Photoresponsive diblock copolymers bearing strong push-pull azo chromophores and cholesteryl groups

Article abstract of DOI:10.1016/j.polymer.2012.05.063

A series of diblock copolymers bearing strong push-pull azo chromophores and cholesteryl groups on the respective blocks was synthesized by reversible addition fragmentation chain transfer (RAFT) polymerization. The liquid crystal phase structure, microphase-separated morphology, and photoresponsive properties of the block copolymers were investigated by using DSC, POM, AFM, XRD and laser irradiation. The results show that the cholesteryl block (PChEMA) forms smectic-A mesophase and the morphology depends on the length of the azo block (PAzoCN). When the azo block is short, such as PChEMA₅₀-b-PAzoCN ₇, no microphase separation can be identified. For PChEMA ₅₀-b-PAzoCN₂₈, PAzoCN appears as the hexagonal-packed nanocylinders embedded in the PChEMA matrix. When the azo block length further increases, the block copolymer PChEMA₅₀-b-PAzoCN₇₃ forms microphase-separated lamellae. The microphase separation shows no obvious restraint on the photoinduced orientation of the azo chromophores, but micron-scale mass transport of the photoresponsive PAzoCN block is inhibited by the phase confinement.

Full text of DOI:10.1016/j.polymer.2012.05.063

Polymer 53 (2012) 3566e3576
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Photoresponsive diblock copolymers bearing strong pushepull azo chromophores
and cholesteryl groups
*
Yu Zhu, Yuqi Zhou, Zhen Chen, Ran Lin, Xiaogong Wang
Department of Chemical Engineering, Laboratory for Advanced Materials, Tsinghua University, Beijing 100084, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of diblock copolymers bearing strong pushepull azo chromophores and cholesteryl groups on
the respective blocks was synthesized by reversible addition fragmentation chain transfer (RAFT)
polymerization. The liquid crystal phase structure, microphase-separated morphology, and photo-
responsive properties of the block copolymers were investigated by using DSC, POM, AFM, XRD and laser
irradiation. The results show that the cholesteryl block (PChEMA) forms smectic-A mesophase and the
morphology depends on the length of the azo block (PAzoCN). When the azo block is short, such as
PChEMA50-b-PAzoCN7, no microphase separation can be identified. For PChEMA50-b-PAzoCN28, PAzoCN
appears as the hexagonal-packed nanocylinders embedded in the PChEMA matrix. When the azo block
length further increases, the block copolymer PChEMA50-b-PAzoCN73 forms microphase-separated
lamellae. The microphase separation shows no obvious restraint on the photoinduced orientation of
the azo chromophores, but micron-scale mass transport of the photoresponsive PAzoCN block is
inhibited by the phase confinement.
Received 16 February 2012
Received in revised form
18 May 2012
Accepted 27 May 2012
Available online 4 June 2012
Keywords:
Block copolymer
Azobenzene
Liquid crystalline
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
effects on the photoresponsive functions and other properties
[1e12,16]. Synthesizing azo polymers with novel molecular archi-
Polymers containing azobenzene and its derivatives (azo poly-
mers for short) have been intensively investigated in recent years as
photoresponsive materials for many applications [1e3]. Azo poly-
mers can show a variety of photoresponsive properties, such as
phase transition [4], chromophore orientation [5], surface-relief-
grating (SRG) formation [6,7], photo-mechanical thin film
contraction and bending [8e10], and photoinduced colloid defor-
mation [11,12]. The photoresponsive variations are triggered by the
trans-cis photoisomerization of azobenzene units and closely
related with the responses of polymeric structures to the stimulus
at different levels and scales [2]. Azo polymers with different
molecular architectures, such as side-chain polymers, main-chain
polymers, dendritic polymers and block copolymers, have been
prepared to explore and optimize these photoresponsive functions
[2,3,13e16]. The studies indicated that hierarchical structures from
molecule to condensed phase play a critical role to affect the pho-
toresponsive properties of the materials. Depending on the back-
bone structures and functional groups, azo polymers can form
different phases and aggregation states that show diversified
tectures and aggregated structures is necessary to understand the
structureeproperty relationship and develop new materials. The
related study is particularly important in the interests of funda-
mental understanding and device applications.
Block copolymers composed of both liquid crystalline (LC) and
isotropic (I) blocks have attracted considerable research interest
because of the ordering at two different scales and unique functions
[17e30]. LC/I block copolymers can form a microphase-separated
structure with coexisting anisotropic and isotropic phases. The LC
blocks of the copolymers form the ordered sub-phase because of
anisotropic alignment of the mesogens. As isotropic polymers are
highly immiscible in the LC phase, the systems are usually strongly
segregated to form the microphase-separated morphology [17].
The competition between the LC order and phase-separated
confinement can result in a rich variety of morphologies and
nanostructures [19e25]. In recent years, LC/I block copolymers
bearing azo chromophores have been prepared by using atom
transfer radical polymerization (ATRP) [26e30]. Diblock copoly-
mers composed of poly(ethylene glycol) (PEG) and LC azo blocks
have been prepared [26]. The PEG block forms well-organized
nanocylinders embedded in a matrix of azobenzene-containing
LC block. As one unique characteristic of the LC/I copolymer, the
nanocylinders of the block copolymers can be aligned or
* Corresponding author. Tel.: þ86 10 62784561; fax: þ86 10 62770304.
E-mail address: wxg-dce@mail.tsinghua.edu.cn (X. Wang).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2012.05.063

Y. Zhu et al. / Polymer 53 (2012) 3566e3576
3567
manipulated by the polarized light irradiation [27,28]. For a triblock
2. Experimental section
2.1. Synthesis
copolymer composed of PEO and azobenzene-containing poly-
methacrylate blocks, photocontrolled microphase separation in
two-dimension has been demonstrated by using monolayer
prepared by LangmuireBlodgett (LB) method [29].
2.1.1. Materials
In the molecular designs of LC/I azo block copolymers reported
before, the liquid crystallinity of the block copolymers is introduced
by using the side-chain azobenzene moieties with mesogenic
feature [21,26e30]. Although this is a well-established strategy, it
cannot be used to synthesize LC/I block copolymers bearing azo-
benzenes with strong pushepull substituents. The pushepull azo
chromophores have been classified as pseudo-stilbene type
according to the spectroscopic characteristics and isomerization
behavior [31]. Polymers containing the pseudo-stilbene type azo
chromophores show interesting photoresponsive properties such
as SRG formation [6,7], photoinduced colloidal deformation [11,12],
and optical nonlinearity [32,33]. However, the polymers function-
alized with the strong pushepull azo chromophores usually cannot
form LC phase because of the less linear shape and strong
dipoleedipole interaction [2]. One possible way to develop LC/I
block copolymers containing the strong pushepull azo chromo-
phores is to covalently link an azo polymer block with a block
containing mesogenic units. By using this molecular design
strategy, various block copolymers can be developed by covalently
bonding the pushepull azo polymers with a typical side-chains LC
polymer. Besides the unique photoresponsive properties related
with the block copolymers, a variety of nanostructures formed by
interplay between different molecular interactions will be some
prosperous and valuable research objects. However, to our
knowledge, a systematic investigation into LC/I block copolymers
containing strong pushepull azo chromophores has not been
reported in the literature yet.
In this study, we synthesized a series of LC/I diblock copolymers
containing both pseudo-stilbene type azo chromophores and cho-
lesteryl mesogens. The diblock copolymers (PChEMAx-b-PAzoCNy),
poly(2-(3-((cholesteryl)oxycarbonyl)propionyloxy) -ethyl methac-
rylate)-block-poly(6-(N-methyl-N-(4-(40-cyanophenylazo)phenyl)
amino)-hexyl methacrylate), were synthesized by reversible addi-
tion fragmentation chain transfer (RAFT) polymerization. The
diblock copolymers show hierarchical orders and morphologies
with variation of the compositions. The syntheses, LC mesophase
behavior, morphology, and photoinduced orientation of the block
copolymers are reported below.
4-Aminobenzonitrile (98%) was purchased from Acros and used
as received. 6-(Methyl(phenyl)amino)hexan-1-ol (1, Scheme 1) and
6-(N-methyl-N-phenylamino)hexyl methacrylate (2, Scheme 1)
were prepared by using the literature method [34]. The synthetic
details and characterization of succinic acid cholesteryl ester (4,
Scheme 2) are given in the Supporting Information. The purity of
the intermediate products was checked using both spectroscopic
methods and chromatography. The 1H NMR spectra of the inter-
mediate products and the monomers are given in the Supporting
Information (Figs. S1 and S2). The chain transfer agent, 2-(2-
cyanopropyl)-dithiobenzoate (CPDB), was synthesized according
to the literature method [35]. Anisole and THF were distilled from
sodium with benzophenone prior to use. Chloroform was washed
with water to remove the stabilizer ethanol, dried with potassium
carbonate and distilled from sodium sulfate. Other solvents were
flash distilled before use. AIBN was recrystallized from anhydrous
methanol before use. All other reagents were commercially avail-
able products and used as received without further purification.
2.1.2. 6-(N-Methyl-N-(4-(40-cyanophenylazo)phenyl)amino)hexyl
methacrylate (AzoCN)
AzoCN (3) was prepared by the route given in Scheme 1. 4-
Aminobenzonitrile (5.9 g, 0.05 mol) was dissolved in a mixture of
hydrochloric acid (15 mL) and water (30 mL). NaNO2 (4.14 g,
0.06 mol) dissolved in 20 mL water was slowly dripped into the
acidified 4-aminobenzonitrile solution. The mixture was stirred in
an ice bath for 5 min and filtrated to obtain a yellowish solution of
the diazonium salt. 6-(N-Methyl-N-phenylamino)hexyl methacry-
late (2) (11.01 g, 0.04 mmol) was dissolved with 50 mL DMF in
a flask immersed in ice bath. The diazonium salt solution was added
dropwise into the DMF solution and the temperature was
controlled to be below 5 ꢀC. After reaction for 5 h, the raw product
was obtained by pouring the reaction solution into an excess of
deionized water. The precipitate was collected by filtration and
washed with plenty of water. After drying, the product was purified
by recrystallization from ethanol and dried under vacuum. Yield:
81%. 1H NMR (300 MHz,
H to eN¼Ne), 7.75 (d, J ¼ 8.6 Hz, 2H, o-Ar H to eCN), 6.79 (d,
d ppm, CDCl3): 7.89e8.03 (overlap, 4H, o-Ar
Scheme 1. Synthetic route for the azobenzene monomer (AzoCN).

3568
Y. Zhu et al. / Polymer 53 (2012) 3566e3576
Scheme 2. Synthetic route for the cholesteryl-containing monomer (ChEMA).
J ¼ 9.3 Hz, 2H, o-Ar H to eNCH3), 6.09 (s, 1H, ¼CH2), 5.55 (s,
1H, ¼CH2), 4.15 (t, J ¼ 6.5 Hz, 2H, OCH2), 3.47 (t, J ¼ 7.2 Hz, 2H,
NCH2), 3.08 (s, 3H, NCH3), 1.94 (s, 3H, ¼CCH3), 1.58e1.76 (m, 4H,
OCH2CH2), 1.32e1.54 (m, 4H, OCH2CH2CH2). IR (KBr, cmꢁ1): 2941
(CeH), 2227 (C^N), 1713 (C]O), 1597, 1518 (benzene ring), 1381
(N]N), 1142 (CeOeC). MS (ESI): calcd. for C24H28N4O2 [M þ H]þ:
m/z ¼ 405.22; found: 405.43.
(I6.13) and 5.37 ppm (I5.37), corresponding to the vinyl proton of the
methacrylate group of the remaining monomer and methylene
proton of the cholesteryl group.
convð%Þ ¼ ð1 ꢁ I6:13=I5:37Þ ꢂ 100%
(1)
The conversion estimated by this method was approximately
100%. Mn (GPC) ¼ 23 200, Mw/Mn (GPC) ¼ 1.15. 1H NMR (300 MHz,
d
ppm, CDCl3): 3.95e4.42 (d, 4H, OCH2CH2O), 2.53e2.72 (m, 4H,
OOCCH2CH2COO), the other resonance signals in 1H NMR spectrum
represent protons of the cholesteryl group and protons on the
main-chain.
2.1.3. 2-(3-((Cholesteryl)oxycarbonyl)propionyloxy)ethyl
methacrylate (ChEMA)
ChEMA (5) was prepared by the synthetic route given in Scheme
2. Succinic acid cholesteryl ester (4, 9.72 g, 0.02 mol), 4-
dimethylamino pyridine (DMAP, 0.122 g, 1 mmol), N, N0-dicyclo-
hexylcarbodiimide (DCC, 4.54 g, 0.022 mol) were dissolved in
dichoromethane (DCM, 200 mL). The solution was cooled in an ice
bath and 2-hydroxyethyl methacrylate (HEMA, 2.863 g, 0.022 mol)
was added dropwise into the DCM solution. After the reaction was
carried out at room temperature for 24 h, the mixture was filtrated
to remove the insoluble carbamide. The filtrate was rotary-
evaporated to remove major part of the solvent. The concentrated
filtrate was dripped into 200 mL cold methanol. The precipitate was
collected by filtration and purified by recrystallization from
2.1.5. PAzoCN
The azo homopolymer (PAzoCN) was prepared for comparison
with the diblock polymers. It was synthesized by using a similar
procedure as the PChEMA-CTA synthesis. AzoCN (2.02 g, 5 mmol),
AIBN (1.6 mg, 0.01 mmol), and CPDB (11.1 mg, 0.05 mmol) were
added into a 50 mL Schlenk flask, followed by the addition of 2.5 mL
of anisole. The mixture was degassed by three freezeepumpethaw
cycles and sealed under vacuum. The flask was then placed in an oil
bath of 75 ꢀC for 72 h, and then the reaction was halted by
immersing the flask into ice water. The reaction solution was
diluted using THF and precipitated in a large amount of methanol to
obtain the crude product. The purification procedure was repeated
three times and the polymer was dried in a vacuum oven for 24 h.
The conversion of the monomers can be calculated by Eq. (2) on the
basis of the integration areas of the peaks at 6.09 ppm (I6.09) and
6.55 ppm (I6.55), representing the vinyl proton of the methacrylate
of the remaining monomer and the two protons of the phenyl at the
ortho-position to the amino group.
methanol. Yield: 74%. 1H NMR (300 MHz,
d ppm, CDCl3): 6.13 (s,
1H, ¼CH2), 5.59 (s, 1H, ¼CH2), 4.34 (s, 4H, OCH2CH2O), 2.55e2.70
(m, 4H, OOCCH2CH2COO), 1.94 (s, 3H, ¼CCH3), all the other peaks
on 1H NMR represent protons of the cholesteryl group. IR (KBr
cmꢁ1): 2939 (CeH), 1740, 1722 (C]O), 1639 (C]C), 1161 (CeOeC).
MS (ESI): calcd. for C37H58O6 [M þ Na]þ: m/z ¼ 621.56; found:
621.41.
2.1.4. Macromolecular chain transfer agent (PChEMA-CTA)
convð%Þ ¼ ð1 ꢁ 2I6:09=I6:55Þ ꢂ 100%
(2)
ChEMA (2.99 g, 5 mmol), AIBN (3.3 mg, 0.02 mmol), and CPDB
(22.1 mg, 0.1 mmol) were added into a 50 mL Schlenk flask, fol-
lowed by the addition of 5 mL of anisole. The mixture was degassed
by three freezeepumpethaw cycles and sealed under vacuum. The
flask was then placed in an oil bath of 75 ꢀC for 3 h, and then the
reaction was halted by immersing the flask into ice water. The
reaction solution was diluted with THF and precipitated into hot
acetone to obtain the crude product. The purification procedure
was repeated three times and the polymer was dried in a vacuum
oven for 24 h. The conversion of the monomers can be calculated by
Eq. (1) on the basis of the integration areas of the peaks at 6.13 ppm
Conversion of the monomer was estimated to be 45% from 1H
NMR. Mn (GPC) ¼ 15 400, Mw/Mn (GPC) ¼ 1.18. 1H NMR (300 MHz,
d
ppm, CDCl3): 7.54e7.88 (m, 6H, o-Ar H to eN¼Ne, o-Ar H to eCN),
6.45e6.68 (overlap, 2H, o-Ar H to NCH3), 3.74e4.05 (d, 2H, OCH2),
3.18e3.39 (d, 2H, NCH2), 2.81e3.06 (s, 3H, NCH3).
2.1.6. Diblock copolymers
The synthesis of PChEMA50-b-PAzoCN50 is given here as
a typical example. The macromolecular chain transfer agent
PChEMA50-CTA (0.232 g, 0.0077 mmol), AzoCN (0.405 g, 1 mmol)

Y. Zhu et al. / Polymer 53 (2012) 3566e3576
3569
and anisole (0.5 ml) were added into a 50 mL Schlenk flask. A
solution (20 L) of AIBN (164 mg, 1 mmol) dissolved in anisole
diffractometer with GADDS as a 2D detector. Calibration was con-
ducted using silicon powder and silver behenate similar to the 1D
WAXD powder experiments. Fiber-drawn samples were mounted
on the sample stage, and the point-focused X-ray beam was aligned
both perpendicular and parallel to the fiber direction. The 2D
diffraction patterns were recorded in a transmission mode at room
temperature. The 1D small-angel X-ray scattering (SAXS) experi-
ment was performed with a high-flux SAXS instrument (SAXSess,
Anton Paar) equipped with Kratky block-collimation system and
a Philips PW3830 sealed-tube X-ray generator (Cu Ka). The scat-
tering pattern was recorded on an imaging plate (IP) with a pixel
size of 42.3 ꢂ 42.3 m2, of which the peak positions were calibrated
with silver behenate. After background subtraction, desmearing
was performed according to the Lake’s method.
The microphase-separated structure was identified using Atom
Force Microscopy (AFM). The AFM experiments were performed on
an SPM 9500-J3 instrument (Shimadzu, Japan). The films were
obtained by casting the chloroform solution (1 mg/mL) of the
diblock copolymers on silica substrates. After the solvent was
removed at room temperature, the films were first annealed to
210 ꢀC, which was above the clearing point of the LC phase, and
then cooled to room temperature with a cooling rate of 1 ꢀC/min.
m
(1 mL) was added into the flask via a syringe. The mixture was
degassed by three freezeepumpethaw cycles and sealed under
vacuum. The flask was then placed in a preheated oil bath at 75 ꢀC
for 16 h. After that, the reaction was halted by immersing the flask
into ice water. The reaction solution was diluted with THF and
precipitated in hot acetone to obtain the crude product. The puri-
fication procedure was repeated three times and the polymer was
dried in a vacuum oven for 24 h. Mn (GPC) ¼ 31 700, Mw/Mn
(GPC) ¼ 1.22. The other diblock copolymers were synthesized via
a similar procedure by changing the monomer feed ratios and
reaction time. All the diblock copolymers were characterized by
GPC, spectroscopic methods, and thermal analyses. The results are
given in the Results and Discussion below and also in the Sup-
porting Information (Fig. S3). The molecular weights and distri-
butions of the diblock copolymers obtained from both GPC and 1H
NMR are summarized in Table 1.
2.2. Characterization
1H NMR spectra were recorded on a JOEL JNM-ECA300 spec-
trometer (300 MHz for proton) by using CDCl3 as the solvent and
tetramethylsilane as the internal standard. Fourier transform
infrared spectrum (FT-IR) measurements were carried out on
a Nicolet 560-IR spectrophotometer by incorporating the samples
in the KBr tablets. UVevis spectra were recorded by using an Agi-
lent 8453 UVevis spectrophotometer. The molecular weights and
molecular weight distributions were measured by using gel
permeation chromatography (GPC). The GPC instrument was
2.3. Photoinduced orientation and SRG formation
The polymer films were prepared by spin-coating their chloro-
form solutions with a weight fraction of 2.5% on clean glass slides.
After the solvent was evaporated at room temperature, the films
were annealed at 150 ꢀC for 24 h in a vacuum oven and then slowly
cooled to room temperature with a cooling rate of 0.5 ꢀC/min. The
film thicknesses were estimated to be around 300 nm by measuring
the surface scratches with AFM. The optical setup for photoinduced
birefringence measurement was similar to those reported previ-
ously [36,37]. The polymer films were irradiated with a linearly p-
polarized beam from a diode-pumped frequency-doubled solid
state laser (532 nm, 25 mW/cm2). To provide a homogeneous beam
profile over the irradiated area of the film, the laser beam was
expanded through a spatial filter and collimated. The photoinduced
change in the refractive index was probed in a real-time manner by
an unpolarized beam from a HeeNe laser (632.8 nm) with low
intensity. The HeeNe laser beam was transmitted through a pair of
crossed polarizers, whose polarization directions were set at ꢃ45ꢀ
with respect to the polarization of the exciting beam. When the
light intensity variation reached the saturated level, the exciting
beam was turned off to investigate the relaxation process. Photo-
induced dichroism was investigated using polarized FT-IR spec-
troscopy. The films were prepared by spin-coating chloroform
solutions of the polymers (weight fraction ¼ 2.5%) on clean CaF2
substrates. After the solvent was removed at room temperature, the
films were annealed at 150 ꢀC for 24 h in a vacuum oven and slowly
cooled to room temperature with a cooling rate of 0.5 ꢀC/min. The
films were subjected to the irradiation of 532 nm light from
a diode-pumped frequency-doubled solid state laser (25 mW/cm2)
for 30 min. The orientation order parameter (S) was calculated by
the following equation
equipped with a PLgel 5 mm mixed-D column and a refractive index
(RI) detector (Wyatt Optilab rEX). The measurements were carried
out at 35 ꢀC and the molecular weights were calibrated with
polystyrene standards. THF was used as the eluent with the flow
rate of 1.0 mL/min. Thermal properties of the polymers were tested
by using TA Instrument DSC 2920 with a heating rate of 10 ꢀC/min
in the nitrogen atmosphere. Polarizing optical microscopic (POM)
observations were conducted on an Olympus microscope equipped
with a CCD camera and a hot stage.
