The structural effects on the pretilt angle of alignment materials with alkylcyclohexylbenzene as a side chain in polyimides

Article abstract of DOI:10.1080/15421400590955073

We have investigated the relationship between the surface property of rubbed polyimides and the variation of pretilt angle according to the side chain and main chain structures. We calculated the surface free energy of synthesized polyimide surface by mea

Full text of DOI:10.1080/15421400590955073

This article was downloaded by: [University of Alberta]
On: 02 January 2015, At: 04:00
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954
Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,
UK
Molecular Crystals and Liquid
Crystals
Publication details, including instructions for
authors and subscription information:
http://www.tandfonline.com/loi/gmcl20
The Structural Effects
on the Pretilt Angle of
Alignment Materials with
Alkylcyclohexylbenzene as a
Side Chain in Polyimides
Jong-Beom Lee a , Hyun-Kyung Lee a , Jae-Chul Park
a & Yong-Bae Kim a
a Liquid Crystal Research Center, Department of
Chemistry , Division of Natural Sciences, Konkuk
University , Seoul, Korea
Published online: 31 Aug 2006.
To cite this article: Jong-Beom Lee , Hyun-Kyung Lee , Jae-Chul Park & Yong-Bae
Kim (2005) The Structural Effects on the Pretilt Angle of Alignment Materials with
Alkylcyclohexylbenzene as a Side Chain in Polyimides, Molecular Crystals and Liquid
Crystals, 439:1, 161/[2027]-172/[2038], DOI: 10.1080/15421400590955073
To link to this article: http://dx.doi.org/10.1080/15421400590955073
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the
information (the “Content”) contained in the publications on our platform.
However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness,
or suitability for any purpose of the Content. Any opinions and views
expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the
Content should not be relied upon and should be independently verified with

primary sources of information. Taylor and Francis shall not be liable for any
losses, actions, claims, proceedings, demands, costs, expenses, damages,
and other liabilities whatsoever or howsoever caused arising directly or
indirectly in connection with, in relation to or arising out of the use of the
Content.
This article may be used for research, teaching, and private study purposes.
Any substantial or systematic reproduction, redistribution, reselling, loan,
sub-licensing, systematic supply, or distribution in any form to anyone is
expressly forbidden. Terms & Conditions of access and use can be found at
http://www.tandfonline.com/page/terms-and-conditions

Mol. Cryst. Liq. Cryst., Vol. 439, pp. 161=[2027]–172=[2038], 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400590955073
The Structural Effects on the Pretilt Angle of
Alignment Materials with Alkylcyclohexylbenzene as a
Side Chain in Polyimides
Jong-Beom Lee
Hyun-Kyung Lee
Jae-Chul Park
Yong-Bae Kim
Liquid Crystal Research Center, Department of Chemistry, Division of
Natural Sciences, Konkuk University, Seoul, Korea
We have investigated the relationship between the surface property of rubbed
polyimides and the variation of pretilt angle according to the side chain and main
chain structures. We calculated the surface free energy of synthesized polyimide
surface by measuring the contact angles of distilled water and methylene iodide
on the rubbed polyimide surface. Consequently, we found that pretilt angles on
rubbed polyimides changed according to the side chain and main chain structures
of the polyimide. Our calculations show that LC pretilt angles of polyimide films
with a liquid crystal structure as a side chain were approximately 90ꢀ when a
linear and rigid polyimide main chain and a side chain of suitable length were
employed.
Keywords: alignment materials; liquid crystal; pretilt angle; side chain type polyimide;
surface free energy
INTRODUCTION
Liquid crystal displays (LCDs) are widely used in devices such as, video
cameras, laptop and desktop computers due to the LCD’s portablility,
flatness, low voltage requirements, and low power consumption. For
optimal electro-optic performance of an LCD, a uniform alignment of
the liquid crystal (LC) molecules is essential. Rubbing technology, which
easily achieves this uniformity, has been used in LCD production [1,2].
Authors thank Dong-Jin Semichem Co., Ltd. for their financial support for this study.
Address correspondence to Yong-Bae Kim, Liquid Crystal Research Center, Depart-
ment of Chemistry, Division of Natural Sciences, Konkuk University, 1, Hwayangdong,
Kwangjinku, Seoul, 143-701, Korea. E-mail: yongbae@kkucc.konkuk.ac.kr
161=[2027]

162=[2028]
J.-B. Lee et al.
Through the rubbing process, LC molecules are aligned parallel with the
rubbing direction, and a definite pretilt angle is generated. This spretilt
angle influences the performance of LCD devices; in addition the angle
should have a specific value applicable to various kinds of LCD modes.
For example, in the case of VA-LCDs, nematic LC molecules are verti-
cally aligned on the substrate at the field-off state [3–5].
Various kinds of organic and inorganic materials were examined for
LC alignment film. Among them, polyimide is one of the most suitable
materials for LC alignment with respect to their heat resistance,
chemical stability, and mechanical strength. Rubbed polyimide film
can align LC molecules with a constant pretilt angle [6–12]. In other
words, when the liquid crystal molecules align in a direction such that
the free energy of the system is minimized without external energy,
the surface aligning energy becomes zero. In general, this aligning
direction is called an easy axis [7]. The chemical structure of polyimide
and the entire conformation of polyimide chain are considered respon-
sible for generating this easy axis on polyimide surface. The chemical
structure of polyimide and its conformation in the surface region are
related to the electronic and steric effect, respectively. Thus, many
investigators have reported that the factors affecting pretilt angles
are both the electronic interaction and the steric effect [7,8].
In previous work, we investigated the vertical alignment on
substrate, introducing various kinds of side chain in polyimide struc-
ture. And we proposed the alignment model for the dependence of
the LC molecule arrangement on side chain length in side chain struc-
ture [9]. On the basis of this model, we synthesized the alignment
films with various structures and lengths of side chain [9,10]. From
the results, we could confirm that pretilt angle increases with the elon-
gation of alkyl chain. In the present paper, we synthesize polyimides
having the liquid crystal structure as a side chain, and its pretilt angle
and surface free energy were measured. We show the relationship
between the surface free energy and the pretilt angle in alignment
film with side chain. From this relationship, we investigate the change
of pretilt angles on rubbed polyimides, depending on the side and main
chain structures.
EXPERIMENTAL
Materials
3,3,4,4-benzophenonetetracarboxylic dianhydride (BTDA), 1,2,4,5-
benzenetetracarboxylic dianhydride (PMDA), cis-1,2,3,4-cyclopentane-
tetracarboxylic dianhydride (CPDA) 1,2,3,4-cyclobutanetracarboxylic

The Structural Effects on the Pretilt Angle
163=[2029]
dianhydride (CBDA) and 4,40-oxydianiline (ODA) were purchased from
the Aldrich Corporation. 5-(2,5-Dioxo-tetrahydro-furan-3-yl)-3a,4,5,
9b-tetrahydro-naphtho[1,2-c]furan-1,3-dione (TDA), 1,40-Methylenebiscy-
clohexylamine (PACM) and 4,40-Methylenebis-2-methylcyclohexylamine
(ANCAMINE) were purchased from the TCI Corporation. The reagents
were recrystallized and sublimed before use. 4-pentylcyclohexylbenzene
and 4-propylcyclohexylbenzene were purchased from the Canto Corpor-
ation, and were used as received. Solvents were distilled in the presence
of a dehydrating agent and stored over molecular sieves. Other reagents
were commercial-grade and were used as received.
Synthesis of Side Chain Monomers
We synthesized side chain diamine of 40-(4-Pentylcyclohexyl) biphenyl-3,
5-diamine. The synthetic route is shown in Figure 1. The chemical struc-
1
ture of the product was identified by H NMR spectroscopy (Bruker
Avance 400 MHz Spectroscope) and EI mass spectroscopy (70eV Hewlett
Packard 5972 MSD).
1-Bromo-3,5-dinitrobenzene(1)[13]
In a 250 ml round-bottomed flask, equipped with overhead stirrer,
was placed 50 ml of trifluoroacetic acid, 100 mmol of 1,3-dinitroben-
zene, and 20 ml of sulfuric acid (98%). The mixture was stirred vigor-
ously, and 150 mmol of NBS was added in portions over an 8 hours
period. The mixture was heated at 48ꢀC for 2 days. The reaction mix-
ture was poured into 200 ml of ice water, the organic layer separated,
and the aqueous layer extracted with CH2Cl2. The combined organic
layers were washed with brine and dried over MgSO4. After the
CH2Cl2 was evaporated, recrystallization gave the product. (53%
yields) Mass spectrum: 247 (Mþ), 246, 200, 155, 75, 63, 51 1H
NMR(CDCl3): d 9.0(s, 1H), 8.7(s, 2H)
1-Bromo-4-(4-pentylcyclohexyl)benzene (2)
In a 500 ml three-necked round bottomed flask equipped with a
dropping funnel was placed 150 mmol of 4-pentylcyclohexylbenzene
in CHCl3 (300ml) and a little iron powder. 100 mmol of bromine liquid
was slowly added and stirred. After 12 h, the reaction mixture was
quenched with NaOH and extracted with CHCl3. After the evapor-
ation of CHCl3, Recrystallization with EA and EtOH gave the white
1
powder. (91% yields) Mass spectrum: 308 (Mþ), 257, 165, 131, 55 H
NMR (CDCl3): d 7.3(d, 2H), 7.0(d, 2H), 2.9(q, 1H), 1.6(t, 4H),
1.3ꢁ0.9(m, 15H)

