The properties of homeotropic alignment materials as the side chain molecular structure in polyimides

Article abstract of DOI:10.1080/15421400701734056

LCD (Liquid Crystal Display) screens are today's most commonly used displays and are available in a wide range of shapes, size, levels of quality, and models. VA-LCD is good to TV applications from among other LCD modes, owing to wide viewing angle, high

Full text of DOI:10.1080/15421400701734056

Mol. Cryst. Liq. Cryst., Vol. 479, pp. 191=[1229]–203=[1241], 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701734056
The Properties of Homeotropic Alignment Materials
as the Side Chain Molecular Structure in Polyimides
Jae-Cheol Park
Electronic Materials Division, Dongjin Semichem, Hwaseong-Si,
Gyeonggi-Do, Korea
Dong-Ju Park
Kyung-Chul Son
Yong-Bae Kim
Liquid Crystal Research Center, Department of Chemistry, Division
of Natural Sciences, Konkuk University, Kwangjinku, Seoul, Korea
LCD (Liquid Crystal Display) screens are today’s most commonly used displays
and are available in a wide range of shapes, size, levels of quality, and models.
VA-LCD is good to TV applications from among other LCD modes, owing to wide
viewing angle, high contrast ratio and fast switching time. To apply for VA-LCDs,
we have synthesized novel homeotropic alignment materials, which all generated
high pretilt angles (>88ꢀ) with and without rubbing. They showed good homeotro-
pic alignment state too. In addition, response time was very fast. In this paper, we
discussed the synthesis and alignment properties of new homeotropic alignment
materials.
Keywords: homeotropic alignment materials; pretilt angle; response time
INTRODUCTION
Two kinds of liquid-crystalline shape diamines, 2,3-Difluoro-4-(4-propyl-
cyclohexyl)-biphenyl-40-propyl 3,5-diamino benzoate (negative liquid
crystalline shape diamine, SP type) and 40-(4-Pentylcyclohexyl)-
biphenyl-3,5-diamine (positive liquid crystalline shape diamine,
R5CPP type), were polymerized with 4,4-oxydianilin (ODA) and
Address correspondence to Jae-Cheol Park, Dongjin Semichem Co., Ltd, Electronic
Materials Division, Dongjin Semichem, 625-3 Yodang-Ri, Yanggam-Myeon, Hwaseong-si,
Gyeonggi-do 445-931, korea. E-mail: interpot@nate.com
191=[1229]

192=[1230]
J.-C. Park et al.
three aromatic dianhydrides, 3,30,4,40-benzophenone tetracarboxylic
dianhydride (BTDA), 4,40-Oxydiphthalic anhydride (ODPA) and
3,30,4,40-Biphenyltetracarboxylic dianhydride (BPDA), in N-methyl-
2-pyrrolidinone (NMP), and the resulting poly(amic acid)s were ther-
mally cyclodehydrated to polyimides.
These polyimides possess high pretilt angle and high electro-optical
properties. They can generate greater pretilt angle than 88ꢀ. They
showed good homeotropic alignment state too. In addition, response
time of SP type polyimide was very fast.
So these negative liquid crystalline shape side chain polyimides
(novel homeotropic alignment materials), which we synthesized, are
possible to use for VA-LCD application.
EXPERIMENTAL
Materials
Diamine monomer, 2,3-Difluoro-4-(4-propyl-cyclohexyl)-biphenyl-
40-propyl 3,5-diamino benzoate was synthesized as following the
Figure 2 and 40-(4-Pentylcyclohexyl)biphenyl-3,5-diamine was syn-
thesized as following the Figure 3. Poly(amic acid) precursors [1]
were prepared by copolymerization from mixing dianhydride,
ODA(4,40-oxydianiline) and synthesized diamine with side chain. The
three dianhydride monomers, 3,3,4,4-Benzophenonetetracarboxylic
dianhydride (BTDA), 4,40-Oxydiphthalic anhydride (ODPA), 3,30,4,40-
Biphenyltetracarboxylic dianhydride (BPDA) were purchased from
Aldrich and purified by recrystallization from THF and MEK.
Measurement
The chemical structures of products were confirmed by 1H NMR spec-
troscopy (Bruker Advance 400 MHz Spectroscope) and GC=MS spec-
troscope (Hewlett Packard 5972 MSD). Voltage holding ratio (VHR)
was measured with 6254 Instruments from TOYO Inc.Co. Pretilt
angles were measured by using crystal rotation method [2] with the
self set up optic system. Figure 1. indicates the principle block dia-
gram of the measuring system of pretilt angle. Response time of LC
was measured with the self set up system using a waveform generator
and oscilloscope. The condition of alignment of LC was observed by the
polarized microscope of Olympus Co. BX-51.

Properties of Homeotropic Alignment Materials
193=[1231]
FIGURE 1 Configuration of polarizer and cell.
SYNTHESIS
Synthesis of Diamines
1-(2,3-difluoro-phenyl)-4-propyl-cyclohexanol (3)
To the 1,3-difluoro-benzene in super dried THF was added 2.5 M Butyl
lithium at ꢁ78ꢀC, stirred for 3 hr at ꢁ78ꢀC and then added the
4-propylcyclohexanone in super dried THF at ꢁ78ꢀC. The tempera-
ture of mixture was raised until ꢁ10ꢀ, added 5% hydrochloric acid
in water and then extracted with ether, washed with 5% sodium
chloride and water. The organic layer was dried over anhydrous
magnesium sulfate, evaporated in vacuo, chromatographed on silica
gel to give compound (3).
Mass: 254(Mþ), 236, 207, 179, 169, 156, 141, 127, 113, 55
FIGURE 2 Synthetic route of SP diamine.

194=[1232]
J.-C. Park et al.
1,2-difluoro-3-(4-propyl-cyclohex-1-enyl)-benzene (4)
To the solution (3) in toluene was added catalytic amount of
p-toluenesulfonic acid, refluxed for 4 hr and then extracted with ether,
washed with 5% sodium bicarbonate and water. The organic layer was
dried over anhydrous magnesium sulfate, evaporated in vacuo, chro-
matographed on silica gel to give compound (4).
Mass: 236(Mþ) 193, 179, 127, 81, 68, 55
2,3-difluoro-4-(4-propyl-cyclohex-1-enyl)-benzyl boronic
acid (5)
To the solution (4) in super dried tetrahydrofuran was added 2.5 M
Butyl lithium at ꢁ80ꢀC, stirred for 3 hr and then added triisopropyl
borate at ꢁ78ꢀC. The temperature of mixture was raised until room
temperature, stirred continuously. The mixture was added 10%
hydrochloric acid in water, stirred for 1 hr, and then extracted ether.
The organic layer was dried over anhydrous magnesium sul-
fate, evaporated in vacuo, chromatographed on silica gel to give
compound (5).
1H NMR (400 MHz, CDCl3): d 7.1 (2H, d), 5.87 (1H, s), 2.57 (4H, s),
2.24 (2H, m), 1.87 (2H, m), 1.65 (1H, m), 1.55 (2H, m), 1.39 (2H, m),
1.32 (2H, m), 0.94 (3H, t)
3-(4-Bromo-phenyl)-propan-1-ol (7)
To the super dried tetrahydrofuran was added LiAlH4, stirred for
30 min and then added 4-Bromocinnamic acid in super dried THF.
The mixture was refluxed for 2 hr and cooled until room temperature.
The mixture was added water, stirred for 1 hr. The mixture was added
10% hydrochloric acid in water, stirred for 1 hr, and then extracted
ether. The organic layer was dried over anhydrous magnesium sulfate,
evaporated in vacuo, chromatographed on silica gel to give compound
(7) [3].
Mass: 214(Mþ), 196, 169, 117, 104, 91, 77, 63, 51
1-Bromo-4-(3-tert-butoxy-propyl)-benzene (8)
MC was cooled until ꢁ50ꢀC, 2-methyl propene was dissolved in MC.
3-(4-Bromo-phenyl)-propan-1-ol in MC was added, and then H2SO4 1
drop was added. The mixture was stirred for 10 hr at 20ꢀC, and then
extracted ether. The organic layer was dried over anhydrous magnes-
ium sulfate, evaporated in vacuo, chromatographed on silica gel to
give compound (8) [3].
Mass: 270(Mþ), 214, 196, 169, 117, 104, 90, 77, 57

Properties of Homeotropic Alignment Materials
195=[1233]
40-(3-tert-Butoxy-propyl)-2,3-difluoro-4-(4-propyl-cyclohex-1-
enyl)-biphenyl (9)
1-Bromo-4-(3-tert-butoxy-propyl)-benzene and 1,2-Difluoro-3-(4-propyl-
cyclohex-1-enyl)-benzene boronic acid were dissolved in DME.
The mixture was added CsF and Pd(PPh3)4 and then refluxed for
3 hr, and then extracted ether. The organic layer was dried over anhy-
drous magnesium sulfate, evaporated in vacuo, recrystallized with EA
and ethanol to give compound (9) [4,5].
Mass: 426(Mþ), 370, 339, 325, 255, 229, 134, 81, 57
2,3-Difluoro-4-(4-propyl-cyclohex-1-enyl)-biphenyl-40-propyl
acetate (10)
40-(3-tert-Butoxy-propyl)-2,3-difluoro-4-(4-propyl-cyclohex-1-enyl)-
biphenyl was dissolved in acetic acid and HBr, stirred for 1 hr at 50ꢀC.
The mixture was extracted ether. The organic layer was dried over
anhydrous magnesium sulfate, evaporated in vacuo, chromatographed
on silica gel to give compound (10) [6,7].
Mass: 412(Mþ), 352, 282, 256, 243, 201, 55
2,3-Difluoro-4-(4-propyl-cyclohex-1-enyl)-biphenyl-40-propyl
alcohol (11)
2,3-Difluoro-4-(4-propyl-cyclohex-1-enyl)-biphenyl-40-propyl acetate was
dissolved in methanol. The mixture was added 3.5 M K2CO3 and
refluxed for 3 hr. The mixture was extracted ether. The organic layer
was dried over anhydrous magnesium sulfate, evaporated in vacuo,
chromatographed on silica gel to give compound (11) [6,7].
Mass : 370(Mþ), 352, 313, 274, 255, 201, 91, 55
1H NMR (400 MHz, CDCl3): d 7.47 (2H, d), 7.28 (2H, d), 7.10 (1H,
m), 7.03 (1H, m), 6.01 (1H, s), 3.71 (2H, t), 2.76 (2H, t), 2.41 (1H, m),
1.90 (4H, m), 1.62 (2H, m), 1.35 (7H, m), 0.93 (3H, t)
2,3-Difluoro-4-(4-propyl-cyclohex-1-enyl)-biphenyl-40-propyl
3,5-dinitro benzoate (12)
2,3-Difluoro-4-(4-propyl-cyclohex-1-enyl)-biphenyl-40-propyl alcohol in
super dried THF was added triethyl amine. The mixture was added
3.5-dinitro benzoyl chloride in super dried THF, refluxed for 3 hr
and then extracted ether. The organic layer was dried over anhydrous
magnesium sulfate, evaporated in vacuo, recrystallized with EA and
ethanol to give compound (12) [8].
1H NMR (400 MHz, CDCl3): d 9.19 (1H, t), 9.08 (2H, d), 7.46 (2H, d),
7.30 (2H, d), 7.03 (2H, m), 6.02 (1H, s), 4.53 (2H, t), 2.86 (2H, t), 2.36

196=[1234]
J.-C. Park et al.
(2H, m), 2.24 (2H, m), 1.87 (2H, m), 1.65 (1H, m), 1.55 (2H, m), 1.39
(2H, m), 1.32 (2H, m), 0.94 (3H, t)
2,3-Difluoro-4-(4-propyl-cyclohexyl)-biphenyl-40-propyl
3,5-diamino benzoate (13)
2,3-Difluoro-4-(4-propyl-cyclohex-1-enyl)-biphenyl-40-propyl 3,5-dinitro
benzoate was dissolved in benzene and ethanol, and palladium
10 wt.% on activated carbon was added. The mixture was maintained
for 12 hr under 4.5 kg=cm2 pressure of H2. The Pd=C was filtered off
and residue was evaporated in vacuo. The product was recrystallized
with ethanol to give compound (13).
1H NMR (400 MHz, CDCl3): d 7.46 (2H, d), 7.28 (2H, d), 7.11 (1H, m),
7.02 (1H, m), 6.77 (2H, d), 6.19 (1H, m), 4.31 (2H, t), 3.68 (4H, s), 2.86
(1H, m), 2.81 (2H, t), 2.10 (2H, m), 1.89 (4H, m), 1.50 (1H, m), 1.35
(4H, m), 1.24 (2H, m), 1.09 (2H, m), 0.91 (3H, m)
40-(4-Pentylcyclohexyl) biphenyl-3, 5-diamine (19) [9]
We synthesized diamine having side chain of 40-(4-Pentylcyclohexyl)
biphenyl-3, 5-diamine. The synthetic route was shown in Figure 3.
1-Bromo-3, 5- dinitrobenzene was synthesized by bromination of 1,
3-dinitrobenzene [10] and reacted with 4-(pentylcyclohexyl) benzyl
boric acid through Suzuki reaction [11]. Diamine which 40-(4-Pentyl-
cyclohexyl) biphenyl-3, 5-diamine (R5CPP) was prepared by reduction
of their coupling compound.
Synthesis of Polyamic Acid
Poly(amic acid)s were prepared from dianhydrides, ODA and the
functional diamines. A typical polymerization procedure was shown
in Figure 4. Dianhydride of three types, ODA and the functional dia-
mine (mole ratio of 3:2:1) are dissolved in 1-methyl-2-pyrrolidinone
(NMP) to 18 wt.% in a three-necked round-bottom flask with a mechan-
ical stirrer in a nitrogen atmosphere. The reactants were stirred for
4 hrs at 0ꢂ5ꢀC.
The prepared poly(amic acid)s were diluted to 6 wt.% in N-methyl-
2-pyrrolidone (NMP) and 2-butoxyethanol. These solutions were
coated on ITO glass substrates using spin coater and dried at 90ꢀC
for 5 min, and then cured at 220ꢀC for 1 hr. Poly(amic acid)s resulting
from co-polymerization is then converted to polyimide by thermal
imidisation. The imidization ratio was measured by FT-IR.
Table
1
represents the names of polyimide synthesized,
respectively.

Properties of Homeotropic Alignment Materials
TABLE 1 The Name of Polyimides
197=[1235]
Diamine with
side chain
Diamine without
side chain
Polyimide
R5CPP-BT
Dianhydride
R5CPP-OD
R5CPP-BP
SP-BT
SP-OD
SP-BP
RESULTS AND DISCUSSION
One of the important parameters of liquid crystal alignment is the pre-
tilt angle which influences the viewing angle and electro-optic proper-
ties of the LCDs. [12] To measure their pretilt angles, a test cell was
located between a polarizer and an analyzer, and rotated from
ꢁ50ꢀC to þ50ꢀC as change of incident angle. We can calculate the

198=[1236]
J.-C. Park et al.
value of pretilt angle from symmetry angle at the transmittance curve
by following Eq. (1).
sin wx
h ¼ sinꢁ1
ð
Þ
ð1Þ
ne þ no
where, h is pretilt angle and wx is the symmetry angle. And ne is the
extraordinary refractive index, and no is the ordinary refractive index.
High pretilt angles were generated by the synthesis alignment
materials as shown in Table 2.
The alignment characteristic was observed by BX-51 microscope
with cross polarizer. The alignment textures were exhibited in
Table 3. The polyimide films were rubbed by a rubbing machine under
the following conditions: the translation speed of the sample was
13.3 mmsꢁ1; the rotation speed of the roller was 500 rpm; the pile
impression depth Prub was 0.1 mm. We used the nematic liquid crystal
having negative dielectric anisotropy (MERCK, MJ961213).
The microscopic textures in off=on state for each cell represented as
shown in Figure 3. In off state, no defect was observed for cells coated
the synthesized polyimides. They represented white state without
defect at the field-on state (a square-wave AC voltage of 5 V). A homo-
erotic alignment of liquid crystal (LC) molecules obtains over wide
areas on the polyimide.
Transmittance is one of the characteristic parameters decides active
voltage of module. We measured V-T characteristics using electro-
optic system after rubbing. Applied voltage was from 0 V to 10 V.
Table 4. shows the threshold voltage and the transmittance versus
applied voltage for the polyimides. SP type polyimides were showed
high threshold and saturation voltage. The threshold voltage (Vth)
usually has some connection with frequency, dielectric dispersion of
LC materials, alignment [13]. We used same LC materials under the
TABLE 2 Pretilt Angle of Polyimides
Pretilt angle (ꢀ)
Rubbing
Polyimide
Non-rubbing
Rubbing
R5CPP-BT
R5CPP-OD
R5CPP-BP
SP-BT
SP-OD
SP-BP
89.4
89.3
89.3
89.7
89.2
89.0
89.2
88.6
88.6
89.5
88.0
88.1

Properties of Homeotropic Alignment Materials
199=[1237]
TABLE 3 Microscopic Textures of Polyimides (Rubbed Cell)
same frequency. So the reason due to increase the interaction between
SP type polyimides and LC molecules than R5CPP type polyimides.
To drive TFT-LCD, voltage holding ratio (VHR) is also important to
define the ability of LC pixel to keep the state activated by electric
field during the frame time [14]. VHR of the polyimides were mea-
sured using 6254 instruments at 60 Hz with 5 V DC-voltage. The
VHR values of polyimides were listed in Table 5.
Vertical Alignment (VA) mode is a matured technology of LCD field.
The switching times can be described by the following formula [15–18],
c1d2
e0DeðV2 ꢁ Vt2hÞ
son
/
ð2Þ
ð3Þ
c1d2
soff
/
e0DeVt2h

200=[1238]
J.-C. Park et al.
FIGURE 3 Synthetic route of R5CPP diamine.
where, son, soff are switching on and off times, V the applied voltage, c1
rotational viscosity, d cell gap, De the dielectric anisotropy, and Vth is
the voltage needed of 10% transmittance change from the off voltage
state.
Numerous contributions have been made to understand the
dynamic behavior of various LCDs. Also Eqs. (3) and (4) on even VA
cells were proved that there exist a good linear relationship between
threshold voltage Vth and switching time.
In order to study the relationship between response time and poly-
imide, switching time measurements were done by using anti-parallel
TABLE 4 Threshold Voltage for the Polyimides
Host
Polyimide
Vth(V10
)
Vsat (V90)
R5CPP-BT
R5CPP-OD
R5CPP-BP
SP-BT
SP-OD
SP-BP
2.1
2.5
2.3
2.5
2.5
2.3
3.8
4.2
4.1
4.5
4.4
4.4

Properties of Homeotropic Alignment Materials
201=[1239]
TABLE 5 VHR of Polyimides (at 25ꢀC)
Polyimides
VHR (%)
R5CPP-BT
R5CPP-OD
R5CPP-BP
SP-BT
SP-OD
SP-BP
98.89
96.45
97.36
97.47
96.06
98.79
rubbed cell with same cell gap (3.9 mm), driving voltage (a square-wave
AC voltage of 5 V, 60 Hz), and LC (MERCK, MJ961213).
According to Table 6, the switching-off time of SP type polyimides
was reduced. This phenomenon is because of side chain structure of
polyimides. SP diamine has large dipole-moment perpendicularly as
the following Figure 5 simulation (semi-empirical quantum chemical
methods, AM1 Hamiltonian in the MOPAC 7.0 package). The side
chain of SP type polyimides were switched with the electric field
because of dipole-moment. And the electric field was cut-off, the side
chain of SP type polyimides came back again because of elasic force as
well as LC (Fig. 6). In consequence of this, we know that switching-off
time of SP type polyimides was improved by the dipole-momoent and
elastic force of side chain.
CONCLUSION
We synthesized 2 types of polyimide with liquid crystalline shape side
chain for fast response time VA mode LCD. One is positive liquid crys-
talline shape side chain type, the other is negative liquid crystalline
shape side chain type. These novel homeotropic alignment materials
TABLE 6 Measurement of Response Time
Host
Polyimide
sonðmsÞ
soff ðmsÞ
sallðmsÞ
R5CPP-BT
R5CPP-OD
R5CPP-BP
SP-BT
SP-OD
SP-BP
8.0
8.3
7.7
6.8
7.2
6.3
12.1
10.1
12.8
8.8
9.3
10.7
20.1
18.4
20.5
15.6
16.5
17.0

202=[1240]
J.-C. Park et al.
FIGURE 4 Synthetic route of polyimide.
which we have synthesized generated high pretilt angle above 88ꢀ. We
could observe the LC cell coated the polyimides aligned vertically
through the polarized microscope. In addition to negative liquid
crystalline shape side chain type showed faster response time than
FIGURE 5 Simulations of the dipole moment (l) and orientation angle (b).

Properties of Homeotropic Alignment Materials
203=[1241]
FIGURE 6 Principle of alignment and switching.
positive liquid crystalline shape side chain type because of its structure
effect. Therefore, negative liquid crystalline shape side chain type poly-
imide is expected to use especially for fast response time VA mode LCD.
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