Roughness and Salt Annealing in a Polyelectrolyte
Multilayer
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Abstract
The surface roughness
of polyelectrolyte multilayers made from poly(diallyldimethylammonium
chloride),
PDADMAC, and poly(styrene sulfonate), PSS, was measured as a function
of film deposition conditions. For dry multilayers, the significant
roughness which builds up for thicker films is much more apparent
for multilayers terminated with PSS. Corresponding roughness for PDADMA-capped
multilayers may be seen by imaging in situ under electrolyte. Roughness
may be substantially reduced, but not eliminated, by annealing in
salt. Annealing does not lead to loss of polyelectrolyte from the
film, even under conditions where the salt concentration is high enough
to place the film properties beyond the glass transition. Roughness
does not correlate with the molecular weight of the polyelectrolyte
and is thus not caused by solution or film polymer chain conformations.
The wavelength of the roughness features is approximately proportional
to film thickness, which supports a mechanism whereby roughness is
generated by anisotropic swelling due to water and polyelectrolyte
addition in a manner similar to water uptake in hydrogels. Roughness
is preserved by the glassy PSS layer and probably incorporated within
the film as it grows