Macromol. Rapid Commun. 2001, 22, 941–947 941

Synthesis and Characterization of a Soluble PolyimideContaining a Photoreactive 4-Styrylpyridine Derivativeas the Side Group

Seung Woo Lee, Taihyun Chang, Moonhor Ree*

Department of Chemistry, Center for Integrated Molecular Systems, BK21 Functional Polymer Thin Film Group, andPolymer Research Institute, Pohang University of Science & Technology, San 31, Hyoja-dong, Pohang 790-784, KoreaFax: 82-54-279-3399; E-mail: ree@postech.edu

IntroductionSome high performance and functional polymers, such asaliphatic-aromatic and aromatic polyimides, have beenwidely used in the liquid-crystal display (LCD) indus-try.[1 – 9] Particularly, oriented layered polymeric materialsare processed as thin films, and liquid-crystal (LC) align-ment is typically induced by rubbing with fabric vel-vets.[1 – 9] This mechanical rubbing process is known as aneffective surface treatment technique to align LC mole-cules on the film surface.[1 – 8] However, this process suf-fers from shortcomings, such as the generation of dust,electrostatic problems, and a poor control over the rub-bing strength.[3 – 8]

To overcome these disadvantages, some photo-inducedLC alignment concepts have been reported[10 – 21] andreviewed recently.[22, 23] One of the approaches is based onthe incorporation of photoreactive cinnamoyl, coumarin,or 2-styrylpyridine moieties as side groups.[10 – 19, 22, 23] Sev-

eral polyvinyl derivatives containing these photoreactivegroups have been reported upon. These polymers undergoa preferential orientation via a directionally selectivephotoreaction of the side groups when exposed to linearlypolarized UV light (LPUVL). The coumarin group canphotodimerize only because of the vinylene linkage dueto the fused ring, while both cinnamoyl and 2-styrylpyri-dine moieties can undergo photodimerization and trans-cis isomerization. Thus, there have been debates on thephotoalignment mechanism with respect to polymersbearing cinnamoyl and 2-styrylpridine groups.[10 – 19, 22, 23]

The second approach utilizes polymers containing azo-benzene moieties, which can undergo trans-cis photoi-somerization only.[20 – 23] Nevertheless, these polyvinyl-based materials are not workable for the production ofLCDs because of a number of unsolved problems, such aslow thermal stability, low anchoring energy, low pretiltangle, and limited processability with LPUVL. Thus, it is

Communication: An aromatic polyimide bearing photo-reactive 4-(2-(4-oxyethylenyloxyphenyl)vinyl)pyridineside groups was synthesized and characterized. The poly-mer is stable up to 3008C and soluble in organic solvents,giving thin films in good quality. When exposed to UVlight, it reorients favorably with an angle of 988 withrespect to the electric vector of linearly polarized UVlight. UV-exposed films align liquid-crystals (LCs) homo-geneously along the preferential orientation of the poly-mer chains on the surface. The pretilt angle of the LCs is0.32–0.928, depending on the exposure dose and anneal-ing. LC alignment is retained up to 2108C. Based on theoptical retardation behavior and spectroscopic measure-ments, a photoalignment mechanism is proposed.

Macromol. Rapid Commun. 2001, 22, No. 12 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2001 1022-1336/2001/1208–0941$17.50+.50/0

UV-Vis spectra of 6F-HAB-ETPVP polymer films exposedto unpolarized UV light (260–380 nm) at varying exposuredose.

942 S. W. Lee, T. Chang, M. Ree

still a big challenge to develop high performance align-ment-layered polymers for LCD industry, which do notdepend on mechanical rubbing. Furthermore, the photo-alignment mechanism needs to be clarified.

In this study, a new soluble polyimide containingphotoreactive 4-(2-(4-oxyethylenyloxyphenyl)vinyl)pyri-dine, a 4-styrylpyridine derivative, as the side group wassynthesized, and its photoreactivity and photoalignmentcharacteristics were investigated. In addition, the LCalignment behavior of films exposed to LPUVL withvarying exposure dose was examined.

Experimental Part

Materials and Characterization

2,2-Bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhy-dride (6F) was purified by sublimation. 3,39-Dihydroxy-4,49-diaminobiphenyl (HAB) was purified by drying in a vacuumoven at 1008C for 1 d. N-Methyl-2-pyrrolidone (NMP) andtetrahydrofuran (THF) were distilled over calcium hydrideunder reduced pressure and under a nitrogen atmosphere,respectively. All other chemicals were used as received.

