Light-Responsive, Shape-Switchable Block Copolymer Particles

Article abstract of DOI:10.1021/jacs.9b07755

A robust strategy is developed for preparing light-responsive block copolymer (BCP) particles in which shape and color can be actively controlled with high spatial and temporal resolution. The key to achieving light-responsive shape transitions of BCP par

Full text of DOI:10.1021/jacs.9b07755

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Article
Light-Responsive, Shape-Switchable Block Copolymer Particles
Junhyuk Lee, Kang Hee Ku, Jinwoo Kim, Young Jun Lee, Se Gyu Jang, and Bumjoon J. Kim
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07755 • Publication Date (Web): 21 Aug 2019
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Light-Responsive, Shape-Switchable Block Copolymer Particles
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Junhyuk Lee,1,‡ Kang Hee Ku,1,2,‡ Jinwoo Kim,1 Young Jun Lee,1 Se Gyu Jang,3 and Bumjoon
J. Kim1,*
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1 Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science
and Technology (KAIST), Daejeon 34141, Republic of Korea
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Department of Chemistry, Massachusetts Institute of Technology (MIT), Massachusetts
02139, United States
3 Functional Composite Materials Research Center, Korea Institute of Science and Technology
(KIST), Jeonbuk 55324, Republic of Korea
* E-mail: bumjoonkim@kaist.ac.kr (B. J. K)
KEYWORDS. block copolymer particles, light-responsive particles, shape-changing particles,
light-active surfactants, hydrogel display
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ABSTRACT
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A robust strategy is developed for preparing light-responsive block copolymer (BCP)
particles in which shape and color can be actively controlled with high spatial and temporal
resolution. The key to achieving light-responsive shape transitions of BCP particles is the
design and synthesis of surfactants containing light-active groups (i.e., nitrobenzyl esters and
coumarin esters) that modulate the amphiphilicity and interfacial activity of the surfactants in
response to light of a specific wavelength. These light-induced changes in surfactant structure
modify the surface and wetting properties of BCP particles, affording both shape and
morphological transitions of the particles, for example from spheres with an onion-like inner
morphology to prolate or oblate ellipsoids with axially stacked nanostructures. In particular,
wavelength-selective shape transformation of the BCP particles can be achieved with a mixture
of two light-active surfactants that respond to different wavelengths of light (i.e., 254 and 420
nm). Through the use of light-emitting, photo-responsive surfactants, light-induced changes in
both color and shape are further demonstrated. Finally, to demonstrate the potential of the light-
triggered shape control of BCP particles in patterning features with microscale resolution, the
shape-switchable BCP particles are successfully integrated into a patterned, free-standing
hydrogel film, which can be used as a portable, high-resolution display.
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INTRODUCTION
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Shape-changing particles have gained a considerable amount of attention in the
production of smart materials for a wide range of applications. For example, the rheological
behavior,1-4 optical properties,5 and capillary interactions6 can be controlled through changes
of particle shape in response to specific external stimuli to afford smart solutions for industrial
paints and coatings, as well as bio-inspired materials for artificial camouflage and drug delivery
systems.7-9 In particular, the development of light-responsive shape-changing particles has been
a long-thought dream for generating active, programmable smart systems. Distinct from
particles that respond to other physicochemical stimuli (i.e., pH, temperature, redox, and
biomolecules), the shapes and properties of the light-responsive, shape-switching particles can
be tuned actively with high spatial and temporal resolution.10-13 Also, the shape and property of
the particles can be tuned without time delay when the light is switched on and off.14-15
Moreover, the capability to precisely modulate the wavelength and intensity of light provides a
powerful strategy for tuning particle shape in a programmed manner.16-18 In the pursuit of light-
responsive systems, extensive progress has been made to develop photo-responsive polymers
and solution assemblies by the use of polymers having photo-cleavable groups or photochromic
units that undergo reversible isomerization upon light irradiation.19-26 However, examples of
shape-tunable polymer particles that respond to light are very limited, especially in the size
range of 100 nm to a few micrometers,27-29 which is ideal for the practical applications described
above.
