Switchable Full-Color Reflective Photonic Ellipsoidal Particles

Article abstract of DOI:10.1021/jacs.0c02398

Full-color reflective photonic ellipsoidal polymer particles, capable of a dynamic color change, are created from dendronized brush block copolymers (den-BBCPs) self-assembled by solvent-evaporation from an emulsion. Surfactants composed of dendritic mono

Full text of DOI:10.1021/jacs.0c02398

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Switchable Full-Color Reflective Photonic Ellipsoidal Particles
Qilin He,$ Kang Hee Ku,$ Harikrishnan Vijayamohanan, Bumjoon J. Kim,* and Timothy M. Swager*
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ABSTRACT: Full-color reflective photonic ellipsoidal polymer particles, capable of a
dynamic color change, are created from dendronized brush block copolymers (den-BBCPs)
self-assembled by solvent-evaporation from an emulsion. Surfactants composed of dendritic
monomer units allow for the precise modulation of the interfacial properties of den-BBCP
particles to transition in shape from spheres to striped ellipsoids. Strong steric repulsions
between wedge-type monomers promote rapid self-assembly of polymers into large
domains (i.e., 153 nm ≤ D ≤ 298 nm). Of note, highly ordered axially stacked lamellae
(i.e., number of layers >100) within an ellipsoid give rise to a near-perfect photonic
multilayer. The reflecting color is readily tunable across the entire visible spectrum by
alteration of the molecular weight from 477 to 1144 kDa. Finally, the photonic ellipsoids
are functionalized with magnetic nanoparticles organized into bands on the particle surface
to produce real-time on/off coloration by magnetic field-assisted activation. In total, the
reported photonic ellipsoidal particles represent a new class of switchable photonic
materials.
tures are a promising direction. In particular, striped ellipsoids
with transverse isotropy exhibit orientation-dependent optical
responses, wherein controlled alignment affords anisotropic
optical behavior that is not possible in spherical particles.
Despite the significant progress toward the shape control of
polymer particles, major challenges in the achievement of shape-
controlled BBCP particles include the (i) obtaining of near-
perfect order over tens of micrometer-scales in confined
geometries and (ii) tailored engineering of particle interfaces
with the supporting medium (solvent). To address these issues,
we have targeted dendritic polymers that exhibit low chain
entanglement as a result of steric repulsion between pendant
substituents, which promotes rapid self-assembly.34,35 These
materials can generate photonic bandgaps at lower total
molecular weights than typical BBCP systems with long
polymeric side chains, and this feature results in lower viscosity
to facilitate self-assembly.35,36 In addition, the dendritic units
provide a simple route to design and synthesize surfactant
molecules capable of tailoring the interactions between
polymers and the surrounding media.
INTRODUCTION
■
Photonic crystals having periodic nanostructures display
selective diffraction/reflection of light and can produce what is
known as structural color.1−7 Recent reports have revealed the
potential of encapsulated colloidal photonic structures, which
can serve as structural colorants for paints with easy processing
and dynamic reconfigurability.8−12 Brush block copolymers
(BBCPs), which promote a highly extended backbone at high
graft density, have emerged as a powerful material for the
creation of photonic crystals. In particular, they assemble into
lamellar structures with domain spacings capable of strong
reflection in the visible region of the electromagnetic
spectrum.13−17 Compared to other photonic materials (e.g.,
colloidal particles or liquid crystals),18,19 the BBCP is more
tunable by numerous parameters including backbone structure,
side-chain length, composition, and grafting density.20−22
Further dynamic control of these periodic structures (i.e.,
domain size and orientation) and the fabrication of composites
with high refractive index contrast offer an attractive approach to
switchable coloration and display.23,24 Therefore, the BBCP is a
promising design element for the creation of colloidal units with
tunable photonic behavior. To date, the majority of studies
about BBCP photonic crystals have focused on bulk and thin
films rather than their assembly within the three-dimensionally
confined geometries.25−27
Herein, we report full-color reflective photonic ellipsoids
based on the self-assembly of dendronized brush block
copolymers (den-BBCP), wherein the color can be rapidly
switched on/off. Our strategy to achieve photonic particles relies
Received: March 1, 2020
We have been interested to expand the formation of
nonspherical polymer particles beyond the confined self-
assembly of linear coil−coil diblock copolymers. In such
systems, the polymer particles usually lack sufficient domain
spacing and number of layers to function as photonic
reflectors.28−33 Therefore, alternative macromolecular architec-
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A

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Figure 1. Schematic illustration for the preparation of photonic ellipsoidal particles by self-assembly of dendronized brush block copolymers (den-
BBCPs), composed of monomers functionalized with alkyl ether wedge (AW) and benzyl ether wedge (BnW) groups, within an interface-engineered
oil-in-water emulsion. Two different surfactants having a selective affinity to AW (SAW) or BnW (SBnW) are used to control the interfacial interactions
between the den-BBCPs and surrounding aqueous media.
on high molecular weight den-BBCP, which is paired to two
different dendronized surfactants with selective affinity to each
block. We then utilize the dynamic emulsion interface to induce
a spontaneous deformation of the particle shape. Specifically, by
adjustment of the interfacial interactions between the den-
BBCPs and the surrounding environment, the particle shapes
are transformed from spheres to ellipsoids. We observe a
remarkable elongation of ellipsoids along the major axis (i.e.,
aspect ratio over 7.0) to minimize the bending strain of den-
BBCP driven by steric repulsion between pendant wedge side
groups. Variation of the molecular weight in a range from 477 to
1144 kDa demonstrates that the reflecting color is tunable across
the entire visible spectrum. Finally, we demonstrate how the
reflecting colors of striped photonic ellipsoidal particles create
an optically switchable system when functionalized with
magnetic nanoparticles. The magnetic field-assisted alignment
in bulk solutions of photonic ellipsoids exhibits an efficient rapid
on/off color reflection.
Table 1. Summary of the den-BBCPs Used in This Study
Mw
Dispersity
BnW
λ
Domain size
d
a
a
b
max c
Polymer
den-
(kDa)
(D
̵
)
(mol %)
(nm)
(D, nm)
477
1.15
1.19
1.11
1.20
1.18
47.3
48.7
51.0
52.2
48.7
369
153
189
235
259
298
BBCP477
den-
BBCP594
den-
BBCP809
den-
BBCP895
den-
BBCP1144
594
426
515
585
659
809
895
1144
a
b
c
1
Determined by GPC. Determined by H NMR. Determined by
d
UV/vis spectrometer. Measured by SEM.
the outer surface of the particle.39−44 To promote favorable
interactions to each block of den-BBCP, we synthesized CTAB-
derived complementary surfactants having AW (SAW) or BnW
(SBnW) groups. (Figure 1b; see the Supporting Information for
synthetic details.) These surfactants were dissolved in DCM
with den-BBCP before the emulsification. Figure 2 shows the
particles prepared with 1 wt % SAW and 9 wt % SBnW (weight
percent relative to polymer), which are dispersed in an aqueous
CTAB continuous phase. As shown in the optical microscopy
image in Figure 2a, more than 90% of the particles have an
ellipsoidal shape (i.e., 92% ellipsoid, 2% cone, and 6% sphere).
Moreover, an extremely ordered axially stacked lamellar
structure extends over the entire area of particles with sizes up
to 50 μm (Figure 2b,c). The formation of a striped ellipsoid is
attributed to a surfactant-balanced interface that leads to the
exposure of both AW and BnW blocks at the particle surface,
generating axial stacking of lamellae.43,45 Further adjustment of
the surfactant ratio of SAW to SBnW allowed the transition of
particle shapes to include sphere or cone shapes (Supporting
Information Figure S1).
