Photocontrolled microphase separation in a nematic liquid-crystalline diblock copolymer

Article abstract of DOI:10.1016/j.polymer.2011.01.053

Using a bromo-terminated poly(ethylene oxide) as a macroinitiator, an amphiphilic liquid-crystalline (LC) diblock copolymer with an azobenzene moiety as a nematic mesogen was prepared by an atom transfer radical polymerization process. In thin films of th

Full text of DOI:10.1016/j.polymer.2011.01.053

Polymer 52 (2011) 1554e1561
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Polymer
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Photocontrolled microphase separation in a nematic liquidecrystalline
diblock copolymer
,
b
,
b
c
Haifeng Yu a , Takaomi Kobayashi , Guo-Hua Hu
*
a Top Runner Incubation Center for Academia-Industry Fusion, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka 940-2188, Japan
b Department of Materials Science and Technology, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan
c Laboratory Reactions and Process Engineering, Nancy University, CNRS-ENSIC-INPL, 1 rue Grandville, BP 20451, 54001 Nancy, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Using a bromo-terminated poly(ethylene oxide) as a macroinitiator, an amphiphilic liquidecrystalline
(LC) diblock copolymer with an azobenzene moiety as a nematic mesogen was prepared by an atom
transfer radical polymerization process. In thin films of the well-defined diblock copolymer with the
mesogenic block as a continuous phase upon microphase separation, the influence of supramolecular
cooperative motion on the microphase-separated nanocylinders was systematically studied. Although
the major phase of the hydrophobic nematic LC block showed only one-dimensional order, it could
endow the separated minor phase of the hydrophilic PEO nanocylinders with three-dimensionally
ordered structures. Both out-of-plane perpendicular and in-plane parallel patternings of the regularly
ordered nanocylinder arrays were successfully fabricated on macroscopic scales by thermal annealing
and photoalignment, respectively. The microphase-separated nanostructures with high regularity
showed excellent reproducibility and mass production, which might guarantee nanotemplated fabrica-
tion processes and would lead to novel industrial applications in macromolecular engineering.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 26 October 2010
Received in revised form
25 January 2011
Accepted 29 January 2011
Available online 24 February 2011
Keywords:
Liquidecrystalline diblock copolymer
Microphase separation
Supramolecular cooperative motion
1. Introduction
manipulated into an ordered array along with the alignment
direction of the mesogenic block.
Liquid crystals (LCs) and block copolymers (BCs) are two kinds of
soft materials with self-organizing nature, which are elegantly
combined in one organic system of liquidecrystalline block
copolymers (LCBCs). On the one hand, LCBCs inherit LC perfor-
mances such as self-assembly, long-range ordered fluidity, molec-
ular cooperative motion, formed large birefringence, anisotropy in
various physical properties (optical, electrical and magnetic fields),
and alignment change by external fields at surfaces and interfaces
[1,2]. On the other hand, they can microphase separate into varie-
ties of nanostructures like spheres, cylinders, lamellae phase
domains with an increase in volume ratio of the LC block [3e9]. In
thin films of LCBCs, the interplay function between the microphase
separation and the elastic deformation of LC ordering, known as
supramolecular cooperative motion (SMCM) [10e12], plays an
important role in formation of hierarchical structures, which
enables their microphase-separated nanostructures to be
According to SMCM, we systematically studied the microphase-
segregated behaviors of a series of novel amphiphilic smectic LCBCs
consisting of flexible poly(ethylene oxide) (PEO) as a hydrophilic
segment and polymethacrylate containing an azobenzene (AZ)
moiety in side chain as a hydrophobic LC block [10e15]. Upon
microphase separation, the PEO-based LCBCs showed regular
patternings of the PEO nanocylinder array as the minor phase,
periodically dispersed in the AZ LC matrix. A vertically orientated
pattern of PEO nanocylinders was obtained upon annealing with
the help of out-of-plane alignment of AZ mesogens showing
a smectic LC phase [10e17]. Whereas a homogeneously orientated
pattern of PEO nanocylinders was fabricated by the in-plane
homogeneous alignment of AZ mesogens induced either by
a rubbing technique or a polarized laser beam [10e12]. It was
believed that the layer structure of the smectic LC block might be
one of the key factors to fabricate the homeotropic and homoge-
neous alignment of PEO nanocylinder arrays on macroscopic scales
[10e12]. The SMCM exerts a great effect on LCBCs and enables the
phase-segregated nanostructures to be arranged in an alignment
direction perpendicular to the layers of the smectic LC with two-
dimensional (2D) order. However, the influence of SMCM on the
* Corresponding author. Department of Materials Science and Technology,
Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-
2188, Japan.
E-mail address: yuhaifeng@mst.nagaokaut.ac.jp (H. Yu).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.01.053

H. Yu et al. / Polymer 52 (2011) 1554e1561
1555
microphase separation of LCBCs possessing a nematic LC phase
with one-dimensional (1D) order still remains unclear.
2.1.3. 6-(4-(4-Ethoxyphenylazo)phenoxy)-hexyl methacrylate
(M6ABOC2)
In this paper, we report on the microphase-separated behaviors
in thin films of a PEO-based amphiphilic LCBC with AZ moieties as
nematic mesogens under the influence of SMCM. Although
a nematic LC shows lower ordering than a smectic one, it often has
a lower viscosity than the latter. Moreover, AZ-containing nematic LC
polymers have been extensively studied as photonic materials
because of their quick response to light [2, 18e22]. It is expected that
microphase-separated nanostructures can also be supramolecularly
assembled in the designed nematic LCBC with well-defined struc-
tures and the formed phase-segregated nanostructures can be
quickly manipulated by polarized light due to the existence of
photoresponsive AZs as nematic mesogens.
4-Ethoxy-40-(6-hydroxy hexyloxy) azobenzene (1.5 g, 4.3 mmol)
and triethylamine (0.26 g, 4.5 mmol) were dissolved in THF (30 mL)
at 0 ꢀC in an ice bath. Then a solution of methacryloyl chloride
(0.23 g, 4. 5mmol) in THF (10 mL) was added dropwise under the
protection of N2. The reaction mixture was stirred overnight.
100 mL deionized water was added, and the product was extracted
with chloroform. After purification by column chromatography on
silica gel (chloroform) and then recrystallization from methanol to
obtain the purified product (1.4 g, 3.4 mmol) in 80% yield. 1H NMR
(d, CDCl3, 300 MHz): 1.17 (t, 3H), 1.36e1.42 (m, 4H), 1.64 (t, 2H), 1.74
(t, 2H),1.88 (s, 3H), 3.97 (q, 2H), 4.03e4.12 (m, 4H), 5.48 (d,1H), 6.04
(d, 1H), 6.92 (m, 4H), 7.78 (m, 4H). lmax (chloroform) ¼ 360 nm.
2.2. Synthesis of the macroinitiator PEO-Br
2. Experimental section
The macroinitiator was prepared by a SchötteneBaumann
reaction between 2-bromo-2-methylpropionyl chloride and poly
(ethylene glycol) monomethyl ether in dichloromethane (CH2Cl2)
with a yield of about 32% (12). The molecular weight (GPC, relative
to PS standards), polydispersity index and the melting temperature
were 7400 g/mol, 1.02, and 64 ꢀC, respectively.
2.1. Materials
2-Bromo-2-methylpropionyl chloride, triethylamine, phenol,
6-chloro-1-hexanol, methacryloyl chloride, N,N-dimethylforma-
mide (DMF), 4-ethyloxaniline, hydroquinone, sodium nitrite,
potassium carbonate and hydrochloric acid were commercially
available (Kanto Chem. Co.) and used without further purification.
Poly(ethylene glycol) monomethyl ether with a number-averaged
molecular weight of about 5000 g/mol (Aldrich) was dried by
azeotropic distillation with toluene. Anisole and tetrahydrofuran
(THF) were purified by distillation from sodium with benzophe-
none. The ligand, 1,1,4,7,10,10-hexamethyltriethylenetetramine
(HMTETA, Aldrich) was used as received without further purifica-
tion. Catalyst copper (I) chloride (Cu(I)Cl) was washed successively
with acetic acid and ether, then dried.
