Photoinduced Alignment of Ferroelectric Liquid Crystals Using Azobenzene Polymer Networks of Chiral Polyacrylates and Polymethacrylates

Article abstract of DOI:10.1021/ma034886d

Two chiral dimethacrylate and one chiral diacrylate monomer containing an azobenzene group were synthesized and dissolved in a commercial ferroelectric liquid crystal (FLC) host. With two of the monomers, thermally induced radical polymerization with the

Full text of DOI:10.1021/ma034886d

9024
Macromolecules 2003, 36, 9024-9032
Photoinduced Alignment of Ferroelectric Liquid Crystals Using
Azobenzene Polymer Networks of Chiral Polyacrylates and
Polymethacrylates
Steve Lecla ir ,† Liza m m a Ma th ew ,† Ma r tin Gigu e`r e,‡ Sh a h r ok h Mota llebi,‡ a n d
Yu e Zh a o*,†
De´partement de Chimie, Universite´ de Sherbrooke, Sherbrooke, Que´bec, Canada J 1K 2R1, and
St-J ean PhotoChemicals, 725 Trotter, St-J ean-sur-Richelieu, Que´bec, Canada J 3B 8J 8
Received J une 27, 2003; Revised Manuscript Received September 19, 2003
ABSTRACT: Two chiral dimethacrylate and one chiral diacrylate monomer containing an azobenzene
group were synthesized and dissolved in a commercial ferroelectric liquid crystal (FLC) host. With two
of the monomers, thermally induced radical polymerization with the mixture exposed to linearly polarized
irradiation results in a bulk alignment of FLC in the direction perpendicular to the polarization of
irradiation light as a result of the photoinduced orientation of azobenzene groups related to the trans-
cis isomerization. The monomer that is most effective for photoalignment of FLC displays a polymer
network formed by strings of colloidal particles (∼250 nm in diameter), which exerts a commanding effect
on the alignment of FLC as being able to reorient the FLC by changing the polarization direction of
subsequent irradiation. The study suggests that the network of colloidal particles is isotropic and that
the photoalignment of azobenzene moieties on the large surface of the network dictates the alignment of
FLC.
In tr od u ction
More recently, we reported that azobenzene polymer
networks could also be used to optically align FLC.17,18
In the latter case, however, an increased segregation of
the polymer network from the FLC host was observed
in aligned samples in the ferroelectric chiral smectic-C
(S*C) phase,18 giving rise to the formation of thick
polymer threads parallel to the FLC alignment direc-
tion. Although the bulk alignment of FLC remains, such
a large-scale phase separation is not desirable because
of the resulting inhomogeneity. In view of the funda-
mental interest and technological importance of FLC,
we have undertaken a systematic investigation to fully
assess this optical and rubbing-free approach. As a
matter of fact, much effort has been devoted to the
development of new alignment methods, other than
surface-stabilized FLC.19 These include polymer-dis-
persed FLC20 microphase-stabilized FLC21 and ferro-
electric elastomers,22,23 for which a mechanical shear is
generally employed to induce the alignment.
All azobenzene monomers used in our previous stud-
ies are achiral diacrylates. In this paper, we report on
the synthesis of a number of chiral azobenzene diacry-
late and dimethacrylate monomers as well as the
investigation on their use to induce optically aligned
FLC without the use of surface orientation layers. It was
expected that the chirality in the azobenzene polymer
network may improve the compatibility with the FLC
in the chiral S*C phase. By doing so, there may be
stronger interactions between the azobenzene groups
and the surrounding FLC molecules, which facilitate the
bulk alignment of FLC through the action of the
photoactive azobenzene moieties.
Polymer-stabilized liquid crystals (PSLC) refer to low-
molar-mass liquid crystals (LC) whose bulk alignment
or texture is stabilized by a low-concentration polymer
network.1-4 PSLC based on nematic,2 cholesteric,1 and
ferroelectric liquid crystals (FLC)2,3 have been studied.
In all cases, the LC alignment or texture was first
obtained by surface orientation layers or an electric
field, followed by polymerization of a monomer dissolved
in the LC host. The polymer network formed in the
aligned LC host can be anisotropic and, in turn, stabilize
the LC alignment or texture. Potential applications of
PSLC for displays and electrically switchable optical
devices have been demonstrated.1,2 In the case of
polymer-stabilized FLC, another interest is to improve
the shock resistance of surface-stabilized FLC cells.4
In recent years, azobenzene-containing polymers,
including amorphous,5,6 liquid crystalline,7-10 and
elastomers,11-13 have attracted a great deal of attention
due to their potential utility in holographic storage as
well as other optical and photonic applications. Being
motivated by this, our group has been exploiting a new
type of PSLC, in which the polymer network bears the
azobenzene chromophore.14-18 We showed that the
presence of an azobenzene polymer in PSLC made
possible the use of the reversible trans-cis photo-
isomerization as well as the related photoalignement
of azobenzene to affect or induce the LC orientation.14-18
Indeed, when a diacrylate monomer containing an
azobenzene moiety is polymerized under linearly polar-
ized irradiation, either in the nematic15 or isotropic
phase,16 the azobenzene network formed can be aniso-
tropic and induce a stable bulk alignment of the LC in
the nematic phase in the absence of rubbed surfaces.
Exp er im en ta l Section
1. Ma ter ia ls. Aminobenzoic acid (99%), sulfamic acid (98%),
sodium acetate trihydrate (99%), ammonium hydroxide (28-
30%), phenyldiethanolamine (97%), (s)-(+)-1-bromo-2-meth-
ylbutane (99%), 1,8-diazabicyclo[5.4.0]undec-7-ene (98%), meth-
acrylic anhydride (94%), 2,6-di-tert-butyl-4-methylphenol (98%),
† Universite´ de Sherbrooke.
‡ St-J ean PhotoChemicals.
* Corresponding author: e-mail yue.zhao@usherbrooke.ca.
10.1021/ma034886d CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/07/2003

Macromolecules, Vol. 36, No. 24, 2003
Ferroelectric Liquid Crystals 9025
F igu r e 1. Chemical structures of chiral azobenzene monomers.