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One promising approach is to design photo-responsive polymers and surfactant
molecules, which can manipulate the shape of polymer particles. Recently, to prepare shape-
transforming, nanostructured polymer particles, self-assembly of block copolymers (BCPs)
confined in soft and mobile emulsion droplets has been utilized.30-44 This method enables the
large-scale production of uniform particles in the size range of 100 nm to several
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micrometers,45-46 which fulfills the important requirements for a variety of practical
applications. The use of the stimuli-responsive BCPs and/or surfactants in these particle
systems can allow fine-tuning of particle shape and nanostructures in response to external
stimuli.47-51 For example, elongation of polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) BCP
particles was achieved by triggering a solubility change of pH-responsive P2VP.47-48 Recently,
our group reported that the use of responsive polymer surfactants can facilitate temperature-
and pH-driven shape transformations of BCP particles.49-50 Therefore, we expected that
judiciously designed, light-responsive surfactants, which adjust the interfacial properties of the
BCP particles in response to light irradiation, would afford polymer particles exhibiting light-
responsive, shape-switchable properties.
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In this work, we develop shape- and color-switchable BCP particles in response to a
light signal. The structural changes of the photochromic unit, which can be reversibly
isomerized between two or more states, are often insufficient to exert a significant effect on
interfacial activity. Therefore, our strategy starts from the design of photo-cleavable surfactants,
which undergo a significant change in amphiphilicity and interfacial activity under light
irradiation. The use of these surfactants significantly changes the interfacial interactions between
the BCPs and surrounding environment in response to the light irradiation, affording shape
transformations of the particles from spheres to striped footballs (prolate particles) or convex
lenses (oblate particles). Wavelength-selective shape transformation of BCP particles is
realized using a mixture of light-active surfactants, which respond to light of two different
wavelengths (i.e., 254 nm and 420 nm). To demonstrate the potential of such light-responsive
BCP particles, the particles are incorporated into a hydrogel film to produce a portable display
with microscale resolution. The shape and color tuning of the BCP particles at the desired
locations in the film is achieved by irradiation through a photomask.
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RESULTS AND DISCUSSION
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Scheme 1. Schematic illustration of the photo-induced shape transformation of PS-b-P2VP
particles enabled by light-active 5-hexyloxy-2-nitrobenzyl-16-N,N,N-trimethylhexadecan-1-
ammonium bromide (N-CTAB) surfactants.
PS102k-b-P2VP97k (number-average molecular weight (Mn,PS) = 102 kg mol-1, Mn,P2VP
= 97 kg mol-1, dispersity (Ð) = 1.15) particles were prepared by emulsifying polymer solution
(1 wt% in chloroform) into aqueous solution containing light-active surfactant, 5-hexyloxy-2-
nitrobenzyl-16-N,N,N-trimethylhexadecan-1-ammonium bromide (N-CTAB). The chloroform
was evaporated, resulting in the production of the solid BCP particles. Then, the particles
dispersed in water were treated by different times of light irradiation with solvent vapor.
Several aspects of the light-active N-CTAB surfactant design were carefully
considered to achieve particle shape transformation in response to light irradiation. First, N-
CTAB is composed of light-cleavable nitrobenzyl ester linkages, which upon irradiation at 254
nm cleave to yield a carboxylic acid-terminated surfactant (i.e., HOOC-CTAB).52 The ability
of the pyridine groups in P2VP to hydrogen bond with carboxylic acids is exploited to provide
preferential interaction between the cleaved surfactants and the P2VP domains of the BCP
particles.48 Therefore, it was hypothesized that the preferential interaction between the BCPs
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and surfactants (i.e., PS with N-CTAB (before light irradiation) and P2VP with HOOC-CTAB
(after irradiation)) could be tailored by light irradiation to transform the interfacial properties,
and therefore, the shape of PS102k-b-P2VP97k BCP particles using these surfactants (Scheme 1).
