Swelling and dimensional changes of poly(2-methylaniline) upon redox cycling. Anion Influence.

L. LIZARRAGA; F. V. MOLINA; D. POSADAS; E.M. ANDRADE

Warshaw

Encuentro; The 51º Annual Meeting of International Electrochemistry Society; 2000

International Electrochemistry Society

We have studied the volume changes of poly(2-methylaniline) during cyclic potential scan between leucoemeraldine and emeraldine forms. These studies were performed using voltammetricexperiments and simultaneous optical microscopy. Two different situations were considered: continuous cycling and waiting at the cathodic limit until the polymer film is completely reduced. The experiments were carried out in different acidic media; the influence of acid concentration was also assessed. In the cell, the electrode was placed face up at the bottom of a channel. The channel top was an optical glass window facing the microscope objective under incident light. With this arrangement, voltammetric experiments were performed and simultaneously video images were captured. These images were processed to detect the area showed by the polymer film. In the video images, the polymer film is seen approximately as a circle darker than the background; this is actually a projection of the film onto the image plane and the volume changes are observed as variations of the area detected in the images. The relative area changes, DA/A, are used as a measure of the volume changes. In continuous potential cycling, the film shows smooth, relatively low volume changes with some hysteresis. The magnitude of this volume change depends on the electrolyte anion. When the polymer is completely reduced and relaxed, its volume reaches a minimum. In the following anodic scan, there is an important volume increase, an order of magnitude higher than in continuous cycling. This behaviour does not depend on the acid used. During the cathodic scan, however, the dimensional changes are almost similar for both situations (continuous cycling and completely reduced film). The changes observed in continuous cycling are affected by both the size and acidity constant of the electrolyte. The latter manifests itself through the voltammetric charge: less strong acids show a lower charge. These results seem to be the consequence of three different effects: pH, ionic exchange with the electrolyte and structural modifications of the polymer backbone. It is well known that, unless the pH is very low, anions ingress into the film upon oxidation and egress during reduction, possibly carrying solvent molecules. This results in polymer swelling/deswelling, and our results indicate that it is thedominant process in the short term. Polymer backbone changes should be related to the amine / quinone imine transition of the aromatic rings upon redox switching. Our results indicate that these processes cause a higher volume change than the swelling/deswelling, but the relaxation from the oxidized (quinone imine) to the reduced (amine) structures has a considerably larger time constant, thus during continuous potential cycling, even at the relatively low scan rate employed here, the main volume effect is due to anion exchange with swelling/deswelling processes.