Studies on polyelectrolyte brushes
University of Sheffield
Mark Geoghegan is a lecturer in
Physics at the University of Sheffield. His research interests
centre on the study of polymer films, polymer diffusion, and
polymer gels using neutron reflectometry, ion beam analysis, and
scanning force microscopy. He is interested in all-polymer
electronic devices (field-effect transistors, photovoltaics, and
LEDs), biopolymer interfaces, "smart" materials (i.e. switchable
polyelectrolytes) as well as related fundamental polymer physics.
The work in polyelectrolytes is part of a wider "soft
nanotechnology" research theme and is performed in close
collaboration with Professor Richard Jones in the Department of
Physics and Astronomy and Professors Tony Ryan and Steve Armes
and Dr Linda Swanson in the Department of Chemistry.
Above right: Archive photo of MG working inside
the CRISP neutron reflectometer beamline at the ISIS spallation
source.
In PolyFilm we intend to improve
our understanding of polyelectrolyte brushes synthesised using
atom transfer radical polymerisation (ATRP) with detailed neutron
reflectometry experiments. ATRP is a synthetic method enabling
the production of dense layers of grafted polymers (brushes). The
pH behaviour of polyelectrolyte brushes will induce collapse or
swelling depending on pH; for example, a polyacid in basic
solution will be stretched because of the Coulombic repulsion
between negative charges located along the polymer chain while in
acidic solution these charges are neutralised and the chains
collapse because they are intrinsically hydrophobic.
Above: Scanning force microscopy (10 µm x 10
µm) image of polymethacrylic acid gel collapsed (38 nm
high) and swollen (250 nm high) in different pH aqueous
environments. Image courtesy of Jon Howse.
We have observed this collapse
and swelling using neutron reflectometry experiments. However, a
detailed comparison of the brush conformation and that of
polymers in dilute solution is presently missing. The complexity
of such a problem is partly because it is unclear what the pH is
within the confined brush layer; there is no reason why it should
be the same as in the surrounding solution. One may suppose that
the polymer will collapse at more extreme values of pH than in
solution. Beyond these basic science studies we intend to tailor
the surfaces with different properties to take advantage of the
pH responsive behaviour. In particular we are interested in
gradient densities in the brushes, lithographed channels within
the brushes, and polymer and small molecule diffusion on or
around the brush. Diffusion measurements can be made by
fluorescence techniques such as fluorescence correlation
spectroscopy.
