Electromechanical response of semiconducting carbon–polyimide nanocomposite thin films

Electromechanically responsive polymer nanocomposite thin films can provide embedded microscale sensing elements for unobtrusive monitoring of strain, torque and pressure particularly for composite structures. Thin nanocomposite carbon–polyimide films with thicknesses up to 90 μm were produced with carbon contents that yield semiconducting behaviour attributable to distance dependent electron hopping between isolated nanoparticles. The tensile modulus and the strain at break indicated minimum interaction between polymer and nanoparticle surfaces. A decreasing storage modulus with increasing temperature indicated increasing free volume inducing polymer chain motions.Well below the percolation threshold the electromechanical sensitivity was linear, while approaching this threshold nonlinearity arose through a contribution to conductivity from limited nanoparticle aggregates. These semiconducting thin films exhibited sensitivities (gauge factors) that ranged to 8.0 depending upon their geometric profile compared to about two for traditional metal foil gauges. This dependency on Poisson’s ratio occurs from competing interparticle spacing effects where longitudinal applied strain increases particle spacings reducing conductivity and transverse contraction decreases spacings enhancing conduction.