The mesogenic phase behavior was characterized by X-ray
diffraction measurements. The 1D WAXD powder experiments
were performed on a Philips X’Pert Pro diffractometer with a 3 kW
ceramic tube as the X-ray source (Cu Ka) and an X’celerator
detector. To record the diffraction patterns, the sample films were
obtained by casting THF solutions of the polymers on clean copper
substrates. The films were then dried in vacuum for 24 h to remove
the remaining solvent. In the measurements, the sample stage was
set horizontally, and the reflection peak positions were calibrated
with silicon powder (2q q
> 15ꢀ) and silver behenate (2 < 10ꢀ).
Background scattering was recorded and subtracted from the
sample patterns. A temperature control unit (Paar Physica TCU 100)
in conjunction with the diffractometer was utilized and the heating
and cooling rates in the WAXD experiments were 1.5 ꢀC/min. The
2D WAXD fiber patterns were obtained using a Bruker D8 Discover
Table 1
ꢀ
ꢁ.ꢀ
ꢁ
Molecular weights and distributions of the macromolecular chain transfer agent and
S ¼ At ꢁ A
At þ 2A
(3)
k
k
diblock copolymers, obtained from GPC and 1H NMR.
where At and A are the absorbance at 2227 cmꢁ1 (C^N stretching
jj
Composition
PAzoCN
(wt%)
Mn
Mw/Mn
(GPC)
Mn
(GPC)/104
(NMR)/104
vibration band) perpendicular and parallel to the polarization
direction of the laser beam.
The polymer films were prepared by the same method as used
for the photoinduced orientation experiments. The experimental
setup is similar to those reported before [6,7]. A linearly polarized
Arþ laser beam (488 nm, 80 mW/cm2) was used as the light source.
PChEMA50-CTA
0
8
27
40
50
2.32
2.35
2.91
3.17
3.55
1.15
1.18
1.17
1.22
1.27
2.99
3.27
4.12
5.01
5.94
PChEMA50-b-PAzoCN7
PChEMA50-b-PAzoCN28
PChEMA50-b-PAzoCN50
PChEMA50-b-PAzoCN73

3570
Y. Zhu et al. / Polymer 53 (2012) 3566e3576
After the p-polarized laser beam was expanded and collimated, half
of the beam was incident on the films directly and the other half of
the beam was reflected onto the films from a mirror. The surfaces of
the films were probed by AFM on an SPM 9500-J3 instrument.
The diblock copolymers were prepared by using PChEMA-CTA to
initiate the AzoCN polymerization. For the clarity of presentation,
only results obtained from one specific PChEMA-CTA (PChEMA50
-
CTA) are reported in this article. Fig. 1 presents the GPC curves of
PChEMA50-CTA and the diblock azo copolymers. The diblock
copolymers were obtained by using different feed ratios of the azo
monomer to the macromolecular chain transfer agent. The increase
of the second block length is indicated by the shift of the unimodal
peaks in GPC curves to the higher-molecular-weight region. The
unimodal feature can be seen for all of the diblock copolymers,
which indicates the controlled growth of the second block. Even
when the conversion of the ChEMA in the preparation of PChEMA-
CTA reaches almost 100%, the obtained macromolecular chain
transfer agent has the ‘living’ chain end with the high reactivity, as
no residual PChEMA is detected from the GPC. The result shows
that the series of the block copolymers with different PAzoCN block
lengths is obtained by this method. The polydispersity index of the
diblock copolymers slightly increases with the increase of the
molecular weights. The molecular weights and distributions of this
series of polymers obtained from GPC are given in Table 1.
3. Results and discussion
3.1. Synthesis and characterization
Reversible addition-fragmentation chain transfer (RAFT) poly-
merization was used to synthesize the diblock azo copolymer. RAFT
polymerization is an effective method to obtain polymers with
functional groups [38,39]. The monomers used for the polymeri-
zation, 6-(N-methyl-N-(4-(40-cyanophenylazo)phenyl)amino)hexyl
methacrylate (AzoCN) and 2-(3-((cholesteryl)oxycarbonyl)propio-
nyloxy)ethyl methacrylate (ChEMA), were prepared through the
synthetic routes given in the Schemes 1 and 2. The synthetic
procedure of the diblock azo copolymers is illustrated in Scheme 3.
The obtained diblock copolymers were characterized by 1H NMR,
FT-IR, UVevis, and GPC. Some representative results are given in
Figs. 1e3 for discussion and the other characterization results are
given in the Supporting Information.
To prepare the diblock copolymers, the macromolecular chain
transfer agents with different molecular weights and low poly-
dispersities (PDI ¼ Mw/Mn, GPC, lower than 1.20) were obtained
using CPDB as the chain transfer agent. It was observed that the
azobenzene-containing monomer AzoCN polymerized at a much
slower rate compared with ChEMA, which was related to the poor
solubility of the azo polymer in anisole and the retardation of the
azo groups to the radical polymerization. Therefore, the diblock
copolymers were synthesized by using the macromolecular chain
transfer agents (PChEMA-CTA) obtained from the polymerization of
ChEMA. The number average molecular weights (Mn) of PChEMA-
CTA were determined by both 1H NMR and GPC. It is supposed
that the proportion of the polymeric chains undergoing termina-
tion is negligible and CPDB is converted completely in the process.
Therefore, Mn (NMR) can be calculated by Eq. (4)
Fig. 2 shows the 1H NMR spectra of the diblock copolymers and
corresponding homopolymers. The structures of the diblock
copolymers are confirmed by the characteristic resonance signals
(Fig. 2A). The assignment of the resonances for the representative
copolymer is given in Fig. 2B. Mn (NMR) of the copolymers is
obtained from the integration areas of the peaks at 6.55 ppm (I6.55
)
and 5.37 ppm (I5.37) by using Eq. (5). These two peaks correspond to
the resonance signals of the two protons at the ortho-position to the
amino group and the methylene protons of the cholesteryl moiety
MnðNMRÞ ¼ 29900 þ 25 ꢂ ðI6:55=I5:37Þ ꢂ 404:5
(5)
where 29,900 and 404.5 are the molecular weights of PChEMA-CTA
and the azo monomer. The number of the repeat units for each
block is obtained from the molecular weights of PChEMA-CTA and
azo blocks. The molecular weights, compositions, and block lengths
obtained from the 1H NMR analysis are summarized in Table 1. Both
1H NMR and GPC analyses reveal that the diblock copolymers with
different Mns and azo block lengths are obtained. The Mn (GPC) is
lower than Mn (NMR) owing to the reason given above, which is
consistent with those reported before [39]. As the Mn (GPC) is not
the absolute molar mass, the block lengths estimated from the 1H
NMR are used here for the following discussions.