The N-CTAB surfactant was synthesized from 5-hydroxy-2-nitrobenzaldehyde as
described in the supporting information, and the photolysis studied at the small molecule level
prior to use for particle shape transformations. Photolysis of N-CTAB surfactant was monitored
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by H nuclear magnetic resonance (1H NMR) spectroscopy and attenuated total reflectance
Fourier transform infrared (ATR-FTIR) spectroscopy. Under light irradiation, N-CTAB
surfactants were cleaved, with the overall conversion of N-CTAB to HOOC-CTAB reaching
92 % in 2 h (200 μW/cm2). As shown in Figure S2a, the intensities of the benzylic proton
resonances on the nitrobenzene ring of N-CTAB at 8.19 and 7.04 ppm decreased, while new
peaks appeared at 8.06 and 7.47 ppm, corresponding to the nitrosoacetophenone photoproduct.
ATR-FTIR spectra (Figure S3) show the appearance of a broad absorption peak at 3500-2500
cm-1, which corresponds to O-H stretching of carboxylic acid, after light irradiation. Temporal
control of the cleavage reaction progress was demonstrated by switching the light on and off
(Figure S4).
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Figure 1. Shape-switching BCP particles induced by light-active surfactants. (a-e) TEM
images of PS102k-b-P2VP97k particles prepared with N-CTAB surfactants. The onion-shaped
particles were treated with light irradiation at 254 nm for (a) 0 h, (b) 0.5 h, (c) 1.0 h, (d) 1.5 h,
and (e) 2.0 h.; (f) The time-dependent mole percent of surfactants (N-CTAB and HOOC-
CTAB) during photo-cleavage of N-CTAB.
Having characterized the photocleavage of these surfactants at the small molecule
level, we then implemented them to stabilize particles and facilitate light-triggered
transformations. First, onion-shaped PS102k-b-P2VP97k BCP particles were prepared with N-
CTAB surfactants (Figure 1a). To provide enough mobility of polymer chains during the shape
transformation, we performed solvent vapor annealing with chloroform, which is a good solvent
for both PS and P2VP blocks (Figure S5).53 The BCP particles in water were subject to the
chloroform-saturated atmosphere with different times of UV light irradiation (0.5, 1, 1.5, and 2
h). Then, the absorbed chloroform was completely released for 36 h. Transmission electron
microscopy (TEM) images in Figure 1 show the shape transformation of PS102k-b-P2VP97k
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BCP particles depending on the light irradiation times. Before irradiation, onion-like spherical
BCP particles with PS at the outermost layer were obtained (Figure 1a). Subsequent light
irradiation led to a remarkable transition of the overall shape as well as the inner morphology
of the BCP particles. After irradiation for 0.5 h, the BCP particles showed a mixture of axially
and radially stacked lamellar morphologies, forming tulip bulb shapes (Figure 1b). In this case,
PS and P2VP lamellae were exposed alternately at the surface with high curvature, while only
PS was surface-exposed on the interface with low curvature. Prolonged irradiation resulted in
the generation of striped prolate particles after 1 h (Figure 1c). As the time of the light exposure
increased to 2 h, inverse tulip bulb-like particles (Figure 1d) and spherical particles with P2VP
outer layers (Figure 1e) were produced.
