Effect of the functional diamine structure on the properties of a polyimide liquid crystal alignment film

Article abstract of DOI:10.1039/c4ra16997b

A novel functional diamine containing triphenylamine moiety and biphenyl as well as a long alkyl chain, 4-dodecyloxy-biphenyl-4′,4′′-diaminotriphenylamine (DBDTA), was synthesized and characterized. A series of polyimides (PIs) were copolymerized from DBDTA, 3,3′-dimethyl-4,4′-methylenedianiline (DMMDA) and 4,4′-oxydiphthalic anhydride (ODPA) via a one-step method. The chemical structures of the diamine and PIs were characterized by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (¹H NMR). Properties such as the solubility, rubbing resistance, thermal stability and pre-tilt angle of the PIs were investigated. Furthermore, the results were compared with the PIs-BZA derived from 4-dodecyloxy-biphenyl-3′,5′-diaminobenzoate (DBPDA). The PIs-TPA derived from DBDTA exhibited better transparency and thermal stability than the PIs-BZA from DBPDA. When the content of functional diamines was only 10%, the PI₁-TPA derived from DBDTA displayed better solubility than the PI₁-BZA from DBPDA. In addition, all PIs could induce uniform vertical alignment of the liquid crystals (LCs) before and after the rubbing process, but PI₁-BZA only induced parallel alignment of the LCs after rubbing process. It is suggested that the PI₁-TPA film is more resistant to the rubbing process than PI₁-BZA. This journal is

Full text of DOI:10.1039/c4ra16997b

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Effect of the functional diamine structure on the
properties of a polyimide liquid crystal alignment
film
Cite this: RSC Adv., 2015, 5, 25348
a
b
a
a
a
a
Ming Liu, Xinguo Zheng, Shiming Gong, Lulu Liu, Zhen Sun, Leishan Shao
and Yinghan Wang*a
A novel functional diamine containing triphenylamine moiety and biphenyl as well as a long alkyl chain,
0 00
4
-dodecyloxy-biphenyl-4 ,4 -diaminotriphenylamine (DBDTA), was synthesized and characterized. A
0
0
series of polyimides (PIs) were copolymerized from DBDTA, 3,3 -dimethyl-4,4 -methylenedianiline
0
(
DMMDA) and 4,4 -oxydiphthalic anhydride (ODPA) via a one-step method. The chemical structures of
the diamine and PIs were characterized by Fourier transform infrared spectroscopy (FT-IR) and nuclear
1
magnetic resonance spectroscopy ( H NMR). Properties such as the solubility, rubbing resistance,
thermal stability and pre-tilt angle of the PIs were investigated. Furthermore, the results were compared
0
0
with the PIs-BZA derived from 4-dodecyloxy-biphenyl-3 ,5 -diaminobenzoate (DBPDA). The PIs-TPA
derived from DBDTA exhibited better transparency and thermal stability than the PIs-BZA from DBPDA.
When the content of functional diamines was only 10%, the PI
better solubility than the PI -BZA from DBPDA. In addition, all PIs could induce uniform vertical
alignment of the liquid crystals (LCs) before and after the rubbing process, but PI -BZA only induced
parallel alignment of the LCs after rubbing process. It is suggested that the PI -TPA film is more resistant
to the rubbing process than PI -BZA.
1
-TPA derived from DBDTA displayed
Received 24th December 2014
Accepted 2nd March 2015
1
1
DOI: 10.1039/c4ra16997b
1
www.rsc.org/advances
1
14
copolymerization. Though these attempts could create a more
soluble modied PI, main-chain-type PIs are still unable to
1
. Introduction
Aromatic polyimides (PIs) are widely used in many elds, such satisfy the requirements of manufacturing high-performance
1,2
3–5
as membrane materials for separation, composites, coat- LCDs due to their small pretilt angles. Recently, it was repor-
6
ings and liquid crystal (LC) alignment layers in liquid crystal ted that LCDs with vertical aligned mode had a faster response
7,8
displays (LCDs) because of their excellent mechanical and time and a higher contrast ratio compared to that with parallel
thermal stability.
aligned mode. PIs containing alkyl side chains exhibited good
One of the key technologies in LCDs is surface alignment of solubility and high pretilt angle, however, the thermal stability
LCs. The pre-tilt angle of LC molecules is a sector angle between of these PIs was low. To the best of our knowledge, the research
the LC molecule and substrate that inuences the electro-optic on the preparation of soluble, thermally stable and rubbing
characteristics of LCDs, such as bias voltage, threshold voltage, resistance PI for vertical alignment of LC has rarely been
9
and response time. Due to their rigid chain characteristics, reported. Many works have been carried out on the effect of
traditional aromatic PIs usually exhibit very low solubility and a length of alkyl groups on pretilt angles, but only a few papers
15
high melting point far above their decomposition temperature. have discussed the effect of the skeleton structures of diamine.
Thus, PI processing is generally carried out with polyamic acid
In this study, we have synthesized a functional diamine
PAA) intermediate and then converted to PI via a rigorous 4-dodecyloxy-biphenyl-4 ,4 -diaminotriphenylamine (DBDTA).
0
00
(
ꢀ
0
0
thermal treatment (250–300 C), which is fatal to the substrate Some PIs were synthesized from 3,3 -dimethyl-4,4 -methyl ene-
used in the LCD. This disadvantage can be overcome by intro- dianiline (DMMDA), 4,4 -oxydi(phthalic-anhydride) (ODPA)
spiroskeleton and DBDTA. Other PIs were synthesized from DMMDA,
linkage and asymmetric units into the backbone or ODPA and functional diamine 4-dodecyloxy-biphenyl-
,5-diaminobenzoate (DBPDA). Then the solubility, trans-
0
10
11
ducing bulky groups,
exible linkages,
12
13
3
mittance and thermal stability of the PIs were investigated.
Meanwhile, alignment ability and pretilt angles of LCs induced
by the PIs were also studied. The structure geometries of two
functional diamines were optimized by the density functional
theory (DFT) calculations using a large basis-set function which
a
State Key Laboratory of Polymer Materials Engineering, College of Polymer Science
and Engineering, Sichuan University, Chengdu 610065, China. E-mail: wang_yh@
scu.edu.cn; Fax: +86-28-8546-0823; Tel: +86-28-8546-0823
b
Railway Engineering Research Institute of China Academy of Railway Science, Beijing
1
00081, China
25348 | RSC Adv., 2015, 5, 25348–25356
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
was shown in Fig. 1. As shown in Fig. 1, the lengths of two (Chengdu, China). Spin-coat process was executed using a KW-4A
functional diamines (DBDTA and DBPDA) were 27.0 and 29.2 A˚ , spinner from institute of microelectronics of Chinese Academy of
respectively, that is, their length was almost the same. And an Science (Peking, China). The pretilt angles of LCs were measured
effect of the skeleton structures of the functional diamines on by crystal rotation method using a pretilt angle tester from
the pretilt angles of the LCs before and aer rubbing process Changchun Institute of Optics, Fine Mechanics and Physics
has been investigated.
(Changchun, China), and at least ve different points on cells
were selected for measurement. The optical micrographs were
obtained from polarizing optical microscope (Shanghai Milli-
meter Precision Instrument Co. Ltd., Shanghai, China) under
room temperature with the magnication of 40 times. The
2. Experimental section
2.1 Materials
4
-Hydroxydiphenyl, benzoylchloride, 4-uoronitrobenzene, surface morphology of the PI lms before and aer rubbing was
fuming nitric acid and cesium uoride were purchased from investigated by atomic force microscopy (AFM) operating in
Aladdin Chemical Reagent Corp. (shanghai, China). Hydrazine tapping mode using an instrument with a SPI4000 Probe Station
hydrate was purchased from Haihong Experimental Instrument controller (SIINT Instruments Co., Japan) at room temperature.
Co., Ltd (Chengdu, China). ODPA (>98%, Shanghai Research Olympus tapping mode cantilevers with spring constants ranging
ꢁ
1
ꢁ1
Institute of Synthetic Resins) was puried by a recrystallization from 51.2 N m to 87.8 N m (as specied by the manufacturer)
from acetic anhydride. DMMDA was purchased from Shanghai were used with a scan rate in the range of 0.8–1.2 Hz.
EMST Corp. (Shanghai, China) and dried before use. DBPDA
16
was synthesized according to our previous work. N-Methyl- 2.3 Synthesis of monomer
-pyrrolodone (NMP), N,N-dimethyl acetamide (DMAc),
2
The synthesis of DBDTA was shown in Fig. 2.
.3.1 Synthesis of 4-benzoyloxybiphenyl (1). A 500 ml ask
equipped with a magnetic stirrer and a condenser was placed
with a mixture of 200 ml of dry THF, 20.40 g (0.08 mol) of
dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF)
and chloroform (TCM) were purchased from Bodi Chemical
Reagents Corp. (Tianjin, China), and then puried by vacuum-
distillation over phosphorus pentoxide. Ethanol, isopropyl
alcohol and tetrahydrofuran (THF) were dried with 4 A molec-
ular sieves before use. 5CB was purchased from Shijiazhuang
Crown Chem-Tech. Co. Ltd.
2
4-hydroxybiphenyl and 16.4 ml (0.05 mol) of dry triethylamine
˚
(
distilled from KOH). Then 12.2 ml (0.10 mol) of benzoyl-
ꢀ
chloride was added dropwise into the mixed solution at 0 C
and the reaction mixture was stirred for 24 h at room temper-
ature. And then the reaction mixture was ltered. THF and
triethylamine were evaporated. The solid was washed with water
2
.2 Measurements
Fourier transform infrared spectroscopy (FT-IR) was recorded and hot methanol, and recrystallized from 1-butanol to afford
1
6
on a Nicolet 560 Fourier transform spectrophotometer. Nuclear 20.41 g (yield: 93%) of white crystals. H NMR (DMSO-d , d),
1
magnetic resonance ( H NMR) spectra were obtained from a 400 ppm: 8.14, 7.51, 7.41 (m, 5H, ArH), 7.55, 7.48, 7.32, 7.25, 7.21
MHz Unity INVOA 400 spectrophotometer. Thermogravimetric (m, 9H, ArH of biphenyl).
0
analysis (TGA) was performed under nitrogen on a DuPont
2.3.2 Synthesis of 4-benzoyloxy-4 -nitrobipheny (2). 20.00 g
TGA2100 thermogravimetric analyzer and the sample was (0.07 mol) of 4-benzoyloxybiphenyl (1) was mixed with 160 ml of
ꢀ
ꢁ1
ꢀ
heated at 20 C min . Differential scanning calorimetry (DSC) glacial acetic acid and heated to 85 C. Fuming nitric acid
was conducted under nitrogen with Netzsch 204 DSC and the (48 ml) was slowly added dropwise into the mixture. The
ꢀ
ꢁ1
sample was heated at 10 C min . The rubbing process was
operated with a rubbing machine from TianLi Co. Ltd.
Fig. 1 Spatial diagram of two functional diamines. Gray, white, red and
blue of each molecular structure display C, H, O and N, respectively.
Fig. 2 Synthesis of functional diamine (DBDTA).
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 25348–25356 | 25349

View Article Online
RSC Advances
Paper
ꢀ
temperature was kept at 90 C and the reaction mixture was
stirred for 30 min. The reaction mixture was ltered when it was
hot. The solid was washed with water and methanol, and
recrystallized from glacial acetic acid to afford 10.52 g (yield:
17
1
4
5%) of light yellow crystals. H NMR (DMSO-d
6
, d), ppm: 8.34,
8.02, 7.92, 7.50 (m, 8H, ArH of biphenyl), 8.18, 7.78, 7.64 (m, 5H,
ArH).
0
2.3.3 Synthesis of 4-hydroxy-4 -nitrobiphenyl (3). 9.98 g
0
(0.03 mol) of 4-benzoyloxy-4 -nitrobipheny (2) was mixed with 60
ml of ethanol and heated to reux. A solution of 6.01 g of KOH
in 20 ml of water was added dropwise at reux. The reaction
mixture was reuxed for 30 min, and then cooled to room
temperature. The resulting purple crystals were ltered and
washed with THF to yield 8.35 g (yield: 98%) of potassium
0 0
-hydroxy-4 -nitrobiphenyl. And then 4-hydroxy-4 -nitro- Fig. 3 The H NMR spectrum (DMSO-d ) of DBDTA.
6
1
4
biphenyl could be obtained by dissolving the salt in water and
adding 50/50 HCl/H O. The yellow solid was ltered, washed
with water, and recrystallized from ethanol.
2
1
H
NMR
7.65 g crude product was obtained (yield: 85%). The crude
(
1
DMSO-d , d), ppm: 8.27–7.03 (m, 8H, ArH of biphenyl), 5.50 (bs,
6
2 2
product was puried by recrystallization from CH Cl /methanol
18
1
H, hydroxy).
as yellow crystal.
6
H NMR (DMSO-d , d), ppm: 8.21, 7.26
0
2.3.4 Synthesis of 4-dodecyloxy-4 -nitrobiphenyl (4). A
(m, 8H, ArH), 7.75, 7.64, 7.31, 7.02 (m, 8H, ArH of biphenyl),
round ask equipped with a magnetic stirrer and a condenser
4
.00 (t, 2H, –O–CH
2
2
–), 1.21–1.85 (m, 20H, –(CH )10–), 0.85
was placed with a mixture of 6.01 g (0.03 mol) of 4-hydroxy-
(
t, 3H, CH3).
0
0
00
4
3
-nitrobiphenyl (3), 7.11 g (0.03 mol) of 1-bromododecane,
2.3.7 Synthesis of 4-dodecyloxy-biphenyl-4 ,4 -diamino-
.92 g (0.03 mol) of potassium carbonate and 50.0 ml of dry
triphenylamine (7). A ask equipped with a magnetic stirrer and
a condenser was charged with a mixture of 6.03 g (0.01 mol) of
4
5
ꢀ
DMF. Then reaction mixture was stirred for 7 h at 90 C. The
reaction mixture was cooled to room temperature, and then the
mixture was poured in distilled water. The precipitate was
collected by ltration and recrystallized from acetonitrile to
0
00
-dodecyloxy-biphenyl-4 ,4 -dinitrotriphenylamine (6), 0.61 g of
% palladium on activated carbon (Pd/C) and 40 ml of isopropyl
alcohol. The mixture was heated to reux and then 4.8 ml
0.08 mol) of 80% hydrazine hydrate was added dropwise into
1
afford 10.15 g (yield: 95%) of pale-yellow crystals. H NMR
(
(
–
CDCl
O–CH
3
, d), ppm: 8.27–7.01 (m, 8H, ArH of biphenyl), 4.00 (t, 2H,
the mixture. Aer about 4 h, the reaction was stopped to allow
the mixture to room temperature, the catalyst was ltered off
and the solvent was evaporated under the reduced pressure. The
crude product was separated out by the column chromatog-
raphy (ethyl acetate : petroleum ether ¼ 10 : 3 (v/v)). At last,
2
–), 1.46–1.86 (m, 20H, –(CH
2
)
10–), 0.98 (t, 3H, CH
3
).
0
2.3.5 Synthesis of 4-dodecyloxy-4 -aminonitrobiphenyl (5).
A ask equipped with a magnetic stirrer and a condenser was
charged with a mixture of 7.73 g (0.02 mol) of 4-dodecyloxy-
4
0
-nitrobiphenyl (4), 0.77 g of 5% palladium on activated carbon
5
.26 g (yield: 95%) of the light gray solid DBDTA was obtained.
(
Pd/C) and 40 ml of isopropyl alcohol. The mixture was heated
2
The FT-IR n ¼ 3399, 3334 (–NH ), 3037 (C–H), 2919, 2848
to reux and then 4.8 ml (0.08 mol) of 80% hydrazine hydrate
was added dropwise into the mixture. Aer about 4 h, the
reaction was stopped to allow the mixture to room temperature,
the catalyst was ltered off, and then the solvent was evaporated
under the reduced pressure. The crude product was recrystal-
lized from ethanol to afford 6.53 g of white crystals (yield: 90%).
1
H NMR (CDCl , d), ppm: 7.45–6.75 (m, 8H, ArH of biphenyl),
3
4
–
.00 (s, 2H, NH
(CH 10–), 0.98 (t, 3H, CH
.3.6 Synthesis of 4-dodecyloxy-biphenyl-4 ,4 -dinitro-
2
), 3.96 (t, 2H, –O–CH
2
–), 1.29–1.83 (m, 20H,
2
)
3
).
0
00
2
triphenylamine (6). A three-necked ask equipped with a
magnetic stirrer, a nitrogen inlet and a distillation head was
charged with a mixture of DMSO (40 ml), p-uoronitrobenzene
(
4.23 g, 0.03 mol) and cesium uoride (CsF) (4.93 g, 0.03 mol),
ꢀ
heated and stirred at 150 C. Then a solution of 5.46 g (0.02 mol)
4
0
-dodecyloxy-4 -aminonitrobiphenyl (5) in 40 ml of DMSO was
added dropwise. The reaction mixture was heated and stirred at
ꢀ
1
50 C for 10 h. The reaction mixture was cooled and poured
into a water–methanol (water : methanol ¼ 1 : 1 (v/v)). The
yellow precipitate was ltered, washed with water and dried. Fig. 4 Synthetic route of the polymers.
25350 | RSC Adv., 2015, 5, 25348–25356
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
(
(
(
(
2
–CH –), 1603, 1500 (aromatics), 1326 (C–N–C), 1267, 1173
C–O–C) and the H NMR spectrum was given in Fig. 3. H NMR
1
1
DMSO-d , d), ppm: 6.82, 6.54 (m, 8H, ArH), 7.45, 7.31, 6.91, 6.64
6
m, 8H, ArH of biphenyl), 4.98 (s, 4H, NH ), 3.90 (t, 2H,
2
–
2 2 3
O–CH –), 1.23–1.83 (m, 20H, –(CH )10–), 0.85 (t, 3H, CH ).
2.4 Representative procedure for the preparation of PIs
PIs were synthesized via one-step method (as shown in Fig. 4). A
typical polymerization procedure was as follows: the diamine
DBDTA (0.054 g, 0.1 mmol), ODPA (0.310 g, 1.0 mmol) and
o-dichlorobenzene (10.0 ml) were added to a three-necked ask
equipped with a stirrer, a nitrogen inlet and a distillation head.
Isoquinoline (0.12 g) was added to the ask. Aer stirring at
ꢀ
ꢀ
8
0 C for 1 h, the solution was heated to 180 C and kept stirring
for 2 h. When the mixture was cooled to room temperature,
DMMDA (0.203 g, 0.9 mmol) was added. The mixture was
reacted at 80 C for 1 h, and then at 180 C for 24 h. The
1
6 2
Fig. 5 The H NMR spectrum (DMSO-d ) of PI -BZA.
ꢀ
ꢀ
resulting PI solution was poured into a large amount of ethanol work has speculated that the m-cresol may act as a nucleophile
with vigorous stirring. The precipitated PI was collected by at the elevated temperature and displaced the carboxylate group
ꢀ
19

ltration and then dried at 80 C for 24 h under reduced pres- from the benzoate position. Thus, the less polar o-dichloro-
sure. The yield was 90–95%. Molar ratios of DBDTA to DMMDA benzene was used as the solvent in the synthesis of the PIs. In
were controlled to 1 : 9 (PI
1
-TPA), 2 : 8 (PI
2
-TPA), and 3 : 7 this case, the PIs remained in solution and no side chain
(
1
PI
: 9 (PI
3
-TPA). Molar ratios of DBPDA to DMMDA were controlled to cleavage occurred.
1
1
-BZA), 2 : 8 (PI
2
-BZA), and 3 : 7 (PI
3
-BZA).
The PI lm samples were used for characterization by H
1
NMR and FT-IR spectroscopy. H NMR spectra of PI
2
-BZA and
2.5 Preparation of lms and assembling of cells
2
PI -TPA were shown in Fig. 5 and 6, respectively. The multiple
peaks at 8.31–7.02 and 8.09–6.85 ppm are the characteristic
2
The glass substrates were cut into 3.0 ꢂ 3.0 cm pieces. The PI
peaks of the phenyl ring protons of the PI -BZA and PI -TPA,
2
2
solutions (5 wt% in NMP) were spin-coated onto indium tin
oxide (ITO) glass substrates at 600 rpm for 15 s and 2500 rpm for
0 s. The PI lms were baked at 120 C for 2 h. Then the PI lm
was rubbed by a rubbing machine covered with a cotton velvet
respectively. The single peak at 3.96 ppm is the characteristic
peaks of methylene protons of the unit of DMMDA and alkyl
ether. The single peak at 2.11 ppm is characteristic peaks of
methyl protons of the unit of DMMDA. The multiple peaks at
ꢀ
3
cloth. The denition of the rubbing strength, RS was given by
1
.78–0.89 and 1.75–0.84 ppm are the characteristic peaks of the
alkyl chain protons of the PI -BZA and PI -TPA, respectively.
Fig. 7 showed the FT-IR spectra of PI -TPA and PI -BZA lms.
2
2
RS ¼ NM(2prn/60v ꢁ 1)
2
2
Both spectra clearly showed the characteristic imide bands in
where N is the number of times the substrates were rubbed
ꢁ1
the vicinity of 1776, 1723 and 1375 cm and C–H stretching
(
N ¼ 1 in this work), M is the depth of the bers of the fabric
ꢁ
1
vibrations of aliphatic groups in 2924, 2850 cm
.
deformed due to the pressed contact (0.3 mm), n is the rota-
tional speed of the drum (700 rpm), v is the translational speed
ꢁ
1
of the substrate (17.2 mm s ) and r is the radius of the roller
(22.5 mm).
LC cells were fabricated with two pieces of the rubbed
substrates in an anti-parallel rubbing direction using a 40 mm
thick spacer. In addition, some LC cells were manufactured
with two pieces of the non-rubbed substrates. A nematic LC
(5CB) was injected into the cell gap, followed by sealing of the
injection hole with epoxy glue. The alignment behaviors of LC
cells were examined through the whole cell by polarizing
microscope. The pretilt angles of the LC cells were determined
using a crystal rotation method.
3. Results and discussion
3.1 Characterization of PI structures
Traditionally, m-cresol was used as the polymerization solvent
in the one-step method of PI polymerization. However, early Fig. 6 The H NMR spectrum (DMSO-d
1
6
) of PI
2
-TPA.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 25348–25356 | 25351

View Article Online
RSC Advances
Paper
a
Table 1 Solubility of the PIs in different solvents
PI
NMP DMSO DMF DMAc m-Cresol TCM THF
PI
1
PI
2
PI
3
PI
1
PI
2
PI
3
-TPA ++
-TPA ++
-TPA ++
-BZA ++
-BZA ++
-BZA ++
++
++
++
++
++
++
++
++
++
+
++
++
+
+
++
++
++
+
++
++
+
++
++
+ꢁ
++
++
++
++
+ꢁ
++
++
++
++
+ꢁ
++
++
a
ꢀ
(
++) soluble at room temperature, (+) soluble upon heating at 60 C,
ꢀ
(+ꢁ) partial soluble upon heating at 60 C.
Fig. 7 FT-IR spectra of PI
2
-TPA and PI
2
-BZA film.
3.3 Solubility of PIs
The solubility of the polymer was listed in Table 1. It could be
found that PI
solvents, however, the PI
1
-TPA and PI
1
-BZA were partly soluble in the polar
-TPA, PI -BZA, PI -TPA and PI -BZA
3.2 UV-vis transmission of PIs
2
2
3
3
showed good solubility. They could not only easily dissolve in
strong polar aprotic solvents, such as NMP, DMAc, DMSO, DMF,
m-cresol, but also dissolve in low-boiling-point solvents, such as
THF, TCM. It can be attributed to the presence of the long side
groups, the pendant methyl group and the exible ether in the
polymer backbone which inhibit close packing and reduce the
The PI resin was dissolved in NMP with stirring to give a
homogeneous PI solution with a solid content of 10%. The PI
solution was ltered by a 0.22 mm Teon syringe lter and then
spread onto a glass plate which was then thermally baked in an
ꢀ
ꢀ
ꢀ
ꢀ
oven at 80 C for 2 h, 120 C for 1 h, 150 C for 1 h and 180 C for
1
h. The PI lm was obtained by immersion the glass plate in
20
ꢀ
chain–chain interaction to enhance solubility. Comparing
PI -TPA with PI -BZA, it was found that the solubility of PI -TPA
was better than that of PI -BZA. The excellent solubility of
water followed by drying in an oven at 100 C. The UV-vis spectra
of the PI lms whose thickness is 20 mm were shown in Fig. 8.
The PIs-TPA displayed better transparent than PIs-BZA and
showed high transmissions (above 80%) in the wavelength
range 500–800 nm. The good optical transparency and lighter
color of the PI lms resulted from the decrease in the inter-
molecular charge transfer complex (CTC) interaction and elec-
tron conjugation due to introduction the side group moieties
1
1
1
1
PI -TPA could be attributed to the existence of the twisted three-
1
dimensional TPA structure that limited the chain packing,
increased the free volume and caused solvent molecules to
penetrate into the polymer chains.
and the pendant methyl group. Besides, the existence of ether 3.4 Thermal properties of PIs
units (–O–) and aliphatic units (–CH –) in the PI backbone
2
Glass transitions temperature (T ) of PIs was listed in Table 2.
g
reduces intramolecular CTC interactions and improves optical
transmittance. Furthermore, the rigid, twisted and non-
coplanar triphenylamine (TPA) structures in the PI main
chain can efficiently hinder the main chain dense packing and
reduce intermolecular CTC interactions. Consequently, the PIs-
TPA derived from DBDTA exhibited more excellent transparency
than the PIs-BZA from DBPDA.
g
The T decreased with the increase in the content of functional
diamine. Because the introduction of bulky side groups reduced
intermolecular interactions and chain dense packing, the free
21
g
volume in polymer systems increased and thus T decreased.
Comparing PI with the same content of side chains, it could be
found that T of PI containing DBDTA was lower than that of the
g
PI containing DBPDA. The reason is that the introduction of
non-coplanar TPA structure in the PI main chain would increase
the free volume in polymer system.
Table 2 Thermal properties of the PIs
a
ꢀ
b
d
ꢀ
b
ꢀ
b
ꢀ
c
PI
T
g
( C)
T
( C)
T
5
( C)
T
10 ( C)
R800 (%)
PI
1
PI
2
PI
3
PI
1
PI
2
PI
3
-TPA
-TPA
-TPA
-BZA
-BZA
-BZA
240
236
227
251
242
231
384
373
365
372
369
354
493
464
423
474
451
418
523
496
471
514
488
466
60.5
55.9
38.9
60.2
45.8
36.1
a
b
T
g
,
glass transition temperature.
T
d
,
the onset thermal
decomposition temperature of the PIs. T and T10 are the temperature
5
c
at a 5% and 10% weight loss of the PIs, respectively.
percentage at 800 C from TGA measurements.
R800, weight
ꢀ
Fig. 8 The UV-vis spectra of the PI films (film thickness: 20 mm).
25352 | RSC Adv., 2015, 5, 25348–25356
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
functional diamine, the density of side chains increased and the
interaction of the side chains enhanced at the same time. The
enhancement in interaction of the side chains increased the
rigidity of the side chains and restricted its incline trend along
the rubbing direction. Aer rubbing process, PI
induce LC vertical alignment, but PI -BZA only induced LC
parallel alignment. It is suggested that PI -TPA lm is more
resistance to rubbing process than PI -BZA.
.5.2 Alignment behavior. The alignment behaviors of the
1
-TPA could still
1
1
1
3
LC cells between crossed polarizers were examined with a
polarizing optical microscope. As shown in Fig. 10a, an alter-
nate and periodical change of transmittance with the rotation of
the substrate stage could be observed, meaning that the LC
Fig. 9 The TGA curves of the PIs.
molecules in cells using the rubbed PI
to the PI surface. Fig. 10b showed that the transmittance of the
LC cells using the rubbed PI -TPA with a rotation of the sample
1
-BZA were aligned planar
1
Thermal resistance was studied by TGA in N . As shown in
2
stage gave no rise to an alternate and periodic appearance of
darkness and brightness, meaning that the LC molecules are
aligned vertical to the PI surface. Conoscope observation of the
cells was used with polarized optical microscope. As shown in
Fig. 10b, dark crossed brush was seen clearly and not moved
with the LC cell rotating before and aer rubbing. The result
Fig. 9, all PIs showed two-step pyrolysis behavior with
increasing temperature. This behavior may reasonably be
presumed to arise from the fact that side chains are degraded in
the lower-temperature range and the main chains are degraded
22
in the higher-temperature range. Table 2 showed the thermal
properties of the PIs with T , T and T10. With an increase of the
side chains content, the gradual decrease in T , T and T10
d
5
further proved the vertical alignment was induced by PI
The same result could be obtained by PI -TPA, PI -TPA, PI
and PI -BZA.
.5.3 Surface morphology. Surface morphology of PI -TPA
1
-TPA.
d
5
2
3
2
-BZA
indicated that thermal stability of PI was greatly affected by the
side chain. Comparing PI containing the same content of side
chains, it could be found that thermal stability of PIs-TPA with
DBDTA was higher than that of PIs-BZA with DBPDA. The
reason is that the PI with DBDTA side chain contained more
3
3
1
and PI -BZA lms was measured by AFM before and aer
1
rubbing treatment. Parts A and B of Fig. 11 were shown AFM
images of pristine PI
PI -TPA and PI -BZA lms were found to have a root-mean-
square (RMS) roughness of 0.35 and 0.33 nm over an area of
1 1
-TPA and PI -BZA lms, respectively. The
benzene rings, thus, higher values of T
Furthermore, the PIs-TPA had higher T values than the corre-
d
were observed.
1
1
5
sponding PIs-BZA, which could be attributed to the fact that less
of the overall weight of each PIs than its corresponding PIs-BZA
derived from alkoxy units. The introduction of bulky, twisted
TPA group into DBDTA increased the rigidity of the backbone of
PIs; thus, higher values of T10 were observed.
2
2
.5 ꢂ 2.5 mm , respectively. The surface morphology and
roughness of the PI lm derive mainly from the characteristics
of the polymer chains that govern the aggregation and molec-
ular ordering that occur during the drying and thermal imid-
9
ization processes aer spin-casting. In contrast, the rubbed PI

lms displayed completely different morphology. As can be
3.5 Alignment ability and pretilt angle of LCs
3
.5.1 Pretilt angle. As shown in Table 3, all PIs induced
ꢀ
pretilt angle of 90 before rubbing process and uniform align-
ment of LCs. But aer rubbing process, PIs with different
composition of functional diamine could induce different pre-
tilt angles of LCs. The pretilt angles increased with the content
of functional diamine. With an increase in the content of the
Table 3 Pretilt angle of the PIs
ꢀ
Pretilt angle ( )
PI
Before rubbing
Aer rubbing
PI
1
PI
1
PI
2
PI
2
PI
3
PI
3
-TPA
-BZA
-TPA
-BZA
-TPA
-BZA
90
90
90
90
90
90
89.5
1.8
89.7
89.6
89.8
89.8
Fig. 10 The optical micrographs on the left hand side were taken for
LC cells using the rubbed PI -BZA (a) PI -TPA (b), the corresponding
1
1
optical micrographs on the right hand side were taken for them after
ꢀ
rotating the stage by 45 .
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 25348–25356 | 25353

View Article Online
RSC Advances
Paper
Fig. 11 AFM images and surface profiles of PI films before and after rubbing: (A) and (a) pristine PI
1
-TPA PI; (B) and (b) pristine PI
1
-BZA PI; (C) and
(
1 1
c) rubbed PI -TPA PI; (D) and (d) rubbed PI -BZA PI. The arrows in the images (C) and (D) denote the rubbing direction, respectively.
seen in Fig. 11C and D, both of the rubbed PI lms exhibited parallel to the biphenyl groups of the side chains in the PI
23,24
groove-like structures and covered with sub-micrometer-scaled lms.
Therefore, it is believed that the molecular chain
spikes along the rubbing direction. The microgroove lines reorientation, especially biphenyl groups and alkoxy tails, had a
that were generated might be due to the shear-induced defor- greater inuence on the LC aligning ability of the PI lm
mation of the polymer lm surface induced by the contact of surfaces than do the microgrooves. Aer rubbing process, PI
bers during the rubbing process. The spikes might be due to TPA could still induce LC vertical alignment, but PI -BZA only
some degree of abrasion accompany the shear-induced PI induced LC parallel alignment. The reason was that the rigid
deformation of the rubbing process. The developed micro- biphenyl groups in the side chain of PI -TPA were directly
grooves of the rubbed PI -TPA and PI -BZA lms were around 35 linked to the backbone, which could restrain the lodging of the
1
-

1
1
1
1
nm or larger in size. The RMS surface roughness values of the biphenyl group of the side chain in the rubbing process.
rubbed PI -TPA and PI -BZA lms were 0.94 and 1.37 nm over However, the side chain of PI -BZA containing exible ester
1
1
1
2
an area of 2.5 ꢂ 2.5 mm , respectively.
First, the LC molecules on the PI
with side chains containing rigid biphenyl groups could achieve
vertical alignment without rubbing process. Second, although 3.6 Molecular structures and molecular orbital energies
the PI -TPA lm had different morphologies before and aer
rubbing process, they could induce LC molecules vertical
alignment. Third, both of the rubbed PI -TPA and PI -BZA lms
had the similar microgrooves, but they induced different
alignment behaviors of LC molecules. Finally, the microgrooves
developed along the rubbing direction are much larger in size
than the LC molecules, which have dimensions of ca. 1.8 nm
bond was easy to lodge along the rubbing direction when the
1
1
-TPA and PI -BZA lms density of side chain was low.
1
It is well known that the materials properties of PI alignment
lms are strongly depended on their molecular structure. In

1
1
order to get a comprehensive understanding of the relationship
between optical and rubbing resistance properties and polymer
structure, the three-dimensional molecular structures of the
model compounds for PIs were investigated. The structure
geometries were optimized by the density functional theory
(
length) and 0.25 nm (diameter). The dimensions of these
[
B3LYP/6-311G (d,p)] calculations using a large basis-set func-
surface structures seem to be large to effectively interact with
and align LC molecules. These facts and the observed LC
alignment suggested that the LC molecules are 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 existing
between LC molecules and the side chains of PIs, such as the
dipole–dipole and p–p interaction between the biphenyl groups
of LC molecules and the biphenyl groups of side chains, and the
van der Waals forces between the alkoxy end-group of PI and the
25
tion.
The molecular orbital energies of these model
compounds were determined by molecular orbital (MO)
calculation.
Fig. 12 depicted the optimized molecular structures of the
model compounds for PI
calculated molecular orbital energies were listed in Table 4, it is
found that the PI -TPA exhibited the much higher 3LUMO values
and smaller D3 values than PI -BZA. In general, the large energy
1 1
-TPA and PI -BZA. Meanwhile, the
1
1
gap is favorable for preventing the CTC formation. However, the
optical transparency of PI -TPA is higher than PI -BZA. It is
presumed that the incorporation of the non-coplanar TPA
1
1

exible alkoxy tail in an LC molecules. Due to the strong mutual
interactions, LC molecules tend to be aligned in the direction
structure in PI
1
-TPA backbone no longer allows to dense chain
25354 | RSC Adv., 2015, 5, 25348–25356
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Fig. 12 Molecular structure of the model compounds for PI
1 1
-BZA and PI -TPA. Gray, white, red and blue color unit display C, H, O and N atom,
respectively.
Table 4 Molecular orbital energies of HOMO and LUMO as well as suggested that PI
-TPA lm is more resistance to rubbing
1
-BZA.
1
energy gaps of aromatic PIs
process than PI
a
b
c
PIs
3
LUMO (eV)
3
HOMO (eV)
D3 (eV)
4
. Conclusions
PI
PI
1
-TPA
-BZA
ꢁ3.812
ꢁ3.844
ꢁ4.710
ꢁ5.097
0.898
1.253
1
A novel functional diamine DBDTA containing TPA, biphenyl
and long alkyl chain was synthesized and characterized. A series
of PIs were prepared based on ODPA, DMMDA and different
contents of DBDTA or DBPDA. The PIs with DBDTA
exhibited high thermal stability with T
a
b
3
LUMO: the lowest unoccupied molecular orbital energy.
3
HOMO: the
c
highest occupied molecular orbital energy. D3: energy gap, D3 ¼
3LUMO ꢁ 3HOMO
.
ꢀ
g
of 227–240 C and T
5
of
ꢀ
423–493 C, respectively. The PIs-TPA with DBDTA displayed
stacking, which decreases inter-molecular CTC interactions. In
addition, as shown in Fig. 12, the dihedral angle between
benzene ring and imide ring of PI
better solubility and transparency than that with DBPDA.
The PIs showed high pretilt angle and good uniform vertical
alignment without rubbing. When the rubbing was performed,
1
-TPA and PI
1
-BZA are 46.9 and
-TPA is
ꢀ
35.3 , respectively. It is suggested that the backbone of PI
1
PI -BZA showed parallel alignment and the rest of PIs showed
1
more twisted, which is capable of efficiently hindering the chain
packing and decreasing electron conjugation and intra-
molecular CTC interactions. It is further supported that PI
has better solubility and optical transparency than PI -BZA.
vertical alignment. The different alignment of LC induced by
rubbed PI -TPA and PI -BZA with the same content of side
1 1
chains was attributed to the distorted backbone and the xed
torsion angle of C–N bond in TPA, which restrained the move-
1
ment of the biphenyl group in PI -TPA lm. It provides a
method of designing molecular structure for LC vertical align-
ment lm with good solubility, high thermal stability and
rubbing resistance.
1
-TPA
1
As shown in Fig. 12, the torsion angle of C–N bond in TPA
structure of the model compound for PI -TPA is about 13.6, 68.7
and 14.5 . The torsion angle of C–N bond in TPA structure
limits the movement of the biphenyl group, so that biphenyl
group always aligned perpendicular to the plane of alignment
1
ꢀ

lm, thereby inducing the vertical alignment of the LC mole-
cules aer rubbing. However, biphenyl group of the side chain
Acknowledgements
in PI -BZA can be rotated around the ester and the bond angle is
1
ꢀ
19.3 . Thus, biphenyl group of the side chain in PI -BZA can be This work was supported by National Natural Science Founda-
1
1
easily reoriented along the rubbing direction, which resulted in tion of China (Grant no. 51173115), the Ministry of Education
change of the conformation of biphenyl group of the PI thin (the Foundation for Ph.D. training, Grant no. 20110181110030)
24,26

lms causing the LC molecules parallel alignment.
The alkyl of China.
side chains of the PI
1
-BZA lm surface, which are absence of
biphenyl rigid support, are prone to lodge and re-orientate
along the rubbing direction inducing LC parallel alignment.
15
References
In short, aer rubbing, the vertical alignment of PI -TPA is
1
1
X. Y. Chen, V. T. Hoang, D. Rodrigue and S. Kaliaguine, RSC
Adv., 2013, 3, 24266.
X. Y. Chen, V. T. Hoang, D. Rodrigue and S. Kaliaguine, RSC
Adv., 2014, 4, 12235.
attributed to the distorted backbone and the xed torsion angle
of C–N bond in TPA structure, which restrains the movement of
the biphenyl group of the side chain in the rubbing process. It is
2
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 25348–25356 | 25355

View Article Online
RSC Advances
Paper
3
4
H. Shima, M. M. Hossain and J. R. Hahn, RSC Adv., 2014, 4, 15 Y. J. Lee, J. G. Choi, I. Song, J. M. Oh and M. H. Yi, Polymer,
1204. 2006, 47, 1555.
S. Ramakrishnan, M. Dhakshnamoorthy, E. J. Jelmy, 16 Z. J. Liu, F. F. Yu, Q. Zhang, Y. Zeng and Y. Wang, Eur. Polym.
4
R. Vasanthakumari and N. K. Kothurkar, RSC Adv., 2014, 4,
743.
Y. Niu, X. Zhang, J. Zhao, Y. Tian, Y. Li and X. Yan, RSC Adv.,
014, 4, 28456.
H. J. Yen, C. L. Tsai, P. H. Wang, J. J. Lin and G. S. Liou, RSC
Adv., 2013, 3, 17048.
Y. Q. Fang, J. Wang, Q. Zhang, Y. Zeng and Y. H. Wang, Eur.
Polym. J., 2010, 46, 1163.
X. Wang, H. Wang, L. Luo, J. Huang, J. Gao and X. Liu, RSC
Adv., 2012, 2, 9463.
J., 2008, 44, 2718.
17 S. H. Ou, V. Percec, J. A. Mann and J. B. Lando, Langmuir,
1994, 10, 905.
18 S. H. Hsiao, G. S. Liou, Y. C. Kung, H. Y. Pan and C. H. Kuo,
Eur. Polym. J., 2009, 45, 2234.
19 D. H. Wang, Z. Shen, M. Guo, S. Z. D. Cheng and
F. W. Harris, Macromolecules, 2000, 40, 889.
20 C. F. Chen, W. M. Qin and X. A. Huang, Polym. Eng. Sci.,
2008, 10, 1151.
9
5
6
7
8
9
2
21 L. Wang, P. Chang and C. L. Cheng, J. Appl. Polym. Sci., 2006,
100, 4672.
B. Chae, S. B. Kim, S. W. Lee, S. I. Kim, W. Choi, B. Lee,
M. Ree, K. H. Lee and J. C. Jung, Macromolecules, 2002, 35, 22 J. Y. Hwang, K. J. Lee, D. S. Seo and T. H. Kim, Jpn. J. Appl.
10119.
Phys., 2003, 42, L6725.
1
1
0 Z. Sun, M. Liu, L. Yi and Y. Wang, RSC Adv., 2013, 3, 7271.
1 P. M. Heregenrother and S. J. Havens, Macromolecules, 1994,
23 X. Wang, P. Zhang, Y. Chen, L. Luo, Y. Pang and X. Liu,
Macromolecules, 2011, 44, 9731.
2
7, 4659.
2 C. P. Yang, Y. Y. Su, S. J. Wen and S. H. Hsiao, Polymer, 2006,
7, 7021.
24 S. B. Lee, G. J. Shin, J. H. Chi, W. C. Zin, J. C. Jung,
S. G. Hahm, M. Ree and T. Chang, Polymer, 2006, 47, 66061.
25 K. Takizawa, J. Wakita, M. Kakiage, H. Masunaga and
S. Ando, Macromolecules, 2010, 43, 2115.
26 S. W. Lee, S. J. Lee, S. G. Hahm, T. J. Lee, B. Lee, B. Chae,
S. B. Kim, J. C. Jung, W. C. Zin, B. H. Sohn and M. Ree,
Macromolecules, 2005, 38, 4331.
1
1
1
4
3 Y. T. Chenrn, J. Y. Tsai and J. J. Wang, J. Polym. Sci., Part A:
Polym. Chem., 2009, 47, 2443.
4 S. Xia, Z. Sun, L. Yi and Y. Wang, RSC Adv., 2013, 3, 14661.
25356 | RSC Adv., 2015, 5, 25348–25356
This journal is © The Royal Society of Chemistry 2015