Microstructural Origin of Nonmonotonic Piezoresistivity in Polymer Nanocomposites

Incorporating conductive nanomaterials into polymers yields a new class of piezoresistive strain-sensing materials. While possessing monotonic resistance-strain behavior is a fundamental requirement for any material used for strain sensing, polymer nanocomposites frequently exhibit nonmonotonic resistance responses under strain, which limits their application prospects. In this study, physical experiments and molecular dynamics simulations are performed to determine a feasible solution to overcome this limitation. The corresponding results demonstrate that regulating the initial inter-nanofiller junction geometry imparts complete control over the monotonic piezoresistive behavior of polymer nanocomposites. Mechanistically, monotonically increasing resistance responses under tension can be achieved by promoting active diffusion that causes van der Waals force-driven barrier crossing of nanofillers (resulting in direct contact between nanofillers, e.g., at elevated curing temperatures) during curing; thus, during deformation, nanofillers primarily move away from one another. Conversely, suppressing diffusion during curing causes barrier crossing of nanofillers, which results in resistance reduction, under deformation owing to stress-driven local rearrangement of polymer molecules in heterogeneous shear transformation zones. The mechanistic insights provided by this study can guide the design of next-generation, advanced strain-sensing materials in the future.