ꢂ
ꢃ
Mn NMR ¼ 598 ꢂ ½Mꢄ0=½CTAꢄ0 ꢂ conv
(4)
where 598 is the molecular weight of the monomer and the
conversion (conv) is obtained from 1H NMR by Eq. (1). The Mn (GPC)
is lower than Mn (NMR) for PChEMA-CTA, which could be attrib-
uted to the deviation caused by using polystyrene standards for the
molecular weight calibration in the GPC characterization. Due to
this reason, Mn obtained from GPC cannot be treated as the absolute
molar mass [39].
Fig. 3 gives the UVevis spectra of the block copolymers in
chloroform (0.04 g/L). The diblock copolymers exhibit typical
spectral characteristics of the pseudo-stilbene type azo chromo-
phores [31]. The maximum absorption band of
pep* transition
Scheme 3. Synthetic route for the diblock azo copolymers.

Y. Zhu et al. / Polymer 53 (2012) 3566e3576
3571
Fig. 2. (A) 1H NMR spectra of the diblock copolymers and the corresponding homo-
polymers in CDCl3, from top to bottom: PAzoCN, PChEMA50-b-PAzoCN73, PChEMA50-b-
PAzoCN50
, PChEMA50-b-PAzoCN28, PChEMA50-b-PAzoCN7, PChEMA-CTA. (B) The
assignment of the 1H NMR spectrum for PChEMA50-b-PAzoCN28 as a typical example.
3.2. Phase behavior and transition temperature
In order to identify the mesophase type and transition
temperature of the diblock copolymers, the phase behavior of the
corresponding monomer (ChEMA) and homopolymers (PChEMA-
CTA and PAzoCN) was studied by DSC, POM and XRD. The
cholesteryl-containing monomer ChEMA melts at 63 ꢀC on the
heating scan. The XRD pattern of the monomer exhibits
Fig. 1. Chemical structure of the diblock copolymer PChEMA50-b-PAzoCNx and GPC
curves of the macromolecular chain transfer agent and diblock copolymers. (curve a)
PChEMA-CTA, (curve b) PChEMA50-b-PAzoCN7, (curve c) PChEMA50-b-PAzoCN28
(curve d) PChEMA50-b-PAzoCN50, (curve e) PChEMA50-b-PAzoCN73
,
.
appears in the visible wavelength range (lmax ¼ 446 nm). The
weak n- * transition band is overlapped with the * band and
cannot be identified in the spectra. The absorbance of the
p
pep
pep*
transition band increases linearly with the increase of the
PAzoCN block length, which confirms the conclusion drawn from
1H NMR and GPC.
Fig. 3. UVevis spectra of PChEMAx-b-PAzoCNy in CHCl3 (0.04 g/L), (a) PChEMA50-b-
PAzoCN7, (b) PChEMA50-b-PAzoCN28
PAzoCN73
, (c) PChEMA50-b-PAzoCN50, (d) PChEMA50-b-
.

3572
Y. Zhu et al. / Polymer 53 (2012) 3566e3576
a diffraction peak in the small angle region, which corresponds to
periodic spacing of 5.03 nm. The DSC curves of homopolymer
PChEMA-CTA on the first cooling and second heating scans are
shown in Fig. 4. PChEMA-CTA shows the glass transition tempera-
ture (Tg) at 47 ꢀC. A broad exothermic transition around 182 ꢀC
(peak value) is observed on the cooling scan of PChEMA-CTA, and
the endothermic transition is around 185 ꢀC (peak value) on the
heating scan (Table 2). POM observation indicates that the
isotropic-LC transition of PChEMA-CTA occurs at around 190 ꢀC.
Batonnet texture first appears at that temperature and gradually
develops into a typical fan-shaped texture as the temperature
decreases (Fig. 5). The XRD pattern was recorded with a casted film
of PChEMA-CTA upon cooling from 230 ꢀC to room temperature
(Fig. 6). At 220 ꢀC, three strong peaks corresponding to the 1st, 2nd,
and 3rd order diffraction of smectic layers become visible in the
small angle region. The layer spacing increases gradually as the
temperature decreases, which is a characteristic of smectic-A (SA)
mesophase. The observation can be attributed to the gradual order
alignment of the spacers that are between the main chains and the
rigid cholesteryl groups. In the wide angle region, the broad scat-
tering peak shows the disordered arrangement of the mesogenic
groups in the smectic layers. The broad scattering peak shows no
change in the cooling process, which excludes the SB and other
smectic phases from option of the mesophase types of
PChEMA-CTA.
To further identify the mesophase type, 2D-XRD was performed
on an oriented sample of PChEMA-CTA (Fig. S4 in the Supporting
Information). The fiber sample was obtained by drawing the
polymer in the liquid crystalline phase. The diffraction arcs can be
seen in the equatorial direction perpendicular to the fiber axis,
which shows that the smectic layers are aligned parallel to the fiber
axis. The characteristics of a SC phase, such as four-spot reflexes,
cannot be observed from the 2D-XRD. The cholesteryl-bearing LC
side-chain polymer is identified to be in SA phase. The layer spacing
of the SA phase at room temperature is 5.03 nm according to XRD
pattern (Fig. 6), which is the same as that of the monomer ChEMA.
As the fully extended length of the monomer is calculated to be
2.77 nm, the side chains of the polymer are arranged according to
a two-layer packing mode with partial overlapping of the alkyl tails.
The homopolymer PAzoCN, corresponding to the azo block of the
block copolymers, doesn’t form any liquid crystalline phase and is
an amorphous polymer with Tg of 37 ꢀC.
the series. The Tg increases with the increase of the PAzoCN block
length and reaches the highest value for PChEMA50-b-PAzoCN50
.
The phase transition temperatures (TLCꢁI) and the enthalpies of the
copolymers, derived from the DSC analyses of the first cooling and
second heating scans, are listed in Table 2. Compared to TLC-I of
185 ꢀC for PChEMA-CTA (the second heating scan), the transition
temperature declines to 169 ꢀC for PChEMA50-b-PAzoCN7, which is
the lowest in the series. With the increase of the PAzoCN block
length, the TLC-I increases and reaches 196 ꢀC for PChEMA50-b-
PAzoCN73. The increases of the phase transition temperatures with
the increase of the PAzoCN block length is related to the micro-
phase separation occurring in the systems, which will be discussed
in the next section. The LC textures of the block copolymers were
observed with POM by annealing the samples within their LC
region (a few degrees below their TLC-I) for 24 h before observation
(Fig. S5 in the supporting information). The fan-shaped texture can
only be observed for PChEMA50-b-PAzoCN7. For other block
copolymers, the samples show birefringence but lack the typical LC
textures. Fig. 7 shows SAXS profiles of the diblock copolymers at
room temperature. The diffraction peaks corresponding to the SA
layer spacing of PChEMA blocks can be seen for all the block
copolymers, which confirm that the LC sub-phase is the SA phase.
