Transport mechanism in anionic conductive ionomers from temperature and pressure conductivity measurements
Résumé
Anionic ionomers exclusively conducting by F−, Cl−, Br−, I−, BF4−, (CF3SO2)2N−, anions are synthesised by grafting a quaternary ammonium salt into an unsatured poly(oxyethylene) matrix. An experimental set up allows accurate measurements of their conductivity versus pressure, P (1–5000 bars) as well as versus temperature, T (290–390 K). Experimental data give access to an “apparent activation volume” decreasing with increasing temperature and currently between 10 and 30 cm3/mol in the temperature range 390–300 K.
A microscopic approach is developed assuming an activated mechanism for charge carrier formation and a free volume model for their mobility. According to this model the apparent activation volume would result from the contribution of two terms. The first one, negative, would be related to the formation of charge carriers, the second one, positive, to their migration. The negative value associated to charge carriers formation can be explained by a volumic contraction induced by the dissociation phenomena. The free volume values necessary to the anionic migration are close to the anion volume at room temperature. This result is in agreement with a weak interaction between polyethers and anions.