164=[2030]
J.-B. Lee et al.
FIGURE 1 Synthetic route of diamine as a side chain.
4-(Pentylcyclohexyl)benzyl Boric Acid (3)
In a 500 ml three-necked round bottomed flask equipped with a
nitrogen inlet, 65 mmol of 1-bromo-4-(4-pentylcyclohexyl)benzene in
dried THF was cooled to ꢂ78ꢀC, and then 77 mmol of 2.5 M BuLi
was slowly added. After the mixture was maintained at ꢂ78ꢀC for
30 min, 130 mmol of triisopropyl borate was added while maintaining
the temperature. The reaction mixture was warmed to room tempera-
ture and neutralized with 10% HCl. The product was extracted with
diethylether. The organic layer was dried over MgSO4 and removed
in vacuo to give a white crude product. The product recrystallized with
hexane was given. (71% yields) 1H NMR (CDCl3): d 7.2( t, 2H),
7.1(d, 2H), 2.9(q, 1H), 1.6(t, 4H), 1.5(q, 1H), 1.3ꢁ0.9(m, 15H)

The Structural Effects on the Pretilt Angle
165=[2031]
3,5-Dinitro-40-(4-pentylcyclohexyl)biphenyl (4) [14]
In a 500 ml three-necked round bottomed flask equipped with reflux
condenser, 20 mmol of 1-bromo-3,5-dinitorbenzene was dissolved in
benzene. Tetrakis (triphenylphosphine) palladium (0.2 g) and 2 M
sodium carbonate (70 ml) were added. 20 mmol of 4-(pentylcyclohexyl)
benzyl boric acid dissolved in ethanol was added. The mixture was
refluxed at 95ꢀC for 24 h. The reaction mixture was quenched into
the NaCl solution, extracted with ether, and dried with anhydrous
MgSO4. The evaporation of the solvent and recrystallization gave
the yellowish solid. (68% yields) Mass: 396 (Mþ ), 270, 257, 165, 131,
1
55 H NMR (CDCl3): d 9.1(s, 1H), 8.9(s, 2H), 7.4(d, 2H), 7.2(d, 2H),
2.8(m, 1H), 1.6ꢁ0.9(m, 20H).
40-(4-Propyl-cyclohexyl)-biphenyl-3,5-diamine(5)
In a high-pressure autoclave was placed 18 mmol of 3,5-dinitro-
40-(4-pentylcyclohexyl)biphenyl dissolved in benzene and ethanol, and
palladium 10 wt% on activated carbon (0.5 g) was added. The mixture
was maintained for 3 h under 4 kg=cm2 pressure of H2. The Pd=C was fil-
tered off and the residue was evaporated in vacuo. The product was
recrystallized with ethanol to yield the yellowish solid product. (95%
yields) Mass: 336 (Mþ ), 270, 211, 115, 71, 55 1H NMR (CDCl3): d 7.4(d,
2H), 7.2(d, 2H), 6.0(d, 2H), 5.6(d, 1H), 1.6 ꢁ 1.5(m, 5H), 1.3ꢁ0.9(m, 15H)
Polymer Synthesis
Preparation of Poly(amic acid)s
In a flame-dried 50 ml flask equipped with a mechanical stirrer and
anitrogen inlet were placed 2 mmol of 40-(4-pentyl-cyclohexyl)-biphenyl-
3,5-diamine, 4 mmol of 4,40-oxydianiline (ODA) and 5 ml of purified
NMP. The mixture was stirred under a slow stream of nitrogen gas until
the diamines were entirely dissolved. 18 mmol of BTDA and an additional
4 ml of NMP were added in one portion. The solution was stirred
mechanically for 1 h at 0ꢀC, becoming clear, then it was stirred at room
temperature for 12 h under nitrogen. The clear and viscous poly (amic
acid) solution (R5CPP-B-O) was poured into deionized water to yield
a white solid. The solid was washed with additional deionized water
and purified methanol. The polyamic acid solid was filtered and dried
overnight under vacuum at 40ꢀC, then at 90ꢀC for 12 h.
Preparation of Polyimide Layer
The polyamic acid solution with 4% solid content in NMP=2-
butoxyethanol (3:1 ratio) was coated on indium-tin oxide glass by
spin-coating. The solvent was evaporated at 80ꢀC for 5 min, and curing

166=[2032]
J.-B. Lee et al.
was carried out at 260ꢀC for 1 h. The approximate thickness of the
˚
resulting polyimide films was about 600 ꢃ 50 A.
Measurements
Pretilt Angle
The rubbing process was performed as described in the paper. The
rubbing strength (RS) was calculated using the empirical expression
[9]. The rubbing conditions include the number of the revolution per
minute (500 rpm), the moving speed of the roller (16.7 mm=s), the
radius of the roller (75 mm), and the depth of rubbing (0.1 mm or
0.3 mm). The test cell was assembled with two pieces of glass sub-
strates, ensuring the rubbing direction antiparallel to each other. We
maintained the cell gap by using microfilms with 48 mm thickness. The
cell was filled with nematic LC (MJ961213: Merck Co.) and sealed
with epoxy adhesive. Pretilt angle was measured with laboratory
equipment set by applying the crystal rotation method [15].
Surface Free Energy
The dispersion and polar force of the rubbed PI films was calculated
by Owens’s formula (Eq. (1)):
ð1 þ cos hÞcL ¼ 2ðcd ꢄ cdÞ þ 2ðcSp ꢄ cLp Þ
ð1Þ
1=2
1=2
S
L
Where the d and p refer to the dispersion force and polar force
components [16,17]. The value of cdS and cSp can be determined from
the measure contact angle h, using more than two different liquids
whose surface free energy (cdL, cLp ) is known. We can get the surface free
energy of the rubbed PI films, using the reference data:
cW ¼ 72.8 dyn=cm; cdW ¼ 21.8 dyn=cm; cpW ¼ 51.0 dyn=cm for distilled
water and cMI ¼ 50.8 dyn=cm; cdMI ¼ 49.5 dyn=cm; cMp I ¼ 1.30 dyn=cm
for methylene iodide. Preparation of polyimide films on the ITO glass
and rubbing process were carried out as above described. Contact
angle was measured by the sessile drop method, using a contact angle-
meter (Phoenix 300: Seo Co.). After liquid was dropped on the rubbed
PI surface, we left it undisturbed to attain the equilibrium state for
2 min. The measurements were made at room temperature.
RESULTS AND DISCUSSION
Table 2 shows the structure of a side chain synthesized and reported
from a previous paper [10,12] and its calculated length. Also the pretilt
angle on polyimide films with these side chains is shown in Figure 3.

The Structural Effects on the Pretilt Angle
167=[2033]
FIGURE 2 The synthetic route of polyimides.
We can see that pretilt angles on polyimide films with an 18-alkyl
ester side chain are significantly lower than those of an 18-alkyl side
˚
chain. Although the length of an 18-alkyl side chain (21.6 A) is
almost the same as the length of an 18-alkyl ester side chain
˚
(24.0 A), we could confirm that the vertical alignment was hindered
by the introduction of the ester group. On the basis of the concept
of an increase in pretilt angle being dependant on side chain elonga-
tion, we assumed that an 18-alkyl ester side chain is held on the
backbone of the polyimide by an ester group. The ester group has

1
6
8
=
[
2
0
3
4
]
J.-B. Lee et al.
FIGURE 3
L
C
p
r
a
e
l
t
i
s
l
u
t
a
n
c
g
e
l
e
f
s
r
a
n
d
e
s
u
r
r
g
f
y
a
)
c
e
t
e
(
n
p
s
o
i
o
n
r
d
f
e
o
p
e
e
n
)
d
a
e
n
n
c
e
o
f
s
i
i
d
s
e
p
c
h
s
a
i
n
n
p
d
s
t
r
u
c
t
u
r
e
s
;
c
(
t
o
t
r
f
a
e
e
n
e
,
c
l
a
r
c
d
c
(
d
e
r
i
o
s
f
o
r
c
e
)
,
r
u
b
b
i
n
g
d
e
p
¼
t
h
0
.
3
m
m
.
a
b
e
n
t
s
t
r
u
c
t
u
r
e
,
w
h
i
c
h
m
a
k
e
s
i
t
d
i
f
f
i
c
u
l
t
f
o
r
i
t
t
o
p
r
o
v
i
d
e
u
p
w
a
r
d
s
o
u
n
r
p
p
p
o
y
o
l
,
r
y
t
i
s
f
m
m
o
i
i
r
d
l
a
n
L
C
m
o
h
8
l
e
a
c
n
l
u
l
e
L
.
C
l
O
s
n
t
e
c
h
c
h
e
h
a
o
t
n
.
h
e
e
r
x
h
h
a
b
n
d
s
s
,
t
h
g
e
h
p
p
r
r
r
t
e
e
h
t
t
i
i
i
s
l
l
t
t
i
a
n
g
g
u
a
l
e
-
t
e
f
i
l
m
s
w
i
t
i
d
a
i
h
i
i
t
h
i
a
n
l
a
i
t
i
a
r
t
o
t
h
e
1
-
a
k
y
s
i
d
e
i
n
T
e
r
e
a
o
n
f
o
s
t
h
t
e
h
e
L
C
A
s
l
i
t
d
h
a
u
e
o
n
b
c
u
h
g
1
a
h
8
a
i
n
t
a
h
a
s
l
a
r
g
e
o
o
d
l
i
o
k
f
a
e
s
t
r
u
L
h
c
C
e
y
t
u
s
L
o
r
i
C
f
e
d
c
e
s
h
c
a
h
e
r
a
a
i
c
C
c
n
h
t
e
r
i
s
t
i
c
o
f
a
n
L
C
m
o
l
-
c
u
l
e
.
h
e
e
n
t
h
a
n
i
s
s
h
o
r
t
e
r
t
h
a
n
t
h
e
l
e
n
a
g
t
y
h
o
o
f
-
t
l
k
y
u
s
i
d
c
h
i
n
,
t
d
i
d
a
i
n
c
a
c
n
a
s
i
t
a
n
d
v
e
r
t
i
-
c
l
l
n
s
s
t
r
e
d
e
t
t
h
e
r
i
g
i
i
t
t
h
e
L
s
i
d
e
h
n
.
T
t
h
a
s
e
f
o
n
a
t
d
c
h
t
o
e
t
r
s
s
e
a
f
e
f
n
e
g
a
c
t
t
g
i
h
n
g
o
m
t
f
e
h
e
p
e
l
r
e
a
e
t
i
c
l
t
r
t
r
a
n
i
u
g
c
l
e
i
c
a
r
e
e
r
[
n
o
t
,
t
i
1
o
n
l
b
y
e
T
t
h
e
e
s
t
e
r
L
h
i
C
c
e
m
f
f
o
e
e
l
t
c
e
t
-
t
,
b
u
l
s
t
r
t
h
l
e
o
n
n
t
a
c
o
n
t
w
e
n
c
u
l
e
a
h
i
r
l
i
n
n
t
y
e
s
r
f
a
e
6
,
8
1
]
.
h
e
h
i
g
p
r
i
l
a
w
n
o
g
r
l
d
e
s
s
,
o
f
a
e
n
p
L
C
a
s
r
u
e
r
s
t
r
c
o
e
n
p
g
r
l
o
y
d
r
u
e
c
l
a
e
t
s
e
a
d
t
o
t
a
h
e
e
l
r
o
p
w
r
s
u
r
t
f
a
a
c
n
e
g
e
n
e
r
g
a
y
n
.
I
n
e
o
n
r
d
o
e
n
r
-
t
h
o
l
a
r
f
a
s
m
l
l
e
t
i
l
l
e
t
h
t
h
p
m
o
o
f
l
a
r
c
t
s
u
r
s
p
r
f
a
a
c
n
e
d
d
u
e
a
e
t
o
l
e
t
h
e
e
p
n
s
t
u
d
t
r
o
r
n
n
g
c
e
l
e
c
t
r
o
i
o
r
d
n
d
e
i
e
c
r
c
s
i
t
n
o
i
h
t
i
n
o
e
d
r
e
s
a
n
t
n
c
t
r
t
i
u
i
i
f
c
n
o
y
t
n
b
h
e
e
,
t
e
w
r
w
e
c
e
t
a
3
n
r
d
.
L
C
c
d
e
e
l
e
u
o
e
l
e
n
f
t
t
h
p
o
y
m
r
s
f
a
e
.
I
n
t
l
e
o
n
i
e
f
e
r
c
r
e
t
i
l
n
g
l
d
n
e
e
e
n
i
o
w
g
t
t
o
e
n
a
-
s
x
i
h
a
u
F
l
r
e
e
e
d
e
s
u
f
a
c
e
t
e
e
n
n
e
r
g
y
n
c
o
t
e
t
s
h
h
Y
n
a
i
s
a
e
s
w
i
g
u
T
h
L
C
p
r
e
t
i
l
a
g
l
e
i
c
r
a
s
e
i
a
n
c
r
a
s
e
i
n
t
o
t
a
s
u
r
f
a
c
e
f
r
e
e
n
e
r
g
y
.
T
h
i
s
t
r
e
n
d
o
f
v
a
r
i
a
t
i
o
n
i
n
L
C
p
r
e
t
i
l
t
a
n
g
l
e
s
d
o
e
s
n
o
t
c
o
r
r
e
l
a
t
e
d
w
i
t
t
h
h
c
e
o
g
n
e
v
n
e
e
n
r
t
a
i
o
t
n
a
n
l
c
o
o
f
n
p
c
r
e
e
p
t
t
s
t
.
H
e
g
n
l
c
e
e
s
,
w
e
n
e
e
e
-
d
c
t
h
o
a
r
e
-
c
t
o
y
n
p
s
e
i
d
p
e
o
r
t
i
h
m
e
i
c
d
o
e
n
f
c
e
m
p
t
.
o
n
i
o
i
l
a
n
o
n
s
i
d
i
n
l
y
i
l

The Structural Effects on the Pretilt Angle
169=[2035]
In the case of the main-chain-type polyimide, the steric effect between
the long side chain and LC molecules is not a factor. That is to say that
the LC molecule lies on the surface of the polyimide backbone without
interacting with the side chain. Thus, the total surface free energy
considerably affects pretilt angles due to the interaction between LC
molecules and the polyimide backbone. However, the pretilt angle on
polyimide with side chain is affected by the steric effect between the
LC molecule and the side chain attached to the polyimide backbone.
If the LC molecule occupies the space formed by the side chain inter-
val, it was attracted to that position by the attractive force between LC
molecules and the polar group in the polyimide backbone. This polar
functional group includes imide in repeat unit of polyimide. This can
be described as the polar force, which is known to greatly contribute
to the pretilt angle rather than the dispersion force. When this attract-
ive force is strong, the LC molecule will be gradually vertical on poly-
imide backbone. Therefore, in the case of side-chain-type polyimides,
the pretilt angle increases with an increase in polar force.
RELATIONSHIP BETWEEN THE PRETILT ANGLE AND MAIN
CHAIN STRUCTURES
We conducted studies on variations in the LC pretilt angles by using
various main chain structures while keeping the same side chain of
liquid crystal molecules. Five dianhydrides (BTDA, PMDA, CPDA,
CBDA and TDA) and three diamines (ODA, PACM and ANCAMINE)
were used to modify the main chain structure of the polyimide. Their
combinations are presented in Table 1. We can see that the main
TABLE 1 Synthesized Polyimide Series
Side chain diamines
R3CPP
Dianhydrides
Main chain diamines
Combination
BTDA
PMDA
CPDA
CBDA
TDA
ODA
ODA
ODA
ODA
PACM
ANCAMINE
ODA
ODA
ODA
R3CPP-B-O
R3CPP-P-O
R3CPP-CP-O
R3CPP-CB-O
R3CPP-TDA-P
R3CPP-TDA-A
R5CPP-B-O
TDA
R5CPP
BTDA
PMDA
CPDA
CBDA
TDA
R5CPP-P-O
R5CPP-CP-O
R5CPP-CB-O
R5CPP-TDA-P
R5CPP-TDA-A
ODA
PACM
ANCAMINE
TDA

170=[2036]
J.-B. Lee et al.
TABLE 2 Side Chain Type and their Structures
a
˚
Side chain type
18-alkyl ester
The structures of side chains
Side chain length (A)
24.0
21.6
18-alkyl
R5CPP-LC
10.8(R3CPP)
13.1(R5CPP)
aAfter the molecular conformation obtained by using MM2 of Chem 3D ultra 7.0, each
length is calculated.
chain structure would contribute significantly to the pretilt angle
variations as shown in Figure 4. As a whole, polyimides of R3CPP-
LC side chain exhibit lower pretilt angle than R5CPP-LC side chain.
This is due to the steric effect depending on the side chain length.
Polyimide films with more linear and rigid main chain structures
FIGURE 4 LC pretilt angles on polyimide films rubbed by a 0.1 mm rubbing
depth.

The Structural Effects on the Pretilt Angle
171=[2037]
(BTDA, PMDA, CPDA and CBDA) exhibit high pretilt angles
(R3CPP: 66–89.7ꢀ, R5CPP: 89.1–89.7ꢀ). They have benzene rings,
cyclopentane rings and cyclobutane rings, giving a rigid structure.
However, asymmetric and flexible main chain structure (TDA-PACM
and TDA-ANCAMINE) leads to a low pretilt angle. (R3CPP: 3.1–3.4ꢀ,
R5CPP: 5.0–11.1ꢀ) Diamines, such as PACM and ANCAMINE, have
an alicyclic structure allowing it to bend easily. In this regard, their
combinations do not have a linear and rigid structure, and thereby
exhibit low pretilt angles. The steric effect of alkyl or LC side chain
strongly affects the pretilt angle when linear and rigid main chain
structure is employed. The reason is that a linear and rigid main
chain structure makes the space for a vertically standing LC molecule.
Therefore, the linear and rigid main chain should be introduced in
order to produce the vertical alignment of LC.
CONCLUSION
We synthesized polyimides having the liquid crystal molecule as a side
chain. We found that polyimide films having a LC side chain can sup-
port standing LC molecules due to the rigidity of the side chain, and
thus exhibit high pretilt angles. Also, the pretilt angles of a side-
chain-type polyimide increase with an increase in the polar force
resulting from the strength of the interaction between the LC molecule
and polyimide backbone. The pretilt angle of polyimides with a LC side
chain changes depending on the main chain structures, so a polyimide
with linear and rigid main chain exhibits high pretilt angles. On the
basis of these findings, we can design other polyimide structures to
achieve vertical alignment.
REFERENCES
[1] Berreman, D. W. (1972). Phys. Rev. Lett., 28, 1683.
[2] Freary, J. M., Goodby, J. W., Kmetz, A. R., & Patel, J. S. (1987). J. Appl. Phys., 62,
4100.
[3] Ohmuro, K., Kataoka, S., Sasaki, T., & Koike, Y. (1997). SID’97 Digest, 845.
[4] Koike, Y., Kataoka, S., Sasaki, T., Chida, H., Takeda, A., Ohmuro, K., Sasabayashi, T.,
& Okamoto, K. (1997). IDW’97, 159.
[5] Cal, C., Line, A., & Andry, P. S. (2001). Jpn. J. Appl. Phys., 40, 6913.
[6] Lee, K. W., Paek, S. H., Lien, A., During, C., & Fukuro, H. (1996). Macromolecules,
29, 8894.
[7] Ban, B. S. & Kim, Y. B. (1999). J. Phys. Chem., 103, 3869.
[8] Seo, D. S. (1999). J. Appl. Phys., 86(7), 3594.
[9] Ban, B. S. & Kim,Y. B. (1999). J. Appl. Polym. Sci., 74(2), 267.
[10] Ban, B. S., Rim, Y. N., & Kim, Y. B. (2000). Liquid Crystals, 27(1), 125.
[11] Nishikawa, M. (2000). Polym. Adv. Technol., 11, 404.

172=[2038]
J.-B. Lee et al.
[12] Han, B. R. & Kim, Y. B. (2002). Korea Liquid Crystal Conference Proceedings, 5, 57.
[13] Duan, J. & Zhang, L. H. (1999). Synlett, 8, 1245.
[14] Miqaura, N., Yanagi, T., & Susuki, A. (1981). Synth. Comm., 3, 1261.
[15] Cuminal, M. & Brunet, M. (1997). Liquid Crystals, 22(2), 185.
[16] Owens, D. K. & Wendt, R. C. (1969). J. Appl. Polym. Sci., 13, 1741.
[17] Bes, L., Rousseau, A., Boutevin, B., Mercier, R., & Kerboua, R. (2001). Macromol.
Chem. Phys., 202, 2954.