Sch em e 1. Syn th etic Rou te to Azoben zen e Mon om er s of
4-(4-{Bis-[2-(2-m eth yla cr yloyloxy)eth yl]a m in o}p h en yla zo)ben zoic Acid 2-Meth ylbu tyl Ester (Azo1) a n d
4-{4-[Bis(2-a cr yloyloxyeth yl)a m in o]p h en yla zo}ben zoic Acid 2-Meth ylbu tyl Ester (Azo2)
triethylamine (99%), 4-(dimethylamino)pyridine (99%), acryl-
oyl chloride (96%), and (s)-(+)-3-bromo-2-methyl-1-propanol
(97%) were used as received from Aldrich. Hydrochloric acid
(EMD, 36-38%), sodium nitrite (BDH, 97%), potassium iodide
(BDH, 99%), sodium hydroxide (Fisher, 98%), acetic acid
(Fisher, 99%), phenol (Lancaster, 99%), and anhydrous potas-
sium carbonate (BDH, 99%) were also used as received. 2,2′-
Azobis(isobutyronitrile) (AIBN, Polysciences) was recrystal-
lized from ethanol prior to use. All solvents were commercially
available and used as received, except THF which was dried
by distillation from sodium and benzophenone under N2.
The ferroelectric liquid crystal (FLC) mixture used was
Felix-SCE8 purchased from Clariant. It has the following
phase transitions: Cr -40 °C SC* 60 °C SA 80 °C N* 102 °C I.
The spontaneous polarization and rotational viscosity of the
FLC measured at 25 °C are -4.5 nC/cm2 and 76 mPa,
respectively (data provided by Clariant).
2. Syn th esis of Ch ir a l Azoben zen e Mon om er s. The
chemical structures of the monomers synthesized are shown
in Figure 1, including two dimethacrylates (Azo1 and Azo3)
and one diacrylates (Azo2). Azo1 and Azo2 have one chiral
center and the azobenzene moiety in the side group, while Azo3
has two chiral centers per molecule and the azobenzene moiety
in the main chain. Their synthetic routes are depicted in
Schemes 1 and 2, with more details given below.
(1.0 g, 0.10 mmol) was then added to neutralize the oxidant.
Then the solution was added drop by drop to a cold mixture of
AcONa (24.1 g, 175 mmol), NH4OH (42.3 g, 350 mmol), and
phenyldiethanolamine (32.7 g, 175 mmol) dissolved in water
(335 g). The reaction lasted 1 h at room temperature. After-
ward, the product was filtered and rinsed with water and
dried. An orange powder was obtained with a yield of 56%;
mp: decomposed before melting. MS (m/e): 329 (M+). λmax (in
THF): 444 nm. 1H NMR: 3.51-3.61 (m, N(CH2CH2OH)2, 8H),
6.86 (d, J ) 9.3 Hz, 2 aromatic H ortho to N(CH2)2), 7.77 (d, J
) 8.7 Hz, 4 aromatic H ortho to NdN), 8.03 (d, J ) 8.5 Hz, 2
aromatic H ortho to C(O)OH).
Syn th esis of 4-{4-[Bis(2-h yd r oxyeth yl)a m in o]p h en yl-
a zo}ben zoic Acid 2-Meth ylbu tyl Ester (2): 4-{4-[Bis(2-
hydroxyethyl)amino]phenylazo}benzoic acid (1.65 g, 5 mmol),
(s)-(+)-1-bromo-2-methylbutane (1.51 g, 10 mmol), and potas-
sium iodide (1.24 g, 7.5 mmol) were first dissolved in DMF
(120 mL). Then, 1,8-diazabicyclo[5.4.0]undec-7-ene (2.66 g, 17.5
mmol) was added under stirring. The solution was stirred and
heated to 70 °C for 48 h. The cooled solution was extracted
with dichloromethane and washed with water and sodium
hydroxide (1 N). The product was purified by column chroma-
tography on silica gel (eluent: acetone/hexane ) 4.5/5.5),
giving an orange solid with a yield of 50%; mp: 100.2 °C. MS
1
(m/e): 399 (M+). λmax (in THF): 442 nm. H NMR: 0.96-1.04
2.1. P r ep a r a t ion of Azo1 a n d Azo2. Syn t h esis of
4-{4-[Bis(2-h yd r oxyeth yl)a m in o]p h en yla zo}ben zoic Acid
(1): Aminobenzoic acid (24.5 g, 175 mmol) was dissolved in a
mixture of water (344 g) and HCl (60.4 g, 613 mmol) and cooled
in an ice-acetone bath under stirring. A cold solution of
NaNO2 (13.15 g, 185 mmol) in water (25 g) was added drop by
drop to keep the temperature under 5 °C, and sulfamic acid
(m, CH3CH2CH(CH3)CH2, 6H), 1.26-1.54 (m, CH3CH2CH-
(CH3)CH2, 2H), 1.86 (m, CH3CH2CH(CH3)CH2, 1H), 3.45 (OH,
2H), 3.71 (t, J ) 4.5 Hz, NCH2CH2OH, 4H), 3.93 (t, J ) 4.9
Hz, NCH2CH2OH, 4H), 4.13-4.24 (m, CH3CH2CH(CH3)CH2,
2H), 6.79 (d, J ) 8.9 Hz, 2 aromatic H ortho to N(CH2)2), 7.86-
7.91 (m, 4 aromatic H ortho to NdN), 8.13 (d, J ) 8.4 Hz, 2
aromatic H ortho to C(O)OCH2).

