Reversion of alignment direction in the thermally enhanced photoorientation of photo-cross-linkable polymer liquid crystal films

Article abstract of DOI:10.1021/ma011439u

Reversion of the in-plane reorientation direction of mesogenic groups has been observed for the first time in novel polymethacrylate liquid crystal (PLC) films substituted with a 4-methoxycinnamoyloxybiphenyl side group. The reversion was generated by irradiation with linearly polarized ultraviolet (LPUV) light and a subsequent annealing. Irradiation with LPUV light induces negative optical anisotropy of the films as a result of an axis-selective photoreaction of the side groups. The direction of the thermally enhanced reorientation is dependent on the degree of photoreaction and the distribution of photoproducts, while the induced orientational order in both directions, S, was larger than 0.5. The distribution of photoproducts in PLC films has been analyzed to elucidate their contribution to the thermally enhanced reorientation behavior. Initially upon photoreaction, thermal enhancement of the photoinduced negative optical anisotropy was observed. However, when the degree of photodimerization was 15% or greater, the direction of the thermally enhanced reorientation was found to be parallel to the polarization direction (E) of LPUV light. It is concluded that a small amount of photoproduct plays a role in the thermal amplification of the photoinduced negative optical anisotropy in a manner identical to that of PLC with azobenzene side groups. In contrast, photodimerized mesogenic groups generated reversion of the orientational direction and enhancement of positive optical anisotropy of the film through annealing.

Full text of DOI:10.1021/ma011439u

706
Macromolecules 2002, 35, 706-713
Reversion of Alignment Direction in the Thermally Enhanced
Photoorientation of Photo-Cross-Linkable Polymer Liquid Crystal Films
Nobu h ir o Ka w a tsu k i,* Koh ei Goto, Tetsu r o Ka w a k a m i, a n d Toh ei Ya m a m oto
Department of Applied Chemistry, Himeji Institute of Technology,
2167 Shosha Himeji, 671-2201 J apan
Received August 10, 2001; Revised Manuscript Received October 30, 2001
ABSTRACT: Reversion of the in-plane reorientation direction of mesogenic groups has been observed
for the first time in novel polymethacrylate liquid crystal (PLC) films substituted with a 4-methoxy-
cinnamoyloxybiphenyl side group. The reversion was generated by irradiation with linearly polarized
ultraviolet (LPUV) light and a subsequent annealing. Irradiation with LPUV light induces negative optical
anisotropy of the films as a result of an axis-selective photoreaction of the side groups. The direction of
the thermally enhanced reorientation is dependent on the degree of photoreaction and the distribution
of photoproducts, while the induced orientational order in both directions, S, was larger than 0.5. The
distribution of photoproducts in PLC films has been analyzed to elucidate their contribution to the
thermally enhanced reorientation behavior. Initially upon photoreaction, thermal enhancement of the
photoinduced negative optical anisotropy was observed. However, when the degree of photodimerization
was 15% or greater, the direction of the thermally enhanced reorientation was found to be parallel to the
polarization direction (E) of LPUV light. It is concluded that a small amount of photoproduct plays a role
in the thermal amplification of the photoinduced negative optical anisotropy in a manner identical to
that of PLC with azobenzene side groups. In contrast, photodimerized mesogenic groups generated
reversion of the orientational direction and enhancement of positive optical anisotropy of the film through
annealing.
1. In tr od u ction
If the in-plane orientation can be controlled both
parallel and perpendicular to E of LP light, a patterning
of the direction and birefringence of the film is enabled.
Reversion of the alignment direction of liquid crystal
(LC) on some types of photo-cross-linkable polymer
surfaces has been explored by changing exposure doses
of LPUV light.6,23-26 This may offer the multidomain
alignment of LC materials. In these cases, the direction
of interaction between the LC and polymer surfaces may
be dependent on the exposure dose. The in-plane and
out-of-plane switching of photoorientation of polymer
films has been observed in some azobenzene-containing
PLCs by adjusting exposure doses of LP light or by
controlling the annealing process,16,17 although the
direction of the in-plane orientation is always perpen-
dicular to E. To date, no photoreactive polymer materi-
als have been reported which can self-induce the
reversion of in-plane molecular orientation.
