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POLYMERS FOR ADVANCED TECHNOLOGIES
Polym. Adv. Technol. 2006; 17: 444–452
nce.wiley.com). DOI: 10.1002/pat.732
Published online 23 June 2006 in Wiley InterScience (www.interscieSynthesis, characterization and liquid crystal-aligning
properties of new poly{3-[4-(n-alkyloxy)phenyloxy]pyromellitimide}s
Jin Kook Lee1, Seong Jun Lee1, Jin Chul Jung1*, Wang-Cheol Zin1, Taihyun Chang2
and Moonhor Ree2
1Polymer Research Institute/Center for Advanced Functional Polymers, Department of Materials Science & Engineering, Pohang University of
Science & Technology (POSTECH), San 31, Hyoja-dong, Pohang, Gyeongbug 790-784, Korea2BK21 Functional Polymer Thin Film Group, Department of Chemistry, Pohang University of Science & Technology (POSTECH), San 31,
Hyoja-dong, Pohang, Gyeongbug 790-784, Korea
Received 30 March 2006; Accepted 10 April 2006
*Correspofor AdvaScience anology (P790-784, KE-mail: jcContract/grant num
Four new polypyromellitimides (MCm-PPIs andMCm-OPIs,m¼ 6, 8) that are singly substituted with
a flexible n-alkyloxy side branch at the pyromellitimide ring were prepared by the two-step
polycondensation of 3-[4-(n-alkyloxy)phenyloxy]pyromellitic dianhydrides (MCm-PMDAs, m¼ 6,
8) with p-phenylenediamine (PDA) and 4,4(-oxydianiline (ODA), respectively. The dianhydride
monomers were synthesized from durene via several reaction steps. Inherent viscosities of the
precursor poly(amic acid)s ranged from 0.57 to 1.58 dl/g. After chemical structures of the polyimides
had been characterized, their thermal properties, crystalline structures, and liquid crystal (LC)
aligning abilities on their rubbed thin films were determined and discussed in comparison to the
polypyromellitimides that are doubly substituted at the pyromellitimide ring with the same side
branches. For all polymers thermogravimetric analysis (TGA) programs showed a typical two-step
degradation behavior with onset temperatures in the 430–4558C range. In X-ray scattering studies all
the samples were found to be amorphous, but the presence of a loosely developed layer structure
could be confirmed, in which two main chains gather together to form a double-strand backbone
layer and n-alkyl branches fill the space between the layers. On the rubbed surfaces of the polyimide
thin films LCs uniformly aligned parallel to the rubbing direction with the pre-tilt angles 5–78 inMCm-OPIs and 18–328 in MCm-PPIs. Copyright # 2006 John Wiley & Sons, Ltd.
KEYWORDS: polypyromellitimides; polyimides; synthesis; LC alignment layers; films
INTRODUCTION
In recent years thin aromatic polyimide films have become of
tremendous importance in manufacturing thin film transis-
tor-liquid crystal display (TFT-LCD) devices, which are
widely used in numerous electronic appliances such as
mobile telephones, television sets, or notebook computers.
This importance relies mainly on the excellent dimensional
stability and liquid crystal (LC) aligning ability of the
surface-treated polyimide films.1–3 Of a number of surface-
treatment methods developed so far for the polyimide films
such as optical microgroove formation or deposition of SiO,4
the rubbing with velvet fabric5,6 is still the major technique
ndence to: J. C. Jung, Polymer Research Institute/Centernced Functional Polymers, Department of Materialsnd Engineering, Pohang University of Science & Tech-OSTECH), San 31, Hyoja-dong, Pohang, Gyeongbugorea.jung@postech.ac.krgrant sponsor: Korea Research Foundation; contract/ber: KRF 2004-005-D00009.
being exclusively taken in the pertaining industries for large-
scale production.
A TFT-LCD device has quite a complex structure, and its
optical and electrical performance is governed by various
factors.7 However, a pre-tilt angle of LCmolecules aligned on
a polyimide film surface is one of the most critical factors in
determining viewing angles, response time and energy
consumption of the devices, and a great deal of effort has
been devoted to developing new aromatic polyimides which
give rise to high pre-tilt angles.3,8,9 The chemical attachment
of flexible side branches to rigid aromatic polyimide
backbones has recently appeared as one of the most
promising approaches for aligning LC molecules with high
pre-tilt angles.10–12 Such successful approaches are based on
some specific interaction between LC molecules and chain
groups of polyimides, both of which possess the rigid rod
structure with a flexible alkyl part. Although the exact
mechanism of the LC alignment on the rubbed polyimide
Copyright # 2006 John Wiley & Sons, Ltd.
New polypyromellitimides 445
surfaces is still unknown, the interaction is expected to
improve LC aligning ability of the polyimides.
The authors have been preparing a number of new
polypyromellitimides that are doubly substituted with
various groups at the 3- and 6-positions of the pyromellitimide
ring10,13 and making studies for applications as LC aligning
layers after the surfaces were rubbed with the standard velvet
fabric. In these studies the authors have attempted to develop
new polyimides with very high pre-tilt angles. For instance,
poly{p-phenylene-3,6-di[4-(n-butyloxy)phenyloxy]pyromelli-
timide} and poly{p-phenylene-3,6-di[4-(n-octyloxy)phenylox-
y]pyromellitimide} (DCm-PPI, m¼ 4, 8) were found to give
pre-tilt angles ranging from 258 to 878, depending on the
rubbing density (unpublished results for DC6-PPI).14,15
In the present article the synthesis, characterization, and LC
aligning behaviors of the polypyromellitimides that are singly
substituted only at the 3-position of the pyromellitimide ring
with (n-hexyloxy)- and (n-octyloxy)-phenyloxy side groups are
reported. This work aims at obtaining a better understanding
of the effect of chemical structures of aromatic polypyromelli-
timides on LC aligning behavior by comparing the present
results with those of the doubly substituted ones. Since the side
branch contents in a repeat unit are reduced to half the doubly
substituted polyimides, the rigid backbone contents naturally
increase and this increase effect can be observed in LC aligning
behaviors of the singly substituted polyimides.
EXPERIMENTAL
Materialsp-Phenylenediamine (PDA, Aldrich) and 4,40-oxydianiline
(ODA, Aldrich) were once recrystallized in ethanol and
vacuum-sublimated three times before use. N,N-Dimethyl-
formamide (DMF) was stirred overnight over dry MgSO4
and vacuum-distilled. N-Methylpyrrolidone (NMP) for
polymerization was purified by vacuum-distillation after
boiling for several hours over CaH2. Durene (Aldrich),
bromine (Junsei), aniline (Aldrich), methanol (Samchun),
Alconox1 (Alconox), ethanol (Samchun), dimethylsulfoxide
(DMSO, Kanto), CH2Cl2 (Samchun), 4-n-pentyl-40-cyanobi-
phenyl (5CB, Aldrich), Disperse Blue 1 (Aldrich), iodine
(Aldrich), and other solvents, inorganic salts, mineral acids
and alkalies were used as received.
MeasurementsMelting points were determined with an IA 9100 digital
melting-point apparatus. Fourier transform infrared (FT-IR)
spectra were obtained from a Mattson Infinity Gold
spectrophotometer. 1H- and 13C-NMR spectra (300MHz
for 1H and 75MHz for 13C) were recorded on a Bruker AM
300 spectrophotometer with tetramethylsilane (TMS) as an
internal standard. Elementary analyses were performed on a
Carlo Erba EA 1108 microanalyzer at the Korea Basic Science
Institute, Daegu, Korea. The inherent viscosities were
measured from 0.2 g/dl poly(amic acid) solutions in NMP
at 258C with an Ubbelohde-type viscometer. Thermal
analyses were conducted at 108C/min scan rate with
Perkin–Elmer DSC-7 and TGA-7 under a nitrogen flow. X-
ray diffractograms were obtained in transmission mode
using Ni-filtered Cu-Ka radiation with a wavelength of
Copyright # 2006 John Wiley & Sons, Ltd.
1.542 A on a Rigaku Geiger Flex D-max X-ray diffractometer
at room temperature.
Preparation of LC cell and measurement of LCalignmentPolyimide films for LC alignment layers were prepared by
spin-casting 4wt% poly(amic acid) solutions in NMP at
3000 rpm for 40 sec onto cleaned indium tin oxide (ITO) glass
plates (1.5� 4 cm2), followed by drying and thermal
imidization. The polyimide films were subsequently rubbed
with a roller (Wande Co.) covered with a rayon velvet fabric
(YA-20-R, Yoshikawa Co., fabric density 2400fiber/cm2,
fiber diameter 15mm, and length 1.85mm) and the rubbing
density was calculated by the equation L/l¼N[(2prn/
60n)�1], where L (in mm) is the total length of the rubbing
cloth that touches a certain point of the film, l (in mm) is the
contact length of the rubbing roller circumference, N is the
cumulative number of rubbings, n (in cm/sec) is the velocity
of the substrate stage, and n (in rpm) and r (in cm) are the
rubbing roller speed and radius, respectively.
LC cells were fabricated from two pieces of the rubbed
polyimide films assembled in an antiparallel rubbing
direction at 50mm cell gap using poly(ethylene terephthalate)
film spacers and filled with 5CB containing 1.0wt% dichroic
dye (Disperse Blue 1) by a capillary method. Optical phase
retardation measurements were, as described earlier,16
carried out with a phase retardation analyzer equipped with
a photoelasticmodulator (model PEM90, Hinds Instruments)
with a fused silica head, a He-Ne laser with a 632.8 nm
wavelength (model 106-1, Spectra Physics), a pair of
polarizers (model 27300, Oriel), a photodiode detector (model
PIN-10DL, UDT Sensors), and a pair of lock-in amplifiers
(model SR510, Stanford Research Systems). The pre-tilt
angles for the fabricated LC cells were measured by a crystal
rotation method using a laboratory apparatus equipped
with a goniometer, a photodiode detector, a He-Ne laser
(632.8 nm), a polarizer-analyzer pair, and a sample stage.
Monomer preparation
Synthesis of 3-bromodurene (1)To a 500ml, round-bottom flask equipped with a dropping
funnel and a reflux condenser were added 67.1 g (0.5mol)
durene, 250ml CH2Cl2 and 1.0 g iodine and stirred on an ice-
water bath to make the flask contents homogeneous. Then
83 g (0.52mol) bromine diluted with 100ml CH2Cl2 were
dropped in over 1 hr and vigorously stirred in the cooled
conditions until no HBr evolves when detected with dilute
aqueous ammonia solution. The reaction mixture was
neutralized with 2% aqueous NaOH solution and phase-
separated. The organic layer was washed with water, dried
with MgSO4, and evaporated. The residue was recrystallized
from ethanol and further purified by steam distillation.
Yield¼ 67%; melting point, mp¼ 578C (60.58C in the
literature17). 1H-NMR (CDCl3, ppm): 6.88 (s, 1H), 2.35
(s, 6H), 2.26 (s, 6H). 13C-NMR (CDCl3, ppm): 135.0, 134.1,
130.5, 129.3, 21.2, 20.4.
Synthesis of 3-bromopyromellitic acid (2)Oxidation of 1 was carried out in the same procedure as
described in the literature.13 The crude products were
Polym. Adv. Technol. 2006; 17: 444–452
DOI: 10.1002/pat
446 J. K. Lee et al.
purified by recrystallization from 1% aqueous HCl
solution.
Yield¼ 47%. IR (KBr, cm�1): 3400–2600 (br, COOH), 1700
(C––O). 1H-NMR (DMSO-d6, ppm): 13.85 (br, 4H), 8.44 (s, 1H).13C-NMR (DMSO-d6, ppm): 167.0, 164.5, 142.2, 130.7, 129.4,
117.0.
Synthesis of 3-bromopyromellitic dianhydride (3)In a 250ml round-bottomed flask equipped with a reflux
condenser and a drying tubewere placed 10 g of compound 2
and 90ml of acetic anhydride. The reaction mixture was
refluxed for 6 hr under argon. As the reaction mixture was
cooled to ambient temperature, white crystals were pro-
duced. They were filtered, washed thoroughly with diethyl
ether, and dried at 1008C for 24 hr in a vacuum.
Yield¼ 82%. IR (KBr, cm�1): 1869, 1770 (C––O, anhydride).
Synthesis of 3-bromo-N;N0-diphenylpyromellitimide (4)In a 250ml round-bottomed flask equipped with a reflux
condenser and a Dean–Stark trap were placed 150ml
toluene, 3.72 g (0.04mol) aniline and 60ml NMP. After the
mixture was cooled on an ice–water bath, 6.0 g (0.02mol) of
compound 3 was added and stirred under nitrogen flow
until it became homogeneous. After this solution was heated
at 1608C for 12 hr, it was cooled to ambient temperature and
poured into excess methanol. The precipitates formed were
filtered, washed thoroughly with methanol, and dried in a
vacuum.
Yield¼ 85%; mp> 3008C. IR (KBr, cm�1): 1781, 1716 (C––O,
imide), 1596, 1500 (C––C, aromatic). 1H-NMR (DMSO-d6,
ppm): 8.27 (s, 1H), 7.54–7.45 (m, 10H). 13C-NMR (DMSO-d6,
ppm): 164.3, 164.1, 138.7, 134.5, 131.5, 129.0, 128.5, 127.4,
116.2, 114.6.
Synthesis of 3-[4-(n-hexyloxy)phenyloxy]-N;N0-diphenylpyromellitimide (5a)In a 100ml round-bottomed flask were added in a nitrogen
atmosphere 10ml of anhydrous methanol, 0.2 g (8.7mmol)
sodium, and 1.69 g (8.7mmol) 4-n-hexyloxyphenol13 and
stirred for 30min at room temperature. Then methanol was
completely distilled off. To the residue 50ml of anhydrous
DMF and 3.96 g (8.7mmol) of compound 4 were added and
heated at 708C for 12 hr. After being cooled to room
temperature, the reaction mixture was poured into 500ml
0.5 N HCl solution to obtain white precipitates. The solids
were filtered, washed with water, and purified by recrys-
tallization from a mixture of CHCl3/ethanol (1:1 v/v).
Yield¼ 71%; mp¼ 2678C. IR (KBr, cm�1): 3062 (C–H,
aromatic), 2960–2850 (C–H, aliphatic), 1726, 1777 (C––O,
imide), 1506 (C––C, aromatic), 1243, 1189 (C–O–C). 1H-NMR
(DMSO-d6, ppm): 0.83 (t, 3H), 1.34–1.23 (m, 6H), 1.63 (quint,
2H), 3.84 (t, 2H), 7.00–6.79 (dd, 4H), 7.53–7.39 (m, 10H), 8.16
(s, 1H). 13C-NMR (DMSO-d6, ppm): 164.9, 163.2, 154.3, 154.5,
152.3, 148.1, 140.0, 131.4, 129.0, 128.7, 128.4, 127.1, 117.1, 114.9,
31.0, 67.8, 28.7, 25.2, 22.1, 13.9.
Synthesis of 3-[4-(n-octyloxy)phenyloxy]-N;N0-diphenylpyromellitimide (5b)This compound was prepared via the same procedure as
for 5a.
Copyright # 2006 John Wiley & Sons, Ltd.
Yield¼ 55%; mp¼ 2598C. IR (KBr, cm�1): 3071 (C–H,
aromatic), 2965–2852 (C–H, aliphatic), 1780, 1721 (C––O,
imide), 1504 (C––C, aromatic), 1239, 1190 (C–O–C). 1H-NMR
(CDCl3, ppm): 8.29 (s, 1H), 7.51–7.37 (m, 10H), 6.98–6.78 (dd,
4H), 3.86 (t, 2H), 1.72 (quint, 2H), 1.41–1.23 (m, 10H), 0.86 (t,
3H). 13C-NMR (CDCl3, ppm): 164.0, 162.3, 155.1, 155.0, 152.1,
150.0, 139.7, 130.4, 128.7, 128.1, 127.9, 125.9, 116.9, 114.8, 67.9,
31.3, 28.9–28.7, 25.6, 22.2, 13.6.
Synthesis of 3-[4-(n-hexyloxy)phenyloxy]pyromelliticacid (6a)In a 100ml round-bottomed flask equipped with a reflux
condenser were placed 50ml 10% aqueous NaOH solution,
50ml ethanol and 2.0 g of compound 5a and heated to reflux
under vigorous stirring for 12 hr. After being cooled to room
temperature, the reaction mixture was acidified with
concentrated HCl to obtain white precipitates. They were
collected by filtration and dissolved into a mixture of 50ml
DMSO and 12ml concentrated HCl. This solutionwas stirred
at 808C for 4 days. After being cooled to room temperature,
the reaction mixture was diluted with 50ml concentrated
HCl and extractedwith diethyl ether. After the ether solution
was washed with water and dried with MgSO4, ether was
evaporated. The solid residue was purified by recrystalliza-
tion from 5% HCl solution.
Yield¼ 42%. IR (KBr, cm�1): 3400–2600 (br, OH), 2931–
2868 (C–H, aliphatic), 1708 (C––O), 1504 (C––C, aromatic),
1246, 1185 (C–O–C). 1H-NMR (DMSO-d6, ppm): 13.5 (br, 4H),
8.26 (s, 1H), 6.81–6.66 (dd, 4H), 3.87 (t, 2H), 1.66 (quint, 2H),
1.41–1.26 (m, 6H), 0.86 (t, 3H). 13C-NMR (DMSO-d6, ppm):
165.7, 165.1, 154.0, 151.9, 148.0, 135.1, 130.2, 117.0, 115.3, 114.8,
67.7, 31.0, 28.7, 25.2, 22.1, 13.9.
Synthesis of 3-[4-(n-octyloxy)phenyloxy]pyromelliticacid (6b)This compound was prepared in the same manner as 6b.
Yield¼ 53%. IR (KBr, cm�1): 3400–2600 (br, OH), 2928–
2865 (C–H, aliphatic), 1716 (C––O), 1504 (C––C, aromatic),
1249, 1184 (C–O–C). 1H-NMR (DMSO-d6, ppm): 13.5 (br, 4H),
8.22 (s, 1H), 6.77–6.23 (dd, 4H), 3.82 (t, 2H), 1.66 (quint, 2H),
1.34–1.22 (m, 10H), 0.86 (t, 3H). 13C-NMR (DMSO-d6, ppm):
165.7, 165.1, 154.0, 152.0, 148.0, 135.1, 130.2, 117.0, 114.8, 114.9,
67.7, 31.2, 28.7, 28.8, 25.5, 22.0, 13.9.
Synthesis of 3-[4-(n-hexyloxy)phenyloxy]pyromelliticdianhydride (MC6-PMDA)A mixture of 0.8 g 6a and 10ml acetic anhydride was stirred
under reflux for 6 hr in argon atmosphere. After being cooled
to ambient temperature the liquid was removed by vacuum
evaporation to obtain a solid residue. This residue was
purified by recrystallization from a mixture of CHCl3 and
cyclohexane (1:1, v/v).
Yield¼ 72%; mp¼ 1848C. IR (KBr, cm�1): 3091 (C–H,
aromatic), 2973, 2868 (C–H, aliphatic), 1859, 1774 (C––O,
anhydride), 1622, 1505 (C––C, aromatic), 1226, 1188 (C–O–C).1H-NMR (acetone-d6, ppm): 8.46 (s, 1H), 7.08–6.87 (dd, 4H),
3.96 (t, 2H), 2.04 (quint, 2H), 1.49–1.31 (m, 6H), 0.89 (t, 3H).13C-NMR (acetone-d6, ppm): 161.8, 159.2, 156.8, 153.2, 152.6,
142.2, 130.3, 118.7, 116.0, 69.1, 32.4, 26.51, 26.50, 23.4, 14.4.
Polym. Adv. Technol. 2006; 17: 444–452
DOI: 10.1002/pat
New polypyromellitimides 447
Synthesis of 3-[4-(n-octyloxy)phenyloxy]pyromelliticdianhydride (MC8-PMDA)This compound was prepared by the same procedure as for
MC6-PMDA.
Yield¼ 77%; mp¼ 1728C. IR (KBr, cm�1): 3091 (C–H,
aromatic), 2927, 2854 (C–H, aliphatic), 1858, 1774 (C––O,
anhydride), 1622, 1505 (C––C, aromatic), 1227, 1189 (C–O–C).1H-NMR (acetone-d6, ppm): 8.46 (s, 1H), 7.09–6.88 (dd, 4H),
3.96 (t, 2H), 1.75 (quint, 2H), 1.48–1.30 (m, 10H), 0.89 (t, 3H).13C-NMR (acetone-d6, ppm): 160.3, 157.8, 155.4, 151.8, 151.2,
140.8, 128.8, 117.2, 116.5, 114.4, 67.6, 31.2, 27.73, 27.72, 27.71,
27.69, 22.0, 13.0.
Polymerization and film preparationA polymerization tube containing 5mmol of solid PDA or
ODA and 5mmol of a Cm-PMDA was sealed with a septum
and filled with argon. Then the calculated volume of NMP
saturated with argon was injected to keep solid contents at
10% (wt/v) in C6-PMDA and 12% in C8-PMDA. The NMP
solutions were magnetically stirred for 72 hr at room
temperature. Then a small portion of each viscous NMP
solution containing precursor poly(amic acid) was taken out
to measure inherent viscosities.
The other portions of the poly(amic acid)s were cast onto
the glass plates (3� 5 cm2), which had been cleaned by
dipping into ultrasonically assisted Alconox solution for 1 hr
and ethanol for 1 hr and thoroughly washing with distilled
water. The cast films were heat-treated for imidization over a
thermal cycle of 808C for 1.5 hr, 1308C for 1 hr, 2008C for 1 hr,
and 3008C for 2 hr. Free-standing polyimide films were
obtained by immersing the plates into distilled water,
followed by drying in a vacuum.
Poly{p-phenylene-3-[4-(n-hexyloxy)phenyloxy]pyromellitimide} (MC6-PPI)IR (KBr, cm�1): 3081 (C–H, aromatic), 2928, 2857 (C–H,
aliphatic), 1778, 1729 (C––O, imide), 1614, 1518 (C––C,
Scheme 1. Synthetic route to
Copyright # 2006 John Wiley & Sons, Ltd.
aromatic), 1366 (C–N–C), 1241, 1190 (C–O–C). Elemental
analysis calcd. for C28H22N2O6: C, 69.70%; H, 4.60%; N,
5.81%. Found: C, 68.58%; H, 4.50%; N, 5.76%.
Poly{p-phenylene-3-[4-(n-octyloxy)phenyloxy]pyromellitimide} (MC8-PPI)IR (KBr, cm�1): 3081 (C–H, aromatic), 2926, 2855 (C–H,
aliphatic), 1779, 1735 (C––O, imide), 1615, 1518 (C––C,
aromatic), 1366 (C–N–C), 1242, 1189 (C–O–C). Elemental
analysis calcd. for C30H26N2O6: C, 70.58%; H, 5.13%; N,
5.49%. Found: C, 69.76%; H, 5.61%; N, 5.57%.
Poly{p-phenyleneoxy-p-phenylene-3-[4-(n-hexyloxy)phenyloxy]pyromellitimide} (MC6-OPI)IR (KBr, cm�1): 3073 (C–H, aromatic), 2929, 2855 (C–H,
aliphatic), 1776, 1729 (C––O, imide), 1598, 1501 (C––C,
aromatic), 1375 (C–N–C), 1241, 1189 (C–O–C). Elemental
analysis calcd. for C34H26N2O7: C, 71.07%; H, 4.56%; N,
4.88%. Found: C, 70.36%; H, 4.56%; N, 4.56%.
Poly{p-phenyleneoxy-p-phenylene-3-[4-(n-octyloxy)phenyloxy]pyromellitimide} (MC8-OPI)IR (KBr, cm�1): 3079 (C–H, aromatic), 2925, 2853 (C–H,
aliphatic), 1776, 1728 (C––O, imide), 1598, 1501 (C––C,
aromatic), 1375 (C–N–C), 1242, 1186 (C–O–C). Elemental
analysis calcd. for C36H30N2O7: C, 71.75%; H, 5.02%; N,
4.65%. Found: C, 71.17%; H, 4.87%; N, 4.58%.
RESULTS AND DISCUSSION
Monomer preparationTwo new pyromellitic dianhydride monomers (MCm-
PMDAs,m¼ 6, 8) that are singly substituted at the 3-position
of the pyromellitic ring with 4-(n-alkyloxy)phenyloxy side
groups were successfully prepared from durene through the
reaction path, as shown in Scheme 1.
dianhydride monomers.
Polym. Adv. Technol. 2006; 17: 444–452
DOI: 10.1002/pat
448 J. K. Lee et al.
When durene was nuclear-brominated, a mixture of 3-
bromodurene and 3,6-dibromodurene was produced. The
monobrominated product was successfully isolated from the
mixture first by fractional recrystallization in ethanol and
consecutive steam distillation. This separation process is
based on the lower crystallization tendency and higher
volatility of 3-bromodurene than 3,6-dibromodurene. The
pure 3-bromodurene was unambiguously identified by 1H-
NMR spectroscopy, in which the absorption peak for one
aromatic proton remaining unreacted is characteristic.
The 3-bromodurene thus obtained was exhaustively
oxidized with KMnO4 in pyridine to obtain 3-bromopyr-
omellitic acid, which was cyclodehydrated to dianhydride
and protected with aniline to obtain the diimide compound.
The diimide was used as substrate for the bromine
displacement reaction with 4-(n-alkyloxy)phenyloxy anions.
The N-phenylimide groups acted as protecting moiety to
hinder the substitution reaction from taking place at C––O.
The protecting imide groups were transformed to anhydride
groups by hydrolysis and ring closure. The hydrolysis was
undertaken in two steps, first with NaOH dissolved in
1:1 ethanol/water mixture and then with aqueous HCl
containing DMSO, because the base-catalyzed hydrolysis
converts imide groups only to amic acids that are highly
resistant to base hydrolysis.10,13
Chemical structures of MCm-PMDAs were identified
through IR and NMR spectra. In Fig. 1 the 1H-NMR
spectrum (acetone-d6) of MC8-PMDAwith peak assignments
is shown. All the absorption peaks were found to be exactly
coincident with the expected chemical structure of MC8-
PMDA and no presence of COOH could be detected.
PolymerizationMC6- andMC8-PMDAswere polymerizedwith conventional
PDA and ODA in NMP by a two-step method, as shown in
Scheme 2. When equimolar mixtures of pure monomer pairs
were dissolved in NMP and stirred at room temperature, the
solutions became gradually viscous to form precursor
poly(amic acid)s. Inherent viscosities were measured using
the precursors and their values ranged from 0.57 to 1.58 dl/g,
as shown in Table 1. These values indicate that polymers
with reasonably high molecular weights have been obtained.
To obtain the polyimides in film form the poly(amic acid)s
solutions were cast onto clean glass plates and thermally
imidized over a routine heating cycle. All the films thus
02468
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e
d
c
O
O
OO
OO
OO
solvent
b~e
TMS
a
fg
H2Oh
ijk
δ (ppm)
k
ih b
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Figure 1. 1H-NMR spectrum of MC8-PMDA in acetone-d6.
Copyright # 2006 John Wiley & Sons, Ltd.
obtained were not soluble in any organic solvents both at
room and at boiling temperatures in spite of prolonged time
lapse of treatment of finely powdered samples and soluble
only in hot, concentrated sulfuric acid. Taking the appli-
cation of the polyimides only in film form, no samples were
prepared by chemical imidization.
The chemical structures of the polyimides were charac-
terized by FT-IR spectroscopy and elemental analysis.
Figure 2 shows the FT-IR spectrum of MC6-OPI as a
representative polymer sample. All the peaks characteristic
of imide linkages could distinctively be identified and no
acid or amide peaks could be detected. In addition, the
elemental analysis results given in the Experimental section
are well coincident with the calculated values.
Thermal propertiesThermal resistances were studied by thermogravimetric
analysis (TGA) in nitrogen atmosphere at a 108C/min scan
rate and their numerical results are summarized in Table 1.
All the TGA pyrograms exhibited a two-step degradation
behavior. In the lower-temperature step, the side branch
splits away from the polyimide backbone and in the higher-
temperature step the rigid backbone degrades. This thermal
behavior is typical in most of the rigid-rod polymers with
flexible side branches.13 Table 1 shows that the first onset
degradation temperatures (To values) lie higher than 451 and
4478C in MCm-PPIs and MCm-OPIs, respectively, indicating
that the polyimides have excellent thermal stability.
To see which part of the side group splits away in the
lower-temperature range, the n-alkyl group contents in a
repeat unit of the polymers were calculated and compared
with the measured values, as shown in Table 1. From Table 1
it is seen that the calculated values are fairly well coincident
with the values measured from the TGA pyrograms. This
result leads to the presumption that the polyimides with (n-
alkyloxy)phenyloxy side branches might be transformed to
thosewith (p-hydroxyphenyl)oxy group or p-quinonyl group
in the first-step thermolysis.
Thermal phase transitions were investigated by differen-
tial scanning calorimetry (DSC) in nitrogen at 108C/min scan
rate, but no transitions could be detected in spite of repeated
scans. However, in a previous study on the analogous
polyimides (DCm-OPIs), in which the same side groups are
doubly substituted at the 3,6-positions of the pyromellitic
ring, glass transition temperatures, (Tg values) could clearly
be determined, for example, at 2318C for m¼ 8.13 It is
reasonable to surmise that both crystal melting temperatures,
if any, and Tg valuesmight lie near or higher than the thermal
degradation temperatures, because MCm-OPIs contain dis-
tinctively less flexible groups, and hence are more rigid than
DCm-OPIs for the same m value.
Crystalline structureIt is generally known that rigid-rod polymers with flexible
side chains can crystallize into the layered structures, in
which two rigid backbones come across to form double-
strand rigid layers and the flexible side chains emanating
from the rigid layers into two lateral directions form a
separate crystal region.
Polym. Adv. Technol. 2006; 17: 444–452
DOI: 10.1002/pat
Scheme 2. Two-step polymerization.
New polypyromellitimides 449
Crystalline structure of the polyimide films was investi-
gated by X-ray diffractometry in transmission mode after
annealed at 2508C for 3 hr. In wide-angle patterns (data is not
shown) all the polyimides exhibited only a broad halo at
q�14.7 nm�1, indicating that they are amorphous and do not
possess any crystal domains.
Small-angle diffractograms of MCm-PPI and MCm-OPI
films are reproduced in Fig. 3(a) and 3(b), respectively. In
Fig. 3(a), MCm-PPIs show a broad peak in the smaller-angle
region and a sharp peak in the wider-angle region and the d-
spacing of the smaller-angle peak increases with the
increasing n-alkyl side chain length, while that of the
wider-angle peak stays constant at 11.7 A.
This result deduces to the conjecture that some layered
structure is developed in MCm-PPIs, because the smaller-
angle peaks are presumably ascribable for the layer spacings
between two neighboring backbones and the wider-angle
peaks are for rigid backbone domain widths. The backbone
width of poly(p-phenylenepyromellitdiimide) (PPI) without
any side substituents has been determined by X-ray
diffractometry to be 5.6 A.18 MCm-PPIs are monosubstituted
with flexible (n-alkyloxy)phenyloxy moiety only at one side
Table 1. Inherent viscosity (hinh) and thermal resistance
measurements of the polyimide films
Polymercode
hinha
(dL/g)
Thermal stabilityb
WR800
(%)First
T0 (8C)SecondT0 (8C)
WL1 (%)
Calculatedc Observed
MC6-OPI 0.57 55 447 581 16.9 16MC8-OPI 1.35 49 455 583 20.7 22MC6-PPI 0.58 50 451 596 20.0 19MC8-PPI 0.83 53 456 596 24.3 23
aDetermined from poly(amic acid) solutions (0.2 g/dl in NMP) at258C.bT0¼onset temperature of degradation, WR800¼ residual weight at8008C, WL1¼weight loss in the lower-temperature range.c Calculated for n-alkyl content.
Copyright # 2006 John Wiley & Sons, Ltd.
of PPI, and hence the very rigid, unsubstituted side of one
PPI-like chain should come close to the same rigid side of the
other chain to form a double-strand backbone. Then this
backbone should have a width value of 2� 5.6¼ 11.2 A. This
value 11.2 A calculated for double-strand PPI backbone
is surprisingly well coincident with the value 11.7 A.
Therefore, the presumption that MCm-PPIs might have a
double-strand backbone structure could be reasonably
accepted.
In Fig. 3(b) MCm-OPIs show two broad peaks around 27
and 15 A, indicating that the layered structure ismore loosely
developed in these samples than in MCm-PPIs. Figure 3(b)
also shows that the d-spacing of the wider-angle peak of
MCm-OPIs (14.8 A) is longer than that of MCm-PPIs (11.7 A).
This is naturally understood since the repeat unit of theMCm-
OPI backbone has a kinked p-phenylox-p-yphenyl group
(about 1208 bent), while the MCm-PPI backbones are fully
rod-like.
BirefringenceThe optical birefringence generated on the polyimide film
surface during the rubbing process wasmeasured in terms of
optical phase retardation (birefringence�phase) using
Figure 2. IR spectrum of MC8-OPI.
Polym. Adv. Technol. 2006; 17: 444–452
DOI: 10.1002/pat
Figure 3. Small-angle X-ray diffractograms of MCm-PPIs (a) and MCm-OPIs (b) taken in
transmission mode at room temperature.
450 J. K. Lee et al.
He-Ne laser at 632.8 nm. In Fig. 4 a polar diagram taken from
the optical phase retardation measurements upon in-plane
rotating MC6-PPI film that was rubbed at a density of 120 is
shown.
Such polar diagrams provide information on the reor-
ientation of polymer chains on the film surface and induced
by rubbing. As Fig. 4 shows, the signal intensity reaches a
maximum value at rotating angles of 08 and 1808 and a
minimum value at 908 and 2708. The same measurements
made for unrubbed films revealed no such anisotropic
orientation at all. This result means that the mechanical
rubbing exerted a noticeable change in chain reorientation on
polyimide surfaces through inducing anisotropic chain
alignment parallel to the rubbing direction. Such an
anisotropic reorientation has also been observed in the
analogous Cm-PPIs.14,19 This chain reorientation is known to
greatly affect LC-aligning ability and direction of the
polyimide films.
Surface morphologyThe surface morphology of polyimide films (3� 3mm2)
before and after rubbing at a density of 180 was investigated
by atomic force microscopy (AFM) and the images of MC8-
-0.010
-0.009
-0.008
-0.007
-0.006
-0.005
0
30
6090
120
150
180
210
240270
300
330
-0.010
-0.009
-0.008
-0.007
-0.006
-0.005
)esah
P X ec
neg
nirferiB(
Rubbing direction
Figure 4. Polar diagram taken from the optical phase retar-
dation measurement of a MC6-PPI film surface rubbed at a
density of 120.
Copyright # 2006 John Wiley & Sons, Ltd.
PPI together with depth profiles taken along the black lines
drawn in the images are represented in Fig. 5(a) and 5(b),
respectively. Figure 5(a) shows that the surfaces of the
polyimide films prepared by spin-coating for this study are
highly smooth and uniform. However, Fig. 5(b) clearly
shows a large groove with 0.3mm width at 2.67mm and a
small microgroove at 1.05mm.
The surface morphology of MC6-PPI surface must result
from the deformational response of polyimide films to the
mechanical shear force caused by contact with the roller
fibers during the pressed rubbing, and thus the resulting
deformation degree should be dependent on ductility of the
polyimide films. As it is well known, poly(p-phenylenepyr-
omellitimide), which corresponds to the backbone structure
of MC6-PPI, is fully rod-like, and hence very hard and
brittle.20 The formation of microgrooves, small or large, on
MC6-PPI surface means that MC6-PPI is more or less ductile.
Such ductility should be ascribed for the effect of appendance
of the flexible side chain.
LC alignment propertiesTo investigate the ability of the polyimide films to align LC
molecules, 50mm thick LC cells were fabricated from two
pieces of polyimide films (1.5� 4.0 cm2) rubbed at four
different rubbing densities and assembled in an antiparallel
rubbing direction.
Using the cells the direction of LC alignment was
determined by polar diagrams taken from the optical phase
retardation measurements. In Fig. 6 a polar diagram of the
linearly polarized He-Ne laser light absorbance of a dichroic
dye (Disperse Blue 1) as a function of rotational angle of the
cell fabricated fromMC6-OPI films is shown. Since the dye is
known to align with 5CB molecules, Fig. 6 clearly shows that
the absorbance reaches a maximum at rotation angles of 08and 1808 (rubbing direction) and minimum at 908 and 2708,and hence it is deduced that the LC molecules are aligned
homogeneously along the rubbing direction. LC cells made
from MC8-OPI, MC6-PPI and MC8-PPI under the same cell
fabrication method gave similar results in their polar
diagrams.
As discussed for Fig. 4, the anisotropic birefringence is
formed on the surfaces of the polyimide films parallel along
the rubbing direction and in Fig. 5 it was shown that the
Polym. Adv. Technol. 2006; 17: 444–452
DOI: 10.1002/pat
Figure 5. AFM images of MC6-PPI film surface (a) before
and (b) after rubbing at a rubbing density of 180.
-0.54
-0.51
-0.48
-0.45
-0.42
0
30
6090
120
150
180
210
240270
300
330
-0.54
-0.51
-0.48
-0.45
-0.42
Ab
sorb
ance
Rubbing direction
Figure 6. Polar diagram for measurement of MC6-OPI about
LC alignment direction at rubbing density 180.
New polypyromellitimides 451
microgrooves are generated again parallel along the rubbing
direction. Therefore, it can be said that the mechanical
rubbing has exerted influence on determining the alignment
direction of 5CB in the cells. As shown in Figs. 4 and 5, the
rubbing generates simultaneously both microscopic grooves
and molecular level chain reorientation through shear force-
induced deformation. However, as already suggested in the
studies with DCm-PPIs,14 it is also believed that themolecular
chain reorientation has greater influence on LC aligning
ability of polyimide film surfaces rather than microgrooves,
because the molecular dimension of 5CB (about 1.8 nm long
and 0.25 nm thick) is roughly similar to that of polyimide
repeat unit but far much smaller than that of microgrooves.
Pre-tilt angles were measured by a crystal rotation method
and their numerical values are plotted against rubbing
Copyright # 2006 John Wiley & Sons, Ltd.
density, as shown in Fig. 7(a) for MCm-OPIs and Fig. 7(b) for
MCm-PPIs. Figure 7 shows that themeasured angles lie in the
5–78 range in MCm-OPIs and above 168 for MCm-PPIs,
depending on rubbing density (pre-tilt angles over the 25–558range could not be measured with the crystal rotation
technique, due to some limitations in its optical setup).
Considering the fact that most polyimides without side
chains have pre-tilt angles lower than 58,1 the high values can
be attributable to the incorporation of side branches.
According to the reports,12,13,21,22 a pre-tilt angle of LC
molecules in contact with a rubbed polymer surface is
affected mainly by van der Waals interactions and the
inclination angle of the polymer backbones. MCm-PPIs have
fully rod-like backbone structures, and thus the contribution
of the backbone inclination toward pre-tilt angles of LC
molecules must be lower in MCm-PPIs than in MCm-OPIs.
However, the LC pre-tilting results revealed that higher
values were achieved in MCm-PPIs than in MCm-OPIs. This
result suggests first that the pre-tilt angles in this study were
mainly governed by van der Waals interactions between the
LCmolecules and the incorporated side branches and second
that the backbone inclination present in MCm-OPIs may
reduce the effective interactions.
Figure 7 also shows that pre-tilt angles for polyimides with
m¼ 8 is remarkably higher in MCm-PPIs and only slightly
higher in MCm-OPIs than those for polyimides with m¼ 6.
This behavior might be attributed to the greater contribution
of van der Waals interaction of n-pentyl part of 5CB with n-
octyl unit than that of n-hexyl unit of polyimides present on
the rubbed surface, because it could reasonably be surmised
that MC8-polyimides are richer in n-alkyl unit than MC6-
polyimides not only in bulk but also on the film surface. The
remarkably higherm value effect observed inMCm-PPIs than
in MCm-OPIs must be related to the higher contribution of
van der Waals interaction of MCm-PPIs than that of MCm-
OPIs.
In previous studies for DCm-PPIs.14,15 the pre-tilt angles
determinedwere in the ranges 25–558 for DC6-PPI and 55–878for DC8-PPI, much higher than those for MCm-PPIs. This
result can be explained by the fact that DCm-PPIs have two
Polym. Adv. Technol. 2006; 17: 444–452
DOI: 10.1002/pat
2602402202001801601401201008060400
2
4
6
8
10
260240220200180160140120100806040
16
20
24
28
Pre
tilt
Ang
le (o )
MC8-OPI
Pre
tilt
Ang
le (o )
Rubbing Density (L/l)
MC6-OPI
(b)
MC8-PPI
MC6-PPI
Rubbing Density (L/l)
(a)
Figure 7. Pre-tilt angles of 5CB molecules on the rubbed
MCm-OPIs (a) and MCm-PPIs (b) as a function of rubbing
density.
452 J. K. Lee et al.
side branches in their repeat units, hence the density of side
branches on the rubbed surface should be much higher in
DCm-PPIs than in MCm-PPIs.
SUMMARY
Two new PMDA derivatives singly substituted with 4-(n-
alkyloxy)phenyloxy groups (-O-Ph-O-n-CmH2mþ1,m¼ 6,8) at
their 3-position were successfully synthesized via consecu-
tive reactions starting from durene and were polymerized
with aromatic diamines, PDA and ODA, to obtain well-
defined polyimide films. The polyimide films showed good
chemical resistance in spite of the unsymmetrical incorp-
oration of a side branch and their inherent viscosities
measured with poly(amic acid) precusors in NMP were in
Copyright # 2006 John Wiley & Sons, Ltd.
the 0.57–1.35dl/g range. All the polyimides showed two-
step pyrolysis, which consisted of a side-chain scission near
4508C, followed by main-chain degradation. The X-ray
diffractograms revealed that the polyimide films had loosely
developed layered structures with double-stranded back-
bones. The phase retardation measurements showed that not
only the birefringence on the rubbed polyimide surface but
also the LC director in LC cells fabricated with the rubbed
polyimide films were parallel to the rubbing direction. The
pre-tilt angles of LC molecules on the rubbed film surfaces
were in the range 5–78 for MCm-OPIs and above 168 for MCm-
PPIs, depending on the alkyl side chain length and the
rubbing density.
AcknowledgmentThis work was supported by the Korea Research Foundation
(KRF 2004-005-D00009).
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