Synthesis of soluble polyimide derived from novel naphthalene diamines for liquid crystal alignment layers and a preliminary study on the mechanism of imidization

Article abstract of DOI:10.1039/c3ra42098a

Novel functional diamine, 6-hexyloxy-naphthalen-3′,5′- diaminobenzoate (N6) containing a rigid naphthalene unit, was molecularly designed and successfully synthesized. PIs (polyimides) were obtained by copolymerization of N6, 3,3′-dimethyl-4,4′-methylenediamine (DMMDA) and 4,4′-oxydiphthalic anhydride (ODPA). The structures of the intermediates, diamines and PIs were confirmed by FT-IR and ¹H NMR spectra. All PIs obtained could be dissolved in polar aprotic solvents and low-boiling-point solvents. PI (polyimide) films attained using a casting method showed favorably high transmittance above 95% in the wave length range of 400-700 nm and could align LCs vertically before and after rubbing treatment. PI-N6 derived from N6, DMMDA and ODPA exhibited a much higher temperature at a 5% weight loss (T₅) compared with the corresponding PI-C6 from 4-hexyloxy-biphenyl-3′,5′-diaminobenzoate (C6). For PI-N6, the weight ratio of the side chains was smaller than that of PI-C6, but a much higher T₅ was attained. The results demonstrated that the introduction of a naphthalene unit into the side chain could effectively improve the thermal stability of PI without sacrificing its solubility. Moreover, the outstanding thermal stability of the PIs was explained in a preliminary manner by the imidization reaction mechanism. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra42098a

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Synthesis of soluble polyimide derived from novel
naphthalene diamines for liquid crystal alignment
layers and a preliminary study on the mechanism of
imidization
Cite this: DOI: 10.1039/c3ra42098a
Senlin Xia, Zhen Sun, Longfei Yi and Yinghan Wang*
Novel functional diamine, 6-hexyloxy-naphthalen-39,59-diaminobenzoate (N6) containing a rigid naphtha-
lene unit, was molecularly designed and successfully synthesized. PIs (polyimides) were obtained by
copolymerization of N6, 3,39-dimethyl-4,49-methylenediamine (DMMDA) and 4,49-oxydiphthalic anhydride
(ODPA). The structures of the intermediates, diamines and PIs were confirmed by FT-IR and 1H NMR
spectra. All PIs obtained could be dissolved in polar aprotic solvents and low-boiling-point solvents. PI
(polyimide) films attained using a casting method showed favorably high transmittance above 95% in the
wave length range of 400–700 nm and could align LCs vertically before and after rubbing treatment. PI–
N6 derived from N6, DMMDA and ODPA exhibited a much higher temperature at a 5% weight loss (T5)
compared with the corresponding PI–C6 from 4-hexyloxy-biphenyl-39,59-diaminobenzoate (C6). For PI–N6,
the weight ratio of the side chains was smaller than that of PI–C6, but a much higher T5 was attained. The
results demonstrated that the introduction of a naphthalene unit into the side chain could effectively
improve the thermal stability of PI without sacrificing its solubility. Moreover, the outstanding thermal
stability of the PIs was explained in a preliminary manner by the imidization reaction mechanism.
Received 29th April 2013,
Accepted 5th June 2013
DOI: 10.1039/c3ra42098a
www.rsc.org/advances
groups with alicyclic groups to form the so-called alicyclic PIs
are by far the most effective approaches.13–16
1. Introduction
Aromatic polyimides (PIs) are considered to be relevant
materials for advanced technologies because of their out-
standing thermal, mechanical, and chemical stability.1,2
However, it is known that fully aromatic PIs are very often
insoluble in most organic solvents because of the rigidity of
their backbone and strong interchain interactions, which
greatly limits their application in many fields such as
microelectronics and liquid crystal displays (LCD).3 The
increasing demands of this application have stimulated
extensive research on soluble PIs that can exhibit certain
advantageous properties. Typical approaches include the
introduction of flexible or kinked linkages,4,5 bulky substitu-
ents,6–8 noncoplanar structures,9,10 and spiro-skeletons11,12
into the polymer backbone.
