Colorimetric and Resistive Polymer Electrolyte Thin Films for Real-time Humidity Sensors

Abstract

We have developed fast responsive, colorimetric and resistive-type polymeric humidity sensors from a series of self-assembled poly(styrenesulfonate-methylbutylene) (PSS-b-PMB) block copolymers with tailored hygroscopic properties. In dry state, the PSS-b-PMB films exhibit hexagonal cylindrical morphology where hydrophobic PMB cylinders are dispersed within a PSS matrix. Under levels of humidity, the PSS-b-PMB thin films self-displayed discernible reflective color changes, covering almost entire visible light regions from violet (RH = 20%) to red (RH = 95%). The sensors also revealed a few orders of magnitude changes in impedance with exposure to humid air by taking advantages of strong polymer electrolytes characteristics. Remarkably, the time to complete the changes in the signals was only a few seconds, as rationalized by good connectivity of the PSS domains and short water diffusion pathways in nanometer scales. Repeated hydration/dehydration tests demonstrated reliable sensor properties, which is in sharp contrast to the poor stability of PSS homopolymer sensors lacking organization.