Micromechanical Cantilever Technique: A Tool for Investigating the
Swelling of Polymer Brushes
Gina-Gabriela Bumbu,
²,§
Markus Wolkenhauer,
²
Gunnar Kircher,
²
Jochen S. Gutmann,*
,²,‡
and Ru ¨diger Berger*
,²
Max Planck Institute for Polymer Research, Ackermannweg 10, D 55128 Mainz, Germany, and Institute
for Physical Chemistry, Johannes Gutenberg UniVersity, Welderweg 11, D 55099 Mainz, Germany
ReceiVed July 21, 2006. In Final Form: NoVember 14, 2006
Polymer brush coatings are well-known for their ability to tailor surface properties in a wide range of applications
from colloid stabilization to medicine. In most cases, the brushes are used in solution. Consequently, efforts were
expended to experimentally investigate or theoretically predict the swelling behavior of the brushes in solvents of
different qualities. Here, we show that the micromechanical cantilever (MC) sensor technique is a tool to perform
time-resolved physicochemical investigations of thin layers such as polymer brushes. Complementary to scattering
techniques, which measure the thickness, the MC sensor technique provides information about changes in the internal
pressure of the brushes during a swelling and deswelling process. We show that the kinetics of both swelling and
deswelling are dependent on solvent quality. Comparing the measured data with its thickness evolution, which was
calculated based on the Flory-Huggins theory, we found that only the first 10% of the thickness increase of the
polymer brush results in a significant pressure increase inside the polymer brush layer.
Introduction
Polymer molecules end-tethered to a surface with a high density
of attachment points force the chains to stretch away from the
solid interface, forming thin films known as polymer brushes.
1
Polymer brush layers play an important role to tailor physical
and chemical properties of surfaces for applications such as
colloidal stabilization,
2,3
corrosion inhibition, nonfouling surface
technology,
4,5
opto-electronic devices,
6
tribology,
7
chromatog-
raphy,
8
rheology,
9
chemical gates,
10
and biomedical science.
11-13
Stimuli-responsive binary polymer brush layers can be used to
create switchable surfaces.
14
The properties of polymer brushes
depend on their environment, e.g., the quality of a solvent.
15
Therefore, both experimental and theoretical swelling and
deswelling studies of polymer brushes are mandatory to
understand and tailor their properties.
One way to gradually vary the solvent quality is by mixing
a solvent and a nonsolvent at different ratios. Birshtein and
Lyatskaya modeled the behavior of a polymer brush immersed
in a mixture of miscible solvent and nonsolvent. In these studies,
the Flory-Huggins interaction parameter between solvent and
polymer,
sp
, was varied between 0 and 2.
16
Results indicated
the existence of two regimes during the swelling process of the
brushes in mixed solvents: the so-called “decollapse regime”,
and the regime of the “swollen brush”. The “decollapse regime”
shows up at low concentrations of solvent and is characterized
by a small increase in the height of the brush upon increasing
the solvent concentration. In this regime, the swelling is realized
mainly by the penetration of the solvent molecules in the brush.
The “swollen brush” regime appears at higher concentrations of
solvent. Here, the increase of the brush thickness is attributed
to a replacement of nonsolvent with solvent molecules.
Experimentally, the swelling of polymer brushes was mainly
studied by X-ray and neutron scattering techniques
17-19
pioneered
by Auroy and Auvray.
17
They investigated the collapse-
stretching transition of grafted poly(dimethylsiloxane) in various
mixtures of dichloromethane/methanol. Knoll et al.
18
looked into
the swelling behavior of polystyrene brushes with different
grafting densities in mixtures of toluene/methanol of various
compositions. However, the neutron scattering technique requires
perdeuterated solvent mixtures to increase the contrast. This
technique is not applicable for mixtures with a high proportion
of solvent, since changes in Kiessig fringes could not be resolved.
Therefore, the swelling of the brushes cannot be followed up to
100% solvent.
Here, we report the investigation of the kinetics of both the
swelling and deswelling behavior of poly(methyl methacrylate),
PMMA, brushes using a micromechanical cantilever sensor
technique.
* To whom correspondence should be addressed. Telephone: +49-06131-
379-114. Fax: +49-06131-379-100. E-mail: gutmann@mpip-mainz.mpg.de;
berger@mpip-mainz.mpg.de.
²
Max Planck Institute for Polymer Research.
‡
Johannes Gutenberg University.
§
Permanent address: P. Poni Institute of Macromolecular Chemistry,
Iasi, Romania.
(1) Milner, S. T. Science 1991, 25, 905.
(2) Napper, D. H. Polymeric Stabilization of Colloid Dispersions; Academic:
New York, 1983.
(3) Pincus, P. Macromolecules 1991, 24, 2912.
(4) Caster, K. C. In Polymer Brushes; Advincula, R. C., Brittain, W. J., Caster,
K. C., Ru ¨he, J., Eds.; Wiley-VCH: Weinheim, 2004; pp 329-371.
(5) Leckband, D.; Sheth, S.; Halperin, A. J. Biomater. Sci., Polym. Ed. 1999,
10, 1125.
(6) Whiting, G. L.; Farhan, T.; Huck, W. T. S. In Polymer Brushes; Advincula,
R. C., Brittain, W. J., Caster, K. C., Ru ¨ he, J., Eds.; Wiley-VCH: Weinheim, 2004;
pp 371-381.
(7) Klein, J.; Kumacheva, E. Science 1995, 269, 816.
(8) van Zanten, J. H. Macromolecules 1994, 27, 6797.
(9) Parnas, R. S.; Cohen, Y. Rheol. Acta 1994, 33, 485.
(10) Ito, Y.; Ochiai, Y.; Park, Y. S.; Imanishi, Y, J. Am. Chem. Soc. 1997, 119,
1619.
(11) Harris, J. M. Poly(ethylene glycol) Chemistry; Plenum Press: New York,
1992.
(12) Galaev, I. Y.; Mattiasson, B. Trends Biotechnol. 1999, 17, 335.
(13) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter,
P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892.
(14) Uhlmann, P.; Ionov, L.; Houbenov, N.; Nitschke, M.; Grundke, K.;
Motornov, M.; Minko, S.; Stamm, M. Prog. Org. Coat. 2006, 55, 168.
(15) Carignano, M. A.; Szleifer, I. J. Chem. Phys. 1994, 100, 3210.
(16) Birshtein, T. M; Lyatskaya, Y. V. Macromolecules 1994, 27, 1256.
(17) Auroy, P.; Auvray, L. Macromolecules 1992, 25, 4134.
(18) Bunjes, N.; Paul, S.; Habicht, J.; Prucker, O.; Ru ¨ he, J.; Knoll, W. Colloid
Polym. Sci. 2004, 282, 939.
(19) Fick, J.; Steitz, R.; Leiner, V.; Tokumitsu, S.; Himmelhaus, M.; Grunze,
M. Langmuir 2004, 20, 3948.
2203 Langmuir 2007, 23, 2203-2207
10.1021/la062137u CCC: $37.00 © 2007 American Chemical Society
Published on Web 01/10/2007