Photoinduced reorientation and polarization holography in photo-cross-linkable liquid crystalline polymer films with large birefringence

Article abstract of DOI:10.1016/j.polymer.2010.04.043

A new photo-cross-linkable liquid crystalline polymer (PLCP) comprised 4-methoxycinnamoyloxy groups connected with a bistolane side group was synthesized to investigate thermally enhanced photoinduced molecular reorientation of a thin film with linearly polarized (LP) 365. nm light exposure. Due to the axis-selective photoreaction of the cinnamate groups followed by the thermally induced self-organization, large molecular reorientation parallel to the polarization of LPUV light (S>0.6) and large birefringence at the non-resonance region (Δ n=0.34 at 632.8. nm) were obtained. The obtained Δ n value is the largest among transparent PLCPs in the visible region. The influence of the degree of photoreaction and the annealing temperature on the thermally enhanced molecular reorientation behavior was explored in detail. Finally, for an application of thin optical devices using the PLCP films, pure polarization holographic gratings with large birefringence, which showed periodic molecularly oriented structure, were fabricated using a 325. nm He-Cd laser in various polarization modes and characterized their optical properties.

Full text of DOI:10.1016/j.polymer.2010.04.043

Polymer 51 (2010) 2849e2856
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Photoinduced reorientation and polarization holography in photo-cross-linkable
liquid crystalline polymer films with large birefringence
,
a
a
b
b
Nobuhiro Kawatsuki a , Ayumi Yamashita , Mizuho Kondo , Taro Matsumoto , Tatsutoshi Shioda ,
*
Akira Emoto b, Hiroshi Ono b
a Department of Materials Science and Chemistry, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji 671-2280, Japan
b Department of Electrical Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka 940-2188, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A new photo-cross-linkable liquid crystalline polymer (PLCP) comprised 4-methoxycinnamoyloxy groups
connected with a bistolane side group was synthesized to investigate thermally enhanced photoinduced
molecular reorientation of a thin film with linearly polarized (LP) 365 nm light exposure. Due to the axis-
selective photoreaction of the cinnamate groups followed by the thermally induced self-organization,
large molecular reorientation parallel to the polarization of LPUV light (S > 0.6) and large birefringence at
Received 5 January 2010
Received in revised form
6 April 2010
Accepted 20 April 2010
Available online 28 April 2010
the non-resonance region (
D
n ¼ 0.34 at 632.8 nm) were obtained. The obtained
Dn value is the largest
among transparent PLCPs in the visible region. The influence of the degree of photoreaction and the
annealing temperature on the thermally enhanced molecular reorientation behavior was explored in
detail. Finally, for an application of thin optical devices using the PLCP films, pure polarization holo-
graphic gratings with large birefringence, which showed periodic molecularly oriented structure, were
fabricated using a 325 nm HeeCd laser in various polarization modes and characterized their optical
properties.
Keywords:
Polymer liquid crystal
Photoinduced orientation
Birefringent film
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
are not utilized for display applications because they are colored in
the visible region. In contrast, thermally enhanced molecular
Much attention has been paid to the photoinduced molecular
reorientation of photoreactive polymeric materials due to large
number of their applications, such as optical and holographic
memories, birefringent optical devices and photoalignment layers
for liquid crystal displays [1e8]. Several types of photoreactive
polymers, including azobenzene-containing polymers [2e4,9e12]
and photo-cross-linkable liquid crystalline polymers (PLCPs) with
cinnamate or coumarin end groups [5,6,13e16], have been inves-
tigated. New approach for the photoinduced molecular reor-
ientation includes the polymers based on the ionic self-assembly
that exhibited very large molecular orientation. [17] The axis-
selective photoreaction of these polymeric films was initiated by
the use of linearly polarized (LP) light irradiation [18]. For the
reorientation parallel to E after the axis-selective photoreaction
occurs in the cinnamate-containing PLCP films due to the LC
characteristics of the material, which causes self-organization [6].
The selectively photoreacted groups parallel to the polarization of
the LP light act as a photo-cross-linked anchor to thermally reorient
the non-reacted mesogenic groups along them [6,14]. Since the
reoriented PLCP films are transparent in the visible region they are
useful for display applications [19,20]. To attain thinner molecularly
reoriented optical devices, we need larger photoinduced optical
birefringence. Therefore, both an efficient molecular orientation
and
materials.
Since the tolane moiety has a larger inherent optical birefrin-
a large inherent birefringence are necessary for useful
azobenzene-containing
polymers,
molecular
reorientation
gence than a biphenyl or phenyl benzoate mesogenic core in LC
materials, several types of tolane-containing LC monomers and
polymers have been investigated [21e31]. Copolymerization of
monomers with tolane side groups is an effective technique for
enhancing photoinduced optical anisotropy, where cooperative
molecular reorientation is needed [24b,25,27]. Ikeda’s group has
synthesized azobenzene-containing polymethacrylates copoly-
merized with monomers containing tolane side groups, which
perpendicular to the polarization (E) of the LP light occurs simul-
taneously via the axis-selective trans-cis-trans photoisomerization
process [2]. However, azobenzene-containing polymeric materials
* Corresponding author. Tel.: þ81 79 267 4886; fax: þ81 79 267 4885.
