Simultaneous Preparation of Multiple Polymer Brushes under Ambient Conditions using Microliter Volumes

Article abstract of DOI:10.1002/anie.201805534

The fabrication of well-defined, multifunctional polymer brushes under ambient conditions is described. This facile method uses light-mediated, metal-free atom-transfer radical polymerization (ATRP) to grow polymer brushes with only microliter volumes req

Full text of DOI:10.1002/anie.201805534

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201805534
German Edition: DOI: 10.1002/ange.201805534
Polymer Brushes
Simultaneous Preparation of Multiple Polymer Brushes under Ambient
Conditions using Microliter Volumes
Benjaporn Narupai, Zachariah A. Page, Nicolas J. Treat, Alaina J. McGrath, Christian W. Pester,
Emre H. Discekici, Neil D. Dolinski, Gregory F. Meyers, Javier Read de Alaniz, and
Craig J. Hawker*
Abstract: The fabrication of well-defined, multifunctional
polymer brushes under ambient conditions is described. This
facile method uses light-mediated, metal-free atom-transfer
radical polymerization (ATRP) to grow polymer brushes with
only microliter volumes required. Key to the success of this
strategy is the dual action of N-phenylphenothiazine (PTH) as
both an oxygen scavenger and polymerization catalyst. Use of
simple glass cover slips results in a high degree of spatial and
temporal control and allows for multiple polymer brushes to be
grown simultaneously. The preparation of arbitrary 3D
patterns and functional/emissive polymer brushes demon-
strates the practicality and versatility of this novel strategy.
overlying copper plate[11] provide partial solutions; however,
no method allows well-defined polymer brush patterns to be
simultaneously produced from microliter reaction volumes
under ambient conditions.
Herein, we report a simple strategy for growing polymer
brush structures using a simple glass cover slip as an oxygen
barrier. Under ambient conditions, this method is shown to be
robust and user-friendly, employing microliter amounts of
reagents and visible light-mediation to achieve polymer brush
growth over large surface areas. Key to the success of this
strategy is the use of organic photoredox systems (PTH) as
both a catalyst and oxygen scavenger. By masking light
activation, complex surface patterns with excellent spatial
resolution (ca. 1 mm) can be achieved while the use of
specifically shaped cover slips allows multiple features to be
prepared on a single substrate.
The procedure for controlled polymer brush growth under
ambient conditions is graphically illustrated in Scheme 1.
First, about 20 mL of monomer/PTH solution (N,N-dimethyl-
acetamide, DMA, 4.7m) is deposited onto an ATRP initiator-
functionalized silicon wafer. A glass cover slip is subsequently
placed on the wafer to uniformly spread the solution and
provide a vertical barrier to oxygen diffusion. Under ambient
conditions, irradiation with visible light (for example, LEDs,
natural sunlight, or a compact fluorescent lamp; Supporting
Information, Figure S2) excites the PTH, which acts as
a scavenger to remove dissolved oxygen as well as a catalyst
for brush polymerization. Significantly, growth of polymer
brushes occurs without degassing and can be controlled by
either the concentration of PTH or by the physical dimensions
of the cover slip.