To attain good quality LCD devices, many factors should be
taken into account such as the viewing angle, optical contrast,
response time, and LC alignment ability in addition to the
solubility and optical transparency. However, the pretilt angle
plays the most important role in determining the optical and
electrical performance of industrial LCD devices. The conven-
tional main-chain-type PI alignment layer can achieve only a
small pretilt angle.17,18 PIs that induce a high pretilt angle are
needed for specially tailored display applications. For example,
a vertical alignment method has been used to improve the
alignment of the LCs with negative dielectric anisotropy for a
faster response time and a higher contrast ratio compared
with those of twisted nematic LCDs.19 Some recent reports
have pointed out that alkyl side chains greatly elevate the
pretilt angle and that alkyl side-chain PIs could be very
promising candidates for excellent LC alignment layers.17,20–24
As widely reported, the long alkyl side chain with more than 6
carbon atoms is well known for its positive effect on the LC
alignment and the attainment of a high pretilt angle.25 Those
materials that induce a high pretilt angle often contain side
chains composed of rigid units and alkyl groups that tend to
stick out of the surface of the PI.26–28 However, the existence of
side chains unfortunately sacrifices the thermal stability of
For PI materials used as alignment layers in the field of
LCD devices, high transparency is also required in addition to
easy processability. To improve the optical transparency of PIs,
methods such as the introduction of the fluorinated groups
and a twisted structure, and the replacement of aromatic
College of Polymer Science and Engineering, State Key Laboratory of Polymer
Materials Engineering, Sichuan University, Chengdu 610065, China.
E-mail: wang_yh@scu.edu.cn; Tel: 028-85460823
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soluble PIs due to the loose chain-packing and week
interchain interactions resulting from the introduction of
long side chains. One of the most significant ideas is that the
thermal stability of soluble PIs can be improved by adjusting
the chemical structure of side chain, while a high pretilt angle
is achieved simultaneously.
Our previous work has been mainly focused on the
solubility of PIs and high pretilt angles generated by the PI
film without serious consideration of its lowered thermal
stability.26 It is well-known that materials used in LCD devices
should be thermally resistant because LCD devices usually
generate a great amount of heat while operating. However, to
the best of our knowledge, the thermal stability of soluble PIs
which can align LCs vertically has not been carefully studied.
So this area of research is worth being paid enough attention
to and carried out.
The objectives of this research were to synthesize organo-
soluble PIs having improved thermal stability and high pretilt
angles at the same time, and to preliminarily investigate the
mechanism of imidization. We focused on a specially designed
novel PI containing naphthalene side chains as the alignment
layer having a high pretilt angle. Properties such as solubility,
optical transparency, surface energy and pretilt angle were
investigated. The focus has especially been on the improved
thermal stability, and the results have been compared with our
previous studies.26 In addition, the favorable thermal stability
of the PIs was preliminarily explained by the imidization
reaction mechanism. The novel PIs obtained will hopefully be
applied in vertical-alignment-mode LCD devices.
2.2 Measurement
Nuclear magnetic resonance (1H NMR) spectra were obtained
using a 400 MHz Unity Inova 400 spectrophotometer using
CDCl3 or DMSO as a solvent. Fourier transform infrared (FTIR)
spectra were recorded with a Nicolet 560 FTIR spectrometer
(Thermo Nicolet Corporation, USA). A spin-coat process was
executed using a KW-4A spinner from the Institute of
Microelectronics of the Chinese Academy of Sciences
(Beijing, China). The pretilt angles of the LCs were measured
using the crystal rotation method using a pretilt angle tester
from the Changchun Institute of Optics, Fine Mechanics and
Physics (Changchun, China), and at least five different points
on cells were selected for measurement. The rubbing process
was operated using a rubbing machine from TianLi Co. Ltd.
(Chengdu, China). Thermogravimetric Analysis (TGA) was
performed using a DuPont TGA2100 at a heating rate of 10
uC min21 under a nitrogen atmosphere. Differential scanning
calorimetry (DSC) testing was performed on a PerkingElmer
DSC7 differential scanning calorimeter at a scanning rate of 10
uC min21 under flowing nitrogen (30 cm3 min21), and the
glass-transition temperatures (Tgs) were read from the DSC
curves at the same time. The contact angles of deionized water
and diiodomethane on the surface of PI films were measured
using a Kruss DSA100 goniometer system (Kruss GmbH,
Germany). Each sample was tested three times and the mean
values of contact angle as well as surface free energy were
calculated. The inherent viscosity of the polymer was
measured at a concentration of 0.5 g dL21 in NMP using an
Ubbelohde-type viscometer at 30 uC. Gel permeation chroma-
tography of soluble polymers was performed using an Applied
Bio system at 70 uC with two PL gel 5 mm mixed-C columns.
2.3 Synthesis of the monomers
2. Experimental
2.3.1 Synthesis of 6-hexyloxy-2-hydronaphthalen (a).
A
mixture of 200 mL of methanol, 9.12 g (0.057 mol) of
naphthalene-2,6-diol and 5.00 g (0.089 mol) of potassium
hydroxide was placed into a 500 mL flask equipped with a
magnetic stirrer and a condenser. The mixture was stirred
until a complete dissolution was achieved and heated to reflux
with an oil bath, and then 4.2 mL of 1-bromopentane was
added dropwise into the mixed solution. After refluxing for 24
h, the resulting mixture containing the precipitate KBr was
filtered off using a Buchner funnel after a transitory cooling.