E-mail address: kawatuki@eng.u-hyogo.ac.jp (N. Kawatsuki).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.04.043

2850
N. Kawatsuki et al. / Polymer 51 (2010) 2849e2856
CH3
Br
showed large birefringence [25]. They also reported large photo-
induced birefringence in polymethacrylates with a tolane moiety
directly attached to azobenzene side groups [23,24]. We have
investigated the thermally enhanced photoinduced reorientation
behavior in polymethacrylate films consisting of a 4-methox-
ycinnamoyloxy group connected with a tolane moiety (P2 in Fig. 1)
[28]. This PLCP film showed a larger thermally enhanced photoin-
O
C
CH +
I
HO
1
Pd(PPh3)2Cl2
CuI
NH3aq
CH3
Br
CH3
O
C
C
+
HC
C
C
OH
HO
duced birefringence (
with a biphenyl moiety (P3 in Fig. 1,
D
n ¼ 0.27) than that of a polymethacrylate
CH3
2
D
n ¼ 0.24). Furthermore, it is
Pd(PPh3)2Cl2
CuI
NH3aq
known that the bistolane moiety possesses higher inherent bire-
fringence than the tolane moiety. Okano et al. have synthesized
a polymethacrylate with a bistolane mesogenic core directly
attached to an azobenzene end group [23,24a]. However, the bis-
tolane moiety has not yet been introduced as the LC core into PLCP
materials, which are transparent in the visible region.
CH3
C
CH3
OH
CH3
O
C
C
C
C
HO
3
NaOH
In this paper, we describe a synthesis of a new PLCP that exhibits
large thermally enhanced photoinduced birefringence as well as its
thermallyenhanced photoinduced molecular reorientation behavior
using LP-365 nm light. We synthesized a polymethacrylate with
a photoreactive 4-methoxycinnamoyloxy group directly connected
to a bistolane side group (P1 in Fig. 1). This PLCP showed the largest
photoinduced birefringence at the wavelength with non-resonance
region among the PLCPs so far. The influence of the degree of the
photoreaction and the annealing temperature on the thermally
enhanced molecular reorientation behavior was explored in detail.
Finally, as a transparent optical application using PLCP films, polar-
ization holographic gratings were fabricated using interference
325 nm light beams in various polarization modes, and their optical
propertieswerecharacterized. Among P1-P3films, thegratingsofP1
films exhibited the most efficient diffraction properties due to the
largest photoinduced birefringence.
CH3
C
OCH3
O
C
C
CH
+
I
O
HO
O
5
4
Pd(PPh3)2Cl2
CuI
NH3aq
CH3
OCH3
O
C
C
C
C
O
HO
O
6
COCl
CH3
C
O
C
OCH3
O
C
C
C
O
O
O
7
4
I
OCH3
+
2. Experimental section
Pd(PPh3)2Cl2
CuI
NH3aq
CH3
2.1. Materials
Starting materials were used as received from Tokyo Kasei
Chemicals and Aldrich Co. Ltd. In addition, we used previously
synthesized PLCPs P2 and P3 [14,28] Additionally,
O
C
C
C
C
OCH3
HO
a poly-
8
methacrylate with 4-methoxybistolane side groups (P4) was
synthesized for the comparison of the photoreaction of the bisto-
lane groups with cinnamate groups in P3. The synthetic route for
methacrylate monomer with bistolane side groups is outlined in
Scheme 1. Monomers 1 and 5 were synthesized according to the
literature [28]. The detailed synthetic procedures for 2-4, and 6-9
COCl
CH3
C
O
C
O
C
C
C
OCH3
O
9
Scheme 1. Synthetic route for methacrylate monomers 7 and 9.
CH3
O
OCH3 P1
C
O
C
C
C
C
O
O
are described in the supporting information. Polymerization was
carried out by free radical polymerization of monomers 7 and 9, in
THF, using AIBN as an initiator at 55 ꢀC for one day. Monomer and
AIBN concentrations were 10 w/v% and 1.2 mol%, respectively. After
the polymerization, we purified the polymers by precipitating
them for several times from a THF solution into methanol and
O
p
O
C
P2
OCH3
O
C C
O
O
O
p
O
C
Table 1
OCH3
P3
O
O
Molecular weight and thermal properties of the polymers used.
O
O
Polymer
Mnꢁ10-4 a
Mw/Mn
Thermal properties (ꢀC)b
p
P1
P2
P3
P4
4.2
2.5
3.2
2.1
2.3
2.2
2.5
1.6
C 114N > 300 I
C 66N > 300 I
G 105N > 300 I
G 96 N 202 I
CH3
C
O
C
P4
O
C
C
C
OCH3
O
p
a
Measured by GPC, polystyrene standards, chloroform eluent.
b
Fig. 1. Chemical structures of PLCPs P1eP3, and P4.
G; glass, C; crystal, N; nematic, I; isotropic. Determined by DSC and POM.