2.3. Nematic LCBC (PEO-b-PM6ABOC2)
Cu(I)Cl (7.43 mg, 0.075 mmol), PEO-Br (125 mg, 0.025 mmol)
and M6ABOC2 (923 mg, 2.25 mmol) were mixed in a 20 mL
ampoule, degassed and filled with argon. HMTETA (20.4
mL,
0.075 mmol) in anisole (5.0 mL) was added. The mixture was
degassed by three freeze-pump-thaw cycles and sealed under
vacuum, and placed in an oil bath preheated at 80 ꢀC for 24 h. Then
the solution was passed through a neutral Al2O3 column to remove
the catalyst. The filtrate was precipitated into methanol. The
copolymer was collected and dried. Yield: 742 mg (71%). Mn
(GPC) ¼ 31, 200, Mw/Mn ¼ 1.17. The LCBC showed good solubility in
organic solvents, such as THF, chloroform, toluene, DMSO, DMF and
anisole.
2.1.1. 4-Ethoxy-40-hydroxyazobenzene
4-Ethoxylaniline (20 g, 0.146 mol) was dissolved in a solution of
hydrochloric acid (280 mL, 2 M) and the resulting solution was
cooled at 0 ꢀC. Sodium nitrite (10.1 g, 0.146 mol) dissolved in water
(200 mL) was added dropwise into the solution to produce diazo-
nium salt. A mixture of phenol (13.7 g, 0.146 mol) and sodium
hydroxide (17.5 g, 0.438 mol) dissolved in waster (250 mL) was
added slowly at 0 ꢀC, and then a yellow solid precipitated. The
reaction mixture was stirred at room temperature for 2 h. After
hydrochloric acid (2 M) was added to the reaction mixture to give
pH ¼ 3, the precipitated solid was collected and extracted with
chloroform. The chloroform layer was dried with magnesium
sulfate. After the solvent was removed, the crude product was
purified by column chromatography on silica gel (ethyl acetate:
n-hexane ¼ 3:1) and then recrystallization from ethanol to obtain
the purified product (22.5 g, 0.092 mol) in 64% yield. 1H NMR
2.4. Characterization
1H NMR spectra were measured using a Lambda-300 spec-
trometer operating at 300 Hz with tetramethylsilane as an internal
reference for chemical shifts. The molecular weights of polymers
were determined by gel permeation chromatography (GPC, JASCO)
with standard polystyrenes in chloroform as eluent. The thermal
properties of the monomers and polymers were analyzed by
a differential scanning calorimeter (DSC, Seiko) at a heating and
cooling rate of 10 ꢀC/min. At least three scans were performed to
check the reproducibility. The LC phases were evaluated with
a polarizing optical microscope (POM, Olympus BH-2) equipped
with a hot stage. The UVevis absorption spectra were measured
using a JASCO V-550 spectrophotometer.
(CDCl3, 300 MHz):
(m, 4H).
d 1.5 (t, 3H), 4.1 (q, 2H), 5.4 (s,1H), 7.0 (m, 4H), 7.9
2.1.2. 4-Ethoxy-40-(6-hydroxyhexyloxy) azobenzene
A mixture of 4-ethoxy-40-hydroxyazobenzene (11.0 g, 45 mmol)
and 6-chloro-1-hexanol (9.3 g, 68 mmol) was dissolved in DMF
(200 mL), and postassium carbonate (9.3 g, 68 mmol) was added to
the solution. The resulting solution was refluxed for 12 h. After the
reaction mixture was cooled to room temperature, the product was
precipitated in water and then extracted with chloroform. The
chloroform layer was dried with magnesium sulfate. After the
solvent was removed, the crude product was purified by recrys-
tallization from methanol to obtain product (13.5 g, 0.039 mol) in
2.5. Film preparation and photoalignment of LCs
The diblock copolymer films with a thickness of about 100 nm
were prepared by spin-coating its toluene solution on clean glass
substrates or silicon wafers. After the solvent was removed at room
temperature, the copolymer film was annealed at 160 ꢀC in
a vacuum oven for 24 h. Both the heating and cooling rates were
controlled at 0.5 ꢀC/min. The microphase separation of the block
copolymer films was explored at room temperature with an atomic
force microscope (AFM, SII 4000) in tapping mode.
