Electrochemical Composite Membranes based on Intrinsically Conducting Polymers

Abstract

Membranes based on intrinsically conducting polymers (ICPs) have been employed in various membrane processes such as gas separation, pervaporation, nanofiltration and electrodialysis. The change in the membrane morphology, hydrophilicity, and ion exchange behaviour based on the oxidation state and doping levels of ICP have been used to enhance permeability and selectivity. In this thesis, a highly permeable membrane with high selectivity was developed by depositing polyaniline (PANI) on the pore walls of a microporous base membrane without blocking the pores. The layering of positively charged polyaniline originates electrolyte polarisation in the pores and permselectivity is achieved by the electrostatic screening of permeating ions through the membrane. Polyaniline (PANI) was deposited on mixed-cellulose ester (ME) microporous membranes by using various in situ chemical oxidative polymerization techniques. These include solution-phase polymerization, vapour-phase polymerization and diaphragmatic polymerization in a two-compartment cell. The composite membranes were characterized by scanning electron microscopy (SEM), gravimetric PANI content measurement, Fourier-transform infrared (FTIR-ATR) spectroscopy and x-ray photoelectron spectroscopy (XPS). The solution-phase and vapour-phase polymerizations yielded PANI layering on the surface of the base membrane whereas PANI was deposited on the pore walls of the membrane by using the two-compartment cell technique. FTIR and XPS results showed PANI deposition in its emeraldine salt state and C1- doping was polymerization time dependent. XPS quantified the extent of PANI layering at the surface that was polymerization time dependent. The solution-phase polymerization yielded an incomplete surface layering as compared to the vapour-phase polymerization. Surface and trans-membrane electrical conductivities were measured by using four-point micro probe and two-point probe techniques, respectively. These conductivities showed dependence on PANI deposition site and extent in the membranes. Electrochemical characterization of the composite membranes was conducted by using electrochemical impedance spectroscopy (EIS) and transport numbers measurements. EIS data were analysed by using equivalent circuit modelling technique. The results showed the dependence of charge transport resistance of the membranes on PANI deposition site, extent and doping levels. In-pore PANI deposition in the membranes showed several orders of magnitude lower levels of resistance and higher capacitance due to the polarisation of pore electrolyte. In addition, the low values of diffusional resistance and high capacitance indicate anion-coupled charge transport in the membrane through PANI polaron/bipolaron transitions. The composite membranes with PANI layering only at the surface or undoped PANI showed higher diffusional resistance and low capacitance due to slow electronic/ionic diffusion inside the bulk membrane. Transport numbers of counter-ions in the composite membranes showed high anion selectivity at low pH (in HCl) as compared to the membranes at high pH (~12). The transport numbers showed the weak dependence on PANI deposition site and levels.