9026 Leclair et al.
Macromolecules, Vol. 36, No. 24, 2003
Sch em e 2. Syn th etic Rou te to Azoben zen e Mon om er of
4-{4-[2-Meth yl-3-(2-m eth yla cr yloyloxy)p r op oxy]p h en yla zo}ben zoic Acid
2-Meth yl-3-(2-m eth yla cr yloyloxy)p r op yl Ester (Azo3)
Syn th esis of 4-(4-{Bis-[2-(2-m eth yla cr yloyloxy)eth yl]-
a m in o}p h en yla zo)b en zoic Acid 2-Met h ylb u t yl E st er
(Azo1): 4-{4-[Bis(2-hydroxyethyl)amino]phenylazo}benzoic acid
2-methylbutyl ester (100 mg, 0.25 mmol) and methacrylic
anhydride (116 mg, 0.75 mmol) were dissolved in acetone (25
mL). A trace of 2,6-di-tert-butyl-4-methylphenol was added,
and the solution was cooled in an ice bath with stirring. Then
a solution of triethylamine (76 mg, 0.75 mmol) and 4-(di-
methylamino)pyridine (6 mg, 0.05 mmol) in acetone (10 mL)
was added to the first solution drop by drop. The reaction was
stirred overnight (14 h) at room temperature. A few drops of
acetic acid were added to obtain a pH of 6. The solution was
extracted with dichloromethane and washed with water and
sodium hydroxide (1 N). The product was purified by column
chromatography on silica gel (eluent: AcOEt/hexane ) 1.5/
8.5). A red oily liquid was obtained: yield 45% (60 mg, 112
mmol); mp -26.0 °C. MS (m/e): 535 (M+). λmax (in THF): 428
nm. 1H NMR: 0.95-1.05 (m, CH3CH2CH(CH3)CH2), 6H),
1.25-1.61 (m, CH3CH2CH(CH3)CH2, 2H), 1.80-1.90 (m,
CH3CH2CH(CH3)CH2, 1H), 1.93 (s, CH3C(CH2)C(O)O, 6H),
3.80 (t, J ) 6.2 Hz, N(CH2CH2O)2, 4H), 4.12-4.26 (m,
CH3CH2CH(CH3)CH2, 2H), 4.39 (t, J ) 6.1 Hz, N(CH2CH2O)2,
4H), 5.59 (s, CH3C(CH2)C(O)O, 2H), 6.10 (s, CH3C(CH2)C(O)O,
2H), 6.89 (d, J ) 9.1 Hz, 2 aromatic H ortho to N(CH2)2), 7.86-
7.91 (m, 4 aromatic H ortho to NdN), 8.15 (d, J ) 8.5 Hz, 2
aromatic H ortho to C(O)OCH2).
before melting. MS (m/e): 242 (M+). λmax (in THF): 358 nm.
1H NMR: 6.96 (d, J ) 8.9 Hz, 2 aromatic H ortho to OH), 7.81-
7.85 (m, 4 aromatic H ortho to NdN), 8.1 (d, J ) 8.5 Hz, 2
aromatic H ortho to C(O)OH).
Syn th esis of 4-[4-(2-Hyd r oxyp r op oxy)p h en yla zo]ben -
zoic Acid 2-Hyd r oxyp r op yl Ester (4): 4-(4-Hydroxyphen-
ylazo)benzoic acid (656 mg, 2.7 mmol) was dissolved in DMF
(50 mL), and then (s)-(+)-3-bromo-2-methyl-1-propanol (1.45,
9.5 mmol), 1,8-diazabicyclo[5.4.0]undec-7-ene (1.24 g, 8.1
mmol), and potassium iodide (1.35 g, 8.1 mmol) were added
to the solution. The solution was stirred and heated to 70 °C
for 48 h. The product was collected after extraction of the cold
solution with dichloromethane, washing with water and
sodium hydroxide (1 N), and recrystallization from toluene
solution. Yield: 69%; mp: 130.7 °C. MS (m/e): 386 (M+). λmax
(in THF): 362 nm. 1H NMR: 1.06-1.09 (m, CH2CH(CH3)CH2,
6H), 2.01 (OH, 2H), 2.14-2.24 (m, CH
2CH(CH3)CH2, 2H),
3.59-3.72 (m, CH(CH3)CH2OH, 4H), 4.02-4.08 (m, C(O)-
OCH2, 2H), 4.33-4.40 (m, ArOCH2, 2H), 7.02 (d, J ) 7.0 Hz,
2 aromatic H ortho to OCH2CH), 7.89-8.15 (m, 4 aromatic H
ortho to NdN), 8.16 (d, J ) 7.0 Hz, 2 aromatic H ortho to C(O)-
OCH2).
Syn t h esis of 4-{4-[2-Met h yl-3-(2-m et h yla cr yloyloxy)-
p r op oxy]p h en yla zo}ben zoic Acid 2-Meth yl-3-(2-m eth yl-
a cr yloyloxy)p r op yl Ester (AZO 3): 4-[4-(2-Hydroxypro-
poxy)phenylazo]benzoic acid 2-hydroxypropyl ester (600 mg,
1.55 mmol) and methacrylic anhydride (838 mg, 5.43 mmol)
were dissolved in acetone (55 mL). A trace of 2,6-di-tert-butyl-
4-methylphenol was added, and the solution was cooled in an
ice bath with stirring. A solution of triethylamine (549 mg,
5.43 mmol) and 4-(dimethylamino)pyridine (38 mg, 0.31 mmol)
in acetone (15 mL) was added to the first solution drop by drop.
The reaction solution was stirred overnight (14 h) at room
temperature. A few drops of acetic acid were added to obtain
a pH of 6. The monomer was obtained after extraction of the
solution with dichloromethane, washing with water and
sodium hydroxide (1 N), and purification by column chroma-
tography on silica gel (eluent: AcOEt/hexane ) 1/4). Yield:
57% (460 mg, 0.88 mmol); mp: 57 °C. MS (m/e): 522 (M+).
Syn th esis of 4-{4-[Bis(2-a cr yloyloxyeth yl)a m in o]p h en -
yla zo}ben zoic Acid 2-Meth ylbu tyl Ester (Azo 2): The
experimental procedure was similar to that for Azo1, except
the use of acryloyl chloride instead of methacrylic anhydride;
1
mp: -35 °C. MS (m/e): 507 (M+). λmax (in THF): 428 nm. H
NMR: 0.94-1.05 (m, CH3CH2CH(CH3)CH2), 6H), 1.25-1.58
(m, CH3CH2CH(CH3)CH2, 2H), 1.85-1.92 (m, CH3CH2CH(CH3)-
CH2, 1H), 3.80 (t, J ) 6.2 Hz, N(CH2CH2O)2, 4H), 4.12-4.36
(m, CH3CH2CH(CH3)CH2, 2H), 4.40 (t, J ) 6.1 Hz, N(CH2-
CH2O)2, 4H), 5.82-5.89 (m, CH2CHC(O)O, 2H), 6.08-6.17 (m,
CH2CHC(O)O, 2H), 6.38-6.45 (m, CH2CHC(O)O, 2H), 6.88 (d,
J ) 9.2 Hz, 2 aromatic H ortho to N(CH2)2), 7.86-7.92 (m, 4
aromatic H ortho to NdN), 8.15 (d, J ) 8.5 Hz, 2 aromatic H
ortho to C(O)OCH2).
