Behaviors of self-assembled diblock copolymer with pendant photosensitive azobenzene segments

Article abstract of DOI:10.1002/pola.23871

A novel monomer, ethyl 4-[4-(11-methacryloyloxyundecyloxy)phenyl azobenzoyl-oxyl] benzoate, containing a photoisomerizable N=N group was synthesized. The monomer was further diblock copolymerized with methyl methacrylate. Amphiphilic diblock copolymer pol

Full text of DOI:10.1002/pola.23871

Behaviors of Self-Assembled Diblock Copolymer with Pendant
Photosensitive Azobenzene Segments
JUI-HSIANG LIU, YI-HONG CHIU
Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan
Received 22 October 2009; accepted 8 December 2009
DOI: 10.1002/pola.23871
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: A novel monomer, ethyl 4-[4-(11-methacryloylox-
yundecyloxy)phenyl azobenzoyl-oxyl] benzoate, containing a
photoisomerizable N¼N group was synthesized. The monomer
was further diblock copolymerized with methyl methacrylate.
Amphiphilic diblock copolymer poly(methyl methacrylate-
block-ethyl 4-[4-(11-methacryloyloxyundecyloxy)phenyl azoben-
zoyl-oxyl] benzoate (PMMA-b-PAzoMA) was synthesized using
atom transfer radical polymerization. The reverse micelles with
place as a result of the reversible E-Z photoisomerization of
azobenzene segments in PMMA-b-PAzoMA. Microphase sepa-
ration of the amphiphilic diblock copolymer in thin films was
achieved through thermal and solvent aligning methods. The
microphases of the annealed thin films were investigated using
atom force microscopy topology and scanning electron micros-
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copy analyses. 2010 Wiley Periodicals, Inc. J Polym Sci Part
A: Polym Chem 48: 1142–1148, 2010
spherical construction were obtained with
2 wt % of the
diblock copolymer in a THF/H2O mixture of 1:2. Under alternat-
ing UV and visible light illumination, reversible changes in mi-
cellar structure between sphere and rod-like particles took
KEYWORDS: AFM; annealing; atom transfer radical polymeriza-
tion (ATRP); block copolymers; diblock copolymers; isomer/
isomerization; polymers; self-assembly; thin films
INTRODUCTION Block copolymers exhibit nanoscale self-
assembly behavior to form microscopically ordered structures
called microphases. Most block copolymers are known to ex-
hibit a microphase separation from a disordered state to an
ordered state upon cooling. The unfavorable interactions
between dissimilar monomers comprising a given block co-
polymer are considered to be the cause for such behavior.
Microphase separation (nanostructure formation) by block
copolymers in thin films has been the subject of intense
study because of interest in the phenomenological aspects of
self-assembly in a confined state.1–6 From a practical view-
point, such nanostructures also received considerable atten-
tion for fabrication of even smaller feature than those
obtained by the photolithography process.7–9 Such nano-
structures have potential applications in high-density data
storage media,10,11 ultrafine filters or membranes,12 as tem-
plates for metal nanowires,13,14 and so forth.15
the polymers are based on the trans-to-cis and the cis-to-
trans photoisomerizations of azobenzene, which lead to con-
siderable changes in their molecular shape and dipole
moments.19–24
Recent studies show that polymers containing azobenzenes
(azo polymers for short) can exhibit fascinating photorespon-
sive variations, such as phase transition,25,26 chromophore
orientation,27 surface-relief-grating formation,28 and photo-
mechanical bending.29 The variations are triggered by the
trans–cis photoisomerization of the azobenzenes. The isom-
erization behavior of the azo chromophores plays a critical
role in affecting the properties of azo polymers.
In this study, a diblock copolymer (PMMA-b-PAzoMA) con-
sisting of poly(methyl methacrylate) and azobenzene was
synthesized using atom transfer radical polymerization
(ATRP). Photoresponsive uniform colloidal spheres were con-
structed from the amphiphilic azobenzene containing diblock
copolymer. Under alternating UV and visible light illumina-
tion, reversible changes in micellar structure between sphere
and rod-like particles took place as a result of the reversible
E-Z photoisomerization of azobenzene segments in PMMA-b-
PAzoMA. Microphase separation of the amphiphilic diblock
copolymer in thin films was achieved through thermal and
solvent aligning methods. The morphology of the annealed
thin films was investigated using atom force microscopy
(AFM) topology and scanning electron microscopy (SEM)
analyses.