This observation is consistent with LC/I block copolymers con-
taining cholesteryl mesogens [19,20]. The lack of smecitc texture
for the other block copolymers can be attributed to microphase
separation reported in the following section.
3.3. Microphase separation and morphology
The morphology of the diblock copolymers due to the micro-
phase separation was investigated by AFM. The typical AFM
images of PChEMA50-b-PAzoCN7, PChEMA50-b-PAzoCN28
, and
PChEMA50-b-PAzoCN50 at a larger scale are given in the Support-
ing Information as Fig. S6. The samples were prepared by casting
the chloroform solution of the diblock copolymers on silica
substrates and annealing. The AFM images were obtained by the
tapping mode. For PChEMA50-b-PAzoCN7, the nano-sheets with
size over 10 mm can be seen. The thickness of the nano-sheets is
about 5 nm estimated by AFM, which matches the SA layer spacing
derived from the XRD. The nano-sheets are the exfoliated smectic
layers with the homeotropic alignment of the mesogens on the
surfaces. The sizes of the LC layers evidently reduce with the
increase of the length of the isotropic PAzoCN block. In the case of
PChEMA50-b-PAzoCN73, where the weight fraction of the PAzoCN
block is the highest, no exfoliated layer structure can be observed
from AFM.
The transition temperature and phase behavior of the diblock
copolymers were investigated by using the similar methods. The
Tgs of the diblock copolymers obtained from the DSC curves (Fig. 4)
are listed in Table 2. PChEMA50-b-PAzoCN7 shows the lowest Tg in
Fig. 4. DSC curves of the homopolymer and block copolymers on the first cooling scan (A) and second heating scan (B) with a heating/cooling rate of 10 ꢀC/min; (curve a) PChEMA-
CTA, (curve b) PChEMA50-b-PAzoCN7, (curve c) PChEMA50-b-PAzoCN28, (curve d) PChEMA50-b-PAzoCN50, (curve e) PChEMA50-b-PAzoCN73
.

Y. Zhu et al. / Polymer 53 (2012) 3566e3576
3573
Table 2
identified. Fig. 8 shows the AFM height and phase images of
PChEMA50-b-PAzoCN28 and PChEMA50-b-PAzoCN73 The block
Thermal analysis data of the synthesized polymers, both the heating and the cooling
rates were 10 ꢀC/min, samples were first heated to 250 ꢀC to remove thermal history.
.
copolymer PChEMA50-b-PAzoCN28, where the weight fraction of
PAzoCN is about 27%, exhibits two levels of self-assembling struc-
tures related with orientation of the mesogenic units and micro-
phase separation of the blocks. The PChEMA block forms the SA
phase matrix, which is evidenced by the stacked equidistance
layers in the AFM height image (Fig. 8a, Fig. S6). The isotropic
PAzoCN block is confined in hexagonal-packed cylinders aligned
perpendicular to the smectic layers. The minority sub-phase
formed by the PAzoCN blocks appears as the bright dots on the
phase image (Fig. 8b). Since the matrix is in the SA phase and the
mesogenic units align perpendicular to the smectic layers, it can be
concluded that the mesogens align parallel to the nanocylinders.
The AFM height and phase images of PChEMA50-b-PAzoCN73 are
given in Fig. 8c and d. The lamellar morphology is evidenced by
both height and phase images. The AFM observations cannot
identify whether the smectic layers lie perpendicular or parallel to
b
Sample
Tg/ꢀCa
TLC-I ꢀC ( H/J gꢁ1
D )
Heating
Cooling
PChEMA50-CTA
47.0
30.2
34.4
50.1
49.2
185.3(2.54)
169.1(1.80)
183.5(3.17)
198.0(3.18)
195.5(3.08)
182.0(3.42)
165.7(1.86)
175.3(3.87)
195.5(4.68)
191.3(3.59)
PChEMA50-b-PAzoCN7
PChEMA50-b-PAzoCN28
PChEMA50-b-PAzoCN50
PChEMA50-b-PAzoCN73
a
Glass transition temperature of the polymers obtained from the second heating
cycle.
b
All the transitions were recorded in the first cooling and second heating cycle.
The enthalpies were normalized to the weight fractions of the ChEMA block.
The microphase-separated morphology of block copolymers is
revealed by AFM through the height and phase images of the
surfaces. For PChEMA50-b-PAzoCN7 with the shortest PAzoCN
block, no regular microphase-separated morphology can be
Fig. 5. Polarizing optical micrographs (magnification ꢂ 40) of PChEMA50 cooling from 200 ꢀC (isotropic phase) to (A) 190 ꢀC, (B) 185 ꢀC, (C) 180 ꢀC and (D) 175 ꢀC. Samples held at
each temperature for 5 min before taking the micrographs.
Fig. 6. X-ray diffraction patterns of PChEMA-CTA on cooling from 230 ꢀC to room temperature.

3574
Y. Zhu et al. / Polymer 53 (2012) 3566e3576
the microphase separation interface. It has been reported that the
orientation of the smectic layers could be parallel or perpendicular
to the lamellae as revealed by TEM observation [21e23]. For the
case of PChEMA50-b-PAzoCN50, which has the weight fraction of
PAzoCN between the two cases mentioned above, no such typical
microphase-separated morphology can be perceived from AFM
investigations.
Typical coilecoil AB diblock copolymers can form thermody-
namically stable morphologies including lamellae, hexagonal-
packed cylinder, body-centered cubic and bicontinuous cubic
gyroid structures depending on the composition [40]. These
structures are formed as a competitive result of the minimization of
the interfacial energy and the maximization of the coil entropy. In
the case of block copolymers composed of side-chain liquid crystal
polymer (SCLCP) and isotropic blocks, competition between the
anisotropic LC packing and phase separation of the block copoly-
mers leads to some unique characteristics. As SCLCP needs higher
surface area per chain than the isotropic block, a highly curved
interface is favorable when the joints of the blocks are in a narrow
interface [17]. Therefore, the stabilization of isotropic cylinders or
spheres in the LC matrix could be expected. The morphology of the
block copolymers also depends on the mesophase of the LC block.
The smectic LC sub-phases will favor the formation of lamellar
morphology as a result of compatible symmetry [22,23]. Otherwise,
the distorted director field of the mesogens will lead to an increase
of the elastic energy of the system [17]. Above understanding can
Fig. 7. SAXS profiles of the diblock copolymers at room temperature. The polymers
were annealed within their liquid crystailline region for 24 h and then quenched.
(curve a) PChEMA50-b-PAzoCN7, (curve b) PChEMA50-b-PAzoCN28, (curve c) PChEMA50
b-PAzoCN50, (curve d) PChEMA50-b-PAzoCN73
-
.
Fig. 8. AFM height and phase images for PChEMA50-b-PAzoCN28 (a and b) and PChEMA50-b-PAzoCN73 (c and d). Image size: 1 ꢂ 1
mm for a, 2 ꢂ 2
mm for b, c, d.

