Year

2000

Degree Name

Doctor of Philosophy

Department

Department of Chemistry

Abstract

The use of Conducting Electroactive Polymers (CEPs) to transport metal ions has been employed in recent years. The use of polyelectrolytes (PEθs) to facilitate this has only been recently exploited. The first part of this work examined the preparation, characterisation and use of a polypyrrole/poly(vinyl phosphate) (PPy/PVP) polymer in the transport of various metal ions. PPy/PVP was prepared electrochemically using potentiodynamic, potentiometric and potentiostatic methods from aqueous solutions of pyrrole and polyvinyl phosphate). Post-polymerisation cyclic voltammograms (CVs) indicated that galvanostatic growth gave rise to thin films that were the most electroactive and subsequent growth experiments employed this technique.

PPy thin films are usually grown from solutions that are 0.20 M in pyrrole. However, in this work PPy/PVP membranes were grown from monomer solutions that contained 0.60 M pyrrole. It was found that this concentration gave rise to the thickest and most conducting films. Growth experiments examined a range of PVP concentrations which found that films grown from solutions with 0.20% (w/w) PVP produced the most electroactive films, as evidenced by post polymerisation CVs.

Physical characterisation methods indicated the polymer had a ratio of 10:1 for nitrogen phosphorous, a water content of 28 ± 3% and a conductivity of 0.28 S cm-1. The polymer, as shown by SEM and AFM, has a rough amorphous morphology. Electrochemical studies using electrodes with thin polymer films indicated that oxidation/reduction of the material varied depending on whether potassium, sodium, lithium, calcium or magnesium ion was present. Transport of each of these ions across PPy/PVP membranes electrodeposited onto platinum coated polyvinylidene fluoride (PVDF) membranes was achieved using electrochemical control. These studies can be performed using a single membrane transport stirred cell and a pulsed potential waveform with potential values of -0.80/+0.45 V and a 50 s pulse width. It was found that transport of metal ions across PPy/PVP composite membranes followed the order: K+ > Na+ > Li+ for the group 1 cations and Ca2+ > Mg2+ for the group 2 cations. Transport of Cu2+ ion was also achieved, with flux values similar to Na+.

There is a wide range of applications for CEPs however they are limited in their ability to be fashioned into devices due to their insolubility in common solvents and their intractability. The second part of this research examined the use of PPy conducting polymer colloids consisting of PPy doped with PVP that were produced by an electrohydrodynamic polymerisation in the absence of an added steric stabiliser using a three compartment flow-through cell.

Elemental analysis indicated that the colloids were more highly doped than the corresponding thin films composed of the same material. Colloidal particles displayed very low conductivities but were electroactive, while their zeta potentials showed that they were negatively charged. Transmission Electron Microscopy was used to characterise the colloids grown. This showed there were small spherical and raspberry-like particles present. In addition, the effects of variations in flow rate and applied potential on the yield of the electrochemically prepared colloids were also investigated. The colloids were shown to adsorb calcium, copper and iron ions from aqueous solutions according to the series Cu2+ > Fe2+ > Ca2+.

The third part of this work investigated electrochemically facilitated transport of aqueous mixtures containing Na+, K+, Ca2+ and Mg2+ across conducting polymer composite membranes. Experiments were also conducted with the same cations but were examined as individual ions. Competition experiments were also performed with Cu2+/Fe2+ mixtures. The composite membranes consisted of PPy doped with poly(styrene sulfonate)/dodecylbenzenesulfonate (1%) (PPy/PSS/DBS), hydroxyquinoline sulfonic acid/sulfuric acid (PPy/HQS/SO/O, poly (vinyl phosphate)/dodecylbenzenesulfonate (1%) (PPy/PVP/DBS) and ρ-toluene sulfonic acid) (PPy/ρTS). These CEPs were deposited onto a platinum sputter coated poly(vinylidene fluoride) filter (0.22μm). All four membranes were utilised in transport experiments using a flow-through cell using constant potential techniques while PPy/PSS/DBS and PPy/PVP/DBS were used in conjunction with the flow cell and pulsed potential waveform values of -0.80 V and +0.45 V. These two composites were also employed with the same pulsed potential waveform and a stirred cell.

The flux of Na+, K+ and Ca2+ ions was significantly higher when transport was examined using the flow-through cell and driven by application of a constant potential. In all systems examined the flux of metal ions followed the sequence K+ > Na+ > Ca2+ > Mg2+. Transport of each metal ion was more facile under all conditions examined across composite membranes containing polypyrrole doped with PPy/PSS/DBS. Atomic Force Microscopic examination of the surface of this membrane showed it to have a significantly smoother surface morphology compared to the other PPy doped materials.

The last part of this research examined the doping of emeraldine base with the sulfonated calixarene host species, calix[4]-ρ-tetrasulfonic acidand calix[6]-ρ-hexasulfonic acid in water or DMSO solvents. This process yielded the conducting emeraldine salts Pan.calix[4]S03H and Pan.calix[6]S03H. The colloidal polyaniline products were characterised by UV-visible and FTIR spectra, cyclic voltammetry, particle size analysis and transmission electron microscopy. The sulfonated dopants conferred enhanced stability on the emeraldine salts towards alkaline de-doping, with only partial conversion emeraldine base occurring even at pH 14. The de-doped mixtures could be re-doped to emeraldine salts using 0.10 M HC1.

The emeraldine salts were readily oxidised by persulfate ion at pH 2 to the pernigraniline salt forms. Oxidation of the dedoped mixtures at pH 12 generated pernigraniline base. Reduction at pH 2 and the de-doped mixtures at pH 12 using hydrazine hydrate yielded the fully reduced leucoemeraldine base.

Share

COinS