Polydopamine and polyethyleneimine comodified carbon fibers/epoxy composites with gradual and alternate rigid-flexible structures: Design, characterizations, interfacial, and mechanical properties

Polymer composites with tailored structure and enhanced properties and functions have attracted great attentions due to their wide applications in materials science, nanotechnology, and engineering science. In this work, we demonstrate the innovative concepts of fabricating both gradual "rigid-flexible" and alternate "rigid-flexible" structures, which are achieved by altering the grafting sequence of flexible polyethyleneimine with high flexibility and polydopamine with suitable rigidity. Then, the effect of different rigid-flexible structures on the structure and properties of the carbon fiber-epoxy (CF/EP) composites are studied. It is found that the composites reinforced by the alternate rigid-flexible polymer layer possess superior interfacial effects and mechanical properties. Compared with the CF/EP without polymer layer modification, the rigid-flexible layer contributes to about 80.3% and 167.3% on the interfacial shear strength and the impact strength of final composites. These great improvements are attributed to the complex crack propagation path, more energy consumption, wider interface region, higher wettability, as well as strong physical and chemical interactions. This work provides practical and scalable potentials for regulating the structure and functions of the CF-reinforced polymer composites by optimal interfacial design.