Photoinduced orientation of a photoinactive liquid crystalline polymer doped with N-benzylideneaniline derivatives

Article abstract of DOI:10.1246/cl.160217

Doping photosensitive N-benzylideneaniline (NBA) derivatives in a photoinactive polymethacrylate with benzoic acid (BA) side groups (P6BAM) produces a photoinduced orientation in a P6BAM film and its patterning upon exposure to linearly polarized (LP) 365

Full text of DOI:10.1246/cl.160217

Received: March 2, 2016 | Accepted: March 15, 2016 | Web Released: March 25, 2016
CL-160217
Photoinduced Orientation of a Photoinactive Liquid Crystalline
Polymer Doped with N-Benzylideneaniline Derivatives
Ryosuke Fujii, Mizuho Kondo, and Nobuhiro Kawatsuki*
Department of Applied Chemistry, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280
(
E-mail: kawatuki@eng.u-hyogo.ac.jp)
Doping photosensitive N-benzylideneaniline (NBA) deriva-
tives in a photoinactive polymethacrylate with benzoic acid
OH
O
O
O
C
H C
C
O
3
(BA) side groups (P6BAM) produces a photoinduced orientation
O
P6BAM
p
in a P6BAM film and its patterning upon exposure to linearly
polarized (LP) 365-nm light and subsequent annealing. Two
types of NBA derivatives are used: NBA1 containing a
carboxylic acid substituent effectively generates a significant
photoinduced reorientation. The H-bonds among the carboxylic
acids of BA and NBA1 play an important role in the
photoinduced orientation.
N
OCH3
C
HO
NBA1
O
N
OCH3
C
H CO
3
NBA2
Keywords: Liquid crystalline polymer
| Photoalignment |
Hydrogen bond
Figure 1. Chemical structure of the P6BAM and NBA
derivatives.
Photoalignment based on axis-selective photoreaction of
photosensitive polymeric films has received much attention due
to its potential in diverse optical and photonic applications.
(Figure 1). We investigate the influence of H-bond formation
between the BA side groups and the NBA derivatives on the
thermally stimulated photoinduced orientation of the P6BAM/
NBA composite films. Exposing the P6BAM/NBA composite
films to LP 365-nm light generates a small optical anisotropy
that can be amplified by annealing at an elevated temperature;
the films exhibit molecular reorientation of the BA side groups,
accompanied by sublimation of the NBA derivatives. Further-
more, a patterned oriented P6BAM film is facilely fabricated
using a photomask because the NBA moiety can be reversibly
oriented by adjusting the polarization of LP 365-nm light.
The synthetic procedure of the NBA derivatives is described
in the Supporting Information, and P6BAM was synthesized
1
Several types of photoalignment materials for liquid crystal
displays (LCDs) have been investigated, including azobenzene,
1
,2
coumarin, chalcone, and cinnamate-containing polymers. Our
research has focused on photoreactive liquid crystalline poly-
mers (PLCPs) with cinnamate and cinnamic acid mesogenic
side groups, which exhibit thermally stimulated photoinduced
molecular reorientations.3 Recently, we investigated PLCPs
with N-benzylideneaniline (NBA) side groups, and found that
a photoinduced molecular reorientation and in situ formation
of NBA groups can facilitate precise photoinduced orientation
4
patterning. In conventional photoalignable PLCPs, exposing
5
the resultant oriented film to UV light alters its optical and
mechanical properties because the photosensitive mesogenic
side groups remain.
according to the literature. Thin films (P6BAM thickness
of 150 nm after annealing) were photoreacted using an
ultrahigh-pressure Hg lamp with a band-pass filter at 365 nm
¹2
To resolve this issue, we have explored the photoinduced
molecular reorientation of composite films consisting of a
photoinactive polymethacrylate with a benzoic acid (BA) side
group (P6BAM) and monomeric cinnamic acid (CA) derivatives,
where the H-bonded CA photoreacts axis-selectively with BA
side groups, and the CA monomers sublime during thermal
amplification, providing a photodurability of the resultant
(30 mW cm ). The generated orientation was evaluated using
the in-plane order parameter, S [= (A¦ ¹ A¬)/(A¦ + 2A¬), where
A¦ (A ) indicates the absorption perpendicular (parallel) to the
¬
polarization (E) of LP 365-nm light]. Detailed experimental
methods are described in the Supporting Information.