1H NMR spectra were taken on a Bruker Aspec 300 MHz.Molecular weights were estimated by means of gel permea-tion chromatography (GPC; Polymer Labs Model PL-GPC210) equipped with a set of four columns (Alltech Jordi100 �, 1000 �, 10000 �, and 100000 �) using a flow rateof 1.0 mL/min, THF as the eluent, and calibration with poly-styrene standards (M

—w = 2800–1260000).

Synthesis of 4-(2-(4-(2-Hydroxyethoxy)phenyl)vinyl)pyridine(ETPVP)

4-(2-Hydroxyethoxy)benzaldehyde (4.003 g, 24.09 mmol),3,4-dihydro-2H-pyran (3.037 g, 36.10 mmol) and pyridiniump-toluenesulfonate (PPTS; 1.206 g, 4.82 mmol) were dis-solved in dry methylene chloride (40 mL) and stirred for 10h at room temperature. The reaction mixture was dilutedwith diethyl ether and washed twice with water to removePPTS, followed by drying over magnesium sulfate. Oily 4-(2-tetrahydropyranyloxyethoxy)benzaldehyde (THPBA; 6.03g, 24.08 mmol) was obtained after evaporating the solvent.The oily product was dissolved in THF and added dropwiseto a mixture of 4-picoline in THF and butyllithium (BuLi) inhexane at –788C under a nitrogen atmosphere. The reactionmixture was stirred for 2 h and then poured into a saturatedammonium chloride solution to eliminate excess BuLi. Theorganic layer was dried, and the solvent was removed byusing a rotary evaporator. The product, 4-(2-(4-((2-te-trahydropyranyloxy)ethoxy)phenyl)-2-hydroxyethyl)pyridine(4.51 g, 13.13 mmol), was recrystallized from diethyl ether.The product and p-toluenesulfonic acid monohydrate(TsOH; 0.5 g, 2.63 mmol) were dissolved in methanol andrefluxed with stirring for 12 h. After cooling to room tem-perature, the reaction mixture was poured into 500 mL ofwater and adjusted to pH 7 with aqueous sodium bicarbo-nate. The resulting yellow solid was washed thoroughly withwater and collected by filtration. The crude product was

recrystallized from methanol, providing ETPVP in 45%yield.

1H NMR (300 MHz, DMSO-d6): d = 8.53 (d, 2 H, PyH),7.62 (d, 2 H, PyH), 7.53 (d, 2 H, ArH), 7.51 (d, 1 H,Ar1CH=), 7.12 (d, 1 H, Py1CH=), 7.00 (d, 2 H, ArH), 4.92(t, 1 H, 1OH), 4.04 (d, 2 H, Ar1O1CH21), 3.73 (d, 2 H,1CH21OH).

Synthesis of Soluble Aromatic Polyimide 6F-HAB

6F (1 molar eq.) and HAB (1 molar eq.) were dissolved indry NMP together with isoquinoline as the catalyst (2 molareq.). The mixture was gently heated to 708C under stirringfor 2 h, followed by refluxing for 5 h. Thereafter, the reac-tion solution was poured into a mixture of methanol andwater under vigorous stirring, giving 6F-HAB polyimide as apowder. The precipitate was filtered, washed with methanol,and dried under vacuum to give 98% of 6F-HAB; M

—w =

53400.1H NMR (300 MHz, DMSO-d6): d = 10.10 (s, 1 H, ArOH),

8.24 (d, 1 H, ArH), 8.03 (d, 1 H, ArH), 7.82 (s, 1 H, ArH),7.43 (d, 2 H, ArH), 7.23 (d, 2 H, ArH).

Synthesis of Photoreactive Polyimide 6F-HAB-ETPVP

According to the Mitsunobu reaction,[24] 6F-HAB (1 molareq.), ETPVP (4 molar eq. per mole polymeric repeat unit)and triphenylphosphine (TPP; 4 molar eq. per mole poly-meric repeat unit) were dissolved in dry THF under a nitro-gen atmosphere. Then, diisopropyldiazocarboxylate (DIAD;4 molar eq. per mole polymeric repeat unit) was added drop-wise. After stirring for 8 h at room temperature, the solutionwas poured into hot methanol under vigorous stirring, givingphotoreactive 6F-HAB-ETPVP polyimide as a powder. Theprecipitate was filtered, washed with methanol, and driedunder vacuum to give 97% of 6F-HAB-ETPVP.

