Flow-induced anisotropy in a carbon black-filled silicone elastomer: Weak structural anisotropy causes strong piezoresistive anisotropy

Carbon black (CB)-elastomer composites can serve as low-cost, highly deformable sensor materials. We report on the flow-induced anisotropy of CB-silicone films generated via doctor blade coating. Cured films exhibited larger conductivity perpendicular to the coating direction ( R ∥ / R ⊥ > 1 ). The piezoresistive sensitivity was 2-3 times larger when stretching perpendicular than parallel to the coating direction, with relative resistance increases of 100–200 %. In contrast, the mechanical stress response to strain was isotropic within the measurement uncertainties. Structural analyses at length scales up to the CB agglomerate level ( < 1 μ m) yielded only weak structural anisotropy and excluded alignment of small, primary CB aggregates ( < 150 nm ) in flow direction. Small structural anisotropy apparently suffices to induce significant (piezo-)electric anisotropy. Atomistic molecular dynamics simulations of CB in a viscous medium under strong shear indicate that the CB aggregates have a weak tendency to align with the flow. This generally leads to increased conductivity parallel to the coating R ∥ / R ⊥ < 1 . Affine deformation in response to small tensile strain reduces conductivity uniformly. Our results show that shear can induce the formation of electrically anisotropic composites but excludes shear alignment as dominating mechanism. We propose that anisotropy is caused by an interplay of extensional flow and weak alignment in the flow-vorticity plane that varies under tensile strain.