Electroactive ion exchange membranes based on conducting polymers

摘要

Ion exchange membranes are indispensable for the separation of ionic species. They candiscriminate between anions and cations depending on the type of fixed ionic grouppresent in the membrane. These conventional ion exchange membranes (CIX) haveexceptional ionic conductivity, which is advantageous in various electromembraneseparation processes such as electrodialysis, electrodeionisation and electrochemical ionexchange. The main disadvantage of CIX membranes is their high electrical resistanceowing to the fact that the membranes are electronically non conductive. An alternativecan be electroactive ion exchange membranes, which are ionically and electronicallyconducting. Polypyrrole (PPy) is a type of electroactive ion exchange material as wellas a commonly known conducting polymer. When PPy membranes are repeatedlyreduced and oxidised, ions are pumped through the membrane.The main aim of this thesis was to develop electroactive cation transport membranesbased on PPy for the selective transport of divalent cations. Membranes developedcomposed of PPy films deposited on commercially available support materials. To carryout this study, cation exchange membranes based on PPy doped with immobile anionswere prepared. Two types of dopant anions known to interact with divalent metal ionswere considered, namely 4-sulphonic calix[6]arene (C6S) and carboxylated multiwalledcarbon nanotubes (CNT). The transport of ions across membranes containingPPy doped with polystyrene sulphonate (PSS) and PPy doped with para-toluenesulphonate (pTS) was also studied in order to understand the nature of ion transport andpermeability across PPy(CNT) and PPy(C6S) membranes. In the course of these studies,membrane characterisation was performed using electrochemical quartz crystalmicrobalance (EQCM) and scanning electron microscopy (SEM). Permeability of themembranes towards divalent cations was explored using a two compartment transportcell.EQCM results demonstrated that the ion exchange behaviour of polypyrrole isdependent on a number of factors including the type of dopant anion present, the type ofions present in the surrounding medium, the scan rate used during the experiment andthe previous history of the polymer film. The morphology of PPy films was found tochange when the dopant anion was varied and even when the thickness of the film wasaltered in some cases. In nearly all cases the permeability of the membranes towardsmetal ions followed the order K+ > Ca2+ > Mn2+. The one exception was PPy(C6S), forwhich the permeability followed the order Ca2+ ≥ K+ > Mn2+ > Co2+ > Cr3+. The abovepermeability sequences show a strong dependence on the size of the metal ions withmetal ions having the smallest hydrated radii exhibiting the highest flux. Another factorthat affected the permeability towards metal ions was the thickness of the PPy films.Films with the least thickness showed higher metal ion fluxes. Electrochemical controlover ion transport across PPy(CNT) membrane was obtained when films composed ofthe latter were deposited on track-etched Nucleopore® membranes as support material.In contrast, the flux of ions across the same film was concentration gradient dependentwhen the polymer was deposited on polyvinylidene difluoride membranes as supportmaterial. However, electrochemical control over metal ion transport was achieved witha bilayer type of PPy film consisting of PPy(pTS)/PPy(CNT), irrespective of the type ofsupport material.In the course of studying macroscopic charge balance during transport experimentsperformed using a two compartment transport cell, it was observed that PPy films werenon-permselective. A clear correlation between the change in pH in the receivingsolution and the ions transported across the membrane was observed. A decrease insolution pH was detected when the polymer membrane acted primarily as an anionexchanger, while an increase in pH occurred when it functioned as a cation exchanger.When there was an approximately equal flux of anions and cations across the polymermembrane, the pH in the receiving solution was in the range 6 - 8. These observationssuggest that macroscopic charge balance during the transport of cations and anionsacross polypyrrole membranes was maintained by introduction of anions (OH-) andcations (H+) produced via electrolysis of water.