A more recent study on small-molecule azobenzene com-
pounds, in which the trans–cis photoisomerization imparts
an amphiphilic character to the molecules, found that azo-
benzenes can display enhanced fluorescence emission in
solution after a prolonged UV irradiation, as a result of light-
driven molecular self-assembly into micellar aggregates. Azo-
benzene-containing polymers are widely studied because of
their potential applications in many fields, which include op-
tical data storage,16 liquid crystal displays,17 and holographic
surface-relief gratings.18 The photoresponsive properties of
Additional Supporting Information may be found in the online version of this article. Correspondence to: J.-H. Liu (E-mail: jhliu@mail.ncku.edu.tw)
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EXPERIMENTAL
[4-(6-hydroxyundecyl)phenylazo] benzoate (2) (C26H36O4N2,
440): Calc.: C, 70.91; H, 8.18; N, 6.36. Found: C, 70.95; H,
8.14; N, 6.37.
Measurements
All new compounds were characterized by 1H NMR, FTIR,
and elemental analysis (EA). FTIR spectra were recorded with
a KBr disk on a Jasco VALOR III (Tokyo, Japan) FTIR spectro-
Ethyl 4-[4-(11-Methacryloyloxyundecyloxy)phenyl
azobenzoyl-oxyl] Benzoate (M1)
1
photometer. H NMR (400 MHz) and H spectra were obtained
Compound 2 (4.4 g, 10 mmol), triethylamine (3.1 g, 30
mmol), and a catalytic amount of hydroquinone were dis-
solved in distilled tetrahydrofuran (THF, 30 mL). The solu-
tion was cooled with an ice/salt bath and then methacryloyl
chloride (2.1 g, 20 mmol), dissolved in distilled THF (10
mL), was added dropwise under vigorous stirring. The mix-
ture was further stirred for 8 h at room temperature. After
completing the reaction, the mixture was poured into cold
water and the precipitate was filtered. The crude product
was washed several times with water and then recrystallized
from a Bruker AMX-400 (Darmstadt, Germany) high-resolu-
1
tion H NMR spectrometer, and chemical shifts were reported
in ppm with tetramethylsilane (TMS) as an internal standard.
Elemental analyses were carried out on a Heraeus CHN-O
(Darmstadt, Germany) rapid elemental analyzer. Gel permea-
tion chromatography (GPC) measurements were performed
at 40 ꢀC with a Hitachi L-4200 (Osaka, Japan) instrument
equipped with TSK gel GMH and G2000H columns. CHCl3 was
used as the eluent in these columns, with a rate of elution of
1.0 mL/min. The instrument was calibrated with a polysty-
rene standard. UV–vis absorption spectra were measured
with a Jasco V-550 spectrophoto-meter. SEM images were
taken with a JEOL HR-FESEM JSM-6700F (Osaka, Japan)
instrument. The morphology of the colloidal spheres and the
surface microphase separation were characterized by trans-
mission electron microscopy (TEM) and AFM.
ꢀ
twice from ethanol. M.p. ¼ 95 C. Yield: 85%.
1H NMR (CDCl3, d in ppm): 1.21–1.53 (18H, CH2), 1.55–1.58
(3H, CH3), 1.67–1.70 (3H, CH3), 4.05–4.45 (6H, OCH2), 5.55
(1H, CH2), 6.09 (1H, CH2), 7.00–7.03 (2H, aromatic), 7.88–
7.95 (4H, aromatic), 8.11–8.13 (2H, aromatic). IR: 3615
m(AOH), 2120 (N¼¼N), 1720 (C¼¼O), and 1640 (C¼¼C). Analy-
sis for ethyl 4-[4-(11-methacryloyloxyundecyloxy)phenyl azo-
benzoyloxyl] benzoate (M1) (C30H40O5N2, 508): Calc.: C,
70.87; H, 7.87; N, 5.51. Found: C, 70.68; H, 7.85; N, 5.53.
1H-NMR spectrum of M1 can be found in the online version,
which is available at www.interscience.wiley.com.
Materials
All chemical reagents are purchased from Tokyo Chemicals
(TCI, Japan). All organic solvents were purchased from
Aldrich Chemical. Analytical thin-layer chromatography is
conducted on Merck aluminum plates with 0.2 mm of silica
gel 60F-254.