Y. Zhu et al. / Polymer 53 (2012) 3566e3576
3575
rationalize the observation of the morphologies mentioned above.
For the block copolymers studied here, the clearly identified
morphologies are the hexagonal-packed cylinders and lamellae.
When the PAzoCN block is short, the interface curvature should be
a dominant factor to result in the cylindrical morphology. On the
other hand, when the PAzoCN block length is increased, the elastic
energy effect of LC sub-phase could play an important role to
stabilize the lamellar morphology.
ensure the occurrence of microphase separation. The films showed
no overall birefringence before the light irradiation. Fig. 9A gives
the birefringence change with the irradiation time for the block
copolymers and homopolymer PAzoCN. Under the same light
irradiation conditions, all the polymers reach saturation at
approximately the same time. The saturated value of birefringence
D
nmax increases with the increase of the PAzoCN content. For
PChEMA50-b-PAzoCN28 PChEMA50-b-PAzoCN50 PChEMA50-b-
PAzoCN73, and PAzoCN, the nmax is 0.022, 0.025, 0.031, and 0.036.
,
,
For LC/I block copolymers, the sub-phase confinement can also
have a significant influence on the LC phase behavior [17,18]. When
confined in spherical sub-phase, the SCLCP blocks only form the
nematic (N) phase instead of the smectic phase observed for the
corresponding homopolymer [19,20]. In this study, the morphology
change doesn’t show the effect to alter the LC type of the PChEMA
sub-phase. But, the variation of the smectic to isotropic transition
temperature (TLC-I), given in Table 2, can be attributed to the effect
of the morphology change. PChEMA50-b-PAzoCN7 has the lowest
TLC-I in the series because it doesn’t show the microphase separa-
tion. PAzoCN component exists as a small amount of impurity in the
LC phase and plays a role as the defects to destabilize the SA phase
of PChEMA. For PChEMA50-b-PAzoCN28, the PAzoCN block forms
the cylinders embedded in the matrix of PChEMA and the meso-
gens align parallel to the interface between the sub-phases. As
a result, the copolymer shows the elevated TLC-I compared with
PChEMA50-b-PAzoCN7. The TLC-I of PChEMA50-b-PAzoCN28 is only
slightly lower than that of the homopolymer (PChEMA-CTA).
PChEMA50-b-PAzoCN50 and PChEMA50-b-PAzoCN73 show the
significantly higher TLC-I compared with that of PChEMA-CTA. It can
be attributed to the compatibility between the lamellar
morphology and the mesogen alignment in the SA phase. In this
case, the confinement of PAzoCN lamellae shows the effect to
stabilize the SA sub-phase and increase the TLC-I of the mesogenic
phase.
D
For the diblock copolymer PChEMA50-b-PAzoCN7 without micro-
phase separation, which has a weight fraction of 8% for PAzoCN
block, shows the lowest Dnmax of 0.008. After the light irradiation,
the films were examined by AFM and no visible variation of the
morphology was detected by this method.
The photoinduced orientation was also characterized by polar-
ized FT-IR spectroscopy. The orientation order parameter was
obtained from the angular-dependent infrared absorption intensi-
ties of the C^N stretching vibration band at 2227 cmꢁ1. For the
diblock copolymers, the measured order parameter is smaller than
that of the homopolymer. Fig. 9B gives the polar plots of the
absorption intensities of PAzoCN and PChEMA50-b-PAzoCN73. The
order parameters are 0.0225 and 0.0174 for PAzoCN and PChEMA50
-
b-PAzoCN73. The result confirms that the birefringence produced by
the light irradiation is caused by the azo chromophore orientation.
3.4. Photoresponsive properties
Azo polymers have been intensively studied for their photoin-
duced orientation behavior, which is a property promising for
applications in optical data-storage and holography [2,3,5,41]. As
a result of repeated cycles of trans-cis-trans isomerization caused by
the light irradiation, the azo chromophores on the polymers are
forced to orientate preferentially in a direction perpendicular to the
polarization of the light. The orientation can also cause some
correlative alignments of adjacent groups or segments in the azo
polymers. For LC block copolymers, when the azobenzene-
containing block is confined in spherical or lamellar domains,
restraint effect on the photoinduced alignment might be observed
[42]. The diblock copolymers studied here have a weight fraction of
PAzoCN varied in the range of 8e50% and show the morphologies of
mixing at molecular level, hexagonal-packing cylinders, and
parallel lamellae. Therefore, it is interesting to find out how the
different structures affect the photoinduced orientation behavior of
the azo block.
The molecular orientation was detected by the photoinduced
birefringence measurement in a dynamic in-situ manner [1,2,41].
The value of birefringence (Dn) was obtained from Eq. (6).
I ¼ I0sin2ð
d n=
p D l
Þ
(6)
where I is the intensity of the probe beam transmitted through the
crossed polarizer, and I0 is the intensity of the probe beam that
Fig. 9. (A) Photoinduced change of birefringence (
diblock copolymers: (curve a) PAzoCN, (curve b) PChEMA50-b-PAzoCN73, (curve c)
Dn) for thin films of PAzoCN and the
transmitted through the first polarizer, d is the film thickness, and
l
PChEMA50-b-PAzoCN50, (curve d) PChEMA50-b-PAzoCN28, (curve e) PChEMA50-b-
is the wavelength of the probe beam. The spin-coated films for the
experiments were measured to have thicknesses around 300 nm.
The samples were annealed below the clearing temperature to
PAzoCN7. (B) Polar plots of the infrared absorbance at 2227 cmꢁ1 of PAzoCN and
PChEMA50-b-PAzoCN73 recorded upon rotation of the polarizer at every 10ꢀ. The arrow
denotes the direction of electric vector of the pump laser.

3576
Y. Zhu et al. / Polymer 53 (2012) 3566e3576
The block azo copolymer films were also irradiated with inter-
and Engineering, Peking University for their help in XRD and SAXS
measurements.
fering laser beams to check whether surface-relief-grating (SRG)
can be formed on the film surfaces. The films of the homopolymer
PAzoCN was tested under the same condition to compare with the
block copolymers. In contrast to the observation of the substantial
SRG formed on the PAzoCN film surface, no surface modulation can
be observed for the block copolymers. SRG formation on the azo
polymer films is caused by the mass transport in micrometer scale
driven by the light irradiation [2,6,7]. For PChEMA50-b-PAzoCN7,
the azo chromophore amount in the system is probably too low to
cause the mass transport. For the other block copolymers, it can be
attributed to the confinement of the PAzoCN blocks in the sub-
phase so that the micron-scale migration of the polymer chains
cannot be achieved by the light irradiation. The suppression of
undesired SRG formation by the microphase separation can be an
attractive feature of the block copolymers as volume holographic
media for photo-storage applications [43].
Appendix A. Supporting information
Supporting Information associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.polymer.
2012.05.063.
References
[1] Xie S, Natansohn A, Rochon P. Chem Mater 1993;5(4):403e11.
[2] Natansohn A, Rochon P. Chem Rev 2002;102(11):4139e75.
