Intrinsic and Ionic Conduction in Humidity-Sensitive Sulfonated Polyaniline

The influence of humidity on the conductivity of sulfonated polyaniline (SPANI) and polyaniline (PANI) is investigated with electrochemical impedance spectroscopy (EIS). Separation of intrinsic (q) and ionic charge (i) mobility was observed using combination of ac and dc impedance measurements at relative humidity concentrations (RH) ranging from 5% to 90%. Nyquist plots for both acid-doped PANI and SPANI show single semicircles at all humidity levels, indicating pure intrinsic charge conduction. These semicircles are well-described with a fairly constant capacitance (C-1=10(-10) F) of the interdigitated electrodes and with a large varying resistance (10 k Omega-10 M Omega) of the polymer bulk with RC times in the order of 10(-3)-10(-6) sec. In all samples the intrinsic charge conductivity increases for increasing humidity, with emeraldine base PANI showing the strongest (2 orders of magnitude) increase upon raising the RH from 5-90%. A partial exchange of protons with sodium ions in SPANI (SPAN-Na) induces a second semicircle in the Nyquist plot at humidity levels above 40% with large RC (T) values (10(-1)-10(-3) sec), which is attributed to a contribution of ionic mobility at the polymer/electrode interface. Blends of SPAN-Na with poly(vinyl alcohol) (PVA) and polyacrylamide (PAA) also show a second semicircle at slightly higher humidity levels from 50% RH onwards. In all cases this second semicircle is best described by a Constant Phase Element (CPE) with a value (order typical 10(-9)-10(-7) F) increasing at higher humidity, and a varying exponential coefficient (0.5<n<0.8), possibly due to a changing dispersion in the representative relaxation times for migration and accumulation of the ionic species. In addition, for SPAN-Na/SPANI samples blended with PVA a third semicircle is seen which can be described by an additional RC element representing a second phase or grain boundary contribution in the film. (C) 2014 Elsevier Ltd. All rights reserved.