Synthesis of a poly(p-styrenesulfonate) brush via surface-initiated polymerization

Article abstract of DOI:10.1021/ma020301o

The synthesis of molecularly thin poly(p-styrenesulfonate) monolayers as an example for a negatively charged polyelectrolyte brush covalently attached to a planar solid surface is described. The polymer monolayers are generated directly at the surface of the substrate by using self-assembled monolayers of an azo initiator and radical chain polymerization of p-styrenesulfonate ethyl ester (ESS) monomer in situ. This grafting from approach yields surface-bound poly(p-styrenesulfonate ethyl ester) (PESS) molecules with high molecular weights and with high graft densities of the attached chains. The PESS monolayers can be transformed into a charged poly(p-styrenesulfonate) (PSS) monolayer through a polymer-analogous saponification reaction under mild conditions, and the reaction can be carried out to quantitative conversion. The thickness of the polyelectrolyte brush can be controlled from 2 to more than 35 nm in the dry, collapsed state.

Full text of DOI:10.1021/ma020301o

1222
Macromolecules 2003, 36, 1222-1227
Synthesis of a Poly(p-styrenesulfonate) Brush via Surface-Initiated
Polymerization
Ma r k u s Biesa lsk i†,‡ a n d J u1 r gen Ru1 h e*,†,‡
Institute for Microsystem Technology (IMTEK), University of Freiburg, Georges-Ko¨hler-Allee 103,
D-79110 Freiburg, Germany, and Max-Planck-Institute for Polymer Research, Ackermannweg 10,
55128 Mainz, Germany
Received February 25, 2002
ABSTRACT: The synthesis of molecularly thin poly(p-styrenesulfonate) monolayers as an example for a
negatively charged polyelectrolyte brush covalently attached to a planar solid surface is described. The
polymer monolayers are generated directly at the surface of the substrate by using self-assembled
monolayers of an azo initiator and radical chain polymerization of p-styrenesulfonate ethyl ester (ESS)
monomer in situ. This “grafting from” approach yields surface-bound poly(p-styrenesulfonate ethyl ester)
(PESS) molecules with high molecular weights and with high graft densities of the attached chains. The
PESS monolayers can be transformed into a charged poly(p-styrenesulfonate) (PSS) monolayer through
a polymer-analogous saponification reaction under mild conditions, and the reaction can be carried out
to quantitative conversion. The thickness of the polyelectrolyte brush can be controlled from 2 to more
than 35 nm in the dry, collapsed state.
In tr od u ction
styrene molecules that were end-functionalized with a
monochlorosilane group to a high area silica gel. Sub-
sequently, they converted the neutral polystyrene layer
into a poly(styrenesulfonate) layer through a polymer-
analogous sulfonation reaction using a mixture of
sulfuric acid and SO3. The degree of sulfonation was
f ) 0.64. As the highest swollen thickness in good
solvent, 75 nm was measured.11 Also, Tran et al.12,13
have used this technique to link poly(styrenesulfonate)
(PSS) onto planar SiO2 surfaces. Other synthetic strate-
gies include the formation surface-attached polymer
brushes via surface-initiated ATRP polymerization14-16
as well as surface-attached polyelectrolyte brushes
through photoemulsion polymerization.17 For a more
detailed overview about the fascinating field of surface-
attached polymeric brush systems, the reader is referred
to recent review articles.18
Polyelectrolyte (PEL) brushes are monolayers where
charged polymer chains are tethered with one anchor
group (or a small number of such groups) to a surface
while the grafting density of the chains is high enough
to induce interchain correlations. As a result of the
electrostatic interactions, the polymer chains are
stretched away from the surface and assume a so-called
“brush”-like conformation.1,2 Many research groups have
focused their work on a theoretical elucidation of a
polyelectrolyte monolayer, in which the charged mol-
ecules are terminally attached to a planar solid sur-
face.3-10 In general, polyelectrolyte brushes show a very
rich and interesting phase behavior. As the conforma-
tion of the surface-attached chains depends on both
interchain correlations and electrostatic interactions,
such a system is expected to show a very interesting
dependence of the physical properties on the structure
and conformation of the polymers. The conformation of
the polymer molecules in the monolayer and therefore
the properties of the surface-attached film depend on
the graft density, the molecular weight, and the charge
density of the attached chains as well as the ionic
strength of the surrounding medium to name just a few
of the important parameters. Polymer brushes are of
potential technological importance for the improvement
of lubrication, adhesion, and wetting properties as well
as for the stabilization of colloids against flocculation.