[3] Delaire JA, Nakatani K. Chem Rev 2000;100(5):1817e45.
[4] Ikeda T, Horiuchi S, Karanjit DB, Kurihara S, Tazuke S. Macromolecules 1990;
23(1):42e8.
[5] Todorov T, Nikolova L, Tomova N. Appl Opt 1984;23(23):4309e12.
[6] Rochon P, Batalla E, Natansohn A. Appl Phys Lett 1995;66(2):136e8.
[7] Kim DY, Tripathy SK, Li L, Kumar J. Appl Phys Lett 1995;66(10):1166e8.
[8] Finkelmann H, Nishikawa E, Pereira GG, Warner M. Phys Rev Lett 2001;87(1):
015501.
4. Conclusion
[9] Yu YL, Nakano M, Ikeda T. Nature 2003;425(6954):145.
[10] Li MH, Keller P, Li B, Wang XG, Brunet M. Adv Mater 2003;15(7e8):569e72.
[11] Li YB, He YN, Tong XL, Wang XG. J Am Chem Soc 2005;127(8):2402e3.
[12] Li YB, Deng YH, He YN, Tong XL, Wang XG. Langmuir 2005;21(14):6567e71.
[13] Jiang DL, Aida T. Nature 1997;388(6641):454e6.
[14] Archut A, Azzellini GC, Balzani V, De Cola L, Vogtle F. J Am Chem Soc 1998;
120(47):12187.
[15] Tsuda K, Dol GC, Gensch T, Hofkens J, Latterini L, Weener JW, et al. J Am Chem
Soc 2000;122(14):3445e52.
In this work, a series of diblock copolymers bearing strong
pushepull azo chromophores and cholesteryl groups were
synthesized by RAFT polymerization. The diblock copolymers
(PChEMAx-b-PAzoCNy) are composed of the same cholesteryl-
containing block (x ¼ 50) and the azo blocks with the four
different lengths (y ¼ 7, 28, 50, 73). Distinct from the previously
reported LC/I diblock azo copolymers, the block containing
pushepull azo chromophores is isotropic and the block bearing the
cholesteryl groups on the side-chains forms the LC phase. For the
annealed thin films, PChEMA block exists in the SA mesophases and
the morphology of the block copolymers depends on the PAzoCN
block length. When the PAzoCN block is short (PChEMA50-b-
PAzoCN7), no obvious microphase separation can be observed. For
y ¼ 28 and 73 as two typical cases, PAzoCN appears as the
hexagonal-packed cylinders embedded in a matrix of the PChEMA
for the former and copolymer shows microphase-separated
lamellar morphology in the latter. The LC-isotropic transition
temperature and enthalpy of the diblock copolymers show
[16] Zhao Y, He J. Soft Matter 2009;5(14):2686e93.
[17] Walther M, Finkelmann H. Prog Polym Sci 1996;21(5):951e79.
[18] Poser S, Fischer H, Arnold M. Prog Polym Sci 1998;23(7):1337e79.
[19] Fischer H, Poser S, Arnold M, Frank W. Macromolecules 1994;27(24):7133e8.
[20] Fischer H, Poser S, Arnold M. Liquid Crystal 1995;18(3):503e9.
[21] Mao GP, Wang JG, Clingman SR, Ober CK, Chen JT, Thomas EL. Macromolecules
1997;30(9):2556e67.
[22] Zheng WY, Albalak RJ, Hammond PT. Macromolecules 1998;31(8):2686e9.
[23] Schneider A, Zanna JJ, Yamada M, Finkelmann H, Thomann R. Macromolecules
2000;33(3):649e51.
[24] Ansari IA, Castelletto V, Mykhaylyk T, Hamley IW, Lu ZB, Itoh T, et al.
Macromolecules 2003;36(23):8898e901.
[25] Hayakawa T, Horiuchi S. Angew Chem Int Edit 2003;42(20):2285e9.
[26] Tian YQ, Watanabe K, Kong XX, Abe J, Iyoda T. Macromolecules 2002;35(9):
3739e47.
[27] Morikawa Y, Nagano S, Watanabe K, Kamata K, Iyoda T, Seki T. Adv Mater
2006;18(7):883e6.
a
dependence on the microphase-separated structures. The
lamellar morphology shows the effect to stabilize the SA mesophase
of the PChEMA sub-phase as indicated by the elevated TLC-I. The
saturated degree of the photoinduced birefringence increases with
the increase of the azo chromophore amount in the block copoly-
mers. When the azo chromophores are confined in the microphase-
separated sub-phase, the photoinduced orientation can be
observed for all the block copolymers, which show similar relaxa-
tion behavior after switching off the excitation light. The polymer
films with photo-active azo polymers confined in nanostructures
could be an appealing candidate for volume holographic recording
and the LC continuous phase with field-tunable feature could add
new functions to the material.
[28] Yu HF, Iyoda T, Ikeda T. J Am Chem Soc 2006;128(34):11010e1.
[29] Kadota S, Aoki K, Nagano S, Seki T. J Am Chem Soc 2005;127(23):8266e7.
[30] Yu HF, Naka Y, Shishido A, Ikeda T. Macromolecules 2008;41(21):7959e66.
[31] Rau H. In: Rabek JF, editor. Photochemistry and photophysics, vol. II. Boca
Raton: CRC Press; 1990.
[32] Wang XG, Chen JI, Marturunkakul S, Li L, Kumar J, Tripathy SK. Chem Mater
1997;9(1):45e50.
[33] Wang XG, Kumar J, Tripathy SK, Li L, Chen JI, Marturunkakul S. Macromole-
cules 1997;30(2):219e25.
[34] Wang DR, Ren HF, Wang XQ, Wang XG. Macromolecules 2008;41(23):
9382e8.
[35] Song JS, Winnik MA. Macromolecules 2006;39(24):8318e25.
[36] Natansohn A, Rochon P, Gosselin J, Xie S. Macromolecules 1992;25(8):
2268e73.
[37] Ye G, Wang DR, He YN, Wang XG. J Mater Chem 2010;20(47):10680e7.
[38] Chiefari J, Chong YK, Ercole F, Krstina J, Jeffery J, Le TPT, et al. Macromolecules
1998;31(16):5559e62.
[39] Zhao Y, Qi B, Tong X, Zhao Y. Macromolecules 2008;41(11):3823e31.
[40] Bates FS, Fredrickson GH. Phys Today 1999;52(2):32e8.
[41] Wu YL, Demachi Y, Tsutsumi O, Kanazawa A, Shiono T, Ikeda T. Macromole-
cules 1998;31(2):349e54.
Acknowledgment
The financial support from the NSFC under Projects 91027024
and 51061130556 is gratefully acknowledged. Authors sincerely
thank professor Er-Qiang Chen, Dr. Zhi-Hao Shen, Dr. Xiao-Fang
Chen and Miss Ling-Ying Shi in Department of Polymer Science
[42] Breiner T, Kreger K, Hagen R, Hackel M, Kador L, Muller AHE, et al. Macro-
molecules 2007;40(6):2100e8.
[43] Hackel M, Kador L, Kropp D, Schmidt HW. Adv Mater 2007;19:227e31.