Molecularly oriented liquid crystalline polymer films based on axis-selective photo-fries rearrangement

Article abstract of DOI:10.1080/15421406.2013.843257

Four kinds of liquid crystalline polymers (LCPs) with phenyl benzoate side groups were synthesized and their photoinduced reorientation behavior was investigated using linearly polarized (LP) UV light based on the axis-selective photo-Fries rearrangement

Full text of DOI:10.1080/15421406.2013.843257

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Molecularly Oriented Liquid Crystalline
Polymer Films Based on Axis-Selective
Photo-Fries Rearrangement
Nobuhiro Kawatsuki a , Hitomi Matsushita a , Naoki Shin a , Mami
Kurita a & Mizuho Kondo a
a Department of Materials Science and Chemistry, Graduate School
of Engineering , University of Hyogo , 2167 Shosha Himeji, Hyogo ,
671-2280 , Japan
Published online: 16 Dec 2013.
To cite this article: Nobuhiro Kawatsuki , Hitomi Matsushita , Naoki Shin , Mami Kurita &
Mizuho Kondo (2013) Molecularly Oriented Liquid Crystalline Polymer Films Based on Axis-
Selective Photo-Fries Rearrangement, Molecular Crystals and Liquid Crystals, 583:1, 1-9, DOI:
10.1080/15421406.2013.843257
To link to this article: http://dx.doi.org/10.1080/15421406.2013.843257
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Mol. Cryst. Liq. Cryst., Vol. 583: pp. 1–9, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.843257
Molecularly Oriented Liquid Crystalline Polymer
Films Based on Axis-Selective Photo-Fries
Rearrangement
NOBUHIRO KAWATSUKI,∗ HITOMI MATSUSHITA,
NAOKI SHIN, MAMI KURITA, AND MIZUHO KONDO
Department of Materials Science and Chemistry, Graduate School of
Engineering, University of Hyogo, 2167 Shosha Himeji, Hyogo 671-2280, Japan
Four kinds of liquid crystalline polymers (LCPs) with phenyl benzoate side groups
were synthesized and their photoinduced reorientation behavior was investigated using
linearly polarized (LP) UV light based on the axis-selective photo-Fries rearrangement
and thermally induced self-organization of the mesogenic side groups of the materials.
Due to different effect of the axis-selectively photoreacted side groups, reorientation
direction and its performance depended on the chemical structure of the side groups.
Both alignment ability and the molecular rotation of the photoreacted side groups played
an important role in the reorientation behavior.
Keywords polymer liquid crystal; photo-Fries rearrangement; photoinduced orienta-
tion; birefringent film
Introduction
Photoalignment of functional materials based on an axis-selective photoreaction of photore-
active materials has received much attention due to its practical application to liquid crystal
displays (LCDs) [1]. Many photoalignment materials are developed for the LC alignment
layers [1–3]. However, most of the photoalignment layers do not exhibit molecular orien-
tation of the film. When combining with thermally induced self-organization of mesogenic
groups in photoreactive liquid crystalline polymer (LCP) films, axis-selective photoreaction
of the film generates uniaxial molecular orientation [4]. In this case, molecular reorien-
tation of the mesogenic groups in the LCP film in a direction parallel or perpendicular
to the polarization of linearly polarized (LP) light is generated, where the axis-selectively
photoreacted mesogenic groups act as command in bulk for the self-organization at the
LC temperature range of the material. We reported that several types of LCPs with cin-
namate or cinnamic acid side groups exhibited efficient thermally enhanced photoinduced
molecular reorientation, which can be applicable to birefringent films (phase retarder), LC
photoalignment layer and polarization-holographic devices [5–8].
It is known that phenyl benzoate derivatives photoreact with exposure to UV light to
produce photo-Fries rearrangement products [9, 10]. This photoreaction undergoes axis-
selectively, and some photopolymers with methacrylamidoaryl side groups were applied
∗Address correspondence to Nobuhiro Kawatsuki. Tel.: +81-79-267-4886; Fax: +81-79-267-
4885. E-mail: kawatuki@eng.u-hyogo.ac.jp
1

2
N. Kawatsuki et al.
Figure 1. Chemical structure of polymers in this study.
to the LC photoalignment layer [11, 12]. Recently, we have found that polymethacrylates
with 4-methoxyphenyl benzoate side group derivatives axis-selectively photoreact under
LPUV light exposure and show sufficient photoinduced molecular orientation [13].
In this study, we synthesized several LCPs comprised of benzoate side groups including
biphenyl moiety (Fig. 1) and investigated the influence of the position of the biphenyl
moieties in the side groups on the photoreactivity and photoinduced orientation behavior
using LP-248 and 313 nm light.
Experimental
Materials and Polymer Synthesis
All starting materials were used as received from Tokyo Kasei Chemicals. Azobisisobuty-
ronitrile (AIBN) was recrystallized from ethanol and stored below 0◦C.
All polymers (Fig. 1) were synthesized by radical polymerization in THF using AIBN
as an initiator. Polymers P1 and P3 were synthesized previously [13, 14], and polymers P2
and P4 were synthesized according to the procedure shown in Scheme 1. Monomer and
polymer synthesis was determined by 1H-NMR. Molecular weight and thermal properties
of synthesized polymers are summarized in Table 1.
Synthesis of monomer M2. Methacrylate monomer with 4-phenylbenzoic acid side groups
(BAM, 3.5 g 9.2 mmol) was dissolved in chloroform (10 ml) and SOCl2 (2.2g, 18 mmol)
and 3 drop of DMF. After stirring for 3 h at r.t., an acid chloride was obtained by evaporating
the excess SOCl2 and solvent. Then, this acid chloride was dissolved in THF (20 ml), and
added to a solution of p-methoxy phenol (1.2 g, 10 mmol), triethylamine (1.4 g, 14 mmol) in
10 ml of THF at 0◦C. The solution was stirred at r.t. for 19 h. After filtration, the solution was
evaporated then extracted by chloroform. The organic layer was dried over sodium sulfate
and evaporated. The crude product was purified by silica-gel column chromatography

Photoinduced Reorientation of LCP based on Photo-Fries Rearrangement
3
Scheme 1.
(eluent: chloroform). The products were purified by recrystallization from ethyl acetate and
hexane. Yield: 1.8 g (40 mol%) C 73 LC 181 I. 1H NMR (CDCl3): δ (ppm) 1.51–1.88 (m,
8H), 1.98 (s, 3H), 3.86, (s, 3H), 4.05 (t, J/Hz = 6.3 Hz, 2H), 4.20 (t, J/Hz = 6.5Hz, 2H),
5.58 (s, 1H), 6.13 (s, 1H), 6.98 (d, J/Hz = 9.0 Hz, 2H), 7.03 (d, J/Hz = 9.0Hz, 2H), 7.18
(d, J/Hz = 8.5 Hz, 2H), 7.63 (d, J/Hz = 8.5 Hz, 2H), 7.71 (d, J/Hz = 8.5 Hz, 2H) 8.26 (d,
J/Hz = 8.5 Hz, 2H). IR (KBr): 2936, 2858, 2837, 1725, 1603, 1509, 1248, 1196 cm−1
.
Monomer M4 was synthesized by a similar method to synthesis of M2 from monomer
1
HBM and 4-methoxybenzoyl chloride. H NMR (CDCl3): δ (ppm) 1.47–1.56 (m, 4H),
1.72–1.76 (m, 2H), 1.80–1.84 (m, 2H), 1.97 (s, 3H), 3.89, (s, 3H), 3.98 (t, J/Hz = 6.4 Hz,
2H), 4.19 (t, J/Hz = 6.6Hz, 2H), 5.58 (t J/Hz = 1.6 Hz, 1H), 6.13 (s, 1H), 6.51 (d, J/Hz =
16 Hz, 2H), 6.97–6.91 (m, 4H), 7.09 (q, J/Hz = 4.9 Hz, 2H), 7.56 (d, J/Hz = 8.5 Hz, 2H)
7.84 (d, J/Hz = 16 Hz, 2H).
Synthesis of polymer P2. Methacrylate monomer (M2) (1.0 g, 2.1 mmol) and AIBN
(3.5 mg, 0.21 mmol) were dissolved in 10 ml of THF, degassed and heated at 55◦C
for 1 day. Polymer was obtained by precipitating to diethyl ether, and purified by repre-
1
cipitations from THF solution to diethyl ether for 2 times. Yield, 0.6 g (60%). H NMR
(CDCl3): δ (ppm) 0.97–1.88 (br, 8H), 1.79 (br, 3H), 3.77 (brs, 3H), 3.91 (br, 2H), 3.98 (br,
2H), 6.87 (br, 4H), 7.07 (br, 2H), 7.47 (br, 2H), 7.55 (br, 2H), 8.12 (br, 2H).
Polymer P4 was synthesized by similar method to P4 from M4. 1H NMR (CDCl3): δ
(ppm) 0.93–1.75 (br, 13H), 3.83 (br, 7H), 6.87 (br, 4H), 7.12 (br, 2H), 7.37 (br, 2H), 7.45
(br, 2H), 8.06 (br, 2H).
Table 1. Molecular weight, thermal and spectroscopic properties of polymers
Molecular weighta)
Thermal propertyb)
(◦C)
λmax
c)
Polymer
MnX10−3
Mw/Mn
(nm)
P1
P2
P3
P4
18.0
31.1
38.0
28.1
1.7
2.0
1.7
2.3
G 45 N 115 I
K 123 N1294 I
G 71 K 143 N 283 I
G 65 K 131 N 284 I
259
287
266
263
a) Determined by by GPC. Polystyrene standards, chloroform eluent. b) Determined by DSC. 2nd
heating. G: glassy, K: crystal, N: nematic, I: isotropic. c) On quartz substrate.

4
N. Kawatsuki et al.
Characterization
Thermal properties were examined using a polarization optical microscope (POM) and a
differential scanning calorimetry (DSC) analyzer. Polarization UV-vis spectra were mea-
sured using a spectrometer equipped with Glan-Taylor polarizing prisms. The in-plane
order parameter, S, is expressed in the form of equation 1;
A|| − A⊥
S =
(1)
A|| + 2A⊥
where A and A are the absorbances parallel and perpendicular to polarization (E) of the
ꢀ
⊥
LP-248 or 313 nm light, respectively. S was calculated by polarized UV-vis spectroscopy
using wavelengths at λmax of the materials. Equation 1 indicates that the molecular reorien-
tation direction is parallel and perpendicular to E when the S value is positive and negative,
respectively. The retardation of the film was evaluated using a polarimeter at 517 nm.
Photoirradiation
Thin films were prepared by spin-coating a THF solution of copolymers (1 – 5 wt/wt-%)
onto quartz substrates. The film thickness was approximately 0.2 – 1.0 μm. The film was
irradiated by light from a Xe lamp with a band-path filter at 248 nm or 313 nm that was
passed through a glass placed at Brewster’s angle. The light intensity was 10 mW/cm2
at 248 nm and 5 mW/cm2 at 313 nm, respectively. For the thermally enhanced molecular
reorientation, the irradiated film was annealed at elevated temperatures for 10 min. The
generated optical anisotropy of the film was measured by POM and polarization UV-vis
spectroscopy.
Results and Discussion
Photoreaction of Polymer Films
All synthesized polymers exhibited nematic LC characteristics (Table 1), and UV absorption
spectra of the films revealed that all the films were transparent in the visible region. Because
of the longer conjugation of mesogenic side groups, λmax of P2 is longer than other PLC
films as summarized in Table 1. Furthermore, the absorption properties of P3 and P4 are
similar to each other due to their similar chemical structure with opposite connection to
the main chain. Therefore, a 248 nm light was used for the photoreaction of P1, P3 and P4
films, and a 313 nm light was used for P2.
All LCPs are photoreacted by irradiation with UV light to form photo-Fries products
(Scheme 2). Several studies have reported photo-Fries rearrangement of phenyl benzoate
derivatives, where the photodegradation occurs followed by the formation of photo-Fries
products (o-hydroxy phenyl ketone) [15, 16] Figures 2a-d shows change in UV absorption
spectra of P1 - P4 films when films were exposed to 248 nm or 313 nm light. For all
films, absorption bands around 250 – 280 nm decreased, λmax shifted to longer wavelength
and absorption at longer wavelength (λ > 300 nm) increased when the exposure energy
increased. This is due to the photo-Fries rearrangement of mesogenic side groups as shown
in Scheme 2. Additionally, the spectrum changes of P3 and P4 films are similar because the
mesogenic side groups of these polymers are similar with opposite connection to the main
chain. Furthermore, the absorbance at the longer wavelength’s region decreased when the

Photoinduced Reorientation of LCP based on Photo-Fries Rearrangement
(a)
5
(b)
1.2
1.2
Exposure energy
(J/cm2 )
Exposure energy
(J/cm2 )
1
1
0
0
0.3
0.1
0.5
5
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
0.1
2.5
7.5
15
26
40
20
40
250 300 350 400 450 500
Wavelength (nm)
250 300 350 400 450 500
Wavelength (nm)
(c)
(d)
1.2
1
1.2
1
Exposure energy
(J/cm2 )
Exposure energy
(J/cm2 )
0
0
0.8
0.6
0.4
0.2
0
1
0.4
1.3
5
0.8
0.6
0.4
0.2
0
3
12
20
28
42
26
33
42
250 300 350 400 450 500
Wavelength (nm)
250 300 350 400 450 500
Wavelength (nm)
Figure 2. Change in UV-vis absorption spectra of polymer films when exposed to 248 nm or 313 nm
light. (a) P1, (b) P2, (c) P3, and (d) P4 films.
Scheme 2.
exposure energy was greater than 5 – 20 J/cm2. This is due to the H-abstraction reaction by
ketone moiety in the photo-Fries products to induce photo-cross-linking of the films [13].
When LP-248 or LP-313 nm was used for the irradiation, photoinduced negative ꢀA
( = A -A ) was observed for all polymer films as shown in Fig. 3. This means that the photo-
ꢀ
⊥
Fries rearrangement axis-selectively occurs for all the films. The maximum photoinduced
optical anisotropy is observed when degree of the photoreaction (DP) is around 40 – 50%.
Additionally, ꢀA of P3 and P4 films are similar to each other due to the similar mesogenic
side groups as described above. However, the generated maximum ꢀA and the resultant
order parameter is less than 0.15 and 0.1, respectively, for all cases because molecular
reorientation does not occur. These photoinduced ꢀA without the molecular reorientation
is similar to the case of axis-selective photoreaction of photopolymer films containing
cinnamate, chalcone and coumarin groups [3, 4, 17–20].

6
N. Kawatsuki et al.
0.05
0
-0.05
-0.1
-0.15
-0.2
P1
P2
P3
P4
0.001 0.01 0.1
1
10 100
Exposure energy (J/cm2)
Figure 3. Photoinduced optical anisotropy (ꢀA) of polymer films as a function of exposure energy.
ꢀA values are at 259 nm for P1, 287 nm for P2, 266 nm for P3, and 263 nm for P4.
Thermal Amplification of Photoinduced Optical Anisotropy of Polymer Films
The photoinduced small optical anisotropy was amplified for a P1 film when the exposed
film (DP∼5%) was annealed at elevated temperatures, where the molecular reorientation
perpendicular was generated as shown in Fig. 4a. Small photoinduced ꢀA ( = −0.012 at
259 nm) was amplified to the same direction (ꢀA = −1.49) after annealing. In contrast,
thermally enhanced molecular reorientation parallel to E was generated for a P3 film when
the DP was around 20% as shown in Fig. 4b. In this case, photoinduced negative ꢀA ( =
−0.104 at 265 nm) was amplified to the positive one (ꢀA = +1.72), where the molecular
reorientation direction was parallel to E. These amplification behaviors were observed when
the exposed films were annealed in the liquid crystal temperature range of the material.
This is due to the thermally induced self-organization of the film, which is similar to the
case of photoinduced reorientation in LCPs with cinnamate and cinnamic acid mesogenic
side groups [4–7]. However, the thermal amplification in P2 and P4 films rarely occurred at
(a)
(b
)
2
1.5
1
2
1.5
1
after annealing( )
after annealing(||)
initial
after exposure( )
after exposure(||)
initial
after exposure( )
after exposure(||)
after annealing( )
after annealing(||)
0.5
0
0.5
0
250 300 350 400 450 500
Wavelength(nm)
250 300 350 400 450 500
Wavelength(nm)
Figure 4. (a) Change in the polarization UV-vis polarization spectrum of a P1 film before photoir-
radiation, after irradiating for 0.01 J/cm2 (thin lines), and after subsequent annealing at 80◦C for
10 min (thick lines). (b) Change in the polarization UV-vis polarization spectrum of a P3 film before
photoirradiation for 0.7 J/cm2 after subsequent annealing at 200◦C for 10 min (thick lines). Solid line
represent A , while dotted lines show A .
ꢀ
⊥