In PEL brushes the stabilization of the colloids would
be achieved with a combination of electrostatic and
steric interactions.10
Several conditions have to be fulfilled in order to
synthetically realize a polyelectrolyte brush. First, the
graft density of the attached polymer films has to be
sufficiently high. Second, the polymer has to cover the
substrate material in a homogeneous manner. A third
prerequisite is that the solubility of all parts of the chain
is similar. Serious problems could arise from the latter
if, for example, the polyelectrolyte is obtained through
a polymer-analogous reaction of a water-insoluble poly-
mer and the conversion of the neutral into the charged
species is not quantitative. In this case microphase
separations can occur inside the film as some parts of
the chain are readily water-soluble and other parts
consist of neutral segments, which are not soluble.
Consequently, the polyelectrolyte monolayer will be
phase-separated on a microscopic level and will become
structurally inhomogeneous. It is obvious that a com-
plete characterization of such a layer remains extremely
difficult and that such a system can hardly be used as
a model system to study the phase behavior of a
polyelectrolyte brush. Therefore, it is an important
target for the realization of a PEL brush system to
obtain monolayers, where the degree of functionalization
is high and well-known.
Despite strong theoretical interest in such systems
now for more than a decade, there are only a limited
number of examples known that describe the experi-
mental realization of such surface-attached charged
monolayers. In one example, Mir et al.11 linked poly-
† University of Freiburg.
‡ Max-Planck-Institute for Polymer Research.
* To whom correspondence should be addressed: e-mail
ruehe@imtek.de.
10.1021/ma020301o CCC: $25.00 © 2003 American Chemical Society
Published on Web 01/29/2003

Macromolecules, Vol. 36, No. 4, 2003
Poly(p-styrenesulfonate) Brush 1223
After the formation of the initiator monolayers the sub-
strates were transferred into Schlenk tubes, which were then
filled with monomer and solvent. After removal of all oxygen
traces from the solution under vacuum during repeated freeze
thaw cycles, the tubes were placed into a thermostat at
60.0 ( 0.1 °C. The polymerization of ESS with the surface-
attached initiator was carried out in acetonitrile using a
monomer concentration of 30 vol %. After desired polymeri-
zation times the substrates were removed from the polymer-
ization solution, rinsed with a good solvent for the resulting
polymer (acetonitrile), and extracted for about 15 h with the
same solvent in a Soxhlet extractor to remove any physisorbed
polymer from the deposited polymer layer.
The saponification of the PESS monolayers was carried out
in 0.5 M aqueous ammonium carbonate solution at 50-70 °C
following the procedure described for the saponification of low
molecular sulfonate esters and of free PESS in solution.24,28,29
Reaction times were chosen between 7 and 60 h, including a
additional heating period of 1 h in pure aqueous solution to
remove the ammonium ions through outgassing of NH3;
otherwise, the surface-attached polyanionic PSS molecules
contain NH4+ as counterions. After completion of the reactions
the samples were rinsed twice with water and acetonitrile to
remove reaction byproducts, such as ethanol and ammonium
carbonate, and dried in a vacuum and in a closed chamber
containing solid potassium hydroxide. The latter procedure
ensures that residual water molecules are removed from the
dry film and that no ambient water vapor enters the film.
