Synthesis of new vulcan-shape aromatic polyimides for IPS-mode and their alignment properties

Article abstract of DOI:10.1080/15421400701675275

In plane switching (IPS)-mode liquid crystal display (LCD) is one of the most useful technologies for a wide range of viewing angles. For IPS-mode application, we synthesized novel diamine containing a methyl group as a bridge and homogeneous alignment ma

Full text of DOI:10.1080/15421400701675275

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Synthesis of New Vulcan-Shape
Aromatic Polyimides for IPS-
Mode and Their Alignment
Properties
Kyung-Chul Son a , Dong-Ju Park a , Yong-Bae Kim a &
Jae-Cheol Park b
a Liquid Crystal Research Center, Department of
Chemistry, Division of Natural Sciences , Konkuk
University , Seoul, Korea
b DONGJIN SEMICHEM CO., LTD., Electronic Materials
Division , Gyeonggi-do, Korea
Published online: 22 Sep 2010.
To cite this article: Kyung-Chul Son , Dong-Ju Park , Yong-Bae Kim & Jae-Cheol Park
(2007) Synthesis of New Vulcan-Shape Aromatic Polyimides for IPS-Mode and Their
Alignment Properties, Molecular Crystals and Liquid Crystals, 479:1, 81/[1119]-93/
[1131], DOI: 10.1080/15421400701675275
To link to this article: http://dx.doi.org/10.1080/15421400701675275
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Mol. Cryst. Liq. Cryst., Vol. 479, pp. 81=[1119]–93=[1131], 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701675275
Synthesis of New Vulcan-Shape Aromatic Polyimides
for IPS-Mode and Their Alignment Properties
Kyung-Chul Son
Dong-Ju Park
Yong-Bae Kim
Liquid Crystal Research Center, Department of Chemistry, Division
of Natural Sciences, Konkuk University, Seoul, Korea
Jae-Cheol Park
DONGJIN SEMICHEM CO., LTD., Electronic Materials Division,
Gyeonggi-do, Korea
In plane switching (IPS)-mode liquid crystal display (LCD) is one of the most
useful technologies for a wide range of viewing angles. For IPS-mode application,
we synthesized novel diamine containing a methyl group as a bridge and homo-
geneous alignment materials. A 1ꢀ2 low pretilt angle manifested by changing
dianhydrides (BTDA and BPDA) and bridged diamines (ODA and PACM). We
measured their pretilt angles using the crystal rotation method with positive liquid
crystal (LC). The electro-optical properties of LC cells using these polyimides as
alignment films were also investigated. Results show all polyimides manifest a
low pretilt angle of below 2 degrees.
Keywords: diamine; IPS mode; polyimide; pretilt angle
INTRODUCTION
TFT-LCD applications were limited to mobile phones and smaller-
than-20-inch computer monitors in the early 1990s. Since 2000,
however, TFT-LCDs grew in size and have become so popular that
they have started to replace conventional CRT devices. Their advan-
tages reside on the fact that they are more compact in size and lighter
in weight than CRT. Despite these advantages, several problems such
Address correspondence to Yong-Bae Kim, Liquid Crystal Research Center,
Department of Chemistry, Division of Natural Sciences, 1, Hway angdong, Kwangjinku,
Seoul 143-701, Korea. E-mail: yongbae@konkuk.ac.kr
81=[1119]

