Interphase engineering of carbon fiber/polyimide composites with Nd2O3-modified polyimide sizing for elevated-temperature performance

An Nd2O3-modified phenylethynyl-terminated polyimide (PI-PEPA) sizing system was developed to regulate the carbon fiber/polyimide (CF/PI) interphase and improve its elevated-temperature interlaminar performance. A two-stage formulation strategy was adopted: the PI-PEPA sizing concentration was first optimized, after which KH560 (γ-glycidoxypropyltrimethoxysilane)-treated Nd2O3 nanoparticles were incorporated at loadings of 0.3–1.0 wt% into the selected 1.0 wt% PI-PEPA formulation. SEM/EDS observations showed a comparatively uniform nanoparticle distribution at 0.3 wt% Nd2O3, whereas increasing nanoparticle loading produced progressively more localized particle-rich regions and coating heterogeneity. XPS confirmed the presence of Nd-containing species on the sized fiber surface, while dynamic contact-angle measurements showed that the apparent surface free energy increased from 50.70 to 55.84 mJ m−2 after incorporation of 0.3 wt% Nd2O3. The same formulation increased the 5% mass-loss temperature of PI-PEPA from 497.05°C to 527.73°C and the residual mass at 800°C from 60.75% to 72.64%. Dynamic mechanical analysis further revealed improved storage-modulus retention at elevated temperatures. The corresponding CF/PI laminate exhibited apparent interlaminar shear strengths of 114.5 MPa at room temperature and 57.6 MPa at 300°C, representing increases of 4.4% and 11.8%, respectively, relative to the PI-PEPA control. These results indicate that 0.3 wt% Nd2O3 provides a favorable balance among surface modification, nanoparticle dispersion, and sizing-layer continuity, whereas excessive loading increases interphase heterogeneity and offsets the benefits of nanoparticle incorporation.