1H NMR (300 MHz, DMSO-d6): d = 8.43 (s, 2 H, PyH),8.07 (d, 1 H, ArH), 7.92–7.10 (m, 8 H, ArH, PyH,Ar1CH=), 7.41 (m, 2 H, ArH), 6.76 (d, 1 H, Ar1CH=), 6.60(s, 2 H, OArH), 4.48 (s, 2 H, Ar1O1CH21), 4.21 (s, 2 H,1CH21O1Ar).

Film Properties, Photoreactivity and Photoalignment

A solution of 6F-HAB-ETPVP in NMP (1.0 wt.-%) wasspin-cast on pre-cleaned quartz substrates and dried for 12 hin a vacuum oven at 1208C, giving a series of films with ca.200 nm thickness. Glass transition (Tg) and decomposition(Td) temperatures were determined (10.08C/min, N2 atmos-phere) using a differential scanning calorimeter (Seiko, DSC220CU) and a thermogravimeter (Seiko, TG/DTA-6300). In-plane and out-of-plane refractive indices (nxy and nz) weremeasured at room temperature using a prism couplerequipped with a He-Ne laser (632.8 nm, that is, 474.08 THzin optical frequency, cubic zirconia prism, n = 2.1677, con-trolled by a personal computer).[25 – 27] UV irradiation (260–380 nm) was performed using a high-pressure Hg lamp sys-tem of 1.0 kW (Altech, ALHg-1000) with an optical filter(Milles Griot, 03-FCG-179). The UV exposure dose appliedwas measured using a photometer (International, IL1350)with a sensor (SED 240). Photoreactivity measurementswere carried out using a UV-Vis spectrometer (Hewlett-

Synthesis and Characterization of a Soluble Polyimide Containing ... 943

Packard, HP8452). IR spectra were measured with an FT-IRspectrometer (ATI Mattson, Research Series 2). LPUVL irra-diation (260–380 nm) was carried out using a linear dichroicpolarizer (Oriel, 27320). Optical retardation and dichroicratio were determined with a plane polariscopic system andUV-Vis spectrometer, respectively.

LC Alignment

Two series of irradiation were performed: (1) irradiationwith unpolarized UV light (260–380 nm, 0.25 J/cm2) andsubsequent LPUVL irradiation (260–380 nm, varying expo-sure dose), and (2) UV irradiation (unpolarized, 260–380 nm, 0.25 J/cm2) and subsequent LPUVL irradiation(260–380 nm, 1.50 J/cm2). For LPUVL exposure, the filmwas positioned with a tilt angle of 458 (tilt angle: anglebetween film plane and propagation plane of the light). Theirradiated films were further annealed in an accumulativestep manner at 60, 80, 120, 150, 180, and 2108C/10 min.Pairs of the film-adhered glass slides were paralleled to theelectric vector of LPUVL and then assembled together with50 lm thick spacers. The assembled cells were filled with 4-pentyl-49-cyanodiphenyl (5CB, Aldrich) containing adichroic dye (Disperse Blue 1, Aldrich; 1.0 wt.-%) by thecapillary method, and sealed with epoxy glue. Measurementswere carried out with a plane polariscope equipped with line-arly polarized He-Ne laser (632.8 nm) as a function of rota-tional angle. Pretilt angle a of the LCs was determinedapplying the crystal rotation method.[3, 4]

Results and Discussion

Synthesis and Properties

New photoreactive monomer ETPVP was synthesizedfrom 4-(2-hydroxyethoxy)benzaldehyde and 4-picoline.The chemical structure was determined by means of 1HNMR spectroscopy. The UV-Vis spectrum of ETPVP (in1,4-dioxane) shows two absorption maxima at 324 and230 nm, which can be assigned to the 4-styrylpyridinemoiety and the aromatic and pyridine rings, respectively.Polyimide 6F-HAB with M

—w = 53400 and a polydisper-

sity of 1.87 was prepared from a one-step polycondensa-tion in NMP.

