Curing Dependent Piezoresistivity in CNT/epoxy Nanocomposites: from Experimental Observation to Numerical Analysis

  • Wong, Ting Yui (The Hong Kong Polytechnic University)
  • Yu, Tao (The Hong Kong Polytechnic University)
  • Zou, Fangxin (The Hong Kong Polytechnic University)

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It has been observed that the resistance change of a CNT/epoxy nanocomposite under tensile loading can be either monotonic or non-monotonic. Although the non-monotonic piezoresistive behaviour may possibly be caused by matrix plasticity, Poisson’s effect induced lateral motion of CNTs, and/or rearrangement of CNT network, the mechanism has not yet been fully understood. While previous works have primarily investigated the effect of nanofiller content, our study examined the impact of curing condition and found that the monotony of the piezoresistivity could be enhanced by using a higher curing temperature. We further explored the effect of curing temperature on the crosslinked molecular network of an epoxy-amine polymer matrix and on the physical properties of the resultant CNT/epoxy nanocomposite. We utilized in situ near-infrared spectrometry to monitor the polymerization kinetics which controls the growths of epoxy network structures, and atomic force microscopy to observe the spatial heterogeneities, in terms of mechanical properties, developed in the epoxy network structures. Correlating the spectrometric and microscopic results indicate that curing temperature controls the mode of network structure growth and hence determines the final degree of spatial heterogeneity. We therefore propose that the curing-dependent piezoresistivity is a result of varying the degrees of spatial heterogeneity which causes the CNT networks inside different epoxy matrices to move differently under tensile deformation. Coarse-grained molecular dynamic simulations were undertaken to decipher the local movements of CNTs under tensile deformation in epoxy network structures with varied degrees of conversion (DOCs). The microscopic piezoresistivity of the CNT/epoxy simulation boxes generated was found to switch from negative to positive when the local DOC increases from 50% to 80%. Then, the spatial heterogeneity of an epoxy matrix was modelled as an assembly of simulation boxes with high and low local DOCs. The macroscopic piezoresistive behaviour of a CNT/epoxy nanocomposite would change from monotonic to non-monotonic when the ratio of high DOC elements to low DOC elements decreases, i.e., the epoxy matrix becoming more heterogeneous. Using this multiscale approach, the curing dependent piezoresistivity can be reproduced qualitatively and the correlation between the piezoresistive behaviour of a CNT/epoxy nanocomposite and its epoxy network structure can be established.