Flow-induced anisotropy in a carbon black-filled silicone elastomer: electromechanical properties and structure

TL;DR

This study investigates how shear flow during coating induces anisotropy in carbon black-filled silicone elastomers, affecting their electrical and piezoresistive properties, with implications for sensor material design.

Contribution

It reveals flow-induced anisotropy in electrical and mechanical properties of CB-elastomers and links it to microstructural changes caused by shear during coating.

Findings

Electrical conductivity is lower parallel to coating direction.

Piezoresistive sensitivity is higher when stretched perpendicular to coating.

Weak structural anisotropy can cause significant electrical anisotropy.

Abstract

Carbon black (CB)-elastomers can serve as low-cost, highly deformable sensor materials, but hardly any work exists on their structure-property relationships. We report on flow-induced anisotropy, considering CB-silicone films generated via doctor blade coating. Cured films showed slight electrical anisotropy, with conductivity parallel to the coating direction being lower than perpendicular to it. Furthermore, piezoresistive sensitivity was much larger for stretch perpendicular to the coating direction than for parallel stretch. Structural analysis for length scales up to the CB agglomerate level yielded only weak evidence of anisotropy. Based on this evidence and insight from CB network simulations, we hypothesize that shear flow during coating fragments the CB network and then induces a preferential aggregate alignment, as well as increased inter-particle distances, parallel to the coating direction. As a practical conclusion, already weak anisotropic structuration suffices to cause significant electric anisotropy.