ETPVP was incorporated into 6F-HAB as a side groupvia the Mitsunobu reaction (Scheme 1). In the 1H NMRspectrum, the hydroxyl protons of 6F-HAB appear around10.0 ppm. However, such chemical shift was not detectedfor the product 6F-HAB-ETPVP, indicating that the OHgroups of 6F-HAB quantitatively reacted with ETPVP.6F-HAB decomposes at 4408C (Td) but does not show aTg over the range 20–4408C. For 6F-HAB-ETPVP, Td =3008C and Tg = 2048C, mainly originating from theincorporation of ETPVP side groups. For a film of 1.8lm, in-plane refractive index nxy = 1.632, while out-of-plane refractive index nz = 1.616. Out-of-plane birefrin-gence D = nxy-nz = 0.0163, indicating that the polymerchains are oriented in the film plane as typically observed

for semi-rigid polyimides.[25 – 27] The results also indicatethat 6F-HAB-ETPVP is a positively birefringent polymerwith the polarization along the chain axis being relativelylarger than that normal to the chain axis.

Photoreactivity

Figure 1 shows the UV-Vis spectra of a 6F-HAB-ETPVPfilm irradiated with UV light at various exposure doses.The spectrum exhibits three absorption maxima at 225,308 and 324 nm, originating from aromatic and pyridinerings, 6F-HAB polyimide backbones, and photoreactiveETPVP side groups, respectively. The absorption peak atkmax = 324 nm decreased drastically with increasing expo-sure doses up to 7.5 J/cm2. Instead, a new absorptionband appeared at 260 nm, which increased with increas-ing exposure dose. This might be due to photoreactions,such as [2+2] cycloaddition and trans-cis isomerizationof the ETPVP moiety.

Scheme 1. Synthetic scheme of photosensitive 6F-HAB-ETPVP.

Figure 1. UV-Vis spectra of 6F-HAB-ETPVP polymer filmsexposed to unpolarized UV light (260–380 nm) at varying expo-sure dose.

944 S. W. Lee, T. Chang, M. Ree

Quina and coworkers[28] reported that styrylpyridiniumsalts in the solid state undergo photodimerization favor-ably. Ichimura et al.[29] and Cockburn et al.[30] alsoreported that poly(vinyl alcohol) films containing styryl-pyridinium salts become photochemically crosslinked viaa [2+2] cycloaddition of the incorporated salts. Further-more, Ichimura and coworkers[17 – 19] found that photodi-merization occurs as the major reaction in polymethacry-lates containing 2-styrylpryridine moieties with variousakylene spacers. Considering these results it is reasonablethat both the decrease in the absorption intensity at kmax =324 nm and the appearance of a new peak at 260 nm aremainly due to a [2+2] photodimerization of the ETPVPside groups. Moreover, the polyimide film becomes insol-uble in NMP when exposed to UV light, which corrobo-rates the photodimerization hypothesis.

In the UV-Vis spectrum of the 6F-HAB-ETPVP film,three isosbestic points appear during the course of irradia-tion with varying exposure dose: (1) at ca. 275 nm, (2) atca. 248 nm, and (3) at ca. 240 nm. The appearance ofmultiple isosbestic points suggests that the trans-ETPVPside groups in the polymer film undergo cis-isomerizationin addition to photodimerization. In particular, the firstisosbestic point around 275 nm is shifted to the lowwavelength region when irradiated to UV light with A 3.0J/cm2. This blue-shift may be caused by photochemicaldegradation.

Figure 2 shows FT-IR spectra of the polymer filmswith and without UV exposure. The unexposed film hasvibrational absorption bands at 1786, 1726, 1592, 1510,1377, and 966 cm – 1. The two bands at 1786 and 1726cm – 1 can be attributed to symmetric and asymmetricstretching vibrations of the carbonyl group of the imidering, which are typical of polyimides.[31] The band at1377 cm – 1 corresponds to the C-N stretching vibration,that at 1510 cm – 1 to C=C stretching vibrations in the aro-matic and pyridine ring, that at 1592 cm – 1 to C=C stretch-ing vibrations of the vinylene linkage and the aromatic

ring, as well as to the C=C stretching vibration of the py-ridine ring. The band at 966 cm – 1 results from the out-of-plane bending of C1H in the vinylene linkage.[32]

For the UV-exposed film, the IR bands of the vinylenelinkage (1592 and 966 cm – 1) decreased gradually withincreasing exposure dose. The peak at 1592 cm – 1 wasshifted to 1602 cm – 1, which is attributed to an increase inthe population of non-conjugated C=N stretching in thepyridine rings due to the photochemical consumption ofvinylene linkages. These results further suggest that[2+2] photodimerization of the ETPVP side groups takesplace indeed. In contrast, characteristic vibrations of thecis-isomeric ETPVP side groups could not be detected.

