Water sorption of acid-doped polyaniline powders and hollow fibers: Equilibrium and kinetic response

Measurements are reported for the sorption equilibrium and transport of water vapor into polyaniline (PANI) powders and asymmetric, microporous hollow fibers. Equilibrium isotherms at 298 K were measured on powders doped with 13 different organic and inorganic acids to change the polyaniline from its emeraldine base, PANI-EB (insulating) form, to the emeraldine salt, PANI-ES (conducting) form. The powders were exposed to air streams of varying relative humidity (RH) from 15 to 80%. The four acid dopants, H ₃PO ₄, HPF ₆, HBF ₄, and CF ₃SO ₃H, provided the highest water sorption figures-of-merit and also displayed nonlinear isotherms. At 50% RH, these four doped powders adsorbed 0.9-1.6 molecules of water per PANI-ES repeat unit. Measurements of the sorption kinetics on the hollow fibers were done with one dopant acid, H ₃PO ₄. The data were obtained under exposure to air streams at an ambient pressure of ∼0.1 MPa with relative humidity ∼50% and temperatures of 300-309 K. The sorption process was well-described by unsteady Fickian diffusion into an infinite hollow cylinder. The asymmetric and porous nature of the hollow fiber's wall was represented by using an effective medium approach. The quantitative adsorption/desorption rates and equilibrium capacities depended on the experimental conditions with the measured water capacities being between 33 and 75 (mg of H ₂O)/(g of dry polymer) under most conditions, though at the highest relative humidity (∼80% at 300 K) the fiber adsorbed almost 250 (mg of H ₂O)/g. For relative humidities ≤ 50%, when the mass loading was recalculated on a per (H ₂O molecule/polymer repeat unit) basis, the fiber had a capacity of ∼0.5-1 H ₂O per PANI-ES repeat unit. In this same range of humidities, the apparent diffusion coefficients varied between 0.21 and 0.67 × 10 ^-12 m ²/s, except for the case of desorption at 80% RH, wherein a value of 2.38 × 10 ^-12 m ²/s was obtained. In general, the apparent diffusion coefficients were always larger during desorption than during adsorption. The water capacity of the polyaniline hollow fibers is at the upper end of what is usually observed for glassy polymers and provides the possibility for exploiting their electronic conductivity, good mechanical strength properties, and desiccant qualities for advanced humidity control and sensing applications.