Synthesis, nanostructures, and functionality of amphiphilic liquid crystalline block copolymers with azobenzene moieties

Article abstract of DOI:10.1021/ma011859j

A series of liquid crystalline (LC) homopolymers of poly{11-[4-(4-butylphenylazo)phenoxy]-undecyl methacrylate} containing an azobenzene mesogen with different degrees of polymerization were synthesized by using the atom transfer radical polymerization (A

Full text of DOI:10.1021/ma011859j

Macromolecules 2002, 35, 3739-3747
3739
Synthesis, Nanostructures, and Functionality of Amphiphilic Liquid
Crystalline Block Copolymers with Azobenzene Moieties
Ya n qin g Tia n , Ka zu h ito Wa ta n a be, Xia n gxin g Kon g, J ir o Abe, a n d
Tom ok a zu Iyod a *
Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan University,
1-1 Minami-Ohsawa, Hachioji, Tokyo, 192-0397, J apan
Received October 25, 2001; Revised Manuscript Received February 8, 2002
ABSTRACT: A series of liquid crystalline (LC) homopolymers of poly{11-[4-(4-butylphenylazo)phenoxy]-
undecyl methacrylate}containing an azobenzene mesogen with different degrees of polymerization were
synthesized by using the atom transfer radical polymerization (ATRP) method. The homopolymers were
prepared with a range of number-average molecular weights from 6100 to 23 500 with narrow
polydispersities of less than 1.17. Thermal investigation showed that the homopolymers exhibit monolayer
smectic A, smectic C, and an unknown smectic X phases. The transition temperatures increase slightly
with the increase of the molecular weights and level off at around 21 500. Novel amphiphilic LC-coil
diblock copolymers with a defined length of a flexible poly(ethylene glycol) segment as the hydrophilic
coil were also prepared by the ATRP method. The LC-coil diblock copolymers exhibit narrow polydis-
persities of less than 1.11. Morphologies of the thin films of the block copolymers were investigated by
using transmission electron microscopy (TEM). Microphase separation with small size in the range of
10-20 nm (nanoseparated structures) was observed. Different photophysical and photochemical behaviors
were observed between annealed homopolymer and block copolymer films, which is thought to be caused
by the formation of nanostructures of the block copolymers.
In tr od u ction
amphiphilic properties in the block copolymers not only
provides novel LC block copolymers but also opens the
opportunities to investigate reproducible nanostructure
formation under the influence of polymer structures,
thermal treatment, and the driving force by self-
assembly. Additionally, azobenzene chromophore is one
of the typical functional units which can undergo
reversible trans-cis isomerization under photoirradia-
tion and can be applied in the fields of information
storage, waveguide switching, optical memories, etc.
Based on the microphase separation of block copolymers
with different morphologies, the azobenzene group will
act as a probe to study whether the photoisomerization
has influence on the microphase separation and whether
the above-mentioned morphologies have some influence
on the photochromism of the azobenzene unit. Though
some studies on nanostructures affected by polymer
sequences and LC properties of the azobenzene block
copolymers5-7 have been reported, the investigation of
the relationship of the photophysical and photochemical
properties of azobenzene block copolymers with the
nanostructures8 has just started.
Block copolymers can produce numbers of microphase-
separated nanostructures with cylinder, lamellar, sphere
morphologies, etc, which are of wide scientific and
technological interests.1-3 If any functional groups/
properties are well assembled into the block copolymers,
the functionality might act as a probe to elucidate
relationship of nanostructures with functionality, i.e.,
both nanostructures controlled by functionality and
nanostructure-specific functionality. Some of the recent
studies in the fields are shifting their gravities to the
function-directed polymer materials utilizing the nano-
structures. For example, J enekhe et al. revealed that
different morphologies of a rod-coil diblock copolymer
exhibited quite different photoluminescence emis-
sions.4 Mao et al. pointed out that the formed cylinder
structures of a series of liquid crystalline (LC) block
copolymers may stabilize the mesophases, resulting in
the increase of isotropic transition temperatures.5
Schneider et al. found that not only the nanoseparated
morphologies but also the domain sizes could be con-
trolled by using different thermal treatment conditions
of a LC block copolymer.6 Along this line, we have set
our object more clearly to explore new type of functional
materials based on the microphase-separated nano-
structures.
For the preparation of the new functional block
copolymers, we chose the recently developed polymer-
ization techniquesatom transfer radical polymerization
(ATRP) methodsbecause the ATRP method can be
handled easily and can be applied to a wide variety of
monomers and lead to interesting polymer architectures
with narrow polydispersity.9-17 Different from many
reports describing the synthesis of amphiphilic block
copolymers and LC block copolymers,5,6,18-21 we adopted
the ATRP method for the first time to the synthesis of
amphiphilic thermotropic functional LC block copoly-
mers.
To further understand the nanostructure-functional-
ity correlation based on functional block copolymers, we
design a novel series of multifunctional block copoly-
mers, which contain both a defined length of poly-
(ethylene glycol) (PEG) as the hydrophilic block and
different length of poly(methacrylate) with an azoben-
ezene unit as the hydrophobic, liquid crystalline, and
photoisomerization block. The combination of LC with
Therefore, in this paper, we will describe the synthesis
of a new series of LC homopolymers of poly{11-[4-(4-
butylphenylazo)phenoxy]undecyl methacrylate} (P 1)
* Corresponding author: Tel +81-426-77-2833; Fax +81-426-
77-2821.
10.1021/ma011859j CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/29/2002

3740 Tian et al.
Macromolecules, Vol. 35, No. 9, 2002
Ch a r t 1
Sch em e 1
and their amphiphilic block copolymers with a defined
poly(ethylene glycol) as the hydrophilic block (P 2) by
using the ATRP method and the investigation and
comparison of their LC properties, nanostructures, and
the relationship of nanostructures with their photo-
physical and photochemical properties (see Chart 1).
Sch em e 2
Exp er im en ta l Section
Ch a r a cter iza tion . 1H NMR spectra were measured by
using a J EOL 270 instrument spectrometer operating at 270
MHz with TMS internal standard as a reference for chemical
shifts. FT-IR spectra were recorded on a Bio-Rad FTS 3000
spectrophotometer. UV-vis absorption spectra were recorded
on a Shimadzu UV-3100S spectrophotometer. Polymer films
for photoreactions were spin-coated from 1 wt % toluene
solutions onto quartz glass plates. Trans-cis photoisomeriza-
tions of azobenzene units were carried out by using a Xe lamp
with cut filters of UV34 and U340 (the light intensity was
about 1.2 mW/cm2 at 365 nm). Cis-trans photoisomerizations
were carried out by the Xe lamp using an L42 and a Y50 cut
filter. Molecular weights of polymers were determined by using
a J ASCO 860 GPC (J apan Spectroscopic Co., Ltd.) equipped
with UV and RI detectors in reference of a series of standard
polystyrenes with THF as eluent.
Thermal behaviors were determined by using a SII Extra
6000 DSC system (Seiko Instruments Inc.) at a scanning rate
of (10 °C/min. Liquid crystalline textures were observed under
a Nikon Microphot-UFX polarized optical microscope (POM)
with a Mettler FP-82 hot stage and a FP-80 central processor.
Wide-angle X-ray diffraction (WAXD) was measured on a MAC
Science MPX 3 X-ray diffractometer equipped with a thermal
controller model 5310.
initiator as shown in Scheme 1. The polymerizations were
carried out under vacuum in degassed and sealed ampules at
80 °C.
Exa m p le of th e P r ep a r a tion of P 1b. 10 mg (0.1 mmol)
of Cu(I)Cl and 492 mg (1.0 mmol) of the monomer 1 were
mixed in a 10 mL ampule bottle, degassed and filled with
nitrogen. 11 µL (14.7 mg, 0.1 mmol) of HMTETA and 7.3 µL
(9.75 mg, 0.05 mmol) of ethyl 2-bromoisobutyrate in 5 mL of
anisole were added through a syringe. The mixture was
degassed by three cycles of freeze-pump-thaw procedures
and sealed under vacuum. After 30 min stirring at room
temperature, the ampule was placed in the preheated 80 °C
oil bath for 20 h. A solution was taken for 1H NMR mea-
surement. Conversion was 87%, determined on the basis of
the intensity of the peak at 2.66 ppm (I2.66) corresponding to
the two methylene protons of the butyl group at the para
position of the phenyl group and that at 6.08 ppm (I6.08
)
representative of a vinyl proton of the methacrylate group by
eq 1.
Transmission electron microscopy (TEM, J EOL 1200 EXII)
experiments were carried out at an acceleration voltage of 200
kV. TEM samples for the observation of nanostructures of the
block copolymers were prepared by spreading 0.5-2 wt %
toluene solutions onto water surface and then transferring the
thin films onto a copper TEM grid. The thin films on the grids
were annealed at 105 °C for 24 h and then exposed to RuO4
vapor at room temperature for 2 min to selectively stain the
PEG block.22
Ma ter ia ls. Anisole as the solvent for solution polymeriza-
tion was purified by distillation from sodium with benzophe-
none. Catalyst Cu(I)Cl (Kanto Chem. Co., J apan) was washed
successively with acetic acid and ether, then dried, and stored
under nitrogen. 2-Bromo-2-methylpropionyl chloride, 11-bromo-
1-undecanol, 4-butylaniline, methacrylic acid, and dicyclo-
hexylcarbodiimide (DCC), commercially available from Kanto
Chem. Co (J apan), were used without further purification.