Synthesis of Diblock Copolymers
PMMA-Br Macroinitiator
Copper (I) bromide (CuIBr, 17.93 mg, 0.13 mmol),
N,N,N0,N0,N"-pentamethyldiethyl entriamin (PMDETA, 43.33
mg, 0.25 mmol), and 1 mL of diphenyl ether in a 15-mL
Schlenk flask were stirred using magnetic stirrer for 5 min
at room temperature. Ethyl-a-bromoisobutyrate (EBiBr, 48.77
mg, 0.25 mmol) was added into the former mixture and then
stirred for further 5 min at room temperatures. MMA (5 g,
50 mmol) in 3 mL diphenyl ether was added into the flask.
The reactor was charged with nitrogen and then degassed
by three freꢀeze–thaw cycles. The reaction mixture was
heated at 85 C for 50 min. At the end of reaction, the flask
was cooled using liquid nitrogen. The flask is opened and
added 5 mL THF to terminate the polymerization. The reac-
tion mixture was removed from the flask by a syringe and
purified by passage through a silica column to remove the
copper catalyst. The product was further purified by repreci-
pitation from cold hexane. The precipitated macroinitiator
was filtered and dried under vacuum. Mn: 9645 g/mol, Tg:
93.1 ꢀC. 1H-NMR spectrum of PMMA-Br macroinitiator can
be found in the online version, which is available at
www.interscience.wiley.com.
Synthesis of Monomer
Ethyl 4-(4-Hydroxyphenylazo) Benzoate (1)
Ethyl 4-aminobenzoate (5 g, 30 mmol) was dissolved in 1 M
aqueous HCl (100 mL) and kept in an ice bath at 0 ꢀC. So-
dium nitrite (3.53 g, 51.16 mmol) in water (20 mL) was
added dropwise to the former solution and stirred for 30
min. Sodium hydroxide (4.4 g, 0.11 mol) and phenol (5.0 g,
53.19 mmol) were dissolved in water (100 mL) and stirred
at 0 ꢀC for 30 min. The former reaction mixture was added
dropwise to the latter solution at 0 ꢀC and then stirred for
1 h. The resulting mixture was poured into water, and the
solution was neutralized with 5% aqueous HCl. The crude
product of compound 1 was filtered and recrystallized twice
from ethanol. M.p. ¼ 154 ꢀC. Yield: 47%. IR: 3615 m(AOH),
1720 (C¼¼O), and 2120 (N¼¼N). Analysis for ethyl 4-(4-
hydroxyphenylazo) benzoate (1) (C15H14O3N2, 270): Calc.: C,
66.67; H, 5.19; N, 10.37. Found: C, 66.64; H, 5.21; N, 10.34.
Ethyl 4-[4-(6-Hydroxyundecyl)phenylazo] Benzoate (2)
Compound 1 (4.96 g, 18.4 mmol) was dissolved in ethanol
(100 mL). Potassium hydroxide (1.2 g, 22.1 mmol) dissolved
in ethanol (30 mL) was then added dropwise to the former
solution. 11-Bromo-1-undecanol (4.8 g, 22.1 mmol) and a
trace of potassium iodide were then added, and the solution
was heated at reflux for 30 h. The resulting mixture was
poured into water and extracted with dichloromethane. After
evaporation and drying, the crude product was recrystallized
twice from ethanol. M.p. ¼ 92 ꢀC. Yield: 52%. IR: 3610
m(AOH), 1720 (C¼¼O), and 2120 (N¼¼N). Analysis for ethyl 4-
Diblock Copolymer PMMA-b-PAzoMA
CuIBr (1.43 mg, 0.01 mmol), hexamethyl-triethylene-tetra-
mine (HMTETA, 6.91 mg, 0.03 mmol), and 1 mL of anisole in
a 15-mL Schlenk flask were stirred at room temperature for
5 min. Next, PMMA-Br (99.59 mg, 0.01 mmol) in 2 mL ani-
sole was added into the flask and then stirred for further 5
min. After that 0.76 g of M1 (1.5 mmol) in 3 mL anisole was
BEHAVIORS OF SELF-ASSEMBLED DIBLOCK COPOLYMER, LIU AND CHIU
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthesis of monomer M1.
