Instability of Surface-Grafted Weak Polyacid Brushes on Flat Substrates

Article abstract of DOI:10.1021/acs.macromol.5b01289

We study the stability of weak polyacid brush (WPAB) gradients in aqueous media covering a range in grafting density (σ) spanning 0.05-0.5 chains/nm² using two analogous surface-anchored bromoisobutyrate-based initiators for atom transfer radical polymerization (ATRP) bearing either an ester or amide linker. Variations in dry thickness of ester-based WPABs as a function of time and pH are consistent with WPAB degrafting via linker hydrolysis catalyzed by mechanical tension in the grafted chains. Sources of tension considered include high σ, as well as swelling and electrostatic repulsion associated with increasing degree of deprotonation (α) of repeat units in the WPAB. Normalized thickness of the WPAB decreases by a maximum amount at intermediate σ between ～0.05-0.15 chains/nm², implying that contributions to tension by α are counterbalanced by charge regulation in the WPAB at high σ. Amide-based WPABs are more stable up to 264 h incubation, suggesting that commonly used ester-bearing ATRP initiators are more susceptible to hydrolysis over the time scales examined. (Figure Presented).

Full text of DOI:10.1021/acs.macromol.5b01289

Article
pubs.acs.org/Macromolecules
Instability of Surface-Grafted Weak Polyacid Brushes on Flat
Substrates
Casey J. Galvin,†,‡ Erich D. Bain,†,§ Adam Henke,∥ and Jan Genzer*,†
†Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695, United
States
‡Okinawa Institute of Science Technology Graduate University, Onna-son, Okinawa 904-0497 Japan
§U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, United States
∥California Institute for Biomedical Research, La Jolla, California 92037, United States
S
* Supporting Information
ABSTRACT: We study the stability of weak polyacid brush (WPAB)
gradients in aqueous media covering a range in grafting density (σ) spanning
0.05−0.5 chains/nm2 using two analogous surface-anchored bromoisobuty-
rate-based initiators for atom transfer radical polymerization (ATRP) bearing
either an ester or amide linker. Variations in dry thickness of ester-based
WPABs as a function of time and pH are consistent with WPAB degrafting via
linker hydrolysis catalyzed by mechanical tension in the grafted chains.
Sources of tension considered include high σ, as well as swelling and
electrostatic repulsion associated with increasing degree of deprotonation (α) of repeat units in the WPAB. Normalized thickness
of the WPAB decreases by a maximum amount at intermediate σ between ∼0.05−0.15 chains/nm2, implying that contributions
to tension by α are counterbalanced by charge regulation in the WPAB at high σ. Amide-based WPABs are more stable up to 264
h incubation, suggesting that commonly used ester-bearing ATRP initiators are more susceptible to hydrolysis over the time
scales examined.
focused at the bottom-most section of the polymer brush close
to the substrate.10,17−19 This action lowers the activation energy
for breaking labile chemical bonds either in the initiator or the
headgroup chemistry of the initiator that links the initiator to
the substrate. For the surface-grafted initiator commonly
employed with silicon substrates, [11-(2-bromo-2-methyl)-
propionyloxy] undecyltrichlorosilane (eBMPUS), shown in
Figure 1b, two possible functional groups are liable to breakage
due to hydrolysis: (1) the siloxane networks at the initiator/
substrate interface and/or (2) the ester group contained within
the initiator. While some researchers have identified siloxane
linkage cleavage in replacement reactions in silane mono-
layers20 and as the point of scission in brush systems,13,15 there
is a possibility that the ester in the initiator can also be activated
for and undergo hydrolysis.12,18,21,22
Regardless of the bond that undergoes hydrolysis and leads
to degrafting of the polymer from the surface, one testable
hypothesis that arises from this mechanism is that a weak
polyacid (e.g., poly(methacrylic acid), PMAA) will exhibit
increasing levels of instability with increasing pH. Since
increasing pH will result in a higher degree of dissociation
(α) within the polyacid brush, and thus increased solubility as
well as electrostatic repulsion, the tension along the polymer
INTRODUCTION
■
Polymer brushes represent a specific class of macromolecules
that are attached to a substrate. Polymer brushes have received
extensive attention in the literature due to their relevance in a
number of research fields and applications.1−3 Since the
polymer chains are typically grafted covalently to a solid
substrate, there has been particular interest in applying polymer
brush systems as coatings in aqueous environments, in
particular the biomedical field4 to direct cell adhesion,5
lubricate artificial joints,6 extend microarrays into a third
dimension7 and prevent biofouling.8 This latter use is also an
attractive route toward solving the outstanding issue of
nonspecific biological fouling of man-made substrates.9 In all
of these cases, the envisioned purpose of the polymer brush
coating requires the coating to survive on the time scale of
years, even decades.
While many brush-solvent systems are thought to be stable,
i.e., the chains remain grafted to the surface during their
operation, some systems have demonstrated undesired
instability, which resulted in the removal of chains from the
surface.10,11 Examples include polyelectrolytes in water12−16
and branched macromolecules,13,17 as well as polyacrylamide in
methanol with a strong base.18 A proposed mechanism12 for
the instability of polyelectrolyte brushes in water, illustrated in
Figure 1a, suggests that strong swelling of the brush due to
electrostatic charging in the brush generates increased tension
along the grafted chain backbone. This tension is ultimately
Received: June 13, 2015
Revised: July 15, 2015
© XXXX American Chemical Society
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leading to potential brush degrafting from the substrate, in ways
that are very challenging to predict.
In this article, we examine the stability of weak polyelec-
trolyte brushes with a gradient in σ at different pH levels.
Employing a sample design that facilitates smooth variation of
σ, i.e., polymer assemblies whose areal density ranges from
densely to sparsely grafted, enables systematic investigation of a
broad parameter space with a minimum of materials, effort, and
sample-to-sample variations. We use these samples to address
the aforementioned hypothesis for densely grafted weak
polyacid brushes and examine simultaneously the interplay
between σ and α. Furthermore, we make use of two analogous
atom transfer radical polymerization (ATRP) initiators
possessing either an ester or amide functional group in order
to provide insight on the bond primarily responsible for
degrafting.