In principle, the generation of a polyelectrolyte mono-
layer, covalently attached to a solid surface, can be
established using two different approaches. One, the
“grafting to” technique, as shown by Mir et al.11 and by
Tran et al.,12,13 uses the chemisorption of polymer chains
from solution. Typically, the polymer molecules are
functionalized with an anchor group at one end of the
molecule and are linked to appropriate sites at the
surface of the substrate.19 Because of the fact that in
such systems the polymer molecules have to diffuse to
the surface against the concentration gradient built up
by already attached chains, further film growth is
kinetically hindered once the surface becomes signifi-
cantly covered. Therefore, only relatively small amounts
of polymer (typically between 1 and 5 nm film thickness)
can be attached to the surface.19,20
Another approach, the so-called “grafting from” tech-
nique, uses initiators that are self-assembled onto a
solid substrate. The polymer layer is then grown in situ
from the surface.21,22 Using this “grafting from” tech-
nique, a wide range of different functional polymer
brushes can be obtained at planar and spherical solid
surfaces with high graft density (>1.5 µmol/m2) and
high molecular mass (Mn > 106) of the surface-attached
chains.21-23 In addition, the synthesis of various posi-
tively charged quarternized poly(4-vinylpyridine) brushes
on planar silicon oxide surfaces has been reported in
several communications.23
Mon ola yer Ch a r a cter iza tion . For a qualitative charac-
terization of the generated monolayers, Fourier transform
infrared (FTIR) transmission measurements were carried out
using a Nicolet Omnic 850 spectrometer. 1 mm thick silicon
wafers polished on both sides were used as substrates. Typi-
In this paper we describe the “grafting from” synthesis
of a poly(styrenesulfonate) brush on planar siliconoxide
surfaces. The monomer p-styrenesulfonate ethyl ester
was polymerized at the surface with the immobilized
initiator. The neutral polymer brush is then transferred
through a polymer-analogous saponification reaction
into the polyelectrolyte brush. The rate and conversion
of the neutral surface-attached polymer chains into the
corresponding polyelectrolytes are studied. A compari-
son between the reaction of surface-attached and non-
attached polymer chains is made.
cally 750 scans were accumulated with a resolution of 4 cm-1
.
As the final polyelectrolyte monolayers are strongly hygro-
scopic, nitrogen purging of the sample chamber had to be
carried out especially careful to keep the intensity of water
adsorption bands low. Additional qualitative information about
the layer composition was obtained by X-ray photoelectron
spectroscopy (XPS). The measurements were carried out on a
Fisions 220 spectrometer using Mg KR radiation. The step
width during accumulation of the spectra was 2 eV. The
analyzer angle was set to 90° relative to the substrate surface.
For the determination of the thickness of the monolayer X-ray
reflectometry measurements were performed. The measure-
ments were carried out using an instrument with a 18 kW
rotating anode (Cu KR, λ ) 0.154 nm). The reflected intensities
were measured as a function of the incidence angle from 0.2°
to 2.0° relative to the sample plane. Above the critical angle a
decrease of the intensity proportional to q-4 can be observed,
which is modulated by periodic oscillations, the so-called
Kiessig fringes. They originate from the interference of beams
reflected at the polymer-air and polymer-substrate inter-
faces. The thickness of the sample and the roughness (rms
roughness) of the various interfaces can be obtained with the
help of a matrix formalism for the analysis of the data. Details
of the instrumental setup, about the instrument performance,
and of the model fit calculations are described elsewhere.30
Exp er im en ta l Section
Ma ter ia ls. The azo initiator used for the preparation of the
self-assembled initiator monolayer was prepared in a three-
step synthesis as described by Prucker and Ru¨he.21 Toluene
(p.a. grade) was dried over sodium-potassium alloy, and
benzophenone was used as an indicator. As substrates for the
deposition of the monolayers, silicon wafers (Aurel; Germany)
were used, which had an approximately 2.5 nm thick silicon
oxide layer on the surface. The monomer, p-styrenesulfonate
ethyl ester (ESS), was prepared in a two-step synthesis
according to procedures described by Woeste.24 Briefly, the
sodium salt of styrenesulfonic acid, which had been recrystal-
lized from 50% aqueous ethanol prior to use, was converted
into the corresponding alkyl ester by forming in a first step
the silver salt of the acid. The silver sulfonate was purified by
extraction with acetonitrile (p.a. grade). In a second step the
sulfonate ethyl ester was formed through reaction of the silver
salt with ethyl bromide.25,26 The final product, the ESS
monomer, was carefully extracted with methylene chloride and
purified over a silica column (1:50, eluent acetonitrile).