87% yield. 1H NMR (CDCl3, 300 MHz):
4.1 (m, 4H), 7.0 (m, 4H), 7.9 (m, 4H).
d 1.2e1.9 (m, 11H), 3.7 (t, 2H),

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H. Yu et al. / Polymer 52 (2011) 1554e1561
Fig. 2 shows thermal properties of the novel amphiphilic
nematic LCBC. In its thermogram by DSC (Fig. 2a), two phase-
transition peaks on heating appeared at 43 and 154 ꢀC, corre-
sponding to the PEO melting point and the nematic LC to isotropic
phase transition, respectively. An obvious glass transition temper-
ature (Tg) was observed at 76 ꢀC, which was attributed to the AZ LC
block. This thermal property is similar to the homopolymer
PM6ABOC2, as reported by Ikeda et al. [23]. On cooling, a clearing
point appeared at 146 ꢀC due to the overcooling effect. Interest-
ingly, the PEO melting point was observed at ꢁ34 ꢀC, far lower than
that in the heating process, which might be originated from the
confined crystallization by microphase separation [24]. A typical
Schlieren texture appeared in its POM image at 120 ꢀC (Fig. 2b)
upon cooling from the isotropic phase, indicating a nematic LC
phase of the obtained LCBC.
In the UVevis absorption spectrum of the LCBC in chloroform
solution (Fig. 3), two peaks appeared at the band of about 358 nm
* and ne * transitions of AZ chro-
and 455 nm, owing to the pep p
mophores, respectively. For the as-prepared film with a thickness of
about 100 nm obtained from spin-coating, a blue shift of about
15 nm occurred in the maximum absorption peak of the
pep
*
transition, due to the formation of H-aggregation [25]. However
a shoulder peak still remained at 358 nm, resulting from the non-
aggregated AZs. Then the as-prepared film was annealed at 160 ꢀC
for 6 h, and cooled down to room temperature with a speed of
0.5 ꢀC/min. After the annealing, multiple peaks were observed as
shown in Fig. 3. The absorbance of the peak at 343 nm decreased
greatly, which might be attributed to the out-of-plane arrangement
of the AZ mesogens in thin films [10e12]. On the other hand, the
absorption peak became broader than that of the as-prepared film.
It is well-known that J-aggregation induces a red shift in the
Fig. 1. Synthetic scheme and 1H NMR spectra of the monomer M6ABOC2, the mac-
roinitiator PEO-Br and the amphiphilic nematic LCBC.
A linearly polarized laser beam from an Arþ laser (488 nm) at an
intensity of 50 mW/cm2 was used to induce alignment of AZ
mesogens. The transmittance of a HeeNe laser at 633 nm with
weak intensity was measured simultaneously during irradiation
through a pair of crossed polarizers, with the sample films between
them. After the photoalignment, the film was annealed at 150 ꢀC to
enhance the alignment. The film anisotropy was characterized with
polarized UVevis absorption spectra and POM images.
3. Results and discussion
Fig. 1 shows the synthetic scheme and 1H NMR spectra of the
monomer M6ABOC2, the macroinitiator PEO-Br and the amphi-
philic nematic LCBC. To ensure a nematic LC phase of the designed
LCBC, a typical AZ compound M6ABOC2 was used as a mesogenic
monomer for a modified atom transfer radical polymerization
(ATRP) with a bromo-substituted PEO (PEO-Br) having about 114
repeat units as a macroinitiator [11,12]. The well-specified molec-
ular structure of PEO-b-PM6ABOC2 can easily be assigned from its
NMR spectra, and about 64 repeat units of the AZ block were
estimated by the integration of the phenyl proton NMR peak at
6.82 ppm (proton c in Fig. 1) of the AZ group to that of the oxy-
ethylene protons of the PEO at 3.58 ppm (proton k and l). Refer-
encing against standard polystyrenes with chloroform as eluent,
the GPC curve yielded a molecular weight of 31 000 g/mol and
a narrow polydispersity index of 1.2, which agrees with the esti-
Fig. 2. Thermal properties of PEO-b-PM6ABOC2. (a) Heating and cooling DSC curves
(the third scan). Tg is the glass transition temperature. (b) POM image obtained at
120 ꢀC. A: analyzer; P: polarizer.
mated results from NMR analysis, indicating
molecular structure of the obtained LCBC.
a well-defined

H. Yu et al. / Polymer 52 (2011) 1554e1561
1557
separation, the as-prepared film was annealed in a vacuum oven at
160 ꢀC, a little higher than the nematic LC to isotropic phase-
transition temperature. After annealing for 2 h, obviously disor-
dered nanostructures with irregular domain sizes were observed
(see Fig. 4b), which might be due to the incomplete microphase
separation within a short annealing time. It must be mentioned
that the dark parts in the images were PEO domains due to the
difference in elastic modulus between the hydrophilic PEO block
and the hydrophobic AZ mesogenic segment. Both nanodots and
nanostripes were found in Fig. 4c, indicating the formation of PEO
nanocylinders upon microphase separation after annealing for 4 h.
