Humidity and Strain Rate Determine the Extent of Phase Shift in the Piezoresistive Response of PEDOT:PSS.

The piezoresistive response of PEDOT:PSS is sensitive to changes in its morphology when exposed to humidity and in response to different strain rates. The piezoresistive response of as-cast PEDOT:PSS transitions from being in-phase to being out-of-phase with applied strain when the relative humidity is reduced from >50% to near zero. At >50% relative humidity, the PSS matrix swells and interrupts the connectivity of electrically conducting PEDOT domains. Stretching PEDOT:PSS at such conditions leads to an increase in resistance with strain. Under dry conditions, PEDOT domains are connected; stretching PEDOT:PSS instead leads to preferential alignment of the conducting domains and a concomitant decrease in resistance. At intermediate humidity, the piezoresistive response of PEDOT:PSS is phase shifted relative to applied strain, with it being out-of-phase at low strain rates (0.34%/min) and in-phase at high strain rates (1.12%/min). We interpret this peculiar and surprising observation as a competition between strain-induced domain separation and alignment, each having a different response time to applied strain. Postdeposition treatment of PEDOT:PSS with dichloroacetic acid removes excess PSS; PEDOT:PSS's piezoresistive response is then invariant with humidity and strain rate. Stabilizing its piezoresistive response can ensure accuracy of PEDOT:PSS-based flexible resistive sensors whose response to small strains is used to monitor environmental and human-health.