Designing Flexible and Self-Healing Electronics Using Hybrid Carbon Black/Nanoclay Composites Based on Diels-Alder Dynamic Covalent Networks

Abstract

The addition of an organo-modified nanoclay to a carbon black-based electrically conductive self-healing composite showed a synergistic improvement of the electrical conductivity and healing ability. The synergistic effect was studied as a function of carbon black (primary) and nanoclay (secondary) filler loadings in a Diels-Alder-based polymer network. The synergistic effect is the greatest when the carbon black particles are organized around the partially exfoliated nanoclay platelets. Too extensive exfoliation and dispersion of the nanoclay by ultrasonication result in a partial loss of electrical conductivity by around 18% and a substantial loss of the healing ability by around 84%, whereas the hybrid composite shows a very poor recovery of only 12% based on the strain at break. Second, the synergy is governed by the compatibility between the organic modifiers of the nanoclays and the backbone chemistry of the polymer network. The incorporation of Cloisite 15A with a hydrophobic modifier in a network based on poly-(propylene oxide) (PPO) results in a greater synergetic improvement of the electrical conductivity and healing behavior than the incorporation of a more hydrophilic nanoclay or the use of Cloisite 15A in a more hydrophilic poly-(ethylene oxide) (PEO) backbone. Finally, the effect of nanoclay as a secondary filler depends also on the chemistry and architecture of the polymer network. By using no platelets at 10 wt % carbon black, CB-filled composites built on PPO- and PEO-based networks show distinctly different electrical conductivities of 6.18 and 15.97 S m-1, respectively. The synergistic effect increases with a decreasing cross-linking density of the polymer network, especially by the improvement of the healing behavior. For instance, introducing Cloisite 15A enhances the mechanical healing efficiency of the PPO-based composite possessing a lower cross-linking density by 44% while diminishing it for the PEO-based composite by 33%. This study provides deep insights into the structure-property relationships that facilitate the optimization of electrically conductive and self-healing composites for a wide variety of applications, in particular for flexible electronics and soft robotic applications.