In the following studies, silicon wafers functionalized with
undecyl-2-bromo-2-methylpropanoate initiator and a visible
LED excitation source (lmax = 405 nm) were utilized (see
details in the Supporting Information). Initial studies exam-
ined the influence of oxygen on polymer brush growth with
two variables being systematically changed: wafer size from
7 ꢀ 7 to 10 ꢀ 10 to 14 ꢀ 14 and 18 ꢀ 18 mm (glass cover slip was
kept constant at 18 ꢀ 18 mm) and the concentration of PTH
(0.5, 1.0 and 5.0 mol%). As shown in Figure 1, for all 7 ꢀ 7 and
10 ꢀ 10 mm wafers, polymerization of 2-(dimethylamino)ethyl
methacrylate (DMAEMA) resulted in homogenous brush
growth across the entire wafer. In contrast, for the 14 ꢀ 14 mm
wafers, only the 5 mol% PTH concentration showed homo-
genous brush growth over the entire wafer surface, with edge
effects (no polymer growth) being observed for the 0.5 and
1.0 mol% PTH samples. This edge effect is further pro-
nounced for the 18 ꢀ 18 mm substrates (glass cover slip is the
T
he ability to precisely control polymer brush growth[1] has
enabled applications in areas ranging from anti-fouling,[2]
anti-fog,[3] sensing,[4] to lubrication.[5] Key to these advances
has been the development of surface-initiated controlled
radical polymerization (SI-CRP)[6] strategies involving the
growth of polymer brushes from initiator-functionalized
surfaces. While successful, major challenges prevent the
widespread adoption of SI-CRP. These include the use of
air-free conditions, requirement for large reaction volumes
(for example, complete submergence of the substrate in
monomer solution), and elaborate strategies for patterning
brush growth.[7] To increase the versatility of SI-CRP, our
group recently reported a visible-light-mediated method for
the preparation of polymer brushes using either iridium[8] or
organic photoredox catalysts (N-phenylphenothiazine,
PTH).[9] While these techniques enable access to patterned
and gradient three-dimensional polymer brush nanostruc-
tures, the need for an inert atmosphere and special equipment
(for example, glovebox, custom-made chambers) for brush
growth significantly limits wide-spread implementation.
Additives that regenerate the catalyst[10] or the use of an
[*] B. Narupai, Dr. Z. A. Page, Dr. N. J. Treat, A. J. McGrath,
Dr. C. W. Pester, E. H. Discekici, N. D. Dolinski, Prof. C. J. Hawker
Materials Research Laboratory, University of California
Santa Barbara, CA 93106 (USA)
E-mail: hawker@mrl.ucsb.edu
B. Narupai, E. H. Discekici, Prof. J. Read de Alaniz, Prof. C. J. Hawker
Department of Chemistry and Biochemistry, University of California
Santa Barbara, CA 93106 (USA)
Dr. G. F. Meyers
The Dow Chemical Company
Midland, MI 48667 (USA)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201805534.
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Scheme 1. Representation of visible-light-mediated polymer brush growth under ambient conditions. N-phenylphenothiazine (PTH) acts as both
a scavenger for oxygen and as a catalyst for SI-CRP, while the glass cover slip acts as a vertical barrier for the diffusion of oxygen.
Figure 2. a) Optical image of initial P(BnMA) brush showing the edge-
effect region (no polymer brush growth: brown region), b) growth of
P(DMAEMA) using a photomask with transparent 20ꢀ200 mm rectan-
gles over full wafer, c) AFM topography images and height profiles for
patterned P(DMAEMA) brushes on the edge-effect region (X) and
d) on P(BnMA) region (Y).
18 ꢀ 18 mm cover slip, and 1 mol% PTH to create the
expected polymer brush functionalized substrate with
a well-defined edge effect, as depicted in Figure 2a. Subse-
quent irradiation of the entire substrate (both regions)
through a photomask consisting of 20 ꢀ 200 mm transparent
Figure 1. a) Representation of P(DMAEMA) brush growth in the pres-
ence of PTH, b) representation of the experimental dimensions using
wafers of different size and 18ꢀ18 mm glass cover slips, c) effect of
wafer size and catalyst loading on the edge effect.
rectangles, resulted in patterned P(DMAEMA) brushes
across both regions as evidenced by optical microscopy
same size as the silicon wafer), with this region increasing in
size with decreasing PTH concentration (Figure 1c). Exclud-
ing edge-effect regions, optical reflectometry indicated uni-
form polymer brush thicknesses ranging from 40 to 90 nm
(Supporting Information, Table S1).
