(506i) Humidity-Dependent Mixed Ionic and Electronic Conduction of Conjugated Polyelectrolytes

Polymeric mixed ionic-electronic conductors (MIECs) are of broad interest to energy storage and conversion applications. A class of polymeric MIECs are conjugated polyelectrolytes (CPEs), which possess a pi-conjugated backbone imparting electronic transport along with side chains comprised of a pendant ionic group to allow for ionic transport. Here, our study focuses on the humidity-dependent structure-transport properties of poly[3-(potassium-n-alkanoate) thiophene-2,5-diyl] (P3KnT) CPEs with varied side-chain lengths of n = 4, 5, 6, and 7. UV-Vis spectroscopy along with electronic paramagnetic resonance (EPR) spectroscopy reveal the infiltration of water leads to a hydrated, self-doped state that allows for electronic transport. The resulting humidity-dependent ionic conductivity (σ_i) of the thin films shows a monotonic increase with relative humidity (RH) while electronic conductivity (σ_e) follows a non-monotonic profile. The values of σ_e continue to rise with increasing RH reaching a local maximum after which σ_e begins to decrease. P3KnTs with higher n values demonstrate greater resiliency to increasing RH before suffering decrease in σ_e. This drop in σ_e is attributed to two factors. First, disruption of the locally-ordered pi-stacked domains observed through in situ humidity-dependent grazing incidence wide angle X-ray scattering (GIWAXS) experiments can account for some of the decrease in σ_e. A second and more dominant factor is attributed to the swelling of the amorphous domains where electronic transport pathways connecting ordered domains are impeded. P3K7T is most resilient to swelling (based on ellipsometry and water uptake measurements) where sufficient hydration allows for high σ_i (1.0 × 10^-1 S/cm at 95% RH) while not substantially disrupting σ_e (1.7 × 10^-2 S/cm at 85% RH and 8.0 × 10^-3 S/cm at 95% RH). Overall, our study highlights the complexity of balancing electronic and ionic transport in hydrated CPEs.