Effect of moisture absorption on piezoresistance of conducting polymer composite thin films

This paper is devoted to the study of the piezoresistance of conducting composite thin films consisting of epoxy resin and needle-like conductive fillers. The surface resistivity of the thin films at room temperature depended on filler volume fraction and relative humidity (rh). The effect of moisture absorption on the piezoresistance of the composite thin films was studied under various rh values. At high rh values, strain dependence of resistance was positive, and this positive coefficient decreased with decreasing rh value. At 30%rh, it became negative and its absolute value increased with decreasing rh. These results suggest that the piezoresistance effect of the conducting thin films is enhanced by moisture absorption by the epoxy matrix that is relatively hydrophilic. From these observations, the piezoresistance effect is considered to be based on the following two strains: one is derived from a mechanical stress and the other is due to environmental stress induced by moisture absorption.