However, the nanocylinders were aligned randomly: some of them
were normal to the substrate and others in plane. In Fig. 4d, all the
nanostripes were changed into nanodots with a diameter of about
6 nm, demonstrating that all the PEO nanocylinders were aligned
normally to the substrate after the annealing for 6 h. The fast
Fourier transform (FFT) image in a small area of Fig. 4d showed
a distorted hexagon, indicating a well-defined morphology with
a modified hexagonal packing of the PEO nanocylinders [14]. No
obvious change in the microphase-separated nanostructure was
observed with a longer annealing time. Unfortunately, perfectly
hexagonal packing of the PEO nanocylinders could not be obtained
in the whole area of the nematic LCBC films, which is slightly
different from that of the smectic LCBC films [10e12].
Fig. 3. UVevis absorption spectra of the LCBC in chloroform, as-prepared and
annealed films.
absorption of chromophores [26]. Therefore, H-aggregation,
J-aggregation and non-aggregated AZs might simultaneously exist
in the annealed film [25e27]. Such aggregation behaviors were
different from the thermally-annealed films of smectic LCBCs,
where H-aggregation and non-aggregated AZs were involved and
no J-aggregation was observed [10e12].
In thin films of smectic LCBCs, previous studies [10e12] carried
out vertical alignment of a PEO nanocylinder array with hexagonal
packing, which might be attributed to the SMCM of the micro-
phase-separated nanocylinder with the out-of-plane arranged
smectic LCs in a 2D order. A similar microphase-separated nano-
structure was observed in the AFM images of the present nematic
LCBC film, as shown in Fig. 4. In this work, all AFM images are phase
images obtained in tapping mode. No nanostructures appeared
clearly in the as-prepared film because the microphase separation
might be blocked in the glassy state at room temperature. To
accelerate the thermodynamically driven process of microphase
Annealing the LCBC films at a temperature in the isotropic phase
is one of the key factors to obtain well-defined morphologies of the
perpendicular orientation of microphase-separated PEO nano-
cylinders. The SMCM functioned as the interaction between the
self-organization of the LC block and microphase separation of the
amphiphilic incomparable diblock copolymers. At an elevated
temperature of 160 ꢀC, the amphiphilic nematic LCBC was liquid-
like, and both the PEO block and the AZ block had sufficient
mobility to self-assemble [28]. However the self-organization of
Fig. 4. AFM phase images of the nematic LCBC with different annealing times. (a) 0 h, (b) 2 h, (c) 4 h, (d) 6 h. The inset picture of (d) is the fast Fourier transform of a small area.

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H. Yu et al. / Polymer 52 (2011) 1554e1561
the LCBC could not reach an equilibrium morphology in a short
time, which led to a disordered nanostructures in Fig. 4b,c. By
gradually decreasing the temperature after the annealing for 6 h,
the AZ block first underwent an isotropic to nematic LC phase
transition and the AZ block formed the major phase in bulk films
due to its large volume ratio. The AZ blocks self-organized in its
nematic LC phase in 1D order, which still exerted a force on the
liquid-like PEO domains according to the SMCM, resulting in
a vertically orientated patterning of nanocylinder arrays in Fig. 4d.
Then the microphase separation was frozen when temperature
went below Tg of the AZ block. Finally, the PEO block slowly crys-
tallized among the glassy nematic LC domains, which did not
influence the finally obtained microphase separation.