RESULTS AND DISCUSSION
■
To prepare den-BBCPs, we synthesized two norbornene wedge-
type monomers functionalized with alkyl ether wedge (AW) and
benzyl ether wedge (BnW) groups and subjected them to ring-
opening metathesis polymerization (ROMP) (Figure 1a) in
accord with published procedures.34,35 The poly(AW-b-BnW)
den-BBCPs were synthesized with the weight-average molecular
weight (Mw) ranging from 477 to 1144 kDa while low dispersity
(D̵) was maintained (Table 1). (See the Methods section and
Supporting Information for the synthetic details.) To produce
solid den-BBCP particles, a dichloromethane (DCM) solution of
den-BBCPs (2 wt %) was emulsified into an aqueous solution
containing surfactants (i.e., cetyltrimethylammonium bromide
(CTAB)) using a vortex for 5 s (Figure 1b). The evaporation of
DCM initiates nucleation of ordered den-BBCP domains near
the interface between the emulsion and the surrounding
aqueous solution, which is followed by the propagation of
polymer ordering into the particle center.37,38 This rapid self-
assembly of den-BBCPs from a volatile solvent affords long-
range-ordered periodic nanostructures capable of reflecting light
(Figure 1c).
It is worth noting that these ellipsoids have a significant
elongation behavior along the major axis. The formation
mechanism of the ellipsoidal BCP particles has been described
by theoretical calculations that quantify the degree of particle
elongation.45 In brief, when the ellipsoidal particle elongates, the
entropic penalty of bending polymer chains can be released by
reducing the curvature of the lamellar layers, but at the expense
Surfactants play a critical role in the emulsification of BCPs as
the nature of the surfactant determines which block is exposed at
B
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Figure 2. (a) Optical microscopy, (b) scanning electron microscopy (SEM), and (c) transmission electron microscopy (TEM) images of ellipsoidal
den-BBCP594 particles prepared with 1 wt % SAW, 9 wt % SBnW (weight percent relative to polymer), and aqueous CTAB solution as the continuous
phase.
of increased surface area of the particle (Figure 3). Therefore,
the competition between these energy contributions dictates the
example, the AR of these ellipsoidal particles can be enhanced up
to 7 with a particle size of 50 μm (Figures S2 and S3), which is a
stark contrast to the previously reported AR values that are
limited to the range from 1.0 to 3.0.
We prepared a series of den-BBCP particles having different
molecular weights (Mw) of 477, 594, 809, 895, and 1144 kDa. All
particles exhibit long-range ordering of axially stacked lamellae
with domain sizes of 153, 189, 235, 259, and 298 nm (Figure 4).
To analyze the relationship between the measured domain
spacing (D) and Mw value, D is plotted as a function of Mw in the
log−log plot (Figure S4). The exponents α in the scaling form D
∼ (Mw)α resulted in the slope of α = 0.82. This value is greater
than the power index of typical linear BCPs which have the
Figure 3. Schematic illustration for the release of the high bending
energy of curved lamellar den-BBCP to have a highly elongated striped
ellipsoid.
scaling relation of D ∼ (Mw)2/3 46 This behavior of den-BBCPs
.
most thermodynamically stable structure. In the case of lamella-
forming linear diblock copolymers, the low bending energy of
curved lamellae within ellipsoidal particles prevents the particles
from having a high anisotropy.41,42,45 In contrast, BBCPs allow
ellipsoidal particles with an exceptionally high shape-anisotropy
and regular structure. Importantly, we find that particle sizes can
increase up to a few tens of micrometers without any defects as a
result of their rapid self-assembly kinetics and large domain size.
Additionally, the wedge-type monomers lead to a considerable
elongation of the polynorbornene backbone, resulting in high
entropic penalty of bending den-BBCP chains. These features
yield ellipsoidal particles with remarkable size, domain spacing,
and aspect ratio (AR) compared to previous reports.28,37,45 For
originates from the rigid architectures that display a reduced
degree of chain entanglement.47,48
The excellent self-assembly of den-BBCPs into defect-free,
long-range-ordered periodic nanostructures with D ranging
from 153 to 298 nm enables light reflection across the visible
spectrum. Figure 5a shows the transmission spectra and the
photographs of den-BBCP films having different Mw. Increasing
Mw from 477 to 1144 kDa causes a commensurate shift in the
peak of reflection (λmax) from 369 to 659 nm (Figure 5b). This is
attributed to the linear increase of λmax as a function of D, which
is in agreement with the previous report.35 For normal incident
Figure 4. Influence of the Mw of the den-BBCP particles on overall shape and inner nanostructure: (a−e) SEM and (f−j) TEM images of ellipsoidal
den-BBCP particles having different Mw of (parts a and f) 477 kDa (domain spacing (D) = 153 nm), (parts b and g) 594 kDa (D = 189 nm), (parts c and
h) 809 kDa (D = 235 nm), (parts d and i) 895 kDa (D = 259 nm), and (parts e and j) 1144 kDa (D = 298 nm).