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To understand the role of the light-active N-CTAB surfactants on the shape transition
of BCP particles, the ratio of N-CTAB to HOOC-CTAB surfactants was monitored as a
function of irradiation time. The ratio of N-CTAB to HOOC-CTAB was calculated from the 1H
NMR spectra, by comparing the integration of the nitrobenzene resonances at 8.19 and 7.04
ppm to those of the nitrosoacetophenone photoproduct at 8.06 and 7.47 ppm; the results are
summarized in Figure 1f. The use of solely N-CTAB surfactants resulted in spherical particles
with preferential localization of the PS domain at the surface, driven by the favorable
hydrophobic interactions between N-CTAB and PS.48, 54 After irradiation for 1 h, the conversion
of N-CTAB to HOOC-CTAB resulted in prolate particles. Importantly, the ratio of N-CTAB
and HOOC-CTAB determined particle shape and morphology. For example, after light
exposure for 1 h, 72 mol % of the N-CTAB had converted to HOOC-CTAB. This condition
produced neutral interfaces such that both PS and P2VP blocks localized at the particle surface,
leading to the perpendicular orientation of the BCPs relative to the surface and generating
ellipsoid-shaped particles with axially stacked lamellar morphologies.34, 38 Lower and higher
percentages of carboxylic acid-terminated surfactants (i.e., 57 mol% after 0.5 h and 82 mol%
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after 1.5 h irradiation) afforded the presentation of primarily PS and primarily P2VP,
respectively, at the particle surface. Therefore, both radially and axially stacked lamellae were
formed within a particle (i.e., one block on the low curvature, and stripe distribution on the high
curvature), producing tulip bulb-like particles.55 Finally, as the ratio of HOOC-CTAB increased
to 0.92 after irradiation for 2 h, inverse onion-like spherical particles with P2VP-outermost
layers were produced driven by hydrogen bonding interactions between HOOC-CTAB and
P2VP (Figure 1e).48 We note that most of the nitrosoacetophenone photoproduct of N-CTAB
migrate to the surrounding aqueous media and are removed by repeated washing steps, therefore
these molecules do not affect the morphology of the BCP particles.
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Figure 2. Light-responsive, shape transformation of cylinder-forming BCP particles. (a-
c) TEM and (d-f) SEM (top-view and side-view) images of PS15k-b-P4VP7k particles prepared
with N-CTAB surfactants. The spherical particles (a, d) were treated with irradiation at 254 nm
for (b, e) 0.5 h, and (c, f) 2 h.
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To investigate the general applicability of the light-active surfactants for producing
particles with various shapes with different BCPs, N-CTAB was applied to encapsulate
cylinder-forming polystyrene-b-poly(4-vinylpyridine) (PS15k-b-P4VP7k) (Mn,PS = 15 kg mol-1,
Mn,P4VP = 7 kg mol-1, Ð = 1.15). Figure 2 shows the scanning electron microscopy (SEM) and
TEM images of PS15k-b-P4VP7k BCP particles before exposure and after light irradiation for
different amounts of time. First, spherical BCP particles with coiled cylinders were produced
(Figure 2a), where the preferable hydrophobic interaction between PS and N-CTAB generated
particles with PS as the outermost layer.36, 56 Subsequently, when the BCP particles were
subjected to light irradiation, the overall particle shape transformed to oblate with hexagonally
packed cylindrical morphology (Figure 2b). Again, conversion of the N-CTAB surfactant to
HOOC-CTAB afforded neutral wetting conditions at the particle surface, however, a different
molar ratio of N-CTAB to HOOC-CTAB was required due to the different relative surface area
of each domain at the surface of the particles with cylindrical morphology relative to those with
lamellar morphology.45, 57 For example, for cylinder-forming BCPs, 40 % conversion of N-
CTAB to HOOC-CTAB (after 0.5 h) was sufficient to generate neutral surfaces allowing the
presentation of both PS and P4VP blocks at the surface. The neutral surfaces created upon
irradiation yielded perpendicularly oriented cylinders, resulting in the formation of oblate
particles.36, 56 Similar to the previous results, prolonged light exposure (> 2 h) changed the
particle shape back to the spherical BCP particles, but with P4VP outermost layers (Figure 2c).
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Figure 3. Wavelength-dependent, orthogonal activation of shape-switching properties. (a)
Chemical structures of two light-active surfactants (C-CTAB and N-CTAB) having different
activation wavelengths (420 nm for C-CTAB and 254 nm for N-CTAB); (b-d) TEM images of
PS102k-b-P2VP97k particles prepared with a mixture of C-CTAB and N-CTAB surfactants (b)
before light exposure, (c) irradiated at 420 nm for 6 h, and (d) irradiated for an additional 2 h at
254 nm. (e) Time-dependent mole fraction of each surfactant (C-CTAB, N-CTAB, and HOOC-
CTAB) during light irradiation.
