New photoalignment materials in LCD's development: Liquid crystal pretilt angle variations by using fluoroalkylmethacrylates

Article abstract of DOI:10.1080/15421400701229909

Fluorinated copolymers based on 4-(N-methacryloylamino)phenyl methacrylate are proposed as materials for the liquid crystal (LC) photoalignment with a wide range of variations of a pretilt angle. As fluorinated chains, 2,2,3,3-tetrafluoropropylmethacrylat

Full text of DOI:10.1080/15421400701229909

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New Photoalignment Materials
in LCD's Development:
Liquid Crystal Pretilt
Angle Variations By Using
Fluoroalkylmethacrylates
a
a
a
L. Vretik , L. Paskal' , V. Syromyatnikov , V.
a
a
b
Zagniy , O. Savchuk , L. Dolgov , O. Yaroshchuk
b
c
&
C.-D. Lee
a
Macromolecular Chemistry Department , Taras
Shevchenko Kyiv National University , Kyiv, Ukraine
b
Institute of Physics, National Academy of Science
of Ukraine , Kyiv, Ukraine
c
Industrial Technology Research Institute, Chutung ,
Hsinchu, Taiwan, Republic of China
Published online: 22 Sep 2010.
To cite this article: L. Vretik , L. Paskal' , V. Syromyatnikov , V. Zagniy , O.
Savchuk , L. Dolgov , O. Yaroshchuk & C.-D. Lee (2007) New Photoalignment
Materials in LCD's Development: Liquid Crystal Pretilt Angle Variations By Using
Fluoroalkylmethacrylates, Molecular Crystals and Liquid Crystals, 468:1, 173/
[525]-179/[531], DOI: 10.1080/15421400701229909
To link to this article: http://dx.doi.org/10.1080/15421400701229909
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Mol. Cryst. Liq. Cryst., Vol. 468, pp. 173=[525]–179=[531], 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701229909
New Photoalignment Materials in LCD’s Development:
Liquid Crystal Pretilt Angle Variations By Using
Fluoroalkylmethacrylates
L. Vretik
L. Paskal’
V. Syromyatnikov
V. Zagniy
O. Savchuk
Macromolecular Chemistry Department, Taras Shevchenko Kyiv
National University, Kyiv, Ukraine
L. Dolgov
O. Yaroshchuk
Institute of Physics, National Academy of Science of Ukraine,
Kyiv, Ukraine
C.-D. Lee
Industrial Technology Research Institute, Chutung, Hsinchu,
Taiwan, Republic of China
Fluorinated copolymers based on 4-(N-methacryloylamino)phenyl methacrylate
are proposed as materials for the liquid crystal (LC) photoalignment with a wide
range of variations of a pretilt angle. As fluorinated chains, 2,2,3,3-tetrafluoropro-
pylmethacrylate and 2,2,3,3,4,4,5,5-octafluoropenthylmethacrylate are used. The
growth of a pretilt angle with the concentration of fluorinated chains, more
pronounced in a series with longer fluorinated chain, is observed. The pretilt
ꢀ
ꢀ
angle of TN LC ZLI 2293 is smoothly varied in the range 0 –20 . Additionally, the
ꢀ
ꢀ
uniform high pretilt angle (89 –90 ) alignment is realized for VA LC MJ961180.
Keywords: fluorinated copolymer; liquid crystal photoalignment; methacrylate;
pretilt angle
Address correspondence to L. Vretik, Macromolecular Chemistry Department, Taras
Shevchenko Kyiv National University, 64 Volodymyrs’ ka Str., Kyiv, 01033, Ukraine.
E-mail: svg@mail.univ.kiev.ua
173=[525]

1
74=[526]
L. Vretik et al.
1. INTRODUCTION
Over the last decade, a big amount of works has been directed towards
the photo-induced liquid crystal (LC) alignment [1–4] as an alternative
route of the rubbing method. The photoalignment effect is due to the
ordering of polymer fragments under the action of polarized ultra-
violet (UV) light. The photoalignment method allows one to induce
an anisotropy on the alignment layer preventing contact, pollution,
electrical charging and mechanical destruction of the surface. It has
been reported that the LC alignment occurs with films having the
cis-trans-isomerization of azo-chromophore or cinnamate chromophore
[
[
5–10]; photo-crosslinking of cinnamate, coumarin [11], chalconyl
12] chromophore by the (2 þ 2) cycloaddition reaction or photo-
crosslinking of anthracenyl chromophore by a (4 þ 4) cycloaddition
reaction [13] on the illumination with UV light; cationic photopolymer-
ization involving the ring opening of epoxide groups for the alignment
fixation [14–16]. But the existing photoaligning materials have a num-
ber of drawbacks, such as low anchoring energy, low thermal stability
and photostability, and hard control of an LC pretilt angle. So many
efforts are directed to the searching for new photoaligning materials
with improved characteristics.
In our previous work [17], we proposed bis-methacrylics (parti-
cularly 4-(N-methacryloylamino)phenyl methacrylate) as a new class
of photoaligning materials. Such materials have two methacrylic
groups with different reactivities. One group is thermally polymerized
in the process of synthesis of the polymer, while the other one aniso-
tropically crosslinks polymer chains under the photoalignment treat-
ment. These materials demonstrate the improved stability of LC
alignment, but, however, impact to an LC small pretilt angle.
In order to control the pretilt angle, we propose to vary the hydro-
phobic properties of methacrylates by means of using their fluorinated
copolymers. 4-(N-methacryloylamino)phenyl methacrylate copolymer-
ized with 2,2,3,3-tetrafluoropropylmethacrylate and 2,2,3,3,4,4,5,5-
octafluoropenthylmethacrylate are considered. Controlling the type and
content of fluorinated chains, we expect to control the LC pretilt angle.
2
2
. EXPERIMENTAL SECTION
.1. Synthesis and Characterization of Alignment Materials
All solvents of p.a. quality (Aldrich) were stored over molecular sieves
˚
of 3 or 4 A. All chemicals were purchased from Aldrich and used
without further purification.

New Photoalignment Materials in LCD’s Development
175=[527]
Monomer synthesis was done by the usual acylation of aminophenole
Fig. 1). Particularly, 4-(N-methacryloylamino)phenyl methacrylate was
(
synthesized via acylation of 4-aminophenole (1) in dry tetrahydrofuran
ꢀ
with methacryloyl chloride (2) at 15–20 C [17].
Copolymers were synthesized by the following method: 4-(N-metha-
cryloylamino)phenyl methacrylate (MAPhM) and 2,2,3,3-tetrafluoro-
propylmethacrylate or 2,2,3,3,4,4,5,5-octafluoropenthylmethacrylate
in a corresponding molar ratio with 1 wt.% of AIBN in dry dimethyl-
ꢀ
formamide were heated to 80 C, and polymer product was isolated
by precipitation into diethyl ether.
By this way, two series of copolymers were synthesized with two
different fluorinated agents (Fig. 2 a, b). Copolymers inside the series
differ from each other by the x:y ratio, i.e. by the mole fraction c of
f
fluorinated tails in the main chain. In the series of copolymers based
on 2,2,3,3-tetrafluoropropylmethacrylate, homologues with c equal to
f
4
2
4
1.2, 57, and 70 mole % were obtained. In turn, in the series with
,2,3,3,4,4,5,5-octafluoropenthylmethacrylate, the c values were 10,
f
1.2, 50, and 70 mole %.
The characterization of polymers was performed by different
methods. Thin layer chromatography was performed with Merck
Kieselgel plates 60F254. NMR spectra were recorded with a Bruker
NMR-spectrometer (500 MHz) with TMS as internal standard. IR
spectra were run on a Nicolet 5SXB FTIR-spectrophotometer.
GPC-measurements were performed using PSS-SDV columns and a
DMF eluent containing LiBr at a flow rate of 1.0 L=min. The cali-
bration curves for GPC analysis were obtained using PSS polystyrene
standards (1000 – 400 000 D). Differential scanning calorimetry was
carried out with a Perkin-Elmer DSC 7.
FIGURE 1 Scheme of synthesis of 4-(N-methacryloylamino)phenyl methacrylate.

1
76=[528]
L. Vretik et al.
FIGURE 2 Groups of copolymers of MAPhM: (a) with 2,2,3,3-tetrafluoropro-
pylmethacrylate; (b) with 2,2,3,3,4,4,5,5-octafluoropenthylmethacrylate.
2.2. Testing of LC Alignment
The quality of LC photoalignment was investigated in the following
way. A 2-wt.% solution of the investigated polymer in dimethylforma-
mide was spincoated (3000 rpm, 1 min) on the glass substrate. The
ꢀ
obtained film was pre-backed during 10 min at 100 C, then backed
ꢀ
at 180 C during 1 h. After that, the polymer film was normally
2
irradiated by polarized UV light (I ¼ 28 mW=cm , t ¼ 15 min) and
2
then obliquely irradiated by unpolarized UV (I ¼ 100 mW=cm ,
t ¼ 1 min). At last, a symmetric optical cell (the cell thickness is
2
0 mm) with antiparallel LC alignment was made from the irradiated
substrates. The cell was filled by liquid crystal ZLI 2293 (De>0) or
MJ 961180 (De<0). The quality of liquid crystal alignment was
estimated by the observation of a sample in polarized light (a light
box and a polarizing microscope). The pretilt angle of a liquid crystal
in the cell was measured by the commonly used crystal rotation
method [18].
3. RESULTS AND DISCUSSION
It was revealed that both MAPhM homopolymer and its fluorinated
copolymers provide the LC alignment of excellent quality (Fig. 3).
Using copolymers with different fluorinated chains allows us to
control the liquid crystal pretilt angle. Figures 4 and 5 demonstrate

New Photoalignment Materials in LCD’s Development
177=[529]
FIGURE 3 Photoalignment of liquid crystal MJ961180 (De < 0) in the cell
based on: (a) poly-MAPhM, planar alignment; (b) MAPhM copolymer with
2
,2,3,3-tetrafluoropropylmethacrylate (c
f
¼ 57), tilted alignment; (c) MAPhM
copolymer with 2,2,3,3,4,4,5,5-octafluoropenthylmethacrylate (c ¼ 70), verti-
f
cal alignment. Symmetrical cells are in crossed polarizers.
the monotonous increase of the pretilt angle with concentration of
fluorinated chains. One can see also that, for TN LC ZLI2293, the
longer fluorinated tail promotes a higher value of the pretilt angle
than the shorter one at the same mole concentration in the poly-
mer. This difference is less pronounced for VA LC MJ961180 having
negative dielectric anisotropy. The pretilt angle of LC ZLI2293 is
ꢀ
ꢀ
varied in the range 0 –20 . In turn, the pretilt angle of MJ961180 runs
ꢀ
ꢀ
the whole range of pretilt angles, i.e., it changes from 0 to 90 . The
values close to 90 are easily achieved, which makes these materials
ꢀ
promising for the VA mode.
Noteworthy, even if the concentration of non-photosensitive
fluorinated monomers is 50–70 mole%, the alignment quality is good.
Thus, the LC pretilt angle changes with the concentration of fluori-
nated tails without noticeable changes in the alignment quality. In
addition, in contrast to copolymers with cinnamate units [19], LC
photoalignment in copolymers with methacrylic reactive groups is
much more stable. These results make such method of pretilt angle
variation extremely promising for applications.

1
78=[530]
L. Vretik et al.
FIGURE 4 Dependence of LC pretilt angle vs. mole concentration c
nated fragments in 2,2,3,3-tetrafluoropropylmethacrylate. (1) ZLI2293
De > 0); (2) MJ961180 (De < 0).
f
of fluori-
(
FIGURE 5 Dependence of LC pretilt angle vs. mole concentration c of fluori-
f
nated fragments in 2,2,3,3,4,4,5,5-octafluoropenthylmethacrylate. (1) ZLI2293
(
De > 0); (2) MJ961180 (De < 0).

New Photoalignment Materials in LCD’s Development
179=[531]
CONCLUSIONS
We have proposed an effective method to control the LC pretilt angle
on the methacrylate copolymer surface. Copolymerization of meth-
acrylic monomers with fluorinated fragments of different lengths
and different concentrations in copolymer is considered. It is revealed
that both the lengthening of a fluorine chain and the increase of the
mole concentration of a fluorine fragment leads to the growth of the
liquid crystal pretilt angle. Fluorinated chains increase the polymer
hydrophobic properties and promote a gradual change of the LC align-
ment from a planar to tilted and then to vertical alignment. The
increase of the pretilt angle with the concentration of hydrophobic
chains is not accompanied with the worsening of the LC alignment
and alignment stability. This fact makes the method extremely
promising for industrial applications.
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