λ
max (in THF): 360 nm. 1H NMR: 1.08-1.18 (m, CH2CH(CH3)-
CH2, 6H), 1.95 (s, CH3CH(CH2)C(O)O, 6H), 2.36-2.48 (m,
CH2CH(CH3)CH2, 2H), 3.96-4.07 (m, C(O)OCH2CH(CH3)-
CH2O, 2H), 4.16-4.34 (m, C(O)OCH2CH(CH3)CH2O and OCH2-
CH(CH3)CH2O, 6H), 5.58 (s, CH3C(CH2)C(O)O, 2H), 6.13 (s,
CH3C(CH2)C(O)O, 2H), 7.02 (d, J ) 8.8 Hz, 2 aromatic H ortho
to OCH2CH), 7.88-7.99 (m, 4 aromatic H ortho to NdN), 8.16
(d, J ) 8.7 Hz, 2 aromatic H ortho to C(O)OCH2).
3. P r ep a r a tion of Op tica lly Align ed F LC. Photoaligned
FLC was prepared by polymerizing azobenzene monomers
dissolved in the FLC host while exposing the mixture to
linearly polarized UV or visible light. The typical procedure
is shown in the following example. In a small flask, Azo1 (1.8
mg, 0.0034 mmol), FLC (60 mg), and AIBN (1.2 mg) were
dissolved in THF (0.3 mL) at room temperature to obtain a
2.2. P r ep a r a tion of Azo3. Syn th esis of 4-(4-Hyd r oxy-
p h en yla zo)ben zoic Acid (3): Aminobenzoic acid (24.5 g, 175
mmol) was dissolved in a mixture of water (344 g) and HCl
(60.4 g, 613 mmol), which was cooled in an ice-acetone bath
with stirring. A cold solution of NaNO2 (13.15 g, 185 mmol) in
water (25 g) was added to the first one drop by drop to keep
the temperature under 5 °C. Sulfamic acid (1.6 g, 0.017 mmol)
was then added to neutralize the oxidant. Afterward, the
solution was added to a mixture of AcONa (24.1 g, 175 mmol),
NH4OH (42.3 g, 350 mmol), and phenol (16.65 g, 175 mmol)
dissolved in water (335 g). The reaction lasted 1 h at room
temperature before addition of acetic acid (30 mL). The
solution was filtered, and the product, an orange powder, was
rinsed with water and dried. Yield: 95%; mp: decomposed

Macromolecules, Vol. 36, No. 24, 2003
Ferroelectric Liquid Crystals 9027
homogeneous solution. The solvent was allowed to evaporate
at room temperature first under the atmosphere pressure for
10-14 h and then in a vacuum oven for 3 h. A drop of the
freshly dried mixture was then placed between two quartz
plates, warmed to 40 °C, and compressed manually to produce
a film of about 60 mm2 large and with a thickness around 5
µm. Afterward, the film was moved into a microscope hot stage
and, without delay, exposed to linearly polarized light (normal
incidence) while being heated to 110 °C in the isotropic phase
for 15 min to complete the thermally induced free radical
polymerization of the azobenzene monomer. With a heating
rate of about 20 °C/min, polymerization started during the
heating but was essentially preceded in the isotropic phase.
Finally, under irradiation the mixture was cooled, at a rate of
about 5 °C/min, to room temperature. After the preparation
procedure was completed, the irradiation light was turned off.
All samples prepared using this procedure were first examined
on a polarizing optical microscope to observe the photoalign-
ment of FLC and then used in different experiments as shown
in the paper.
The concentration of azobenzene monomer in the FLC was
between 1 and 10 wt %. Linearly polarized irradiation light
was obtained by using an UV-vis curing system (Novacure)
combined with an UV or visible interference filter (10 nm
bandwidth, Oriel) and an UV linear dichroic polarizer (Oriel).
Both polarized UV irradiation at 360 nm (∼10 mW/cm2) and
polarized visible exposure at 440 nm (∼4 mW/cm2) were
produced. For all experiments of photoisomerization and
photoalignment in this study, polarized UV light was used for
Azo3, whereas polarized visible light was used for Azo1 and
Azo2.
4. Ch a r a cter iza tion s. Mon om er s: All azobenzene mono-
mers as well as the compounds used for their synthesis were
characterized by a number of techniques. Their 1H NMR
spectra in CDCl3 were recorded on a Bruker-AC300 (300 MHz)
or a Bruker DMX-600 (600 MHz) spectrometer. Mass spectra
were recorded on a Micromass ZAB-1F high-resolution mass
spectrometer. A differential scanning calorimeter (Perkin-
Elmer DSC-7) was used to investigate their thermal behavior,
using indium as the calibration standard and a heating or
cooling rate of 10 °C/min. All compounds were found to display
only a crystalline phase before melting. Their reported melting
temperature was determined as the maximum of the endo-
thermic peak during the second heating scan. UV-vis spectra
in THF solution were recorded on a Hewlett-Packard 8452A
diode array spectrophotometer.
F igu r e 2. UV-vis spectra of azobenzene monomers in (a)
tetrahydrofuran solution and (b) the ferroelectric liquid crystal.
Resu lts a n d Discu ssion
1. Ch ir a l Azoben zen e Mon om er s. The UV-vis
spectra of all azobenzene monomers dissolved in THF
are shown in Figure 2a. Because of the electron-donor
and electron-acceptor groups, Azo1 and Azo2 have the
maximum absorption of trans-azobenzene (π-π* transi-
tion) appearing in the visible region (428 nm), whereas
Azo3 displays the maximum absorption (π-π* transi-
tion) at lower wavelengths in the UV range (360 nm)
and weak absorption in the visible range (450 nm, n-π*
transition). Given in Figure 2b are the UV-vis spectra
of 5% Azo1 and 3% Azo3 dissolved in the FLC host. For
Azo1 and Azo2, the absorption band of azobenzene
shows little shift in the FLC (432 nm) and is well
separated from the strong absorption of the latter at
lower wavelengths, while the absorption of Azo3 in the
FLC is noticed as a shoulder due to overlapping of FLC.
For all monomers, the occurrence of photoisomerization
at the used irradiation wavelengths was confirmed.