Subsequently the filtrate was poured into a large amount of
water, the white solid precipitate was filtered off and washed
with 5% dilute hydrochloric acid and then with distilled water,
then dried in an oven to yield crude compound a. The purified
compound a (mp: 147–149 uC, yield: 41%) was obtained by
recrystallization from ethanol.
2.1 Materials
4,49-Oxydiphthalic anhydride (ODPA; .98%, Shanghai
Research Institute of Synthetic Resins), was purified by
recrystallization from acetic anhydride and dried under
vacuum prior to use. 3,39-Dimethyl-4,49-methylenediamine
(DMMDA) was purchased from Shanghai EMST Corp.
(Shanghai, China) and purified by recrystallization from
ethanol. Naphthalene-2,6-diol was acquired from Shanghai
Jingsai chemical Corp. (Shanghai, China). 1-Bromopentane
was obtained from Shanghai Reagent Co. Ltd. (Shanghai,
China). 3,5-Dinitrobenzoyl chloride was attained from Taixing
Re-fined Chemical Co. Ltd. (Jiangsu, China) and 5% palladium
activated carbon was gained from Guoyao Corp. (Shanghai,
China). Triethylamine (TEA) was obtained from Chengdu
Kelong Chemical Reagent Corp. (Chengdu, China). N-Methyl-
2-pyrrolidone (NMP) was distilled under reduced pressure
after stirring over calcium hydride for 24 h and stored over 4 Å
molecular sieves before use. Dimethylsulfoxide (DMSO),
dimethylformamide (DMF), dimethylacetamide (DMAc), tetra-
hydrofuran (THF), chloroform (TCM) and m-cresol were used
as received. Nematic LC E7 (no = 1.521, Dn = 0.22, Tn21 = 60 uC)
was purchased from Shijiazhuang Crown Chem-Tech. Co. Ltd.
FT-IR (KBr, pellet), cm21: 3416 (O–H); 2938 (C–H); 1607,
1
1501 (aromatics); 1248, 1177 (C–O–C). H NMR (CDCl3), ppm:
7.43–6.84 (m, 6H, Ar–H of naphthalene); 5.18 (s, 1H, hydroxy);
4.00 (m, 2H, –CH2–O–); 1.38–1.81 (m, 8H, –(CH2)4–); 0.95 (t,
3H, methyl).
2.3.2 Synthesis of 6-(hexyloxy)naphthalen-2-yl 3,5-dinitro-
benzoate (b). THF (20 mL), triethylamine (1.6 mL) and
6-hexyloxy-2-hydronaphthalen (4.48 g, 0.02 mol) were added
to a 100 mL flask placed in an ice bath. After the mixture was
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cooled down to 0 uC, a solution of 3,5-dinitrobenzoyl chloride
(7.89 g, 0.03 mol) in THF (15 mL) was added dropwise, so that
the mixture was maintained between 0 and 5 uC. After the
addition was completed, the solution was stirred for 12 h at
room temperature and then filtered. The filtrate was added
slowly to a vigorously stirred solution of sodium carbonate (32
g) in water (400 mL). The solid was collected by filtration and
washed with distilled water three times. The product was
recrystallized from a mixed solvent of ethanol and water (7 : 3,
v/v) to afford 7.45 g (83%) yellow need-like crystals: mp 164–
166 uC.
120 uC for 30 min, 200 uC for 1 h and 250 uC for 2 h) to produce
a fully imidized PI film. The chemical imidization method was
carried out by the addition of acetic anhydride and pyridine
(diamine : acetic anhydride : pyridine = 4 : 36 : 9, molar ratio)
to the aforementioned PAA solution (with magnetic stirring) at
the ambient temperature for 30 min and via heating at 80 uC
for 16 h. The PI solution was poured into methanol. The
precipitate was collected by filtration, washed thoroughly with
hot methanol and dried at 80 uC in a vacuum oven to give PI.
Yields were virtually quantitative in every case.
FT-IR (KBr, pellet), cm21: 1749, 1735 (CLO); 2937 (C–H);
1542, 1340 (NO2); 1609, 1501 (aromatics); 1263, 1138 (C–O–C).
1H NMR (CDCl3), ppm: 9.37–9.33 (m, 3H, NO2–ArH); 7.55–6.93
(m, 6H, Ar–H of naphthalene); 4.01 (m, 2H, –CH2–O–); 1.39–
1.81 (m, 8H, –(CH2)4–); 0.95 (t, 3H, methyl).
