epoxy resin and decrease the crosslinking density of epoxy resin; the co-polyimide can form nano-sized separated phase via self-assembly, which could activate additional energy dissipative mechanisms to enhance the resistance of failure for the epoxy resins. With 5% BPADA-ODA/HAB, the resultant epoxy resins exhibit simultaneous enhancements in impact strength and tensile strength, 184% and 150% higher than those of neat epoxy resin. This study provides an efficient method for preparing tough and strong tetrafunctional epoxy resins, which are expected to be applied more broadly in industries.