Poly(ethylene glycol) methyl ether with number-average mo-
lecular weight of about 2000 (Aldrich) was dried by azeotropic
distillation with toluene before use. The ligand 1,1,4,7,10,10-
hexamethyldiethylenetriamine (HMTETA, Aldrich) was used
as received without further purification.
conv (%) ) (I2.66 - 2I6.08)/I2.66 × 100%
(1)
The solution was passed through a neutral Al2O3 column with
THF as eluent to remove the catalyst. The yellow filtrate was
concentrated under reduced pressure and reprecipitated twice
into methanol. The yellow polymer was collected by filtration
and dried under vacuum. Yield: 270 mg (54%). Mn (GPC) )
8300, Mw/Mn ) 1.10.
P r ep a r a tion of P EG Ma cr oin itia tor s (5). The synthesis
of the macroinitiator is shown in Scheme 2.
A solution of 1.8 g (7.7 mmol) of 2-bromo-2-methylpropionyl
chloride in 10 mL of dry THF was added to a mixture of
1.1 g (10 mmol) of triethylamine and 10 g (5 mmol) of PEG
methyl ether with an Mn of 2000 in 30 mL of THF at 0 °C,
and then the mixture was stirred for 18 h. After the mix-
ture was filtered, half of the solvent was evaporated, and
the PEG macroinitiator was precipitated into cold ether.
After dissolution in ethanol, the solution was stored in
refrigerator to recrystallize the product. Yield: 60%. 1H NMR
(CDCl3), δ (ppm): 4.33 (dd, 2H, -OCH2COO-), 3.73 (m, 158H,
-CH2-), 3.38 (s, 3H, -OCH3), 1.94 (s, 6H, (CH3)2CBrCOO-).
Mn(GPC) (RI detector) ) 3500, Mw/Mn ) 1.03, and Mn(NMR)
) 1900.
P r ep a r a tion of Azobezen e Mon om er . The synthesis of
the monomer is illustrated in Scheme 1 according to the
normal organic synthetic procedure.23 The purity of the
monomer 11-[4-(4-butylphenylazo)phenoxy]undecyl methacry-
late (1) was guaranteed by IR, NMR, and elemental analy-
sis.24
P r ep a r a tion of th e Block Cop olym er s. Block copolymers
were synthesized by using the analogous procedure of the
homopolymers of P 1 except for a use of 5 as the macro-
initiator.
P r ep a r a tion of Hom op olym er s. Homopolymers, P 1a -
P 1e, were synthesized by using Cu(I)Cl complexed with
HMTETA as the catalyst and ethyl 2-bromoisobutyrate as the
Exa m p le of th e Syn th esis of P 2d . A 4 mg (0.04 mmol) of
Cu(I)Cl, 19 mg (0.01 mmol) of 5, and 570 mg (1.15 mmol) of
the monomer 1 were mixed in a 10 mL ampule bottle,

Macromolecules, Vol. 35, No. 9, 2002
LC Block Copolymers with Azobenzene Moieties 3741
Ta ble 1. Exp er im en ta l Con d ition sa a n d P r op er ties of th e P olym er s Syn th esized by th e ATRP Meth od
b
i
sample
[M]0/[I]0
timec
convd (%)
Mn(GPC)e
Mn (calc)
Mn (NMR)h
Mw/Mn
LC content (%)
yield (%)
P 1a
P 1b
P 1c
P 1d
P 1e
P 2a
P 2b
P 2c
P 2d
10
20
40
60
80
115
115
115
115
18
20
20
20
20
2
4
8
12
90
87
88
90
80
20
32
60
80
6 100
8 300
4 600f
8 800f
1.12
1.10
1.13
1.16
1.17
1.08
1.09
1.09
1.11
100
100
100
100
100
86j
66
54
62
68
64
10
24
45
63
13 600
21 500
23 500
15 900
18 700
27 800
35 600
17 500f
23 800f
31 700f
13 200g
20 000g
35 800g
47 200g
13 700
21 600
34 400
53 600
91j
94j
96j
a
[Initiator]:[HMTETA]:[Cu(I)Cl] ) 1:2:2. Initiators for P 1 and P 2 are ethyl 2-bromoisobutyrate and 5, respectively. Polymerization
temperature is 80 °C. Feed molar ratio of the monomer [M]0 to initiator [I]0. c Polymerization time in hours. Conversion determined
b
d
by 1H NMR from eq 1 in the text. e Number-average molecular weight, Mn(GPC), determined by GPC. f Mn(calc) calculated according to
g
Mn(calc) ) 195 + [M]0/[I]0 × conv × 492, where 195 and 492 are the molecular weights of initiator and monomer, respectively. Mn(calc)
calculated according to eq 3. Mn(NMR)determined by 1H NMR according to eq 2. Polydispersity determined by GPC. j LC weight fraction
h
i
determined by 1H NMR.
degassed, and filled with nitrogen. An 11 µL (15 mg, 0.1 mmol)
of HMTETA in 6 mL of anisole was added through a syringe.