2.5 Fabrication of the liquid crystal cells
PAA solutions diluted to a solid content of 5% in NMP were
spin-coated on the clean ITO-coated glass or CaF2 glass at a
speed of 600 rpm for 15 s and 2500 rpm for 30 s followed by
heating at 80 uC for 30 min to evaporate the solvent. The films
thus obtained spin-coated on glass substrates were cured at
250 uC for 2 h to complete thermal imidization. The prepared
PI films on glass were subsequently rubbed with a roller
covered commercial rubbing cloth, and the rubbing strength
(RS) was calculated as follows:
2.3.3 Synthesis of 6-hexyloxy-naphthalen-39,59-diamino-
benzoate (N6). Reduced iron powder (4.46 g, 0.08 mol),
NH4Cl (2.14 g, 0.04 mol) and a mixed solvent of ethanol and
water (3 : 1, v/v, 200 mL in total) was added to a 500 mL flask
equipped with a magnetic stirrer and a condenser. After the
reaction the mixture was stirred for 1 h at 80 uC and 6-
(hexyloxy)naphthalen-2-yl 3,5-dinitrobenzoate (4.38 g, 0.01
mol) was added. The reaction mixture was refluxing in an oil
bath for another hour and was monitored by thin layer
chromatography (TLC). After the reaction was completed, the
hot solution was filtered to remove the impurities and poured
into distilled water under vigorous stirring. The crude
diamines precipitated as a clear pale yellow solid, which was
filtered, thoroughly washed with water, and recrystallized from
ethanol; consequently the pure light-yellow solid was obtained.
Yield: 78.6%, mp: 152–154 uC.
RS = NM(2prn/60v 2 1)
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; M (0.3 mm) was the contact length of the rubbing roller
circumference; v (17.2 mm s21) 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 assembled from two pieces of rubbed
substrates in an anti-parallel rubbing direction with 20 mm
thick spacers. In addition, some non-rubbed substrates were
used to fabricate LC cells for comparison. LC E7 was injected
into the cell gap using a capillary method at 80 uC, followed by
sealing of the injection hole with photo-sensitive epoxy glue. In
this study, LC cells were pre-heated to 80 uC before injection to
eliminate flow marks in the LC cells. The pretilt angles of LCs
were measured using a crystal rotation method.
FT-IR (KBr, pellet), cm21: 3439, 3366 (NH2); 2921 (C–H);
1725 (CLO); 1608, 1498 (aromatics); 1227, 1202 (C–O–C). 1H
NMR (CDCl3), ppm: 7.13–6.95 (m, 6H, Ar–H of naphthalene);
5.97–6.72 (s, 3H, NH2–ArH); 4.00 (m, 2H, –CH2–O–); 3.65 (s,
4H, 2–NH2); 1.39–1.81 (m, 8H, –(CH2)4–); 0.97 (t, 3H, methyl).
2.4 Synthesis of the polymers
The PIs were synthesized from various functional diamines
(N6 and C6), dianhydride ODPA and assistant diamine
DMMDA via a two-step method. The molar ratios of N6 and
DMMDA were controlled to be 0 : 10 (PI0–N6), 2 : 8 (PI2–N6),
3 : 7 (PI3–N6), 5 : 5 (PI5–N6), 7 : 3 (PI7–N6) and 10 : 0 (PI10–N6)
to obtain the PI–N6s. The molar ratios of 4-hexyloxy-biphenyl-
39,59-diaminobenzoate (C6) and DMMDA were controlled to be
2 : 8 (PI2–C6), 3 : 7 (PI3–C6) and 5 : 5 (PI5–C6) to attain the PI–
C6s.
The synthesis of PI5–N6 is used as an example to illustrate
the general synthetic route to produce the PIs. 5.0 mmol of
DMMDA was dissolved in 15 mL of NMP in a 100 mL three-
necked flask with mechanical stirring and 10 mmol of ODPA
was added to the solution. After a clear solution was formed,
5.0 mmol of N6 was added, and the mixture was stirred at
room temperature under a gentle flow of dry nitrogen for 24 h
to yield viscous poly(amic acid) (PAA) solutions. PAA was
converted into a PI via thermal or chemical imidization
methods. For the thermal imidization method, PAA solution
was cast onto a clean glass plate and heated (80 uC for 30 min,
3. Results and discussion
3.1 Monomer synthesis
The diamine 4-hexyloxy-biphenyl-39,59-diaminobenzoate (C6)
used for comparison was synthesized according to our
previous work.25 The novel functional diamine N6 was
synthesized from naphthalene-2,6-diol through a three-step
synthetic route according to Scheme 1.
First, 6-hexyloxy-2-hydronaphthalen was prepared by the
addition of the corresponding 1-bromopentane and potassium
hydroxide to
a solution of the naphthalene-2,6-diol in
methanol; the isolation procedure was carried out using the
distinct solubility of the 6-hexyloxy-2-hydronaphthalen and the
chief byproduct 6-hexyloxy-2-hexyloxynaphthalen in the alka-
line water solution. Subsequently, all 6-hexyloxy-naphthalen-
39,59-dinitrobenzoates were obtained by esterification of
6-hexyloxy-2-hydronaphthalen and 3,5-dinitrobenzoyl chloride
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Fig. 1 The 1H NMR spectra of the functional diamine N6.
ring-opening polymerization at room temperature for 24 h
yielded PAAs, followed by sequential heating to 250 uC or the
addition of
a mixture of Ac2O and Py to obtain the
corresponding polymers. Transformation from PAA to PI was
possible via the thermal or chemical cyclodehydration; the
merits of the former were the product was easy to handle and
to cast into thin films, and the latter was suited to prepare
soluble PIs. The chemical imidization method allowed high
yields and high-molecular-weight PIs to be achieved in every
case. The yields and molecular weight of the isolated PIs have
been summarized in Table 1.
All of the polymers were characterized by spectroscopic
techniques, in particular IR and NMR spectroscopy. As an
example, the 1H NMR spectrum of one polymer is shown in
Fig. 2, where all the signals could be readily ascribed to the
protons of the polymer. IR spectra of the various polymers
showed absorption bands typical of aliphatic C–H stretching at
2927–2863 cm21, corresponding to the methyl groups of the
diamine N6. Strong bands at 1778 and 1724 cm21 due to the
Scheme 1 The synthetic route of the functional diamine N6.
in the presence of TEA acting as a catalyst, and the products
could be purified by recrystallization.
The final step to obtain the ultimate diamine was the
reduction of the corresponding nitrocompound using
a
neutral reducing agent (Fe/NH4Cl), allowing the synthesis of
the diamine in a quantitative yield and pure enough for
polycondensation. The oxygen anion is quite stable under the
strong conjugate effect from a naphthalene unit, leading to the
easy reduction of the ester bond in alkaline or acidic
conditions, so a neutral reducing agent was applied to the
final step instead of the common reduction systems, such as
Pd/C in hydrazine hydrate (a strongly alkaline reducing agent)
and SnCl2 in hydrochloric acid (a strongly acidic reducing
agent).
The chemical structures of all above compounds were
confirmed by melting point analysis, FT-IR and 1H NMR
spectra. All the spectroscopic data obtained were consistent
with the proposed structures. The 1H NMR spectrum of the
functional diamine is displayed in Fig. 1.
3.2 Polymer synthesis
New PIs were prepared from N6 or C6, assistant diamine
DMMDA and commercially available aromatic dianhydride
(ODPA) via a conventional two-step 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. The
Scheme 2 The synthetic route for the polymers.
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Table 1 Intrinsic viscosity, molecular weight and elemental analysis of PIs
Elemental analysisb
C (%)
a
a
Sample
ginh (dL g21
)
Yieldsa (%)
Mn (61024 g Mol21
)
Mw/Mn
1.91
1.67
1.89
1.83
1.85
1.77
H (%)
N (%)
c
PI0
1.12
0.34
0.25
0.23
—
95
90
87
85
82
81
1.81
1.55
1.36
1.09
0.89
0.63
Calc.
74.40
73.91
71.84
70.55
71.66
70.01
71.34
69.86
71.06
69.39
70.71
68.88
4.00
3.89
3.83
3.71
3.90
3.77
4.03
3.91
4.15
4.02
4.29
4.06
5.60
5.81
6.71
6.89
6.43
6.52
5.94
6.41
5.53
6.26
5.00
5.84
Found
Calc.
PI2–N6
PI3–N6
PI5–N6
PI7–N6
PI10–N6
Found
Calc.
Found
Calc.
Found
Calc.
Found
Calc.
—
Found
a
b
Measured at a polymer concentration of 0.5 g dL21 in NMP at 30 uC, PIs were obtained by chemical imidization. Samples prepared by
c
thermal imidization. PI0 was prepared by polymerization of ODPA and DMMDA.
symmetrical and unsymmetrical vibrations of the two imide
carbonyl groups, were also observed for all polymers, along
with a sharp absorption band around 725 cm21 associated to
the skeletal vibration of the imide rings. Absorption bands
appearing at 1375 cm21 and 1244–1103 cm21 belonged to the
vibrations of C–N and C–O–C units, respectively. Spectroscopic
data are also in agreement with an essentially complete
imidization (Fig. 3).
The data of inherent viscosity and GPC measurements listed
in Table 1 indicate high molecular weights in the range of
0.63–1.81 6 104 g mol21 for Mn. The elemental analyses
obtained agree quite well with calculated values for the
proposed structures of the PIs. This is consistent with the
fact that creasable films could be obtained by casting and
solvent evaporation of polymer solutions.
polar solvents, such as cresols, NMP, or DMAc at room
temperature in most cases and even showed outstanding
solubility in common organic solvents, like THF, chloroform
and so on. The favorable solubility is most likely due to the
combination of the steric and polar nature of the carbonyl
units of both functional dimines (N6 and C6), which greatly
favors solubility in polar organic media. Also, the steric
hindrances of the –CH3 and flexible segments (–CH2–) twists
the rings of the main chains dramatically out of plane,
resulting in a noncoplanar and contorted conformation
capable of efficiently hindering the chain packing. All factors
reduce the chain–chain interaction and enhance solubility.
PI films fabricated by casting from the NMP solutions were
characterized using X-ray diffraction (XRD), showing only an
amorphous halo and no indication of crystallinity (Fig. 4),
which further explains the good solubility observed.
3.3 Solubility
The solubility of the PIs was investigated in different solvents
(Table 2). Satisfactorily, all polymers could be dissolved in
Fig. 2 The 1H NMR spectra of a polymer.
Fig. 3 The FT-IR spectra of PI2–N6.
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Table 2 Solubility of the PIs in different solvents
PIsa
PI0
PI2–N6 ++
PI2–C6 ++
PI3–N6 ++
NMP DMSO DMF DMAc m-cresol TCM THF
+2
+2
++
++
++
++
+2
++
++
++
++
+2
++
++
++
++
+2
++
++
++
++
++
++
++
++
++
+
++
++
++
++
PI3–C6
++
a
Qualitative solubility was determined with 10 mg of the polymer in
1 ml of solvent. (++) soluble at room temperature, (+) soluble upon
heating, (+2) partially soluble upon heating.
3.4 Optical transparency
Fig. 5 The transmittance of PI–N6.
The optical transparency of the novel PI–N6 films was studied
using UV-visible spectra. The films obtained by casting from a
5 wt% solution on a calcium fluoride (CaF2) piece all showed
high transmission (.95%) in the wavelength range of 400–700
nm. As shown in Fig. 5, the curves of optical transparency were
shifted toward the low wavelength with the an increasing side-
chain content. This was attributed to the incorporation of the
bulky hexyloxy-naphthalen groups which decreased the
charge-transfer complex (CTC) effect between backbones. On
the other hand, the introduction of an aliphatic unit (–CH2–)
into the PI backbone would also facilitate less intramolecular
CTC interactions, lowering the color of these PIs. The high
optical transparency of PI films is advantageous in LCD
devices.
technique to provide the homogeneous alignment of the LC
molecules onto the polymer surface. Thus, a lot of research
has been carried out in order to understand the physical
mechanism responsible for the LC alignment on the polymer
surface. In this study, a series of PAAs with molecularly
designed structures were synthesized and the effect of the
rigid part (naphthalene unit) of N6 on the pretilt angle after
the rubbing process was investigated. Furthermore, the
relationship between the pretilt angle and the molar fraction
of N6 to total diamines used has been studied.
3.5.2 Effect of the content of functional diamine on the
pretilt angle. As illustrated in Fig. 6, the pretilt angles of the LC
cells fabricated using the novel PIs before the rubbing
treatment ranged from 89 to 90u except for PI0. The pretilt
angle of PI0 was not detected before the rubbing treatment
because the LC was not aligned on the surface of the PI0 film.
However, the pretilt angles of the LC cells fabricated with
PI2–10–N6 containing N6 equal to or greater than 20 mol% to
the total amount of the diamines used ranged from 89 to 90u
3.5 Characterization of the liquid crystal display cells
3.5.1 Pretilt angle. The pretilt angle is an important
parameter that determines the optical properties of LCD
devices. The pretilt angle can be affected by various factors,
such as surface energy, surface morphology, steric effects and
the electronic interaction of the LC with the alignment layer,
etc. This might be caused by a processing condition like the
rubbing treatment as well as the chemical structure of the PI.
As is well known, the rubbing treatment is a widely used
Fig. 6 Pretilt angles of the LC cells fabricated with the PIs as LC alignment layers
Fig. 4 Wide-angle XRD patterns of the polymers.
(a) before and (b) after the rubbing treatment (rubbing strength was 56.9 mm).
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Table 3 Variation of the surface energy of the polyimides before and after the
rubbing treatment
Contact angle (u)
˜
Polyimide
Water
Diiodomethane
Surface energy (dyn cm1)
Before the rubbing treatment
PI0
70.8
78.1
78.3
78.8
78.7
79.1
30.1
46.0
46.7
46.2
50.8
50.8
46.9
38.7
38.3
38.3
36.4
36.2
PI2–N6
PI3–N6
PI5–N6
PI7–N6
PI10–N6
After the rubbing treatment
PI0
68.6
77.6
77.6
77.7
78.3
79.0
22.1
46.0
46.7
47.3
48.0
50.1
49.9
38.8
38.6
38.2
36.6
36.4
PI2–N6
PI3–N6
PI5–N6
PI7–N6
PI10–N6
Fig. 7 Variation in the surface energy of the polyimides films.
very high pretilt angle above 89u even after the rubbing
process. The novel PI with a long alkyl chain was connected
with a particularly rigid naphthalene unit. That is, the rigid
naphthalene connecting group may restrict the movement of
the long alkyl chain into the polymer surface, which results in
the low surface energy and an enrichment of non-polar alkyl
side chains at the outmost layer of the PI surface. As a result,
the PI obtained from N6 with a rigid naphthalene group
showed a high pretilt angle above 89u even after rubbing
treatment.
Conoscope observations of the LC cells were obtained with a
polarized optical microscope. As shown in Fig. 8, the dark
crossed brush was seen clearly and did not move with the LC
cell rotating before and after rubbing. The result further
proved the vertical alignment was induced by PI with
naphthalene side chains.
before the rubbing treatment. This was due to the very low
surface energies of the PI2–10–N6 ranging from 36.2 to 38.7 dyn
˜
cm1. Table
3 shows the contact angles of water and
diiodomethane on the PI films, from which the surface
energies were calculated by the harmonic mean equation
before and after the rubbing treatment. As shown in Table 3,
with an increase in the content of the hexyloxy-naphthalen
groups, the water contact angles on the PIs were increased
both before and after rubbing, which was due to a decrease in
the surface polarity of the PI films with the introduction of the
hexyloxy-naphthalen groups. In particular, the surface energy
˜
of the PI was remarkably decreased from 46.9 to 38.7 dyn cm1
with the introduction of 20 mol% of N6. Furthermore, the
˜
surface energy slightly decreased to 36.2 dyn cm1 with an
increased content of naphthalene side chains, which resulted
in high pretilt angles above 89u. This revealed similar results to
our previous studies, which showed that the pretilt angle
increased with a decrease in the surface energy of the PI films
or an enrichment of the long alkyl side chains on the PI
surface.23
3.6 Thermal properties and the mechanism of imidization
DSC and TGA methods were applied to evaluate the thermal
properties of the PIs, and thermal analysis data from the TGA
and DSC curves of the PIs are summarized in Table 4. DSC
revealed that rapid cooling from 300 uC to room temperature
produced predominantly amorphous samples. The Tg values of
3.5.3 Effect of rubbing treatment on the pretilt angle. The
surface of PIs is the core of research into the alignment layers
since it straightforwardly contacts, aligns LC molecules and
generates pretilt angles for LC molecules. The surface energies
can supply some information about the surface. As shown in
Fig. 7, the surface energies of the PI films were compared
before and after the rubbing treatment. As summarized in
Table 3, they were slightly increased by the rubbing treatment,
which resulted in an increase in the surface polarity of the PI
film and a decrease of the pretilt angles. That is, the rubbing
process induced the increase in the polarity of the PI films by
an insertion of the naphthalen side group of the N6 into the
polymer bulk phase during the rubbing process. This resulted
in an increase in the polarity of the PIs containing long alkyl
side chains and the increase in the flatness of the thin film to
make the LC molecules lie flatter. However, the surfaces of the
PI2–10–N6 films were saturated by the naphthalen side group
above a composition of 20 mol% of N6, which resulted in a
Fig. 8 Polarized optical microscopic image of vertical LC. All of the pictures were
captured under crossed polarizers. The conoscopic images are shown in the
corner of the picture. (a) Before rubbing, (b) after rubbing. The alignment film is
PI3–N6.
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Table 4 The thermal properties of the PIs
PIsa
Tg (uC)
Td (uC)
T5 (uC)
T10 (uC)
Rw (%)
b
c
c
c
c
PI2–N6
PI2–C6
PI3–N6
PI3–C6
PI5–N6
PI5–C6
244
243
235
232
230
221
373
371
354
355
338
333
443
435
438
421
416
409
470
466
451
446
437
430
57.9
56.9
56.6
54.8
49.5
48.1
a
Measured samples were obtained by the thermal imidization
b
method. Tg, the glass-transition temperature, was measured using
DSC at a heating rate of 10 uC min21 in air. Td, decomposition-
c
starting temperature; T5 and T10, temperature at a 5 or 10% weight
loss; Rw, residual weight (%) at 800 uC in nitrogen.
Fig. 9 The TGA curves of the PI–N6s.
PI–N6 were in the range of 221–244 uC. As we expected, the Tg
values of these PIs depended on the structure of the different
components and decreased with increasing flexibility of the PI
backbones according to the applied structure of the monomer.
Compared with the PIs from C6, PIs from N6 show a slightly
higher Tg. N6 is shorter than C6 when they possess the same
length of alkyl chain, so N6 brought about smaller steric
configuration, resulting in tighter chain packing of PI–N6. The
segments of the chain could only move under higher
temperatures, so higher Tgs were observed for PI–N6.
that the introduction of a naphthalene unit into the side chain
could effectively improve the thermal stability of soluble PIs.
The TGA curves of PI–N6 from thermal imidization are
displayed in Fig. 9. The favorable thermal stability from the
thermal imidization method was possibly due to the presence
of partial intermolecular crosslinking, as shown in Fig. 10. The
difference between PIs produced by thermal and chemical
imidization can be explained by the imidization reaction
mechanism. PAA was synthesized by dianhydride and diamine
via a polyaddition reaction in the amide type polar solvent at
room temperature. This kind of PAA was an unstable polymer,
a nucleophilic substitution reaction was caused, substituting
the hydroxyl group of the carboxylic acid group and amino
group of amide in PAA, respectively, with the carbonyl group of
PAA. The former (hydroxyl group with the carbonyl group) was
a reversible reaction, which could reduce to form anhydride
and amine at room temperature or in the heating process, and
the latter (amino group with the carbonyl group) was an
irreversible reaction, which would dehydrate to form imide
only in the heating process. At room or low temperatures, the
equilibrium of the former tends to become PAA solution with
little chance for a decomposition reaction to occur. However,
when PAA solution was heated, the reversible and irreversible
reactions increased with temperature, and more anhydride
ends and amine ends in PAA were produced. The ketone group
of the side chain was an electrophilic group, which condensed
to the amine group to form an imine group; therefore, PAA
with the ketone group is connected with imine linkage to form
nonlinear or crosslinking PI during thermal imidization. The
crosslinking reaction increased with a higher concentration of
PAA after most solvents were volatilized in the imidization. If
PAA is tested with Ac2O and Py via chemical imidization,
causing the Ac2O to react with amic acid to form amide-
anhydride, then the imide is formed by a nucleophilic reaction
of the amide-anhydride group with separating acetic acid. That
is, a crosslinking reaction would not occur in chemical
imidization. According to the preceding description, this type
of crosslinking was due to the ketone group of PAA being
connected with the imine group by thermal imidization.
In order to investigate the thermal stability of the PIs, the
thermal imidization method was adopted to produce PIs
rather than chemical imidization method, because soluble PIs
from the chemical imidization method are usually linear
polymers which tend to absorb water which may exert
unexpected influence on the results. The difference in the
PIs produced by thermal and chemical imidization could be
explained by the imidization reaction mechanism as discussed
later in this section. Table 4 gives the temperatures of the
initial decomposition (Td) and 5% (T5) and 10% (T10)
gravimetric losses in nitrogen. From Table 4, we can see PI–
N6 exhibited slightly higher Td and T10, but much higher T5
compared with the corresponding PI–C6. Research has shown
that PIs containing side chains exhibit distinct two-step weight
loss behaviors.29 The weight loss percentage during the first
degradation was equivalent to the weight fraction of the side
chain, indicating that the values of Td and T5 had a close
relationship with the structure of side chains. Both functional
diamines possessed the same long alkyl chains and the
backbone of PIs were also the same, so similar values of Td
were observed. The planar naphthalene unit was more rigid
than the twisted biphenyl unit, so the introduction of
naphthalene groups into N6 increased the rigidity of the side
chains of PI–N6, but decreased the weight ratio of the side
chains simultaneously. Higher values of T5 demonstrated that
PI–N6 was more thermally-stable than PI–C6. The second
degradation corresponded to the thermal degradation of the
polymer main chain.29 As they contain the same alkyl chain
and are similar in structure, the two kinds of PIs obtained
from the two functional diamines had similar structures, so
close values of T10 and Rw were found. The TGA results show
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Fig. 10 The mechanism of chemical and thermal imidization.
chains was smaller than that of PI–C6, but a much higher T5
was attained. The results demonstrated that the introduction
of a naphthalene unit into the side chain could effectively
improve the thermal stability of the PI without sacrificing its
solubility. In addition, the difference between PIs produced by
4. Conclusion
Novel functional diamine (N6) containing a rigid naphthalene
unit, was molecularly designed and successfully synthesized .
PIs were obtained by copolymerization of N6, DMMDA and
ODPA. The structures of the intermediates, diamines and PIs
were confirmed using FT-IR and 1H NMR spectra. All PIs
obtained could be dissolved in polar aprotic solvents and low-
boiling-point solvents. PI films attained by a casting method
showed favorably high transmittance above 95% in the wave
length range of 400–700 nm and could align LCs vertically
before and after rubbing treatment. PI–N6 exhibited a slightly
higher Td and T10, but much higher T5 compared with the
corresponding PI–C6. For PI–N6, the weight ratio of the side
thermal and chemical imidization was explained in
a
preliminary manner by the imidization reaction mechanism.
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.
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