A distinct advantage of the light-activated, shape-transforming BCP particles is the
ability to selectively engineer and trigger multiple transformations depending on the light
wavelength and intensity. To demonstrate the potential of the wavelength-dependent shape-
switching behavior of BCP particles, we developed a model system to prepare BCP particles
with a mixture of two light-active surfactants having different activation wavelengths. For this
purpose, we designed another light-responsive surfactant, 7-(diethylamino)-4-(N,N,N-
trimethylhexadecan-1-ammonium bromide)coumarin (C-CTAB), composed of a coumarin
ester group, which can be activated and deprotected at 420 nm (Figure 3a).58-59 Similar to the
N-CTAB surfactants, cleavage of C-CTAB leads to the generation of carboxylic acid groups,
forming HOOC-CTAB (Figures S2-4). Accordingly, the use of C-CTAB surfactants in the
preparation of PS102k-b-P2VP97k particles produced similar light-induced shape transformations
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of the particles from spheres to striped prolate particles, however, using lower-energy 420 nm
light to trigger this transformation (Figure S6).
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We then monitored the orthogonal, wavelength-triggered shape change of PS102k-b-
P2VP97k BCP particles stabilized by a mixture of N-CTAB and C-CTAB surfactants at a molar
ratio of 1:3. As expected, the BCP particles were initially spherical with PS outermost layers
due to the preferential hydrophobic interactions of PS with alkyl chains of both N-CTAB and
C-CTAB (Figures 3b).48, 54, 60 The BCP particles were then sequentially irradiated at 420 nm,
followed by 254 nm to investigate the resulting shape and morphological changes (Figures 3b-
d). Irradiation at 420 nm led to a transformation of particle shape from spheres with onion-like
morphology to prolate ellipsoids with stacked lamellar morphology. During irradiation at 420
nm, C-CTAB molecules were selectively converted to HOOC-CTAB, resulting in a ratio of
HOOC-CTAB/C-CTAB = 0.73/0.02 while the fraction of N-CTAB (0.25) remained unchanged
after 6 h. Photolysis of C-CTAB was clearly proved by 1H NMR spectra (Figure S2b), where
the intensities of the proton resonances on 7-(diethylamino)-4-(hydroxymethyl)coumarin of C-
CTAB (i.e., 4.79 and 3.38 ppm) decreased according to the light irradiation. We note that the
N-CTAB surfactants were stable under 420 nm light irradiation (Figure 3e). The second
irradiation step at 254 nm decreased the ratio of N-CTAB, resulting in the overall ratio of
HOOC-CTAB/N-CTAB/C-CTAB to be 0.95/0.03/0.02. In this case, inverse onion-like
spherical particles with P2VP-outermost layers were obtained. Therefore, the shape transition
of BCP particles from onion-like spheres to striped prolate particles was achieved by irradiation
at 420 nm light, followed by another shape transition from prolate particles to spheres at 254
nm, highlighting the excellent wavelength-selectivity of the shape transformations of these
particles.
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Figure 4. Light-triggered, shape-dependent optical properties of BCP particles. (a) PL
spectra and (b-d) photographs showing the fluorescence of PS102k-b-P2VP97k particles prepared
with C-CTAB surfactants before and after irradiation at 420 nm for (b) 0 h (black line, onion-
like particles with PS outermost layers), (c) 3 h (red line, prolate particles), and (d) 6 h (blue
line, reverse onion-like particles with P2VP outermost layers).