Photoinduced Reorientation of LCP based on Photo-Fries Rearrangement
7
Table 2. Fabrication conditions, maximum thermally enhanced S values and generated
birefringence
Exposure energy
(J/cm2)a)
DP
Annealing
Polymer
DP (%) temperature (◦C) S (λ, nm) Birefringence b)
P1
P2
P3
P4
0.01
—
0.7
0.7
3
80
0.57 (259)
<0.01
0.75 (266)
0.08 (263)
0.15
—
0.23
<0.05
—
18
18
200
200
210
a) P1, P3 and P4 films were exposed to 248 nm light, while P2 was exposed to 313 nm light. b)
Measured at 517 nm by a polarimeter.
any exposure and annealing conditions. Table 2 summarizes maximum order parameters,
generated birefringence and their fabricated condition of polymer films.
The thermal amplification of the photoinduced optical anisotropy depended on the
exposure doses. Figure 5 plots thermally amplified S values of LCP films as a function
of exposure doses when the exposed films are annealed at the LC temperature range of
each film. For P1, sufficient molecular reorientation perpendicular to E was observed
when the exposure energy was 0.007 – 0.07 J/cm2, where DP was around 5%. But the
molecular reorientation parallel to E was not observed at any exposure doses. For P2 and
P4, photoinduced anisotropy was rarely amplified at all the exposure energy. In contrast,
for P3, thermal amplification of the molecular reorientation reversely occurred when the
exposure energy was 0.5 – 3 J/cm2, where DP was around 15 - 25%. Sufficient amount
of axis-selectively formed photo-Fries products generated the reorientation parallel to
E. Additionally, reorientation perpendicular to E was observed at the early stage of the
photoreaction similar to the case of P1, but the enhanced S values were less than 0.2.
In case of P1, small amount of photo-Fries products parallel to E acted as impurity to
inhibit the self-organization in the parallel direction, resulting in the self-organization per-
pendicular direction in which the film showed greater LC characteristics. Since photo-Fries
products from P1 do not reveal LC characteristics and do not possess the reorientation abil-
ity along them, self-organization occurs in the perpendicular direction. The rotational effect
0.8
0.6
0.4
0.2
P1
P2
P3
P4
0
-0.2
-0.4
-0.6
-0.8
0.001 0.01 0.1
1
10 100
Exposure energy (J/cm2)
Figure 5. Thermally enhanced S values at λmax of photoreacted polymer films as a function of
exposure energy. Annealing temperatures: 80◦C for P1, 200◦C for P2 and P3, 210◦C for P4.

8
N. Kawatsuki et al.
Figure 6. Schematic explanation of thermally enhanced molecular reorientation mechanism. (a) P1,
(b) P2, (c) P3, and (d) P4.
of the photo-Fries products at elevated temperature upon the self-organization process plays
a role in determining the reorientation behavior (Fig. 6a). Similar thermal amplification of
photoinduced optical anisotropy in the same direction was observed in other type of LCP
films comprised of cinnamates, cinnamic acid, and azobenzene side groups [4, 6, 21–25].
However, photo-Fries products parallel to E for P2 contain biphenyl moieties, which may
possess non-negligible alignment ability. Therefore, thermally induced self-organization in
both directions does not occur as illustrated in Figure 6b.
In contrast, parallel reorientation is generated for P3 because sufficient amount of
photo-Fries products parallel to the polarization act as photoreacted anchors to “lock”
the reorientation direction (Fig. 6c). Certain amounts of photo-Fries products with bulky
photoreacted end groups cannot rotate to inhibit the parallel reorientation at elevated tem-
perature, which induce thermally generated self-organization parallel to them. At the initial
stage of the photoreaction of a P3 film, photo-Fries products did not control the parallel
reorientation, but they acted as impurities similar to P1 to induce small reorientation per-
pendicular to E. On the other hand, photoreacted products cannot control the reorientation
direction for P4, where the self-organization did not occur since 4-methoxyphenyl end
groups easily rotate as illustrated in Fig. 6d. Namely, the rotation of end groups compete
with parallel reorientation ability of the biphenyl groups that are close to the main chain,
resulting in the lower reorientation order.
Conclusion
LCPs with benzoate side groups were synthesized and thermally enhanced photoinduced
molecular reorientation of the thin films was investigated based on the axis-selective photo-
Fries rearrangement. Similar axis-selective photoreaction of the thin films was observed

Photoinduced Reorientation of LCP based on Photo-Fries Rearrangement
9
for all the films but the thermally amplified reorientation behavior was different among the
polymers. For P1 and P3, sufficient reorientation was obtained, but reorientation perfor-
mance of P2 and P4 was poor. The axis-selectively generated photo-Fries products played
an important role in the reorientation ability and self-organization direction. The reoriented
films are applicable to birefringent films and patterned retarder films for LCDs by means
of patterned LPUV exposure.
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