Mixtu r es of Mon om er /F LC a n d P olym er ized Sa m p les:
DSC was also used to monitor the polymerization of the
azobenzene monomers dissolved in the FLC. In each case, a
freshly dried mixture (about 15 mg) was inserted into a DSC
pan, and the first heating scan was recorded from 20 to 160
°C to complete the polymerization. Then the sample was cooled
to room temperature, and a second heating scan was conducted
to observe the changes. A polarizing optical microscope (Leitz
DMR-P, magnification 100×) equipped with an Instec hot
stage was used to examine the photoalignment of FLC after
the irradiation light was turned off at room temperature (see
the conditions described above for the preparation of optically
aligned samples). Optical micrographs were taken using a
Leica DC-300 digital camera. The photoisomerization of
azobenzene monomers (before polymerization) and polymers
(after polymerization) in the FLC was also investigated using
the UV-vis spectrophotometer. UV-vis spectra of thin films
(about 5 µm thick) cast between two quartz plates were
recorded immediately after the UV or visible irradiation, and
for polarized UV-vis measurements, a polarizer (Oriel) was
placed in front of the optically aligned sample. Finally, a
scanning electron microscope (J EOL J SC-840A) was used to
observe the azobenzene polymer networks. For these experi-
ments, the films between quartz plates were dipped in hexane
to slowly remove the FLC; the two plates were then carefully
separated, and the polymer network remained on both sub-
strates was dried before use.
The chiral nature of all monomers was confirmed by
mixing them with a nematic LC. They all act as a chiral
dopant and induce a chiral nematic (N*) phase. Figure
3 shows polarizing optical micrographs for a nematic
LC, BL006 (Merck), containing 5% azobenzene mono-
mer, taken at 100 °C. The fingerprint texture charac-
teristic of the induced N* phase is observed in all
mixtures. Moreover, a stronger helical twisting in the
mixture with Azo3 is clearly the result of a higher
concentration of chiral centers, twice as many as Azo1
and Azo2. The helical pitch estimated from Figure 3 is
about 6.6 µm with Azo3 and 12 µm for both Azo1 and
Azo2. Note that if an initiator (AIBN) is added in the

9028 Leclair et al.
Macromolecules, Vol. 36, No. 24, 2003
colored due to the azobenzene chromophore, and no
phase separation can be detected. This suggests a fine
dispersion of the formed polymer network in the FLC.
However, when the mixture is cooled into the LC
phases, where the liquid crystal domains develop, the
polymer network apparently is segregated from the FLC
and move into the many defect or disclination regions
associated with the polydomain structure (no bulk
alignment). On reheating the mixture into the isotropic
phase, the thickened polymer network becomes visible
on the bright-field optical microscope, as is shown in
Figure 5. It is apparent that the azobenzene polymer
networks have different morphologies, depending on the
chemical structure of the azobenzene monomer. Azo1
gives the most prominent 3D-like network, with con-
nection of long and flexible filaments. The polymer of
Azo2 seems to be concentrated in certain areas, while
for Azo3 the network appears as an assembly of short
and irregularly shaped aggregates. SEM observations
confirmed the formation of very different polymer
networks. Figure 6 shows SEM images of networks
formed from polymerization of 3% monomer in the
mixture, the FLC host being extracted in hexane. Even
though theses pictures may not reflect the exact state
of the network in the FLC because alternations such
as collapse may happen during the FLC removal, the
differences are evident. For Azo1, the network is formed
by long strings of interconnected spherical particles
(about 250 nm in diameter); it is likely that such strings
of particles may be flexible and easy to arrange into a
uniform and open network structure. In the case of
Azo2, larger and compact-looking aggregates coexist
with noninterconnected particles of irregular shapes,
and the network of Azo3 is formed by twisted aggregates
of rice-grain particles. These features are consistent
with the optical microscopic observations (Figure 5). As
is shown below, the polymer network morphology has
profound effects on the optical alignment of FLC. It was
noted that the polymerization kinetics of the three
azobenzene monomers in the FLC, as revealed by the
nonisothermal DSC measurements (exemplified in Fig-
ure 4), showed no noticeable differences. Moreover,
using Azo1, it was found by SEM that similar morphol-
ogy resulted from polymerizations carried out at differ-
ent temperatures in the isotropic phase. These results
suggest that the different polymer networks formed
from the three monomers should mainly be caused by
different polymer interactions and compatibility with
the FLC, which affect the phase separation behavior.
The more twisted network structure of Azo3 may be
related to the greater chirality of this monomer than
Azo1 and Azo2.
F igu r e 3. Polarizing optical micrographs, taken at 100 °C,
of 5% azobenzene monomers dissolved in a nematic liquid
crystal. The fingerprint texture arises from the induced chiral
nematic phase. Picture area: 470 µm × 470 µm.
F igu r e 4. DSC heating curves (10 °C/min): (a) second heating
scan for the pure ferroelectric liquid crystal (FLC); (b) first
heating scan for the mixture of FLC with 5% 4-(4-{bis-[2-(2-
methylacryloyloxy)ethyl]amino}phenylazo)benzoic acid 2-methyl-
butyl ester (Azo1); and (c) second heating scan for the mixture
of FLC/Azo1 (5%).
mixture for polymerization, the conversion of the chiral
monomer into a chiral polymer network reduces the
chirality transfer to the nematic BL006, and most
helices are unwinded.
2. P olym er iza t ion a n d P olym er Net w or k s in
F LC. Before investigating the optical alignment of FLC,
we performed a series of experiments without irradia-
tion to make sure that polymerization of all chiral
monomers did take place in the FLC and give rise to
polymer networks. Figure 4 compares the DSC heating
curves of the pure FLC and its mixture with 5%
azobenzene monomer, Azo1. The occurrence of polym-
erization during the first heating scan of the mixture
is clearly revealed by the exotherm starting at about
65 °C, which is superimposed with the mesophase
transition endothermic peaks of the FLC. It is noticeable
that the SA-N* and N*-I transition temperatures in
the mixture with the monomer are lowered by several
degrees as compared with that of the pure FLC, which
is caused by a disrupting effect of the monomer dis-
solved in the FLC. After polymerization, on the second
heating scan, only the transition peaks are observed,
and the transition temperatures rebound but still
remain lower than those of pure FLC due to the
interaction between the FLC and polymer network.
These results indicate that the azobenzene monomer is
dissolved in the FLC and can be polymerized at tem-
peratures in either the N* or the isotropic phase. Similar
results were obtained for other monomers.
