Theoretical quantification of pH-responsiveness of blend membrane

Abstract

In this work, a simple predictive mathematical model has been presented that quantifies the membrane parameters, e.g., permeability, effective pore diameter and number of charged units in a polymer chain in a pH-varying environment devoid of added salt. This has been validated experimentally for the case of poly(vinylidene fluoride) (PVDF) membranes blended with poly(methyl methacrylate)-co-poly(acrylic acid) (PMMA-co-PAA) additive copolymer. The effect of Donnan potential was incorporated into the model and estimated using electrokinetic characterizations. It varied considerably from 0.25 to −1.93 units (dimensionless) from pH 2 to 11. Variations of surface morphology and the electronic structure of the blend membrane with pH were studied in detail. Molecular simulations were performed to estimate the effective distance between the neighboring units in the charged/uncharged states and for confirming the copolymer chain behavior in different pH environments. Adsorption experiments were conducted to compare the variation of the number of uncharged units with pH with those estimated using the model. A pH-dependent permeability hysteresis behavior was observed which was explained using an intermolecular hydrogen bonding mechanism. About 80 % variation in the permeability of the blend membrane was observed between the pH limits which was in close agreement with the model results.