Curing behavior, thermal, and mechanical properties of epoxy/polyamic acid based on 4,4′‐biphtalic dianhydride and 3,3′‐dihydroxybenzidine

Abstract Different synthesis routes were studied to obtain 4,4′‐biphtalic dianhydride/3,3′‐dihydroxybenzidine polyimide precursors (polyamic acids [PAAs]) with different inherent viscosities (IVs) and imidization degrees. The synthesized PAAs were introduced as a thermoplastic modifier into an epoxy (EP) resin. Different loadings of PAA were used to investigate the curing behavior, heat resistance, and mechanical properties. The onset curing temperature of the EP by adding 20 wt% PAA diminished by around 15°C. Thermogravimetric analysis revealed that the initial and 10 wt% weight loss temperature for EP with 5 wt% PAA improved by 13°C and 7.7%, respectively. Further, the results of tensile and plane‐strain fracture toughness tests indicated that as the amount of PAA increased, the strength and toughness of EP decreased. These improvements were due to the high heat resistance and mechanical properties of PI precursor introduced into the EP, which formed a three‐dimensional structure together. The interlaminar shear strength (ILSS) of the system experienced a reduction; however, after adding 2 phr nanosilica to the system containing PAA with average IV and imidization degree, ILSS showed 4.4% increment.