The purpose of this paper is to develop a novel photo-
cross-linkable PLC that can generate high orientational
order and induce the reversion of the in-plane reorien-
tational direction. The influence of the photoisomeriza-
tion reaction on the reorientational behavior of the PLC
containing the cinnamoyl group was then clarified. To
this end, novel photo-cross-linkable methacrylates with
a 4-methoxycinnamoyloxybiphenyl side group was syn-
thesized, and reversion of the in-plane reorientation
direction of the mesogenic groups of PLC films gener-
ated by irradiation with LPUV light and subsequent
annealing was demonstrated. The orientational behav-
ior of the film was studied using polarization UV-vis
and FT-IR spectroscopies. Since the mesogenic group
in this study contains a 4-methoxycinnamoyl group, in
which the absorption band is separated from that of the
biphenyl group, spectral analysis was performed to
investigate the photoproduct influence on reorienta-
tional behavior. We have determined that the thermally
The controlled alignment of molecules in photoreac-
tive polymer films induced by photoirradiation gener-
ates unusual birefringent characteristics which may
prove to be useful in various kinds of optical devices,
such as optical memory devices, holographic elements,
and phase retarders for liquid crystal displays.1-6
Several types of materials have been reported, including
azobenzene-containing polymers and photo-cross-link-
able polymers, which can generate optical anisotropy
of their films upon irradiation with linearly polarized
(LP) light.7-22 In particular, numerous studies of azoben-
zene-containing polymers have been performed to char-
acterize the axis-selective E-to-Z photoisomerization
reaction which leads to in-plane, high molecular reori-
entation in a direction perpendicular to the electric
vector (E) of LP light.7-15 Alternatively, we studied a
photo-cross-linkable polymer liquid crystal (PLC) con-
taining a cinnamoyloxyethoxybiphenyl side group and
its copolymers which exhibit thermally stable reorienta-
tion of mesogenic side groups by the use of LP ultra-
violet (LPUV) light and subsequent annealing.20-22 The
photo-cross-linked mesogenic groups were found to
realign in the direction parallel to E of LPUV light,
caused by the axis-selective photoreaction of the cin-
namoyl substituent.21 However, the influence of the
photoisomerization of the cinnamoyl group has not been
elucidated due to the difficulty of spectrum analysis for
such a mesogenic group. This system demonstrates
three-dimensional orientation of the film by slantwise
LPUV exposure, no absorption in the visible region, and
high thermal stability;23 however, the induced reorien-
tational order of the films is low compared to that of
azobenzene-containing PLCs.
* Corresponding author.
10.1021/ma011439u CCC: $22.00 © 2002 American Chemical Society
Published on Web 12/29/2001

Macromolecules, Vol. 35, No. 3, 2002
Polymer Liquid Crystal Films 707
Ta ble 1. Yield , Molecu la r Weigh t, a n d Th er m a l
P r op er ties of Syn th esized P LCs
Sch em e 1
mol wta
thermal propertyb
c
yield Mw × 10-3
Tg
Tm
(°C)
Ti
(%)
(g/mol) Mw/Mn
(°C)
phase (°C)
P 2CB 60
P 6CB 51
78.6
67.8
2.9
3.1
148
105
153
116
N
N
>300
>300
a
Molecular weight of hydrolyzed polymer (main part is poly-
(methacrylic acid)). Determined by GPC using polystyrene stan-
b
dard. THF was used for eluent. Determined by polarization
optical microscopy and DSC. Tg ) glass transition, Tm ) melting
point, Ti ) clearing point, N ) nematic. Both polymer did not
exhibit Ti up to 300 °C. c Both polymer did not exhibit clear glass
transition. It seems that the side groups become a crystalline
structure above Tg.
(CH2)2-(CH2)2-(CH2)2-O-), 1.70-1.88 (m, 4H, -O-CH2-
CH2-(CH2)2-CH2-CH2-O-), 1.95 (s, 3H, CH2dC(CH3)), 3.86
(s, 3H, Ph-OCH3), 4.00 (t, J ) 6.6 Hz, 2H, -COO-CH2-),
4.17 (t, J ) 6.6 Hz, 2H, -CH2-O-Ph), 5.55 (s, 1H, CH2d
C(CH3)), 6.10 (s, 1H, CH2dC(CH3)), 6.52 (d, J ) 15.8 Hz, 1H,
-CHdCH-Ph), 6.93-6.95 (m, 4H, Ph), 6.99-7.2 (m, 2H, Ph),
7.21-7.22 (m, 2H, cinnamoyl-Ph), 7.51-7.52 (m, 2H, Ph-Ph),
7.55-7.83 (m, 2H, Ph-Ph), 7.85 (d, J ) 15.8 Hz, 1H, -CHd
CH-Ph). IR (KBr): 2935, 1722, 1632, 1599, 1500, 1145, 829
cm-1
.
2.2. P olym er iza tion . The polymerization of methacrylate
monomers 2CB and 6CB was performed utilizing a free radical
solution polymerization in tetrahydrofuran (THF) with 2,2′-
azobis(isobutyronitrile) (AIBN) as the initiator. The concentra-
tion of monomers and AIBN were were 10% (w/v) and 2 mol
%, respectively. As an example of a general polymerization
procedure, the synthesis of polymer P 6CB is given below.
6CB and AIBN were dissolved in THF, and the reaction
mixture was treated with a gentle stream of nitrogen. After
sealing, the mixture was heated to 60 °C for 24 h. The resulting
homogeneous solution was added dropwise into excess amount
of diethyl ether to precipitate the polymer. After two additional
precipitations from a dichloromethane solution into diethyl
ether, the polymer was extracted with diethyl ether for 1 day.
The polymer was dried at 25 °C under vacuum for 48 h. In
the case of the polymerization of 2CB, the resultant polymer,
P 2CB, was precipitated during the polymerization because of
its low solubility in THF at 60 °C. Table 1 summarizes the
yield, molecular weight, and thermal properties of synthesized
PLCs. NMR and IR spectroscopic characteristics of P 2CB and
P 6CB are as follows:
enhanced reorientation direction is dependent on the
distribution of photoproducts and the degree of photo-
reaction. Furthermore, the greatest orientational order
of a photo-cross-linkable PLC system has been observed.
2. Exp er im en ta l Section
2.1. Ma ter ia ls a n d Mon om er Syn th esis. All starting
materials were used as received from Tokyo Kasei Chemicals.
The syntheses of methacrylate monomers (nCB) and com-
pounds 1-3 are outlined in Scheme 1.
4-Acetoxy-4′-methoxybiphenyl (1) was synthesized by a
conventional method from 4-methoxy-4′-hydroxybiphenyl and
acetic anhydride, while methyl (E)-4-methoxycinnamate (2)
was generated from 4-methoxycinnamoyl chloride and metha-
nol. The syntheses of these compound were confirmed by 1H
NMR and FTIR spectroscopies. The model compound 4′-
methoxylbiphenyl (E)-4-methoxycinnamate (3) was synthe-
sized from 4-methoxycinnamoyl chloride and 4-methoxy-4′-
hydroxybiphenyl in 51% yield; mp 202 °C, Ti > 300 °C. 1H
NMR (CDCl3): δ (ppm) 3.86 (s, 6H, Ph-OCH3, Ph-Ph-OCH3),
6.5-6.53 (d, J ) 15.8 Hz, 1H, -CHdCH-Ph), 6.93-6.96 (m,
2H, cinnamoyl-Ph), 6.97-6.99 (m, 4H, Ph-Ph), 7.2-7.21 (m,
2H, cinnamoyl-Ph), 7.5-7.51 (m, 4H, Ph-Ph), 7.83-7.86 (d,
J ) 15.8 Hz, 1H, -CHdCH-Ph). IR (KBr): 2966, 1722, 1631,
P 2CB: 1H NMR (CDCl3/hexafluoro-2-propanol ) 1/1): δ
(ppm) 1.0 (brs, 3H, CH2-C(CH3)), 1.26 (brs, 2H, CH2-C(CH3)),
4.12 (m, 2H, -COO-CH2-), 4.50-4.51 (m, 2H, -CH2-O-Ph),
6.37 (m, 1H, -CHdCH-Ph), 6.85-6.97 (m, 6H, Ph), 7.26-
7.39 (m, 6H, Ph-Ph), 7.74 (m, 1H, -CHdCH-Ph). Signal of
Ph-OCH3 group is overlapped with the solvent. IR (KBr):
2930, 1723, 1632, 1600, 1495, 1245, 1134, 826 cm-1. P 6CB:
1H NMR (CDCl3): δ (ppm) 1.47-1.57 (m, 6H, -O-(CH2)2-
(CH2)2-(CH2)2-O- and CH2-C(CH3)), 1.70-1.88 (m, 4H, -O-
CH2-CH2-(CH2)2-CH2-CH2-O-), 1.74 (s, 3H, CH2C(CH3)),
3.51 (s, 3H, Ph-OCH3), 3.64 (t, J ) 6.6 Hz, 2H, -COO-CH2-),
3.81 (t, J ) 6.6 Hz, 2H, -CH2-O-Ph), 6.41 (d, J ) 15.8 Hz,
1H, -CHdCH-Ph), 6.93-6.96 (m, 2H, Ph), 7.0-7.2 (m, 4H,
Ph), 7.42-7.49 (m, 4H, cinnamoyl-Ph), 7.51-7.52 (m, 2H, Ph),
7.85 (d, J ) 15.8 Hz, 1H, -CHdCH-Ph). IR (KBr): 2930, 1723,
1599, 1494, 1137, 830 cm-1
.
4′-(4-Methoxycinnamoyl)-4-phenylphenoxyalkyl methacry-
lates (2CB and 6CB) were synthesized from 4′-hydroxy-4-
phenylphenoxyalkyl methacrylate27 and 4-methoxycinnamoyl
chloride according to the usual method.27 Yield, mp, NMR, and
IR spectroscopic characteristics are as follows: 2CB: yield
39%, mp 145 °C. 1H NMR (CDCl3): δ (ppm) 1.96 (S, 3H, CH2d
C(CH3)), 3.86 (s, 3H, Ph-OCH3), 4.27 (t, J ) 4.75 Hz, 2H,
-COO-CH2-), 4.52 (t, 2H, J ) 4.75 Hz, -CH2-O-Ph), 5.60
(s, 1H, CH2dC(CH3)), 6.16 (s, 1H, CH2dC(CH3)), 6.52 (d, J )
15.84 Hz, 1H, -CHdCH-Ph), 6.93-6.95 (m, 2H, cinnamoyl-
Ph), 6.99-7.0 (m, 2H, Ph-Ph), 7.21-7.22 (m, 2H, cinnamoyl-
Ph), 7.51-7.52 (m, 2H, Ph-Ph), 7.55-7.83 (m, 4H, Ph-Ph),
7.85 (d, J ) 15.84 Hz, 1H, -CHdCH-Ph). IR (KBr): 2935,
1723, 1631, 1598, 1495, 1145, 829 cm-1. 6CB: yield 38%, mp
89 °C. 1H NMR (CDCl3): δ (ppm) 1.47-1.57 (m, 4H, -O-
1632, 1599, 1495, 1245, 1145, 829 cm-1
.
2.3. Ch a r a cter iza tion . 1H NMR spectra were measured
with Bruker DRX-500 FT-NMR apparatus. As the synthesized
polymers were hardly soluble in THF at room temperature,
they were hydrolyzed before molecular weight determination
by gel permeation chromatography (GPC). The hydrolysis was
carried out by refluxing the polymers in 10/80/10 wt/wt/wt %
sodium hydroxide/THF/water solution for 2 days, resulting in
1
a soluble polymer in THF. By the H NMR analysis, the main
part of the hydrolyzed polymer was poly(methacrylic acid),
although further identification was not carried out at present.

708 Kawatsuki et al.
Macromolecules, Vol. 35, No. 3, 2002
Ta ble 2. Sp ectr a l An a lysis of Mod el Com p ou n d 3
absorption coefficients (ꢀ)a
wavelength
(nm)
E-isomer
Z-isomer
PF-Product
315
330
365
3.65 × 104
2.57 × 104
0
1.90 × 104
1.53 × 104
0
0.16 × 104
0.55 × 104
1.11 × 104
Absorption coefficients (L mol-1 cm-1) in methylene chloride
a
solution.
The molecular weight of these hydrolyzed polymers was
measured by GPC (Tosoh HLC-8020 GPC system with Tosoh
TSKgel column; eluent, THF) calibrated using polystyrene
standards. Thermal properties were examined using a polar-
ization optical microscope (Olympus BHA-P) equipped with a
Linkam TH600PM heating and cooling stage in addition to
differential scanning calorimetry (DSC; Seiko-I SSC5200H)
analysis at a heating and cooling rate of 10 K/min. Polarization
FTIR spectra were recorded through a J ASCO FTIR-410
system with an attached wire-grid polarizer. Polarization UV-
vis spectra were measured using a Hitachi U-3030 spectrom-
eter equipped with Glan-Taylor polarizing prisms. The in-
plane order parameter, S, is expressed in the form of eq 1:23
F igu r e 1. UV absorption spectrum of model compounds 1-3
in a methylene chloride solution.
Ap - As
A(large) + 2A(small)
S )
(1)
where Ap and As are the absorbances parallel and perpendicu-
lar to E, respectively, while A(large) is taken to be the larger of
Ap and As, while A(small) is taken to be the smaller. S was
calculated by polarized UV-vis spectroscopy at a wavelength
of 320 nm.
2.4. P h otoir r a d ia tion . Thin films of P n CB were prepared
by spin-coating a methylene chloride solution or a hexafluoro-
2-propanol (6FP) solution of polymers (∼5 wt %) onto a quartz
or CaF2 substrate. The film thickness was 0.2-0.5 µm, which
was determined by the stylus contact method using Taly-Step
(Rank Taylar Hobson). The film was set on a Linkam TH600PM
heating stage and irradiated by light from a 250 W high-
pressure Hg-UV lamp which passed through Glan-Taylor
polarizing prisms with a cutoff filter under 290 nm. The light
intensity was about 50 mW/cm2 at 313 nm. The optical
anisotropy of the film was measured by polarizing microscopy
and polarization UV-vis and FT-IR spectra.
2.5. Sp ectr a l An a lysis. Thin films of P n CBs were irradi-
ated with UV light, and the absorption spectra were measured
at intervals. The molar extinction coefficient of the Z-isomer
and photo-Friez (PF) product of 3 at each wavelength were
calculated using the determination of the E/ Z and E/ PF ratios
by means of HPLC analysis. Table 2 summarizes the results.
Spectral analysis of P n CB was performed using the procedure
given by Ichimura et al.28 according to the following expression:
F igu r e 2. Possible types of photoreaction of the mesogenic
group of 4-methoxycinnamoyloxybiphenyl.
by the spin-coating method from a methylene chloride
solution for P 6CB and from a 6FP solution for P 2CB.
To analyze the photoreaction of P n CB films, we
initially examined the UV spectra of model compounds
1-3 in methylene chloride solution as shown in Figure
1. Model compound 3 has a mesogenic side group
identical to that of P n CB, and the spectrum is similar
to that of P 6CB. It reveals that 3 exhibits several
absorption peaks that could be considered as the
absorption bands for 1 and 2. Since absorption of 2
longer than 305 nm does not overlap with that of 1, the
absorption of the cinnamate group in 3 could ap-
proximately be separated from the absorption of the
biphenyl group. This may allow evaluation of the
photoreaction of the cinnamate group of 3 using absorp-
tion bands longer than 305 nm.
D315nm ) ꢀE(315nm)[E] + ꢀZ(315nm)[Z] + ꢀPF(315nm)[PF]
D330nm ) ꢀE(330nm)[E] + ꢀZ(330nm)[Z] + ꢀPF(330nm)[PF]
D365nm ) ꢀE(365nm)[E] + ꢀZ(365nm)[Z] + ꢀPF(365nm)[PF]
It is well-known that irradiation of phenyl cinnamate
derivatives with UV light produces three types of
photoproducts as shown in Figure 2, i.e., a photodimer-
ized product (photodimer), a photoisomerized product
(Z-isomer), and a product from photo-Friez (PF) rear-
rangement.27,29 Figure 3 illustrates the spectral change
of P 6CB film upon a quartz substrate. Film absorption
was found to decrease monotonically, exhibiting a slight
increase at longer wavelengths. This increase in the
absorption at longer wavelengths is attributed to forma-
tion of a very small amount of PF product. The sup-
pression of the photo-Friez rearrangement reaction in
the film state is reported when using a methacrylate
polymer with 4-cinnamoyloxybiphenyl side groups.27
where D315nm, D330nm, and D365nm are the absorbances during
photolysis, ꢀE(315nm), ꢀZ(315nm), and ꢀPF(315 nm) are the absorbance
coefficients of E-isomer, Z-isomer, and photo-Friez products
at the corresponding wavelength, and [E], [Z], and [PF] are
their concentrations.
3. Resu lts a n d Discu ssion
3.1. P h otoch em istr y of P n CB F ilm s. The synthe-
sized PLCs exhibit a nematic liquid crystalline phase
as summarized in Table 1. Both polymers are insoluble
in THF at room temperature, while P 6CB is soluble in
chloroform, methylene chloride, and dimethylforma-
mide. However, P 2CB is insoluble in such solvents and
only soluble in 6FP. Therefore, thin films were prepared

Macromolecules, Vol. 35, No. 3, 2002
Polymer Liquid Crystal Films 709
F igu r e 3. UV absorption spectral changes in P 6CB film
during the course of irradiation with UV light.
F igu r e 5. UV polarization spectrum of P n CB films before
photoirradiation, after irradiation with 30 mJ cm-2 doses (thin
lines), and after subsequent annealing (thick lines). Ap is
shown as the solid lines, and As is shown as the dotted lines.
(a) P 2CB film, annealed at 170 °C for 1 min; (b) P 6CB film,
annealed at 130 °C for 1 min.
F igu r e 4. Fractions of E- (O) and Z- (b) isomers, photodimers
(0), and photo-Fries products (2) in thin films of P 6CB as a
function of exposure energy.
Therefore, the major photoproducts will come from
photodimerization and photoisomerization reactions of
the mesogenic group.
negative ∆A, and subsequent annealing will yield
pronounced adjustment of spectral intensities.
Figure 5a shows polarized UV-vis spectral differ-
ences between a P 2CB film irradiated with 30 mJ cm-2
doses of LPUV light and that film following subsequent
annealing at 160 °C for 5 min. After exposure, a small
negative ∆A is generated as annealing induces enhance-
ment of the negative anisotropy. The enhanced reori-
entation direction of mesogenic groups is perpendicular
to E of LPUV light, and the orientational order param-
eter S is amplified from -0.009 to -0.61. The thermal
enhancement of negative ∆A is also observed for PLCs
with azobenzene side groups, where separate mecha-
nisms have been elucidated with respect to E-to-Z
photoisomerization and thermal reorientation.7-15 The
first mechanism was clarified through use of an LP light
at a wavelength of 436 nm, where the major component
of the rod-shaped E-isomer of azobenzene resides in a
direction perpendicular to E and amplifies the negative
∆A.14 The second mechanism involves the generation
of considerable amounts of the Z-isomer in a direction
parallel to E by the use of 365 nm LPUV light which
lowers the phase transition temperature by accelerating
thermal motion of the azobenzene side groups.15 In the
case of P 2CB irradiated with 30 mJ cm-2 doses, the
photogenerated Z-isomer and the photodimer are 1.5%
and 1.9%, respectively. Reorientation of the mesogenic
groups during photoirradiation will scarcely occur as the
mobility of the mesogenic groups at room temperature
Spectral analysis was performed according to Ichimu-
ra’s method using spectral parameters as listed in Table
2.28 Results for the P 6CB film plotted in Figure 4 reveal
both photodimerization and photoisomerization take
place at an early stage of photoirradiation, although the
amount of the photodimer produced is larger than that
of the Z-isomer. Additionally, the amount of Z-isomer
produced was found to have an upper limit of 18-20%
over a prolonged exposure. The formation of PF products
amounts to less than 1% over all exposure doses. Similar
results were obtained for P 2CB films. A greater degree
of photodimerization relative to photoisomerization had
been reported for methacrylates with cinnamate side
group having a para substituent.28
3.2. En h a n cem en t a n d Rever sion of P h otoin -
d u ced Op tica l An isotr op y. Irradiation of a polymer
containing cinnamoyl groups with LPUV light leads to
negative optical anisotropy as a result of axis-selective
[2 + 2] photodimerization as well as E-to-Z photo-
isomerization.5,6,23,28,30 If a polymer with cinnamoyl
groups exhibits a liquid crystalline phase, the photoin-
duced optical anisotropy generated in an amorphous
state of the as-coated polymer film may change when
the film is annealed at elevated temperatures owing to
5,20-23
its liquid crystalline nature.
Accordingly, for
P n CB, irradiation with LPUV light generates a small

710 Kawatsuki et al.
Macromolecules, Vol. 35, No. 3, 2002
F igu r e 6. UV polarization spectrum of P n CB films before
photoirradiation, after irradiation with 450 mJ cm-2 doses
(thin lines), and after subsequent annealing (thick lines). Ap
is shown as the solid lines, and As is shown as the dotted lines.
(a) P 2CB film, annealed at 180 °C for 1 min. (b) P 6CB film,
annealed at 155 °C for 1 min.
F igu r e 7. FT-IR spectra of P n CB films on CaF2 substrates
before photoirradiation (dotted lines), after irradiation with
30 mJ cm-2 doses (thin lines), and after subsequent annealing
(thick lines). (a) P 2CB film, annealed at 170 °C for 1 min. (b)
P 6CB film, annealed at 130 °C for 1 min.
is quite low while the photoinduced ∆A is very small.
Therefore, thermal enhancement of negative ∆A will be
triggered by a small amount of Z-isomer and photo-
cross-linked photodimer in a direction parallel to E,
which may lower the Tm of the film analogously to the
parallel to E. The axis-selective irradiation decreases
mobility of the photo-cross-linked mesogenic groups that
can control the reorientational direction as reported
previously,20-22 while the greatest values of enhanced
order parameters are obtained to date for photo-cross-
linkable PLC films. The thermally enhanced optical
anisotropy of P 2CB is revealed to be smaller than that
of P 6CB in Figure 6. The mobility of mesogenic groups
during thermal reorientation with a shorter methylene
spacer will be limited. Additionally, aggregation is
suppressed for P 6CB as a certain amount of photore-
action products will restrain molecular aggregation as
a result of the decrease in liquid crystallinity of the PLC.
behavior of the polymer with azobenzene groups.15
A
detailed discussion is provided in section 3.5. In addi-
tion, the thermal enhancement of negative ∆A is also
observed for films of P 6CB, although a red shift of the
absorption band takes place, as shown in Figure 5b.
This red shift is a consequence of the J -aggregation of
mesogenic groups caused by annealing and results in a
lower ∆A in comparison with that of P 2CB.
In contrast, reversion of the amplification of the
thermal reorientation order is observed when the ir-
radiation dose is 450 mJ cm-2, as shown in Figure 6.
In these cases, the amounts of photogenerated Z-isomer
and photodimer are 7% and 17% for P 2CB and 8% and
17% for P 6CB, respectively. This shows that the pho-
toinduced negative optical anisotropy after exposure is
larger than in films with 30 mJ cm-2 doses because of
the larger degree of axis-selective photoreaction. This
strongly suggests that an efficient, axis-selective, photo-
cross-linking reaction of the cinnamoyl group is occur-
ring. After annealing, S is amplified from -0.05 to +0.60
for P 2CB and from -0.03 to +0.68 for P 6CB. This
phenomenon is attributed to the thermally enhanced
reorientation of unreacted mesogenic groups along the
photo-cross-linked mesogenic groups in a direction
3.3. F T-IR Sp ectr oscop y. The reversion of reorien-
tation direction was further investigated by polarized
FT-IR spectroscopy measurements. Parts a and b of
Figure 7 show FT-IR spectral differences of P 2CB and
P 6CB films prior to irradiation with LPUV light and
after irradiation with 30 mJ cm-2 doses and subsequent
annealing. After irradiation, a slight decrease in the
absorption of the cinnamoyl group at 1632 cm-1 (νCd
C) is observed for P 2CB, although differential spectra
(Ap - As) reveal a very small dichroism, indicating that
reorientation of the mesogenic groups can be negligible.
