A synergistic effect of MXene/MWCNT enables self-healable and low percolation elastomer sensor: A combined experiment and all-atom molecular dynamics simulation

Abstract

Flexible conductive composites have garnered significant interest as wearable strain sensors due to their potential in diverse application fields, such as next-generation robotics automation, electronic skin, and human body detection. Nevertheless, the practical usage scenarios often entail tear and fracture, posing a persistent challenge to their performance stability. Consequently, there exists a pressing demand to engineer conductive composites that not only exhibit flexibility, stretchability, and sensing capabilities but also demonstrate effective self-healing properties. This pursuit presents a formidable task, considering the complex requirements imposed on the materials. This study presents a novel methodology for fabricating a conductive silicone elastomer composite. The approach involves the initial utilization of an excess of amino-capped Polydimethylsiloxane (PDMS) to undergo a reaction with Toluene diisocyanate (TDI) and Isoflurane diisocyanate (IPDI), leading to the formation of amino-capped polyurea (TPU). Subsequently, the self-healing elastomer IPDI/TDI/TA (ITT) is synthesized through chain expansion with Terephthalaldehyde (TA) in tetrahydrofuran (THF). To complement the experimental investigations, all-atom molecular dynamics (AAMD) simulations are employed to develop a comprehensive model elucidating the mechanical characteristics and self-healing capabilities of the elastomers. To introduce hybrid fillers into the elastomer composite, MXenes are electrostatically modified with L-glutamine (negatively charged), while multi-walled carbon nanotubes (MWCNTs) are modified with cetyltrimethylammonium bromide (positively charged). These modified fillers self-assemble in water and are subsequently combined with the aforementioned rubber THF solution after drying. Through ultrasonication and drying, the ITT elastomers/MXene/MWCNT composite is successfully prepared. The formation of hydrogen bonds and dynamic imine bonds is verified using FT-IR and AAMD simulations. By maintaining a fixed ratio of TA to PDMS, we observe a positive correlation between TDI concentration and the fracture strength of the ITT elastomer, while the elongation at break decreases with increasing TDI. Furthermore, the successful modification of MXene and MWCNT is confirmed through FT-IR, XRD, and zeta potential measurements, and the resulting composites exhibit significant electrical conductivity and self-healing properties. Tensile and electrochemical measurements demonstrate that the composite possesses promising characteristics suitable for sensor applications, and its mechanical and electrical properties can recover after self-healing. Notably, the resulting composites exhibit a very low conductivity threshold (3.5 wt%). The composite system exhibits desirable tensile properties and efficient self-healing ability due to the reversibility of multiple hydrogen and imine bonds. Overall, this study aims to introduce a novel elastomer-based composite that possesses excellent properties in terms of self-healing, stretchability, sensing capability and flexibility.