P r ep a r a tion of th e P olym er Mon ola yer s. The im-
mobilization of the azo initiator at the surface of the silicon
substrate was carried out in dry toluene at room temperature
under argon. The reaction time was 15 h, and the concentra-
tion of the initiator was approximately 0.5 mmol/L.27 Trieth-
ylamine (dried over LiAlH4 and distilled prior to use) was
added as acid scavenger and catalyst for the condensation
reaction. Nonattached initiator and other byproducts of the
reaction were removed after completion of the reaction by
careful extraction with toluene and methanol.
For the determination of the dry thickness of the PESS
brushes before and after saponification null ellipsometry
measurements were carried out using a commercial ELX-1
ellipsometer (Riss, Germany). The commercial software sup-
plied with the instrument was used to model the derived
ellipsometric parameters in order to calculate the dry layer
thickness using Fresnel formalism.
Resu lts a n d Discu ssion
Figure 1 schematically describes the synthesis of the
charged surface-attached polymer brushes. An azo
initiator with a monochlorosilane headgroup was im-
mobilized on the surface of a silicon (oxide) substrate.
After the generation of the self-assembled initiator
monolayer, the neutral PESS monolayer was formed by

1224 Biesalski and Ru¨he
Macromolecules, Vol. 36, No. 4, 2003
F igu r e 2. (A) XP spectrum of a 35 nm thick PESS monolayer
on a silicon wafer. The monolayer was prepared on the wafer
by radical chain polymerization of p-styrenesulfonate ethyl
ester in acetonitrile solution, 1/2 (v/v) at T ) 60 °C. After the
polymerization the substrate was extracted with acetonitrile
for 15 h. (B) Transmission FTIR spectra of (a) a PESS
monolayer on silicon wafer, prepared under the same condi-
tions as in (A). The films are deposited on both sides of a silicon
wafer (polished on both sides). (b) The PESS bulk material in
a KBr pellet.
F igu r e 1. Synthesis strategy of negatively charged PSS
monolayer covalently attached to planar silicon oxide surface
by using a “grafting from” approach.
thermally induced radical chain polymerization of p-
styrenesulfonate ethyl ester (ESS) monomer using the
surface-attached initiator. The ester route was chosen
as it has distinct advantages over the well-known direct
sulfonation of polystyrene.24 The sulfonation of polysty-
rene has to be carried out under rather drastic condi-
tions (H2SO4/SO3). Because of the harsh reaction con-
ditions, a number of side reactions can take place. It is
well-known that double (or even triple) substitution of
the phenyl rings occurs even if conversion is well below
100%, and a large number of phenyl moieties remain
still unchanged.31-33 This makes the determination of
the degree of functionalization and thereby the deter-
mination of the charge density on the polymer chain
difficult. Further problems arise from the occurrence of
degradation reactions such as breaking of polymer
chains or partial cross-linking31-33 and/or the cleaving
off of polymer chains due to the strongly acidic medium.
