Photoinduced reversible transmittance modulation of rod-coil type diblock copolymers containing azobenzene in the main chain

Article abstract of DOI:10.1039/c2cc17265h

Photoinduced reversible transmittance modulation was achieved with the self-assembled block copolymer micelles. A large conformational change of the well-defined rod-coil diblock copolymers containing azobenzene and ether groups in the main chain of the r

Full text of DOI:10.1039/c2cc17265h

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COMMUNICATION
Photoinduced reversible transmittance modulation of rod–coil type
diblock copolymers containing azobenzene in the main chainw
Jaewon Heo, Yun Jun Kim, Myungeun Seo, Seonhee Shin and Sang Youl Kim*
Received 22nd November 2011, Accepted 10th February 2012
DOI: 10.1039/c2cc17265h
Photoinduced reversible transmittance modulation was achieved
with the self-assembled block copolymer micelles. A large con-
formational change of the well-defined rod–coil diblock copoly-
mers containing azobenzene and ether groups in the main chain
of the rod block induced a remarkable macroscopic change
which can be observed with the naked eye.
However, preparation of well-defined condensation polymers
has been very difficult until the development of chain-growth
condensation polymerization (CGCP),9 while controlled radical
polymerizations of diverse vinyl monomers have been well
established.10 Simple combination of CGCP and atom transfer
radical polymerization (ATRP) allows straightforward synthesis
of rod–coil type diblock copolymers composed of condensation
and addition blocks with controlled molecular weights and
molecular weight distributions.11
Nanostructured materials responsive to external stimuli such as
temperature,1a pH,1b chemicals,1c and light1d have attracted
considerable research interest due to their potential application
in many areas including integrated optical devices2a and biomedical
drug delivery.2b Among many stimuli, light has distinctive advan-
tages such as controllable selectivity of illumination areas and
periods, remote incidents, and so forth. Photoresponsive polymeric
materials which perceive and shut off the light of undesired
wavelengths have tremendous potential for intelligent optical
devices.3 However, the development of nanostructured polymeric
materials to adjust light transmittance based on the amount of
external irradiation is still a great challenge. Recently, photo-
responsive morphological changes of micellar aggregates in
solutions by UV and visible light were achieved by incorporating
azobenzene into polymeric materials.4 The photoresponsive
changes were mainly attributed to the dipole moment changes of
molecules induced by photoisomerization of azobenzene groups
together with hydrophilic–hydrophobic balance shifts which
destabilize the self-assembled aggregates.
Here we report the reversible dramatic transmittance
changes resulting from photoinduced aggregation and segregation
of self-assembled block copolymer micelles composed of well-
defined rod–coil type diblock copolymers. The block copolymers
were synthesized through CGCP for rod segments containing
azobenzene units in the main chain, and subsequent ATRP of
styrene with a proper macroinitiator for coil blocks. A nucleophilic
aromatic substitution (SNAr) reaction was a key for synthesis of
well-controlled poly(arylene ether azobenzene) (PAEAz) by
displacement of the leaving group (i.e. F or NO2) activated by
a trifluoromethyl group.12 In cyclohexane, uniform micellar
aggregates of the diblock copolymer were formed as the
spherical structure consisted of the core rod and the periphery
coil. Irradiated by UV light, the transmittance of the homo-
genous solution decreased down to maximum ca. 22%, and
became turbid, but returned to a clear solution when exposed
to visible light. The solution obstructs incident light whenever
a certain amount of UV irradiation is illuminated.
Self-assembled nanostructures of block copolymers both in
thin films5 and in solutions6 have been intensively studied in
perspective of development of nanostructured organic materials,
and construction of complex nanostructured materials by self-
assembly usually requires precisely defined nanomolecules where
the molecular structures become primary factors to determine
morphologies of self-assembled nanostructures.7 Among many
different types of diblock copolymers, self-assembly behavior of
diblock copolymers consisting of a rigid rod and a flexible coil is
interesting because of the conformational asymmetry between each
block and various functionalities on main chains of rod segments.8
Well-defined rod–coil type diblock copolymers (P1–3) prepared
in this study are shown in Scheme 1, and their physical properties
are summarized in Table 1. Details of the synthetic methods and
Department of Chemistry, Korea Advanced Institute of Science and
Technology (KAIST), 373-1, Guseong-dong, Yuseong-gu, Daejeon,
305-701, Korea. E-mail: kimsy@kaist.ac.kr; Fax: +82-42-350-8177;
Tel: +82-42-350-2834
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc17265h
Scheme 1 (a) Synthesis of PAEAz through CGCP. (b) End-group
modification of PAEAz. (c) ATRP of styrene with the macroinitiator
for synthesizing well-defined block copolymers.
c
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Chem. Commun., 2012, 48, 3351–3353 3351