3. P h otoa lign m en t of F LC. As mentioned earlier,
the photoisomerization and photoalignment of the
azobenzene chromophore were thought of as what may
impart a bulk alignment of FLC contained by an
azobenzene polymer network. The photoisomerization
of azobenzene monomers as well as azobenzene moieties
on the polymer network in the FLC host was confirmed
by UV-vis spectroscopy. An example is given in Figure
7, displaying changes in the UV-vis spectrum of 5%
Azo1 polymerized in the FLC, upon irradiation at 440
nm. The trans-cis isomerization of azobenzene moieties
on the network (likely on the surface) is indicated by
the diminution of the absorption of trans-azobenzene
around 432 nm, which is almost superimposed with the
n-π* transition of cis-azobenzene at about 450 nm.
The formation of an azobenzene polymer network can
also be observed on an optical microscope. After polym-
erization in the isotropic phase (110 °C), a uniform
liquid film is seen on bright field, which is orange-

Macromolecules, Vol. 36, No. 24, 2003
Ferroelectric Liquid Crystals 9029
F igu r e 5. Azobenzene polymer networks formed by 3% monomer dissolved in the ferroelectric liquid crystal, viewed on bright
field optical microscope at 110 °C. Picture area: 780 µm × 625 µm.
F igu r e 6. Scanning electron microscope images of azobenzene polymer networks formed from 3% monomer dissolved in the
ferroelectric liquid crystal (FLC). The FLC host was removed in hexane. The scale bar is 1 µm.
actually have a strong tendency to be phase separated
from the FLC at room temperature. If the phase
separation precedes the polymerization under linearly
polarized irradiation, no bulk alignment of FLC can be
achieved. It is therefore crucial to carry out the experi-
ment on freshly prepared homogeneous mixtures. Quick
evaporation of the solvent was found to be effective to
produce a kinetically controlled homogeneous mixture.
Figure 8a shows the apparent texture of such a mixture
compressed between two quartz plates, with no devel-
oped smectic texture. From this state, if the mixture is
heated to the isotropic phase and cooled back to room
temperature without irradiation, the monomer is simply
polymerized, but there is no bulk alignment of FLC in
the S*C phase (Figure 8b). If linearly polarized irradia-
tion is applied to the mixture during the whole process
F igu r e 7. UV-vis spectra for a sample of ferroelectric liquid
of heating and cooling, a bulk alignment of smectic
domains can be obtained along the direction perpen-
dicular to the polarization of irradiation light (Figure
8c). Clearly this optical alignment of FLC must be
attributed to the action of azobenzene moieties.
crystal with 5% azobenzene polymer network formed from 4-(4-
{bis-[2-(2-methylacryloyloxy)ethyl]amino}phenylazo)benzoic acid
2-methylbutyl ester (Azo1) before and after irradiation. Ir-
radiation times are indicated.
Mention also that in another series of experiments
photoinduced birefringence was observed for mixtures
of chiral azobenzene monomers and polymers (except
Azo3) in a FLC similar to the one used in this study
(excitation using an Ar+ laser at 488 nm).24 All these
confirm the occurrence of the photoisomerization of
azobenzene monomers as well as azobenzene moieties
on the polymer networks in the FLC. As shown later, a
preferential orientation of azobenzene groups in the
direction perpendicular to the polarization of irradiation
light may be induced as a result of the conventional
photoorientation process.
Of the three chiral azobenzene monomers, Azo1 and
Azo2 were found to be able to induce optically aligned
FLC without rubbed surfaces. Azo3 cannot. Using the
mixture with 1% Azo2, Figure 8 illustrates the condi-
tions and the typical results. Whatever the azobenzene
monomer used, a good dissolution or fine dispersion in
the FLC host is indispensable for the success of photo-
alignment. Quite surprisingly, the chiral monomers
Better photoinduced bulk alignment of FLC was
observed using Azo1 than with Azo2. Figure 9 shows
the representative results at various concentrations of
Azo1. Good alignment was obtained with up to 5% Azo1,
but a higher concentration leads to more defects, where
the azobenzene polymer network is located. No bulk
alignment was observed with 10% Azo1, the polymer
network of which seems to be too dense to allow the
alignment to develop. Two significant differences are
emerged between chiral and achiral azobenzene poly-
mers investigated for the photoalignment of FLC. First,
in the case of the achiral polymer network,17,18 there is
an enhanced segregation of the polymer network from
aligned FLC in the SC* phase, which results in the
appearance of thick polymer threads (∼ several mi-
crometers in width) running parallel to the aligned
smectic domains. No such increased phase separation
was observed for aligned FLC with the chiral azoben-
zene polymer networks.

9030 Leclair et al.
Macromolecules, Vol. 36, No. 24, 2003
F igu r e 8. Polarizing optical micrographs for a film of the mixture of ferroelectric liquid crystal and 5% 4-{4-[bis(2-acryloyloxyethyl)-
amino]phenylazo}benzoic acid 2-methylbutyl ester (Azo2) at room temperature: (a) before polymerization and irradiation; (b)
after polymerization without irradiation; and (c) after polymerization under irradiation. Picture area: 780 µm × 700 µm.
visible. The network appears randomly aligned and
displays no noticeable difference in the two areas with
orthogonal alignment of smectic domains. Moreover,
SEM observations made on aligned samples found no
evidence of anisotropic polymer network. A typical
result is shown in Figure 11, obtained after extraction
of FLC from an aligned FLC with 3% Azo1. At higher
magnifications, the formation of network by intercon-
nected spherical particles, like in Figure 6, could be
seen.
Polarized UV-vis spectroscopic measurements con-
firmed that the reorientation of smectic domains de-
scribed above is related to the photoinduced reorienta-
tion of azobenzene moieties on the polymer network.