The mixture was degassed three times using the freeze-
pump-thaw procedure and sealed under vacuum. After 30 min
stirring at room temperature, the ampule was placed in the
1
preheated 80 °C oil bath for 12 h. A solution was taken for H
NMR measurement. Conversion was 80%. The solution was
passed through a neutral Al2O3 column with THF as eluent
to remove the catalyst. The yellow filtrate was concentrated
under reduced pressure and reprecipitated twice into acidic
methanol. The yellow polymer was collected by filtration and
dried under vacuum. Yield: 365 mg (63%). Mn(GPC) ) 35 600,
Mw/Mn ) 1.11.
Resu lts a n d Discu ssion
Syn th esis. For the study of the influence of the
F igu r e 1. GPC curves of the macroinitiator 5 (a) and its
degree of polymerization on the liquid crystalline prop-
erties, five homopolymers with azobenzene units, P 1a -
P 1e, were synthesized by using ethyl 2-bromoisobu-
tyrate as the initiator and CuCl complexed by HMTEMA
as the catalyst. Detailed experimental conditions and
results are given in Table 1. For all of the polymeriza-
tions, the initial monomer concentration ([M]0) was kept
constant ([M]0 ) 0.2 M), whereas the monomer/initiator
molar ratio, [M]0/[I]0, was systematically changed from
10 for P 1a to 80 for P 1e, to control their molecular
weights by the [M]0/[I]0. The molar mass characteristics
were determined by using GPC. All of the homopoly-
mers, P 1a -P 1e, show monomodal GPC curves that
shift toward lower values along the elution volume
scales as the increases of the [M]0/[I]0. Number-average
molecular weights (Mn(GPC)) from 6100 to 23 500 g/mol
and narrow polydisperisties (Mw/Mn) in the range of
1.10-1.17 were evaluated, which implied the polymer-
ization of 1 could be controlled well under the above
conditions.
diblock copolymers P 2a (b), P 2b (c), P 2c (d), and P 2d (e). For
5, Mn(GPC) ) 3500, Mw/Mn ) 1.03. For P 2a , Mn(GPC) )
15 900, Mw/Mn ) 1.08. For P 2b, Mn(GPC) ) 18 700, Mw/Mn )
1.09. For P 2c, Mn(GPC) ) 27 800, Mw/Mn ) 1.09. For P 2d ,
Mn(GPC) ) 35 600, Mw/Mn ) 1.11.
F igu r e 2. First-order kinetic plots for the ATRP of azobenzene
monomer 1 initiated by 5 mediated by HMTETA and copper-
(I) chloride in anisole at 80 °C.
Four amphiphilic LC-coil diblock copolymers, P 2a -
P 2d , were synthesized from 5 as the hydrophilic coil
macroinitiator by controlling the degree of polymeriza-
tion of 1 as a function of time. Detailed experimental
conditions and the measured average molecular weights
are also given in Table 1. Figure 1 shows the GPC
profiles of the four block copolymers P 2a -P 2d and their
starting macroinitiator 5. The GPC curves correspond-
ing to the block copolymers P 2a -P 2d shifted to high
molecular weights with the increase of polymerization
time, implying a well-controlled living polymerization.
No residual precursor 5 was detected, suggesting that
the macroinitiator 5 work efficiently to initiate the
copolymerizations. Figure 2 gives the kinetic plot of
polymerization of the azobenzene monomer 1 with 5 as
the macroinitiator in anisole solution. A nearly linear
relationship was observed, implying the controlled living
character of the block copolymerization by using the
ATRP method. Figure 3 gives the dependence of molec-
ular weight and polydispersity on monomer conversion.
It can be seen clearly that the block copolymers exhibit
narrow polydispersities in the range from 1.08 to 1.11,
and the molecular weight of the block copolymers
increases regularly with the increase of the conversion
of the azobenzene monomer 1, indicating the living
character of the block copolymerization. Estimation of
the relative compositions of 1 to 5 in the block copoly-
mers was derived from the integration of the oxymeth-
ylene protons at 3.9 ppm (I3.9) (c and e in Figure 4) of
the azobenzene block to that of the oxyethylene pro-
tons of PEG block at 3.7 ppm (I3.7) (b in Figure 4). The

3742 Tian et al.
Macromolecules, Vol. 35, No. 9, 2002
F igu r e 3. Dependence of molecular weight (Mn(GPC)) of
P 2a -P 2d and polydispersity (Mw/Mn) on the conversion of
azobenzene monomer 1.
