Moisture diffusion and hygrothermal aging in bismaleimide matrix carbon fiber composites—part I: uni-weave composites

Moisture diffusion and hygrothermal aging in uni-weave bismaleimide composites were systematically studied in this research. The moisture weight gain curves of the composites were compared with those of the neat resin in order to determine the interface effect on moisture absorption. Both the neat resin and composites display similar two-stage diffusion behavior, with the first and second stages being diffusion- and relaxation-controlled, respectively. When the diffusivities of the composites were compared to theoretical predications of an appropriate mathematical model, it is found that the fiber-matrix interface has little effect on the short term moisture diffusion. However, the long term absorption is suppressed in the composites. The rigid fiber–matrix interface seems to constrain long-range segmental motions of the matrix and in so doing reduce water absorption in the relaxation-controlled second stage. Although the fiber–matrix bonding is initially very strong, prolonged water absorption at elevated temperatures eventually damage the interface and causes interfacial cracking.