Both a decrease in molecular mass and the cleavage of
chains from the surface would lead to a decrease of the
resulting dry film thickness. The polymer-analogous
cleavage of a sulfonate alkyl ester, on the other hand,
which gives essentially the same final product, can be
carried out under relatively mild conditions. No side
reactions have been reported, and the saponification of
such a polymer in solution can be carried out to almost
quantitative conversion.24
The chemical identity of the surface-attached PESS
monolayers was proven by FTIR and XPS measure-
ments. The XP spectrum of an 35 nm thick (dry
thickness) PESS layer on a silicon wafer is shown in
Figure 2A. After the deposition of a PESS layer on the
silicon substrate, the spectrum is dominated by the
O(1s) signal at 520 eV, the C(1s) signal at 285 eV, and
the S(2s/2p) signals around 200 eV due to the presence
of the polymer. No signals from the underlying substrate
are visible, indicating that the surface is homogeneously
covered with the neutral polymer monolayer. Figure 2B
shows the FTIR spectrum of a 35 nm thick PESS brush
and the spectrum of bulk PESS (in KBr pellet). The
transmission spectrum of the sample shows typical
vibrational bands of the PESS that arise from the C-H
stretching (around ν ) 3000 cm-1). Also, the vibrational
bands arising from the symmetric and asymmetric
sulfonate ester vibrations at ν ) 1355 cm-1 and at ν )
1180 cm-1 are detected. All other observed absorption
bands are in good agreement with the bulk spectrum.

Macromolecules, Vol. 36, No. 4, 2003
Poly(p-styrenesulfonate) Brush 1225
F igu r e 4. Thickness of PESS monolayer as a function of
polymerization time, measured by X-ray reflectometry. The
polymerization was carried out in acetonitrile solution, in a
monomer concentration of p-styrenesulfonate ethyl ester of 1/2
(v/v) at T ) 60 °C. After polymerization all substrates were
extracted for 15 h with acetonitrile. The solid line represents
the graft density Γ of the polymer molecules calculated from
the decomposition kinetics and radical efficiency of the initia-
tor.
F igu r e 3. X-ray reflectometry spectra of (a) 12, (b) 15, (c) 20,
and (d) 37 nm thick PESS monolayers on silicon wafers. The
monolayers were prepared on the substrates by radical po-
lymerization of p-styrenesulfonate ethyl ester in acetonitrile,
1:2 (v/v) at T ) 60 °C for t ) 3, 6, 9, and 15 h. After
polymerization the substrates were extracted for 15 h in
acetonitrile.
monomer or solvent and from thermal polymerization
in solution). Control samples consisting of spin-cast
films of the free polymer showed that during such an
extraction procedure almost all polymer could be re-
moved from the surface, and only films of a few
nanometers thickness remained.23 Besides dry thickness
also the model calculations of the reflectivity measure-
ments yield an average roughness of the interfaces
studied. In this case here the root-mean-square rough-
ness of the PESS monolayers was about 1 nm, which is
only slightly higher than the roughness of the bare
substrate.
The thickness of the surface-attached polymer mono-
layer is a function of the number of polymer chains per
surface area (graft density) and the molecular weight
of the polymer chains.21,34 As a consequence, the layer
thickness can be controlled by adjusting polymerization
parameters such as the polymerization time, the con-
centration of the monomer, and the grafting density of
the initiator. When the concentration of the monomer
and the reaction temperature are set to a fixed value,
the conversion of the initiator and therefore the polym-
erization time controls the graft density of the attached
chains and as a result of that the thickness of the
polymer layer.
The neutral monomer ESS was polymerized with the
surface-immobilized initiator using acetonitrile as sol-
vent for different periods of time. The thickness of the
prepared monolayers was measured by means of X-ray
reflectometry experiments on the prepared samples.
Examples of such X-ray-reflectivity curves are shown
in Figure 3. All samples have been prepared individually
under identical conditions; only the polymerization time
was varied. Clearly, it can be seen that with increasing
polymerization time the frequency of the Kiessig fringes
increases, indicating an increase in the dry thickness
of the surface-attached layers. Calculated reflectivity
curves obtained from Fresnel modeling were compared
to the experimental data. The model-fit calculations are
superimposed to the data in Figure 3 as solid lines.
