Effect of spacer length on photoinduced reorientation behavior of photocrosslinkable liquid crystalline polymer films comprising 4-(4-methoxycinnamoyloxy)biphenyl side groups

Article abstract of DOI:10.1080/15421400802612136

Methacrylate polymer liquid crystals containing 4-(4-methoxycinnamoyloxy) biphenyl (MCB) side group with different alkylene spacer length between the polymethacrylate backbone and MCB side groups are synthesized. Influence of the alkylene spacer length on

Full text of DOI:10.1080/15421400802612136

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Effect of Spacer Length on
Photoinduced Reorientation
Behavior of Photocrosslinkable
Liquid Crystalline Polymer
Films Comprising 4-(4-
Methoxycinnamoyloxy)biphenyl
Side Groups
Nobuhiro Kawatsuki a & Junpei Horii a
a Department of Materials Science and Chemistry ,
Himeji Institute of Technology, University of Hyogo ,
Shosha Himeji, Japan
Published online: 22 Jul 2010.
To cite this article: Nobuhiro Kawatsuki & Junpei Horii (2009) Effect of Spacer Length
on Photoinduced Reorientation Behavior of Photocrosslinkable Liquid Crystalline
Polymer Films Comprising 4-(4-Methoxycinnamoyloxy)biphenyl Side Groups, Molecular
Crystals and Liquid Crystals, 498:1, 49-58, DOI: 10.1080/15421400802612136
To link to this article: http://dx.doi.org/10.1080/15421400802612136
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Mol. Cryst. Liq. Cryst., Vol. 498, pp. 49–58, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802612136
Effect of Spacer Length on Photoinduced Reorientation
Behavior of Photocrosslinkable Liquid Crystalline Polymer
Films Comprising 4-(4-Methoxycinnamoyloxy)biphenyl
Side Groups
Nobuhiro Kawatsuki and Junpei Horii
Department of Materials Science and Chemistry, Himeji Institute of
Technology, University of Hyogo, Shosha Himeji, Japan
Methacrylate polymer liquid crystals containing 4-(4-methoxycinnamoyloxy)biphenyl
(MCB) side group with different alkylene spacer length between the polymethacrylate
backbone and MCB side groups are synthesized. Influence of the alkylene spacer
length on the thermally enhanced molecular reorientation behavior of thin films is
investigated using linearly polarized ultraviolet (LPUV) light exposure and sub-
sequent annealing. In-plane reorientation preferentially occurs when the alkylene
spacer is short, and the induced in-plane order parameter is greater than 0.6. On
the other hand, when the alkylene spacer is long, thermally induced out-of-plane
reorientation is observed as well as the in-plane reorientation.
Keywords: optical anisotropy; photocrosslinking; photoinduced reorientation; polymer
liquid crystal
INTRODUCTION
Photo-control of the molecular orientation in a polymeric film has
received much attention because it can be applicable to birefringent
optical devices, the photoalignment layer of liquid crystal displays,
optical memories, and holographic data storage devices [1–3]. Based
on a polarization-axis selective photoreaction of a photoreactive poly-
meric film containing cinnamate groups, a small optical anisotropy is
created, [4] and it can be amplified by annealing at elevated tempera-
tures if the material exhibits a liquid crystalline (LC) nature [5–7].
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
49

50
N. Kawatsuki and J. Horii
We have carried out systematic studies on the photoinduced
molecular reorientation in photo-cross-linkable polymer LC (PPLC)
films containing mesogenic side groups terminated with cinnamate
derivatives [5–7]. Irradiating the PPLC films with linearly polarized
(LPUV) light generates a small photoinduced optical anisotropy, and
a subsequent thermal treatment enhances the molecular reorien-
tation. Among them, polymethacrylates with 4-(4-methoxycinnamoy-
loxy)-biphenyl (MCB) side groups exhibit efficient in-plane molecular
reorientations [6,7]. In this case, the axis-selective photoreacted
groups act as a photo-cross-linked anchor that thermally reorients
the non-photoreacted mesogenic groups parallel to the polarization
(E) of LPUV light. However, the influence of the alkylene spacer
length between the methacrylate backbone and the MCB mesogenic
side groups on the thermally enhanced photoinduced optical ani-
sotropy has yet to be explored. This paper describes the synthesis of
photo-cross-linkable liquid crystalline polymethacrylates comprised
of MCB side groups with various alkylene spacer lengths and the
influence of the alkylene spacer length on the thermal property and
the thermal enhancement of the molecular reorientation film.
EXPERIMENTAL
Materials
Figure 1 shows chemical structure of PPLCs used in this study. Meth-
acrylate monomers with MCB groups were synthesized according to
Scheme 1. Chemical structure of the monomers was confirmed by
1H-NMR and FT-IR spectroscopies. PPLCs were synthesized by a
radical polymerization in THF solution using AIBN as an initiator.
Weight average molecular weight of PPLC is between 40,000 and
90,000 g=mol. Table 1 summarizes thermal property of PPLCs.
FIGURE 1 Chemical structure of PPLCs used in this study.

