Experimental Study on Moisture Diffusion and Mechanical Properties of Polymer Matrices Under Seawater Immersion

ABSTRACT The durability of polymer matrices is a critical weak link in the hygrothermal aging of composites. This study investigates the mechanisms of moisture diffusion and the evolution of mechanical properties in polymer matrices under seawater immersion. The experiments involve three types of epoxy resins (EP) cured at medium‐high temperature (EP‐MHT), medium temperature (EP‐MT), room temperature (EP‐RT), and one type of vinyl ester resin (VE‐MHT), with seawater temperatures set at 25°C, 40°C, and 60°C. Comparing epoxy resins, the behaviors of moisture diffusion reveal that resins with higher curing temperatures have lower equilibrium water absorption. One of the reasons is that the number of polar groups decreases with the increase of curing temperature, and the maximum water absorption is closely associated with the quantity of polar groups, which govern the formation of bound water. In addition, due to the limitation of water absorption caused by high curing temperature, the corresponding mechanical properties retention rate is also higher, and similar conclusions were obtained at different aging temperatures. Different from the Fickian diffusion of EP‐MHT, the moisture diffusion of VE‐MHT exhibits non‐Fickian diffusion, which shows a sustained decline in the later stage. This primarily stems from the synergistic effects of water absorption and hydrolysis. Besides, VE‐MHT exhibits a similar extent of mechanical properties degradation to EP‐MHT, with retention rates primarily above 90%.