Evolution of morphology, segmental dynamics, and conductivity in ionic liquid swollen short side chain perfluorosulfonate ionomer membranes

J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1273-1280 Many authors have observed a critical uptake where both the conductivity and electroactive responses of ionic liquid (IL) swollen ionomer actuators improved markedly. In this work, the mechanical dynamics, conductivity, and morphology of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMI-Tf) swollen Aquivion membranes are explored. The absorbed EMI-Tf is mainly bounded in the cluster region of Aquivion. Despite the observed critical uptake effects, the segmental relaxation frequency still plays a major role for the ionic conduction in these IL swollen membranes over a broad uptake range.

We investigate the morphology, segmental dynamics, and conductivity of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMI-Tf) swollen short side chain perfluorosulfonate ionomer (Aquivion) over a broad uptake range using small angle X-ray scattering (SAXS), dielectric relaxation spectroscopy, and transient current measurement. The SAXS data indicate that the absorbed EMI-Tf is mainly bounded in the ionic region of Aquivion. At low uptakes, EMI-Tf acts as an effective plasticizer lowering the cluster T_g and markedly shifting the segmental relaxation to a high frequency; however, at high uptakes, the additional EMI-Tf acts like a filler instead. A time-domain model was employed to quantify the conductivity of these membranes containing two mobile ion species, that is, cations and anions. The conductivity of both neat EMI-Tf and EMI-Tf swollen membranes exhibits Vogel-Fulcher-Tamman relation, revealing different activation parameters for ionic conduction. Furthermore, membranes containing different EMI-Tf uptakes have similar conductivity over the reduced T_g/T axis and also follow Debye-Stokes-Einstein relation. Therefore, despite the abrupt change in conductivity near the critical uptake (29 wt %), both cluster T_g and segmental motion remain the key factors for the ionic conduction in these EMI-Tf swollen membranes.