Microstructural origin of non-monotonic piezoresistivity in CNT/epoxy nanocomposite*

Epoxy mixed CNTs has been attracting considerable interest as a structural material with self-strain sensing capability. However, the material often exhibits non-monotonic piezoresistive behaviours in a seemingly uncontrollable manner, hindering its application in the real world. Our previous work (Wong et al. Compos. Commun., 2023, 39: 101557.) demonstrated that the monotonicity of the piezoresistive behaviour in CNT/epoxy nanocomposite displays a dependence on curing temperature. Although this dependency was preliminarily attributed to the heterogeneity, in terms of crosslink density, in the network structure of the epoxy matrix, the underlying microstructural mechanism remained ambiguous. In this study, we establish the relationships between curing temperature, microstructure, and the piezoresistivity in CNT/epoxy nanocomposite through a combination of physical experiments and numerical simulations. While the experimental observations verify that the level of heterogeneity in the epoxy matrix is determined by the thermal-sensitive curing kinetics, coarse-grained molecular dynamic simulation results show that heterogeneity further governs how the configuration of the CNT network, and hence the piezoresistive behaviour of CNT/epoxy nanocomposite, would evolve under mechanical deformation. All in all, this study offers molecular-level insight into the monotonicity of the piezoresistivity in CNT/epoxy nanocomposite.