Effect of Poly(ether imide) on the Mechanical Properties of Epoxy Resin under Extreme Environmental Conditions

The high- and low-temperature mechanical properties of epoxy resin (EP) play a critical role in determining the service performance of carbon fiber-reinforced composites (CFRP) in extreme environments. In this study, the poly(ether imide) (PEI) thermoplastic was incorporated into the epoxy matrix to improve the mechanical properties of the epoxy resin. Curing kinetics analysis demonstrated that the addition of PEI lowered the apparent activation energy of the curing reaction. Rheological testing revealed that PEI increased the viscosity of the epoxy resin and shortened its gel time. Dynamic mechanical analysis (DMA) indicated that the addition of 8.0 wt % PEI raised the glass transition temperature (Tg) of the epoxy to 207.7 °C. Tensile testing demonstrated that the epoxy resin modified with 8.0 wt % PEI achieved tensile strengths of 139.43 MPa at RT, 127.85 MPa at 90 K, 93.03 MPa at 373 K, and 46.05 MPa at 423 K, representing improvements of 33.7%, 7.0%, 8.5%, and 24.9%, respectively, compared to the unmodified epoxy resin. Similarly, the elongation at break reached 5.66% at RT and 1.54% at 90 K, representing 38.7% and 11.6% enhancements compared to the unmodified epoxy. Moreover, modulus measurements showed values of 3.53 GPa at 373 K and 2.36 GPa at 423 K, exceeding the unmodified epoxy by 1.7% and 14.0%, respectively. The mechanism of PEI in improving the high- and low-temperature mechanical properties of epoxy resins can be summarized as follows: (i) At low temperatures, the enhanced mobility of PEI molecular chains improves the toughness of epoxy resins; (ii) At high temperatures, the greater rigidity of the PEI molecular chain improves the heat resistance of the epoxy resin.