Polymethacryloylaminoarylmethacrylates: New concept of photoalignment materials for liquid crystals

Article abstract of DOI:10.1080/15421400701680846

New class of highly efficient polymers for liquid crystal (LC) photoalignment is proposed. These polymers are based on methacryloylaminoarylmethacrylates having two methacryloyl groups of different reactivity. The more active O-methacryloyl group is subje

Full text of DOI:10.1080/15421400701680846

Mol. Cryst. Liq. Cryst., Vol. 479, pp. 121=[1159]–134=[1172], 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701680846
Polymethacryloylaminoarylmethacrylates: New
Concept of Photoalignment Materials
for Liquid Crystals
L. Vretik
V. Syromyatnikov
V. Zagniy
L. Paskal
Macromolecular Chemistry Department, Kyiv National Taras
Shevchenko University, Kyiv, Ukraine
O. Yaroshchuk
L. Dolgov
V. Kyrychenko
Institute of Physics, NASU, Kyiv, Ukraine
C.-D. Lee
Industrial Technology Research Institute, Chutung, Hsinchu,
Taiwan, R.O.C.
New class of highly efficient polymers for liquid crystal (LC) photoalignment is
proposed. These polymers are based on methacryloylaminoarylmethacrylates
having two methacryloyl groups of different reactivity. The more active O-methacryloyl
group is subjected to polymerization, while the less active NH-methacryloyl group
undergoes photocrosslinking reaction in combination with Fries rearrangement.
The latter process is anisotropic in case of polarized light irradiation that results
in LC photoalignment with readily controlled pretilt angle and anchoring energy.
The induced alignment shows high thermal and photo-stability.
Keywords: liquid crystal photoalignment; photocrosslinking; polymethacrylate
These studies were in part supported by NAS of Ukraine (grant VC89).
Address correspondence to L. Vretik, Macromolecular Chemistry Department, Kyiv
National Taras Shevchenko University, Volodymrs’ka Str. 60, 01033 Kyiv, Ukraine.
E-mail: vretik@list.ru
121=[1159]

1
22=[1160]
L. Vretik et al.
1. INTRODUCTION
The ability of the films of some photosensitive materials to align liquid
crystals after exposing to polarized actinic light is known over fifteen
years [1–4]. This alignment method, called as photoalignment, is one
of the most attractive alternative to rubbing process. It yields highly
uniform LC alignment with controllable easy axis and anchoring
energy. The photoalignment is free of many drawbacks of rubbing,
such as deterioration of alignment layers and their electric charging.
Besides, photoalignment suggests easy patterning procedure. The
photoalignment effect is caused by the orientational order formed in
the alignment film under illumination with a polarized actinic light
[
5–8]. This process is usually accompanied by alteration of film’s top-
ology, which also is a factor of LC alignment [9]. The molecular orien-
tational ordering in the photoalignment films occurs due to the
angularly selective photochemical reactions, which are distinctly dif-
ferent for the photosensitive species of different type. Based on the
prevailing photochemistry the classification of photoalignment materi-
als can be given. The first group form materials with photosensitive
species undergoing trans-cis photoisomerization. First of all, this
group includes azocompounds: chemically [1] and physically [10]
adsorbed azodyes, azodye blends and azopolymers [5–8]. The second
group is formed with materials having photosensitive fragments pre-
disposed to photodestruction, such as photooxidation, chain scission,
etc. The examples of these materials are photosensitive polyimides
[
11] and polysilanes [12]. The third and the mostly studied group
includes materials undergoing photo-crosslinking of cycloaddition
type. The materials containing cinnamate [2,3], coumarin [13] and
chalconyl [14] chromophore undergo [2 þ 2] cycloaddition. The [4 þ 4]
cycloaddition reaction is typical for anthracenyl chromophore [15].
Each class of materials has own advantages and disadvantages. The
azo compounds usually give excellent LC alignment at low exposure
dose (less than 0.5 J), which is, however, not sufficiently stable against
heat and light, because of reversible photochemistry. The photodes-
tructive materials commonly require high exposure dose (more than
2
1
0 J=cm ) and demonstrate enhanced image sticking. Compared with
them, materials undergoing photo-crosslinking demonstrate the best
promise. They combine good photosensitivity and high resistance of
LC alignment to heat and light, because of irreversible photo-
crosslinking products.
In spite of big progress in development of photoalignment materials
they are not used yet in commercial LCD. The problems still unsolved
are low anchoring strength, image sticking and insufficient alignment

New Concept of Photoalignment Materials for LC
123=[1161]
stability. For industrial implication the materials with radically
improved alignment characteristics are required. As an attempt in
this direction we propose new polymers capable of photo-crosslinking.
The crosslinking in these polymers may arise due to radical polymer-
ization of double bonds in addition to cycloaddition crosslinking earlier
considered. More particularly, we deal with 4-(N-methacryloylami-
no)aryl methacrylates containing two methacryloyl fragments. One
group is thermally polymerized in the process of polymer synthesis,
while the other one crosslinks polymer chains under photoalignmet
treatment. This provides polymer films with thermally stable and
reproducible LC alignment. Systematic variation of methacryloylami-
noarylmethacrylate molecule structure is made for better understand-
ing of ‘‘structure-photoaligning properties’’ correlation. The attempts
are also paid to reduce exposure dose for LC alignment.
2. EXPERIMENTAL SECTION
Materials
As photoalignment materials we used polymers poly-M2-poly-M6
obtained by polymerization of monomers M2–M6 depictured in
Figure 1. The different structures were considered to clarify the influ-
ence of flexible alkyl spacer on the reactivity of O-methacryloyl and
FIGURE 1 Chemical formulas of studied monomers.

1
24=[1162]
L. Vretik et al.
FIGURE 2 Schematic representation of photo-active polymer formation via
selective polymerization.
NH-methacryloyl groups in conditions of radical polymerization of
monomer as well as the influence of aromatic core structure (naphtha-
lene or phenylene) and presence of the spacer groups on LC photoalign-
ment properties of corresponding polymers. Polymer material poly-M1
obtained from monomer M1 earlier developed and tested [16] is used just
for comparison purpose.
Because of different reactivity of O- and NH- methacryloyl groups
in monomers M2–M6, at low degree of monomer conversion one
can realize selective polymerization of O-methacryloyl groups with
formation of linear soluble polymer products bearing photo- and
thermoactive double bonds of NH-methacryloyl groups (Fig. 2). As
we believed, these active bonds may react under photoalignment
treatment causing surface ordering and LC alignment effect.
The detailed description of monomer synthesis and the root of
polymerization are given in the next section.
Synthesis Procedure
The monomer syntheses was carried out via one step acylation of
starting materials with double excess of methacrylic anhydride=
methacryloyl chloride (syntheses of M3–M5) or by two step acylation
with isolation in a pure state of intermediate methacrylamide product
(
syntheses of M2, M6). The effectiveness of this synthesis root was
preliminarily checked [17]. More precisely, monomers were obtained
and polymerized as follow.

New Concept of Photoalignment Materials for LC
125=[1163]
Synthesis of 4-(methacrylamido)naphthol methacrylate (M2). 1.7 g
(
0.02 mol) of Na acetate and 3.1 g (0.02 mol) of methacrylic anhydride
were added in the solution of 4.0 g (0.02 mol) of 1,4-aminonaphthole
hydrochloride in 30 ml of acetic acid. The reaction mixture was boiled
during 2 hours. After cooling to room temperature a residue was formed.
The obtained white powder was filtered off and washed with hexane.
2
2
.3 g (51%) of 4-(methacrylamido)naphthol were formed. M.p. ¼ 225–
ꢀ
1
27 C (dec.). R ¼ 0.29 (acetone:hexane:benzene ¼ 4:5:7). H NMR
f
identification (400 MHz, DMSO-d ), ppm: 9.98 (s, 1H, NH), 8.17 (d,
6
1
=
H, Ar), 7.77 (d, 1H, Ar), 7.45 (p, 2H, Ar), 7.20 (d, 1H, Ar), 7.20 (d, 1H,
CH ), 6.84 (d, 1H, =CH ), 2.04 (s, 3H, CH ).
2
2
3
In the solution of 2.3 g (0.01 mol) of 4-(methacrylamido)naphthol in
0 ml of DMF was added 1.7 g (0.015 mol) of triethylamine. Then 1.9 g
1
(
0.015 mol) of methacryloyl chloride was added dropwise with stirring
ꢀ
and permanent cooling to 5–10 C. The reaction mixture was kept 5
hours at room temperature and the insoluble TEAꢁHCl was removed
by filtration. Filtration was poured into 200 ml of distilled water and
oily residue was formed. The residue was washed with distilled water
till neutral reaction in litmus. The obtained product was separated
from water phase by filtration. After drying 1.6 g of 4-methacrylamido)-
ꢀ
naphthyl methacrylate (M2) was obtained (54%) M.p.: 122 C. Rf
value (acetone:hexane:benzene ¼ 4:5:7) ¼ 0.50.
1
H NMR (400 MHz, DMSO-d ), ppm: 9.85 (s, 1H, NH), 7.98 (d, 1H,
6
Ar), 7.82 (d, 1H, Ar), 7.57 (d, 2H, Ar), 6.49 (m, 2H, Ar and 1H, ¼ CH2),
6
.02 (d, 1H, =CH ), 5.97 (s, 1H, =CH ), 5.55 (s, 1H, =CH ), 2.15 (s, 3H,
2 2 2
CH3), 2.07 (s, 3H, CH3).
Synthesis of 5-(Methacrylamido)naphthyl methacrylate (M3). The
solution of 4.78 g (0.03 mol) of 5-amino-1-naphthol and 10 ml (0.094mol)
ꢀ
of methacrylic anhydride was heated under the stirring at 90 C during 4
hours at the presence of 0.05 g of H SO as catalyst and phenothiazine
2
4
as inhibitor of polymerization. Then a reaction mixture was poured into
00 ml of distilled water. The precipitated black residue formed in water
5
was filtered off and washed with water till neutral reaction in litmus.
The dry black residue was boiled in toluene: hexane mixture (2:5) dur-
ing 5 minutes and slightly yellowish solution was decanted from the
black oily residue. Yellowish precipitate was formed after filtrate cool-
ing. This residue was filtered off and dissolved in 5 ml of acetone. The
acetone solution was poured into 100 ml of water containing 5 wt.% of
NaOH. The formed white precipitate was filtered off and purified by
two time recrystallization with toluene:hexane mixture. 1 g of 5-(metha-
crylamido)naphthyl methacrylate (M3) was obtained. Yield 11%,
ꢀ
m.p. ¼ 134–135 C, R ¼ 0.89 (eluent acetone:hexane:benzene ¼ 4:5:7).
f

1
26=[1164]
L. Vretik et al.
1
H NMR identification (400 MHz, DMSO-d ), ppm: 9.87 (s, 1H, NH),
6
7
.87 (d, 1H, Ar), 7.79 (d, 1H, Ar), 7.56 (d, 1H, Ar), 7.53 (t, 2H, Ar), 7.29
(
d, 1H, Ar), 6.47 (s, 1H, =CH ), 6.00 (s, 1H, =CH ), 5.96 (m, 1H, =CH ),
2 2 2
5
.53 (s, 1H, =CH ), 2.14 (s, 3H, CH ), 2.05 (s, 3H, CH ).
2 3 3
Synthesis of 4-(N-methacryloylamido)phenyl methacrylate (M4). To
the solution of 54.5 g (0.5 mol) of 4-aminophenole in 250 ml of dry
tetrahydrophurane was added 50.5 g (1 mol) of triethylamine. Then
1
04.5 g (1.1 mol) of methacryloyl chloride was added dropwise to the
solution at the room temperature with stirring and permanent cooling.
The reaction mixture was kept 3 days at room temperature and
then poured into 1 l of distilled water. The obtained residue was
filtered off and washed with water till neutral reaction in litmus.
Recrystallization in toluene mixture with silica gel gave 70.3 g of
ꢀ
4
-(N-methacryloylamido)phenyl methacrylate (57%). M.p.: 103–105 C.
R –value (acetone:hexane:benzene ¼ 4:5:7) ¼ 0.93.
f
1
H NMR (400 MHz, DMSO-d ), ppm: 9.71 (s, 1H, NH), 7.72 (d, 2H,
6
Ar), 7.01 (d, 2H, Ar), 6.27 (s, 1H, =CH ), 5.81 (m, 2H, =CH ), 5.45
2
2
(
s, 1H, =CH ), 2.02 (s, 3H, CH ), 1.97 (s, 3H, CH ).
2
3
3
Synthesis
of
4-(methacrylamido)phenethyl
methacrylate
(
4
M5). 3.95 g (0.088 mol) of triethylamine was added to the solution of
g (0.029 mol) of 4-aminophenethyl alcohol in 50 ml of dry acetone.
Then a solution of 9.2 g (0.088 mol) of methacrylchloride in 5 ml of
dry acetone was added dropwise with stirring and permanent cooling
ꢀ
to 5–10 C. The reaction mixture was kept 3 hours at room temperature.
After this the insoluble TEA ꢁ HCl was removed by filtration. A clean
solution was then poured into 500 ml of distilled water and oily residue
was formed. Fresh distilled water was added three times till neutral
reaction in litmus. The obtained product was then separated from a
water phase by dissolving in benzene. The benzene solution was dried
with Na SO and the solvent was removed by distillation. 5 g of
2
4
4
-(methacrylamido)phenethyl methacrylate was obtained. Yield 63%,
ꢀ
ꢀ
m.p. ¼ 36 –37 C, R ¼ 0.85 (eluent acetone:hexane:benzene ¼ 5:1:1).
f
1
H NMR identification (400 MHz, DMSO-d ), ppm: 9.58 (s, 1H, NH),
6
7
5
.58 (d, 2H, Ar), 7.13 (d, 2H, Ar), 5.99 (s, 1H, =CH ), 5.77 (s, 1H, =CH ),
2 2
.77 (s, 1H, =CH ), 5.58 (m, 1H, =CH ), 4.25 (t, CH -O), 2.89 (t,
2
2
2
Ar-CH ), 1.95 (s, 3H, CH ), 1.87 (s, 3H, CH ).
2
3
3
Synthesis of 4-(2-methacrylamidoethyl)phenyl methacrylate (M6). M6
was prepared in two steps. 4.4 g (0.044 mol) of triethylamine was added
to the solution of 4 g (0.029 mol) of 4-(2-aminoethyl)phenol in 10 ml of
dry acetone and 2 ml of DMF. Then 4.6 g (0.044 mol) of methacrylchlo-
ride was added dropwise with stirring and permanent cooling to

New Concept of Photoalignment Materials for LC
127=[1165]
ꢀ
–10 C. The reaction mixture was kept 3 hours at room temperature
5
and the insoluble TEA ꢁ HCl was removed by filtration. The filtered sol-
ution was then poured into 500 ml of distilled water that resulted in
precipitation of solid residue. The obtained product was separated from
a water phase by filtration. 4 g of 4-(2-methacrylamidoethyl)phenol
was obtained with a yield of 67%. After recrystallization from toluene,
ꢀ
m.p. ¼ 133 C, R ¼ 0.53 (eluent acetone:hexane:benzene ¼ 3:2:10).
f
1
H NMR identification (400 MHz, DMSO-d ), ppm: 8.93 (s, 1H, OH),
6
7
5
3
.76 (s, 1H, NH), 6.94 (d, 2H, Ar), 6.62 (d, 2H, Ar), 5.59 (s, 1H, =CH2),
.23 (s, 1H, =CH ), 3.25 (t, CH -O), 2.62 (t, Ar-CH ), 1.85 (s,
2
2
2
H, CH3).
.48 g (0.0147 mol) of triethylamine was added to the solution of 2 g
0.0098 mol) of 4-(2-aminoethyl)phenol in DMF. Then 1.54 g (0.0147 mol)
1
(
of methacrylchloride was added dropwise with stirring and permanent
ꢀ
cooling to 5–10 C. The reaction mixture was kept 3 hours at room
temperature. After that the insoluble TEA ꢁ HCl was removed by fil-
tration. A filtered solution was poured into 200 ml of distilled water that
resultedin formationofsolid residue. The obtained productwas separated
from a water phase by filtration. 2 g of 4-(2-methacrylamidoethyl)phenyl
methacrylate was obtained with a yield of 75%. After recrystallization
ꢀ
from the acetone:water mixture, m.p. ¼ 93 C, R ¼ 0.65 (eluent acetone:
f
hexane:benzene ¼ 3:2:10).
1
H
NMR identification (400 MHz, DMSO-d ), ppm: 7.86 (m, 1H,
6
NH), 7.23 (d, 2H, Ar), 7.00 (d, 2H, Ar), 6.23 (d, 1H, =CH ), 5.81 (d,
2
1
H, =CH ), 5.60 (d, 1H, =CH ), 5.24 (d, 1H, =CH ), 3.36 (m, 2H,
2 2 2
CH -NH), 2.78 (m, 2H, CH -Ar), 2.01 (s, 3H, CH ), 1.85 (s, 3H, CH ).
2
2
3
3
Polymerization procedure. The polymerization root given below
for monomer M2 was employed as a general procedure for synthesis
of homopolymers of M2–M6 (polymers poly-M2-poly-M6) with vari-
ation of polymerization time (see section ‘‘Results and discussion’’).
For the polymerization 0.5 g of 4-(methacrylamido)naphthyl meth-
acrylate (M2) and 0.005 g of azobisisobutironitrile (AIBN) were dis-
solved in 5 ml of dimethylformamide. The reaction vessel with the
solution was subsequently closed air-tight. The solution was heated
ꢀ
to 80 C for 90 minutes. Thereafter, the reaction vessel was opened
and the solution was added dropwise to 100 ml of ethanol while stir-
ring at room temperature. The separated polymer was filtered off,
dried and dissolved in 3 ml of dimethylformamide. Subsequently, this
solution was added dropwise to 50 ml of ethanol. Filtration and drying
ꢀ
at 40 C in a vacuum gave 0.085 g (17%) of poly(4-(methacrylamido)-
ꢂ1
naphthyl methacrylate) (poly-M2). IR identification (KBr), cm
:
broad 3460, 3340 (NH), 1750 (ꢂOꢂCOꢂ), 1680 (amide I), 1660,

1
28=[1166]
L. Vretik et al.
1
1
500, 1250, 947, 661 (aryl), other signals 2930, 1621, 1530, 1390, 1340,
320, 1220, 1130, 814, 760, 594.
Material Characterization
Thin layer chromatography was performed with Merck Kieselgel
plates 60F254. NMR spectra were recorded with a Bruker AM400 FT-
NMR-spectrometer (400 MHz) with TMS as internal standard. IR
spectra were run on a Nicolet 5SXB FTIR-spectrophotometer. The
UV=Vis spectra of polymers were measured by Ocean Optics spec-
trometer in the spectral range 220–600 nm. GPC-measurements were
performed using PSS-SDV columns and DMF eluent containing LiBr
at a flow rate of 1.0 mL=min. The calibration curves for GPC analysis
were obtained using PSS polystyrene standards (1000–400000 D).
Differential scanning calorimetry was carried out with a Perkin-Elmer
DSC 7.
LC Cells and Photoalignment Tests
The poly-M2-poly-M6 polymers were dissolved in DMF at concen-
tration 1 wt% and filtered to 0.2 mm by syringe filter. The polymer
films were obtained by spin coating of the polymer solutions on the
ꢀ
quartz or ITO covered glass plates and subsequent backing at 150 C over
1
h. The photoalignment action was provided by a broad band irradiation
from a high pressure mercury lamp DRS-500 linearly polarized by a
Glann-Tompson prism. To impart unambiguous direction of LC pretilt
the substrates were irradiated in two steps: first with polarized UV light
2
(
(
14 mW=cm , 10 min) and then with non-polarized UV light
2
ꢀ
78 mW=cm , 1 min) by sample rotation of 90 around its normal. Both
ꢀ
irradiations were carried out obliquely at the incidence angle of 45 .
Two types of LC cells were constructed. In the most common case
LC cell was made by sandwiching the LC between a pair of glass=ITO
substrates coated with photoalignemnt layer and irradiated as
described above. To obtain a uniform director orientation across the
cells, they were assembled in an antiparallel fashion. This means that
direction of irradiation with non-polarized light were antiparallel to
each other. Cell thickness was adjusted by spacers with a diameter
of 20 mm. These cells, termed symmetrical cells, were used to deter-
mine the type of LC alignment (homeotropic, planar or tilted), and also
to measure pretilt angle of LC.
To define the alignment direction in the cell plane and the value of
azimuthal anchoring energy we also constructed cells consisting of one
rubbed polyimide substrate and a photoaligned substrate (asymmetri-
cal cells). The easy axis of the photoalignment substrate was turned

New Concept of Photoalignment Materials for LC
129=[1167]
ꢀ
0 with respect to the rubbing direction of the polyimide substrate.
9
The rubbed substrate was used as a reference one with predetermined
alignment direction and strong azimuthal anchoring. The thickness of
these cells was reduced to 6 mm to increase upper limit of anchoring
energy measurement.
The cells were filled at room temperature with nematic mixture
ZLI2293 from Merck. We judged the alignment quality by cell obser-
vation in a light box and polarizing microscope using evaluation scale
with five grades: (1) excellent; (2) good (single alignment faults);
(
3) satisfactory (minor alignment faults); (4) bad (big number of align-
ment faults in a form of inversion walls, flowing patterns, etc.) and
5) no alignment. The pretilt angle was measured by crystal rotation
(
method, while azimuthal anchoring coefficient was estimated by
spectroscopic method described in Ref. [10].
3. RESULTS AND DISCUSSION
Monomer Structure and Polymerization Rate
According to synthetic procedure described above, linear soluble in
organic solvents polymer materials were obtained with satisfactory
yields. The maximum polymerization yield, when linear soluble pro-
duct still could be obtained, is strongly dependent on the chemical nat-
ure of aromatic core (naphthalene or phenylene) and presence of alkyl
tail in monomer molecule.
Naphthalene-containing monomers M2 and M3 could be radically
polymerized into soluble in DMF linear polymer products up to
7
0 wt.% monomer conversion. Free C=CH2 potentially photoactive
double bonds were registered in IR spectra of corresponding polymers
ꢂ1
poly-M2 and poly-M3 at 1621 and 1630 cm respectively.
The maximum yield of soluble polymer for phenylene-containing
monomers M4–M6 falls down to 30 wt% of monomer conversion. Free
C=CH double bonds were registered in IR spectra of polymers poly-
2
ꢂ1
M4, poly-M5 and poly-M6 at 1621, 1622 and 1623 cm respectively.
The total polymerization rate of O-methacryloyl group for monomer
M5 is 20 times higher comparing with other studied monomers. This
means that selective polymerization process is hardly controlled.
1
Due to broad overlapped signals in IR and H NMR spectra of poly-
M2-poly-M6 it is not possible to make correct calculations for the ratio
of non-reacted and reacted NH-methacryloyl and O-methacryloyl
fragments.
We will show later that polymer yield could be considered as an
indirect determinant of the efficiency of LC photoalignment with

1
30=[1168]
L. Vretik et al.
TABLE 1 Polymerization Yield, GPC Data and LC Alignment Results for
poly-M4
Low molecular
Polymer
weight fraction,
yield, wt.%
M
w
M
n
M =M
w n
wt.%
Alignment quality
1
2
3
2
3
5
29 400
50 500
60 000
12 600
28 600
33 600
2.3
1.8
1.9
12
36
22
excellent
good
excellent=good
poly-M2-poly-M6 films. To clear this correlation, several samples
with different polymer yield were prepared for each of M2–M6 mono-
mers changing time of polymerization.
The GPC results presented in Table 1 for poly-M4 show quasilinear
grow of polymer weight with the yield of polymerization. Interestingly,
in addition to relatively high maximum corresponding to fraction with
high molecular weight for which masses presented in Table 1 are cal-
culated, GPC curve contains additional maximum in a low molecular
weight region. The relative content of this fraction clearly correlates
with LC alignment as discussed below.
The UV=Vis spectra (Fig. 3) show that polymers are transparent
in visible and near UV spectral range that predetermines good
FIGURE 3 UV=Visible spectra of poly-M4 film before (1) and after
2
(
2) irradiation with non-polarized UV light (70 mW=cm , 15 min).

New Concept of Photoalignment Materials for LC
131=[1169]
photostability of these compounds and, predictably, their orientational
structures.
Photoalignment Peculiarities
The properties of poly-M2-poly-M6 are strongly dependant on
polymerization yield. At high polymerization yield (usually higher
than 70 wt.%) the polymer products are highly crosslinked and so
insoluble in organic solvents used for film coating purpose. At lower
yield (typically, 70–40 wt.%) material can be dissolved, but quality of
LC alignment is rather poor. Finally, if the yield is lower than some
critical value, acceptable LC alignment is detected. The quality of
ꢀ
LC alignment is excellent or good in both parallel and 90 twist con-
figuration (Fig. 4). For all polymers the easy axis of LC alignment
was induced perpendicularly to the direction of light polarization Eexc
.
The critical values of polymerization yield for poly-M2-poly-M6
are presented in Table 2. Analyzing these data the following conclu-
sions can be drawn:
(
1) The highest critical yield (50 wt.%) is obtained for naphthalene
containing polymer poly-M3.
FIGURE 4 Photo of asymmetric cell containing rubbed PI and photoaligned
poly-M4 layer viewed between a pair of crossed polarizers. The photoalign-
ment substrate contains 3 domains: (1) non exposed area; (2) easy axis is
induced parallel to rubbing direction; (3) easy axis is induced perpendicular
to rubbing direction.

1
32=[1170]
L. Vretik et al.
TABLE 2 Polymerization Yield, Solubility and Characteristics of LC
Alignment for poly-M4-poly-M6
Monomer
Yield, wt.%
Solubility
Alignment quality
LC pretilt angle
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
M2
M3
M4
M5
M6
48
50
35
29
30
DMF
DMF
DMF
DMF
DMF
good=excellent
good
excellent
excellent
excellent=good
0.3 –0.8
ꢀ
0 –0.6
ꢀ
0.4 –1.2
ꢀ
0 –0.5
ꢀ
1.0 –2.2
(
2) The incorporation of phenyl core instead of naphthalene core
poly-M4-poly-M6) leads to improvement of LC alignment
(
quality.
(
3) The incorporation of alkyl tail between O or NH methacryloyl
group and aromatic core (M5, M6) reduces the yield of soluble
polymer (poly-M5, poly-M6) giving some enhancement of LC
pretilt angle.
Taking into account these results we were further focused on
polymer poly-M4 giving the best promise. Table 1 comprises GPC
data and a grade of LC alignment for poly-M4 samples obtained
FIGURE 5 Azimuthal anchoring coefficient as a function of exposure dose for
poly-M4 alignment film.

New Concept of Photoalignment Materials for LC
133=[1171]
with different polymerization yield. For all polymer samples the
molecular weight distribution is bimodal consisting of a higher mol-
ecular weight fraction (for which M , M and M =Mn values are
w
n
w
derived) and a broad lower molecular weight fraction. One can see
that LC alignment improves when the lower molecular weight frac-
tion decreases. This fraction, in turn, grows with polymerization
yield.
The azimuthal anchoring strength on poly-M4 alignment films
monotonically grows with an exposure dose reaching saturation level
2
ꢂ5
2
at about 20 J=cm (Fig. 5). The maximal value W ¼ 2ꢃ10 J=m
a
approaches maximal values obtained for photoalignment materials
with photocrosslinking fragments [18]. The minimal dose needed for
good alignment was about 2 J. To reduce the exposure dose, poly-
M4 was doped with Michler’s ketone (MK) earlier used as photosensi-
tizer for cinnamate polymer [19]. By adding 25wt.% of MK we
obtained double decrease of exposure dose. This shows effective way
for the further material improvement.
The poly-M2-poly-M6 alignment layers demonstrate good align-
ꢀ
ment stability. The LC cells based on them endure aging at 120 C over
2
h without any noticeable alignment deterioration. This strongly
contrasts with poor thermal stability of poly-M1 layers having no
crosslinking fragments in the molecule [16]. Consequently, one can
conclude important role of the crosslinking in the maintenance of
photooriented state of polymer film and LC alignment.
Let us finally discuss possible photoalignment mechanisms determ-
ined by the processes of photochemical transformation. By analogy
with cinnamates [20] one can believe that film’s anisotropy and LC
alignment are mainly determined by the non-reacted anisotropic frag-
ments orientationally ordered in the direction perpendicular to UV
light polarization due to the angularly selective transformations.
The studied polymers are potentially able to undergo Freies
rearrangement and photocrosslinking via [2 þ 2]-cycloaddition and=or
or photopolymerization of NH-methacryloyl fragments. Excellent LC
alignment on the films of poly-M1 capable to only Freies rearrange-
ment suggests important role of this photochemical process in gener-
ation of LC alignment. To provide good LC alignment [16], Fries
rearrangement should be highly polarization sensitive process. The
photocrosslinking appearing in poly-M2-poly-M6 radically enhances
stability of the induced order. But it is unclear now whether crosslink-
ing fragments are anisotropically distributed in the irradiated film
and so participate in the alignment process. A detailed study of the
mechanisms underlying LC alignment on poly-M2-poly-M6 films is
currently being pursued in our laboratory.

1
34=[1172]
L. Vretik et al.
4. CONCLUSIONS
Principally new concept for the design of LC photoalignment polymers
is proposed and experimentally proven by the use of methacryloylami-
noarylmethacrylates having two methacryloyl groups of different reac-
tivity. The more active O-methacryloyl group is subjected to
polymerization, while the less active NH-methacryloyl group under-
goes crosslinking reaction under photoirradiation. These materials
provide excellent LC alignment with low pretilt angle and widely var-
ied azimuthal anchoring excelling conventional materials by high
thermal stability. The materials have flexible design allowing further
optimization. Among the most important tasks is a search of new
synthesis roots giving increased yield of polymerization.
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