Effective addition of nanoclay in enhancement of mechanical and electromechanical properties of SWCNT reinforced epoxy: Strain sensing and crack-induced piezoresistivity

Many studies were performed to improve CNT dispersion into epoxy using different mechanical dispersion methods as well as CNT functionalization. In this study, a novel method is introduced to enhance CNT dispersion using 2D nanoclay as a secondary filler. Hence, this study was aimed to investigate the effect of nanoclay platelets on electrical, mechanical, and piezoresistive characteristics of the SWCNTs doped epoxy nanocomposites. Two different types of nanocomposites were prepared for comparison including binary (SWCNT/epoxy) and hybrid (SWCNT-nanoclay/epoxy) states. CNT content of 0.1 wt% was used for the binary and hybrid states while two different nanoclay loadings (0.5 wt% and 1.0 wt%) were employed in the hybrid nanocomposites. Tensile and mode I fracture tests were performed for the mechanical and electromechanical characterization using two probe techniques while electron microscopy and X-ray diffraction were used for microstructural analysis. Results showed severe CNT agglomeration in the binary state whilst a homogenous CNT dispersion was achieved in the ternary states. The binary nanocomposite showed weak performance in terms of electrical, mechanical and piezoresistive properties caused by severe CNT aggregates. On the other hand, addition of nanoclay into CNTs doped epoxy manifested a significant increase in the electrical, mechanical and piezoresistive-sensitivity performance of the hybrid nanocomposites compared to the binary one. The best performance was achieved at 0.5 wt% nanoclay loading where electrical conductivity increased by six orders of magnitude, UTS increased by 37%, KIC and GIC increased by 34% and 64%, respectively, with respect to the binary nanocomposite. Crack-pinning and crack deflection were accounted for the fracture toughness increase in ternary composites. Nonlinear piezoresistivity resulting from the predominate effect of tunneling resistance ruled piezoresistivity in the hybrid nanocomposites. A sensitivity of 2.1 and 2.0 at strain of 0.01 were obtained for 0.5 wt% and 1.0 wt% nanoclay contents, respectively, whereas no sensitivity was achieved for the binary composite.