N. Kawatsuki et al. / Polymer 51 (2010) 2849e2856
2851
diethyl ether. The synthetic yield was around 50 wt.%. Table 1
summarizes the molecular weight and thermal properties of the
polymers.
2.2. Photoreaction
Thin polymer films (approximately 150 nm thick) were
prepared by spin-coating a methylene chloride solution of poly-
mers (1.0 w/w%) onto quartz or CaF2 substrates. The photoreactions
were performed using an ultrahigh-pressure Hg lamp equipped
with Glan-Taylor polarizing prisms and a band-pass-filter at
365 nm (FWHM ¼ 10 nm) to obtain LP-365 nm light. This light
intensity was 25 mWcmꢂ2 at 365 nm. The photoreactivity of the
cinnamate and tolane groups of the film was estimated by moni-
toring the decrease in absorbances at 1639 cmꢂ1 (vibration of the
cinnamate double bond) and 2200 cmꢂ1 (vibration of the carbone
carbon triple bond), respectively, using FT-IR spectroscopy.
Fig. 2. Experimental set up for polarization holography.
is illustrated in Fig. 2. After the exposure, the sample film was
annealed at elevated temperatures to generate a molecular reor-
ientation. The grating constant (L) was set to 2e15
mm by adjusting
the angle (2 ) between two light beams. The holographic gratings
q
2.3. Characterization
were analyzed using a probe beam emitted from a HeeNe laser at
a wavelength of 632.8 nm. For first order diffraction, the diffraction
efficiency was measured using a photo-detector, and the polariza-
tion states were evaluated using a polarizer placed behind the
sample plane.
1H NMR spectra using a Bruker DRX-500 FT-NMR and FT-IR
spectra (JASCO FTIR-410) confirmed the monomers and polymers.
The molecular weight of the polymers was measured by GPC
(Tosoh HLC-8020 GPC system with a Tosoh TSKgel column; eluent-
chloroform) calibrated using polystyrene standards. The thermal
properties were examined using a polarization optical microscope
(POM; 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 ꢀCminꢂ1. The polarization absorption spectra were measured
with a Hitachi U-3010 spectrometer equipped with Glan-Taylor
3. Results and discussion
3.1. Synthesis, thermal and spectroscopic properties of PLCPs
PLCP P1 was synthesized by free radical polymerization in a THF
solution. A methyl substituent at the bistolane moiety was intro-
duced to improve solubility of the polymer in common organic
solvents, including chloroform, toluene, and DMF [36]. As summa-
rized in Table 1, all polymers revealed a nematic LC phase. The
nematiceisotropic transition of P1, P2 and P3 was above 300 ꢀC and
was accompanied by a partial thermal decomposition. In contrast,
P4 exhibited nematic liquid crystalline phase between 96 and
202 ꢀC. DSC curves and POM photographs of the PLCPs are shown in
the supporting information (Figs. S1 and S2).
Fig. 3 shows the absorption spectra of P1-P4 films on quartz
substrates. The absorption maximum of P1 was at a longer wave-
length than that of P2 and P3 due to the conjugation of the bisto-
lane moiety. It contains absorption maxima around 340 nm, and
the absorption around 365 nm is large. Additionally, the absorption
spectrum of a P4 film revealed absorption maxima around 320 nm,
and it contains moderate absorption around 365 nm. These results
indicate that the absorption band of cinnamate group shifted to
polarization prisms. The photoinduced optical anisotropy,
DA,
which was evaluated using polarization absorption spectra, is
expressed as Eq. (1):
D
A ¼ A ꢂ At
(1)
jj
where A and At are the absorbances parallel and perpendicular to
k
E, respectively. The thermally enhanced molecular reorientation
was conducted by annealing an exposed film at an elevated
temperature for 5 min. The in-plane order was evaluated using the
reorientational order parameter, S, which is given in Eq. (2). This
equation shows that the reorientation direction is parallel to E of
the LPUV light for S > 0, but perpendicular for S < 0.
A ꢂ At
ðlargeÞ
jj
S ¼
(2)
A
þ 2A
ðsmallÞ
where A and At are the absorbances parallel and perpendicular to
k
E, respectively, while A(large) is the larger value of A and At, and
k
A(small) is the smaller one. Additionally, this equation expresses
appropriately the orientation order of the mesogenic groups for
1.2
P2
P4
both directions. The birefringence (Dn) of a reoriented film was
measured by the Senarmont method at 633 nm [32].
1
P3
P1
0.8
2.4. Polarization holography
0.6
0.4
0.2
0
Polarization holographic grating was recorded with orthogonal
linear (OL) exposure using two beams with s- and p-polarized
components [32e34]. Orthogonal circular (OC) exposure consisting
of two opposing circular polarization (R- and L-) beams was also
carried out. A HeeCd laser (Kinmon Koha, IK3501R-G-S) at
a wavelength of 325 nm was used as the light source. The polari-
zation of the laser beams was adjusted by t he use of half- and
quarter-wave plates. The optical set-up for polarization holography
250
300
350
400
450
Wavelength (nm)
Fig. 3. Absorption spectra of P1eP4 films on quartz substrates.

