The structure of layer-by-layer (LbL)
deposited nanofilm coatings consists of alternating polyethylenimine
(PEI) and polystyrenesulfonate (PSS) films deposited on a single crystal
quartz substrate. LbL-deposited nanofilms were investigated by neutron
reflectomery (NR) in contact with water in the static and fluid shear
stress conditions. The fluid shear stress was applied through a laminar
flow of the liquid parallel to the quartz/polymer interface in a custom-built
solid–liquid interface cell. The scattering length density
profiles obtained from NR results of these polyelectrolyte multilayers
(PEM), measured under different shear conditions, showed proportional
decrease of volume fraction of water hydrating the polymers. For the
highest shear rate applied (ca. 6800 s<sup>–1</sup>) the water
volume fraction decreased by approximately 7%. The decrease of the
volume fraction of water was homogeneous through the thickness of
the film. Since there were not any significant changes in the total
polymer thickness, it resulted in negative osmotic pressures in the
film. The PEM films were compared with the behavior of thin films
of thermoresponsive poly(<i>N</i>-isopropylacrylamide) (pNIPAM)
deposited via spin-coating. The PEM and pNIPAM differ in their interactions
with water molecules, and they showed opposite behaviors under the
fluid shear stress. In both cases the polymer hydration was reversible
upon the restoration of static conditions. A theoretical explanation
is given to explain this difference in the effect of shear on hydration
of polymeric thin films