Figure 12 shows the angular dependence of the absor-
bance of azobenzene moieties at 432 nm for a sample
with 3% Azo1, before and after the reorientation. The
angle is that between the polarizations of the beam of
spectrophotometer and the irradiation light used for the
first irradiation. Within experimental errors, the highest
absorbance found at 90° after the first irradiation
indicates the orientation of azobenzene moieties in the
expected direction, i.e., perpendicular to the polarization
of irradiation light, which is also the alignment direction
of smectic domains. After the second irradiation with
the polarization changed by 90°, the results in Figure
12 show a 90° reorientation of azobenzene moieties that
correlates with the observed 90° rotation of the smectic
domains (Figure 10). Some other features about this
photoinduced reorientation of FLC need to be empha-
sized. On one hand, the reorientation process takes
place only when the sample is heated to either the N*
phase or the isotropic phase, while it does not happen
in both the smectic A (SA) and the SC* phase. This is
understandable because the reorientation of smectic
domains, which have higher molecular order and rota-
tional viscosity than the nematic phase, through pho-
toalignment of azobenzene is more energetically de-
manding than reorienting nematic domains. The diffi-
culty of reorienting initially ordered LC domains is
known even with the azobenzene chromophore as part
of the LC structure.25 While in the present case, the
azobenzene polymer is phase-separated from the FLC,
and the effect of photoalignment of azobenzene on the
surrounding FLC molecules would be provided mainly
by azobenzene moieties on the surface of the network.
On the other hand, if the aligned sample is heated to
the isotopic phase and cooled without irradiation, the
initial orientation cannot be recovered. In other words,
the orientational memory effect, known for anisotropic
polymer networks,11 is absent for optically aligned FLC
with the chiral azobenzene network.
F igu r e 9. Photoinduced bulk alignment of ferroelectric liquid
crystal in the chiral smectic C phase (SC*) using various
concentrations of azobenzene polymer network formed from
4-(4-{bis-[2-(2-methylacryloyloxy)ethyl]amino}phenylazo)-
benzoic acid 2-methylbutyl ester (Azo1). The alignment of
smectic domains is perpendicular to the polarization of ir-
radiation light. Picture area: 860 µm × 700 µm.
Second, the chiral azobenzene polymer network exerts
a commanding effect on the alignment of FLC, since
photoinduced reorientation can be achieved by changing
the polarization direction of irradiation light, which was
not observed for achiral azobenzene polymer networks.
Some results are presented in Figure 10 to explain the
way it works. All samples were first photoaligned in one
direction. Then, after covering a part of the sample (the
upper part in the photos) by an aluminum sheet, the
film was exposed to a second irradiation with the
polarization rotated by 90°, and during the process the
film was heated either to the N* phase (90 °C) or the
isotropic phase (110 °C) before being cooled to room
temperature. Polarizing photomicrographs a and b show
the 90° reorientation of smectic domains in the uncov-
ered lower part for a film containing 1% Azo1 heated to
the N* and isotropic phase, respectively. In the latter
case, the initial horizontal alignment in the covered area
was lost after the isotropization process. The reorienta-
tion occurred in the N* phase for a sample with 3% Azo1
is shown in photo c. Because of the higher concentration
of azobenzene polymer network, the reorientation bound-
ary between the two areas appears less shaper. Photo
d is for the same sample as in photo c but was taken
after removal of crossed polarizers. On bright field, the
orange-colored azobenzene polymer network becomes

Macromolecules, Vol. 36, No. 24, 2003
Ferroelectric Liquid Crystals 9031
F igu r e 10. Optical micrographs at room temperature showing photoinduced reorientation of ferroelectric liquid crystal by changing
polarization of irradiation light: (a) 1% azobenzene polymer network formed from 4-(4-{bis-[2-(2-methylacryloyloxy)ethyl]amino}-
phenylazo)benzoic acid 2-methylbutyl ester (Azo1) with reorientation occurred in the N* phase; (b) 1% Azo1 with reorientation
occurred in the isotropic phase; (c) 3% Azo1 with reorientation occurred in the N* phase; and (d) bright field picture for the same
film as in (c). See text for details. Picture area: 625 µm × 770 µm.
F igu r e 13. Schematic illustration of the photoinduced reori-
entation of ferroelectric liquid crystal with a network of
colloidal particles of azobenzene polymer.
F igu r e 11. Scanning electron microscope image of the
azobenzene polymer network formed from 3% 4-(4-{bis-[2-(2-
methylacryloyloxy)ethyl]amino}phenylazo)benzoic acid 2-meth-
ylbutyl ester (Azo1) in an aligned ferroelectric liquid crystal.
The scale bar is 10 µm.
surface azobenzene moieties that can undergo the
photoisomerization. The photoinduced alignment of the
surface azobenzene moieties under linearly polarized
irradiation may bring the FLC molecules in the N*
phase to align in the same direction, and this alignment
remains in the lower-temperature SA and SC* phases.
Changing the polarization of irradiation light changes
the orientation of the surface azobenzene moieties and,
as a result, reorients the FLC molecules. When such
an aligned sample is heated into the isotropic phase
without irradiation, the orientation of azobenzene groups
would be relaxed, and subsequent cooling into the LC
phases cannot recover the initial bulk alignment of FLC.
It is easy to picture that a uniform network of strings
of colloidal particles would maximize the interaction
with the surrounding FLC molecules leading to effective
alignment of FLC. This appears to be the case for the
chiral Azo1 polymer network (Figures 6 and 11). The
observed differences between chiral and achiral azoben-
zene polymers are likely to find the origin in the
interactions between the network and the FLC. As
mentioned in Introduction, it was anticipated that chiral
azobenzene polymers might have an enhanced compat-
ibility with the chiral FLC host, which affects the
morphology of polymer network after phase separation
and, consequently, the photoalignment behavior of the
FLC.
F igu r e 12. Angular dependence of absorbance of azobenzene
moieties on the network at 432 nm for an aligned ferroelectric
liquid crystal containing 3% azobenzene polymer network
formed from 4-(4-{bis-[2-(2-methylacryloyloxy)ethyl]amino}-
phenylazo)benzoic acid 2-methylbutyl ester (Azo1) after two
irradiations with the polarization changed by 90°.