F igu r e 5. DSC curves of the homopolyers P 1 on the first
cooling (A) and second heating (B) process with a heating/
cooling rate of (10 °C/min. Curves a, b, c, d, and e are for
P 1a , P 1b, P 1c, P 1d , and P 1e, respectively.
On further cooling below 100 °C, slight texture change
but clear birefringence change were observed as shown
in Figure 6B, suggesting a smectic C phase (SmC). On
the further cooling below 74 °C, the color caused by the
birefringence changed and the textures (Figure 6C)
became obscure, indicating an other kind of smectic
phase (SmX).25 Results of WAXD also confirmed the
smectic phases. Figure 7 shows the typical temperature-
dependent WAXD patterns of P 1e. At room temperature
the diffraction pattern is constituted by three diffraction
peaks (d001, d002, and d003) in the small-angle regions
at a periodicity of d001 ) 3.10 nm. The layer spacing of
d001 is a little shorter than the length of the fully
extended monomer molecular length (L ) 3.37 nm,
calculated by MM2 force field method), thus suggesting
a tilted monosmectic layer (SmX) of the side group with
a tilted angle of 23° to the layer normal. On heating to
90 °C, the layer spacing d001 changed to 3.21 nm,
suggesting the slightly tilted SmC phase with an angle
of 15° to the layer normal. Actually, the tiled angles
decrease from 23° to 0° with the increasing of the
temperature from 70 to 115 °C, confirming the tilted
SmC phase. On heating to 115 °C, the small-angle
Bragg diffraction shifts to d001 ) 3.39 nm, suggesting
the monolayer SmA phase. It can also be seen that on
heating to SmA and SmC phases the wide-angle diffu-
sion at 2θ ≈ 20° becomes much broader, which sug-
gested a loss of the lateral correlation among the
molecular side groups within the SmA and SmC phases.
The strong lateral correlation among the LC segment
at room temperature suggested the SmX is a high-order
smectic phase; however, we did not do further research
of its nature. Hence, the mesophase sequence of poly-
mers 1 can be summarized as follows: SmX-SmC-
SmA-I. A possible schematic representation of molec-
ular orientation in the LC phases is given in Figure 8.
The transition temperatures and transition enthalpies
of the homopolymers are collected in Table 2. The
transition temperatures increase with the increase of
molecular weights and level off at an Mn(GPC) less than
21 500, while the transition enthalpies level off at an
Mn(GPC) less than 13 600.
F igu r e 4. Typical 1H NMR spectrum of block copolymer P 2c
in CDCl3.
calculated molecular weight based on NMR (Mn(NMR))
is given by eq 2:
Mn(NMR) ) 1900 + 40 × (I3.9/I3.7) × 492
(2)
where 1900 and 492 are the molecular weights of PEG
initiator and the azobenzene monomer, respectively. The
theoretical molecular weight (Mn(calc)) is given by the
conversion and feed ratio of the monomer to initiator
by using eq 3:
Mn(calc) ) 1900 + [M]0/[I]0 × conv × 492 (3)
When the conversion is lower than 60%, the Mn(calc)
and Mn(NMR) are in accordance well based on the
controlled living character.
Th er m a l P r op er ties. Thermal properties of the
polymers were investigated by using differential scan-
ning calorimetry (DSC), polarized optical microscopy
(POM), and wide-angle X-ray diffraction (WAXD).
The azobenzene monomer 1 melted at 80 °C on
heating and crystallized at 72 °C on cooling with no
indication of LC character. However, all of the polymers
exhibit LC properties. Figure 5 shows the DSC curves
of the polymer P 1a -P 1e on their second heating and
first cooling procedures. In the case of P 1e, on heating,
two well-defined endothermic transitions at 74 and 123
°C and a very small transition (transition enthalpy
around 0.2 J g-1) around 109 °C are observed, respec-
tively, which are reversible on cooling with a few degree
of supercooling effect. These transitions were attributed
to three LC transitions. POM was used to identify the
mesophases. On cooling of P 1e, focal conic-fan shaped
textures (Figure 6A) were observed in the temperature
range 122-101 °C, suggesting a smectic A (SmA) phase.
Figure 9 gives the DSC curves of the block copolymers
P 2a -P 2d on the second heating process. Two clear
endothermic transitions at about 70 and 120 °C and a
very small transition at about 100 °C were observed,
which should be associated with the SmX-SmC, SmC-

Macromolecules, Vol. 35, No. 9, 2002
LC Block Copolymers with Azobenzene Moieties 3743
results also confirmed the transitions and mesophases.
