Co-authors​ :

 Kazuya KITAMOTO (JAPAN), Shu MINAKUCHI (JAPAN), Tomohiro YOKOZEKI (JAPAN) 

Polymer-based composites such as carbon fiber reinforced polymer (CFRP) have been widely used for aerospace, marine, civil engineering, and automotive applications due to their superior material properties (e.g., high specific stiffness, high specific strength, low thermal expansion, and high thermal conductivity). However, polymer and polymer-based composites are susceptible to moisture absorption, leading to changes in the physical swelling and mechanical properties (e.g., reduction in elastic modulus, strength, and glass transition temperature). Therefore, understanding the mechanisms of moisture transport, swelling, and mechanical degradation of polymers is considerably crucial for formulating a physics-based model. Such a model has the potential to predict the long-term swelling and mechanical behavior of polymeric components accurately and reliably during hygrothermal aging.
This study investigated water-polymer molecular interactions and mechanical properties of epoxy-based CFRP in the moisture absorption-desorption cycle, developing the high-fidelity simulation model based on the physics linking the molecular to the macroscopic swelling and mechanical behaviors. First, nuclear magnetic resonance (NMR) analysis was employed to identify the two states, the bound or free water, from the mobility of the absorbed water in the CFRP in the moisture absorption and the subsequent desorption processes. The results indicated that the dominantly diffusing water molecules are different between the moisture absorption and the subsequent desorption processes, with bound water diffusing in the moisture absorption process, whereas free water diffusing in the desorption process (Fig. 1). Second, positron annihilation lifetime spectroscopy (PALS) is a valuable tool for studying the microstructure of polymers, polymer networks, and water-polymer interactions by detecting the molecular-sized free volume in the polymer. This result reveals that irreversible changes in the polymer microstructure are small in the moisture absorption and the subsequent desorption processes. Last, to evaluate the macroscopic behavior, the moisture content by gravimetric analysis and the moisture-induced strain by optical fiber sensor, fiber Bragg grating (FBG), were monitored in the moisture absorption and desorption processes. The data indicated non-linear strain behavior with the moisture content and the hysteresis of the strain in the moisture absorption and desorption processes. In addition, the elastic modulus (plasticization) by DMA was studied. These results were combined with previous studies to formulate a two-phase diffusion model based on moisture transport with water-polymer molecular interactions. The model was validated on a strut for a truss structure at the structural element level. Consequently, the results indicated that the model could accurately predict the moisture-induced deformation behavior in the moisture absorption and desorption processes (Fig. 2).