C
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Figure 5. (a) Transmission spectra and photographs of den-BBCPs films. (b) Plots of λmax as a function of Mw of den-BBCPs. (c−e) Reflection
micrographs of individual ellipsoidal den-BBCP particles and the photographs of particle suspension in water with different Mw of 1144 kDa (c, red
color), 809 kDa (d, green color), and 594 kDa (e, blue color). The light source and the camera were kept at a fixed angle of 0°.
Figure 6. (a) SEM image and (b) corresponding EDX spectrum of Fe3O4NP-attached den-BBCP540 ellipsoids (inset figure is the TEM image of the
Fe3O4 NPs). (c) Magnetic field-assisted alignment of Fe3O4/den-BBCP540 particles depending on the time: (i) random assembly, (ii) linear alignment,
and (iii) side-by-side stacking. (d, e) Real-time on/off or blue-shift coloration of the particle suspension by precise control of the particle orientation:
photographs of the suspension of (part d) den-BBCP540 ellipsoids and (part e) den-BBCP1144 ellipsoids under the magnetic field illustrating the light
incident angle-dependent color reflection. The light source and the camera were kept at a fixed angle of (part d) 0° and (part e) 100°.
beam, the relationship between λmax and D is described by the
particles, while slightly broader and blue-shifted reflection is
expected due to the random orientation of the particles. As
shown in Figure 5c−e, individual den-BBCP particles display
red, green, and blue colors in reflective optical microscopy as a
result of the highly ordered multilayer (more than 100) lamellar
structures within an individual particle. For 1D photonic
crystals, the wavelength of reflected light changes with the
following equation13
λmax = 2(n1d1 + n2d2)
where ni is the refractive index of component i and di is the
thickness of the ith layer. In comparison with the den-BBCP
films, the trend in the reflecting color is almost preserved in the
D
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angle of light incidence with respect to the plane of the photonic
layer, causing the bandgap to be angle-dependent.
the spherical particles react to the magnet; however, they do not
display color switching.
We note that in the case of these particle suspensions, the
distribution of the orientation of the lamellae is random and the
grain size is small. In detail, by considering the shape of the
particle as a perfect ellipsoid, the effective number of the
photonic layers (Neff) is expressed as a function of incident angle
θ as in the following equation
CONCLUSIONS
■
We have developed a robust and effective strategy to prepare
photonic ellipsoids with a rapid color-switching response to
magnetic field. Rapid, confined self-assembly of dendronized
BBCPs generated highly elongated ellipsoids having a near-
perfect ordering of axially stacked lamellae. Simple tuning of Mw
afforded the formation of domain sizes that ranged from 153 to
298 nm, and the reflecting color was readily tunable across the
entire visible spectrum. Fe3O4-attached hybrid photonic
particles provided controlled alignment of the photonic
ellipsoids, and their bulk reflectance displayed dynamic on/off
color-switching assisted by directional magnetic field. We
envision that the magnetic NPs-functionalized photonic
ellipsoids represent a new class of magnetically responsive
photonic crystals, providing promising potential in smart
photonic pigments and photonic devices.
Ä
É
Å
Å
Ñ
Ñ
Å
Å
Ñ
Ñ
Ncosθ
Å
Å
Å
Ñ
Ñ
Ñ
N
=
eff
Å
Ñ
(cosθ)2 + (AR × sinθ)2
Å
Å
Ñ
Ñ
Å
Ñ
Å
Ç
Ñ
Ö
where N is the number of photonic layers in the particle. The
photons incident at a significant angle from the particle axis
interact with fewer layers, so the reflections are much weaker.49
Therefore, regardless of the direction of the incident light, the
bulk suspension of the particle exhibits the predominant
reflecting color. The theoretical simulation of the reflectance
spectrum of the particle suspension is provided in the
Supporting Information.