For example, in the case of P 2c, the layer spacing (d001
)
at 30, 90, and 110 °C are 3.12, 3.17, and 3.38 nm,
respectively, which are in accordance with those of
homopolymers. The melting point of the PEG segment
in the block copolymers was not detected by thermal
analysis. It may be attributed to the high weight
fraction of the LC block as shown in Table 1 and/or the
enthalpy loss caused by the disordered interface be-
tween the PEG and LC blocks.5,19-21,26
Based on the thermal investigations, their phase
transitions and transition enthalpies of the block co-
polymers are also shown in Table 2. According to Figure
9 and Table 2, it can be seen that the transition
temperatures and enthalpies of P 2a -P 2d increase with
the increase of LC fractions. However, the enthalpies
associated with the LC transitions are lower than that
expected on the basis of the weight compositions of the
LC blocks in the block copolymers. The phenomenon
becomes much clearer if we consider the molecular
weights of the azobenzene blocks, as shown in Figure
10. Such a kind of phenomenon was observed in several
kinds of LC-coil block copolymers,5,19-21,26 which was
explained as resulting from the disordering of the LC
structures near the microphase interface.
TEM Obser va tion of Na n ostr u ctu r es. The nano-
structures were imaged by TEM observation of their
thin films. Figure 11 gives the typical TEM images of
P 2a -P 2d . For all of the four block copolymers, PEG
blocks exhibit as cylinders and/or spheres with a width/
diameter about 2-3 nm dispersed into the LC matrix.
For P 2a (Figure 11A), the PEG cylinders have a per-
pendicular orientation to the surface, and the cylinders
exhibit several different directions on the surface. The
orientation of the cylinders within the film can be nicely
illustrated by tilting the sample in the electron micro-
scope. The boundaries of the different directions of the
cylinders on the surface before tilting become much
clearer after the samples were tilted to 10° (Figure 11B)
and 25° (Figure 11C); meanwhile, the parallel oriented
cylinders become visible when the samples are tilted.
Therefore, the above results clearly illustrated the PEG
domains are perpendicularly oriented cylinders. For
P 2b, PEG cylinders with orientations parallel and
perpendicular to the surface (Figure 11D) were ob-
served. For P 2c, the morphologies become much sim-
pler; only uniform aligned hexagonal cylinders with
perpendicular orientation to the surface (Figure 11E)
were observed. With the further increase of the LC
fraction to 96 wt % (P 2d , Figure 11F), the PEG block
exhibits as spheres dispersed into the LC matrix,
because no morphological change was observed after the
sample was titled. The cylinder structures in the present
thin films with high LC fractions (86-94 wt %) are
unusual, different from the widely accepted correlation
between LC fraction and nanostructure in the bulk. A
further investigation to explain it is still required, but
the following two issues are added here: At high LC
fractions the smectic alignment of the side chain plays
a great effect on the nanostructures of the LC-coil block
copolymers.27 Very recently, during the reviewing pro-
cess of the paper, Anthamatten et al. reported a block
copolymer with 85 wt % of LC fraction exhibiting
hexagonal coil polystyrene domains dispersed into bi-
layer LC structures.27 The morphology of the thin film
may be different from that of bulk film because the
surface free energy also plays an important role for the
F igu r e 6. Typical polarized optical micrographs of P 1e at 115
(A), 90 (B), and 30 °C (C) (magnification: ×200).
F igu r e 7. Temperature-dependent wide-angle X-ray power
diffraction of P 1e at 30 (a), 90 (b), and 115 °C (c).
SmA, and SmA-I transitions based on the thermal
properties of P 1. The temperature-dependent WAXD

3744 Tian et al.
Macromolecules, Vol. 35, No. 9, 2002
F igu r e 8. A possible schematic illustration of the layer structures of the LC phases. L: extended molecular length, 3.37 nm,
calculated by the MM2 force field method. d001: the layer spacing elucidated from the X-ray patterns of Figure 7. θ: tilt angle of
the smectic phases to the layer normal. The closed and open ellipsoids show the mesogenic groups directed upward and downward,
respectively.