Figure 1. (a) Strong swelling due to increased solubility and
electrostatic repulsion among neighboring chains generates tension
along the polymer backbone, which is focused at the grafting chemistry
of the linker (green circle). This tension activates chemical bonds in
the grafting chemistry for hydrolysis by hydroxide ions. When
hydrolysis occurs, the chains are degrafted from the surface. (b)
Possible reaction mechanisms of hydrolysis in the grafting chemistry of
an ester-based, chlorosilane ATRP surface-bound initiator. (c)
Increasing σ of a polymer brush increases the inherent tension of
the system.
EXPERIMENTAL SECTION
■
General Methods and Materials. Ethanol, dimethyl sulfoxide
(DMSO), diethyl ether (Et2O), tert-butyl methacrylate (tBMA),
N,N,N′,N′,N″-pentamethyldiethylenetriamine (PMDETA), CuCl,
CuBr, trifluoroacetic acid (TFA), dichloromethane (DCM), magne-
sium sulfate (MgSO4), triethylamine (Et3N), undecyl bromide, 2-
bromo-2-methylpropionyl bromide, lithium aluminum hydride
(LiAlH), sodium methacrylate (NaMA), bipyridine (BiPy), and
inhibitor remover packing were purchased from Sigma-Aldrich and
used as received. Monosodium phosphate and disodium phosphate
were purchased from Fisher Scientific and used as received. n-
octyltrichlorosilane (OTS) was purchased from Gelest and used as
received. The 0.5 mm thick, 100 mm diameter silicon wafers
(orientation [100]) were purchased from Silicon Valley Micro-
electronics.
backbone will also increase. This increased tension will result
ultimately in higher susceptibility of the grafting chemistry to
hydrolysis. The rate of hydrolysis depends also on the
concentration of hydroxide ions in solution.23 Thus, higher
pH levels should result in greater instability in a weak polyacid
brush.
Sample Preparation. Initiator Synthesis. Ester BMPUS (eBM-
PUS), [11-(2-Bromo-2-methyl)propionyloxy] undecyltrichlorosilane
(eBMPUS) was synthesized according to the method of Matyjaszewski
and co-workers.31 Amide BMPUS (aBMPUS) was synthesized by
following the procedure outlined below (see also Scheme 1). 10-
An additional source of tension exists in dense polymer brush
systems. Because of the proximity of neighboring chains, the
grafted polymer chains stretch away from the substrate.24 This
perturbation from the Gaussian coil conformation induces
tension along the polymer chain backbone.17 Such a
phenomenon is illustrated in Figure 1c, where chains in the
farthest left region of the brush exhibit the least extent of
stretching. As the grafting density (σ) of the polymer chains on
the surface increases (i.e., moving right along the sample in the
figure), the chains stretch away from the substrate due to
increased excluded volume effect from neighboring chains.24
When the distance between chains is significantly less than the
gyration radius of the grafted chains, the chains will adopt a
highly extended conformation, thus increasing tension along
the chain backbone, which is supported by a single grafting
point to the substrate.25
Scheme 1. Synthesis of aBMPUS
In the absence of any other effects, one might expect that the
densest region of the brush would show the highest levels of
instability, which has been observed in a nonionic polymer
brush system.18 However, σ and α are not mutually
independent parameters in surface-grafted weak polyelectro-
lytes; σ modulates α at different regions within the brush
through charge regulation.26−30 Briefly, charge regulation
occurs when a densely grafted polymer brush does not attain
the bulk α value predicted from solution pH due to strong
crowding and stretching near the substrate. As a result, the
degree of dissociation in weak polyelectrolyte grafts close to the
substrate is lower than that in the upper portion of the brush.
Therefore, in a weak polyelectrolyte brush, σ and α may
influence the swelling behavior due to increased polymer
solubility and electrostatic repulsion, and thus increased tension
undecenyl azide (compound 1 in Scheme 1) was prepared by
dissolving/suspending NaN3 (1.63 g, 25.08 mmol) in DMSO (50 mL)
at 25 °C. Undecylbromide (5 mL, 22.80 mmol) was added and the
resulting reaction mixture was stirred at room temperature until
starting material was fully consumed (6 h). Reaction progress was
followed by GC analysis. The reaction was quenched with H2O (100
mL) [slightly exothermic] and stirred until it cooled to room
temperature. The mixture was extracted with Et2O (3 × 30 mL). The
combined organic layers were washed with water (2 × 40 mL), brine
(1 × 40 mL), and dried over MgSO4. After removing solvent in vacuo,
the crude product 1 in Scheme 1 was obtained as a colorless oil and
was used in next step without further purification. To produce 1-
amino-10-undecene (compound 2 in Scheme 1), a solution of the
crude azide 1 in dry diethyl ether (30 mL) was added drop-wise to a
stirred mixture of LiAlH4 (1.45 g, 38.20 mmol) in dry diethyl ether (70
mL) at 0 °C. The mixture was allowed to attain room temperature,
B
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stirred for 3 h, and then carefully quenched by the addition of H2O (4
mL), 15%, NaOH (4 mL), and H2O (10 mL), followed by filtration.
The filtrate was washed with H2O (2 × 20 mL), brine (1 × 20 mL),
and dried over MgSO4. After removing solvent in vacuo, the crude
product 2 in Scheme 1 (4.3 g) was obtained as a yellowish oil (used in
next step without further purification). N-(10-undecenyl) 2-bromo-2-
methylpropanamide (compound 3 in Scheme 1) was prepared as
follows. A solution of the crude amine 2 (4.3 g) and Et3N (4.2 mL,
29.60 mmol) in dichloromethane (80 mL) was cooled down to 0 °C
under atmosphere of argon. 2 bromo-2-methylpropionyl bromide
(2.82 mL, 22.80 mmol) was added drop-wise to this solution.
Resulting reaction mixture was stirred 30 mins at 0 °C. The reaction
was then quenched with water (10 mL), and after separation, the
organic layer was washed with 3% HCl (20 mL), 1:1 H2O and brine
mixture (30 mL), brine (20 mL) and dried over MgSO4. After
removing solvent in vacuo, the crude product was purified by column
chromatography (hexane:Et2O:acetone = 10:1:2) to afford 6.65 g
amide 3 in Scheme 1 as a colorless oil (92 % overall yield after 3
polymerization was run such that the longest period of immersion was
30 min.