In contrast to spherical particles with concentric lamellae, the
photonic behaviors (i.e., reflecting color and intensity) of the
ellipsoidal particles having axially stacked lamellae are strongly
dependent on the incident light angle to the photonic layer. To
illustrate this feature, we demonstrated magnetic field-assisted
versatile alignment of photonic particles by attaching Fe3O4
magnetic nanoparticles (NPs). We first synthesized 6.4 nm-
sized Fe3O4 NPs capped with 1,2-hexadecanediol (Figure S5).50
Then, a solution of den-BBCP and Fe3O4 NPs (volume fraction
∼3%) were emulsified to yield hybrid magnetic photonic
ellipsoids. As a result of the large entropic barriers arising from
polymer stretching to incorporate NPs and additional enthalpic
gain from the preferential affinity with the surrounding medium,
the NPs segregated on the surface of the den-BBCP particles and
the overall ellipsoidal shape and inner lamellar structure were
maintained.39,51 Highly magnified SEM image (Figure 6a) and
the energy-dispersive X-ray spectroscopy (EDX) analysis of the
Fe (Figure 6b) confirm that the NPs localize at the AW block’s
aqueous interface, forming bands of Fe3O4 NPs. This
partitioning is a result of the 1,2-hexadecanediol giving
predominately alkyl character to the NPs.50 When subjected
to the magnetic field, the hybrid particles are rapidly magnetized
and further interact with each other forming a side-by-side
stacking pattern (Figure 6c). The ellipsoids are magnetized
along the minor axis so that the magnetic NPs assembling on the
surface could achieve maximum interactions between par-
ticles.52
Finally, we explored the particle orientation-dependent
photonic behavior by varying the direction of the magnetic
field. The suspension of den-BBCP540 ellipsoids shows a strong
reflected violet color when the light incident angle is small (i.e., θ
∼ 0°, the direction of the magnetic field is perpendicular to the
incident light), while there is almost no reflection at θ ∼ 90°,
leading to a rapid on/off response in coloration activated by the
magnetic field (Figure 6d and Supporting Information Movie
S1). The ellipsoids with higher Mw show the angle-dependent
blue-shift coloration and finally switch off the color (Figure 6e
and Supporting Information Movie S2). In this case, the light
source and the camera were put at an angle of 100° to observe
the blue-shifted color. The significance of the lamellar structure
in ellipsoids can be emphasized by comparing the magnetic
response of the Fe3O4 NPs-attached spherical particles having
concentric lamellae as a control sample. As shown in Figure S6,
METHODS
■
Synthesis of Dendronized Brush Block Copolymers (den-
BBCPs). Detailed synthetic procedures are provided in the Supporting
Information. Alkyl wedge monomers (AW) and benzyl wedge
monomers (BnW) were synthesized according to the previous report.35
Under the rapid stirring, the Grubbs Catalyst (3rd Generation) was
added into the solution of BnW in tetrahydrofuran (THF), followed by
the addition of AW. The polymerization was allowed to proceed for 5
min. The product was precipitated with methanol to afford den-BBCP.
Synthesis of SAW and SBnW Surfactants. A mixture of AW (or
BnW) precursor, 12-bromo-1-dodecanol, 4-dimethylaminopyridine,
N,N′-dicyclohexylcarbodiimide in DCM was stirred at room temper-
ature overnight. The product was purified by column chromatography
with hexane/ethyl acetate to yield 12-bromododecyl 3,4,5-tris-
(decyloxy)benzoate (66 mg, 46% yield) (or 12-bromododecyl 3,4,5-
tris(benzyloxy)benzoate (123 mg, 52% yield)). The products were
added in a sealed tube with trimethylamine and THF and stirred for 4 d
at room temperature. The product was purified with a short column
using DCM and methanol to yield a solid white powder of SAW (40 mg,
60% yield) or SBnW (100 mg, 77% yield).
