ABSTRACT
Polythiophene
derivatives are amongst the most technologically promising conducting
polymers both for their high stability in both their doped and
undoped states, and for their processability. For these reasons
polythiophene derivatives are excellent candidates for use as
electrochromic conducting polymers.
It is
well known that the inclusion of metal moieties in conducting polymer
structures can alter the polymer's properties. In order to explore
this phenomenon, polythiophene derivatives with pendant transition
metal sulfonate groups were synthesised from the ammonium salt. These
polymers were then studied by cyclic voltammetry (CV) and their key
electrochromic properties were examined. The colour change produced
upon doping was measured, as were the kinetics of this colour change
and the stability of the doped state. The different metal ions caused
small changes in the colour of the undoped and doped polymer. The
interband absorption peaks of the more ionic metal salts were
red-shifted relative to the more covalent ones. In addition, salts of
metals good at complexing were insoluble in water, unlike the salts
of metals that are not. The ability to complex also correlated with
the stability of the doped state, the better complexing metal salts
being more stable. The switching times were a few seconds for most of
the polymers, although the chromium, copper and tin salts were
significantly slower.
Poly(3-alkylthiophene)s
(P3AT) are of great commercial and scientific interest because of
their electrical conductivity, stability in air and processability.
The most cost effective method of their synthesis is oxidative
polymerisation using iron (III) chloride. It has been found that this
can be enhanced by small amounts of nickel and cobalt chloride used
during polymer synthesis. In order to understand this phenomenon, the
mechanism of the oxidative polymerisation of 3-hexylthiophene (3HT)
and the effects of reaction modifiers on the resulting polymer
properties were investigated. Gel permeation chromatography (GPC),
UV-visible spectroscopy and proton NMR spectroscopy were employed to
investigate the effects of specific organic inhibitors and of cobalt
chloride catalyst. The mechanism of polymerisation was identified
using inhibitors that each affected one specific reaction pathway.
Polymerisation was observed to proceed by two different mechanisms,
one of which was greatly inhibited by cobalt chloride whilst the
other was enhanced. Cobalt chloride was found to increase the
molecular weight of the polymer and to make it more regioregular. As
a result, the conductivity of the iodine-doped polymer was strongly
enhanced.
