Synthesis of a novel polyimide used as liquid crystal vertical alignment layers

Article abstract of DOI:10.1039/c5ra08521g

A novel functional diamine containing a triphenylamine moiety and a biphenyl group, N,N-bis(4-aminophenyl)-4-(biphenyl)-4′-aminophenyl ether (N0), was successfully synthesized and characterized. A series of polyimides (PIs) and poly(amic acid)s (PAAs) were synthesized from cyclobutane-1,2,3,4-tetracarboxylic dianhydride (CBDA), 2,2′-bis(trifluoromethyl)-[1,1′-biphenyl]-4,4′-diamine (TFDB), 4-dodecyloxy-phenyl-4′,4″-diaminotriphenylamine (C12) and the newly synthesised diamine monomer (N0) through a conventional two-step procedure that included a ring-opening polyaddition to give polyamic acids, followed by thermal cyclodehydration. Pretilt angles of a liquid crystal (LC) cell fabricated with the PIs were measured, and the rubbing resistance of PIs containing different sidechain structures was investigated. PI1 with a rigid biphenyl side chain could induce LC parallel alignment after the rubbing process. Although PI3 containing a flexible alkyl side chain could induce LC vertical alignment before the rubbing process, it only induced LC parallel alignment after the rubbing process. However, PI2 containing both rigid biphenyl side chains and flexible alkyl side chains could induce LC vertical alignment before and after the rubbing process. A new method for improving the rubbing resistance of PIs and a generation mechanism of rubbing resistance have been proposed. In addition, all PI films showed high thermal stability and high transmittance in the wavelength range of 400-700 nm.

Full text of DOI:10.1039/c5ra08521g

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Page 1 of 22
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Synthesis of a novel polyimide used as liquid crystal vertical
alignment layers
a
a
a
a
a
Qing Gong , Shiming Gong , Heng Zhang , Lulu Liu , Yinghan Wang *
a
State Key Laboratory of Polymer Materials Engineering, College of Polymer
Science and Engineering, Sichuan University, Chengdu 610065, China. Email:
wang_yh@scu.edu.cn; Tel: +86-28-8546-0823; Fax: +86-28-8546-0823
A novel functional diamine containing triphenylamine moiety and biphenyl,
N,Nꢀbis(4ꢀ aminophenyl)ꢀ4ꢀ(biphenyl)ꢀ4’ꢀaminophenyl ether (N0), was successfully
synthesized and characterized. A series of polyimides (PIs) and poly(amic acid)s
(
PAAs) had been synthesized from cyclobutaneꢀ1,2,3,4ꢀtetracarboxylic dianhydride
CBDA), 2,2’ꢀbis(trifluoromethyl)ꢀ[1,1’ꢀbiphenyl]ꢀ4,4’ꢀdiamine (TFDB),
ꢀdodecyloxyꢀphenylꢀ4’,4’’ꢀdiaminotriphenylamine (C12) and the newly synthesised
(
4
diamine monomer (N0) through a conventional twoꢀstep procedure that included a
ringꢀopening polyaddition to give polyamic acids, followed by thermal
cyclodehydration. Pretilt angles of liquid crystal (LC) cell fabricated with the PIs
were measured, and rubbing resistance of PIs containing different side chain structure
was investigated. PI1 with rigid biphenyl side chain could induce LC parallel
alignment after rubbing process. Although PI3 containing flexible alkyl side chain
could induce LC vertical alignment before rubbing process, it only induced LC
parallel alignment after rubbing process. However, PI2 containing both rigid biphenyl
side chain and flexible alkyl side chain could induce LC vertical alignment before and
after rubbing process. A new method for improving rubbing resistance of PIs and a
generation mechanism of rubbing resistance had been proposed. In addition, all PI
films showed high thermal stability and high transmittance in the wavelength range of
4
00ꢀ700 nm.
1
. Introduction
Aromatic polyimides (PIs), which have outstanding thermal, mechanical, and

