Fabrication of complex three-dimensional polymer brush nanostructures through light-mediated living radical polymerization

Article abstract of DOI:10.1002/anie.201301845

A facile approach to unique 3D, patterned polymer brushes is based on visible-light-mediated controlled radical polymerization. The temporal and spatial control of the polymerization allows the patterning of polymer brushes from a uniform initiating layer using a simple photomask (see picture). Furthermore, gradient polymer brushes, patterned block copolymers, and complex 3D structures can be obtained by modulating light intensity. Copyright

Full text of DOI:10.1002/anie.201301845

Angewandte
Chemie
Communications
Photochemistry
A facile approach to unique 3D, patterned
polymer brushes is based on visible-light-
mediated controlled radical polymeri-
zation. The temporal and spatial control
of the polymerization allows the pattern-
ing of polymer brushes from a uniform
initiating layer using a simple photomask
(see picture). Furthermore, gradient
polymer brushes, patterned block
copolymers, and complex 3D structures
can be obtained by modulating light
intensity.
J. E. Poelma, B. P. Fors, G. F. Meyers,
J. W. Kramer,
C. J. Hawker*
&&&& — &&&&
Fabrication of Complex Three-
Dimensional Polymer Brush
Nanostructures through Light-Mediated
Living Radical Polymerization
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DOI: 10.1002/anie.201301845
Photochemistry
Fabrication of Complex Three-Dimensional Polymer Brush
Nanostructures through Light-Mediated Living Radical Polymeri-
zation**
Justin E. Poelma, Brett P. Fors, Gregory F. Meyers, John W. Kramer, and Craig J. Hawker*
Surface-initiated polymerizations (SIPs) have received sig-
nificant attention as a robust and effective strategy for the
extended when compared to sparsely grafted areas, resulting
in varying brush heights on the substrate. A key characteristic
of these routes to patterned and gradient brushes is the
uniform distribution of initiators on the substrate. Though less
common, gradient brushes have also been obtained from
a uniform layer of the initiators by varying the chain density
[1,2]
fabrication of polymer brushes.
These polymerizations are
generally performed on substrates modified with a self-
assembled monolayer of initiators, giving polymer chains that
are tethered to the substrate by one end. By employing
controlled radical polymerization techniques, such as atom-
[26,27]
through the time of exposure to UV light,
or by
[
3]
transfer radical polymerization, reversible addition-frag-
manipulating the contact time of the surface with the
[4]
[28]
mentation chain transfer polymerization, and nitroxide-
monomer and catalyst solution. While these approaches
[
5]
mediated polymerization, brush architectures, such as block
copolymer brushes and a variety of polymer brushes pat-
can readily give rise to gradient surfaces, it is technically
challenging to produce complex 3D structures.
[6]
[7]
terned in the x- and y-dimensions, are accessible. The
Recently, our group reported the living radical polymer-
versatility of these synthetic routes has led to a wide range of
ization of methacrylates regulated by visible light using an Ir-
[8,9]
[29,30]
applications, including antifouling coatings,
chemical sens-
based photoredox catalyst.
In this system the propagating
[
10]
[11,12]
ing,
biofunctional interfaces,
and stimuli-responsive
polymer chains are efficiently returned to their dormant state
when the light source is removed, and can be reinitiated upon
subsequent exposure to light, affording temporal control over
chain growth.
Herein, we demonstrate facile, temporally and spatially
controlled brush formation from a uniform initiating layer
through a visible light-mediated radical polymerization, and
illustrate a set of key differentiating features of this approach
compared with previous strategies (Figure 1a). For example,
the use of light to control the polymerization allows the brush
height to be determined by the exposure time. Through the
use of a traditional photomask, brush growth can also be
spatially confined to exposed regions (Figure 1b), however,
[13,14]
materials.
While progress has been made toward advanced brush
architectures, current surface initiation strategies lack tem-
poral and spatial control and, therefore, rely on a prepatterned
initiator layer to template brush formation. Prepatterning has
been demonstrated on a variety of substrates using top-down
[15]
lithographic techniques, such as photo- and interference
[6]
[16–18]
lithography,
probe lithography,
electron-beam lithography,
scanning-
In these
[19–21]
[22]
and soft lithography.
cases, polymerization only occurs in regions where the
[22]
initiator is present, resulting in patterned polymer brushes.
With greater difficulty, the patterning of polymer brushes
can be extended to three-dimensional (3D) nano- and micro-
structures by patterning a concentration gradient of the
initiating species by lithographic techniques or through
[
23–25]
controlled vapor deposition of initiators.