Photoinduced Reorientation of PPLC Films
51
SCHEME 1 Synthetic route of methacrylate monomers.
TABLE 1 Thermal Property of Synthesized
PPLCs
PPLC
Thermal property (ꢀC)a
P0
P2
P6
C 290 N 300 < I
C 165 N 300 < I
C 101 N 297 I
P10
P12
C 116 SmA 291 I
C 115 SmA 285 I
aC: crystalline, N: nematic, SmA: smectic A, I:
isotropic.

52
N. Kawatsuki and J. Horii
Characterization
Thermal properties were examined using a polarization optical micro-
scope (POM) and a differential scanning calorimetry (DSC) analyzer.
Polarization UV-vis spectra were measured using a spectrometer
equipped with Glan-Taylor polarizing prisms. The in-plane order
parameter, S, and the out-of-plane order parameter, Sh, are expressed
in the form of Eqs. 1 and 2, respectively;
A ꢁ A
?
k
S ¼
ð1Þ
ð2Þ
A þ 2A
?
k
Aannealed
Sh ¼ 1 ꢁ
Airradiated
where A and A are the absorbances parallel and perpendicular to E,
k
?
respectively [6]. Aannealed and Airradiated are the average absorbances of
A and A from the annealed film and the initial irradiated film, respec-
k
?
tively. S and Sh were calculated by polarized UV-vis spectroscopy using
a wavelength of 314 nm. To analyze the three-dimensional orientation of
the mesogenic groups, polarized UV absorption spectroscopy was per-
formed after changing the angle of the incident p- and s-polarized
probe-beams according to a method previously described [8].
Photoirradiation
Thin films of PPLC were prepared by spin-coating a methylene chlor-
ide solution of polymers (1 wt%) onto a quartz substrate. The film
thickness was approximately 0.2 mm. The film was irradiated by light
from a 250 W high-pressure Hg-UV lamp that was passed through
Glan-Taylor polarizing prisms with a cut-off filter below 290 nm. The
light intensity was about 150 mW=cm2 at 365 nm. After exposing,
the film was annealed at elevated temperatures for 10 min. The optical
anisotropy of the film was measured by polarizing microscopy and
polarization UV-vis spectroscopy.
RESULTS AND DISCUSSION
Thermal and Spectroscopic Properties of PPLCs
All the synthesized PPLC show LC phase as summarized in Table 1.
P0, P2 and P6 exhibit a nematic LC phase, while P10 and P12 show
a smectic A phase, which is confirmed by POM observation. This is due
to a long alkylene spacer, as observed in other LC polymers with long

Photoinduced Reorientation of PPLC Films
53
alkylene spacer [9]. For P0, nematic LC phase is seen at high tempera-
tures on a POM observation, but a thermal decomposition of the
material simultaneously occurs. Figure 2 shows absorption spectra
of thin PPLC films. All films reveal similar absorption bands of
MCB side groups.
Axis-Selective Photoreaction of PPLC Film
It is well known that the irradiating a cinnamate-containing polymeric
film with LPUV light leads an axis-selective photoreaction, resulting
in an induction of small negative optical anisotropy of the film [4].
Figure 3 plots degree of the photoreaction of PPLC films, which is
estimated by monitoring the absorption intensity at 314 nm, as a
function of exposure doses. It shows that the photoreaction is slightly
faster for P0 film than other PPLC films. Figure 4 plots the photoin-
duced DAð¼ A ꢁ A Þ of PPLC films as a function of the exposure
?
k
doses. All the films show a generation of a negative DA due to the
axis-selective photoreaction of the MCB side groups, and maximum
DA is obtained when the irradiation energy is approximately
3–10 Jcmꢁ2, where the degree of the photoreaction is approximately
40–50 mol%.
Thermal Amplification of the Photoinduced Optical Anisotropy
When the exposed film with degree of the photoreaction of around
10–20 mol% is annealed at the LC temperature range of the film,
FIGURE 2 Absorption spectra of thin PPLC films on quartz substrate.

54
N. Kawatsuki and J. Horii
FIGURE 3 Degree of the photoreaction of PPLC films as a function of
exposure doses.
the photoinduced optical anisotropy is amplified parallel to the polar-
ization of LPUV light, or the out-of-plane reorientation is generated,
except P0. Because the LC temperature range of P0 is too high,
annealing procedure induces the thermal decomposition of the film.
Figures 5a and 5b show changes in the polarization UV-vis absorption
spectra of the P2 and P6 films irradiated with LPUV light and after
subsequent annealing at LC temperature range of the PPLC for
10 min. The conversion is approximately 15 mol% for both films and
in both cases, irradiating with LPUV light induces a small negative
FIGURE 4 Photoinduced DAð¼ A ꢁ A Þ of PPLC films as a function of the
?
k
exposure doses.

Photoinduced Reorientation of PPLC Films
55
FIGURE 5 UV polarization spectrum of PPLC films before irradiation (thin
lines), after irradiation (thin lines), and after subsequent annealing (thick
lines). A is shown as the solid lines and A is shown as the dotted lines. (a)
k
?
P2 was irradiated for 1.0 Jcmꢁ2 and was annealed at 240ꢀC, (b) P6 was irra-
diated for 0.75 Jcmꢁ2 and was annealed at 150ꢀC, (c) P10 was irradiated for
1.5 Jcmꢁ2 and was annealed at 120ꢀC. (d) P12 was irradiated for 1.5 Jcmꢁ2
and was annealed at 120ꢀC.
optical anisotropy, while subsequent annealing reversely generates
the molecular reorientation parallel to E. The amplified in-plane S
values are 0.66 for P2 and 0.70 for P6, respectively. In these cases,
the out-of-plane order Sh values are ꢁ0.07 and ꢁ0.35, respectively,
indicating that the most of the mesogenic groups reorient in the
in-plane direction. The axis-selectively photoreacted MCB side groups
act as photo-cross-linked anchors to induce the in-plane molecular
reorientation parallel to E [6,7].
In contrast, the thermally enhanced in-plane molecular reorien-
tation is small for P10 and P12 films, as shown in Figures 5c and 5d.

56
N. Kawatsuki and J. Horii
FIGURE 6 Angular dependence of UV absorption spectrum of P12 film as a
function of the angle of the monitoring polarized light. Annealing temperature
was 120ꢀC.
The conversion is approximately 15 mol% for both films. After the
annealing, the amplified in-plane S values are 0.30 for P10 and 0.23
for P12, respectively, while the out-of-plane order Sh values are
þ0.13 and þ0.34. Sh values are positive and much greater than that
of P2 and P6 films, suggesting the out-of-plane molecular reorien-
tation. The out-of-plane molecular reorientation is confirmed by
measuring the UV absorption with changing the angle of the incident
p- and s-polarized probe-beams, as shown in Figure 6. The long
alkylene spacer that causes the smectic character of the PPLC will
thermally generate the out-of plane reorientation as well as the
in-plane molecular reorientation.
Influence of the Annealing Temperature on the Thermal
Amplification of the Photoinduced Optical Anisotropy
The annealing temperature also affects the thermally amplified
in-plane and out-of-plane reorientation behavior. Figures 7a and 7b
plot the S and Sh values after the annealing as a function of the
annealing temperature. For all cases, the degree of the photoreaction
is approximately 15 mol%. For P2 and P6 films, large enhancement of
the in-plane molecular reorientation is observed when the exposed
film is annealed in all the LC temperature range. In contrast, for
P10 and P12 films,
S value decreases when the annealing
temperature increases, while Sh value increases. This means
that the out-of-plane reorientation becomes predominant when the

Photoinduced Reorientation of PPLC Films
57
FIGURE 7 Thermally amplified S and Sh values of the annealed films as a
function of the annealing temperature. (a) S values, (b) Sh values. Films were
annealed for 10 min.
annealing temperature is high. Long alkyl chain of these PPLCs will
induce the thermally generated out-of-plane reorientation.
CONCLUSIONS
Methacrylate PPLCs containing MCB side groups with different alky-
lene spacer length are synthesized. All the PPLCs show a nematic or a
smectic A LC phase. Irradiation of PPLC films with the LPUV light
results in the photoinduced optical anisotropy of the film, and it is
thermally amplified when the exposed film is annealed in the LC tem-
perature range except P0. When the alkylene spacer is short (P2 and
P6), in-plane reorientation is predominant, while out-of-plane reorien-
tation is generated for the PPLC with long alkylene spacer (P10 and
P12). Long alkylene spacer plays an important role in the thermally
generated out-of-plane reorientation.
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