(59d) Salt- and pH-Induced Swelling Behavior of a Poly(Acrylic acid) Brush Via Quartz Crystal Microbalance w/ Dissipation (QCM-D)

Salt- and
pH-Induced Swelling Behavior of a Poly(Acrylic acid)
Brush via Quartz Crystal Microbalance w/ Dissipation (QCM-D)

¹Hollingsworth,
N. R.; ^2,3Wilkanowicz, S. I.; ^1,3,4,*Larson,
R.G.

^* EN-US">Corresponding author

¹ EN-US">Department of Macromolecular Science & Engineering, University of
Michigan, Ann Arbor, Michigan 48109, United States

² EN-US">Warsaw University of Technology, Faculty of Civil Engineering, Mechanics
and Petrochemistry, Institute of Chemistry, Plock, Poland

³ EN-US">Department of Chemical Engineering, University of Michigan, Ann Arbor,
Michigan 48109, United States

⁴ EN-US">Biointerfaces Institute, University of Michigan, Ann Arbor, Michigan
48109, United States

Polyelectrolyte
brushes have a wide variety of uses in areas such as aqueous lubricants and
water treatment. We investigated the swelling/de-swelling behavior of weak
polyelectrolyte brushes of short- (2 KDa), medium-
(14 kDa), and long-chain (39 KDa)
a weakly ionizable polyelectrolyte, poly(acrylic
acid) (PAA), in the presence of K⁺ and Cl^- at KCl at various pH values, and at grafting densities from
0.12 to 2.15 chains/nm² using a Quartz Crystal Microbalance with
Dissipation (QCM-D). The brushes were
created using a thiol-gold linkages to form a polymer self-assembled
monolayer (SAM) directly on the gold-coated QCM-D crystal rather than a
traditional surface-initiated polymerization method, which allows for control
of the molecular weight of the brush by avoiding synthesis of the brush layer in situ. After correcting for the
brush-free solvent effecton the underlying crystal, we compared our experimental findings to
those predicted by scaling theory. From the dissipation and frequency shifts
upon adding KCl electrolyte to the solvating water,
the brush swells at low KCl ionic strength at
pH 7 & 9 (above the pKa bold">), consistent with the well-known osmotic brush regime. At higher salt
concentrations, in the salted brush regime, the brush de-swells with further
increases in ionic strength, exhibiting the well-known ‘polyelectrolyte effect’
induced by charge screening from K⁺ counterions. At pH 3,
below the pKa, the brush is found to be charge
neutral as it remains relatively unaffected. At pH 7, in the low-salt osmotic
brush regime, the brush height scales with a 0.33 power law dependence at lower
grafting densities, in agreement with the
experiments of Wu et al.² but contrary to the scaling theory
of Zhulina et al.,¹ which gives a -1/3
power law dependence. In the high-salt salted
brush regime, the brush thickness is predicted to scale with a -1/3 power law
dependence, and our results demonstrate this. With respect to salt concentration,
we find that brush height scales with -1/3 power law dependence in this salted
brush regime, in agreement with theory. With respect to chain length, we find a
linear, increasing relationship between brush height and chain length in both
the osmotic and salted brush regimes. Ultimately, this work confirms and
extends previous experimental findings, and provides a comprehensive data set
for testing theories and for comparison against other weak and strong
polyelectrolyte brushes and multilayers.

References

(1)  Zhulina, E. B.; Birshtein, T. M.;
Borisov, O. V. Theory of Ionizable Polymer Brushes. Macromolecules 1995,
28 (5), 1491–1499.

(2)  Wu,
T.; Gong, P.; Szleifer, I.; Vlček,
P.; Å ubr, V.; Genzer, J.
Behavior of Surface-Anchored Poly(acrylic acid)
Brushes with Grafting Density Gradients on Solid Substrates: 1. Experiment. Macromolecules
2007, 40 (24), 8756–8764.