RESULTS AND DISCUSSION
added into the flask. The flask was tightly sealed with a rub-
ber septum, degassed by thrꢀee freeze–thaw cycles. The mix-
ture was polymerized at 80 C for 5 days. After polymeriza-
tion, the reaction was terminated by adding 10 mL THF into
the flask. The product was purified by passage through a
silica gel column. The synthesized copolymer was reprecipi-
tated from cold methanol and dried under vacuum. Mn:
27,561 g/mol, Tg: 59.7 and 95 ꢀC. Both 1H-NMR spectrum
and thermal analyses of diblock copolymer PMMA-b-AzoMA
can be found in the online version, which is available at
www.interscience.wiley.com.
Synthesis of Diblock Copolymer PMMA-b-PAzoMA
As shown in Scheme 1, ethyl 4-[4-(11-methacryloyloxyunde-
cyloxy)phenyl azobenzoyloxyl] benzoate (M1) containing an
azobenzene segment was synthesized. The molecular struc-
ture was confirmed using FTIR, NMR, and EA. To synthesize
the amphiphilic diblock copolymer, as shown in Scheme 2, a
macromolecular initiator was synthesized in the presence of
a CuIBr catalyst. The synthesized macromolecular initiator,
PMMA-Br, was used to synthesize the amphiphilic diblock
copolymer PMMA-b-PAzoMA. The molecular weight and the
Tg of the macromolecular initiator and diblock copolymer
were estimated using GPC and dynamic scanning calorimetry
(DSC), respectively. The results are shown in the Experimen-
tal section. Two significant glass transition temperatures of
Self-Assembly of Diblock Copolymer PMMA-b-PAzoMA
Self-Assembly Formed in Solvent
PMMA-b-PAzoMA (0.2 mg) in 3.3 mg THF was added to 6.6
mg water. The mixture was stirred at room temperature for
30 min and then ultrasonically agitated for 30 min, after
ꢀ
59.7 and 95 C were found in the PMMA-b-PAzoMA diblock
copolymer, which are ascribed to the existence of the sepa-
rated amphiphilic segments. Molecular weight distributions
of macroinitiator PMMA-Br and copolymer PMMA-b-PAzoMA
can be found in the online version, which is available at
www.interscience.wiley.com.
ꢀ
which it was put in an oven at 50 C for 30 min. After cool-
ing, one drop of the solution was set on a coppꢀer grid for
TEM analysis. UV exposure was performed at 30 C in THF/
H2O ¼ 1:2 followed by standing at 30 ꢀC for 2 h. The self-
assembled sample was collected and dried for analysis. For
visible light recovering exposure, samples were irradiated
with a sunꢀlight lamp at 30 ꢀC in THF/H2O ¼ 1:2 and then
kept at 30 C for 1 day.
Self-Assembly of Diblock Copolymer PMMA-b-PAzoMA
in Solvent
To investigate the self-assembly of the synthesized diblock
copolymer, PMMA-b-PAzoMA was dissolved in THF, and a
certain amount of water was then added dropwise to the
system. Weight ratios of THF/H2O of 1:2, 1:1, and 2:1 were
investigated. It was found that reverse micelles with spheri-
cal construction were obtained in the condition of 2 wt %
diblock copolymer in THF/H2O ¼ 1:2. Under other condi-
tions, however, no phase-separated constructs were found. In
the case of 2 wt % copolymer, a reverse micelle should be
formed. The hydrophobic PMMA segment could be enclosed
inside the core of the spherical particles. Because of surface
tension of the solvated surface, nanoparticles with spherical
construction were formed. Furthermore, a 10 wt % polymer
concentration caused the formation of bigger nanoparticles
and the aggregation of phase-separated nanoparticles. High
concentration of nanoparticles usually leads to aggregation.
Thermal Annealing
PMMA-b-PAzoMA in toluene (1 wt %) was spin coated on a
silicon substrate. The film thickness was controlled by con-
trolling the spin-coating speed between 2000 and 4000 rpm
for 30 s. After spin coating, the thin film was thermally
ꢀ
annealed at 130 C in a vacuum for 3 days. The morphology
of the thermal annealed thin film was examined with AFM.