The band at 1786 cm – 1 was shifted to the low fre-quency region, while the peak at 1726 cm – 1 becamebroad, which might be caused by a degradation of imiderings. These results suggest that UV exposure causes cer-tain damage to the polymer backbone.

Photoalignment Behavior

Figure 3a shows UV dichroic ratios of 6F-HAB-ETPVPfilms with and without LPUVL exposure (dichroic ratio =(Ak - A//)/(Ak + A//) where Ak and A// are the absorbances atkmax = 324 nm measured perpendicular and parallel to theelectronic vector of LPUVL, respectively). All thedichroic ratios measured are positive for 0.0–3.0 J/cm2.The ratio increased rapidly with increasing exposure doseand reached a maximum at 1.0 J/cm2. Thereafter, thedichroic ratio decreased slightly with further increasingexposure dose, indicating that the ETPVP side groupslocated in parallel to the electric vector of LPUVL wereconsumed more rapidly than those positioned perpendicu-larly. The direction-selective consumption of ETPVP sidegroups might take place via [2+2] cycloaddition andtrans-cis isomerization.

As shown in Figure 3b, the optical retardationincreased sharply with increasing exposure dose, reaching2.3 nm at 0.5 J/cm2. Retardation is then leveled off ordecreased slightly with further increasing dose. Overall,all the films revealed positive retardation, which can beattributed to an optical anisotropy (in-plane birefrin-gence) generated by exposing to LPUVL. The opticalretardation might result from three major factors. (1) 6F-HAB-ETPVP is a positively birefringent polymer chain,as discussed in a preceding section. This means thatpolarization along the backbone is larger than along theside group. The trans-isomeric side group has a relativelylarge polarization along its longer axis, as compared to itscis-isomer. The cis-isomerization of the side groups dueto LPUVL exposure may thus enhance the positive bire-fringence of the polymer chain. The side groups arebound to the polymer backbone at an angle that is near tothe normal rather than to the parallel direction. It turnsout that such local cis-isomerizations of the side groupscontribute positively to create an optical anisotropy in the

Figure 2. FT-IR spectra of 6F-HAB-ETPVP polymer filmsexposed to unpolarized UV light (260–380 nm) at varying expo-sure dose.

Synthesis and Characterization of a Soluble Polyimide Containing ... 945

film plane. (2) The direction-selective cis-isomerizationof the side groups is accompanied by local motions,which may propagate onto their polymer chain back-bones, perhaps causing a reorientation of the polymerchains to some extent. Similar phenomena were observedin polymers containing azobenzene derivatives as sidegroups, which can undergo trans-cis isomerizationonly.[20 – 23, 33] A polymer chain reorientation may contri-bute to the optical retardation. (3) The direction-selective[2+2] photodimerization of the side groups requires a cer-tain movement of the polymer chains. Thus, photodimer-ization is also accompanied with a reorientation of thepolymer chains, leading to a positive optical retardationin the film plane.

All three factors together may contribute to the genera-tion of a positive optical retardation. However, they maycompete with each other, meaning, the overall opticalretardation would be controlled by one or two predomi-nant factor(s).

LC Alignment

Figure 4 illustrates a polar diagram of LC cells fabricatedfrom 6F-HAB-ETPVP films exposed to unpolarized UVlight (0.25 J/cm2) and, subsequently, to LPUVL (1.5 J/cm2). The diagram indicates that the LC molecules arealigned homogeneously on the film surface, and theirmain director has an angle of 988 with respect to the elec-

tric vector of LPUVL. Similar polar diagrams wereobtained for the other LC cells fabricated from films irra-diated with LPUVL of various exposure doses. Theseresults suggest that the LPUVL exposure initiates direc-tion-selective photoreactions, causing polymer mainchains and side groups oriented enough to induce homo-geneous LC alignment on the surface.