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Table 1 Characterization of the block copolymers
Samples Mn [GPC]a Mn [NMR]b Mw/Mn
Tgc/1C Td5d/1C
a
P1
P2
P3
11 300
15 500
28 900
12 700
16 100
25 200
1.18
1.24
1.25
104.2
104.4
108.0
354
347
362
a
b
Estimated by THF-GPC based on PS standards. Calculated by
1H NMR spectra of diblock copolymers in CDCl3. Measured by
c
DSC at the second scan with a heating rate of 5 1C minꢀ1 under
nitrogen flow. 5% weight loss temperature measured by TGA with a
d
heating rate of 10 1C minꢀ1 under nitrogen flow.
characterization of the block copolymers are described in
the ESI.w Polycondensation of the AB0 type monomer (1)
proceeded in a chain-growth manner under the certain reaction
conditions (ESIw). CGCP of 1 produced well-defined PAEAz
(Mn: 2.0 kDa, PDI: 1.09) with the proper feed ratio. The
molecular weights of the polymers were well matched with the
theoretical values as the conversion of the monomers increased,
and the constant number of propagating species was main-
tained. However, it was difficult to obtain high molecular
weights (45 kDa) of polymers with a narrow polydisperse
index (PDI) presumably because of the transetherification
caused by the strongly activated ether bond between an initiator
moiety and a monomer. For ATRP of coil blocks, the end
groups of the polymers were modified to macroinitiator,
PAEAzNBr, and used for polymerization of styrene monomers.
GPC analysis of the polymerization product revealed that all
the macroinitiators were not initiated, indicating that the
efficiency of the amide-linked macroinitiator was rather low in
ATRP of styrene.13 However, the unreacted macroinitiator was
easily separated from the block copolymer by extraction with
cyclohexane (ESIw).
Fig. 1 (a) DLS plots, (b) TEM images on a carbon-coated copper
grid, and (c–e) SEM images on a Si wafer of the self-assembled diblock
copolymer micelles of P2 prepared in cyclohexane. (a) The blank and
patterned columns are the number and intensity MSD values, respectively.
(b) With scale bars of 200 nm. The inset is a magnified image. (c) With a
scale bar of 800 nm. (d) After UV light irradiation with a scale bar of
900 nm. (e) After UV and subsequent visible light irradiation with a scale
bar of 400 nm.
The synthesized PAEAz is soluble in many organic solvents
including NMP, THF, ethylacetate, toluene, and chloroform,
but is not soluble in cyclohexane that dissolves polystyrene.
Therefore, block copolymer micelles of the rod–coil diblock
copolymers were prepared by addition of cyclohexane into the
THF solution of the diblock copolymers and subsequent
dialysis to remove THF. Because the block copolymers (e.g. P3)
having molecular weight above 20 kDa were not dissolved in
cyclohexane, only the diblock copolymer micelles of P1 and P2
were prepared. A dynamic light scattering (DLS) study (Fig. 1a) of
the cyclohexane solutions shows the hydrodynamic diameter of
190 nm of the self-assembled block copolymer micelles with quite
narrow distributions (PDI: 0.005), and TEM images (Fig. 1b)
reveal the regular spherical shape of the micelles consisting of azo
blocks in the core and PS blocks in the shell. The SEM image
(Fig. 1c) also shows that the diblock copolymer micelles have
narrow size distribution with the diameter of about 90 nm
which is less than that of the DLS result due to a contraction
of the micelles.
Fig. 2 (a) Photographs, (b) changes in turbidity, and (c) DLS plots of the
photoinduced reversible transmittance modulation of the self-assembled
diblock copolymer micelles prepared in cyclohexane. (a) (top) The trans-
parent initial or visible-exposed solution; (bottom) the turbid UV-exposed
solution of P2, (b) for P1 (circles) and P2 (squares). The turbidity was
measured at 700 nm in UV-vis spectra. (c) (squares) The UV-exposed
solution; (circles) the subsequent visible-exposed solution.
The diblock copolymer micelle showed photoresponsive
aggregation and segregation behavior that was large enough
to be detected with the naked eye, unlike the previously