Steric interac-
tions between chains cause densely packed areas to be highly
[
*] J. E. Poelma, Dr. B. P. Fors, Prof. C. J. Hawker
Materials Research Laboratory and Materials Department
University of Califormia
Santa Barbara, CA 93106 (USA)
E-mail: hawker@mrl.ucsb.edu
Dr. G. F. Meyers, Dr. J. W. Kramer
The Dow Chemical Company
Midland, MI 48667 (USA)
[**] We thank the MRSEC program of the National Science Foundation
(
DMR 1121053, J.E.P. and C.J.H.) and the Dow Chemical Company
through the Dow Materials Institute at UCSB for financial support.
B.P.F. thanks the California NanoSystems Institute for the Elings
Prize Fellowship in Experimental Science. J.E.P. thanks the NSF
Graduate Research Fellowship for funding.
Figure 1. Patterning of polymer brushes from substrates uniformly
functionalized with trichlorosilane-substituted a-bromoisobutyrate-
based initiators (a) using b) a photomask for patterns or c) a neutral
density filter for gradient structures. DMF=N,N-dimethylformamide,
ppy=2-phenylpyridine.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301845.
2
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the unexposed regions still contain active initiating species
that can be utilized for subsequent polymerizations. In prior
studies, the initiating groups were typically destroyed in areas
where the polymer brushes were not grown. Finally, neutral
density filters can be used to modulate the intensity of
incident light and, therefore, the kinetic rate of polymeri-
zation from the surface. These factors allow the direct
formation of gradient brush structures and arbitrary 3D
features in a single step over large areas (Figure 1c). In
combination with the uniform density of initiating groups, this
method leads to homogeneous stretching of the polymer
chains with varying molecular weights, in direct contrast to
the variable stretching in prior studies. The unique properties
of this process, which leads to nanoscale features that are
molecularly distinct from those achieved previously, offer
significant scope for applications ranging from photolithog-
raphy to one-step, high-throughput fabrication of patterned
substrates.
exposure time of 30 minutes. Similarly, a substrate was cycled
between three dark periods of 5 minutes and two intervals of
5 minutes of exposure to light (10 minutes total exposure to
light). In all cases, brush thickness was determined only by the
total irradiation time (Figure 2, &). This ability to “pause” and
“restart” surface-initiated polymerizations has profound
implications for patterning polymer brushes and clearly
demonstrates that iridium-based photocontrolled polymeri-
zation affords excellent temporal control of brush growth
from a surface.
One of the most attractive features of a photochemically
controlled route to polymer brushes is the potential for direct
spatial control over brush growth. As a simple illustration,
initiator-functionalized substrates in a solution of MMA and
[
31]
[Ir(ppy) ] in DMF were irradiated through photomasks
3
containing rectangular patterns of different sizes. Optical
micrographs show clear patterning of the poly(methyl meth-
acrylate) (PMMA) brushes (Figure 3), thus demonstrating
spatial control over brush formation from a uniform initiating
layer and the ability to pattern a range of features over large
areas. Patterning could be achieved at submicrometer levels
To demonstrate the capabilities of this new concept,
silicon oxides were uniformly functionalized with trichloro-
silane-substituted a-bromoisobutyrate-based initiators (Fig-
[
7]
ure 1a). Initially, the relationship between film thickness
and irradiation time was determined by a series of separate,
yet comparable experiments, in which exposure time to the
light of a commercial 26 Watt fluorescent lamp (available
from any hardware store) was varied for a solution of methyl
methacrylate (MMA), DMF, and fac-[Ir(ppy) ] in contact
3
with silicon wafers uniformly functionalized with a covalently
bound initiating species. Film thickness increased linearly
with time upon continuous irradiation (Figure 2). Signifi-
cantly, the use of light as an external mediator of polymer-
ization enables the control of film thickness without the
addition of a “sacrificial” untethered initiator or a deactivat-
ing species to the monomer and catalyst solution. To further
confirm the facile nature of this process and establish that
brush growth only occurs when irradiated by light, a series of
Figure 3. Optical microscopy image of patterned PMMA brushes
obtained using a negative photomask with a) 20 mm by 200 mm and
b) 2.5 mm by 25 mm rectangles.
“
on”–“off” experiments were conducted. First, a sample was
irradiated for 20 minutes, left in the dark for 10 minutes, and
then re-exposed to light for 10 minutes, resulting in a total
and was only limited by the wavelength of the light. The key
to minimizing the impact of diffusion on the resolution in this
system is the short excited state lifetime of the [Ir(ppy)3]
catalyst(ca. 50 ns). Based on an upper limit for the diffusion
coefficient of the Ir catalyst, which is the self-diffusion
coefficient of water (D = 2.3 ꢀ 10 m s ), the catalyst is
expected to diffuse less than approximately 20 nm during its
excited-state lifetime. This distance is significantly shorter
than the wavelength of light and leads to the high degree of
fidelity observed in this system.