Ta ble 2. Th er m a l P r op er tiesa of th e Syn th esized P olym er s
transition temp (T/°C)
enthalpies (∆H/J g-1
∆HSmC-SmA
)
sample
Tm/°C
TSmX-SmC
TSmC-SmA
TSmA-I
∆HSmX-SmC
∆HSmA-I
8.00
8.42
8.53
8.48
8.49
P 1a
P 1b
P 1c
P 1d
P 1e
5
P 2a
P 2b
P 2c
P 2d
65
69
72
74
74
105b
113
117
122
123
1.99
2.20
2.27
2.47
2.40
104
106
108
109
≈0.2
≈0.2
≈0.2
≈0.2
52
70
70
74
74
100
102
107
109
116
118
122
124
0.53 (2.06)c
1.00 (2.18)c
1.00 (2.26)c
1.00 (2.30)c
d
d
d
d
6.50 (7.30)e
7.51 (7.73)e
7.52 (7.98)e
7.98 (8.15)e
a
b
Transition temperatures and enthalpies on the second heating process were determined by DSC at 10 °C/min. Mixed transitions of
c
SmC-SmA and SmA-I. The ∆HSmX-SmC shown in parentheses was calculated from the weight fraction of LC block and the ∆HSmX-SmC
of homopolymer P 1e. Not detected. e The ∆HSmA-I shown in parentheses was calculated from the weight fraction of LC block and the
d
∆HSmA-I of the homopolymer P 1e.
F igu r e 9. DSC curves of the macroinitiator 5 (a) diblock
F igu r e 10. Relationship of the transition enthalpies at
copolymers P 2a (b), P 2b (c), P 2c (d), and P 2d (e) on the second
SmA-I transitions of homopolymers (b) and block copolymers
heating process. Heating rate is 10 °C/min.
([) with the degrees of the polymerization (DP) of the
azobenzene units derived from the Mn(calc).
nanostructures in the thin film state.28 The distances
between PEG domains of P 2a , P 2b, P 2c, and P 2d are
about 10, 12, 15, and 19 nm, respectively, increasing
with the increase of the LC fractions as expected.
Therefore, the influence of the LC fractions in the block
copolymers on the size and arrangement of the nano-
structures were observed clearly by using the four
amphiphilic block copolymers. The successful control-
lability of the nanostructures provides the possibility
of the investigation of nanostructure-specific functional-
ity by using photophysical and photochemical properties
as the probe.
tures with functionality, homopolymers and block
copolymers were compared. As a typical example, the
results of homopolymer P 1e and diblock copolymer P 2c
with close molecular weights of the azobenzene units
(Mn(calc) as shown in Table 1) were described. Figure
12 illustrates the UV-vis spectra of the as-cast films
and annealed films. It can be seen that the as-cast films
of the two polymers exhibited similar absorption maxima
at 337 nm due to the π-π* long axis transition of the
azobenzene unit. After being annealed at 105 °C in its
smectic phase, the homopolmyer P 1e film exhibited an
18 nm blue shift, implying a strong H-aggregation (face-
to-face) of the azobenzene chromophore in LC state;
meanwhile, the absorbance of the fresh as-cast P 1e film
Na n ost r u ct u r e-Sp ecific P h ot och em ica l F u n c-
tion of Azoben zen e Ch r om op h or e. To obtain infor-
mation about the relationship between the nanostruc-

Macromolecules, Vol. 35, No. 9, 2002
LC Block Copolymers with Azobenzene Moieties 3745
F igu r e 11. TEM micrographs of the block copolymers P 2a (A), P 2a (B, tilted angle is 10°), P 2a (C, tilted angle is 25°), P 2b (D),
P 2c (E), and P 2d (F). PEG block appeared as dark. The solid lines in A, B, and C represent the domain boundaries.
F igu r e 12. UV-vis spectra of the as-cast (a) and annealed films (b) of homopolymer P 1e (A) and block copolymer P 2c (B). The
fresh film was cast from 1 wt % solution in toluene onto quartz glass. Annealing condition is at 105 °C (smectic phase) for 24 h.
decreased more than 60% after annealing, indicating
homeotropic alignment of the chromophores to the
substrate plan. However, only a slight decrease of
absorbance and no blue shift were observed in the case
of block copolymer P 2c. The other block copolymers
exhibited consistent results with that of P 2c. These
differences might be attributed to the formed mi-
crophase-separated structures between the PEG block
and azobenzene side-chain LC block in the nanosepa-
rated thin film. Probably, the formed nanostructures
hinder the formation of larger LC domain and further
chromophore aggregation within the small LC domain
of the microphase-separated block copolymer films,
which caused the different photophysical behavior
between the homopolymers and block copolymers.8
decrease of the absorbance maximum at 337 nm and
the increase of the absorbance maximum at 460 nm,
with the irradiation of 340 nm light. However, their
photoreactive processes are quite different between their
annealed films as shown in Figure 13. A complicated
photochemical procedure was observed in the case of the
annealed P 1e film with the irradiation of λ ≈ 340 nm.