The number of chains per surface area (graft density
Γ) is expressed in mol/m2. The film growth kinetics is
well described by the relation Γ(t) ) f Γ0(1 - exp(-kzt)),
where t is the polymerization time, Γ0 is the graft
density of the initiator, f is the radical efficiency factor,
and kd is the velocity constant of the initiator decom-
position. The parameters entering the equation have
been measured for the system styrene/toluene with the
same surface-attached initiator as Γ0 ) 1.8 µmol/m2,
f ) 0.4, and kz ) 9.6 × 10-6 s-1 21,22
.
Using these
parameters, we have calculated the graft density (solid
line in Figure 4) of the surface-attached PESS brush.
The good agreement between the measured dry thick-
ness and the calculated graft density shows that the
latter can be reliably and reproducibly adjusted by the
polymerization time. Note that the absolute values for
Γ as well as a calculated dry thickness can only be given
with the exact knowledge of the molecular mass of the
surface-attached chains. A more quantitative descrip-
tion of detailed studies on the molecular mass of the
surface-attached polymer molecules is in progress and
will be reported separately.
The outcome of the measurements and model calcula-
tions are summarized in Figure 4. The experimental
error of the measurement technique is within the size
of the symbols. Note that all substrates were extracted
for at least 15 h after the deposition of the PESS layers
with a good solvent for the free polymer (acetonitrile).
The extraction is carried out to remove physisorbed
polymer (originating from the second, free radical
formed during decomposition of the azo compound or
from chain transfer of surface-attached radicals to
After characterization of the polysulfonate ester mono-
layers the negative charges were introduced into the
layer by cleavage of the sulfonate alkyl ester groups.
The saponification reactions were carried out under mild
conditions (aqueous solution of ammonium carbonate,
0.5 M; pH ) 9.5; 70 °C), to prevent degrafting of the
polymer chains from the silicon oxide surface. If the
reactions were carried out using more strongly basic

1226 Biesalski and Ru¨he
Macromolecules, Vol. 36, No. 4, 2003
F igu r e 6. Conversion of the saponification reaction as a
function of the reaction time of a 35 nm thick PESS monolayer
on the surface of a silicon wafer (b). The reaction was carried
out with 0.5 M (NH4)2CO3 at T ) 70 °C. The conversion was
calculated by the integral intensity of the absorbance maxi-
mum at ν ) 1355 cm-1 (asymmetrical vibrational band of the
PESS sulfonate ester). In addition, the conversion (∆) calcu-
lated from the loss of mass during the saponification reaction
of different PESS brushes is shown (see also Table 1).
Ta ble 1. Con ver sion of th e Sa p on ifica tion Rea ction a
Ca lcu la ted fr om th e Loss of Ma ss of th e Mon ola yer
F igu r e 5. FTIR spectra of a 2 × 35 nm thick PESS monolayer
attached to both sides of a silicon wafer. The spectra were
measured during the saponification of the surface-attached
PESS monolayer at a given reaction time (denoted in the
figure). The reaction was carried out with 0.5 M (NH4)2CO3 at
T ) 70 °C.
thickness
thickness
tp [h]/ts [h]b
PESSc [nm] PSSc [nm]
∆d [nm]
conv [%]
6/1.5
6/2
9/3
15.7
15.5
18.3
21.5
15.0
14.6
17.3
19.0
0.7 ( 0.5
0.9 ( 0.5
1.0 ( 0.5
2.5 ( 0.5
33 ( 24
45 ( 25
40 ( 20
84 ( 17
20/14
conditions (pH > 11), high temperatures (T > 80 °C),
and longer reaction times (t > 40 h), the substrates were
not sufficiently stable, and a significant amount of the
polymer chains was cleaved from the surface.
a
The conversion is calculated from the thickness of the PESS
b
layer and the mass ratio of the PESS/PSS repeat units. tp
)
polymerization time; ts ) saponification time. c The thicknesses
of the monolayers were measured with ellipsometry.