Solvent Annealing
Diblock copolymer PMMA-b-PAzoMA 5 wt % in THF was
spin coated on a glass substrate at 2000 rpm for 30 s. After
drying at room temperature in air atmosphere for 1 h, the
spin-coated film was annealed in saturated acetone vapor for
4 h, and the resultant film sample was dried in vacuum. The
morphology of the film sample annealed under various con-
ditions was examined with AFM and SEM.
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SCHEME 2 Synthesis of diblock copolymer PMMA-b-PAzoMA.
Interestingly, UV irradiation caused a deformation of the
nanoparticles. As seen in Figure 1(a,b), after UV exposure,
rod-like particles were formed. In addition, the rod-like par-
ticles were converted to spherical constructions when the
sample was exposed to a sunlight lamp and then kept at 30
ꢀC for 2 h with violent stirring. To describe the phenomena,
UV–vis experiments of the diblock copolymer in THF were
investigated. Figure 2(a,b) shows the results of the UV–vis
experiments with the diblock copolymer in THF both before
and after irradiation at 365 nm. As seen in Figure 2(a), the
decrease in absorption around 365 nm is due to the E-Z
isomerization of the azobenzene segment. It is known that
UV-induced E-Z isomerization causes variations on configura-
tional structure, polarity, and surface tension.30,31 In our
case, UV irradiation may cause the E-Z isomerization of the
diblock copolymer PMMA-b-AzoMA leading to the change of
both polarity and hydrophilicity of the segments. The varia-
tion in construction after UV irradiation, shown in Figure
1(b), could be ascribed to the variation of the hydrophilicity
of pendant groups of the diblock copolymer. Hydrophilicity
changes of the segments may cause a rearrangement of
diblock copolymers. Figure 3 shows schematic representa-
tion of construction variation of self-assembly of the diblock
copolymers before and after UV irradiation. After UV irradia-
tion, hydrophilicity of the PAzoMA of the diblock copolymer
could be changed leading to the aggregation of micelles and
then the deformation of the aggregated spheres to rod-like
construction. As seen in Figure 3, PAzoMA is located at the
outer surface of nanoparticles. After UV irradiation, the
hydrophilicity of PAzoMA segment may be increased leading
to the aggregation of nanoparticles. The increase of solubility
of PAzoMA in water may increase the mobility of the
PAzoMA segment moving to the surface of the rod-like nano-
particle. Simultaneously, the stability of the rod-like particles
is increased as well. These phenomena are quite similar to
those reported for morphology variations in proper solvent
before and after laser exposure.32
It is noteworthy that UV irradiation of the dry spherical
diblock copolymer particles under an air atmosphere caused
no significant change in construction. From the results of
UV–vis analysis, UV irradiation caused significant variations
on UV–vis spectra of the diblock copolymer film. The varia-
tion showed the progress of the E-Z isomerization in the film
state. The stability of the spherical structure after UV expo-
sure is attributed to the rigidity of the dry spherical par-
ticles. In case of the dry state, UV irradiation may still cause
E-Z isomerization. However, the rigidity of the dry nanopar-
ticles may restrict the rearrangement of diblock segments.
As noted in the literature, the morphology of the nanopar-
ticles was kept in poor solvent even after UV exposure.33
FIGURE 1 TEM images of self-assembled nanoparticles (a)
before and (b) after UV exposure in THF/H2O ¼ 1:2.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 3 Schematic representation of the self-assembly of
PMMA-b-PAzoMA in THF/H2O ¼ 1:2 (a) before and (b) after
365-nm exposure.
FIGURE 2 UV–vis spectra of PMMA-b-PAzoMA in THF (a) irradi-
ated at 365 nm and (b) stability in the dark after 365-nm
exposure.
Thermal Annealing of Diblock Copolymer
To investigate the thermal annealing of the diblock copoly-
mer, PMMA-b-PAzoMA in toluene (1 wt %) was spin coated
on a silicon substrate. Afteꢀr spin coating, the thin film was
thermally annealed at 130 C under vacuum for 3 days. The
morphology of the thermally annealed thin film was exam-
ined with an AFM. Figure 4(a–c) shows the AFM morphology
texture of 0.2 wt % copolymer with 4000 ppm, 0.2 wt % co-
polymer with 2000 ppm, and 1 wt % copolymer with 2000
ppm coated films, respectively. Figure 4(a–c) shows AFM
height image before thermal annealing, and Figure 4(d–f)
shows AFM height image after thermal annealing. As seen in
Figure 4, after sufficient thermal annealing, phase separation
occurred and leading to the formation of the microseparated
patterns. Figure 5 shows a schematic representation for the
formation of microphase separation. Heat-melted copolymers
allow the rearrangement of both the hydrophilic and the lip-
ophilic segments of the diblock copolymers. Because of the
FIGURE 4 AFM height images of self-assembled PMMA-b-
PAzoMA (a–c) ‘‘before’’ and (d–f) ‘‘after’’ thermal annealing.
