Interfacial enhancement by constructing a “flexible-rigid” structure between high-modulus fillers and low-modulus matrix in carbon fiber/silicone rubber composites

Silicone rubber (SR) has excellent resistance to high temperatures, and carbon fiber (CF) is a commonly reinforced material. However, the application of carbon fiber in SR composites is limited by the large difference in modulus between the two materials. In order to improve the interfacial bonding of the composites and to exert the reinforcement of carbon fibers, a modulus transition interface, a "flexible-rigid" structure was constructed between CF and SR by coating polydopamine-polyethyleneimine@silica (PDA-PEI@SiO2) on the carbon fiber surface. The results show that the "flexible-rigid" structure of the PDA-PEI@SiO2 layer not only enhanced the mechanical properties but also improved the thermal resistance and wave-transmission properties of the CF/SR composites. Surface microscopy and structural analysis showed that the "flexible-rigid" structure was successfully coated on the fiber surface, and significantly improved the roughness of the fiber surface. It provided the mechanical meshing between the fiber and matrix when the CF/SR composites were subjected to the tensile loads. The tensile strength and elongation of the SR composite with CF-A-E@8SiO2 were 37.16 % and 27.46 % higher than that with unmodified CF, respectively. The electrical test showed that the "flexible-rigid" structure could reduce the dielectric constant and dielectric loss of CF/SR composites, thereby improving the wave-transmission properties. Further mechanism analysis of the interfacial bonding showed that abundant hydrogen bonds in the "flexible-rigid" structure at the interface promoted stress dissipation and avoided stress concentration. Meanwhile, the formation of Si-O-Si with high bonding energy also protected the interface from destruction. The thermogravimetric analysis (TGA) found that the introduction of Si-O-Si bonds could significantly improve the thermal resistance of the CF/SR composites at 350 °C for 2 h. It provides guidance for the improvement of the interface bonding in the fiber/rubber composite as well as its functionality.