2852
N. Kawatsuki et al. / Polymer 51 (2010) 2849e2856
1.2
1
2
Exposure energy (Jcm-2)
A||, after annealing
0
-0.1
-0.2
Initial
0.25
1.5
5.0
22.5
1.5
0.8
0.6
0.4
0.2
0
before exposure
after exposure: ⊥
after exposure: ||
1
0.1
1
10 100
Exposure energy (Jcm-2)
A⊥, after annealing
0.5
0
250 300 350 400 450 500
Wavelength (nm)
250 300 350 400 450 500
Wavelength (nm)
Fig. 4. Change in the absorption spectra of a P1 film upon LP-365 nm light exposure.
The inset plots photoinduced optical anisotropies at 336 nm as a function of exposure
energy.
Fig. 6. Change in the polarization absorption spectra of a P1 film before and after
irradiating with LP-365 nm light, and after subsequent annealing. Exposure energy and
annealing temperatures were 0.15 J cmꢂ2 at 230 ꢀC. A and At indicate absorption
k
parallel and perpendicular to E of the LP-365 nm light.
long wavelength due to the bistolane moiety and the absorption
band of the bistolane moiety overlaps with that of the cinnamate
groups.
much slower: 95% of the tolane moieties remained when DPc was
50%. Furthermore, Fig. 5c shows change in the FT-IR spectra of a P4
film, exhibiting that the photoreaction of the bistolane moiety
occurred although larger exposure energy was required as
compared to the photoreaction of the cinnamate for P1. These
results indicate that the photoreaction of the bistolane moieties
requires more exposing energy than that of the cinnamate moiety.
3.2. Photoreaction of PLCP films
It is well known that exposing a polymeric film with cinnamate
side groups to LPUV light leads to an axis-selective [2 þ 2] photo-
dimerization and photoisomerization [17,37]. Additionally, the
tolane moiety also axis-selectively photoreacted with LPUV light to
give photodimerized products [28b,31]. Fig. 4 shows changes in the
absorption spectra of a P1 film when it was exposed to LP-365 nm
light. The film became insoluble in organic solvents even though
the exposure dose was 0.25 J cmꢂ2, suggesting that the photo-
cross-linking reaction of the mesogenic side groups occurred at the
early stage of the photoreaction. The insets of Fig. 4 plot the
photoinduced optical anisotropies at 336 nm as a function of
exposure energy. It shows negative optical anisotropy due to the
axis-selective photoreaction under LP-365 nm light.
To clarify the detailed photoreaction of the mesogenic side
groups of a P1 film, the photoreaction of the cinnamate and bis-
tolane groups was estimated by FT-IR spectroscopy. Fig. 5a shows
change in the FT-IR spectra of a P1 film. As seen in the figure, the
absorption at 1639 cmꢂ1 (cinnamate groups) gradually decreased,
while the decrease in the absorbance at 2200 cmꢂ1 (tolane groups)
was very slow. The degree of the photoreaction (DP) of cinnamate
(DPc) and bistolane groups (DPt) is plotted in Fig. 5b. It shows that
the DPc gradually increased, whereas the increase in the DPt was
3.3. Thermal enhancement of the photoinduced optical anisotropy
of P1 film
We have previously reported that a small photoinduced optical
anisotropy of the P2 and P3 films, which is derived by irradiating
with LPUV light, can be reversely enhanced when the exposed films
are annealed in the LC temperature range of the material [14,28].
The resultant molecular reorientation was parallel to the E of LPUV
light and the order parameter was greater than 0.6. This revers-
ibility is due to the self-organization of the non-reacted mesogenic
side groups thermally reoriented along the photo-cross-linked
groups parallel to E, which acted as a photo-cross-linked anchor.
The sufficient molecular reorientation was derived when the DPc
was around 1% for P2, but more DPc (10e15%) was required for P3
[14,28]. This indicates that a small amount of photo-cross-linked
anchors with tolane moieties more effectively induced the self-
organization of all the mesogenic side groups parallel to E than that
with biphenyl moieties [28].
a
b
100
cinnamate
Exposure energy (Jcm-2)
Cinnamate
Initial
80
Bistolane
2.6
bistolane
14.5
350
60
40
20
0
3000
2500
2000
1500
0.1
1
10
100
Exposure energy (Jcm-2)
Wavenumber (cm-1)
Fig. 5. (a) Change in the FT-IR spectra of a P1 film upon LP-365 nm light exposure. (b) Degree of the photoreaction (DP) of the cinnamate (DPc) and bistolane groups (DPt) of a P1
film. (c) Change in the FT-IR spectra of a P4 film upon LP-365 nm light exposure.

N. Kawatsuki et al. / Polymer 51 (2010) 2849e2856
2853
Table 2
0.8
Optical properties of PLCP films with thermally enhanced, photoinduced
reorientation.
0.6
0.4
0.2
0
Polymer
DPa (%)
Annealing temperatureb (ꢀC)
Sc
D
nd
P1
P2
P3
2.5
0.8
14.5
230
200
155
0.64
0.69
0.68
0.34
0.27
0.24
a
Degree of photoreaction.
Annealing for 5 min.
