Reinforcing rubber with carbon nanotubes

We suggest a new reinforcing mechanism for carbon nanotube (CNT)/rubber compounds, based on a comparison of CNT reinforcement of natural rubber (NR) with that by carbon black (CB). The mechanical properties of the NR/CNT compounds were significantly higher, but the amount of bound rubber, an indication of the level of filler-rubber interaction, was lower than with CB. Moreover, the CNT-filled compounds showed a greater degree of strain-softening (Payne effect) and stresssoftening (Mullins effect) and higher permanent set than the CB-filled compounds, indicating weaker bonding. In scanning electron microscope studies on cryogenicallyfractured surfaces, the CNT bundles were seen to protrude out of the surface when the sample was stretched and slid back in when the deformation was removed. We infer that interfacial interactions between CNT and NR are weak in comparison with the relatively strong adhesion between rubber and CB, as indicated by the high amount of bound rubber in that case. Thus, reinforcement by CNTs is attributed to their large aspect ratio and physical entanglement with rubber molecules, rather than to strong interfacial interaction.