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1
chemical stability, attracted a lot of scientific and commercial interest . These
polymers have been widely used in many fields such as membrane materials for
2
,3
4ꢀ6
7
separation, composites, high temperature coatings as well as alignment layers in
8
,9
liquid crystal displays (LCDs).
When it comes to LCDs, there is so much wish for its good performance such as
a wide viewing angle, a high contrast ratio and a fast response time. However, the
conventional LCD like STNꢀLCD performance has been unsatisfactory due to a
narrow viewing angle and a slow response time. Thus, several approaches for
improving the viewing angle have been brought up, such as the addition of
1
0
11
birefringence films, the domain divided twisted nematic, the inꢀplaneꢀswitching
1
2,13
14ꢀ18
(IPS) mode
and the multi domain vertical alignment (MVA) mode.
. The
MVAꢀLCD could be very promising to achieve a wide viewing angle, fast response
14
time, and high contrast ratio at the same time. In this kind of display, a homeotropic
PI layer has been used as the liquid crystals (LCs) alignment layer, which aligns LCs
o
vertically at the fieldꢀoff state and the pretilt angle has to be controlled above 88 after
a rubbing process. It is usually accepted that the generation of the pretilt angles is
1
9
highly dependent upon the interactions of the PIs with LCs. Some reports further
pointed out that the high pretilt angle could be achieved using the PIs with bulky side
2
0
chains composed of rigid units and long alkyl chains as the alignment layer. Xia
Senlin and his coworkers synthesized series of PIs containing
ꢀhexyloxyꢀnaphthalen side chain, which had good aligning ability and the pretilt
a
a
6
o 8
angles were above 89 . Liu Zhijie and her coworkers prepared a series of PIs with
(nꢀalkyloxyl)biphenyl group, which could align LCs vertically when n was more than
21
6
. However, previous works mainly focused on the solubility of PIs and high pretilt
angles without serious consideration of the equally important thing rubbing resistance.
It is well known that rubbing process is currently the only technique applied in the
LCD industry to treat PI film surfaces in the massꢀproduction of flatꢀpanel LCD
devices. The rubbing treatment could enhance uniformity of LCs alignment and give
an excellent performance to LCDs. Therefore, PI used as LCs vertical alignment
layers must have a good rubbing resistance, so that PI films could still induce LCs

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vertical alignment after rubbing process. Obviously, the approach to enhance the
rubbing resistance is worth paying enough attention to.
The objective of this work was to synthesize a novel polyimide used as liquid
crystal vertical alignment layers and put forward a new method by introducing rigid
biphenyl chains into PIs to improve the rubbing resistance of PIs. Consideration to
high pretilt angle and good rubbing resistance, we focused on a specially designed PI
obtained
from
copolycondensation
among
N,Nꢀbis(4ꢀ
aminophenyl)ꢀ4ꢀ(biphenyl)ꢀ4’ꢀaminophenyl ether (N0) containing rigid biphenyl
moiety, 4ꢀdodecyloxyꢀphenylꢀ4’,4’’ꢀdiaminotriphenylamine (C12) containing flexible
alkyl moiety, 2,2’ꢀbis(trifluoromethyl)ꢀ[1,1’ꢀbiphenyl]ꢀ4,4’ꢀdiamine (TFDB) and
cyclobutaneꢀ1,2,3,4ꢀtetracarboxylic dianhydride (CBDA) as the alignment layer.
Properties such as optical transparency, thermal stability, and pretilt angle were
investigated. In addition, the mechanism of rubbing resistance was preliminarily
explained by a diagram of side chain structure in PIs. The new method for improving
rubbing resistance of PI films will hopefully be applied in verticalꢀalignmentꢀmode
LCD devices.
2
. Experiment section
2
.1 Materials
4
ꢀfluoronitrobenzene, biphenylꢀ4ꢀol, 1ꢀchloroꢀ4ꢀnitrobenzene, potassium carbonate
2 3
(K CO ), hydrazine hydrate and cesium fluoride (CsF) were purchased from Aladdin
Chemical Reagent Corp. (shanghai, China). Dimethyl sulfoxide (DMSO) and
Nꢀmethylꢀ2ꢀpyrrolodone (NMP) were obtained from Bodi Chemical Reagents Corp.
(Tianjin, China), and then distilled at a reduced pressure after stirring over calcium
hydride for 24 h and stored over 4 Å molecular sieves before use. CBDA (>98%) was
attained from Zhongtiansheng Technology co., Ltd., and purified by recrystallisation
from acetic anhydride and then dried in vacuum prior to use. TFDB was gained from
Shanghai EMST Corp. (Shanghai, China) and used after recrystallisation from ethanol.
o
Nematic LC E7 (n =1.521, ꢀ n = 0.22, TNꢀL = 60°C) was obtained from Yantai

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Xianhua ChemꢀTech Co., Ltd. (Yantai, China). All the other solvents and chemicals
were used as received and without further purification, unless otherwise specified.
2
.2 Measurements
1
Nuclear magnetic resonance ( HꢀNMR) spectra was measured on a 400 MHz Unity
INVOA 400 spectrophotometer with DMSOꢀd or CDCl ꢀd as the solvent and
tetramethylsilane (TMS) as the internal reference. Fourier transform infrared spectra
6
3
(FTꢀIR) was recorded on a Nicolet 560 FTꢀIR spectrophotometer. Thermogravimetric
o
analysis (TGA) was taken from a DuPont TGA 2100 at a heating rate of 10 C/min
under a nitrogen atmosphere. The rubbing process was operated with a rubbing
machine from TianLi Co. Ltd. (Chengdu, China). Spinꢀcoating process was executed
using a KWꢀ4A spinner from institute of microelectronics of Chinese Academy of
Science (Beijing, China).The pretilt angles of LC cells were measured by crystal
rotation method using a pretilt angle tester from Changchun Institute of Optics, Fine
Mechanics and Physics (Changchun, China), and at least five different points on cells
were selected for measurement. The surface morphology of the PI films before and
after rubbing was investigated by atomic force microscopy (AFM) operating in
tapping mode using an instrument with a SPI4000 Probe Station controller (SIINT
Instruments Co., Japan) at room temperature. Olympus tapping mode cantilevers with
spring constants ranging from 51.2 N/m to 87.8 N/m (as specified by the manufacturer)
were used with a scan rate in the range of 0.8–1.2 Hz. Polarising microscope from
Shanghai Millimeter Precision Instrument Co. Ltd. (Shanghai, China) was used to
evaluate the alignment behaviours of LCs. Contact angles of a deionized water and
methylene iodide on the surface of the polyimide films were measured by a Kruss
DSA100 goniometer system (Kruss GmbH, Germany). A Young’s harmonicꢀmean
equation was applied to predict the surface tensions from the contact angles. Gel
permeation chromatography (GPC) of soluble polymers was performed using an
o
Applied Bio system (Foster City, CA, USA) at 70 C with two PL gel 5ꢁm mixedꢀC
columns (Agilent, Santa Clara, CA, USA).

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2
.3 Synthesis of monomer
2
.3.1 Synthesis of 4-(p-nitrophenoxy)biphenyl (a). A mixture of 150 mL of DMF,
7
.25 g (46.0 mmol) of 1ꢀchloroꢀ4ꢀnitrobenzene, 7.53 g (44.3 mmol) of biphenylꢀ4ꢀol
2 3
and 11.06 g (80.0 mmol) of K CO was placed into a 250ꢀmL roundꢀbottomed flask
equipped with a magnetic stirrer and a condenser. The mixture was stirred and heated
o
to 150 C. After 8 h, the reaction was stopped and the mixture was allowed to cool to
room temperature. Then the mixture was poured into 300 mL of distilled water to
yield paleꢀyellow precipitates. The precipitate was collected by filtration and
repeatedly washed with distilled water. Finally, the product was recrystallized with
dichloromethane to gain 11.35 g (yield: 88 %) of crystals.
1
6 2
HꢀNMR (DMSOꢀd ), δ (ppm): 7.29, 8.28 (m, 4H, ArH–NO ), 7.70, 7.20, 7.79,
ꢀ
1
7
.49, 7.39 (m, 9H, Ar–H of biphenyl). FTꢀIR (KBr, pellet), cm : 1568, 1506, 1484
aromatics), 1247, 1168 (C–O–C).
.3.2 Synthesis of 4-(p-aminophenoxy)biphenyl (b). A mixture of 10.14 g
34.8 mmol) of 4ꢀ(pꢀnitrophenoxy)biphenyl (a), 1.02 g of 5 % palladium on activated
(
2
(
carbon (Pd/C) and 80 mL of ethanol was placed into a 250ꢀmL roundꢀbottomed flask
equipped with a magnetic stirrer and a condenser. The mixture was stirred and heated
to reflux, and then 80 % hydrazine hydrate (8.70 g, 174.0 mmol) was added dropwise
into the mixed solution. The mixture was refluxed for 4 h and filtered subsequently to
remove Pd/C. Then the solvent was evaporated, and the crude product was finally
recrystallized with ethanol to gain 8.46 g (yield: 93 %) of solid.
1
6 2
HꢀNMR (DMSOꢀd ), δ (ppm): 5.03 (s, 2H, NH ), 6.61, 6.81 (m, 4H, ArH–O–),
ꢀ
1
6
.93, 7.31, 7.43, 7.60 (m, 9H, Ar–H of biphenyl). FTꢀIR (KBr, pellet), cm : 3398,
326, 3221 (NH ), 1503, 1484 (aromatics), 1224, 1199 (C–O–C).
.3.3 Synthesis of N,N-bis(4-nitrophenyl)-4-(biphenyl)-4’-aminophenyl ether
c). A mixture of the aromatic etherꢀamine 4ꢀ(pꢀaminophenoxy)biphenyl (b) (7.49 g,
8.6 mmol), pꢀfluoronitrobenzene (8.47 g, 60.0 mol), CsF (12.96 g, 85.8 mmol), and
3
2
2
(
2
DMSO (60 mL) was placed into a 250ꢀmL threeꢀnecked flask equipped with a
magnetic stirrer, a nitrogen inlet, and a condenser. The mixture was stirred and heated

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o
to 150 C. After 8 h, the reaction mixture was cooled and poured into 70 mL of
methanol. The yellow precipitate was collected by filtration. The product was washed
with water and dried. Finally, the crude product was purified by recrystallisation from
methanol to afford 10.53 g (yield: 79 %) yellow crystals.
1
HꢀNMR (DMSOꢀd ), δ (ppm): 8.20, 7.22 (m, 8H, ArHꢀNO ), 7.31, 7.18 (m, 4 H,
6
2
–
O–ArH), 7.74, 7.66, 7.47, 7.46, 7.25 (m, 8 H, ArꢀH of biphenyl). FTꢀIR (KBr, pellet),
ꢀ
1
cm : 1500, 1485 (aromatics), 1580, 1340 (NO
.3.4 Synthesis of N,N-bis(4- aminophenyl)-4-(biphenyl)-4’-aminophenyl
(N0). mixture of 9.67 (19.2 mmol) of
2
), 1241, 1112 (C–O–C).
2
ether
A
g
N,Nꢀbis(4ꢀnitrophenyl)ꢀ4ꢀ(biphenyl)ꢀ4’ꢀaminophenyl ether (c), 0.97 g of 5 % Pd/C
and 55 mL of ethanol was placed into a 100ꢀmL roundꢀbottomed flask equipped with
a magnetic stirrer and a condenser. The mixture was stirred and heated to reflux, and
then 80 % hydrazine hydrate (9.62 g, 192.0 mmol) was added dropwise into the
mixed solution. The mixture was refluxed for 4 h and filtered subsequently to remove
Pd/C. Then the solvent was evaporated. The crude product was finally recrystallized
with ethanol to gain 7.58 g of crystals (yield: 89 %).
1
6 2
HꢀNMR (DMSOꢀd ), δ (ppm): 4.96 (s, 4H, NH2), 6.54, 6.82 (m, 8H, NH –ArH),
6
.66, 6.86 (m, 4H, −O–ArH), 6.99, 7.32, 7.44, 7.61 (m, 9H, Ar–H of biphenyl). FTꢀIR
ꢀ1
(
KBr, pellet), cm : 3456, 3335 (NH
2
), 1623, 1495 (aromatics), 1228, 1110 (C–O–C).
2
.4 Synthesis of the polymer
The PIs and PAAs were synthesized from commercial dianhydride CBDA, the
auxilliary diamine TFDB and two functional diamines (N0 or C12). The molar ratios
of N0, C12 and TFDB were controlled to be 2:0:8 (PI1 or PAA1), 1:1:8 (PI2 or
PAA2) and 0:2:8 (PI3 or PAA3) to obtain the PIs and the PAAs.
The synthesis of PI2 and PAA2 was used as an example to illustrate the general
synthetic route used to prepare the PIs and PAAs. The typical procedure was as
follows: 0.07 mmol of N0, 0.07 mmol of C12 and 0.56 mmol of TFDB were
dissolved in 2.05g of NMP in a 50ꢀmL threeꢀnecked flask with a stirrer. After a clear
solution was formed, 0.7 mmol of CBDA was added to the solution. And the final

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mixture was stirred at room temperature under a gentle flow of dry nitrogen for 24 h
to yield viscous PAA2 solutions. The PAA2 was converted into polyimide via a
thermal imidization process. For the thermal imidization method, the PAA2 solution
o
o
was cast onto a clean glass plate and heated (80 C for 30 min, 120 C for 30 min, 200
o
o
C for 1 h, 240 C for 2 h) to produce a fully imidized PI2 film.
2
.5 Fabrication of the LC cells
The PAA solutions were diluted to a solid content of 5 wt% in NMP. The solutions
attained were coated onto clean glass substrates by spinꢀcoating at a rotation speed of
o
6
00 rpm for 15 s, then 2500 rpm for 30 s, followed by heating at 80 C for 30 min to
o
evaporate the solvent. Then the films were heated at 250 C for 1 h to complete
thermal imidization. The cured PI films on glass were rubbed with a roller covered
with rayon velvet fabric, and the rubbing strength (RS) was calculated as follows
(RS=28.5 mm in this work):

2
π
rn 
ꢀ1
RS = NM



60
ν

where RS (mm) was the total length of rubbing cloth that touched a certain point of
the films; N was the cumulative number of rubbings (N=1 in this work); M (0.3 mm)
was the contact length of the rubbing roller circumference;
ν
(17.2 mm/s) was the
velocity of the substrate stage; n (700 rpm) and r (22.5 mm) were the rubbing roller
speed and radius, respectively.
The LC cells were made from two pieces of the rubbed PI films in an antiparallel
rubbing direction with 45ꢀꢁmꢀthick PI film spacers. Besides, some nonꢀrubbed
substrates were also used to fabricate LC cells for comparison. The LC cells gap were
o
filled with a nematic LC E7 using a capillary method at 80 C, and then the injection
hole was sealed with photosensitive epoxy glue. The LC cells were then heatꢀtreated
o
for 20 min at 80 C, which was higher than the nematicꢀtoꢀisotropic transition
temperature of E7, to eliminate flow marks. The alignment behaviors of the LC cells
were examined with polarized optical microscopy. The pretilt angles of the LCs were
measured using a crystal rotation method.

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3
. Results and discussion
3
.1 Monomer synthesis
The diamine 4ꢀdodecyloxyꢀphenylꢀ4’,4’’ꢀdiaminotriphenylamine (C12) used as one of
2
2
the functional diamine was synthesized according to our previously work. Four steps
were employed in synthesizing the novel functional diamine N0 from biphenylꢀ4ꢀol,
as shown in Scheme1.
Scheme 1 Synthetic route to the functional diamine N0
First, 4ꢀ(pꢀnitrophenoxy)biphenyl (a) was prepared via an aromatic nucleophilic
substitution (S
the presence of K
ꢀ(pꢀaminophenoxy)biphenyl (b) by means of Pd/C and hydrazine hydrate. The target
diamine monomer N,Nꢀbis(4ꢀaminophenyl)ꢀ4ꢀ(biphenyl)ꢀ4’ꢀaminophenyl ether (N0)
N
Ar) reaction of 1ꢀchloroꢀ4ꢀnitrobenzene with biphenylꢀ4ꢀol in DMF in
2
CO as an acid acceptor. Subsequently, was reduced to
3
a
4

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was synthesized by double Nꢀarylation reactions of
b with two equivalents of
pꢀfluoronitrobenzene in the presence of CsF, followed by hydrazine hydrate
Pd/Cꢀcatalyzed reduction of the intermediate dinitro compound
N,Nꢀbis(4ꢀnitrophenyl)ꢀ4ꢀ(biphenyl)ꢀ4’ꢀaminophenyl ether (
c
).
The chemical structures of all of the above compounds were confirmed by their
1
FTꢀIR and HꢀNMR spectra. All of the spectroscopic data obtained were consistent
1
with the proposed structures. A typical HꢀNMR spectrum of compound N0 was
illustrated in Fig.1.
1
Fig. 1 The HꢀNMR (400 MHz) spectrum of the functional diamine N0 in DMSOꢀd
6
3
.2 Polymer synthesis
New PIs were prepared from N0, C12, TFDB, and CBDA via a conventional twoꢀstep
procedure, as shown in Scheme 2. The polymerization was carried out by reacting
stoichiometric amounts of the diamines with aromatic dianhydrides at a concentration
of 15 % solids in NMP. Ringꢀopening polymerization at room temperature for 24 h
yielded the PAA solution, followed by a thermal imidization, subsequent gradually
o
heating from room temperature to 240 C to obtain the corresponding PIs.

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Scheme 2 Synthetic route to the polymers
1
The formation of the PAAs and PIs was confirmed with HꢀNMR and FTꢀIR. As
1
an example, the HꢀNMR spectrum of PAA2 was shown in Fig.2. The broad peak at
1
0.1–10.3 ppm was the characteristic peak of the amide protons, the peak at 10.6ꢀ10.9
ppm was the characteristic peak of the carboxyl protons and the peaks at 6.8–8.4 ppm
were the characteristic peaks of the phenyl ring protons of the PAA. The FTꢀIR
spectrum of PIs presented in Fig. 3 showed typical absorption bands of aliphatic C–H
−1
stretching at 2928–2851 cm corresponding to the methyl groups of the diamine C12.
The peak intensity of PI2 was weaker than PI3’s since the content of the diamine C12
−1
−1
of PI2 was half of PI3. Absorptions at 1781 cm and 1724 cm were observed due to
the asymmetric and symmetric C=O stretching vibrations of the imide groups,
−1
respectively. Absorption at 1370 cm was assigned to C–N stretching, absorptions at
−1
−1
1
174–1135 cm to C–F stretching, and a sharp absorption band at around 748 cm to
the skeletal vibrations of the imide rings. There were no characteristic absorption

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−1
bands of the amide group near 3200–3363 cm (N–H stretching) and 1650–1674
−1
23,24
cm (C=O stretching), indicating that the polymers had been fully imidized.
In addition, the GPCs of all PAAs were investigated, and the data was listed in
4
Table 1, which indicated high molecular weights in the range 1.41ꢀ1.64×10 g/mol for
M . The molecular weights were quit up to requirement of the liquid crystal alignment
n
layers.
1
Fig. 2 HꢀNMR (400 MHz) spectrum of PAA2 in DMSOꢀd
6
Fig. 3 Thin film FTꢀIR spectrum of PIs

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Table 1 Formula and GPC molecular weights of PAAs
ꢀ4
PAAs
N0:C12:TFDB:CBDAa
Mn (10 g/mol)b
Mw/Mn
PAA0
PAA2
PAA3
20:0:80:100
10:10:80:100
0:20:80:100
1.41
1.44
1.64
1.93
1.97
2.05
a
Composition of monomers were calculated as mole ratio.
b
Numberꢀaverage molecular weight calculated by GPC in THF versus polystyrene
standards.
.3 Optical transparency
3
The transmittances of PIs were investigated using a UV–vis spectrophotometer at
room temperature. All PI films were obtained by casting from a 5 wt% PAA solution
2
on a calcium fluoride (CaF ) piece at room temperature, and subsequent sequential
o
heating to 240 C for 2 h, then PI films were gained by immersion the glass plate in
o
water followed by drying in an oven at 100 C. The thickness of all PIs was around
10 ꢁm . The UV–vis spectra of the PI films was shown in Fig.4. All of the PI films
showed high transmissions in the wavelength range 400–700 nm, which was
attributed to existence of aliphatic dianhydride (CBDA) that blocked charge
transformation in the molecular chains, fluorine atoms of TFDB that was good for
optical transparency, and large side units that increased the distance and blocked
charge transformation between molecular chains. The similar optical transmittance of
PIs was attributed to the same content of the function diamines (N0 and C12). The
high optical transparency of the PI films was advantageous in LCD devices.

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Fig. 4 UV–vis transmission spectra of PI films (film thickness: 10 ꢁm
)
3
.4 Thermal properties
Thermal stabilities of the PIs films were studied by TGA under a nitrogen atmosphere
and their programs were reproduced in Fig.5. The initial thermal decomposition
temperature (T
d
), temperature at which a weight loss of 5% occurred (T
5
), temperature
o
at which a weight loss of 10% occurred (T ), and the residual weight at 800 C (R )
1
0
w
were summarized for all PIs in Table 1. As Fig.5 showed, twoꢀstage thermal
decomposition behavior was observed, which was similar to the behavior of those
2
5,26
other reported polymers.
The first step at relatively lower temperatures,
presumably, was ascribable to degradation of the side chains. As the results of
Differential Thermal Gravity (DTG) show, the speed on loss in weight of PIs
increased with the increasing content of the long alkyl side chains. Compared with
PI1, PI2 and PI3 contained the same number of functional diamine (N0 or C12), the
PI1 exhibited the best thermal stabilities since its side chains contained more benzene
rings and fewer alkyl units. The second step at relatively higher temperatures
probably resulted from the thermal degradation of the rigid aromatic backbones. The
introduction of bulky, twisted triphenylamine (TPA) group into PIs increased the
rigidity of the backbone of PIs, thus, all PIs exhibited a higher values of T10. The TGA
results showed that all PIs performed good thermal stability.

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o
Fig. 5 TGA thermograms of PIs, obtained at a scan rate of 10 C/min (inset:
Differential Thermal Gravity (DTG) of PIs).
Table 2 The thermal properties of the PIs
a
b
d
o
b
o
b
o
b o
PIs
T
( C)
T
5
( C)
T
10 ( C)
w
R ( C)
PI1
PI2
PI3
373
360
328
420
422
410
444
50.0
49.3
46.8
440
435
a
Measured samples were obtained by the chemical imidization method.
b
d 5
T was the decomposition onset temperature; T and T10 were the temperatures at
o
which weight losses of 5% and 10% occur; R
in nitrogen.
w
was the residual weight (%) at 800 C
3
3
.5 Alignment properties of LCs
.5.1 pretilt angles. The pretilt angle, which greatly determines the electroꢀoptical
properties of LCD devices, is affected by a lot of factors, such as the surface
morphology, steric effects and the electronic interaction of the LC with the alignment
layer. All of these factors are related to the processing conditions used, such as the
rubbing treatment, as well as the chemical structure of the PI. As is well known,
rubbing treatment is a technique that is widely used to obtain homogeneous alignment
of the LC molecules on the polymer surface. This paper concentrated on the effect of
rubbing on the pretilt angles of the LC cells.

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All pretilt angles before and after rubbing process were measured by a crystal
rotation method and their numerical values were listed in Table 2. As shown in the
table, the LCs was not aligned on the surface of the PI1 film without the rubbing
treatment. On the other hand, the pretilt angles of the LC cell fabricated with PI2 and
o
PI3 with long flexible alkyl side chain were 90 before the rubbing process. These
results indicated that long flexible alkyl side chain was necessary for vertical
alignment of LCs. After rubbing process, the pretilt angle of LCs on PI1, PI2 and PI3
o
o
o
film were 4.5 , 89.7 and 2.4 , respectively. That is to say, PI2 containing both rigid
biphenyl side chain and flexible alkyl side chain could still induce LC vertical
alignment after rubbing process, but PI1 only containing rigid biphenyl side chain and
PI3 only containing flexible alkyl side chain induced LC parallel alignment.
Table 3 Pretilt angles of the LC cells fabricated with the PIs
o
Pretilt angle θp ( )
PIs
Before rubbing
After rubbing
PI1
PI2
PI3
No alignment
4.5
89.7
2.4
90
90
3
.5.2 Alignment behavior. A high pretilt angle is crucial to obtain good optical
and electrical performances for vertical alignment LCDs. Besides, the uniformity of
LCs vertical alignment at a large area is also important. Consequently, it is necessary
to investigate the uniformity of LCs alignment on the PI films. The uniformity of LCs
alignment was examined between crossed polarisers with a polarising optical
microscope. As shown in Fig. 6
a and c, after rubbing process, an alternate and
periodical change of transmittance with the rotation of the substrate stage could be
observed, meaning that the LC molecules in cells using PI1 and PI3 were aligned
planar to the PI surface. Fig. 6
b showed that the transmittance of the LC cells using
the rubbed PI2 with a rotation of the sample stage gave no rise to an alternate and
periodic appearance of darkness and brightness, meaning that the LC molecules were

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aligned vertical to the PI surface. Conoscope observation of the cells was used with
polarized optical microscope. In Fig.6 , the dark crossed brush was clearly seen and
b
did not move with the LC cell rotating under conoscope. The result further proved
that PI2 containing both rigid biphenyl side chain and flexible alkyl side chain could
introduce the LCs vertical alignment, and it also suggested that PI2 film was more
resistance to rubbing process.
Fig. 6 Polarized optical microscopic images of LC cells using the rubbed PI1 (a), PI2
(b) and PI3(c). All pictures were captured under crossed polarizers. The conoscopic
images are shown in the corners of the images. The micrographs on the right hand
side were taken for cells after rotating the stage by 45°.
3
.5.3 Surface energy. The surface energy could supply some information about
the surface of PI films for researching the alignment behavior of the LCs. As shown in
Fig. 7, the surface energy of the PI films was measured before and after the rubbing
treatment. Compared with PI1, the surface energy of PI2 and PI3 was significantly
decreased with the introduction of flexible alkyl side chains, which resulted in a high
o
pretilt angle 90 . That is, the surface polarity of the PI films was decreased with the
introduction of the nonpolar alkyl side chains. After rubbing treatment, the surface
energy of PI1 and PI2 were slightly increased, and the surface energy of PI3 had a

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more marked increase compared to PI1 and PI2. That is, for PI3, the rubbing process
induced the increase of the polarity of the polyimide films by an insertion of the
nonpolar alkyl side chains into the polymer bulk phase during the rubbing process,
which resulted in change of a geometric structure of the polyimide thin films and the
27
increase of a flatness of thin film to make the liquid crystal molecules to lie flater.
For PI2, the insertion of the nonpolar alkyl side chains into the polymer bulk phase
during the rubbing process was difficult to appear because of the existence of the rigid
biphenyl side chains, which could restrict the movement of the alkyl chains. So, the
results of surface energy also suggested that PI2 film was more resistance to rubbing
process.
Fig. 7 Surface energy of the PIs before and after the rubbing process.
3
.5.4 Surface morphology. Surface morphology of PI1, PI2 and PI3 films was
measured by AFM before and after rubbing process. Parts A, B and C of Fig. 7 were
shown the AFM images of PI1, PI2 and PI3 films before rubbing treatment,
respectively. It was found that the PI1, PI2 and PI3 films had a rootꢀmeanꢀsquare
2
(RMS) roughness of 0.31, 0.46 and 0.34 nm over an area of 2.5
×
2.5
ꢁ
m . The
surface morphology and roughness of the PI film derived mainly from the
characteristics of the polymer chains that govern the aggregation and molecular
ordering that occurred during the drying and thermal imidization processes after
2
8
spinꢀcasting. In contrast, after rubbing treatment, as show in Fig. 7 D, E and F, all
PIs films exhibited microgrooves along the rubbing direction. The formation of

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microgroove lines might be attributed to the shearꢀinduced deformation of the
polymer films surface induced by the contact of fibers during the rubbing process.
The developed microgrooves of the rubbed PI1, PI2 and PI3 films were around 38 nm
or larger in size. The RMS surface roughness values of the rubbed PI1, PI2 and PI3
2
films were 1.31, 1.29 and 1.36 nm over an area of 2.5
×
2.5
ꢁ
m , respectively.
From the above alignment results of LCs and surface morphology results of PIs
films, although the PI1, PI2 and PI3 films had same morphologies after rubbing
treatment, only PI2 could induce LC molecules vertical alignment, and PI1 and PI3
induced LCs parallel alignment. Besides, the microgrooves developed along the
rubbing direction were much larger in size than the LC molecules. These facts
suggested that the LC molecules were more likely to be aligned by interactions with
the polymer chain segments, rather than by the microgrooves. Further, some reports
had pointed out that there were some intermolecular interactions between LC
molecules and the side chains of PIs, such as the dipoleꢀdipole and
π ꢀπ interaction
between the biphenyl groups of LC molecules and the biphenyl groups of side chains,
as well as the van der Waals forces between the alkoxy endꢀgroup of PI and the
flexible alkoxy tail in an LC molecules. Due to the strong mutual interactions, LC
molecules tended to be aligned in the direction parallel to the biphenyl groups of the
2
5, 29
side chains in the PI films.
Therefore, it was believed that the molecular chain reorientation, especially the
side chain, had a greater influence on the LCs aligning ability on the PI film surfaces
than the microgrooves did. After rubbing process, PI2 containing both rigid biphenyl
side chain and flexible alkyl side chain could still induce LCs vertical alignment, but
PI3 only containing flexible alkyl side chain induced LCs parallel alignment. The
reason was that, in PI3, the flexible alkyl side chain might easily be reoriented toward
the polymer bulk phase by the rubbing process. However, in PI2, the rigid biphenyl
side chain might restrict the movement of the alkyl chain into the polymer surface,
which resulted in an enrichment of alkyl side chain at the outmost layer of the PI
surface.

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Consequently, based on the result of pretilt angles, surface energy and surface
morphology, a generation mechanism of rubbing resistance could be proposed, which
was shown in Scheme 3. As shown, the alkyl side chain of the PI2 containing the rigid
biphenyl side chain might be more difficultly reoriented toward polymer bulk phase
than that of the PI3 with flexible alkyl side only.
Fig. 8 AFM images and surface profiles of PI films before and after rubbing: (A) and
(a) pristine PI1; (B) and (b) pristine PI2; (C) and (c) pristine PI3; (D) and (d) rubbed
PI1; (E) and (e) rubbed PI2; (F) and (f) rubbed PI3. The arrows in the images (D), (E)
and (F) denote the rubbing direction, respectively.

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Scheme 3 Proposed mechanism of the microscopic molecular reorientation of the PIs
surface induced by the rubbing process (a) the PI with rigid biphenyl side chain only,
(b) the PI with flexible alkyl side chain only, (c) the PI containing both rigid benzene
biphenyl side chain and flexible alkyl side chain.
4
. Conclusions
We have synthesised a novel functional diamine (N0) containing a TPA unit and
biphenyl unit. Furthermore, a series of PAAs and PIs were synthesized from CBDA,
TFDB, N0 and C12. All PI films showed high transmittance in the wavelength range
of 400–700 nm. Besides, an effect of the side chain structure on pretilt angles of the
LC cells fabricated with the PIs was investigated. PI2 containing both rigid biphenyl
side chain and flexible alkyl side chain performed a better rubbing resistance than PI3
only containing flexible alkyl side chain. PI2 could align LCs vertically before and
after rubbing treatment, but PI3 could induce LCs vertical alignment only before
rubbing treatment, after rubbing process, PI3 aligns LCs parallel. PI1, only after
rubbing process, could induce LCs parallel alignment. We can propose that
introduction of rigid biphenyl side groups within the PI films used as vertical LCs
alignment layer could improve rubbing resistance. Besides, all PIs could align LCs

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uniformly. Furthermore, thermal stability of PIs was improved with the increase of the
rigid biphenyl side chains. The PIs with a good rubbing resistance and other excellent
performance, like PI3, could be used for advanced LCDs like vertically aligned
LCDs.
Acknowledgements
This work was supported by National Natural Science Foundation of China
(Grant no. 51173115), the Ministry of Education (the Foundation for Ph.D. training,
Grant no. 20110181110030) of China.
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