Mechanical and Electrical Analysis of Carbon Black Networking in Elastomers Under Strain

The influence of quasi-static straining and relaxation at fixed strain on carbon black networking in elastomers is investigated by simultaneous stress and piezo-resistivity measurements. The mechanical and the electrical response are analyzed in the framework of the dynamic flocculation model considering the cyclic breakdown and re-aggregation of filler–filler bonds. In this model, the mechanical hysteresis is related to the stress contribution of stretched filler clusters, which is positive in the up-cycle, but negative in the down-cycle because of stretching of clusters in lateral direction. The analysis is supported by dielectric relaxation spectroscopy delivering information about the morphology of the filler network and the gap distance between adjacent filler particles, i.e., the length of filler–filler bond. This is shown to be relevant for a qualitative understanding of piezo-resistivity and mechanical relaxation behavior of the investigated rubbers. During the relaxation phase at fixed strain, the mechanical stress decreases during the up-cycle and increases during the down-cycle, which in both cases results from the relaxation of cluster stress leading to a homogenization of the stress field. The decrease of the resistivity in the relaxation phase independent of cycle direction is shown to be related to a retarded re-aggregation of filler–filler bonds in compression directions. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers