(417a) Investigating the Swelling Behavior of a Poly(Acrylic acid) Brush Via Quartz Crystal Microbalance w/Dissipation (QCM-D)

Polyelectrolyte brushes are
desirable for their swelling characteristics in aqueous conditions, making them
suitable for colloidal stabilizers, water-based lubricants, and biomaterials.
Weak polyelectrolyte brushes, or annealed brushes, are modifiable as their
charge density and resultant swelling behavior is altered by the environmental
pH or ionic strength, which affect the extent of chain ionization. Furthermore,
the swelling or structural behavior of weak polyelectrolyte brushes has been
shown to depend on a variety of factors, including the type of salt ion, the
valence of salt ion, and the brush grafting density. The effect of this
behavior has been demonstrated to have a significant influence on properties
like the coefficient of friction of the brush as well as the overall adhesion
or wettability. Weak polyelectrolytes have also been demonstrated to exhibit
specific behavior at certain salt concentrations according to the Hofmeister, or lyotropic, series,
which describes ions in terms of their ionic hydration strength. The Hofmeister phenomenon has been previously observed in
polyelectrolyte brushes, where swelling from ionic hydration is observed in
accordance with the series; however, it is not well understood, and Hofmeister hydration should be accounted for in both the
polyelectrolyte brush and the ionic solution. A popular technique for studying
the swelling behavior of weak polyelectrolyte brushes is Quartz Crystal
Microbalance with Dissipation (QCM-D), which can be used to measure subtle
changes in adsorbed mass and material viscoelasticity. However, in the previous
literature, the effect of the flow medium is often ignored in a blank crystal,¹ where the flow medium is susceptible to
significant changes in density from Hofmeister
hydration that cannot be neglected.

In this work, the effect of the
ionic strength of the flow medium was evaluated on a bare QCM-D crystal and
accounted for to properly assess the swelling behavior of a weak
polyelectrolyte brush in the presence of ionic solution. This effect was
investigated for multiple ion types, for various brush grafting densities, and
for varying extents of brush ionization. We measure
swelling/de-swelling behavior of a weak polyelectrolyte brush of short-chain Poly(acrylic acid)
(PAA) self-assembled on a gold electrode in the presence of K⁺ and
Na⁺ ions at ionic strengths ranging from 10² to 10³mM using QCM-D, accounting for the effect of the salt
type and ionic strength on the brush-free QCM-D response, which was ignored in
previous work.¹

The PAA brush was formed by the
deposition of end-thiol-terminated PAA onto the gold
substrate, and the grafting density was varied by diluting the deposition
solution. Variable Angle Spectroscopic Ellipsometry
(VASE) was used to characterize the resultant grafting densities, and
demonstrated that grafting densities ranging from moderate to extremely high
were achieved. Even in the absence of the brush, using a bare gold surface,
significant salt type and concentration effects on QCM-D frequency and
dissipation are observed which roughly
parallel the effect of salt on fluid density, in accordance with the Hofmeister series. This effect is especially strong
for ions such as Ca²⁺ or Li⁺, and for this reason these
ions were not investigated in this study. Correcting for this effect, we measure swelling/de-swelling behavior as a
function of brush grafting density, ion type, ionic strength, and extent of
ionization. At pH 7, for KCl at lower grafting
densities, based on the measured dissipation, the brush swells at low ionic
strength, evidencing the so called  osmotic brush regime, but de-swells
with increasing ionic strength, exhibiting the well known polyelectrolyte
effect induced by charge screening from K⁺counterions.
For NaCl at pH 7, the polyelectrolyte brush swells
with increasing ionic strength at all grafting densities, which is consistent
with known tendency of Na⁺ to hydrate more strongly than does K⁺
(Figure 1).

Figure 1- Schematic Diagram depicting the effect of salt type (Na⁺ or K⁺)
and grafting density, σ (chains/nm²) on swelling behavior at pH
7.

References

1) Zhang, Z.; Moxey, M.;
Alswieleh, A.; Morse, A. J.; Lewis, A. L.; Geoghegan, M.; Leggett, G. J. Effect
of Salt on Phosphorylcholine-based Zwitterionic Polymer Brushes. Langmuir
2016, 32 (20), 5048–5057.