Web Release Date: October 26,
Behavior of Surface-Anchored Poly(acrylic acid) Brushes with Grafting Density Gradients on Solid Substrates: 1. Experiment
ek,
ubr, and
Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393, and Institute of Macromolecular Chemistry, Prague, Czech Republic
Received May 4, 2007
Revised Manuscript Received July 1, 2007
Abstract:
We describe experiments pertaining to the formation of surface-anchored poly(acrylic acid) (PAA)
brushes with a gradual variation of the PAA grafting densities on flat surfaces and provide detailed analysis of
their properties. The PAA brush gradients are generated by first covering the substrate with a molecular gradient
of the polymerization initiator, followed by the "grafting from" polymerization of tert-butyl acrylate (tBA) from
these substrate-bound initiator centers, and finally converting the PtBA into PAA. We use spectroscopic ellipsometry
to measure the wet thickness of the grafted PAA chains in aqueous solutions at three different pH values (4, 5.8,
and 10) and a series of ionic strengths (IS). Our measurements reveal that at low grafting densities, 
, the wet
thickness of the PAA brush (H) remains relatively constant, the polymers are in the mushroom regime. Beyond
a certain value of , the macromolecules enter the brush regime, where H increases with increasing . For a
given , H exhibits a nonmonotonic behavior as a function of the IS. At large IS, the H is small because the
charges along PAA are completely screened by the excess of the external salt. As IS decreases, the PAA enters
the so-called salt brush (SB) regime, where H increases. At a certain value of IS, H reaches a maximum and then
decreases again. The latter is a typical brush behavior in so-called osmotic brush (OB) regime. We provide
detailed discussion of the behavior of the grafted PAA chains in the SB and OB regimes.
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