Electrical Conductivity in Rubber Double Networks

The consequences on electrical conductivity of the various processing steps used to form a double-network rubber are summarized in Table IV. Although the objective was not achieved herein, the potential for using double-network rubbers to attain enhanced conductivity remains. Alternate procedures enabling residual extensions exceeding 100% are suggested for future work. Specific conclusions drawn from this study are as follows: 1. The time dependence of the electrical resistivity after imposition of a tensile strain depends on the magnitude of the strain. The observed behavior is consistent with breakup of carbon-black floc at low strains (with concomitant reduction in conductivity) and with promotion of interparticle contacting at higher strains. The latter engenders enhanced longitudinal conductivity. The enhancement may be due to orientation of the filler phase, but this remains speculative. The effect of deformation on the transverse resistivity could not be reproducibly characterized. 2. The rate dependence of the electrical resistivity was also dependent on the magnitude of the rubber deformation. At the low strains associated with disruption of the filler phase, higher rates (stresses) increase the maximum in the resistivity. At higher elongations for which the resistivity declines, the effect of deformation velocity is less apparent. 3. Subjecting a filled rubber to heating after mixing reduces the electrical resistivity. The irreversible portion of this reduction is attributed to an acceleration in the recovery of an equilibrium level of filler-particle contacts. The resistivity acquires an invariance to temperature after the initial heating that persists for at least several hours. 4. The fact that extension followed by retraction of a carbon-black-reinforced elastomer results in a permanent increase in electrical resistivity negated in this work the possibility of achieving enhanced electrical conductivity via a double-network structure. 5. Consistent with the strain optical properties, orientational crystallization behavior, and stress-strain response previously found for unfilled rubbers containing a double-network structure, carbon-black-reinforced double-network rubbers exhibit electrical resistivities more sensitive to strain than conventionally cured elastomers.