Investigation of lateral strain dependent Inter-nanotube conductivity of carbon nanotube networks prepared on an elastomer substrate

Abstract

Recently, it has been reported that lateral strain applied to carbon nanotube networks results in change in the network conductivity. Since the conduction mechanism within the nanotube network is by percolation, a detailed investigation on the inter-network conductivity of carbon nanotube networks depending on nanotube density and the state of surface functionalization should be made for possible application of carbon nanotube networks to strain sensors. Here, we investigate the external strain dependent conductivity of single walled carbon nanotube networks (NTNs) formed on a stretchable silicone elastomer surface for possible mechanical deformation sensor application. The NTNs were dispersed in 0.1% sodium dodecyle sulphate and sodium dodecylbenzene sulfonate solution under ultrasonic agitation. The NTN was initially formed, using vacuum filtration, on ~20 nm pore alumina membrane filter to prepare networks with uniform density and distribution. To transfer the SWCNT bundle thin-film onto a flexible substrate, poly-dimethylsiloxane (PDMS) was cured directly on the filter. After filter removal, a highly flexible NTN was formed on the PDMS surface. We measured the change in NTN conductivity depending on the sample elongation. For 24 % strain applied to the PDMS, a conductivity change of ~10 % was observed. Since the vacuum filtration process produces nanotube networks with random orientation the net strain applied to the nanotubes reduces from the local strain on the PDMS by a factor of cos2θ, where θ is the orientation of the nanotube relative to the strain direction. Also, since the surface of the nanotubes have been functionalized to become hydrophilic, the hydrophobic PDMS surface strain will not all be translated to the nanotube strain. We will also present strain dependent conductivity results on samples with different nanotube densities. The mechanical coupling strengths of different layers of nanotubes on the PDMS surface will result in a graded response to applied external strain.