Direct, colorimetric visualization of particle shape change is highly beneficial for the
clinical and biological applications, such as drug delivery.7, 49 In our system, the light-cleavable
coumarin ester group of C-CTAB is strongly emissive, imparting tunable optical properties to
the BCP particles and allowing monitoring of the light-triggered shape transformation by color
change. In order to demonstrate the correlation of particle shape to the optical signal, we
examined the emission properties of the BCP particles during the irradiation-induced particle
shape transformation. Figure 4a shows the photoluminescence (PL) spectra of the BCP
particles prepared with C-CTAB surfactants before light exposure and the following irradiation
at 420 nm for 3 and 6 h. Note that the PL spectra were acquired after removal of excess
surfactants by repeated centrifugation. The spherical particles stabilized with C-CTAB showed
a strong blue emission peak at 440 nm from the coumarin chromophore of the C-CTAB
surfactants, as depicted in Figure 4b. After irradiation for 3 h, the particle deformed to a striped
prolate, while the fluorescence decreased by 74 %, resulting in the dark blue BCP particle
suspension shown in Figure 4c. After prolonged irradiation (6 h), the suspension of spherical
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BCP particles with reverse onion morphology (i.e., with a P2VP outermost layer) became nearly
colorless and the fluorescence intensity decreased by 92% (Figure 4d). This result corresponds
closely with the conversion ratio of C-CTAB to HOOC-CTAB (91%) after 6 h of irradiation.
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Figure 5. Fabricating portable, high-resolution displays by incorporating light-responsive
BCP particles into patterned hydrogel matrices. (a) Photograph of a particle-containing
hydrogel film exhibiting blue fluorescence; (b) cross-sectional SEM images of a freeze-dried
hydrogel film; (c) schematic illustration of the preparation of patterned hydrogel film by UV
light irradiation through a patterned photomask; (d) optical micrograph of the patterned
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photomask; (e) fluorescence images of the hydrogels after irradiation for 3 h at 420 nm through
the photomask; (f, g) high-magnification SEM and TEM images of the freeze-dried hydrogel
on the (f) light-unexposed region (boxed blue region in e) and (g) light-exposed region (boxed
black region in e); and (h) schematic illustration of the hydrogel film containing shape-
switching, fluorescent BCP particles.
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Finally, to demonstrate the potential of the light-triggered shape control of BCP
particles in patterning features with microscale resolution, we fabricated hydrogel films
containing the BCP particles, irradiated it through a photomask, and examined the local shape
and fluorescence characteristics in the exposed and unexposed regions. We chose a hydrogel as
the host matrix because the elastic and moldable properties of hydrogels allow for successful
embedding and immobilization of the polymer particles; further, the swelling of hydrogels in
aqueous media provides sufficient mobility of the surfactants to facilitate the particle shape
transition.61-63 Also, the hydrogel film is free-standing and flexible, thus can be engineered for
desired applications. The BCP particle-containing hydrogel film was prepared by UV-initiated
polymerization of the aqueous suspension of poly(ethylene glycol) diacrylate, sodium acrylate,
and BCP particles. After polymerization, the un-reacted components and excess surfactants
were removed by repeated washing of the hydrogel with deionized (DI) water (See the
experimental section and the Supporting Information for details). Figure 5a shows the
photograph of the hydrogel film containing PS102k-b-P2VP97k particles stabilized by C-CTAB.
Due to the homogenous embedding of BCP particles within the matrix, a blue emission was
observed over the entire hydrogel film. This was also demonstrated by the cross-sectional SEM
image of the freeze-dried hydrogel film shown in Figure 5b. Given the good dispersion of the
BCP particles within the hydrogel matrix, each particle can act as a photographic pixel, such
that changes in the particle shape and fluorescence response can be displayed as a fluorescence
map with microscale resolution. To explore the local transitions of particle shape and optical
properties, a square lattice photomask with 100 µm features was placed on top of the pre-cured
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hydrogel film, which was irradiated at 420 nm through the mask (Figures 5c to e). The photo-
responsive shape transformation of the BCP particles was realized successfully within the
hydrogel film (Figures 5f-h and S7-8). As shown in the cross-sectional SEM images of the
freeze-dried hydrogels (Figure 5f and Figure S7a), spherical particles were observed in the
light-unexposed region. In contrast, prolate ellipsoids were observed in the light-exposed region
(Figure 5g and Figure S7b). Consistent with the results in Figure 4 showing reduced
fluorescence upon conversion of C-CTAB to HOOC-CTAB, the fluorescence intensity
decreased only in the light-exposed area, as seen by the dark square patterns shown in Figure
5e.
