Composite
polyelectrolyte multilayers of chitosan and low molecular
weight poly(acrylic acid) (PAA) have been assembled by sequential
adsorption as a first step toward building a surface anchored chitosan
gel. Silane chemistry was used to graft the first chitosan layer to
prevent film detachment and decomposition. The assembly process is
characterized by nonlinear growth behavior, with different adsorption
kinetics for chitosan and PAA. In situ analysis of the multilayer
by means of surface sensitive total internal reflection Raman (TIRR)
spectroscopy, combined with target factor analysis of the spectra,
provided information regarding composition, including water content,
and ionization state of weak acidic and basic groups present in the
thin composite film. Low molecular weight PAA, mainly in its protonated
form, diffuses into and out of the composite film during adsorption
and rinsing steps. The higher molecular weight chitosan shows a similar
behavior, although to a much lower extent. Our data demonstrate that
the charged monomeric units of chitosan are mainly compensated by
carboxylate ions from PAA. Furthermore, the morphology and mechanical
properties of the multilayers were investigated in situ using atomic
force microscopy operating in PeakForce tapping mode. The multilayer
consists of islands that grow in lateral dimension and height during
the build-up process, leading to close to exponentially increasing
roughness with deposition number. Both diffusion in and out of at
least one of the two components (PAA) and the island-like morphology
contribute to the nonlinear growth of chitosan/PAA multilayers
