Year

2006

Degree Name

Doctor of Philosophy

Department

Department of Chemistry - Faculty of Science

Abstract

The fully sulfonated polyaniline, Poly(2-methoxyaniline-5-sulfonic acid) (PMAS), is a watersoluble and electroactive conducting polymer. Its high water solubility makes it a potentially very attractive material for a range of applications. However, to date problems with its synthesis from MAS monomer and the impure nature of the PMAS product have hindered its exploitation. This thesis successfully addresses these problems and then subsequently exploits the pure high molecular weight PMAS obtained as a novel electroactive dopant in composite materials with other conducting polymers. The initial focus of this thesis was the development of a rapid purification technique capable of fractionating in gram scale quantities the crude mixture of PMAS and low molecular weight oligomers formed in establish d syntheses of PMAS. A new method was successfully established using cross flow tangential filtration to obtain, for the first time, the high purity, high molecular weight (HMWt) and low molecular weight (LMWt) PMAS fractions. Gel permeation chromatography, UV-visible spectroscopy and four point probe conductivity were used for characterisation of these fractions. The new separation technique was found to be a more efficient, rapid and scalable method for purification of PMAS than the conventional dialysis previously employed. Results have shown that the presence of the LMWt PMAS has an unexpected influence upon the electronic properties of the HMWt PMAS such that conductivity increases by two fold after its effective removal. Detailed optimisation studies were then performed on the synthesis of PMAS via the chemical polymerisation of 2-methoxyaniline-5-sulfonic acid (MAS) monomer, using ammonium persulfate as oxidant. These demonstrated that experimental variables such as time, temperature, monomer oncentration, pH, and the rate of addition of oxidant influence the yield, molecular weight and conductivity of the resultant PMAS. Employing the optima l synthetic conditions resulted in a HMWt PMAS product with enhanced conductivity of 1.74 ± 0.09 S/cm, Mp 16,900 and recovered yield (after tangential cross flow filtration) of 50 % w/w. The pure HMWt and LMWt PMAS fractions obtained following the above purification were fully characterised using techniques such as cyclic voltammetry (CV), electron spin resonance (ESR), photoluminescence (PL) spectroscopy and transmission electron microscopy. The HMWt PMAS fraction (Mn 13,400 kDa, Mw 33,600 kDa, Mp 16,900 kDa and PDI 2.5) was found to be paramagnetic due to the presence of radical cations (polarons). In contrast, the LMWt PMAS fraction (Mn 2,400 kDa, Mw ,700 kDa, Mp 2,800 kDa and PDI 1.1) was non-electroactive and non-conducting with no paramagnetic properties. Both HMWt and LMWt PMAS fractions exhibited nanodimensional morphology. The recently reported photoluminescence properties of PMAS have been reassessed in this thesis, revealing that the LMWt PMAS fraction in the presence of HMWt PMAS was the source of the unexpected photoemission reported for aqueous PMAS. In contrast, pure HMWt PMAS was found to statically quench the photoluminescence of LMWt PMAS. These optimisation and purification studies provided an opportunity to study, for the first time, the incorporation of pure HMWt PMAS as a conducting polyelectrolyte dopant without the complication of oligomeric impurities, which were shown above to dramatically influence the properties of PMAS. Pure HMWt PMAS was incorporated as a dopant during chemical and electrochemical polymerisation of substituted polyaniline (PAn). PAn/PMAS materials were prepared as water-insoluble films using electrochemical polymerisation, and as highly dispersed nanomaterials via chemical polymerisation of aniline in the presence of HMWt PMAS. The incorporation of PMAS into the polyaniline was confirmed by elemental analysis, UV-vis spectroscopy and CV studies. The conductivity of the PAn/PMAS nanomaterial was found to be 3.0 ± 0.18 S/cm, approximately two orders of magnitude higher than that reported for polyaniline in the presence of non-conducting olyelectrolytes, such as poly styrenesulfonate). Signifcantly, this new class of material exhibited conductivity at neutral pH values where polyanilineitself is de-doped to give a non-conducting material. The enhanced PAn/PMAS stability was xploited in this thesis in the area of electrochromics, giving an enhanced electrochromic contrast (57.0 %ΔT at 770 nm). Its extended pH stability will also permit future studies into the utility of these materials in biological environments, which to date have been unavailable in unsubstituted PAn due to its lack of conductivity and electroactivity.

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