In-plane order parameter at 336 nm for P1, and at 314 nm for P2 and P3.
b
c
d
Birefringence at 632.8 nm, as measured by the Senarmont method.
50 100 150 200 250 300 350
Annealing temperature (°C)
Fig. 6 shows the change in the polarization absorption spectra of
a P1 film before and after irradiating with LP-365 nm light, and
after the subsequent annealing, when the maximum thermally
enhanced molecular reorientation was obtained. The exposure
energy was 0.15 J cmꢂ2 (DPc ¼ 2.5%, DPt < 1%) and the annealing
temperature was 230 ꢀC. After the photoirradiation, the film was
insoluble in organic solvents, suggesting the occurrence of photo-
cross-linking reaction. The photoinduced negative optical anisot-
ropy after the irradiation with LP-365 nm light was very small
Fig. 8. Thermally enhanced S values for P1 (at 336 nm) films as a function of the
annealing temperature. DPc was 2.5%. Arrow shows the LC temperature range of the
PLCP.
3.4. Effect of the degree of photoreaction on molecular reorientation
The DP influenced thermally enhanced molecular reorientation
behavior. Fig. 7 plots the thermally enhanced S values of P1 films as
a function of DPc, in which films were exposed to LP-365 nm light
and subsequently annealed. For comparison, the thermally
enhanced S values of P2 and P3 films are also plotted. The annealing
temperatures were 230 ꢀC for P1, 200 ꢀC for P2 and 155 ꢀC for P3.
For P1, sufficient molecular reorientation parallel to E (S > þ 0.6)
was observed when DPc was around 2.5e4.5%. The generated
maximum S values decreased as the DPc further increased and the
molecular reorientation did not occur when the DPc was greater
than 10%. The molecular mobility for the self-organization reduced
due to the large amount of photo-cross-linked products. For P2,
smaller amount of photoreaction was required for the maximum
molecular reorientation parallel to E, where the required DPc was
around 1%. In contrast, larger amount (DPc ¼ 10e15%) of photore-
action was needed for the sufficient molecular reorientation
parallel to E for P3 films, while molecular reorientation perpen-
dicular to E was observed when the DPc was several %. In this case,
small amount of photoproducts act as impurities to reduce the
liquid crystalline property parallel to E of LPUV light, resulting in
the thermally enhanced self-organization perpendicular to E. This
(DA < 0.01), and reversely amplified by the annealing procedure.
This thermal amplification is similar phenomena to that of P2 and
P3 films. The generated in-plane order parameter, S, was þ0.64 and
the birefringence,
S and
n value of the reoriented P1 film is the largest among the PLCPs
Dn, of P1 film was 0.34. The maximum generated
D
n values of the P1-P3 films are summarized in Table 2. The
D
that are transparent in the visible region, although the S value is
somewhat smaller than that of P2 and P3. The molecular motion of
the long bistolane moieties would be difficult for the sufficient self-
organization at elevated temperatures. Despite the smaller S value
of the P1 film, the largest
to the large inherent birefringence and refractive indices of the
bistolane moieties. Additionally, the large photoinduced n (>0.4)
Dn value of the reoriented P1 film is due
D
had been obtained in polymethacrylate with the bistolane moiety
attached with azobenzene side groups [23], although it was
a colored material and very large
Dn value was due to the wave-
length measured near the resonance region.
It is worth noting that DPc required to attain large thermally
enhanced molecular reorientation for a P1 film was much lower
than that for P3 [14]. Additionally, the effective molecular reor-
ientation for the P2 film was achieved at an early stage of the
photoreaction [28]. These results indicate that a small amount of
axis-selective, photo-cross-linked mesogenic groups with (bis)tol-
ane moieties parallel to E effectively act as the photo-cross-linked
anchors. Further detailed results are discussed in the following
section.
0.8
P1
P2
0.6
P3
0.4
0.2
0
-0.2
-0.4
1
10
100
Degree of photoreaction, DP (%)
c
Fig. 9. (a) Polarization modulation of the interference light beams under OL and OC
conditions. (b) POM photographs of the polarization gratings of P1 films with OL
condition. The arrows show the direction of the polarizers. (c) AFM image of the
gratings of P1 films with OL condition.
Fig. 7. Thermally enhanced S values of P1 (at 336 nm), and P2 and P3 (at 314 nm) films
as a function of DPc. The annealing temperatures were 230 ꢀC for P1, 200 ꢀC for P2 and
155 ꢀC for P3.

2854
N. Kawatsuki et al. / Polymer 51 (2010) 2849e2856
Table 3
a
b
The fabrication conditions and optical properties of polarization gratings.
0th
0th
90
90
100
80
60
40
20
0
100
80
60
40
20
0
Polymer
Two beamsa
Exposure energya (mJ cmꢂ2
)
h
(%)b
D
nc
120
60
120
60
1st
P1
P1
P2
P2
P3
P3
OL
OC
OL
OC
OL
OC
40
40
22
22
400
400
1.5
2.8
1.0
1.9
0.7
1.3
0.33
0.32
0.27
0.26
0.23
0.23
30
1st (x20)
30
(x20)
0
0
a
Film thickness 150 nm. Exposure was by 325 nm He-Cd laser.