In the early stage of the irradiation for 0.5 min, the
absorbance at 338 nm decreased successively, indicating
simple trans-cis isomerization. However, with further
irradiation (0.5-2 min), the absorbance increased
smoothly, and the spectra became much broader. (The
shoulder at 354 nm was assigned to the isolated
monomeric absorption.) These processes might be re-
lated with the reorganization of the azobenzene chro-
mophores during the trans-cis isomerization due to
insufficient free volume caused by the strong H-ag-
The as-cast P 1e and P 2c films exhibit a similar
simple trans-cis photoreactive process with successive

3746 Tian et al.
Macromolecules, Vol. 35, No. 9, 2002
F igu r e 13. Photochemical processes of the annealed films of homopolymer P 1e (A and C) and block copolymer P 2c (B and D)
with the irradiation of 340 nm light (A and B) and 440 nm light (C and D), respectively. For A and B: (a) initial spectrum before
irradiation; (b) 0.5 min irradiation; (c) 2 min irradiation; (d) 8 min irradiation corresponding to the photostationary state. For C
and D: the irradiation times of a, b, c, d, e, f, g, and h are 0 s, 10 s, 20 s, 30 s, 1 min, 2 min, 4 min, and 8 min, respectively.
gregation and perpendicular alignment of the chro-
mophores.8 With further irradiation to 8 min, the
reaction reached its photostationary state. However,
simple photochemical process was observed in the case
of the annealed film of block copolymer P 2c, similar to
its fresh as cast film, implying its simple trans-cis
isomerization process. These findings are analogous to
the results reported by Moriya et al.8 by using polysty-
rene-b-poly{6-[4-(4-cyanophenylazo)phenoxy]hexyl meth-
acrylate}.
The back-reaction processes of the two polymer films
with the irradiation by using visible light at 440 nm are
also given in Figure 13. For each of the two polymers
isolated monomer absorption with a maximum at 354
nm was observed clearly during the cis-trans proce-
dure. The absorbance of the final state of P 1e is much
higher than the original annealed film, showing that
the homeotropic alignment and the H-aggregation of the
azobenzene chromophores formed by annealing in its
smectic state were broken by the photoisomerization
process. However, the absorbance of the photostationary
state of block copolymer, P 2c, is about 90% of its
original annealed film.
cylinder and/or spherical morphologies of PEG block
dispersed into LC block were clearly observed after
annealing at 105 °C (smectic phase) for 24 h. The sizes
of nanoseparated structures are in the range of 10-20
nm, increasing with the increase of LC fraction. Pho-
tophysical and photochemical investigations revealed
the annealed homopolymer thin films exhibited strong
H-aggregations of the azobenzene chromophores and
complicated trans-cis photochemical processes; how-
ever, the annealed block copolymers did not show the
obvious H-aggregations and exhibited simple photo-
chemical processes. The significant different behaviors
among block copolymers and azobenzene homopolymers
are considered to be attributed to the formed nanostruc-
tures of the block copolymers. Such findings may
provide the opportunities of new application of azoben-
zene block copolymers as for new optical storage and
holography materials.
Ack n ow led gm en t. We thank Mr. Yoshishige Mis-
aki at Tokyo Metropolitan University for his assistance
of the TEM measurement. This work was partially
supported by Grants-in-Aid for Scientific Research (A)
(No. 11305061) and Scientific Research on Priority Area
(B) of “Development of Molecular Conductors and
Magnets by Spin-Control” (No. 730/11224207) from the
Ministry of Education, Science, Sports, and Culture,
J apan. Y.-Q.T. and X.K. thank the J apan Society for The
Promotion Science for a Postdoctoral Fellowship and
financial support from the Ministry of Education, Sci-
ence, Sports, and Culture, J apan.
Therefore, the above results clearly show that the
homopolymers and block copolymers exhibit quite dif-
ferent photophysical and photochemical properties. Such
different behaviors might relate with the formed nano-
structrures of the block copolymers. Further investiga-
tions of the effect of photochemical process on nano-
structures are in progress, which will be reported in
another paper.
Con clu sion s
Refer en ces a n d Notes
We have synthesized a new series of well-defined LC
homopolymers and amphiphilic LC-coil diblock copoly-
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polydispersities in the range of 1.08-1.11 by using the
newly developed ATRP method. Thermal investigation
shows that each of the polymers exhibits monolayer
SmA, SmC, and SmX phases. For the block copolymers,
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1
(24) Yellow crystal; mp 82.0-82.2 °C. H NMR (CDCl3), δ (ppm):
7.90-7.81 (dd, 4H, phenyl proton), 7.31-7.28 (d, 2H, phenyl
proton), 7.00-6.97 (m, 2H, phenyl proton), 6.10 (s, 1H,
H-C)), 5.54, (s, 1H, H-C)), 4.11 (t, 2H, CH2O), 4.03 (t, 2H,
CH2O), 2.68 (t, 2H, CH2Ph), 1.94 (s, 3H, CH2dC(CH3)-), 1.82
(m, 2H, CH2), 1.70-1.31 (m, 20H, (CH2)10), 0.94 (t, 3H,
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MA011859J