Having demonstrated the growth of polymer brushes
under ambient conditions, we examined the extent of initiator
retention in the edge-effect regions. Using a substrate with
a defined edge effect, secondary brush growth over the full
wafer was performed (Figure 2). Specifically, benzyl meth-
acrylate (BnMA) polymer brushes were first prepared using
an 18 ꢀ 18 mm ATRP initiator-functionalized silicon wafer, an
(Figure 2b). Moreover, characterization using atomic force
microscopy (AFM) further confirmed patterned polymer
brushes on the edge-effect region (X) (Figure 2c) and on the
P(BnMA) brush region (Y) (Figure 2d) showing oxygen
tolerance and retention of initiating ability in the edge-effect
regions.
From these experiments, we hypothesize that larger cover
slips (relative to the size of the wafer) and increased PTH
loading allows for enhanced scavenging of the initial dissolved
oxygen present in the solutions, as well as any oxygen that
subsequently diffuses in from the edges of the cover slip/
silicon wafer during polymerization. The relationship
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between brush growth and irradiation time was then exam-
ined using a traditional, multi-wafer approach as well as
a single wafer, multiple cover slip strategy. In the first
approach, a 1 mol% PTH/DMAEMA mixture was added to
multiple 6 ꢀ 6 mm silicon wafers, each wafer covered with 18 ꢀ
18 mm cover slips and was irradiated for different lengths of
time. At a PTH catalyst loading of 1 mol%, an inhibition
period of 1 h was observed, followed by a steady increase in
thickness over time resulting in a maximum brush height of
approximately 160 nm (6 h; Supporting Information, Fig-
ure S4). This SI-CRP behavior was further confirmed through
a novel single wafer kinetic study. In this approach, P-
(DMAEMA) brush growth kinetics was investigated on
a single, 2-inch silicon wafer, by sequentially placing individ-
ual drops (ca. 10 mL) of a 5 mol% PTH polymerization
mixture together with 0.5-inch diameter circular glass cover
slips on the wafer at different time periods after starting
irradiation (15, 30, 45, 60, 120, and 180 min). Significantly,
a reproducible and controlled increase in brush thickness with
increasing time is again observed and at this higher PTH
loading level, little or no incubation period is observed
(Supporting Information, Figure S5). These results further
illustrate the dual role of PTH, acting initially as an oxygen
scavenger, and in the absence of oxygen, functioning as
a photoredox polymerization catalyst for SI-CRP.[9a]
Figure 3. Patterning using glass cover slips of different shape. Digital
images showing the preparation of “MRL” brushes on a 4-inch
diameter ATRP-initiator functionalized silicon wafer. a) Initiator-func-
tionalized silicon wafer, b) “MRL” shaped glass cover slips on the
wafer prior to brush growth, c) resulting polymer brushes after removal
of cover slips and extensive washing and d) digital micrograph of
purified polymer brushes illustrating fidelity with “MRL”-shaped cover
slips.
ability to spatially control the synthesis of polymer brushes
under ambient conditions.
An enabling feature of this novel strategy is the central
role of a transparent cover slip for mediating brush growth.
The presence of a cover slip opens up new opportunities for
controlling SI-CRP and the resulting brush features. For
example, extension of transparent cover slips to photomask
allows patterned P(BnMA) brushes with feature resolution
down to about 1 mm to be obtained. AFM confirmed the
presence of patterned brushes with varied surface topography
and corresponding height profiles (Supporting Information,
Figure S6). Furthermore, the use of gradient photomasks
leads to more unique three-dimensional polymer brush
architectures. The gradient profile was confirmed through
AFM and profilometry demonstrating the expected linear
increase in brush height along the length of the structure
(Supporting Information, Figure S7). The success of these
photomodulated experiments prompted a broadening of this
cover slip concept to multiple cover slips and arbitrary shapes.