Pretilt angle a of the LCs in the cells was measured bymeans of the crystal rotation method. a varies in the range0.32–0.588, depending on the exposure dose of LPUVLirradiation. As shown in Figure 5a, a initially increasedwith increasing exposure dose but leveled off above 2.0J/cm2.

Angle a was also measured for the LC cells fabricatedfrom films irradiated with 1.5 J/cm2, which were subse-quently annealed at various temperatures. In the non-annealed film, a = 0.458 but raises to 0.70–0.928 in theannealed film, depending on the annealing history over60–2108C (see Figure 5b). The pretilt angle is retainedup to 2108C, which is higher than Tg = 2048C of the poly-mer film. Furthermore, all cells fabricated from annealedfilms reveal polar diagrams very similar to that observedfor the cell of the non-annealed film.

Both the polymer backbone and the side group consistof mesogen units, which can interact favorably with LCsbased on biphenyl mesogens. The side group possesses a4-styrylpyridine mesogen moiety. Its effective length isrelatively longer in the trans-isomeric form than the cis-isomeric form, so that the former may interact morefavorably with LCs than the latter. The polymer backbonehas a 6F and an HAB moiety per chemical repeat unit,which can act as mesogen units, so that the polymermain-chain may interact favorably with LCs. Therefore,the LC alignment measured might be induced mainly bythe polymer backbone and the trans-isomeric side groups.Such properly oriented units might originate partiallyfrom the reorientation process via direction-selectivephotoreactions (photodimerization and photoisomeriza-

Figure 3. (a) Dichroic ratios and (b) optical retardations of 6F-HAB-ETPVP polymer films exposed to LPUVL (260–380 nm)at varying exposure dose.

Figure 4. Polar diagram of LC cells fabricated from 6F-HAB-ETPVP films irradiated sequentially with unpolarized UV light(0.25 J/cm2) and LPUVL (1.50 J/cm2).

946 S. W. Lee, T. Chang, M. Ree

tion) of the side groups, and partially from the fact thatbackbones and side groups are oriented at an angle of 988with respect to the electric vector of the LPUVL.

The [2+2] photodimerization is a crosslink process,increasing the thermal and dimensional stability of thepolymer film. Thus, the LPUVL-caused reorientation ofthe main chains may cause a relatively high resistanceagainst any influences of thermal annealing. In contrast,the photoisomerization is a reversible process, meaningthe photochemically induced cis-isomerization of the sidegroups may be reverted through thermal annealing. Thisreversible process is also accompanied by a reorientationof the polymer chains back to the original orientation.Thus, thermal annealing may influence severely both theorientation of the polymer chains and the cis-isomers.

The alignment behavior of the LCs in the annealedfilms is very similar to that in the non-annealed film,regardless of the annealing history. Similar results werealso observed for a polyimide bearing a coumarin deriva-tive as the side group, which undergoes photodimeriza-tion only.[34]

Therefore, the alignment of LCs in the film mightresult (1) from the favorable interactions with the poly-mer backbone reoriented by means of photodimerization,and (2) from the fact that the backbone and side groupresidues are oriented at an angle of 988 with respect tothe electric vector of the LPUVL.

ConclusionsA novel photosensitive ETPVP moiety was synthesizedand incorporated successfully into the aromatic polyimide6F-HAB to give the soluble, photoreactive polyimide 6F-HAB-ETPVP.

The polyimide films are highly sensitive towards UVlight, resulting in a preferential orientation of the polymerchains via direction-selective photoreactions on LPUVLexposure. The photoalignment of the polymer chainsseems to be mainly driven by a photodimerization of theETPVP side groups in parallel to the electric vector ofLPUVL. LCs are homogeneously aligned on the film sur-face along a direction with an angle of 988 with respectto the electric vector of the LPUVL. Depending on theUV exposure dose, the LCs show a pretilt angle of 0.32–0.588.

In conclusion, the new photoreactive polyimide, 6F-HAB-ETPVP, is a good candidate as an orientationallayered material for LC display devices with in-planeswitching mode.

Acknowledgement: This study was partially supported by theCenter for Integrated Molecular Systems (KOSEF) and by theMinistry of Industry & Energy and the Ministry of Science &Technology (Electronic Display Industrial Research Association– G7 Project Program).

Received: February 12, 2001Revised: May 22, 2001

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