210
Input
330
210
Input
330
b
First-order diffraction efficiency at 633 nm. The Incident beam was linearly
polarized.
240
120
300
240
120
300
60
c
Birefringence at 632.8 nm.
270
90
270
90
c
d
100
80
60
40
20
0
0th
0th
60
100
80
60
40
20
0
thermal enhancement perpendicular to E was observed in azo-
benzene-containing LC polymers and PLCPs with H-bonded
mesogenic side groups [38e40]. For P1 and P2, thermal enhance-
ment perpendicular to E did not occur. A small amount of photo-
reacted side groups parallel to E act as the photo-cross-linked
anchor for the self-organization rather than acting as the impuri-
ties. The thermally enhanced molecular reorientation of the (bis)
tolane moieties parallel to E is initiated by a small amount of
photoreaction of the cinnamate end groups. Additionally, the larger
required DPc for the effective orientation of the P1 film than that of
P2 could be due to the methyl substituent at the bistolane moiety.
A similar phenomenon has been observed for P2 analogues, when
the tolane moiety had a methyl substituent [28b]
+1st (x7)
30
30
1st (x10)
0
0
-1st
210
Input
330
210
Input
330
240
300
240
300
270
270
Fig. 11. Polarization analysis of the beams diffracted from the polarization gratings of
the P1 film. (a) Grating by OL, linear incident polarization. (b) Grating by OL, R-
circularly polarized incident beam. (c) Grating by OC, linear incident polarization. (d)
Grating by OC, R-circularly polarized incident beam.
3.5. Effect of the annealing temperature on molecular reorientation
reorientation of the PLCP films according to the polarization
modulation direction, which form the pure polarization gratings.
The grating constant was set to 2e15 mm by adjusting the angle of
two beams [41].
The annealing temperature also influenced thermally enhanced
molecular reorientation, as plotted in Fig. 8. Here, DPc for P1 was
2.5%. The figure shows that the thermal amplification of the
molecular reorientation occurred when annealing temperature was
in the LC temperature range of the material. This is quite similar to
the case of other PLCP films that exhibited thermal amplification of
the photoinduced optical anisotropy based on a photo-cross-linked
anchor.
Fig. 9b shows POM photographs of fabricated P1 gratings with
40 mJ cmꢂ2 doses of OL beam when the grating constant was 11
m.
It reveals that a bright area appears every 5.5 m, which corre-
m
m
sponds to the half pitch of the grating constant. A bright area
implies that the uniaxial molecular reorientation and the neigh-
boring orientation directions are orthogonal to each other. AFM
image revealed that the height of the surface relief formation was
less than 3 nm as shown in Fig. 9c, due to the uniform light
intensity of the polarization holography. The grating fabricated
with the OC beam showed a similar results (Fig. S3). Thus, polari-
zation holographic recording was successfully fabricated on a P1
film for both recording conditions. Additionally, polarization
grating of P2 and P3 films were also fabricated. Table 3 summarizes
3.6. Polarization holography
Polarization gratings were recorded using two beam interfer-
ometry with OL and OC beams using P1-P3 films. In these
recording conditions, light intensity is not modulated, but the
polarization of the interferometric light beams is, as illustrated in
Fig. 9a. These holographic recordings induce a periodic molecular
1.2
1
0.8
0.6
0.4
P1
P3
0.2
0
0
2
4
6
8
10 12 14 16
Grating constant (µm)
Fig. 10. Dependence of the induced birefringence on the grating constant for P1 and
P3 films. The graph is normalized at maximum birefringence in polarization holograms
fabricated with OL beam.
Fig. 12. Summary of the polarization states of the ꢃ1st order diffracted beams on
polarization gratings using P1 film. The diffraction efficiencies (DEs) of each diffraction
beam appear in parentheses.

N. Kawatsuki et al. / Polymer 51 (2010) 2849e2856
2855
the fabrication conditions and optical properties of polarization
gratings of P1-P3 films when the maximum diffraction efficiency
was obtained. The n values were calculated by the theoretical
consideration using the value of the diffraction efficiency [35b]. It
shows that the energy required for fabricating satisfactory gratings
of P1 and P2 films was much lower than for P3 films because of the
low exposure energy required for the effective molecular reor-
Acknowledgments
D
This work was partially supported by a grant-in-aid of Scien-
tific Research in Priority Areas “New Frontiers in Photochromism
(No. 471)” from the Ministry of Education, Culture, Sports, Science
and Technology (MEXT), and a grant-in-aid of Scientific Research
B (No. 21350129) and S (No. 21225006) from the Japan Society for
the Promotion of Science.
ientation. Additionally, the
Dn values generated were similar to
those estimated from the uniaxially molecular-reoriented films for
all the gratings, since polarization holography induced the efficient
molecular reorientation. Furthermore, due to its larger Dn values of
Appendix. Supplementary material
reoriented P1 film, the diffraction efficiency was the highest among
P1-P3 films.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.polymer.2010.04.043.
