Phase-microstructure-mechanical properties relationship of carbon fiber reinforced ionic liquid epoxy composites

The effect of liquid phases on the mechanical properties of carbon fiber reinforced composites (CFRP) is still unclear. In this study, various mass percentages of ionic liquid have been incorporated into epoxy resins and used to fabricate CFRPs. A brittle-ductile transition is demonstrated in tensile behavior of the CFRPs, accompanied by the change of failure mechanisms among fiber breakage, a combination of fiber breakage, splitting and delamination, and delamination. These ionic liquid-induced behavior change arouses our enormous interest in investigating the microstructure change of CFRP. On one hand, ionic liquid leads to microstructural transformation of the epoxy-ionic liquid mixtures. Differential scanning calorimetry (DSC) characterizations help to confirm one transition from one phase (ionic liquid confined within epoxy network) to two phases (separate ionic liquid and epoxy phase) in the matrices at the critical mass percentage of 35 wt%. On the other hand, the interfacial bonding strength and effective bonding area of matrix to carbon fibers are decreased, as confirmed by the decrease of short beam strength with the increase of ionic liquid. This work presents a representative example of the systematic analysis of the modified CFRP, which lays good foundations for the optimization of structural electrolytes for structural power composites.