Synthesis and photoinduced reorientation of polymethacrylates comprising photocrosslinkable phenylbenzoate mesogenic side groups

Article abstract of DOI:10.1080/154214091009671

This paper describes synthesis and photoreaction of photocrosslinkable liquid crystalline (LC) polymethacrylates comprising phenyl benzoate mesogenic side groups attached with a photoreactive end group. All synthesized polymers exhibited a nematic LC natu

Full text of DOI:10.1080/154214091009671

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Synthesis and Photoinduced
Reorientation of
Polymethacrylates Comprising
Photocrosslinkable
Phenylbenzoate Mesogenic
Side Groups
Nobuhiro Kawatsuki a , Yuki Kitani a & Akihiro Oda a
a Department of Materials Science and Chemistry ,
Himeji Institute of Technology, University of Hyogo ,
Shosha Himeji, Japan
Published online: 17 Oct 2011.
To cite this article: Nobuhiro Kawatsuki , Yuki Kitani & Akihiro Oda (2005) Synthesis
and Photoinduced Reorientation of Polymethacrylates Comprising Photocrosslinkable
Phenylbenzoate Mesogenic Side Groups, Molecular Crystals and Liquid Crystals, 441:1,
153-161, DOI: 10.1080/154214091009671
To link to this article: http://dx.doi.org/10.1080/154214091009671
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Mol. Cryst. Liq. Cryst., Vol. 441, pp. 153–161, 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/154214091009671
Synthesis and Photoinduced Reorientation of
Polymethacrylates Comprising Photocrosslinkable
Phenylbenzoate Mesogenic Side Groups
Nobuhiro Kawatsuki
Yuki Kitani
Akihiro Oda
Department of Materials Science and Chemistry,
Himeji Institute of Technology, University of Hyogo,
Shosha Himeji, Japan
This paper describes synthesis and photoreaction of photocrosslinkable liquid crys-
talline (LC) polymethacrylates comprising phenyl benzoate mesogenic side groups
attached with a photoreactive end group. All synthesized polymers exhibited a
nematic LC nature. Irradiating the polymer film with linearly polarized ultraviolet
(LPUV) light induced a negative optical anisotropy due to an axis-selective photo-
reaction of the mesogenic groups. When the exposed film was annealed at the LC
temperature range reversion of the reorientation direction was observed, which
was triggered by the photocrosslinked mesogenic groups act as ‘‘command in bulk’’.
Keywords: optical anisotropy; photocrosslinking; photoinduced orientation; polymer
liquid crystal
INTRODUCTION
Photoinduced reorientation of photosensitive polymeric films has
received much attention because of its potential application to optical
devices such as holographic and optical memories and birefringent
films for liquid crystal display [1–4]. Several types of photoreactive
polymeric materials have been investigated including azobenzene
containing polymers and photocrosslinkable polymers. Among them,
we have investigated thermally stable molecular reorientation of
photocrosslinkable polymer liquid crystal (PPLC) films based on an
Address correspondence to Nobuhiro Kawatsuki, Department of Materials Science
and Chemistry, Himeji Institute of Technology, University of Hyogo, 2167 Shosha
Himeji, 671-2201 Japan. E-mail: kawatuki@eng.u-hyogo.ac.jp
153

154
N. Kawatsuki et al.
axis-selective photocrosslinking reaction using LP ultraviolet (LPUV)
light [5–7]. The resultant birefringent films are applicable as phase
retarders in liquid crystal displays [7].
During the course of our systematic study on thermally enhanced
photoinduced reorientation of PPLC films, we clarified that irradiating
with LPUV light generated an axis-selective photocrosslinking reaction
of the film. The resultant photocrosslinked mesogenic groups act as
the command groups in bulk, which controlled the thermally enhanced
reorientation of the mesogenic groups [5,6]. For example, a high
in-plane reorientational order was achieved in a methacrylate PPLC
with a 4-(40-methoxycinnamoyloxy)biphenyl side group (PMCB6M)
[8]. Additionally, cooperative reorientation of a non-photoreactive
mesogenic group on the reorientation behavior of a PPLC was also
reported [6,9]. However, all of the PPLC investigated so far contain
biphenyl mesogenic groups combined with photocrosslinkable groups.
The purpose of this paper is to apply the concept of thermally
enhanced molecular reorientation based on the ‘‘command in bulk’’
to other type of PPLCs. New series of PPLCs comprising phenylbenzo-
ate mesogenic group were synthesized and their axis-selective photo-
reaction was investigated using LPUV light. Thermal enhancement
and reversion of the photoinduced optical anisotropy were estimated
by polarization UV-vis spectroscopy.
EXPERIMENTAL SECTION
Materials and Polymer Synthesis
All starting materials from Tokyo Kasei Chemicals were used as
received. The methacrylate monomers were synthesized according to
1
the Scheme. All synthesized monomers were confirmed by H-NMR
and FT-IR spectroscopy. The radical polymerization was carried out
using AIBN as an initiator in THF solution at 50–60ꢀC for 1 day.
Synthesized polymers were purified by washing with diethyl ether
and reprecipitation method. Table 1 summarizes molecular weight
and thermal properties of synthesized polymers.
Photoreaction
The polymer was spin-coated on a quartz substrate from a methylene
chloride solution, resulting in a 200 nm-thick film. Photoreactions
were conducted by irradiating with a 250 W high-pressure mercury
lamp attached with a cut-off filter below 300 nm. Intensity was
150 mWcmꢁ2 at 365 nm. To irradiate with LPUV light, Glan-Taylor
polarizing prisms were used.

Photoinduced Reorientation of Polymer Liquid Crystals
155
SCHEME

156
N. Kawatsuki et al.
TABLE 1 Molecular Weight, and Thermal and Spectroscopic Properties of
Synthesized PPLCs
Molecular weighta
b
Thermal property
(ꢀC)
kmax
(nm)
PPLC
n
R
Mn ꢂ 10ꢁ4
Mw ꢂ 10ꢁ4
P1a
P1b
P1c
P1d
P1e
P1f
P2
0
0
0
0
1
1
–
H
5.0
4.2
8.5
3.8
1.4
12.0
11.8
30.6
14.3
2.6
G 146 N 213 I
G 56 N 270 I
G 88 N 305 I
G 68 N 208 I
G 94 N253 I
G 65 N 300 I
G 63 N 150 I
268
274
261
CH3
CF3
OCH3
H
OCH3
–
263, 315
267
c
c
c
–
2.0
–
4.5
–
276, 322
aDetermined by GPC, polystyrene standards, chloroform as eluent.
bThin film on quartz substrate.
cNot measured.
Characterization
The optical anisotropy of the film was estimated by POM (Olympus
BX-51) and polarization UV-vis spectra (Hitachi U-3030 spectrometer
equipped with Glan-Taylor polarizing prisms).
The in-plane order parameter, S, is expressed in Eq. (1);
A ꢁ A
?
==
S ¼
ð1Þ
A
þ 2A
ðlargeÞ
ðsmallÞ
where A== and A are the absorbances parallel and perpendicular to
?
the polarization (E) of LPUV light, respectively, while A(large) is the
larger of A== and A , while A(small) is the smaller. These values were
?
calculated by polarized UV-vis spectroscopy at kmax of the film.
RESULTS AND DISCUSSION
Thermal and Spectroscopic Properties of PPLCs
PPLCs were synthesized from corresponding monomers by radical
polymerization in THF solution. Table 1 summarizes molecular
weight, and thermal and spectroscopic properties of synthesized
PPLCs. The P1e and P1f were precipitated during the polymerization.
The P1f was insoluble in common organic solvent such as toluene and
chloroform. Other PPLCs were purified by precipitaion from chloro-
form solution to diethyl ether.
All synthesized PPLC showed nematic liquid crystalline nature after
the glass transition. Thin films of P1a-P1e, and P2 were prepared by

Photoinduced Reorientation of Polymer Liquid Crystals
157
FIGURE 1 Absorption spectrum of synthesized PPLC films P1a-P1e, and P2.
spin-coating method from a methylene chloride solution. The spin-
coated films were transparent and amorphous in nature. Figure 1
shows absorption spectra of PPLC films. It exhibits that kmax for
P1a, P1b, P1c and P1e were detected around 260–270 nm, and that
for P1d and P2 exhibited two separated peaks which were assigned
absorption bands of a phenylbenzoate and a photoreactive group.
Photoreaction
Photoreaction of a PPLC was carried out using a thin film. Figures 2a
and 2b plot change in the polarization absorption spectra of P1a
FIGURE 2 Change in polarization absorption spectra of PPLC film under
irradiating with LPUV light. A==: solid line, A : dotted line. Inset figures exhi-
?
bit photoinduced dichroism as a function of degree of photoreaction. (a) P1a,
(b) P1d.

158
N. Kawatsuki et al.
and P1d films, which were used to monitor the photoreaction of the
film exposed to LPUV light. The resultant films became insoluble
in organic solvents because of a photocrosslinking reaction of
the mesogenic groups. Figure 2a reveals that irradiating with
LPUV light generates a small negative optical anisotropy DA
(¼ A==-A < 0, where A== and A are absorption parallel and perpen-
?
?
dicular to the polarization (E) of LPUV light, respectively) around
270 nm due to an axis-selective photoreaction. The DA increased
as the photoreaction proceeded but decreased when the degree of
the photoreaction became greater than 50 mol-% as plotted in inset
of Figure 2a. For P1d, DA appeared around 315 nm, which was
ascribable to the photoreaction of 4-methoxycinnamoyl group.
The change around 263 nm (phenylbenzoate group) was scarcely
observed. This is because that the axis-selective photoreaction of
4-methoxycinnamoyl groups occurred but the photoreaction of phenyl
benzoate groups was suppressed. Additionally, inset of Figure 2b
exhibits similar DA to the P1a film. It should be noted that the
photoreaction of P1d was faster than that of P1a since the absorp-
tion band of the photoreactive groups did not overlap with the phenyl
benzoate group.
The photoreaction behavior of P1b, P1c, and P1e was similar to
that of P1a, where the absorption band of the photoreactive groups
overlapped with the phenylbenzoate group. The photoreaction of P2,
which comprised calcone group, was much faster than other films
and also exhibited the axis-selective photoreaction.
Thermally Enhanced Reorientation of the Exposed
PPLC Films
We had reported that annealing process enhanced the photoinduced
optical anisotropy of PPLC film with 4-cinnnamoyloxybiphenyl (CB)
and 4-(2-cinnamoyloxy)ethoxybiphenyl (CEB) side groups [5–8]. The
photoinduced negative optical anisotropy was reversed and enhanced
by annealing the exposed film at the LC temperature range of the
PPLC. This is because that the photocrosslinked mesogenic groups
act as the ‘‘command in bulk’’, which controls the direction of molecu-
lar reorientation.
Figures 3a–3f show absorption spectra of PPLC films before irra-
diating, after irradiating with LPUV light and after subsequent
annealing at the LC temperature of each PPLCs. The degree of the
photoreaction was adjusted approximately 15 mol%, in which an
efficient thermal enhancement of the photoinduced reorientation
was observed for PPLCs with CB and CEB side groups.

Photoinduced Reorientation of Polymer Liquid Crystals
159
FIGURE 3 UV polarization spectrum of PPLC films before photoirradiating
(thin line), after irradiating with LPUV light (thin lines), and after subsequent
annealing (thick lines). A== is the solid lines and A is the dotted lines. (a) P1a
?
with 0.75 J=cm2 doses,ꢀannealed at 150ꢀC for 10 min. (b) P1b with 1.05 J=cm2
doses, annealed at 140 C for 10 min. (c) P1c with 9 J =cm2 doses, annealed at
150ꢀC for 10 min. (d) P1d with 0.3 J=cm2 doses, ꢀannealed at 180ꢀC for
10 min. (e) P1e with 44 J=cm2 doses, annealed at 180 C for 10 min. (f) P2 with
0.05 J=cm2 doses, annealed at 120ꢀC for 10 min.

160
N. Kawatsuki et al.
In all cases, negative optical anisotropy was generated after exposing
to LPUV light, and annealing procedure enhanced the positive optical
anisotropy except P1c film. For P1a, the enhanced in-plane order para-
meter, S, was þ0.38. The increasing of A== and decreasing of A after
?
annealing mean the reorientation of the mesogenic groups parallel to
the photocrosslinked mesogenic groups, which were parallel to E.
Additionally, the strong decreasing of A implies an out-of-plane reori-
?
entation of the mesogenic groups. Namely, the annealing procedure
enhances the biaxial-reorientation. This reorientation behavior was
observed for a polymethacrylate with CB side groups [10].
The influence of the substituent groups at the end of the mesogenic
groups was investigated using P1b, P1c, and P1d. In case of the
PPLC with 4-(40-methoxycinnnamoyloxy)biphenyl or 4-(40-methylcinn-
namoyloxy)biphenyl end groups, a large thermally enhanced in-plane
reorientation was observed [10]. However, for P1b and P1d, thermally
enhanced S were þ0.19 and þ0.14, respectively. For P1d, A== around
260 nm did not increase after annealing although A== at 315 nm
increased. This suggests the occurrence of an out-of-plane reorien-
tation and that thermal enhancement of the in-plane reorientation
of phenylbenzoate groups is inferior to that of biphenyl group.
Additionally, for P1c, annealing process generated the out-of reorien-
tation of the mesogenic groups. The same behavior was observed for
PPLC with 4-(40-trifluoromethylcinnnamoyloxy)biphenyl side groups
[10]. The trifluoromethyl group will inhibit the in-plane reorientation.
For P1e, photoreaction was slower than that of P1a. The spectrum
change shown in Figure 3e reveals that molecular motion by the
annealing process was small and thermally enhanced S was þ0.09.
The thermal enhancement of the photoinduced optical anisotropy
was observed in a calcone-containing PPLC, P2 (Fig. 3f). The
enhanced S value was þ 0.14. By comparison with the results of
P1b, thermally enhanced reorientation of the calocne group is smaller
than that of the cinnamate group.
CONCLUSION
New PPLC comprising phenyl benzoate mesogenic groups were synthe-
sized. All PPLCs exhibited nematic LC nature. The exposing a PPLC
film to LPUV light induced an optical anisotropy of the film due to
an axis-selective photocrosslinking reaction. When the exposed film
was annealed at LC temperature range of the material, photoinduced
optical anisotropy reversed and enhanced to parallel to the polarization
of LPUV light. However, the enhanced orientational value was smaller
than that obtained from a film with 4-(40-methoxycinnnamoyloxy)-

Photoinduced Reorientation of Polymer Liquid Crystals
161
biphenyl side group. This thermally enhanced photoinduced molecular
reorientation is based on ‘‘command in bulk’’ of the photocrosslinked
mesogenic groups.
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