In thin films of the nematic LCBC, the modified hexagonal
packing of the PEO nanocylinder array with perpendicular orien-
tation to the substrate can be observed in the whole area of the
substrates. For microphase separation in amorphous block copol-
ymer films, electric or magnetic field [29,30], temperature gradient
[31], crystallization [32], modified surface of substrates [33],
mechanical shearing [34], solvent evaporation [35], roll casting [36]
have been explored to obtain long-range orders due to the absence
Fig. 5. AFM phase image of PEO-b-PM6ABOC2 film after annealing for 6 h with a large
area of 3
m
m ꢂ 3
mm.
Fig. 6. Photoalignment of the nematic LCBC. (a) Photoinduced change in transmittance with irradiation time. (b) Pictures of the aligned film sandwiched with two crossed
polarizers. The arrow is the polarization direction of the linearly polarized beam. (c) Polarized UVevis absorption spectra of the aligned film before and after annealing. Pt and P//
are the absorption curves perpendicular and parallel to the polarization direction of the linearly polarize beam, respectively. (d) Scheme of photoinduced alignment and its
enhancement of AZ LC mesogens.

H. Yu et al. / Polymer 52 (2011) 1554e1561
1559
Fig. 7. AFM phase images of photoalignment films of PEO-b-PM6ABOC2 with different annealing time. (a) 0 h, (b) 2 h, (c) 4 h, (d) 6 h.
of SMCM. Although no specific external driving forces were exerted
on the nematic LCBC films, the vertically patterning of the PEO
nanocylinders was obtained in a macroscopic scale, which is similar
to the work of Stamm and Laforgue [37e39]. Owing to the AFM
circular image with a diameter of about 2.0 mm was clearly observed
in the irradiated area. Whereas a dark image appeared when the
sample film was tilted by 0ꢀ or 90ꢀ, such alternating bright and dark
sphere images appeared with a periodicity of 90ꢀ. No change was
observed in T in the un-irradiated area when rotating the sample
film, demonstrating that the photoalignment occurred only in the
laser-irradiated area.
resolution, a large area of 3
dotted patterning of PEO nanocylinders could be easily achieved
m
m ꢂ 3
mm is given in Fig. 5. In fact, such
over several centimeters, which were confirmed by acquiring AFM
images spaced by 100
m
m over the whole substrate.
After the photoalignment, the nematic LCBC film exhibited
intensive anisotropy in its polarized UVevis spectra as shown in
Fig. 6c, which indicates that the homogeneous LC alignment was
To fabricate parallel patterning of the PEO nanocylinders in plane,
the LC block should be homogeneously aligned based on the SMCM
[10,11]. This work chose a non-contact method of photoalignment of
the nematic AZ mesogens. The reason is that since the viscosity of
the nematic AZ mesogens was lower than that of the smectic LCs,
they might allow obtaining quick photoresponse. The as-prepared
film was irradiated with a linearly polarized beam with a diameter of
about 2.0 mm. A HeeNe laser at 633 nm with weak intensity was
adopted as a probe beam since there was no absorption at this
wavelength in the UVevis absorption spectrum of the LCBC film
(Fig. 2a). Fig. 6a shows the photoinduced change in transmittance (T)
of the LCBC film, measured simultaneously during the irradiation
through two crossed polarizers with the sample films between
them. Before the irradiation, no T was observed because no align-
ment of the mesogens was induced in film-forming process of spin
coating. Upon irradiation, photoalignment of AZs with their transi-
tion moments almost perpendicular to the polarization direction of
the incident light occurred according to the Weigert effect [40],
which led to a quick initial increase in T. After 300 s, T gradually
increased to a plateau value and no obvious increase was induced
with a longer irradiation time, indicating a photostationary state. The
photoalignment of LCs can also be confirmed by POM pictures (see
Fig. 6b). When the sample film was tilted by 45ꢀ with respect to the
polarization direction of the laser beam and the polarizer, a bright
Fig. 8. AFM phase image of photoalignment films PEO-b-PM6ABOC2 after annealing
for 6 h with a large area of 3
mm ꢂ 3
mm.