A similar change is observed for P 6CB. Large adjust-
ment in absorbance appeared after annealing. For Ap
of P 2CB film, intensities of absorption bands at 1632
cm-1, 1600 cm-1 (νPh-Ph), 1495 cm-1 (νPh), and 1245

Macromolecules, Vol. 35, No. 3, 2002
Polymer Liquid Crystal Films 711
F igu r e 9. Difference in the order parameters S of P n CB films
after irradiation with LPUV light (open points) and after
subsequent annealing (closed points), as a function of the
exposure energy. P 2CB films were annealed at 180 °C for 1
min and P 6CB films were annealed at 155 °C for 1 min for
all cases. Circles and squares denote S for films of P 2CB and
P 6CB, respectively.
sogenic groups including methylene spacers reorient
thermally in a direction parallel to E. Furthermore, S
values at 1495 cm-1 are +0.53 for P 2CB and +0.65 for
P 6CB. These values are also in good agreement with
that obtained by UV-vis spectra.
3.4. Effect of Exp osu r e En er gy. To elucidate the
effect of exposure energy on the reversion of direction
for thermally enhanced reorientation of mesogenic
groups, films were irradiated with LPUV light over
various exposure doses. Results are shown in Figure 9
where a plot of the order parameter S after irradiation
and after subsequent annealing as a function of expo-
sure energy is given. All irradiated films of P 2CB and
P 6CB were annealed at 180 and 155 °C for 1 min,
respectively.
F igu r e 8. FT-IR spectra of PnCB films on CaF2 substrates
after irradiation with 450 mJ cm-2 doses (thin lines), and after
subsequent annealing (thick lines). (a) P 2CB film annealed
at 180 °C for 1 min. (b) P 6CB film annealed at 155 °C for 1
min.
Figure 9 reveals the S value after irradiation is
negative regardless of exposure dose and that increasing
exposure doses for both PLCs results in increasing S
values. This is due to the axis-selective photoreaction
of the cinnamoyl group as described in section 3.2. When
the exposure doses were less than 100 mJ cm-2, an-
nealing amplified the negative value of S, with a degree
of photo-cross-linking of the side groups being less than
about 13% and with the amount of Z-isomer being less
than about 5%. Further irradiation inverts the reori-
entation direction after annealing. The maximum posi-
tive S is obtained when the degree of photo-cross-linking
of the side groups was 15-20%, and the amount of
Z-isomer was 7-10%. In addition, annealing of the films
irradiated with doses of 2 J cm-2 scarcely enhanced
molecular reorientation as a high degree of photo-cross-
linking restricts mobilization of the mesogenic groups
at elevated temperatures. These results suggest that the
thermally enhanced reorientation behavior is strongly
dependent on the degree of photoreaction and on the
ratio of the type of photoreacted products, which may
affect the thermal property of exposed film.
cm-1 (νPh-O) are pronouncedly decreased, while a
slight increase at 1723 cm-1 (νCdO) and around 2930
cm-1 (νC-H) can be observed. As absorption bands,
however, exhibit opposite changes. The differential
spectrum reveals clear enhancement of photoinduced
dichroism in the same direction by annealing. This
result indicates that mesogenic side groups thermally
reorient in a direction perpendicular to E. S is amplified
at 1495 cm-1 to -0.61, in good agreement with the value
obtained by UV-vis spectra. For P 6CB films, a similar
tendency for adjustment in the absorption bands is
observed, but the difference between Ap and As after
annealing was found to be relatively small. This is a
consequence of the aggregation of mesogenic groups as
described in the above section.
When films are irradiated with 450 mJ cm-2 doses,
there is a greater photoinduced dichroism than arises
from 30 mJ cm-2 doses as shown in Figure 8a. In
contrast to Figure 7, annealing leads to increases in
absorption bands at 1632 cm-1, 1600, 1495, and 1245
cm-1, with decreases observed at 1723 cm-1 for Ap of
both PLC films. As, however, exhibits opposite adjust-
ment to these absorption bands. The differential spec-
trum reveals these changes are entirely in the opposite
direction as compared to films with 30 mJ cm-2 doses.
In addition, absorption around 2930 cm-1 (νC-H) in the
differential spectrum in Figure 8b distinctly displays
negative dichroism. These results suggest that me-
3.5. Effect of An n ea lin g Tem p er a tu r e. As de-
scribed above, thermally enhanced molecular reorienta-
tion is triggered by the photoinduced optical anisotropy
of the irradiated film. Therefore, mobility of the me-
sogenic side groups will depend on the annealing
temperature. Accordingly, we evaluated the reorienta-
tional behavior of films annealed at various tempera-

712 Kawatsuki et al.
Macromolecules, Vol. 35, No. 3, 2002
F igu r e 11. Model illustration of the reversion of thermally
enhanced in-plane reorientation of PnCB films.