Initial studies with mm-sized hexagon-, circle-, star-, dia-
mond-, and triangle-shaped cover slips (5 mol% PTH) allows
brushes of P(DMAEMA) to be prepared with a high degree
of fidelity, reproducing the original shape of the glass cover
slips (Supporting Information, Figure S8). Significantly, this
strategy permits multiple cover slips of arbitrary shape to be
employed simultaneously on the same silicon wafer. In
demonstrating this additional level of control, M-, R-, and
L-shaped cover slips were fabricated, and used to control
brush growth of P(DMAEMA) from select areas of a 4-inch
diameter silicon wafer. As shown in Figure 3a–d, reproduc-
tion of the “MRL” cover slips as a polymer brush pattern is
achieved by adding monomer droplets to an initiator-func-
tionalized silicon wafer. Placement of the individual M-, R-,
and L-cover slips then conformally spreads the monomer
solution under the cover slips, and brush growth is achieved
by irradiation at 405 nm. These results clearly illustrate the
Of equal importance is the ability to prepare these
polymer brushes over large surface areas. Uniform coverage
of a 4-inch (10 cm) silicon wafer could be achieved using only
200 mL of reaction solution and a 4-inch diameter glass cover
slip (ca. 2.5 mLcmÀ2). Significantly, optical reflectometry
indicates formation of a uniform polymer brush layer
(41 nm; Supporting Information, Figure S9) which demon-
strates the scalability of this approach. The versatility of this
platform was also exemplified by the growth of a wide variety
of functional polymer brushes. Importantly, the monomer
scope is not limited by the solubility of PTH in this system. As
shown in the Supporting Information, Table S2, hydrophobic,
hydrophilic, and reactive methacrylate derivatives could be
polymerized under ambient conditions, resulting in uniform
polymer brush thicknesses (20–60 nm). By measuring the
ratio between swollen and dry polymer brush heights of
poly(methyl methacrylate) PMMA, the grafting density was
estimated to be 0.28 chainsnmÀ2 (see the Supporting Infor-
mation, Figure S11 for detailed calculations).[12]
The versatility of this platform then opens up the
possibility of simultaneously growing multiple polymer
brushes from minimal reaction volumes, a critical feature
for expensive or synthetically challenging monomers. To
demonstrate this advantage, the one-step synthesis of multi-
ple, emissive copolymer brushes from sub-milligram quanti-
ties of iridium-based monomers was examined. Taking
advantage of a library of Ir-complexes recently developed in
our group,[13] four different monomer solutions (10–15 mL),
each containing a distinct Ir-complex (0.5 mol% relative to
benzyl methacrylate), were placed on a functionalized 2-inch
silicon wafer and covered with four individual cover slips.
Irradiation at 405 nm for 2 h, followed by extensive washing,
led to four well-defined features composed of different
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photoluminescent polymer brushes with colors (orange, blue,
green and red) spanning the visible spectrum (lex = 254 nm;
Figure 4). It should be noted that the combined mass of Ir-
complexes used, less than 1.0 mg, is sufficient for the synthesis
of circa cm2 areas of functional polymer brushes with the
phosphorescent properties of the emitting chains being
defined by the different Ir-comonomers used. This ability to
simultaneously perform multiple polymerizations on the
same wafer using minimal amounts of functional building
blocks highlights a major advantage of this strategy.
diameter cover slip then leads to growth of a second P-
(MTEMA) layer after irradiation at 405 nm. This sequence
was then repeated using DMAEMA (1.0-inch diameter cover
slip) and MTEMA (0.5-inch (12.7 mm) diameter cover slip)
to give a novel, three-dimensionally patterned polymer brush.
As shown in Figure 5c, the size and shape of the different
cover slips is reproduced with a high degree of fidelity. This
sequential growth could also be monitored by optical
reflectometry and profilometry, which confirms the step-
wise increase in brush thickness, while X-ray photoelectron
spectroscopy (XPS) and secondary ion mass spectrometry
(SIMS) shows the expected order of monomer incorporation
(Supporting Information, Figures S15–S18) with the change
in monomer functionality being clearly identified through the
presence/absence of the N1s signal for DMAEMA and the
S 2s and S 2p signals for MTEMA in XPS (Supporting
Information, Figures S15, S17). Significantly, other monomer
sequences such as DMAEMA-BnMA-DMAEMA-BnMA or
combination of shapes (square–circle) could also be realized,
with the thickness of each layer being controlled by the
irradiation time (Supporting Information, Figures S16, S17).