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H. Yu et al. / Polymer 52 (2011) 1554e1561
Similar results were obtained in the photoalignment film of the
present nematic LCBC. Following the SMCM, although only 1D
order from the continuous phase of the nematic LC block was
converted to the PEO nanocylinders, its parallel patterning in plane
exhibited a macroscopic order, as shown in Fig. 8. About 260
nanocylinders with
a
length of
3
mm were homogeneously
patterned in an area of 3
nanocylinders patterning in plane could be extended to the whole
irradiated area (a circle with a diameter of about 2 mm).
m
m ꢂ 3
mm. In fact, such a regularity of
The possible functions of SMCM in the nematic LCBC film can be
schematically illustrated in Fig. 9, in which both the LC alignment
and microphase-separated nanostructures are described. The
thermal annealing in an isotropic phase can induce vertical align-
ment of nanostructures, whereas photoalignment with a linearly
polarized light causes in plane alignment of nanocylinders.
Although the major phase of the hydrophobic nematic LC block
shows only 1D order, it can endow the minor phase of the hydro-
philic PEO nanocylinders with 3D order. Both homeotropic and
homogenous patterns of the microphase-separated nanocylinders
were successively fabricated, coinciding with the alignment
direction of the nematic LC block.
Fig. 9. Possible mechanism of the SMCM in the present LCBC with a nematic phase.
achieved perpendicular to the polarization of the actinic light. The
order parameter (S) of 0.22 at 343 nm was calculated by
S ¼ (AN ꢁ A//)/(AN þ 2 A//), where AN and A// are the absorbance
perpendicular and that parallel to the polarization direction of the
laser beam, respectively [22]. No homogeneous alignment of LCs
occurred in the un-irradiated area because S ¼ 0 was obtained.
After being annealed at 150 ꢀC for 6 h, S ¼ 0.58 was calculated in the
irradiated area of the film, indicating the enhanced alignment of
ordered mesogens upon annealing. Fig. 6d depicts a possible
scheme of photoalignment and its enhancement of nematic LC
mesogens upon annealing. The chosen annealing temperature of
150 ꢀC was just slightly lower than the nematic LC to isotropic
phase transition, which is very important for preparing the parallel
patterning of the nanostructures [41]. Firstly, the viscosity of the
film is decreased and the molecular mobility is improved at the
elevated temperature, which leads to the enhancement of the LC
ordering pre-aligned by the actinic laser beam owing to the
molecular cooperative motion, one of the inherent properties of LCs
[2]. Secondly, the microphase separation might proceed quickly in
the less viscous film. Thirdly, the nematic phase have a strong
influence on the SMCM of the LCBC, transferring the LC ordering to
the microphase-separated nanostructures.
4. Conclusions
In summary, an amphiphilic LCBC with an azobenzene moiety as
anematicmesogenwas prepared byamodified atomtransfer radical
polymerization. Upon microphase separation, the LC block formed
the major phase in the well-defined LCBC. Both thermal annealing
and photoalignment were used to control the alignment of AZ
mesogens in films. Although the LC block showed only 1D ordering,
it could endow the minor phase of the hydrophilic PEO nano-
cylinders with 3D order under the action of SMCM. Because of the
fact that the viscosity of the nematic LCBC was lower than that of the
smectic one, both homeotropic and homogenous patterns of the
microphase-separated nanocylinders were successfully obtained
within a short annealing time. Such macroscopically ordered
nanostructures in films provide a convenient and low-cost method
to fabricate ordered patterning of nanosize domains smaller than
100 nm by top-down nanotechnology. The microphase-separated
nanostructures with high regularity in the obtained nematic LCBC
showed excellent reproducibility and mass-production capability,
which might guarantee nanotemplated fabrication processes
[42e44], and lead to novel industrial applications in macromolec-
ular engineering.
Fig. 7 shows the microphase-separated nanostructures of the
photoalignment films with different annealing times. Without
annealing, ambiguous nanostructures were observed in Fig. 7a
because of the frozen microphase separation in the LCBC film. After
2 h of annealing, parallel patterning of the PEO nanocylinders with
irregularity and many defects could be dimly recognized from
Fig. 7b, due to the uncompleted microphase separation within the
short annealing time. Although defects still existed in Fig. 7c after
4 h of annealing, almost regularly parallel patterning of the PEO
nanocylinders in plane was indicated by the FFT image in the whole
area. Then almost homogeneous alignment in plane of the nano-
cylinders was obtained in Fig. 7d after 6 h of annealing. Being
dispersing in the major phase of the AZ nematic LC block, about 86
nanocylinders with a diameter of about 6 nm were detected in an
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