Volume Changes of Aryl Amine Polymers upon Redox Switching

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

Salt Lake City, Utah

Encuentro; 202º Meeting of The Electrochemical Society; 2002

The Electrochemical Society

Aryl amine conducting polymers such as poly (aniline) (PANI) and derivatives undergo dimensional changes when subject to redox switching (1-3). Here we report volume change studies of poly(2-methoxyaniline) (PMOA) and PANI. Thick polymer films of ~ 2 C cm-2 where synthesized from 0.5 M monomer solutions in 1.5 M HClO4 + 1.5 M H2SO4. Cyclic voltammetry with simultaneous microscopical observation was performed in a suitably designed cell (Fig. 1) using 1 M HClO4 or H2SO4 as electrolyte. A hydrogen electrode in the same solution was used as reference, and a platinum foil as auxiliary. was used as reference, and a platinum foil as auxiliary. Initially, the potential was held at the cathodic limit (0.1 V) for 90 min. so as to have the film fully reduced and relaxed (4). A cyclic potential scan between 0.1 and 0.6 V at 12 mV s-1 was subsequently started, recording the current and simultaneously capturing video images at a frame rate of 0.5 s-1. The volume changes were measured through the relative area changes, DA/A0, of the observed polymer film image. Fig. 2 shows the results obtained in H2SO4. In both cases the change in the first cycle is greater than that of the stationary regime, but for PMOA it is about an order of magnitude higher. When 1 M HClO4 is used as electrolyte, the results for both polymers are similar to those of Fig. 2(a). Qualitatively, the behavior of these polymers is similar to that of poly(2-methylaniline) (3). The larger change in the first cycle is assigned to conformational changes of the polymer backbone; the reverse process is slow and is not observed during the following cycles, occurring only after a long holding time at the negative potential limit. On the other hand, the smaller change in the stationary cycles is assigned to the polymer swelling/shrinking due to the exchange of electrolyte and water with the external solution, caused by polymer oxidation/reduction. However, the larger changes found for the PMOA/H2SO4 system cannot be explained by the previous considerations. To investigate further this case, ex-situ diffuse reflectance FTIR studies were conducted following a procedure previously employed (5). The results for oxidized PMOA in HClO4 and H2SO4 after baseline subtraction are shown in Fig. 3. A noticeable broadening at 2700-3300 cm-1 is observed in the presence of H2SO4, indicating the presence of Hbonding. This suggests that H-bonded structures, possibly between the OH of the HSO4-ion with the polymer -OCH3 group, causing steric hindrances and so a greater film expansion upon oxidation.