In Figure 5 transmission FTIR spectra of a PESS
monolayer attached to both sides of a silicon substrate
(film thickness 35 nm on both sides) are shown. The
spectra were measured during the saponification after
different periods of time as noted in the figure. The
decrease of the ester vibrational bands of the PESS
monolayer at ν ) 1355 cm-1 and ν ) 1180 cm-1 and
the appearance of new adsorption bands at ν ) 1025
cm-1 and ν ) 1130 cm-1 indicate the cleavage of the
ester bonds and the formation of the polymeric sulfonic
acid. The spectrum of the resulting surface-attached free
acid is in good agreement with the bulk spectrum.35 The
conversion of the reaction at different reaction times is
shown in Figure 6. This conversion was calculated by
two different methods. The first was FTIR spectroscopy.
IR spectra were normalized to the CH vibrational bands
above ν ) 3000 cm-1. The conversion was then calcu-
lated from the sulfonate ester adsorption band at ν )
1355 cm-1, as this is the only characteristic adsorption
band that has no significant overlap with other bands.
From the complete disappearance of the band it can be
concluded that after approximately 25 h the polymer-
analogous saponification is quantitative.
transfer of the surface-attached PESS layer to the
corresponding polyelectrolyte monolayer, the loss of
adsorbed mass and, consequently, the conversion of the
saponification reaction can be calculated (Table 1). The
results of these calculations are also depicted in Figure
6.
Note that no significant decrease of the integral
intensities of the C-H vibrational bands above ν ) 3000
cm-1 can be observed if the spectra are normalized to
the sulfonate acid vibrational band at ν ) 1025 cm-1
.
This suggests that no polymer molecules are cleaved off
from the surface under the chosen reaction conditions.
Con clu sion s
Poly(p-styrenesulfonate) monolayers as examples for
a negatively charged polyelectrolyte brush were pre-
pared using a “grafting from” approach. Self-assembly
of a radical chain initiator at the surface of the substrate
and radical polymerization initiated from the surface-
attached initiator allows the buildup of monolayers with
high graft densities of the attached chains. By adjusting
the polymerization time during formation of the neutral
precursor brush, we are able to control the graft density
of the surface-attached polyelectrolyte chains.
The transformation of the sulfonate ester groups
under mild conditions leads to a surface-attached mono-
layer consisting of negatively charged poly(p-styrene-
sulfonate) (PSS) molecules. The reaction is quantitative,
and no side reactions such as the loss of polymer
molecules originating from an attack to the anchor
groups or the sission of polymer chains can be observed.
In addition, the conversion of the saponification
reaction can be calculated from the measurement of the
loss of mass. As the mass per monomer unit is decreased
during saponification from 212 g/mol for the sulfonate
ester to 183 g/mol for the free acid, the PESS layer
should show a significant decrease in the dry (solvent
free) thickness. Therefore, sulfonate ester monolayers
with different thicknesses were prepared. From the
measurement of the dry thickness before and after

Macromolecules, Vol. 36, No. 4, 2003
Poly(p-styrenesulfonate) Brush 1227
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The latter two show the excellent stability of the layer
even under good solvent conditions. Moreover, the
covalent attachment of the chains allows for the attach-
ment of polyelectrolyte molecules independent of the
surface charge of the substrate.
In conclusion, the generation of densely grafted PSS
molecules through a polymer-analogous saponification
of a sulfonate ester precursor system represents a
further example for the synthetic establishment of
charged polymeric monolayers covalently attached to
planar solid surfaces. As already performed with posi-
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surface-attached weak polyacid brushes,37 interesting
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