Figure 1: Azure A (C14H14N3S).
Such polymers, also termed as Intrinsically Conducting Organic Polymers, show said properties because of the presence of doping impurities and alternating single and double bonds throughout the carbon backbone. This way, impurities may favour the formation of accessible electronic levels in the band gap. This, in turn, changes the polymer from insulating (un-doped form) to conducting (doped form) upon applying an electrical voltage. This is the reason why most of such materials behave as semiconductors.
On the other hand, mobility and stability of the doping ions play a fundamental role in doping and de-doping processes: indeed, deactivation of the electrochemical activity for many polymers depends on them and knowledge of this piece of information is of paramount importance toward technological applications.
Azure A has been successfully electropolymerised on Indium-Tin Oxide electrodes (ITO) in the past, the reference electrode being Ag|AgCl|KCl(sat). Under these conditions, the potential onset is at about 1.2 V and characteristic redox processes happen between 0.5 V and -0.5 V.
In this work, electropolymerisation of Azure A on tITO and gold electrodes has been carried out from aqueous solution. The resulting polymer poly(Azure A) was characterised by electrochemistry (Cyclic Voltammetry, Chronoamperometry) Quartz Crystal Microbalance and spectroelectrochemistry, which gave insight about its electrochemical activity. This does not decrease because of electrode degradation but rather changes because of “in-and-out” ion exchange through the polymer matrix. In situ vis-NIR spectra were employed to deconvolute CV peaks. Self-electrchromic process and hydrogen production are separated by means this way. In addition, mass changes and motional resistance during the electrochemical polymerisation were monitored by Quartz Crystal Microbalance (E-QCM). Microcontainers formed by bubbles of the hydrogen electrocatalysed hydrogen evolution are observed by means SEM. This opens new and promising possibilities for using these clay of polymer material.
The repeated swelling-shrinking cycles that PAA underwent, due to “in-and-out” solvent may have been responsible for morphological changes of the polymer itself and lowered its interchain interactions, thus influencing the inter-monomer bonds. This is not uncommon: deactivation of poly(o-aminophenol) films has been related to alterations of its molecular structure because of solvent molecules crossing the polymer-surroundings interphase . Polymer hydration, along with breaking of inter-monomer bonds, may be producing less ion movement (especially anions) and therefore reduced electro-activity. This hypothesis could explain the restoration of PAA’s electrochemical activity following its dehydration, where ions become more mobile again.
[1] J. Agrisuelas, C. Gabrielli, J.J. Garcia-Jareno, D. Gimenez-Romero, H. Perrot, F. Vicente, Spectroelectrochemical Identification of the Active Sites for Protons and Anions Insertions into Poly-(Azure A) Thin Polymer Films, J. Phys. Chem. C. 111 (2007) 14230–14237.
[2] J. Agrisuelas, C. Gabrielli, J.J. García-Jareño, H. Perrot, O. Sel, F. Vicente, Polymer dynamics in thin p-type conducting films investigated by ac-electrogravimetry. Kinetics aspects on anion exclusion, free solvent transfer, and conformational changes in poly(o-toluidine), Electrochimica Acta. 153 (2015) 33–43.
[3] J. Agrisuelas, C. Gabrielli, J.J. García-Jareño, H. Perrot, O. Sel, F. Vicente, Electrochemically induced free solvent transfer in thin poly(3,4-ethylenedioxythiophene) films, Electrochimica Acta. 164 (2015) 21–30.
ACKNOWLEDGEMENTS
Part of this work was supported by FEDER-CICyT CTQ2015-71794-R J.