To evaluate the resolution of the polarization gratings, the
influence of the grating constant on the polarization holography
was investigated [41]. Fig. 10 plots the induced birefringence at
632.8 nm of P1 and P3 films for various grating constant, where the
References
[1] (a) Shibaev VP, Kostromin SG, Ivanov SA. In: Shibaev VP, editor. Polymers as
electroactive and photooptical media. Berlin: Springer; 1996. p. 37e110;
(b) MacArdle CB. In: MacArdle CB, editor. Applied photochromic polymer
systems. New York: Blackie; 1991. p. 1e30;
(c) Krongauz V. In: MacArdle CB, editor. Applied photochromic polymer
systems. New York: Blackie; 1991. p. p121e73.
[2] (a) Ichimura K. Chem Rev 2000;100:1847e73;
induced
Dn was normalized at a maximum value. As the grating
constant becomes larger, the induced birefringence also becomes
larger until it is approximately saturated at a grating constant of
11 mm in both materials. It seems that the resolution of P1 might be
lower than that of P3, when the grating constant was small.
Because P1 contains elongated and rigid mesogenic side groups,
thermally enhanced reorientation process possesses a limitation for
the sufficient periodical molecular orientation.
One of the features of the polarization gratings is the
conversion of the polarization of the diffracted beams. Fig. 11aed
show polar-plots of the 0th and ꢃfirst-order diffracted laser
beams for P1 polarization gratings, which represent the optical
(b) Natansohn A, Rochon P. Chem Rev 2002;102:4139e76;
(c) Ikeda TJ. Mater Chem 2003;13:2037e57.
[3] O’Neill M, Kelly SM. J Phys D Appl Phys 2000;33:R67e84.
[4] Seki T. In: Nalwa HS, editor. Handbook of photochemistry and photobiology,
vol. 2. Stevenson Ranch, CA: American Sci Publishers; 2003. p. 435e65.
[5] (a) Anderle K, Birenheide R, Eich M, Wendrorff JH. Makromol Chem Rapid
Commun 1989;10:477e83;
(b) Shi Y, Steier WH, Yu L, Chen M, Dalton LR. Appl Phys Lett 1991;59:2935e7.
[6] Kawatsuki N, Ono H. In: Nalwa HS, editor. Organic electronics and photonics,
vol. 2. Stevenson Ranch, CA: American Sci Publishers; 2007. p. 301e44.
[7] Schadt M, Schmitt K, Kozinkov V, Chigrinov V. Jpn
1992;31:2155e64.
[8] Schadt M, Seiberle H, Schuster A. Nature 1994;381:212e5.
[9] Gibbons WM, Shannon PJ, Sun ST, Swetlin BJ. Nature 1991;351:49e50.
[10] (a) Wu Y, Demachi Y, Tsustusmi O, Kanazawa A, Hisono T, Ikeda T. Macro-
molecules 1998;31:4457e63;
(b) Wu Y, Demachi Y, Tsustusmi O, Kanazawa A, Hisono T, Ikeda T. Macro-
molecules 1998;31:1104e8.
[11] Han M, Ichimura K. Macromolecules 2001;34:90e8.
[12] Natansohm A, Rochon P, Pézolet M, Audet P, Brown D, To S. Macromolecules
1994;27:2580e5.
[13] (a) Kawatsuki N, Takatsuka H, Yamamoto T, Sangen O. J Polym Sci Part
A Polym Chem 1998;36:1521e6;
(b) Kawatsuki N, Ono H, Takatsuka H, Yamamoto T, Sangen O. Macromole-
cules 1997;30:6680e2.
[14] Kawatsuki N, Goto K, Kawakami T, Yamamoto T. Macromolecules
2002;35:706e13.
[15] (a) Jackson PO, O’Neill M, Duffy WL, Hindmarsh P, Kelly SM, Owen GJ. Chem
Mater 2001;13:694e703;
(b) Bergmann G, Jackson PO, Hogg JHC, Stirner T, O’Neill M, Duffy WL, et al.
Appl Phys Lett 2005;87:061914.
[16] Trajkovska A, Kim C, Marchall KL, Mourey TH, Chen SH. Macromolecules
2006;39:6983e9.
J Appl Phys
intensity distribution along
a polarization azimuth angle as
polarization states. The characteristics of the fabricated polari-
zation gratings and the first-order diffraction efficiencies are
shown in Fig. 12. For the grating fabricated with OL beams, the
polarization of the diffracted beams was rotated by 90 deg, when
the linearly polarized probe beam was used as plotted in Fig. 11a.
This grating reversed the rotation direction of the ꢃfirst-order
diffracted beams, when the probe beam was circularly polarized
(Fig. 11b). For the grating of fabricated with OC beams, the polar
plot of Fig. 11c shows that the circularly polarized, diffracted
beams with opposite circular rotations are obtained for a linearly
polarized probe beam. Interestingly, when the probe was
R-circularly polarized, only the þfirst order diffraction beam was
observed and the rotation direction was reversed, but the -first
order was invisible, as plotted in Fig. 11d. Furthermore, the
diffraction efficiency of the þfirst order beam was twice that of
a linearly polarized probe beam. These experimental results are
consistent with the theoretical predictions for pure polarization
gratings [33e35].
