UNDERSTANDING THE EFFECTS OF STRUCTURAL MODIFICATION ON THE ELECTRICAL AND OPTICAL PROPERTIES OF POLYMER-ACID-TEMPLATED CONDUCTING POLYMERS

Abstract

Conducting polymers exhibit electrical conductivities approaching those of metals while maintaining the chemical, mechanical, and processing attributes of plastics. Complexing conducting polymers with polymer acids yields electrically conducting particles that are water dispersible. Polyaniline that is complexed with poly(2-acrylamido-2methyl-1-propane sulfonic acid), PANI-PAAMPSA, and poly(3,4-ethylenedioxythiophene) complexed with poly(styrene sulfonate), PEDOT:PSS, are the primary conducting polymers studied in this thesis. We examined the effects of structural modification on the electrical and optical properties of conducting polymers as active components in flexible sensors, thermoelectric generators, and electrochromic windows. The processing-structure-function relationships developed in this thesis can guide the synthesis of new materials and the development of new processing routes to access next-generation conducting polymers with tailored properties.

Conducting polymers are inherently piezoresistive; we showed that both the magnitude and polarity of the piezoresistive response of conducting polymers can be tuned by their crystallinity, particulate nature, and the compositional distribution in thin films, either at the onset of synthesis or with processing. Such tunability allowed us to attain a high gauge factor for strain sensing and a near-zero gauge factor for thermo-/chemo-resistive sensing applications with a single class of material. Additionally, we enhanced the stability of the piezoresistive response of PANI-PAAMPSA and PEDOT:PSS to humidity and strain rate via structural modification.

We also elucidated the correlations between the structure and thermoelectric properties of conducting polymers. The thermoelectric power factor of conducting polymer complexes is a strong function of both ion and hole/electron conductivities and is therefore very sensitive to processing-induced structural changes. We leveraged this sensitivity to decouple the electrical conductivity and the Seebeck coefficient of PANI-PAAMPSA and PEDOT:PSS, and increased their power factor by two orders of magnitude.

PANI-PAAMPSA and PEDOT:PSS can be combined in a single electrochromic window (ECW) as the electrochromic and charge-balancing layers, respectively. Their optical complementarity enhances the switching contrast of the ECWs without reducing the transparency of the clear state. We quantified the stability and kinetics of electrochromic switching, and showed that PANI-PAAMPSA/PEDOT:PSS-based ECWs can be powered by near-UV organic solar cells to regulate the transmission of visible light and near-infrared heat in a self-powered smart window prototype.