Water uptake and interface hydrothermal aging of thermoplastic CFRPMMA composite bars: Macro property degradation and micro structure evolution

Continuous carbon fiber-reinforced polymethyl methacrylate (CFRPMMA) composite rods combine the molding ease of thermosets with the thermoformability and recyclability of thermoplastics, offering an alternative to traditional CFRP rods. This study systematically investigated the hydrothermal durability of CFRPMMA rods via accelerated water-immersion tests, complemented by multi-scale microscopic characterization, with particular emphasis on water uptake behavior and interfacial degradation mechanisms across different temperatures and exposure times. Experimental results revealed a two-stage water absorption behavior, with the diffusion dynamics well-described by a dual-phase diffusion-relaxation model. Although interlaminar shear strength (ILSS) generally degraded with increasing immersion time and temperature, a partial recovery was observed after 3000 h at 80°C, surpassing the 2000 h values and remaining higher than those at 40°C and 60°C. Scanning electron microscopy (SEM) observations revealed the evolution of interfacial damage, from fiber-matrix debonding to complex failure involving void formation, matrix cracking, and interfacial separation with increased hydrothermal aging. Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) confirmed ester bond hydrolysis (C-O cleavage, evidenced by a 171.5 % increase in O-H content) and carbonyl oxidation (C O functional group transformation), leading to chemical bond reconfiguration and consequent interfacial weakening. The recovery of ILSS after 3000 h exposure at 80°C was attributed to the synergistic effects of post-curing cross-linking, silane condensation, and resin plasticization, which facilitated microcrack closure and reduced interfacial degradation. Micro-computed tomography (Micro-CT) analysis revealed significant increases in internal porosity due to hydrothermal aging, with porosity at 40°C and 80°C after 3000 h exceeding reference specimens by 2.54 and 2.31 times, respectively.