Understanding Correlations Between Structure and Redox Properties in Aqueously-Dispersible, Electrically-Conductive, Polymer-Acid-Doped Polyaniline

Abstract

Template synthesis of polyaniline, or PANI, on poly(2-acrylamido-2-methyl-1-propanesulfonic acid), or PAAMPSA, yields aqueously-dispersible PANI-PAAMPSA particles. Through pH-resolved cyclic voltammetry and UV-vis/NIR spectroscopy measurements, PANI-PAAMPSA exhibits stable and reversible transitions to and from PANI's fully oxidized, intermediate, and fully reduced oxidation states of pernigraniline, emeraldine salt, and leucoemeraldine, respectively, in buffer solutions across a pH range of 3-7. Above pH 7, PANI-PAAMPSA exhibits direct transitions between its pernigraniline and leucoemeraldine states. Each of these states possesses unique optical properties, thus imbuing PANI-PAAMPSA with polyelectrochromism without the need to incorporate any comonomers. Transitions between each of PANI's oxidation states approach 95% completion within 10 seconds. Hysteresis, however, is observed in the electrochromic response as the film is subjected to random cycling, a conditioning effect that is attributed to the gradual relaxation of PANI-PAAMPSA particles as the electrostatic interactions between the two polymers is electrochemically moderated. Solvent-annealing PANI-PAAMPSA in dichloroacetic acid (DCA) induces dramatic structural relaxations, resulting in significant enhancements in terms of stability and reversibility in PANI-PAAMPSA's polyelectrochromic response. This DCA treatment equilibrates the structure within PANI-PAAMPSA films, obviating the dynamic relaxation processes that occur during polyelectrochromic switching with untreated films.

The influence of internal film structure on PANI-PAAMPSA's polyelectrochromic ability is further investigated as a function of PANI-PAAMPSA particle size by controlling PAAMPSA's molecular characteristics. The kinetics of PANI-PAAMPSA's electrochromic transitions exhibit an inverse relationship between reaction rate and particle size. By modeling the transmission response, analogies are drawn between polymer crystallization kinetics and the propagation of reaction fronts between PANI's electrically insulating and conducting forms. Specifically, PANI-PAAMPSA's switching kinetics are limited by interparticle contacts, suggesting that the reaction front propagates across particles faster than between particles. Following DCA treatment, PANI-PAAMPSA's electrochromic response is hastened, stabilized, and invariant to the original particle size. As-spun films also demonstrate size-exclusivity with respect to the ionic radius of the buffer cation; these size-exclusion effects are eliminated following DCA-treatment, further confirming the significant influence of internal film structure on electrochromic kinetics.

Lastly, having established the dependence of redox properties on film structure, the redox chemistries of PANI-PAAMPSA can be manipulated to affect its structural, optical, and electrical properties. By chemically reducing PANI-PAAMPSA films, the electrostatic interaction between PANI and PAAMPSA can be eliminated. PAAMPSA subsequently relaxes in the presence of water vapor, after which the surfaces of the films smoothen dramatically. Following relaxation, the films are chemically oxidized back to their conductive state, during which the optical and electrical properties of the films undergo changes that are directly analogous to those associated with PANI films treated with performance-enhancing DCA. The interrelationships between the structural, optical, electrical, and redox properties of polymer-acid-doped conductive polymers revealed in this work provide novel insight into the behavior of these systems, and will help guide the development of future organic electronic materials.