30 mJ cm-2 doses, R values annealed at Tm - 5 are close
to that at Tm and at Tm + 5. This shows that thermally
enhanced reorientation occurs below Tm of the PLC
when the irradiation energy is 30 mJ cm-2, indicating
Tm of the PLC decreased after exposure as described in
section 3.2. In contrast, R values for PLC irradiated with
450 mJ cm-2 doses gradually increase with increasing
annealing temperature, and R values at Tm - 5 are
much less than for films with 30 mJ cm-2 doses. It is
suggested that the larger amount of photo-cross-linked
side groups restrict the mobility of mesogenic side
groups rather than lowering the Tm of the film; i.e.,
photo-cross-linked side groups control the thermally
enhanced molecular reorientation in a direction parallel
to E.
For P 2CB film irradiated for 90 mJ cm-2, it is
interesting that the enhanced negative value of S
decreased with increasing annealing temperature and
became positive with an annealing temperature of 250
°C. This indicates reorientation in a direction perpen-
dicular to E is generated by a similar manner with film
irradiated at 30 mJ cm-2 when an annealing temper-
ature of about Tm is used. However, nonmobile photo-
cross-linked groups could act as the command group for
thermal reorientation in a direction parallel to E when
the annealing temperature is increased.
F igu r e 10. Change in in-plane order parameters (S) of P n CB
film as a function of annealing temperature at various irradia-
tion doses: O, 30 mJ cm-2; b, 450 mJ cm-2; 9, 90 mJ cm-2
(a) P 2CB; (b) P 6CB.
.
Ta ble 3. Ra tio of Or d er P a r a m eter S a t Va r iou s
An n ea lin g Tem p er a tu r es
Sx/S(Tm + 35)a
dosesb
(mJ cm-2
)
Tm - 5c
Tm
Tm + 5c
c
4. Con clu sion s
P 2CB
P 6CB
30
450
30
0.71
0.25
0.70
0.27
1.0
1.01
0.65
0.82
0.76
The reversion of in-plane reorientation of PLC films
was investigated by irradiation with linearly polarized
ultraviolet (LPUV) light and subsequent annealing,
whereby a high orientational order in both directions
was achieved. Spectral analysis of the films suggested
that that both photoisomerization reactions and the
photo-cross-linking reactions occurred in the mesogenic
groups of the films, with the degree of the photo-cross-
linking reaction occurring to a greater extent than that
of others and with the photo-Friez rearrangement
occurring to the least extent. The mechanism of rever-
sion of the orientational direction can be related to the
degree and type of photoreaction of the mesogenic
groups, as illustrated in Figure 11. Thermal amplifica-
tion of the perpendicular orientation of the films is
induced by a small amount of photoproducts in a
direction parallel to E, resulting in reorientation of the
mesogenic groups perpendicular to E. On the other
hand, when the degree of the photo-cross-linking be-
comes sufficient in a direction parallel to E, reversion
of orientation direction is generated because of the
decrease in mobility at an elevated temperature in a
direction parallel to E. We anticipate these photoreac-
tive PLC films may find application in new optical
devices such as birefringent films for LCD and in optical
memory devices.
0.53
0.81
0.42
450
a
Ratio of order parameter annealed at Tx °C and at Tm + 35
°C. Irradiation energy. c Annealing temperature, °C.
b
tures, using doses of 30, 90, and 450 mJ cm-2. Results
shown in parts a and b of Figure 10 plot the order
parameter S for both films as a function of the annealing
temperature. For all cases, an explicit magnification of
S appears when the annealing temperature is larger
than Tm of the PLC. When films are annealed above Tm,
the direction of reorientation is found to be perpendicu-
lar to E when irradiated with 30 mJ cm-2 or parallel
for films subjected to 450 mJ cm-2 doses, respectively.
S of P 2CB films with 90 mJ cm-2 doses, however, are
initially negatively enhanced and then positively in-
crease upon increase of annealing temperature.
Looking closely at Figure 10, the threshold temper-
ature for thermal reorientation varies with exposure
energy. Table 3 summarizes the ratio of the thermally
enhanced order parameter (R ) Sx/S(T +35)) with differ-
ent exposure doses, where S(T
Tm + 35 (°C), and Sx is S annealed at Tm - 5, Tm, and
Tm - 5 (°C) of the PLCs. For both PLCs irradiated with
m
is taken as S at
m+35)

Macromolecules, Vol. 35, No. 3, 2002
Polymer Liquid Crystal Films 713
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Ack n ow led gm en t. This work was supported by a
Grant-in-aid for Scientific Research from the Ministry
of Education, Science and Culture of J apan.
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