Contrarily to Azo1, the absence of photoalignement
of FLC using Azo3 may be explained by a combination
of several of factors. First, the overlap of the absorption
of Azo3 with that of the FLC host may reduce its
excitation and hinder its photoalignment. Second, in
contrast with Azo1 and Azo2, the lack of electron-donor
and electron-acceptor substituents on the azobenzene
unit of Azo3 results in the poor overlap of π-π* an n-π*
absorptions (Figure 2) and longer lifetime of the cis
All the results together suggest a different mechanism
for the photoinduced alignment of FLC from the chiral
Azo1 polymer network, which is schematically illus-
trated in Figure 13. The network is isotropic and formed
by connection of spherical polymer particles, whose
colloidal dimension (∼250 nm diameter) ensures a high
surface-to-volume ratio and thereby a large number of

9032 Leclair et al.
Macromolecules, Vol. 36, No. 24, 2003
isomer, the consequence of which is a less efficient
photoinduced orientation of azobenzene moieties.25 Third,
probably more damaging for the Azo3 polymer network
is the fact that azobenzene moieties are part of chain
backbone by fixation of both ends. The lack of freedom
for azobenzene moieties on the surface may also prevent
their photoalignment from happening. In another study
using azobenzene divinyl ethers and diepoxides,26 pho-
toalignment of FLC was observed for monomers with
similar structures. However, as compared with these
monomers, the shorter spacers of Azo3, which bear two
chiral centers, may restrict the mobility of azobenzene
moiety. Finally, note that the surface property of the
substrate has a profound effect on the photoalignment
of FLC. Best results were achieved so far with untreated
quartz plates, whereas only partial alignment was
obtained with glass plates coated with indium-tin oxide
(ITO). Work is underway to modify the surface tension
of ITO in order to obtain uniform photoalignment of FLC
required for electrooptical measurements.
dispersed in the host, the action of the surface azoben-
zene moieties actually provides a volume effect on the
surrounding FLC molecules, which is different from the
so-called commanding surfaces by optically aligning
azobenzene groups of thin layers coated on the sub-
starte.6
Ack n ow led gm en t. We thank the Natural Sciences
and Engineering Research Council of Canada and le
Fonds que´be´cois de la recherche sur la nature et les
technologies (Que´bec) for financial support.
Refer en ces a n d Notes
(1) Yang, D.-K.; Chien, L. C.; Fung, Y. K. In Liquid Crystals in
Complex Geometries Formed by Polymer and Networks;
Crawford, G. P., Zumer, S., Eds.; Taylor & Francis: London,
1996; pp 103-143.
(2) Hikmet, R. A. M. In ref 1, pp 53-83.
(3) Hikmet, R. A. M. Adv. Mater. 1995, 7, 300.
(4) Guymon, C. A.; Dougan, L. A.; Martens, P. J .; Clark, N. A.;
Walba, D. M.; Bowman, C. N. Chem. Mater. 1998, 10, 2378.
(5) Natansohn, A.; Rochon, P.; Gosselin, J . J .; Xie, S. Macromol-
ecules 1992, 25, 2268.
Con clu sion s
(6) Kim, D. Y.; Li, L.; J iang, X. L.; Shivshankar, V.; Kumar, J .;
Tripathy, S. K. Macromolecules 1995, 28, 8835.
(7) Ichimura, K. Chem. Rev. 2000, 100, 1847.
(8) Eich, M.; Wendorff, J . H. Makromol. Chem. Rapid Commun.
1987, 8, 59.
(9) Wu, Y.; Zhang, Q.; Kanazawa, A.; Shiono, T.; Ikeda, T.;
Nagase, Y. Macromolecules 1999, 32, 3951.
(10) Fischer, T.; Lasker, L.; Stumpe, J .; Kostromin, S. Photochem.
Photobiol. A: Chem. 1994, 80, 453.
(11) Bai, S.; Zhao, Y. Macromolecules 2001, 34, 9032.
(12) Zhao, Y.; Bai, S.; Dumont, D.; Galstian, T. Adv. Mater. 2002,
14, 512.
(13) Bai, S.; Zhao, Y. Macromolecules 2002, 35, 9657.
(14) Corvazier, L.; Zhao, Y. Macromolecules 1999, 32, 3195.
(15) Zhao, Y.; Chenard, Y.; Paiement, N. Macromolecules 2000,
33, 1049.
(16) Zhao, Y.; Chenard, Y. Macromolecules 2000, 33, 5891.
(17) Zhao, Y.; Paiement, N. Adv. Mater. 2001, 13, 1891.
(18) Zhao, Y.; Paiement, N.; Se´vigny, S.; Leclair, S.; Motallebi, S.;
Gigue`re, M.; Bouchard, L. SPIE Proc. 2003, 5003, 150.
(19) Clark, N. A.; Larerwall, S. T. Appl. Phys. Lett. 1980, 36. 899.
(20) Molsen, H.; Kitzerow, H.-S. J . Appl. Phys. 1994, 15, 710.
(21) Mao, G.; Wang, J .; Ober, C. K.; Brehmer, M.; O’Rourke, M.
J .; Thomas, E. L. Chem. Mater. 1998, 10, 1538.
(22) Semmler, K.; Finkelmann, H. Macromol. Chem. Phys. 1995,
196, 3197.
Three chiral dimethacrylate and diacrylate monomers
containing an azobenzene group in their structure were
synthesized and investigated for photoinduced align-
ment of FLC in the absence of surface orientation layers.
When dissolved in a FLC host and polymerized under
linearly polarized irradiation, two of the monomers are
able to induce a bulk alignment of FLC in the SC* phase,
with a concentration as low as 1%. Polymerization of
the monomer Azo1, which is most effective in inducing
photoalignment, results in a polymer network formed
by strings of colloidal particles with diameters in the
range of 250 nm. Experimental evidence suggests that
the photoinduced bulk alignment of FLC related to the
chiral azobenzene polymer networks may be achieved
through a different mechanism as compared with achiral
azobenzene polymers.14-18 Instead of the anisotropic
polymer network observed in the latter case, which
serves as a framework for the induction and stabiliza-
tion of FLC, the network from chiral Azo1 may be
isotropic, and it is the photoinduced orientation of the
large number of azobenzene moieties on the surface of
the network, i.e., at the interface with the FLC, that is
responsible for the alignment of FLC. This mechanism
accounts for the observed reorientation of FLC by
changing the polarization of irradiation light as well as
the absence of the orientational memory effect charac-
teristic of an anisotropic network. In such azobenzene
polymer-stabilized FLC, since the polymer network is
(23) Brehmer, M.; Zentel, R. Macromol. Chem. Phys. 1994, 195,
1891.
(24) Unpublished results obtained using Felix015, another FLC
from Clariant.
(25) Natansohn, A.; Rochon, P. Chem. Rev. 2002, 102, 4139.
(26) Se´vigny, S.; Bouchard, L.; Motallebi, S.; Zhao, Y. Macromol-
ecules 2003, 36, 9033.MA034886D
MA034886D