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CONCLUSIONS
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In this work, we developed a robust and effective method to prepare particles that
exhibit light-triggered changes in shape and color. The photo-responsive surfactants N-CTAB
and C-CTAB were designed such that irradiation changed their molecular structure, resulting
in modulation of their amphiphilicity and interfacial activity and corresponding changes to the
shape of the PS-b-P2VP or PS-b-P4VP BCP particles. The light-responsive shape
transformation of the BCP particles from spheres to prolate or oblate ellipsoids was achieved
with high temporal resolution and wavelength selectivity. In addition, the use of light-emitting,
photo-responsive surfactants allowed direct, colorimetric visualization of the shape change of
these BCP particles. And, we demonstrated the potential of the shape- and color-changing BCP
particles by integrating them into the free-standing patterned hydrogel film, which can be used
as a portable display with micrometer-scale resolution. Nevertheless, the non-reversible
character of the photo-cleavable surfactants can limit the use of these particles in some practical
applications, which require the particles with reversible shape and color transformation.
Therefore, the design of photo-active surfactants containing the photochromic units that can be
reversibly isomerized between different states upon light irradiation (i.e., spiropyran
derivatives)26, 64 is an important future direction to realize the light-active BCP particles with
different switching modes.
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EXPERIMENTAL SECTION
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Synthesis of N-CTAB and C-CTAB. Detailed synthetic procedures are provided in the
Supporting Information. 5-hexyloxy-2-nitrobenzyl-16-bromo-hexadecanoate or 7-
(diethylamino)-4-(bromo-hexadecanoate)coumarin (3.0 g, 5.3 mmol) was added into a sealed
tube with trimethylamine (5.4 g, 53.0 mmol) and ethyl acetate (15 mL). The reaction mixture
was held under argon for 7 d at 60 °C. The products were purified by recrystallization from
water to obtain N-CTAB as a light yellow powder (2.2 g, yield 67.4%) or C-CTAB as a white
powder (2.3 g, yield 67.9%).
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Preparation and shape transformation of PS-b-P2VP BCP particles. A chloroform solution
(50 μL) containing PS-b-P2VP (0.5 mg) was emulsified in DI water (1 mL) containing 0.1 wt%
of light-responsive surfactant (N-CTAB or C-CTAB) using a homogenizer. Then, the
chloroform was slowly evaporated for 36 h to obtain solid particles. The shape transformation
of the BCP particles is performed by the following procedure: a vial containing the particle
suspension (1 mL) in water was placed inside a larger vial containing chloroform (2 mL) to
produce a chloroform-saturated atmosphere, and irradiated under 254 or 420 nm for different
times (0.5-6 h). Then, the absorbed chloroform was completely removed for 36 h. The particles
were washed by repeated centrifugation (13000 rpm) to remove remaining surfactants.
Fabrication of BCP particle-containing hydrogel films. An aqueous solution (5 mL) of
poly(ethylene glycol) diacrylate (Mn ≈ 1000 g mol-1, 0.9 g), sodium acrylate (0.1 g), 2-hydroxy-
2-methylpropiophenone (10.0 mg), and BCP particles prepared by C-CTAB (5.0 mg) was
injected into the gap between two glass slides spaced by Kapton spacers (1 mm), and irradiated
by UV light (254 nm, 200 μW/cm2) for 10 min to initiate polymerization and gelation.61 To
remove unreacted residues, the resulting hydrogel was rinsed with DI water. An additional
patterned hydrogel film was prepared by placing a square lattice photomask on top of the BCP
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particle-containing pre-cured hydrogel films, followed by irradiation at 420 nm for 3 h. Solvent
vapor annealing was then performed using chloroform for 12 h.
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ASSOCIATE CONTENT
Supporting Information: Experimental details and additional characterization data (NMR,
FT-IR, TEM, and SEM). This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*bumjoonkim@kaist.ac.kr
Author Contributions
‡These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This research was supported by the National Research Foundation (NRF) Grant, funded by the
Korean
Government
(2016R1E1A1A02921128,
2012M3A6A7055540
and
2017M3D1A1039553). The authors thank Prof. Rachel Letteri for helpful discussion.
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