Rubber-Filler Interactions in Carbon Black-Filled Rubber Composites Studied by Scanning Transmission X-ray Microscopy

Carbon black (CB) filled in a rubber matrix can dramatically enhance the mechanical properties of rubber composites. The main reason for reinforcement has been hypothesized to be the covalent bonding between the functional groups on the CB surface and the rubber chain terminals, i.e., the rubber-filler interactions that can effectively transmit external stress. In this study, the CB samples that underwent high-temperature graphitization and oxidation treatment by concentrated nitric acid were investigated for different surface properties compared with those of the untreated CB using synchrotron-based scanning transmission X-ray microscopy (STXM). The results showed that the CB with inert surface properties by graphitization exhibited good miscibility in the nonpolar natural rubber (NR) through weak rubber-filler interactions and the volume filling reinforcing effect. However, the CB with active surface properties by oxidation demonstrated poor miscibility and inhomogeneous dispersion in the nonpolar NR attributed to strong rubber-filler interactions, which may be one of the main reasons for its reduced tensile strength. The strong interactions also induced changes in the C-H group orientation distribution and modified the functional group structures. The insightful information provides a valuable perspective on the CB reinforcing mechanisms and the design of new rubber composites with specific properties.