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ARTICLE
FIGURE 5 Schematic representation of self-assembled micro-
phase separation of PMMA-b-PAzoMA after thermal annealing.
[Color figure can be viewed in the online issue, which is avail-
able at www.interscience.wiley.com.]
hydrophilicity, PMMA segment interacted with hydrophobic
silicon substrate. As seen in Figure 5, the domain patterns
are attributed to the self-assembled PAzoMA core segments.
The results are consistent with the AFM images shown in
Figure 4. In case of high copolymer concentration film shown
in Figure 4(c), the interaction from the silicon substrate on
self-assembly of hydrophilic and hydrophobic segments may
extremely decrease because of the increase of film
thickness.26
FIGURE 6 AFM (a) 3D height images and (b) height section
analysis of thermally annealed PMMA-b-PAzoMA. [Color figure
can be viewed in the online issue, which is available at
www.interscience.wiley.com.]
the thin film was performed in a closed vessel without sol-
vent inside. For acetone annealing, the thin film was put in a
closed vessel with a cup of acetone to saturate the atmos-
phere with acetone vapors for 4 h. As seen in Figure 7(a,b),
the phase separation of the diblock copolymer is quite differ-
ent from film annealed with acetone vapor. Sufficient acetone
vapor treatment caused the microphase separation leading
to the formation of the open pores. Figure 8 shows a sche-
matic representation for the solvent annealing of the diblock
copolymer. Hydrophobic PMMA segment may be dissolved in
acetone and form separated domains. During the slow evapo-
ration of acetone, the dissolved PMMA was pushed to the
periphery of the domain. Solvent-dissolved PMMA segment
may precipitate at the periphery of the open cores. As seen
in Figure 7(a,b), in case of drying without saturated solvent
vapor, bigger phase-separated domains were found because
of lower amount of THF content in the system. The opened
hole is somewhat complicated but can be ascribed to insuffi-
cient phase separation in the systems. In case of Figure 7(c),
Figure 6(a) shows the 3D AFM height image of the thermally
annealed film. The inset photograph is the 2D image of the
surface height pattern. The exact height of the domains is
shown in Figure 6(b). Domains with various heights were
formed on the thin film surface.
Solvent Annealing of Diblock Copolymer Films
To investigate the solvent annealing of diblock copolymers,
PMMA-b-PAzoMA was dissolved in THF and then was spin
coated on a glass substrate. Solvent annealing of the thin
film of the PMMA-b-PAzoMA diblock copolymer was per-
formed at room temperature for 4 h in a closed vessel full of
saturated acetone vapor leading to microphase separation.
Figure 7(a–c) shows the mesoporous structures that were
prepared by ambient drying, slow drying, and acetone
annealing, respectively. In the case of ambient drying, the
thin film was put in the air atmosphere. The slow drying of
FIGURE
7 SEM
images
of
PMMA-b-PAzoMA on
a
glass
substrate prepared via (a) ambi-
ent drying, (b) slow drying, and
(c) acetone annealing.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
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2D and 3D AFM height images of (a) 0.2 wt %, and (b) 1 wt
% PMMA-b-PAzoMA diblock copolymer in toluene can be
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The amphiphilic diblock copolymer PMMA-b-PAzoMA was
synthesized using ATRP. Reverse micelles with spherical con-
struction were obtained using 2 wt % diblock copolymer in
THF/H2O ¼ 1:2. Under alternating UV and visible light illu-
mination, reversible changes in micellar structure between
sphere and rod-like particles took place as a result of the re-
versible E-Z photoisomerization of the azobenzene segments
in PMMA-b-PAzoMA. Microphase separation of the amphi-
philic diblock copolymer in thin films was achieved through
thermal and solvent aligning methods.
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