Fabrication of Photonic Ellipsoidal Particles. DCM solutions of
den-BBCPs (20 mg/mL), SBnW (1 mg/mL), and SAW (1 mg/mL) were
mixed to prepare stock solution where the weight ratio of SAW and SBnW
to den-BBCP were adjusted as follows: the weight percentages of SBnW
and SAW to den-BBCP were 9 and 1 wt % for den-BBCP477, den-BBCP594
den-BBCP809, and den-BBCP895, and 10 wt % of SBnW for den-BBCP1144
,
.
Then, the mixture (50 μL) was emulsified in an aqueous solution of
CTAB (0.1 wt %, 1.0 mL) by vortexing for 5 s. Following this the
evaporation of the organic solvent at room temperature for 24 h
generated a dispersion of solid particles. The excess surfactants were
removed by repeated washing before SEM and TEM characterization.
Fabrication of Fe3O4 NPs-Attached Photonic Ellipsoids and
Their Color-Switching under Magnetic Field. Detailed synthetic
procedures for magnetic NPs are provided in the Supporting
Information. First, the 6.4 nm-sized, surface-modified Fe3O4 NPs
were synthesized according to the previous report.50 A DCM solution
of Fe3O4 NPs was prepared and added to the stock solution of den-
BBCPs, SBnW and SAW, where the volume fraction of Fe3O4 NPs was
adjusted to be ∼3% for den-BBCP540 and ∼1.5% for den-BBCP1144. The
mixture was emulsified and evaporated in the same condition as that of
the preparation of the photonic ellipsoids. For the illumination of
particle dispersions, a white LED as the light source was provided in a
fixed direction. A magnetic field was applied by placing a neodymium
magnet near the vials (i.e., ∼2 cm distance).
Characterization. NMR spectra were recorded on a Bruker
Advance Neo 400. The chemical shift data are reported in units of δ
(ppm) relative to the residual solvent. SEM (Merlin and Crossbeam
E
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540 Zeiss) and TEM (FEI-Tecnai) were used to characterize the overall
shape and internal structures of the photonic ellipsoidal particles. The
samples were prepared by drop-casting polymer particle suspensions
onto the silicon wafers or TEM grids. For the TEM analysis, the
prepared samples were exposed to RuO4 vapor to stain the benzyl ether
group of the den-BBCPs selectively. The molecular weight of the
polymer was measured in an Agilent 1260 Infinity gel permeation
chromatography calibrated with a polystyrene standard and THF as the
eluent. A Leica DMRXP polarized-light microscopy was used to take
optical images. Transmission spectra were measured by an Agilent Cary
4000 UV/vis spectrophotometer.
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AUTHOR INFORMATION
■
Corresponding Authors
Timothy M. Swager − Department of Chemistry, Massachusetts
Institute of Technology (MIT), Cambridge, Massachusetts
02139, United States; orcid.org/0000-0002-3577-0510;
Email: tswager@mit.edu
Bumjoon J. Kim − Department of Chemical and Biomolecular
Engineering, Korea Advanced Institute of Science and Technology
(KAIST), Daejeon 34141, Republic of Korea; orcid.org/
0000-0001-7783-9689; Email: bumjoonkim@kaist.ac.kr
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Microstructured Particles Through Colloidal Crystallization. Science
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Koay, N.; Burgess, I. B.; Kolle, M.; Weitz, D. A.; Aizenberg, J. Color
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Colloidal Assemblies. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 10845.
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Authors
Qilin He − Department of Chemistry, Massachusetts Institute of
Technology (MIT), Cambridge, Massachusetts 02139, United
States
Kang Hee Ku − Department of Chemistry, Massachusetts Institute
of Technology (MIT), Cambridge, Massachusetts 02139, United
States; orcid.org/0000-0002-6405-8127
Harikrishnan Vijayamohanan − Department of Chemistry,
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This research was supported by a Vannevar Bush Faculty
Fellowship to T.M.S. (Grant # N000141812878). B.J.K.
acknowledges the support from National Research Foundation
Grant (2019R1A2B5B03101123) from the Korean Govern-
ment.
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