Ion exchange membranes are indispensable for the separation of ionic species. They can
discriminate between anions and cations depending on the type of fixed ionic group
present in the membrane. These conventional ion exchange membranes (CIX) have
exceptional ionic conductivity, which is advantageous in various electromembrane
separation processes such as electrodialysis, electrodeionisation and electrochemical ion
exchange. The main disadvantage of CIX membranes is their high electrical resistance
owing to the fact that the membranes are electronically non conductive. An alternative
can be electroactive ion exchange membranes, which are ionically and electronically
conducting. Polypyrrole (PPy) is a type of electroactive ion exchange material as well
as a commonly known conducting polymer. When PPy membranes are repeatedly
reduced and oxidised, ions are pumped through the membrane.
The main aim of this thesis was to develop electroactive cation transport membranes
based on PPy for the selective transport of divalent cations. Membranes developed
composed of PPy films deposited on commercially available support materials. To carry
out this study, cation exchange membranes based on PPy doped with immobile anions
were prepared. Two types of dopant anions known to interact with divalent metal ions
were considered, namely 4-sulphonic calix[6]arene (C6S) and carboxylated multiwalled
carbon nanotubes (CNT). The transport of ions across membranes containing
PPy doped with polystyrene sulphonate (PSS) and PPy doped with para-toluene
sulphonate (pTS) was also studied in order to understand the nature of ion transport and
permeability across PPy(CNT) and PPy(C6S) membranes. In the course of these studies,
membrane characterisation was performed using electrochemical quartz crystal
microbalance (EQCM) and scanning electron microscopy (SEM). Permeability of the
membranes towards divalent cations was explored using a two compartment transport
cell.
EQCM results demonstrated that the ion exchange behaviour of polypyrrole is
dependent on a number of factors including the type of dopant anion present, the type of
ions present in the surrounding medium, the scan rate used during the experiment and
the previous history of the polymer film. The morphology of PPy films was found to
change when the dopant anion was varied and even when the thickness of the film was
altered in some cases. In nearly all cases the permeability of the membranes towards
metal ions followed the order K+ > Ca2+ > Mn2+. The one exception was PPy(C6S), for
which the permeability followed the order Ca2+ ≥ K+ > Mn2+ > Co2+ > Cr3+. The above
permeability sequences show a strong dependence on the size of the metal ions with
metal ions having the smallest hydrated radii exhibiting the highest flux. Another factor
that affected the permeability towards metal ions was the thickness of the PPy films.
Films with the least thickness showed higher metal ion fluxes. Electrochemical control
over ion transport across PPy(CNT) membrane was obtained when films composed of
the latter were deposited on track-etched Nucleopore® membranes as support material.
In contrast, the flux of ions across the same film was concentration gradient dependent
when the polymer was deposited on polyvinylidene difluoride membranes as support
material. However, electrochemical control over metal ion transport was achieved with
a bilayer type of PPy film consisting of PPy(pTS)/PPy(CNT), irrespective of the type of
support material.
In the course of studying macroscopic charge balance during transport experiments
performed using a two compartment transport cell, it was observed that PPy films were
non-permselective. A clear correlation between the change in pH in the receiving
solution and the ions transported across the membrane was observed. A decrease in
solution pH was detected when the polymer membrane acted primarily as an anion
exchanger, 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 polymer
membrane, the pH in the receiving solution was in the range 6 - 8. These observations
suggest that macroscopic charge balance during the transport of cations and anions
across polypyrrole membranes was maintained by introduction of anions (OH-) and
cations (H+) produced via electrolysis of water