NBA1 reveals LC characteristics between 224 and 284 °C
due to the H-bonded dimeric structure, whereas NBA2 (m.p.
177 °C) does not. When the NBA derivatives are doped
in P6BAM, UV absorption spectra of the P6BAM/NBA1
composite films exhibit an absorption band at 355 nm (NBA
absorption), which increases as the NBA1 content increases
(Figure 2a). Similar spectra are obtained for the P6BAM/NBA2
films (Figure S1). The DSC curves of the composites show
one isotropic temperature (Ti) around 170 °C, which slightly
increases as the NBA1 content increases, although the melting
temperature of NBA1 is above 200 °C (Figure 2b). This is
because P6BAM and NBA1 are completely miscible due to the
H-bonds between BA and the carboxylic acid of NBA1, which
5
oriented films. H-bond formation is a prominent method
to introduce LC characteristics when fabricating functional
6
materials. Several types of azobenzene monomeric derivatives
doped in non-photoreactive polymeric films via H-bonds have
been reported for the photoinduced molecular orientation and
surface relief formation (SR).7 Additionally, ionic bonding
between the non-LC polymers and the azobenzene or photo-
crosslinkable side groups has also been investigated to generate
,8
9
photoinduced functionalities. However, only a few studies have
focused on removing the photoreactivity after photoinduced
reorientation and SR formation.5
,8
This letter describes new photoalignable composites
consisting of P6BAM and photosensitive NBA derivatives
increases T of the composites. In contrast, the second heating
DSC curves of the P6BAM/NBA2 composite show two peaks,
i
Chem. Lett. 2016, 45, 673–675 | doi:10.1246/cl.160217
© 2016 The Chemical Society of Japan | 673

(
a)
(b)
2(a)
0.8(b)
1
.2
P6BAM
5%-NBA1
0%-NBA1
20 wt%
20 wt%
1
After exposure
1
NBA1: 355 nm
NBA1: 262 nm
NBA2: 355 nm
NBA2: 262 nm
1
.5
After annealing (⊥)
0.6
0.4
0.2
0
NBA1 (wt%)
0.8
0.6
0.4
0.2
0
20%-NBA1
30%-NBA1
NBA1
0
5
After exposure (⊥)
Initial
After exposure (||)
1
1
2
3
0
5%-NBA2
NBA2
Annealed
0
0
10%-NBA2
0%-NBA2
2
After annealing (||)
NBA1
Annealed
3
0%-NBA2
0.5
0
NBA2
2
r.t. 20 J/cm
250 300 350 400 450 500 50
100
150
200
250
300
250 300 350 400 450 500
Wavelength/nm
0.01
0.1
1
10 –2100
Wavelength/nm
Temperature/°C
Exposure energy/J cm
Figure 2. (a) UV absorption spectra of the P6BAM/NBA1
films with various NBA1 compositions. (b) Second heating DSC
curves of the P6BAM/NBA1 and P6BAM/NBA2 composites.
Figure 4. (a) Changes in the polarized UV­vis absorption
spectra of the P6BAM/NBA2 (20 wt %) composite film before
and after irradiation with LP 365-nm light with exposure energy
of 20 J cm and subsequent annealing at 160 °C. (b) Photo-
induced in-plane order parameter (S) value of the P6BAM/NBA1
(NBA2) (20 wt %) composite films after LP 365-nm light
irradiation and after subsequent annealing at 170 (160) °C as a
function of exposure energy.
¹
2
2(
a)
(b)
0.8
0.6
0.4
0.2
0
After annealing (⊥) 20 wt%
NBA1 (wt%)
5
wt%
0 wt%
1
.5
1
After exposure (⊥)
Initial
20 wt%
30 wt%
1
.5
0
After exposure (||)
Additionally, significant thermal amplification is observed
when the annealing temperature is 140­170 °C (Figure S4a).
Since the reorientation is accompanied by the sublimation of
NBA1, self-organization can be induced at the LC temperature
range of the P6BAM even though the composite exhibits higher
LC temperature range.
0
After annealing (||)
2
r.t. 20 J/cm
250 300 350 400 450 500
0.01
0.1
1
10 –2100
Wavelength/nm
Exposure energy/J cm
Figure 3. (a) Changes in the polarized UV­vis absorption
spectra of the P6BAM/NBA1 (20 wt %) composite film before
and after irradiating with LP 365-nm light with an exposure
Doping NBA2 also generates the thermal amplification of
the photoinduced reorientation of the P6BAM/NBA2 film, but
its performance is not significant. Figure 4a plots the change in
the absorption spectrum of the P6BAM/NBA2 (20 wt %) film
¹
2
energy of 20 J cm and subsequent annealing at 170 °C. (b)
Thermally stimulated photoinduced in-plane order parameter (S)
value of the P6BAM/NBA1 composites films at 262 nm after LP
¹2
after exposure to LP 365-nm light for 20 J cm and subsequent
annealing (S = 0.25). In this case, maximum orientation is
obtained when the annealing temperature is 160 °C because
3
1
65-nm light irradiation and subsequent annealing at 170 °C for
0 min as a function of exposure energy.
the T of the composite is lower than 170 °C (Figure S4b). As
i
indicating that partial phase-separation occurs upon heating due
to the inability of NBA2 to form H-bonds with the BA side
groups.
plotted in Figure 4b, the thermally amplified S values of the
P6BAM/NBA2 films are much lower than those of the
P6BAM/NBA1 films. Consequently, the P6BAM/NBA2 films
require larger exposure doses for Smax when the composition of
the P6BAM/NBA derivative is same. For the P6BAM/NBA2
films, the photoinduced optical anisotropy after the LP 365-nm
light exposure is much smaller than that using NBA1. The
photoinduced Smax for the P6BAM/NBA2 films is <0.05, while
that for the P6BAM/NBA1 films is >0.1 (Figure 4b). Exposure
to LP 365-nm light generates less photoinduced reorientation of
P6BAM/NBA2 due to the lack of H-bonds, but photoinduced
molecular reorientation of the H-bonded BA and NBA1 side
groups occurs in the P6BAM/NBA1 films. Therefore, thermal
amplification for the P6BAM/NBA2 films is insufficient
because the axis-selectively photoreacted NBA2 moieties cannot
effectively interact to self-organize the BA side groups.
Finally, a patterned reoriented P6BAM film was fabricated
(Figure 5). Since the NBA molecules reversibly reorient when
changing the polarization direction of LP 365-nm light,
irradiation a P6BAM/NBA1 film using a patterned photomask
facilely attains a non-photosensitive patterned oriented film.
In summary, doping photosensitive NBA derivatives real-
izes a photoinduced molecular reorientation of photoinactive
P6BAM films. A significant reorientation is obtained using
NBA1, which forms H-bonds with the BA side groups. The
axis-selective photoreaction of the P6BAM/NBA1 composite
film using LP 365-nm light produces the photoinduced
We have reported that LP 365-nm light exposure to a
polymethacrylate film with NBA side groups generates a
photoinduced molecular orientation due to the axis-selective
photoisomerization of the NBA moiety.10 Figure 3a plots the
change in the absorption spectrum of a P6BAM/NBA1
¹2
(
20 wt %) film after exposure to LP 365-nm light for 20 J cm
and subsequent annealing at 170 °C. After exposure to LP
65-nm light, A¦ (A««) slightly increases (decreases), indicating
3
photoinduced orientation of the BA and NBA moieties.
Annealing significantly amplifies the optical anisotropy (S =
0.045 to 0.66 at 262 nm) and the absorbance at 355 nm
diminishes. A similar thermally stimulated photoinduced ori-
entation is generated as the P6BAM/MBA1 composition varies,
and the generated maximum orientation order (Smax) is greater
than 0.4 (Figures 3b and S2a­S2c).
Thermal amplification is caused by the small amounts of
photoinduced reorientation structure in the film, where thermally
stimulated self-organization occurs and the 355-nm absorption
disappears, implying that thermal amplification of the orienta-
tion of the BA side groups is accompanied by sublimation of the
NBA1 molecules. FTIR spectroscopy confirms the sublimation
of NBA1 at 170 °C for all compositions as the absorption bands
¹
1
at 1621 and 1501 cm (C=N vibration) diminish in intensity
Figure S3).
(
674 | Chem. Lett. 2016, 45, 673–675 | doi:10.1246/cl.160217
© 2016 The Chemical Society of Japan

Yaroshchuk, Y. Reznikov, J. Mater. Chem. 2012, 22, 286.
a) W. M. Gibbons, P. J. Shannon, S.-T. Sun, B. J. Swetlin,
Nature 1991, 351, 49. b) M. Schadt, K. Schmitt, V.
Kozinkov, V. Chigrinov, Jpn. J. Appl. Phys. 1992, 31,
2
2
6
155. c) T. Seki, S. Nagano, M. Hara, Polymer 2013, 54,
053. d) T. Ikeda, J. Mater. Chem. 2003, 13, 2037. e) M. Ibn-
Elhaj, S. Chappellet, F. Lincker, Solid State Phenom. 2012,
81-182, 3. f) A. Ryabchun, A. Bobrovsky, S.-H. Chun, V.
1
Figure 5. POM photograph of a patterned oriented P6BAM
film. P6BAM/NBA1 (20 wt %) composite film is exposed to LP
Shibaev, J. Polym. Sci., Part A: Polym. Chem. 2013, 51,
4031. g) C. Kim, J. U. Wallace, S. H. Chen, P. B. Merkel,
Macromolecules 2008, 41, 3075.
a) N. Kawatsuki, Chem. Lett. 2011, 40, 548. b) N.
Kawatsuki, K. Goto, T. Kawakami, T. Yamamoto, Macro-
molecules 2002, 35, 706.
a) N. Kawatsuki, H. Matsushita, M. Kondo, T. Sasaki, H.
Ono, APL Mater. 2013, 1, 022103. b) N. Kawatsuki, K.
Miyake, M. Kondo, ACS Macro Lett. 2015, 4, 764.
S. Minami, M. Kondo, N. Kawatsuki, Polym. J. 2016, 48,
267.
T. Kato, J. M. J. Fréchet, J. Am. Chem. Soc. 1989, 111, 8533.
a) A. V. Medvedev, E. B. Barmatov, A. S. Medvedev, V. P.
Shibaev, S. A. Ivanov, M. Kozlovsky, J. Stumpe, Macro-
molecules 2005, 38, 2223. b) J. E. Koskela, J. Vapaavuori, J.
Hautala, A. Priimagi, C. F. J. Faul, M. Kaivola, R. H. A. Ras,
J. Phys. Chem. C 2012, 116, 2363. c) J. Gao, Y. He, F. Liu,
X. Zhang, Z. Wang, X. Wang, Chem. Mater. 2007, 19, 3877.
N. Zettsu, T. Ogasawara, N. Mizoshita, S. Nagano, T. Seki,
Adv. Mater. 2008, 20, 516.
¹
2
3
65-nm light (5 J cm ) without a mask, exposed to LP 365-nm
¹
2
light (changed the polarization, 20 J cm ) with a mask, and
subsequently annealed at 170 °C. Black arrows indicate the
polarizer directions and red arrows indicate orientation directions.
3
4
5
orientation of NBA1 and the BA side groups. The H-bonds
among the carboxylic acids of BA and NBA1 play an important
role in the sufficient amplification of photoinduced orientation.
The reoriented P6BAM films are useful as the photoalignment
layer and birefringent films for display applications.
6
7
This work was partially supported by a Grants-in-Aid for
Scientific Research from Japan Society for the Promotion of
Science (JSPS) (Nos. B15H03869 and S23225003).
Supporting Information is available on http://dx.doi.org/
1
0.1246/cl.160217.
8
9
References
a) V. G. Chigrinov, V. M. Kozenkov, H.-S. Kwok, Photo-
a) Q. Zhang, X. Wang, C. J. Barrett, C. G. Bazuin, Chem.
Mater. 2009, 21, 3216. b) H. Kong, X. Lu, S. Xiao, Q. Lu,
Polymer 2009, 50, 1166.
1
alignment of Liquid Crystalline Materials, John Wiley &
Sons, West Sussex, England, 2008. b) M. O’Neill, S. M.
Kelly, J. Phys. D: Appl. Phys. 2000, 33, R67. c) O.
10 N. Kawatsuki, T. Washio, J. Kozuki, M. Kondo, T. Sasaki,
H. Ono, Polymer 2015, 56, 318.
Chem. Lett. 2016, 45, 673–675 | doi:10.1246/cl.160217
© 2016 The Chemical Society of Japan | 675