Tuning the Magnitude and the Polarity of the Piezoresistive Response of Polyaniline through Structural Control.

We demonstrate the tunability of both the polarity and the magnitude of the piezoresistive response of polyaniline that is template-synthesized on poly(2-acrylamido-2-methyl-1-propanesulfonic acid), PANI-PAAMPSA, by altering the template molecular weight. Piezoresistivity is quantified by gauge factor, a unitless parameter that relates changes in electrical resistance to applied strain. The gauge factor of PANI-PAAMPSA decreases linearly and becomes negative with decreasing PAAMPSA molecular weight. The polarity of PANI-PAAMPSA's gauge factor is determined by macroscopic connectivity across thin films. PANI-PAAMPSA thin films comprise electrostatically stabilized particles whose size is determined at the onset of synthesis. An increase in the interparticle spacing with applied strain results in a positive gauge factor. The presence of PANI crystallites increases connectivity between particles; these samples instead exhibit a negative gauge factor whereby the resistance decreases with increasing strain. The tunability of the piezoresistive response of these conducting polymers allows their utilization in a broad range of flexible electronics applications, including thermo- and chemoresistive sensors and strain gauges.