82=[1120]
K.-C. Son et al.
as viewing angle, brightness, response time, and contrast ratio still
need to be resolved before TFT-LCD technology can be applied to
TVs [1].
The patterned vertical alignment (PVA) and IPS modes, which are
being commercialized now, were developed as alternatives to the tra-
ditional twisted nematic (TN) and super-twisted nematic (STN)
modes. At present, the image quality of TFT-LCD is the same as that
of CRT, and many requirements have been met owing to various tech-
nical developments. Typically, the IPS mode is one of the most valu-
able technologies for a wider viewing angle. In the case of the
existing TN and STN modes, the displayed images differ according
to the direction of the eyesight. This is because the LC molecules
between two glasses are twisted. On the other hand, in the IPS mode,
the LC molecules are parallel; i.e., the LC is switched on the glass
plane. Consequently, the transmitted light penetrating the glass from
all directions can help with the optical contribution of the LC mole-
cules, thereby facilitating a wider viewing angle [2].
In this article, 1,1-Bis-(4-amino-phenyl)-2-phenyl ethane was
synthesized as a novel diamine for IPS-mode application. Novel dia-
mine contains a methyl group that functions as a bridge between
the phenyl group and phenyl group with amine group. Because this
portion bends to 111 degrees, the LC molecule is also inclined owing
to the interaction and exhibits 1ꢀ2 low pretilt angle and good align-
ment property. To achieve a suitable pretilt angle, the surface energy
and the steric effect are considered and the polyimide is synthesized by
changing the bridged chain diamine and dianhydride [3,4].
The homogeneous alignment material, which shows 1ꢀ2 degree
low pretilt angle, is assessed for suitability in terms of its thermal
characteristics, pretilt angle, surface energy, and alignment condition.
Moreover, test cells were created and their electro-optical properties
were measured.
EXPERIMENTAL
Materials
3,3,4,4-benzophenonetetracarboxylic dianhydride (BTDA), 3,3,4,
4-biphenyltetracarboxyic dianhydride (BPDA), and 4,40-oxydianiline
(ODA) were purchased from Aldrich Corporation. 1,40-Methylenebis-
cyclohexylamine (PACM) was purchased from the TCI Corporation.
The reagents were recrystallized and sublimed before use. Benzyl
bromide, 4,4-dichlorobenzopenone, lithium bis(trimethylsilyl) amide,
bis(dibenzylidene acetone) palladium, and tricyclohexyl phosphine

Synthesis of New Vulcan-Shape Aromatic Polyimides
83=[1121]
were likewise purchased from Aldrich Corporation and were used in
their purchased form. Solvents were distilled in the presence of a
dehydrating agent and stored over molecular sieves. Other reagents
were commercial-grade and were used in their purchased form.
Synthesis of Side-Chain Monomers
The side-chain diamine of 1,1-Bis-(4-amino-phenyl)-2-phenyl ethane-
diamine was synthesized. The synthetic route is shown in Figure 1.
1
The chemical structure of the product was identified using H NMR
spectroscopy (with a Bruker Avance 400-MHz spectroscope) and EI
mass spectroscopy (with a 70-eV Hewlett Packard 5972 MSD).
1,1-Bis-(4-chloro-phenyl)-2-phenyl-ethanol (1)
Added to magnesium turnings (3.5 g, 0.1433 mol) and an iodine
crystal was a solution of benzyl bromide (20.4 g, 0.1194 mol) in ether
(100 ml). After being heated under reflux for 2 hours, the Grignard
FIGURE 1 Synthetic route of vulcan-shape diamine (VP).

84=[1122]
K.-C. Son et al.
reagent was cooled, and a solution of 4,40-dichlorobenzophenone (20 g,
0.0796 mol) in tetrahydrofuran (150 ml) was added in a rapid drop-
wise fashion. Reheating was then initiated with a reflux period of 1
hour, which was required for the consumption of 4,40-dichlorobenzo-
phenone as indicated in the TLC analysis. The reaction mixture was
cooled and quenched with saturated NH4Cl (10 ml). The suspension
was stirred for 30 minutes and filtered through celite, and the filtrate
was concentrated in vacuo [5]. The product was extracted using chloro-
form and an NaCl solution. The organic layer was dried over MgSO4
and removed in vacuo to yield a crude yellow product (90% yield).
Mass: 324 (Mþ), 288, 253, 212, 176 and 126
1H NMR (CDCl3): d7.21 (d, 2H), 7.20 (d, 4H), 7.13 (d, 4H), 7.12
(d, 2H), 7.08 (t, 1H), 3.32 (d, 2H) and 2.00 (s, 1H)
1,1-Bis-(4-chloro-phenyl)-2-phenyl Ethylene (2)
In a 500-ml three-necked, round-bottom flask equipped with a
reflux condenser, 1,1-Bis-(4-chloro-phenyl)-2-phenyl-ethanol (35 g,
0.1020 mol) was dissolved in toluene. P-toluenesulfonic acid 1-ydrate
(2.7 g, 0.014 mol) was added. The mixture was refluxed at 130ꢁC for
12 hours. The reaction mixture was added to the NaCl solution,
extracted with ether, and dried with anhydrous MgSO4. The evapor-
ation of the solvent yielded a crude brown product (95% yield).
Mass: 324 (Mþ), 288, 254, 212, 176, 151, 126, 75 and 51
1H NMR (CDCl3): d 7.42 (d, 2H), 7.36 (d, 4H), 7.27 (d, 4H), 7.26
(d, 2H), 7.11 (t, 1H), 6.92 (s, 1H)
1,1-Bis-(4-amino-phenyl)-2-phenyl Ethylene (3)
In a 500-ml three-necked, round-bottom flask equipped with a
reflux condenser, 1,1-Bis-(4-chloro-phenyl)-2-phenyl-ethylene (20 g,
0.06149 mol), lithium bis(trimethylsilyl)amide (43.2 g, 0.2583 mol),
tricyclohexylphosphine (2.6 g, 0.0092 mol) and bis(dibenzylidene
acetone) palladium (5.3 g, 0.0092 mol)were dissolved in toluene. The
mixture was refluxed at 90ꢁC for 12 hours. The reaction progress
was monitored using GC=MS. The crude reaction mixture was diluted
with ether and the silylamide was de-protected by adding one drop of
aqueous 1 N HCl. The mixture was transferred to a separatory funnel
and washed with aqueous 1 N NaOH [6]. The organic layer was dried
over MgSO4, filtered, and concentrated at reduced pressure. The
product was recrystallized with ethanol and ethylene acetate to yield
a yellowish product (50% yield).
Mass: 286 (Mþ), 267, 193, 180 and 165
1H NMR (CDCl3): d 7.42 (t, 2H), 7.26 (d, 2H), 7.17 (d, 4H), 7.11
(d, 1H), 6.92 (s, 1H), 6.46 (d, 4H) and 3.74 (s, broad, 4H)

Synthesis of New Vulcan-Shape Aromatic Polyimides
85=[1123]
1,1-Bis-(4-amino-phenyl)-2-phenyl Ethane (4)
Placed in a high-pressure autoclave was 1,1-Bis-(4-amino-phenyl)-
2-phenyl ethylene (10 g, 0.349 mol)dissolved in toluene and ethanol,
to which was added 10-wt% palladium on activated carbon (0.1 g).
The mixture was maintained for 5 hours under 4 kg=cm2 H2 pressure.
The Pd=C was filtered and the residue was evaporated in vacuo. The
product was recrystallized with ethanol and ethylene acetate to yield
a whitish product (40% yield).
Mass: 288 (Mþ), 197, 180, 168, 91 and 95.
1H NMR (CDCl3): d 7.25 (t, 2H), 7.13 (d, 2H), 7.07 (d, 1H), 7.00
(d, 4H), 6.58 (d, 4H), 4.06 (t, 1H), 3.74 (s, broad, 4H) and 3.30 (d, 2H).
Synthesis of Polymers
Preparation of Polyamic Acids
All the polyamic acids were synthesized using conventional polycon-
densation, as illustrated in Figure 2. In a typical example, placed in a
flame-dried 50-ml flask equipped with a mechanical stirrer and a
nitrogen inlet were 0.00208 mol of 1,1-Bis-(4-amino-phenyl)-2-phenyl
FIGURE 2 Synthetic route of polyimides.

86=[1124]
K.-C. Son et al.
ethane, 0.00416 mol of 4,40-oxydianiline(ODA), and 12.2 g of purified
NMP. The mixture was stirred under a slow stream of nitrogen gas
until the diamines were entirely dissolved. 0.00624 mol of BTDA and
an additional 13.2 g of NMP were added to one portion. The solution
was stirred mechanically for 1 hour at 0ꢁC until it became clear, after
which it was again stirred at room temperature for 12 hours under
nitrogen. The clear and viscous polyamic acid solution was poured into
deionized water to yield a yellowish solid. The solid was washed with
additional deionized water and purified methanol. The polyamic acid
solid was filtered and dried overnight under vacuum at 50ꢁC, then
at 90ꢁC for 12 hours.
Preparation of the PI Alignment Layer
The synthesized polyamic acid solution was used as the LC
alignment material. It was dissolved in NMP=2-butoxyethanol
(3:1 ratio) to form 4-wt% solutions. The solutions were spin-coated
onto indium tin oxide (ITO) glass substrates at 2,600 rpm. The solvent
was evaporated at 80ꢁC for 5 minutes, and main curing was carried out
at 210ꢁC for 1 hour and 30 minutes. The approximate thickness of the
˚
resulting polyimide films was about 600 ꢂ 50 A.
Fabrication of the LC Cell
The LC cell of the pretilt angle was fabricated from two pieces of
rubbed polyimide films assembled in an anti-parallel rubbing direction
with a 50-mm-thick microfilm. To determine its electro-optical proper-
ties, the LC cell was fabricated from two pieces of rubbed polyimide
films assembled in a parallel rubbing direction with glass substrates
and inter-digital electrode substrates with a 5-mm electrode width
and a 10-mm electrode distance. The cell gap was maintained using
4-mm polymer beads. The LC cell was filled with positive LC
(MJ991735: Merck Co.) and sealed using epoxy adhesive.
Characterization and Processing
Gel permeation chromatography was carried out using the Jasco GPC
system equipped with an RI detector and a phenogel column in the
solution (LiBr=DMAC ¼ 1 g=1 L) at 60ꢁC. The average molecular
weight of the polyamic acids was calculated using standard materials
(PM-1: Easical, Polymer Lab) and Jasco software. The thermal proper-
ties of the polyamic acids were examined with TGA (Perkin Elmer
TGA7) in an N2 atmosphere. The pretilt angle of the positive LC
(MJ991735: Merck Co.) was measured with an optical instrument
set using the crystal rotation method [7]. The contact angles were

Synthesis of New Vulcan-Shape Aromatic Polyimides
87=[1125]
measured using the sessile drop method with a contact anglemeter
¨
(KRUSS DSA100) and highly pure distilled water and methylene iod-
ide. The surface free energy was calculated using Owens’ formula [8].
The voltage holding ratio (VHR) was measured with 6254 Instruments
from TOYO Co., Inc. The V-T characteristics were measured using
a self-installed electro-optic system setup at room temperature.
A voltage of 0–10 V was applied.
RESULTS AND DISCUSSION
Polymer Properties
Molecular Weight and Thermal Characterization
The weight-average molecular weight (Mw) of the VP was examined
using gel permeation chromatography (GPC) and summarized in
Table 1 with the results of the thermal analysis of the VP. Figure 3
shows the pyrogram of the polyimide and polyamic acid. VP thermal
stability was analyzed using a theꢁrmogravimetric analyzer (TGA) in
the range of 30ꢀ800ꢁC within a 10 C minꢃ1 heating rate. The thermal
stability standard was estimated at a 5% weight loss temperature
using a TGA measurement [9]. The 5% weight loss temperature of
the VP was determined in the range of 380ꢀ440ꢁC. Since the 5%
weight loss temperature of polyimide occurred at over 400ꢁC, it is
believed that the polyimide can be thermally stable up to 400ꢁC. The
polyimide of the ODA series had a higher decomposition temperature
than that of the PACM series. This is because the polyimide of the
PACM series, which is characterized by an alicycic structure rather
than the aromatic structure of the ODA, consists of single bonds,
which are weaker than double bonds. Moreover, the pyrogram shows
that the curves of the ODA and PACM series have distinct features.
TABLE 1 Molecular Weight and Thermal Properties of Polyimides based
on VP
ꢁ
ꢁ
T5b(ꢁC)
TGAa T10 ( C)
Td ( C)
c
d
Polyimides
GPC Mw
VP-ODA-BTDA
VP-ODA-BPDA
VP-PACM-BTDA
VP-PACM-BPDA
133500
95200
56300
283200
427
435
380
400
522
533
448
447
569
575
456
457
aMeasured at a heating rate of 10ꢁC minꢃ1 in N2.
bTemperature at 5% weight loss.
cTemperature at 10% weight loss.
dTemperature at decomposition.

88=[1126]
K.-C. Son et al.
FIGURE 3 Thermogravimetric diagrams of polymers: a) VP-ODA-BTDA-PI;
b) VP-ODA-BPDA-PI; c) VP-PACM-BTDA-PI; d) VP-PACM-BPDA-PI; e) VP-
ODA-BTDA-PAA; f) VP-ODA-BPDA-PAA; g) VP-PACM-BTDA-PAA; h) VP-
PACM-BPDA-PAA.
This reflects the structural difference between the aromatic and
alicycic structures discussed above. Since the polyimide decomposition
temperature of the PACM series exhibits more sharp weight loss than
that of ODA series, the thermal decomposition temperature of the
polyimide is influenced by dianhydride of the main chain rather than
the main diamine.

Synthesis of New Vulcan-Shape Aromatic Polyimides
89=[1127]
LC Alignment, Pretilt Angle, and Surface Properties
The alignment characteristic of the IPS cell was observed using a
BX60 microscope equipped with a cross-polarizer. Figure 4 shows
the microscopic textures of off-state and on-state polyimides. In the
off-state polyimides represented by the black areas in Figure 4, no
defect was observed in the cells that coated the synthesized polyimide.
The pretilt angle is an important factor that has an impact on the
viewing angle and the electro-optical properties. The typical pretilt
angle of the alignment material used in the IPS-mode is 1ꢀ2 degrees.
To induce a low pretilt angle, the structure of the polyamic acid chain
was kept similar to the structure of the main-chain type rather than
the side-chain type which shows almost a 90 degree pretilt angle. It
is known that showing a low pretilt angle of 1–2 degrees is more diffi-
cult than synthesizing polyimide displaying a high pretilt angle.
Accordingly, it is important to synthesize novel diamine to obtain a
low pretilt angle of 1–2 degrees. The steric effect of the polyimide
and the electric effect alignment model should be considered. A low
pretilt angle occurs when the side chain is shorter than the LCs’ major
axis. Reportedly, an alignment layer with a surface energy similar to
the surface tension of LC molecules is likely to manifest a medium pre-
tilt angle. However, an alignment layer with a surface energy larger
than the surface tension of LC molecules is likely to have a low pretilt
angle [10,11,12].
FIGURE 4 The microscopic textures of off-state and on-state of the cells
coated the synthesized polyimides: a) VP-ODA-BTDA; b) VP-ODA-BPDA;
c) VP-PACM-BTDA; d) VP-PACM-BPDA.

90=[1128]
K.-C. Son et al.
FIGURE 5 The three dimensional model of the novel diamine by Chem office
3D (CS MM2=MOPAC2000).
With the novel diamine we synthesized, the side chain maintained
111 degrees rather than 180 degrees by using methyl group as the
bridge between phenyl groups, so that LC equally aligned. Since the
LC molecule is inclined together by the inclined phenyl group, a low
pretilt angle could be maintained. Figure 5 shows a three-dimensional
model of the novel diamine confirmed by Chem office 3D.
The synthesized polyimide was designed considering the steric
effect and the electric effect alignment model. Measuring the surface
energy of polyimide to certify that and in order to investigate the effect
of the methyl group on the pretilt angle, the change in pretilt angle
was measured with the curing temperature. Figure 6 shows the
changes in pretilt angle. In this measurement, the total surface ener-
gies of the PACM and ODA series were approximately 46 dyn=cm2 and
47 dyn=cm2, respectively. All polyimides exhibited high surface energy
above 45 dyn=cm2. Moreover, all polyimides manifested a low pretilt
angle below 2 degrees at other curing temperatures [13,14,15].
Electro-Optical Properties of PI
The test cell was subjected to five volts for 60 ms at intervals of
16.67 ms using Toyo Company’s LCD measurement system 6254,
and the VHR was measured at 16.67 ms after voltage removal.
Table 2 shows the VHR of each polyimide. If the polarization of the
alignment layer is high, the ionic carriers are polarized by the applied
voltage and adhere to the alignment layer [16]. Hence, the polyimides
of the PACM series have a higher VHR than those of the ODA series.
Consequently, the ODA series has a more aromatic structure than the

Synthesis of New Vulcan-Shape Aromatic Polyimides
91=[1129]
FIGURE 6 The pretilt angle dependent on curing temperature and surface
tension of polyimides: a) VP-ODA-BTDA; b) VP-ODA-BPDA; c) VP-PACM-
p
BTDA; d) VP-PACM-BPDA; cs (total surface free energy), cs (polar force),
d
cs (dispersion force).
polyimides of the PACM series, so that more ionic carriers adhere to
the alignment layer, reducing the VHR.
To get the transmittance values, a self-developed optical equipment
was used in which a polarizer and an analyzer were crossed at a right
angle, a photodiode detected the intensity of radiation after a 632.8-nm
laser passed through the test cell, and an analyzer and a digital

92=[1130]
K.-C. Son et al.
FIGURE 7 The V-T curve of the test cell with various alignment films:
a) VP-ODA-BTDA; b) VP-ODA-BPDA; c) VP-PACM-BTDA; d) VP-PACM-
BPDA.
multimeter received the signal. Assuming that the transmittance is
100% when the driving voltage is off and 0% when the driving voltage
is on, a 10% transmittance (V10) is defined as the optical threshold
voltage, and a 90% transmittance (V90) is defined as the optical satu-
ration voltage. The voltage was applied to the test cell with a 1-KHz
frequency at a 0.1-V=s rate from 0 to 10 V, and subsequently, a test cell
was made after rubbing at 45 degree. Vth and Vsat in accordance with
polyimide alignment layer are shown in Figure 7 and Table 2.
CONCLUSION
This article is mainly about the synthesis and property of homogenous
alignment materials, which has a low pretilt angle of 1–2 degrees for
IPS-mode application.
TABLE 2 V-T Diagram of Polyimides Based on VP
45 degree
Polyimides
VHR (%)
Vth(V10
)
Vsat(V90)
VP-ODA-BTDA
VP-ODA-BPDA
VP-PACM-BTDA
VP-PACM-BTDA
93.02
98.11
98.05
99.54
2.0
2.0
2.0
1.9
5.9
5.9
5.2
5.5

Synthesis of New Vulcan-Shape Aromatic Polyimides
93=[1131]
In the novel diamine we synthesized, the side chain maintains 111
degrees rather than 180 degrees using a methyl group as the bridge
between phenyl groups, so that LC is equally aligned. Since the LC
molecule is inclined together by inclined phenyl group, a low pretilt
angle could be maintained.
Based on the surface energy measurements, all polyimides exhibit
high surface energy above 45 dyn=cm2. The high surface energy also
contributes to the low pretilt angle. The ODA series showed a higher
surface energy than the PACM series. This is because the ODA series
contained the benzene ring that constitutes p-conjugation, so that
mutual interactions between the liquid crystal and the alignment
increased and the surface energies became larger. After measuring
the pretilt angles based on the curing temperatures, the ODA and
PACM series proved suitable for IPS-mode alignment.
Finally, the main-chain structure of polyimides is very important
for an alignment to have ideal electro-optical properties. The ODA
series contained more benzene rings at its side chain than the PACM
series, so its polarity increased and its VHR was lower than that of the
PACM series. Typically, when polarity increases, a polarization field is
generated and the VHR tends to decrease.
The polyimides we synthesized with novel diamine manifested a low
pretilt angle of 1–2 degrees as well as good electro-optical properties,
making them potentially useful for IPS-mode application.
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