reported4 coil type block copolymer micelles. The clear cyclohexane
solution of diblock copolymer micelles became turbid when it was
exposed to UV light, but returned to a clear solution with exposure
to visible light. Fig. 2a shows a photoinduced reversible
transmittance modulation of the self-assembled block copolymer
micelles. The reversible changes in turbidity of the solution were
observed with alternating UV and visible light irradiation
(Fig. 2b). While the transmittance of the cyclohexane solution
c
3352 Chem. Commun., 2012, 48, 3351–3353
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segment plays an important role in photoinduced aggregation
and segregation of the self-assembled diblock copolymer
micelles in cyclohexane. The diblock copolymers containing
azobenzene in the main chain are expected to find many
interesting applications.
Fig. 3 Photographs of advancing contact angle goniometry for
homopolymer PAEAz thin films (a) as spin casting (951). (b) From
the UV-exposed solution (821).
This work was supported by NRF through ERC (R11-2007-
050-04001-0) and NRL (R0A-2008-000-20121-0) programs.
of the P2 micelle reduced to 30%, the transmittance of the P1
micelles decreased to 22% because of the larger volume
fraction of the azobenzene units in P1. The photoinduced
transmittance change was the result of the aggregation and
segregation of the self-assembled block copolymer micelles
with the size change from nanometres to micrometres. The
PDI values also increased dramatically from 0.005 to 0.229
(Fig. 1 and 2). Photoisomerization of azobenzene units in the
main chains of the rod block directly altered the conformation
of the polymer backbone. The changed conformation not only
disturbed the self-assembled structures but also induced
different geometrical packing parameters.4c And also there is
another important change of physical properties as the
conformation of azobenzene changed to cis from trans. It is
well known for the azobenzene without substitutions that the
elongated trans configuration has a zero dipole moment, while
the bent cis has a dipole moment of B3 D.14 To check the
dipole moment change of the diblock copolymers, the thin
films of the polymer were prepared and their contact angles
were measured (Fig. 3). The advancing contact angle was
greater for the films prepared from the solution without
irradiation of UV light than for the films prepared from the
solution with UV-exposure. It seems that the increased hydro-
philicity of a polar cis conformation causes the further aggre-
gation of diblock copolymer micelles in a thermodynamically
unstable state in the nonpolar solvent, cyclohexane. Even
though the volume fraction of the rod blocks is smaller than
that of the coil, the conformational changes of rod blocks are
significant and amplified through consecutive conformational
changes of the rigid backbone.
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To monitor the photoisomerization of azobenzene units of the
diblock copolymers, we investigated the UV-visible absorption
spectra of the block copolymers (ESIw). As expected, trans-to-cis
isomerization of the azobenzene groups occurred upon irradia-
tion of UV light, and the p–p* transition absorption bands of the
trans-azobenzene moieties at around 360 nm decreased with the
concomitant increase of the n–p* transition absorption bands of
the cis isomer at around 440 nm. After UV light irradiation for
several minutes, baseline shifted abruptly because the solution
became turbid. However, subsequent irradiation of visible light
to the heterogeneous solution restored the original spectrum with
stable baseline in a few seconds.
In conclusion, we have demonstrated photoinduced reversible
transmittance modulation of the rod–coil diblock copolymer
micelle. The well-defined rod–coil type diblock copolymers
were synthesized through combination of CGCP and ATRP.
The dipole moment change induced by photoisomerization of
azobenzene incorporated along the backbone into the rod
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3351–3353 3353