[17] (a) Xiao S, Lu X, Lu Q, Su B. Macromolecules 2008;41:3884e92;
(b) Xiao S, Lu X, Lu Q. Macromolecules 2007;40:7944e50;
(c) Zhang Q, Bazuin CG, Barret CJ. Chem Mater 2008;20:29e31.
[18] Barachevsky VA. Proc SPIE 1991;1559:184e93.
[19] Kawatsuki N, Kawakami T, Yamamoto T. Adv Mater 2001;13:1337e9.
[20] Kawatsuki N, Matsuyoshi K, Hayashi M, Takatsuka H, Yamamoto T. Chem
Mater 2000;12:1549e55.
4. Conclusion
[21] Gerhard P. In: Demus D, Goodby J, Gray GW, Spiess HW, editors. Handbook of
liquid crystals, vol. 2A. New York: Wiley-VCH; 1998. p. 128e41.
[22] Ye F, Orita A, Yaruba J, Hamada T, Otera J. Chem Lett 2004;33:528e9.
[23] (a) Okano K, Shishido A, Ikeda T. Adv Mater 2006;18:523e7;
A thermally enhanced in-plane molecular reorientation with
large birefringence was investigated in a thin PLCP (P1) film
comprised of a 4-methoxycinnamoyloxy group directly connected
with a bistolane moiety. Based on the axis-selective photoreaction
of the cinnamate groups, the generated in-plane reorientational
(b) Okano K, Tsutsumi O, Shishido A, Ikeda T.
J Am Chem Soc
2006;128:15368e9.
[24] (a) Okano K, Shishido A, Ikeda T. Macromolecules 2006;39:145e52;
(b) Yoneyama S, Yamamoto T, Tsutsumi O, Kanazawa A, Shiono T, Ikeda T.
Macromolecules 2002;35:8751e8.
[25] (a) Shishido A, Ishiguro M, Ikeda T. Chem Lett 2007;36:1146e7;
(b) Ishiguro M, Sato D, Shishido A, Ikeda T. Langmuir 2007;23:332e8.
[26] Yu H, Kobayashi T, Ge Z. Macromol Rapid Commun 2009;30:1725e30.
[27] Kitani Y, Kitamura C, Yoneda A, Kawatsuki N. Mol Cryst Liq Cryst
2005;443:181e9.
order was greater than 0.6, and the birefringence was
D
n ¼ 0.34 at
633 nm. This is the largest n value for thermally enhanced
D
molecular reorientation for materials transparent in the visible
region. In addition, we have demonstrated optical device applica-
tions, such as polarization holography. Due to the larger generated
birefringence, efficient diffraction efficiency was obtained. Further
applications using P1 films for thinner birefringent devices is
currently under investigation.
[28] (a) Kawatsuki N, Fujii Y, Kitamura C, Yoneda A. Chem Lett 2006;35:52e3;
(b) Kawatsuki N, Yamashita A, Fujii Y, Kitamura C, Yoneda A. Macromolecules
2008;41:9715e21.

2856
N. Kawatsuki et al. / Polymer 51 (2010) 2849e2856
[29] Okano K, Mikami Y, Yamashita T. Adv Funct Mater 2009;15:3804e8.
[30] Michinobu T, Noguchi H, Fujii N, Tokita M, Watanabe J, Shigehara K. Chem Lett
2008;37:356e7.
[31] Obi M, Morino S, Ichimura K. Chem Mater 1999;11:1293e301.
[32] El-Hosseiny FJ. Opt Soc Am 1975;65:1279e82.
[33] (a) Fleck B, Helgert M, Wenke LJ. Opt A Pure Appl Opt 2000;2:16e20;
(b) Helgert M, Fleck B, Wenke L. Opt Commun 2000;177:95e104.
[34] Cipparrrone G, Mazzulla A, Blinov LM. J Opt Soc Am B 2002;19:1157e61.
[35] (a) Kawatsuki N, Hasegawa T, Ono H, Tamoto T. Adv Mater 2003;15:991e4;
(b) Ono H, Emoto A, Kawatsuki N, Hasegawa T. Appl Phys Lett 2003;94:1359e61.
[36] (a) Broer DJ, Heynderickx I. Macromolecules 1990;23:2474e7;
(b) Hikmet RA, Lub J, Higgins JA. Polymer 1993;34:1736e40.
[37] Ichimura K, Akita Y, Akiyama H, Kudo K, Hayashi Y. Macromolecules
1997;30:903e11.
[38] Kidowaki M, Fujiwara T, Ichimura K. Chem Lett 1999:641e2.
[39] StumpeJ,FischerT,RutlohM,RosenhauerR,MeierJG.ProcSPIE1999;3800:150e63.
[40] (a) Uchida E, Kawatsuki N. Polymer 2006;47:2322e9;
(b) Uchida E, Kawatsuki N. Macromolecules 2006;39:9357e64.
[41] Emoto A, Matsumoto T, Yamashita A, Shioda T, Ono H, Kawatsuki N. Appl Phys
Lett 2009;106:073505.