PMAA Brush Synthesis from tert-Butyl Methacrylate (tBMA). The
polymerization solution comprised 45 mL of tBMA (purified by
passing through a column containing inhibitor remover), 45 mL of
DMSO, 40 μL of PMDETA, and 0.058 g of CuCl ([tBMA]:[CuCl]:
[PMDETA] = 470:1:0.33). This solution was degassed by bubbling
with N2 gas for 30 min, then charged to a custom-built glass reactor
containing an initiator-modified wafer using a degassed glass syringe.
The SI-ATRP proceeded for 1 h, at which point the reactor was
opened and the wafer removed. The wafer was rinsed thoroughly with
ethanol, then sonicated in ethanol for 20 min, followed by drying with
a stream of N2 gas. The PtBMA brush was then characterized using
ellipsometry, followed by hydrolysis for a total of 40 min by a 50 vol %
solution of TFA in DCM to yield the PMAA brush.
Incubation Experiments. Buffer solutions with strengths of 10
mM were prepared using sodium phosphate salts and adjusted to pH
4, 7.4, and 9 using minute quantities of HCl and KOH. The pH of
each buffer was measured to 0.02 using an Accumet AB15 pH meter
(Fisher Scientific) equipped with a platinum pH electrode. Periodic
measurement of the buffer solutions were done to ensure no pH drift
occurred. The parent polymer brush samples were segmented into 1
cm wide specimens and placed individually into glass vials containing
buffer solution that had been filtered using 0.2 μm syringe filters. The
vials were then sealed and stored in the dark at room temperature for
the duration of the incubation. After a certain incubation time (i.e., 24
or 120 h), the samples were removed, rinsed briefly with DI water and
dried with a stream of N2 gas and stored for further characterization.
Characterization Techniques. Spectroscopic Ellipsometry.
Measurements were performed on a variable angle spectroscopic
ellipsometer (J.A. Woollam Co.) controlled by WVASE32 software
(J.A. Woollam Co.). For brush thicknesses >30 nm, data were
collected at incidence angles of 65, 70, and 75° over wavelengths
ranging from 400 to 1000 nm. These data were fit to a model
comprising a Si substrate, SiOx layer (thickness 1.5 nm), and a Cauchy
layer. The Si and SiOx layers used material files supplied with the
WVASE32 software. The Cauchy layer was fit using thickness, and the
Cauchy parameters An and Bn. For thicknesses <30 nm, the thickness
and Cauchy parameters cannot be independently fit. Thus, data were
collected at 632.8 nm over a range of incidence angles from 60 to 80°
in 1° increments. These data were fit to a model comprising a Si
substrate, SiOx layer (thickness 1.5 nm) and a Cauchy layer. The
Cauchy parameters, An and Bn, were held constant using values
obtained at the thickest part of the brush. Only thickness was used as a
fitting parameter.
Infrared Variable Angle Spectroscopic Ellipsometry (IR-VASE).
Measurements were performed on an IR-VASE (J.A. Woollam Co.)
controlled by WVASE-IR software (J.A. Woollam Co.) at a 50° angle
of incidence with a resolution of 4 cm−1.
Atomic Force Microscopy. Surface topography measurements were
conducted in air using an Asylum MFP-3D system (Asylum Research)
in tapping mode using Si tips (Model AC160TS; Asylum Research)
with a resonance frequency of 300 kHz and spring constant of
42 N/m. Scans were collected over an area of 4 μm × 4 μm at a scan
rate of 1.0 Hz and resolution 512 lines/scan. Data were analyzed in the
Gwyddion software package.33
Contact Angle Goniometry. Contact angles of DI water were
measured using a Rame−Hart goniometer. A water droplet of volume
́
6−8 μL was dispensed on the surface, and the contact angle of the
droplet determined using automated computer software. Measure-
ments were collected at various points along the Si substrate, and the
location of these measurements were determined using a rule placed
adjacent to the substrate during measurements.
1
steps). H NMR (400 MHz, CDCl3) δ 6.70 (bs, 1H), 5.78 (ddt, J =
16.9, 10.2, 6.7 Hz, 1H), 4.97 (ddt, J = 17.1, 3.7, 1.6 Hz, 1H), 4.90 (ddt,
J = 10.2, 2.3, 1.2 Hz, 1H), 3.23 (td, J = 7.1, 5.8 Hz, 2H), 2.05 − 1.97
(m, 2H), 1.93 (s, 6H), 1.51 (p, J = 7.3 Hz, 2H), 1.41 − 1.20 (m, 12H).
13C NMR (101 MHz, CDCl3) δ 172.0, 139.4, 114.3, 63.9, 40.7, 34.0,
32.9, 29.7, 29.6, 29.5, 29.4, 29.3, 29.1, 27.0. HR HR-MS (ESI) m/z
calculated for C15H28ONBrNa: 340.1247, found 340.1246. Finally,
[11-(2-bromo-2-methyl)propanamide] undecyltrichlorosilane (aBM-
PUS) was obtained by hydrosilylation of compound 3 in Scheme 1. To
a solution of 0.5 g of 3 in 10 mL trichlorosilane was added 1 or 2 drops
of Karstedt catalyst (platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisi-
loxane complex, 3 wt% solution in xylenes). The solution was heated
to reflux for 5 hours. The excess silane was then removed by
distillation. Anhydrous dichloromethane (distilled over activated
molecular sieves) was added, and the product solution was filtered
through a plug of anhydrous sodium sulfate under nitrogen.
Dichloromethane was removed by vacuum, and the product was
dried under vacuum and weighed. Anhydrous toluene (stood over
activated molecular sieves) was added to make a 5 wt% solution. The
solution was transferred to vials and stored in a freezer until use.