À9
2
À1
The ability to spatially control brush formation also opens
up the intriguing possibility of combining spatial (x,y dimen-
sions) with intensity (z dimension) modulation to produce
well-defined three-dimensional nanostructures in a single
step. When compared to previous strategies, this represents
a more practical and versatile synthetic approach. As an
initial example, a grayscale photomask that contains an array
of squares of varying optical density was used to probe the
relationship between brush height and light intensity for
a given exposure time. The brush height was found to be
inversely proportional to the optical density of the mask
(Figure 4). Regions of the substrate that are exposed to more
Figure 2. Brush height as a function of irradiation time measured by
spectral reflectance. *
: continuous irradiation, &: brushes that were
obtained by “on”–“off” cycles (see text for details).
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Figure 4. Brush height as a function of optical density of the photo-
mask, as measured by profilometry.
Figure 5. a) Optical micrograph of nanoscale-inclined plane formed
from a 3D polymer brush; b) 3D AFM image of nanoscale-inclined
plane, and c) height along hashed line across feature as shown in (a).
light experience an increase in the kinetic rate of brush
formation, resulting in higher molecular weight brushes and,
as a result, an increase in polymer brush thickness. Because
the initiator density is uniform over the entire substrate,
variances in brush thickness are a result of variations in the
molecular weights of the polymer brushes in different regions.
To support this hypothesis, the grafting densities were
À2
determined to be consistent (0.27 Æ 0.02 chainsnm ) for
a range of different features/samples, as well as within the
[26]
same sample, regardless of light intensity or exposure time.
These grafting densities compare favorably with reported
[
32]
values for moderately dense polymer brushes.
A powerful consequence of this novel mechanism is the
possibility to fabricate complex and arbitrary three-dimen-
sional patterns by modulating the intensity of light to control
the molecular weight of the brushes rather than the density of
the initiator. Using a grayscale lithography mask, a variety of
features could be prepared, including inclined planes, micro-
prisms, gradients, and arrays of microlenses (Figure S3). The
optical micrograph and 3D atomic force microscopy (AFM)
image of an inclined plane (Figure 5) illustrate the compelling
nature of this technique for patterning 3D polymer brush
structures from a uniform initiating layer in a single step.
Importantly, the linear relationship between feature height
and the optical density of the mask is maintained, as
evidenced by a height profile along the length of the structure
Figure 6. a) Schematic of patterned block copolymer brushes and
conversion of PMMA-b-PtBuMA to PMMA-b-PMAA. b) Optical micro-
graph of PMMA-b-PtBuMA brushes patterned from a uniform PMMA
initiating layer. c) Selective wetting of PMMA-b-PMAA regions after
exposure to water vapor.
(
Figure 5c).
An additional attractive feature of this strategy is the
initiating layer with these block copolymer domains clearly
being visible by optical microscopy (Figure 6b).
nondestructive nature of the patterning process, which allows
retention of initiator activity after initial polymer brush
formation. As a result, this technique represents a facile route
to patterned block copolymer brushes in which both chemical
functionality and surface topography can be tuned (Fig-
ure 6a). To investigate this capability, a uniform PMMA
brush with a height of approximately 40 nm was initially
prepared. Exposure of this surface through a TEM grid as
a photomask was then used to grow poly(tert-butyl meth-
acrylate) (PtBuMA) chains specifically in the irradiated areas.
This technique gives regions of PMMA-b-PtBuMA where the
PtBuMA domains are 12 nm thicker than the PMMA
The ability to selectively initiate the fabrication of
a second block shows that the first PMMA layer still contains
active alkyl bromide chain ends, which can be efficiently re-
initiated to afford spatially defined block copolymer brushes.
As with the temporal control observed by cycling between
light and dark periods, no untethered initiator or deactivating
species was required to maintain control over the polymer
chain ends. To illustrate the variations of surface properties
that can be readily obtained, PtBuMA was converted to
polymethacrylic acid (PMAA) by immersion in a 1:1 mixture
of dichloromethane and trifluoroacetic acid (TFA) for
30 minutes. After deprotonation with a 0.1 molar aqueous
4
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KOH solution, selective wetting of the PMAA regions was
observed by optical microscopy (Figure 6c), further verifying
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In summary, a facile approach to patterned polymer
brushes has been developed by taking advantage of the
temporal and spatial control afforded by a “living” visible
light mediated radical polymerization. Through modulation
of the light intensity, complex and arbitrary 3D structures can
be fabricated. Furthermore, patterned block copolymer
structures can be formed for tuning surface properties.
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