Permeability hysteresis of polypyrrole-polysulfone blend ultrafiltration membranes: study of phase separation thermodynamics and pH responsive membrane properties

Polypyrrole (PPy) is a promising conducting polymer with tunable redox properties that were exploited to prepare pH sensitive smart membranes. These membranes exhibited permeability hysteresis having > 75% variation between the two extreme pH values. The morphological changes were studied using field emission scanning electron microscopy and atomic force microscopy. Membrane zeta potential also changed from -10 mV at pH 3 to -25 my at pH 9. The synergistic effect of pore constriction along with electrostatic interaction due to high negative membrane zeta potential resulted into increase in bovine serum albumin rejection from 68% at pH 3 to 98% at pH 12. The mechanism of pore shrinking was proposed based on structural changes of PPy and confirmed using Fourier transform infra red spectroscopy. Thermodynamic modeling was carried out using Flory-Huggins theory of polymer solution to obtain ternary phase diagram of PPy-polysulfone membrane systems. Solubility parameter of PPy, required for modeling, was determined by group contribution method. Binodal curves at varying PPy concentration were used to quantify thermodynamic hindrance. Trade-off between thermodynamic hindrance and kinetic enhancement of phase inversion was used to predict membrane morphology and permeation characteristics. Model based predictions were validated using morphology and permeability trends available in literature.