Initiator Deposition. Silicon wafers were cut into 4.5 cm × 5 cm
rectangles and were sonicated in methanol, dried with a stream of N2
gas, and treated in a UV−ozone apparatus for 20 min. For initiator
gradient samples, wafers were then placed horizontally next to a well
containing a mixture of OTS in mineral oil (1:4) for 7 min. After OTS
deposition, the wafer was immediately placed into a solution of 30 μL
of 5 vol % eBMPUS in anhydrous toluene and 30 mL of anhydrous
toluene and incubated at −20 °C overnight. An identical approach was
used for aBMPUS. Our experience suggests it is important to use
ethanol for the sonication of aBMPUS-modified wafers (as opposed to
methanol or toluene, for example) to remove properly physisorbed
material. For homogeneous initiator substrates used to form molecular
weight gradients, no OTS deposition step was performed. The wafer
was then removed from solution, rinsed with ethanol, dried with a
stream of N2 gas, then sonicated in ethanol for 20 min and dried with a
stream of N2 gas. The wafer was then immediately analyzed by contact
angle using deionized (DI) water as a probing liquid, then dried with a
stream of N2 gas before polymerization.
PMAA Brush Synthesis from Sodium Methacrylate (NaMA).
Silicon wafer samples functionalized with either eBMPUS or aBMPUS
were inserted in solutions containing CuBr (80 mM), CuBr2 (16
mM), bipyridine (200 mM), and 6 M sodium methacrylate in DI
water, which had been titrated to pH 9 using HCl and degassed by
bubbling with N2 gas for 30 min. For grafting density gradient samples,
the samples were removed after 24 h, rinsed copiously with DI water,
and dried under a stream of dry N2 gas. To produce molecular weight
gradient samples, substrates were immersed into the polymerization
solution at a controlled speed using a dipping apparatus described
previously.32 The polymerization followed the surface-initiated (SI)
ATRP reaction scheme. The polymerization time at a given point on
the wafer depends on the speed of the immersion process. The
RESULTS AND DISCUSSION
■
The data in Figure 2 plot the ex situ dry thickness of two
gradient polymer assemblies, determined by spectroscopic
ellipsometry, against either polymerization time for the
molecular weight (MW) gradient sample (panel a) or against
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molecule. These SI-ATRP were conducted at pH 9, well above
the pKa of PMAA.34 Thus, the polymer brush is expected to
exhibit significant charging.
The MW gradient sample exhibits an increase in thickness
with increasing polymerization time through about 20 min,
which is expected for ATRP reactions.35 At polymerization
times longer than 20 min, when the brush has reached an ex situ
thickness in excess of 200 nm, there is first a reduction in the
rate of thickness increase, followed by a decrease in thickness
with increasing polymerization time. In the σ gradient sample,
an analogous phenomenon occurs, wherein the thickness of the
brush first increases with increasing σ, the thickness then begins
to decrease with increasing σ. Interestingly, this value of σ
appears to be at a point when the brush would exceed 200 nm
in ex situ thickness, which is comparable to a critical thickness
observed in poly(hydroxyethyl methacrylate) (PHEMA)
polymer brushes.16
The dry thickness of a polymer brush (h) scales as the
product of MW and σ, i.e., h = (MW*σ)/(ρ*NA), where ρ is
polymer density and NA is Avogadro’s number. Thus, in order
for the polymer brush to exhibit a thickness decrease, either
MW or/and σ must decrease. Since the aliphatic backbone of
methacrylate polymers is unlikely to break down under the
current experimental conditions, the brush MW should remain
constant. In contrast, the silane bonding chemistry between the
initiator and silicon substrate or the ester group in the eBMPUS
initiator may undergo hydrolysis. Recent work has illustrated
Figure 2. Thickness profiles derived from ellipsometric data for a
PNaMA brush expressing either a gradient in MW (left panel) or σ
(right panel). After a certain MW or σ level, there is a decrease in the
dry thickness of the grafted polymer layer. The lines are intended as
guides for the eye.
the expected grafting density (σ; calculation described in
Supporting Information) for the σ gradient sample (panel b).
These samples were prepared using eBMPUS initiators, which
contain an ester functional group as part of the initiator
Figure 3. (a) Thickness values derived from VASE data show the height profile of an eBMPUS-based brush expressing a gradient in σ as it is
modified from PtBMA (green) to PMAA (orange) and subsequently incubated in pH 9 buffer for 24 h then dried and measured in air (black). (b)
IR-VASE data providing chemical information on the modification of PtBMA (green) to PMAA (orange) and subsequent incubation at pH 4, 7.4,
and 9 for 24 h (blue, red and black, respectively). (c) “Dry” ellipsometric thicknesses after 24 h of incubation normalized by preincubation
thicknesses for samples incubated at pH 4, 7.4, and 9 for 24 h (blue, red, and black, respectively). The lines are intended as guides for the eye.
D
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that this bond can be activated for hydrolysis by mechanical
tension.21 Furthermore, swelling of polymer brushes can
generate significant tension along the polymer backbone,
which focuses at the grafting point of the polymer brush.17,19
This tension is directly proportional to the MW of the grafted
chains, and longer chains of a nonionic polymer in the presence
of a strong base have been shown to cleave preferentially from a
solid substrate.18 In the case of polyelectrolytes, pH-induced
swelling can produce sufficient tension to activate the grafting
chemistry for hydrolysis.12,15,17,19
The σ of a polymer brush can also generate tension. At low σ,
when the distance between neighboring chains (d) is much
greater than the gyration radius of the grafted chains (Rg), the
grafted chains will adopt a Gaussian conformation. At high σ,
when d ≪ Rg, the chains extend normal to the substrate due to
excluded volume interaction. This extension will induce tension
along the chains and can lead to degrafting of the tethered
chains.18
Since both brushes were polymerized at pH 9, brush swelling
due to electrostatic repulsion and highly solubilized chains
generates tension along the chain backbone. This tension is
modulated by variations in the MW and σ levels of the grafted
chains. When critical values of these parameters are exceeded,
sufficient tension is focused at the grafting point of the brush to
catalyze hydrolysis either in the silane headgroup or the ester
bond of the initiator. We hypothesize, then, that the thickness
reductions observed in Figure 2 are due to a reduction in σ as a
result of chains degrafting from the substrate.