This merger of efficient reinitiation with simple cover slip
patterning clearly illustrates the retention of active bromine
chain ends and opens up the design space available to this
strategy.
In summary, this work demonstrates the simplicity of
using metal-free ATRP (PTH acting as both an oxygen
scavenger as well as a polymerization catalyst) in combination
with transparent cover slips for the synthesis of complex and
arbitrary three-dimensional polymer brush structures. Key
advantages include minimal reaction volumes, applicability to
large area substrates and ambient conditions. In addition, the
use of visible light and multiple cover slips of arbitrary shape
allows isolated and spatially defined functional polymer
brushes to be concurrently prepared on the same wafer.
This straightforward and versatile synthetic platform will
allow non-experts access to polymer brush architectures that
are difficult to achieve using other techniques.
Acknowledgements
Figure 4. Chemical structures of the different monomeric Ir-complexes
used for copolymer brushes formation (top), fluorescence image
(254 nm UV excitation) of the four copolymer brushes P(BnMA-co-Ir-
complex). Brushes were simultaneously prepared under ambient
conditions on a 2-inch (5 cm) ATRP initiator-functionalized silicon
wafer using individual cover slips (bottom).
We thank the MRSEC program of the National Science
Foundation (DMR-1720256) and the Dow Chemical Com-
pany through the Dow Materials Institute at UCSB for
financial support. B.N. thanks the Development and Promo-
tion of Science and Technology Talents Project, Royal
Government of Thailand Fellowship for funding. N.J.T. and
E.H.D. thanks the NSF Graduate Research Fellowship for
funding. The authors also appreciate fruitful discussion with
Dr. Jimmy Lawrence and Dr. Yingdong Luo.
The controlled nature of this polymerization process was
then examined through a step-wise sequence of growth-
washing-reinitiation steps using progressively smaller cover
slips and different monomers. This is illustrated in Figure 5,
where DMAEMA (5 mol% PTH) was initially polymerized
from a 2-inch (5 cm) diameter wafer using a 2-inch diameter
circular glass cover slip followed by extensive washing to give
a homogeneous P(DMAEMA) brush. Addition of a droplet
of 2-(methylthio)ethyl methacrylate (MTEMA; ca. 30 mL,
5 mol% PTH) followed by placement of a 1.5-inch (38 mm)
Conflict of interest
The authors declare no conflict of interest.
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Figure 5. Sequential polymerization of DMAEMA-MTEMA-DMAEMA-MTEMA under ambient conditions. a) Representation of the stepwise
synthesis, b) corresponding digital images of the polymerization process, c) digital images of polymer brushes on a 2-inch (5 cm) ATRP initiator-
functionalized silicon wafer during stepwise brush growth.
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Keywords: copolymerization ·
light-mediated radical polymerization · photochemistry ·
polymer brushes
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Manuscript received: May 13, 2018
Revised manuscript received: July 14, 2018
Accepted manuscript online: August 28, 2018
Version of record online: && &&, &&&&
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Communications
Polymer Brushes
A simple approach to complex, arbitrary,
and multifunctional three-dimensional
polymer brushes under ambient condi-
tions is reported. N-phenylphenothiazine
serves the dual purpose as oxygen scav-
enger and photocatalyst. The use of cover
slips to mediate these reactions and
minimal reaction volumes allows excel-
lent spatiotemporal control of brush
growth and large surface area coverage.
B. Narupai, Z. A. Page, N. J. Treat,
A. J. McGrath, C. W. Pester,
E. H. Discekici, N. D. Dolinski,
G. F. Meyers, J. Read de Alaniz,
C. J. Hawker*
&&&& — &&&&
Simultaneous Preparation of Multiple
Polymer Brushes under Ambient
Conditions using Microliter Volumes
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ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers!