synthesis, characterization and liquid crystal-aligning properties of new...

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.interscie
Synthesis, 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, [email protected] 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


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


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.



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).


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.


DOI: 10.1002/pat


Synthesis of 3-[4-(n-octyloxy)phenyloxy]pyromelliticdianhydride (MC8-PMDA)This compound was prepared by the same procedure as for

MC6-PMDA.


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


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,




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,




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,




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.


DOI: 10.1002/pat


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

j f

e

d

c

O

O

OO

OO

OO

solvent

b~e

TMS

a

fg

H2Oh

ijk

δ (ppm)

k

ih b

ga

Figure 1. 1H-NMR spectrum of MC8-PMDA in acetone-d6.


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.


DOI: 10.1002/pat

Scheme 2. Two-step polymerization.


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.


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.


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.


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.


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


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.


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


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


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.


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


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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DOI: 10.1002/pat

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