The continued reduction in thickness after the critical values
for both samples is worth further consideration, since one
might expect the brush to obtain an equilibrium thickness at the
critical values of MW and σ. In the case of the MW gradient,
the continued reduction in σ (i.e., thickness) at longer
polymerization times, and thus higher MWs, may be due to
the longer incubation period in the pH 9 polymerization
solution. The σ gradient sample requires a more subtle
explanation, since all points of the sample experience identical
polymerization/incubation times. However, denser regions of
the sample will reach the critical stretching level sooner than
less dense regions of the brush. These denser regions will
experience a longer period of incubation above this critical
level, and thus demonstrate a greater extent of degrafting.
To further explore the influence of σ on stability of weak
polyacid brushes, we opted for a nonaqueous synthesis route
using tert-butyl methacrylate (tBMA) and subsequent hydrol-
ysis of PtBMA brushes using trifluoroacetic acid (TFA) in
dichloromethane.36 To minimize systematic errors among
multiple samples, “parent” polymer brushes of PtBMA were
synthesized on a large substrate, which was then cut into
smaller segments and individual samples derived from this
“parent” sample were subjected to hydrolysis reaction for
various incubation times at different pH values.
density of the initiator centers), the differences in thickness of
the PtBMA brush originate from differences in σ and ultimately
from the variation in BMPUS fraction on the substrate. The IR-
VASE data shown in Figure 3b, which plots the ellipsometric
parameter Ψ, confirm the chemical identity of the grafted
polymer layer as PtBMA based on the green data in the panel
labeled “PtBMA”. Previously, the parameter Ψ has been shown
to be sensitive to the chemical functional groups on a surface.37
The distinct carbonyl peak at 1720 cm−1 is clearly visible and
characteristic of methacrylate-based macromolecules.38 The
series of peaks located between 2900 and 3000 cm−1 are
associated with the multiple C−H bonds in the tert-butyl group
in the polymer side chain.
The thickness maximum at 1.5 cm (abscissa) is a real feature
based on visual inspection of the samples. A darker coloration,
consistent with a thicker polymer brush, is visible just after the
gradient region (position <1.5 cm) compared to the lighter
coloration in the homogeneous portion of the brush (position
>2.0 cm). Importantly, this feature appeared in the PtBMA
brush, prior to any further chemical modification or exposure to
water. We do not have a clear explanation for this result at this
time.
The orange data in Figure 3a depict the thickness of the
PMAA samples derived by hydrolyzing the PtBMA brushes
with TFA. For all samples, the shape of the PMAA thickness
profile replicates that of the original PtBMA brush, in which
thickness of the chains increases with increasing position along
the wafer (i.e., increasing σ). The reduction in thickness stems
from the removal of the bulky tert-butyl group from the
polymer side chain. The homogeneous portion of the samples
(i.e., small variation in σ; position >2.0 cm) shows a consistent
reduction in thickness for all three samples to ≈0.58 of the
PtBMA brush thickness, suggesting uniform modification
across all samples. This level of reduction also suggests that
nearly all of the PtBMA repeat units have been hydrolyzed to
PMAA, and degrafting of the chains is not substantial during
this modification step (cf. Supporting Information). The
gradient portion of the samples (position on the abscissa
<1.5 cm) follows a less consistent pattern, and does not
indicate a clear trend in behavior. Note that these samples were
not exposed to aqueous solution prior to the measurements
from which these data are derived. While two of the samples
show a smaller thickness reduction in the gradient region
compared to the homogeneous region, the remaining sample
shows a greater thickness reduction. The origin of these
differences is not apparent from the data.
This chemical modification is confirmed by the orange data
in Figure 3b, in the panel labeled “PMAA”, wherein the peaks
near 3000 cm−1 associated with the tert-butyl group are
reduced, with residual signal at this wavenumber due to the
methyl group on the polymer backbone. A small, broad peak
has appeared at ∼3400 cm−1, which is characteristic of hydroxyl
stretches associated with water.38 The continued presence of
the carbonyl peak at ∼1700 cm−1 confirms that the brush layer
has “survived” the hydrolysis reaction, and it exists in its
protonated state (vide infra). The reduction in intensity of this
peak may originate from either reduced concentration of
carbonyl groups at the surface due to chain degrafting, or due
to a reduction in film thickness. We note these data are derived
from ellipsometric measurements, and thus film thickness will
influence their value. In either case, all subsequent samples were
treated equally to this point, and thus start from the same initial
condition prior to pH buffer incubation.
The thickness of a representative, eBMPUS PtBMA brush
sample plotted as a function of distance from the OTS reservoir
is shown in Figure 3a (green). This specific sample is used for
pH 9 incubation studies and is presented as an example, while
the data for the other samples are presented in the Supporting
Information. A smooth variation in thickness as a function of
position on the substrate is seen in each sample, and is
consistent with the variation in BMPUS fraction on the
substrate prior to polymerization. Assuming that the MW of the
grafted polymer chains is comparable along the substrate (i.e.,
the polymerization rate does not depend dramatically on the
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Following hydrolysis of the PtBMA brushes to PMAA
brushes, the samples were incubated in phosphate buffer
solutions at pH 4, 7.4, or 9 (all 10 mM of total salt) for 24 h.
After incubation, the samples were removed from the buffer
solutions, dried by a stream of N2 gas, and characterized by
ellipsometry. The black data in Figure 3a plots the resulting
thickness of the brush incubated at pH 9 as a function of
substrate position. There is a clear increase in sample thickness
following 24 h incubation, which is also observed in the sample
incubated at pH 7.4. The sample incubated at pH 4 shows no
significant change in thickness.
Figure 4 presents atomic force microscopy (AFM) images
collected from the dense portion of the initial PtBMA and
The IR-VASE data presented in Figure 3b confirm residual
charging within the brushes incubated at pH 7.4 and 9
following removal from incubation solution. The data for the
pH 4 sample (blue) do not show any distinct difference from
the PMAA brush prior to incubation (orange). In contrast, the
pH 7.4 (red) and pH 9 (black) data exhibit two significant
changes in the Ψ data. First, the carbonyl peak at ∼1700 cm−1
has diminished in intensity, while a peak at ∼1550 cm−1 has
appeared. This latter peak is associated with carboxylate groups
in PMAA, and has been used to track changes in situ in PMAA
brushes as a result of changing pH.37 The second change in the
Ψ data is the increased intensity of the peak at ∼3400 cm−1,
which is associated with the hydroxyl stretch in water. Thus, we
conclude that incubation of the PMAA brushes at pH 7.4 and
pH 9 leads to dissociation of the proton in the pendant
carboxylic acid groups, resulting in charging along the brush.
Even after removing the samples from the buffer solutions,
rinsing with DI water and drying with N2 gas, these charges
remain stabilized, possibly due to complexation with sodium
ions from the buffer salts. These residual charges lead to the
presence of water in the weak polyelectrolyte brush in ambient
lab conditions.39−41 Whether this water is retained in the brush
during the washing procedure or absorbed from the
atmosphere following drying is not clear from the data.
To more readily compare across pH values, Figure 3c plots
the thickness for all three pH levels normalized by the initial
thickness. The sparsest region of the brushes shows no
significant increase in thickness for all pH values. The sample
incubated at pH 4 exhibits a slight increase within the gradient,
and essentially no swelling in the homogeneous portion of the
substrate. In contrast, the samples incubated at pH 7.4 and pH
9 show a trend of increasing swelling with increasing wafer
position. Furthermore, the sample at pH 9 exhibits a higher
swelling factor than the pH 7.4 sample. This finding is
consistent with expected differences in charging within the
brushes incubated at these pH levels. Although free PMAA
chains in aqueous solutions exhibit a pKa of ≈5.0−5.5 (i.e., well
below pH 7.4 and pH 9), charge regulation shifts the pKa of the
polyacid brush to a higher value and leads to a gradient in pKa
along the length of the brush.28 As a result, measured values of
pKa ≈ 7.0 for the “bulk” PMAA brush and pKa ≈ 4.6 for the
brush/water interface have been reported.34 As α inside the
brush increases with increasing pH, the brush thickness
increases as well due to repulsion among charges within the
brush and increased solubility of the charged polymer chains.
This increased charging persists following the removal from
incubation solution (vide supra) and leads to a greater
normalized thickness when “dry”. If Na+ ions are complexed
with the residual, charged carboxylate groups, then an increased
quantity of these ions may also contribute to the greater
thickness at pH 9.
Figure 4. AFM micrographs of the PtBMA brush and PMAA brush
prior to incubation, as well as the “dry” PMAA brushes following 24 h
incubation at pH 4, 7.4, and 9. Scale bars represent 1 μm in length.
The lower panel plots the RMS roughness value derived from each
micrograph.
PMAA brushes, and the samples incubated at pH 4, pH 7.4, and
pH 9 for 24 h. The root-mean-square (RMS) roughness
calculated from each of these images is plotted in the lower
panel. The morphology of the PtBMA and PMAA brushes is
similar, although the PMAA brush exhibits a marked reduction
in the RMS from 1.85 to ∼1.0 nm for PtBMA. The sample
incubated at pH 4 for 24 h shows little change from the
preincubation RMS roughness value, consistent with the results
seen from VASE and IR-VASE measurements, and presents a
comparable morphology. The samples incubated at pH 7.4 and
9 exhibit a significant reduction in RMS roughness value to
∼0.4 nm, accompanied by changes in the surface morphology.
These findings confirm that the grafted film structure has been
altered as a result of incubation in different pH buffers, and that
these changes persist following standard drying procedures.
Given the film swelling and residual moisture content seen in
the VASE and IR-VASE data, we reason that the samples
incubated at pH 7.4 and pH 9 present a flatter polymer/air
interface. This smoothness may be induced by an attempt to
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minimize the surface area between the partially hydrated
polymer film and air, which is hydrophobic.
The samples were further incubated and examined ex situ
using VASE at periodic time intervals. The fitted thickness from
the ellipsometric data for the densest portion of the brush
(position 3.0 cm along the abscissa) are plotted in Figure 5.
reduction than pH 7.4. Thus, the data indicate that the stability
of a densely grafted, weak polyacid brush decreases with
increasing pH. Increasing pH of the buffer solution increases α
within the brush, as confirmed by the IR-VASE data, resulting
in higher levels of tension at higher pH. This correlation
supports the hypothesis put forth previously12 and illustrated in
Figure 1, namely, that tension generated by strong swelling due
to electrostatic repulsion and increased solubility activates the
surface grafting chemistry for hydrolysis, the rate of which
increases with increasing pH.
In the lower panel of Figure 5, we present schematics
illustrating the mechanism seen in our data. Starting with the
upper left illustration and following the path of arrow 1, a
PMAA brush with initial thickness h0 is incubated in a buffer of
given pH. The thickness h0 corresponds to the orange data in
Figure 3a. During incubation, the polymer chains attain a
certain α, and Na+ ions from the buffer salt penetrate into the
brush to screen the charges on deprotonated acid groups.
Moving along the path of arrow 2, removal of the sample from
the incubation solution and drying leads to residual charging
and moisture within the brush, stabilized by the Na+ ions,
resulting in a swollen “dry” thickness of h1. The thickness h1
corresponds to the black data in Figure 3a, and the data point at
24 h in Figure 5. Continuing along arrow 3, upon further
incubation of this sample, tension generated along the polymer
chain by strong swelling due to charge repulsion and increased
solubility leads to hydrolysis of the grafting chemistry at the
substrate/polymer interface. Finally, following arrow 4, the
degrafting process leads to a localized reduction in σ, resulting
in relaxation of the neighboring polymer chains following
removal from solution, which is observed as a reduction in
thickness, as illustrated for thickness h2. The thickness h2
corresponds to the various time points in Figure 5.
This mechanism raises the question of how removal of a
grafted chain affects the stability of adjacent chains. The
reduction in σ will result in a local reduction of tension of the
remaining chains, which suggests the adjacent chains are less
likely to degraft. However, in weak polyelectrolyte brushes,
such as the ones studied here, α is known to be a function of σ
(as well as distance along the brush’s vertical direction) through
charge regulation.26−28 As a result, due to diminished charge
regulation, the reduction in σ may lead to an increase in α
relative to the densest regions of the brush, leading to increased
tension that promotes degrafting.
Figure 6 displays the thickness derived from ellipsometric
data for the entire length of the eBMPUS σ gradient samples
incubated at pH 9 through 554 h. Data for pH 4 and pH 7.4 are
available in Figure S6 of the Supporting Information. The data
are normalized to the thickness measured after 24 h of
incubation (red). The data in Figure 5 are in the cluster of data
points near 52 nm PtBMA thickness. In addition, we plot the
results from aBMPUS σ gradient samples incubated through
264 h. These initiator species are shown in Figure 7. We have
plotted the thickness as a function of initial PtBMA thickness,
which is a proxy for σ of the PMAA brush. Ideally, in order to
elucidate the effect of σ on the stability of PMAA brushes, we
would plot changes in normalized thickness as a function of σ
of the PMAA brush. Unfortunately, there is currently no known
way to measure σ nondestructively.42,43
Figure 5. Evolution of dense brush thickness for samples incubated in
pH 4, 7.4 and 9 buffer solutions (black, red and blue data,
respectively). The lower illustrations present a potential explanation
of the data in Figure 3 and the dense brush incubation data contained
in this figure. Starting from the upper left illustration, the sequence
follows the numbered arrows. See the text for a detailed discussion of
each step. The lines are intended as guides for the eye.
The data are normalized to the swollen “dry” thickness
observed at 24 h (cf. Figure 3c). Prolonged incubation leads to
a strong dependence of ex situ thickness on pH. The sample at
pH 4 stays essentially unchanged over a period of several weeks.
In contrast, the samples at pH 7.4 and pH 9 show a reduction
in normalized thickness. On an absolute basis, the thickness at
the longest incubation period (554 h) at pH 9 is below the
initial, preincubation PMAA thickness. This point suggests
strongly a removal of chains from the substrate as the origin of
this thickness reduction. Furthermore, on a normalized basis,
the sample incubated at pH 9 shows a greater thickness
As mentioned earlier, in a grafted polymer film, the dry
thickness (h) is proportional to MW*σ. Assuming that the MW
of the PtBMA chains does not depend on σ, any variation in h
is due primarily to variation in σ. Therefore, we plot the PMAA
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Figure 6. Thickness profiles of PMAA brush samples prepared using eBMPUS and aBMPUS expressing a gradient in σ incubated at pH 9
normalized by the thickness measured for each point at 24 h (red). The initial PtBMA thickness for each sample is used as a proxy for the σ level of
each sample (see text). The estimated σ level derived from the PtBMA thickness is plotted on the upper x-axis (see text). The left panel presents data
for eBMPUS initiators and the right panel data for aBMPUS initiators. Data for pH 4 and pH 7.4 are available in the Supporting Information. The
lines are intended as guides to the eye.
from 24 h onward. These findings are consistent with the
trends seen for the densest region of the brush. The samples at
pH 7.4 and pH 9 also exhibit a maximum reduction in thickness
at an intermediate σ value (i.e., at neither the densest nor
sparsest region of the brush). This minimum first develops in
the data at 264 h of incubation (green), and is located between
10 to 20 nm PtBMA thickness (≈0.1−0.2 chains/nm2).
Figure 7. Initiator species, eBMPUS and aBMPUS, used to examine
Examination of the data in Figure 3a reveals these data points
are at the onset of the σ gradient. Increasing PtBMA thickness
from this point (i.e., increasing σ) leads to a fairly smooth
increase in normalized thickness. Likewise, decreasing σ also
results in a higher normalized thickness, although these
normalized thicknesses are all below the 24 h thickness.
Finally, the apparent rate at which this minimum appears in the
data varies between pH 9 and pH 7.4. While the sample at pH
7.4 shows a gradual reduction in thickness at each measurement
time, the sample at pH 9 shows a rapid decrease in thickness at
the 264 h measurement time. Further incubation at pH 9
beyond 264 h does not result in a significant change of
thickness at the point of maximum decrease, but does so in the
denser region of the brush. For both pH 9 and pH 7.4, the
thickness at the sparsest region of the grafted film appears to
reach a steady value after 264 h.
In contrast to the eBMPUS data, the aBMPUS data at pH 7.4
and pH 9 show continued swelling through 144 h of incubation
(orange), followed by thickness reductions at 264 h (green).
Unlike the eBMPUS data, these thickness reductions do not
show an intermediate minimum value and appear consistent
across all values of σ. The “shape” of the thickness profile at 144
h is largely preserved at 264 h. Interestingly, the data at 144 h of
incubation does show a point of maximum swelling for both pH
7.4 and pH 9, and it occurs around a similar σ value (≈0.2
chains/nm2) as the point of maximum reduced thickness seen
in the eBMPUS data. The difference in the evolution of the
thickness profiles between eBMPUS and aBMPUS suggests
there are significant differences between ester and amide
functional groups in susceptibility to hydrolysis. The reduction
in thickness in the aBMPUS samples suggests that hydrolysis of
the siloxane network may occur in both initiator species, or that
amide bonds may be partially susceptible to hydrolysis under
tension.
the susceptibility of the linkage group (i.e., ester vs amide) in BMPUS
to hydrolysis.
thickness against PtBMA thickness, the latter of which is
proportional to σ, such that increasing PtBMA thickness
corresponds to increasing σ. By plotting the data in this way,
Figure 6 depends on parameters inherent to the polymer brush
and serve to generalize the observations and conclusions
reached through analysis of the data. Assuming that PtBMA
thickness is directly proportional to σ allows us to estimate σ by
considering a value of σ at the densest portion of the brush,
which we have taken to be the substrate position furthest from
the silane reservoir during the OTS deposition procedure.
Recent measurements for PMMA brushes grown with an
identical initiator and deposition procedure42 provide a value of
∼0.5 chains/nm2. We can then scale this value of σ for a given
point on the substrate by the PtBMA brush thickness at that
point. The upper axis of the panels in Figure 6 plots this
estimated σ scaling. Note that due to the anomalous thickness
observed at a position of 1.5 cm on the eBMPUS PtBMA
brush, the thickest measurement point of the eBMPUS PtBMA
is not in the region expected to be the densest portion of the
brush based on WCA measurements (cf. Figure S2 in the
Supporting Information). Interestingly, as seen below, this
point behaves as though it is indeed at a σ value consistent with
its position on the substrate.
Focusing first on the samples incubated at pH 4, the
eBMPUS data do not show any distinct trends with incubation
time or σ, and the normalized thickness remains above 0.8. The
same is true for the aBMPUS data, other than there appears to
be a relatively small, but consistent reduction in thickness with
incubation time. The eBMPUS data for pH 7.4 and pH 9 show
trends in both incubation time and σ. For both samples, there is
a general reduction in thickness with increasing incubation time
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Figure 8 depicts AFM images collected at three regions of the
eBMPUS samples incubated at pH 4 and pH 9 for 144 h. The
reduction in σ. This concept is illustrated schematically in
Figure 9.
Figure 9. Charge regulation contributes to maximum degrafting at
intermediate σ. As σ decreases from the dense portion of the brush
(furthest right on sample in figure), α increases, resulting in an
increase in tension and activation of the grafting chemistry for
hydrolysis. Further decreasing σ reduces crowding due to larger
distances between neighboring chains, and the grafted chains are not
forced to adopt a highly stretched configuration. Not drawn to scale.
At the highest grafting densities (right edge of the sample
schematic in Figure 9), neighboring charges can suppress the
formation of additional electrostatic charges in the brush layer.
This suppression results from a competition between the
enthalpic benefit of charge formation and the energy penalties
due to stretching the polymer chain normal to the surface and
increasing osmotic pressure due to an increased concentration
of counterions confined in the brush.27,28 When the stretching
and osmotic pressure penalties exceed the benefit of forming an
additional electrostatic charge, the polymer achieves an α below
that corresponding to a bulk chain in the same buffer solution.
This reduction in α due to σ is termed charge regulation in
polymer brush systems. Furthermore, the same confinement
that leads to charge regulation can produce significant
variations in pH and reaction equilibria in the microenviron-
ment at the initiator/brush interface at different σ levels,28
which also may influence the stability of polyelectrolyte brushes
at different pH levels. It is worth emphasizing that the effect of
σ, osmotic pressure and solution pH and ionic strength
contribute in simultaneous and coupled ways to the local
microenvironments within the brush, such that while some
parameters may contribute more significantly to the free energy
of the system, all the parameters are important.44
With decreasing σ the entropic stretching penalty imposed
on the grafted chains is reduced, while simultaneously the
density of counterions confined in the brush decreases due to a
reduction in the density of repeat units. As a result, α increases
with decreasing σ. Thus, at some critical σ level, the grafted
chains can achieve a maximum α. Note that this maximum α is
not necessarily equal to the bulk α. At this point, the grafted
chains will experience a maximum in tension generated along
the backbone, which has been shown to focus at the grafting
point17 and activate ester groups for hydrolysis.21 This tension
originates from the strong swelling of the polymer chains,
which is modulated by a combination of σ and α. At the
sparsest regions of the grafted film, the chains cannot feel their
adjacent neighboring grafts and are not subjected to extension
away from the substrate due to σ. Without this additional
source of tension, the grafting chemistry is not liable to
breakage by hydrolysis. This behavior is in contrast to nonionic
Figure 8. AFM images show portions of gradient samples incubated in
pH 4 (top row) and pH 9 (bottom row) buffers for 24 h. The scale bar
in each images corresponds to 1 μm. The thickness values correspond
to the PtBMA thickness of the samples prior to any modification steps.
The bottom plot illustrates the evolution of the RMS of AFM images
collected along the gradient samples.
initial PtBMA thickness of each point is indicated above the
images. The PtBMA thicknesses of each point are comparable,
allowing for comparison of morphologies across pH buffer
levels. Plotted below the AFM images are RMS roughness
values extracted from the images for PtBMA, PMAA, and the
pH 4 and pH 9 samples plotted as a function of initial PtBMA
thickness. Since the pH 4 and pH 9 samples came from
different segments of the PtBMA brush, the PtBMA and PMAA
data were selected from the pH 9 sample. The morphology
depends strongly on σ and grows increasingly smooth with
increasing σ. This point is seen in the RMS roughness data as
well, which exhibit a decreasing function with increasing
PtBMA thickness (i.e., increasing σ). As before, morphology
also depends strongly on pH incubation level. While the
morphology and RMS roughness values for the pH 4 sample do
not differ appreciably from the PMAA specimen, the sample
incubated at pH 9 shows markedly lower RMS roughness
values. Furthermore, the morphology at 18 nm PtBMA
thickness differs significantly from that of the pH 4 sample,
and resembles the morphology seen for the pH 9 brush at 52.4
nm PtBMA thickness.
Taken together, the results presented in Figure 6 and Figure
8 suggest that the polymer chains within the gradient behave
comparably to the dense brush regions incubated at the same
pH and bear residual charges and retained moisture after
removal from the incubation solution. We conclude that chains
in the gradient regions are stretched more strongly compared to
the dense region during incubation due to a reduction in charge
regulation (i.e., increase in σ), leading to a relatively larger
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polymer systems, which have exhibited decreased degrafting
rates with decreasing σ.18
Grant no. DMR-1404639 is also acknowledged. CJG acknowl-
edges support from the Micro/Bio/Nanofluidics Unit of the
Okinawa Institute of Science and Technology Graduate
University while preparing this manuscript.
One question that remains unanswered by the experiments in
this article is whether the tension generated by strongly swelling
the grafted chains with increasing pH originates with
electrostatic repulsion or increased solubility or both. The
presence of 10 mM of sodium ions in the incubation solution
leaves the possibility for charge screening to occur in the brush,
which would diminish the impact of electrostatic repulsion and
possibly lower tension in the polymer brush. However, higher
levels of ionic strength enable higher levels of α within the
brush,27 and has been observed to increase instability of dense
PMAA brushes at 37 °C.14 Without in situ swelling data to
identify screening, incubation data with varying ionic strength
or data using nonionic buffering agents to prevent screening,
we cannot comment on the interplay of σ and ionic strength in
the PMAA gradients employed in this study. This line of
inquiry has significant implications for the application of these
systems, since the ocean and the body both contain significant
quantities of ions.
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DOI: 10.1021/acs.macromol.5b01289
Macromolecules XXXX, XXX, XXX−XXX