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Synthesis of alkyl-branched main chain copolyimides and their effect on the pretilt angles of liquid crystal alignment

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Synthesis of alkyl-branched main chain

copolyimides and their effect on the pretilt angles

of liquid crystal alignment

Wen-Chin Lee , Jiun-Tai Chen , Chain-Shu Hsu & Shin-Tson Wu Published online: 11 Nov 2010.

To cite this article: Wen-Chin Lee , Jiun-Tai Chen , Chain-Shu Hsu & Shin-Tson Wu (2002) Synthesis of alkyl-branched main chain copolyimides and their effect on the pretilt angles of liquid crystal alignment, Liquid Crystals, 29:7, 907-913, DOI: 10.1080/02678290110116880

To link to this article: http://dx.doi.org/10.1080/02678290110116880

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Synthesis of alkyl-branched main chain copolyimides and their

eVect on the pretilt angles of liquid crystal alignment

WEN-CHIN LEE, JIUN-TAI CHEN, CHAIN-SHU HSU*

Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30050, ROC

and SHIN-TSON WU

School of Optics/CREOL, University of Central Florida, Orlando, FL 32816, USA (Received 16 May 2001; in Ž nal form 20 October 2001; accepted 23 October 2001) Three series of copolyimides containing long alkyl branches were synthesized using the two-step method via poly(amic acid) precursors and chemical imidization. Most of the copolyimides prepared are soluble in polar organic solvents. Good liquid crystal alignment was achieved by buYng the copolyimide Ž lms spin-coated onto indium tin oxide glass substrates. The measured liquid crystal pretilt angles range from 0.16ß to 15.54ß . The Titan simulation program was used to calculate the dipole of each dianhydride structure and correlate with the observed pretilt angles. Main chain copolyimides with a long alkyl side chain, small dipole, and linear, symmetric and rigid core structures are favourable for generating large pretilt angles.

1. Introduction In this paper, we report systemati c studies on LC pretilt Uniform molecular alignment plays a crucial role in angles using diVerent main chain structures while keep-the electro-optic performance and panel fabrication of a ing the same alkyl side branch. We have prepared several liquid crystal display (LCD) device [1]. Rubbed poly- soluble copolyimides (coPIs) based on 4(octadecyloxy) -imide (PI) Ž lms are commonly used as the alignment 1,3-benzenediamine (18OBD) which has a long linear layers for large LCD panels because of their outstanding alkyl group with 18 carbon atoms. Three dianhydrides thermal stability, low dielectric constant, excellent chemical (BCDA, 6FDA, BPDA) and four diamines (ODA, resistance and high productivity. In addition, the rubbing MDA, PPD, DDS) were used to modify the main chain process provides a stable pretilt angle on the PI Ž lm, structures of the coPIs. The coPI Ž lms were obtained preventing reverse tilt disclination of LC molecules with from their solutions by evaporating the solvents at applied voltage. In order to satisfy speciŽ c pretilt angle low temperature. Their application as LCD alignment requirements for various LCD modes, several approaches layers was evaluated, and the correlation between coPI have been developed for controlling the pretilt angle on backbone structures and LC pretilt angles is discussed. rubbed PI Ž lms [2–11].

Polyimides with long alkyl [3–8] and  uorinated

2. Experimental

alkyl [9–11] side groups were found to exhibit high LC

2.1. Materials

pretilt angles; these may originate from the low surface

energy on the alkyl-branched PI Ž lms [4, 9]. Steric In our experiments, 4,4¾ -( hexa uoroisopropylidene)-interaction between LC molecules and branched long diphthalic anhydride (6FDA) was purchased from Aldrich. alkyl side chains is another possible cause for high pretilt 4,4¾ -Biphthalicanhydride (BPDA), bicyclo[2,2,2]oct-7-ene-angles. However, detailed mechanisms governing how the 2,3,5,6-tetrac arboxylicdianhy dride (BCDA), bis(4-amino-long alkyl side chains aVect the LC pretilt angle have phenyl )sulfone (DDS) and 1-chloro-2,4-dinitrobenzen e not been fully understood. Recently, new studies on were obtained from TCI. 4,4¾ -Methylene dianiline (MDA) pretilt angles for PIs having various side group lengths and p-phenylene diamine (PPD) were purchased from

have been reported [6–8], but, little is known about the Janssen. 4,4¾ -Diaminodipheny l ether (ODA) was obtained eVect of main chain structure on the LC pretilt angle. from Chriskev; 1-octadecanol was obtained from Showa. The nematic LC, 4¾ -pentyl-4-cyanobipheny l (5CB) was supplied by Merck.

*Author for correspondence.

L iquid Crystals ISSN 0267-829 2 print/ISSN 1366-585 5 online © 2002 Taylor & Francis Ltd

http://www.tandf.co.uk/journals DOI: 10.1080/02678290110116880

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908 W.-C. Lee et al.

ODA, MDA, PPD and DDS were recrystallized from ethanol and BCDA from acetonitrile. N-methyl-2-pyrrolidone (NMP) was dried via the benzene azeo-trope. Anhydrous tetrahydrofuran and pyridine were dried by heating at re ux with sodium under nitrogen, followed by vacuum distillation, and stored with molecular sieves. All other reagents and solvents were used as received.

2.2. Synthesis

As shown in Ž gure 1, the linear alkyl group-containin g diamine, 4-(octadecyloxy)-1,3-benzenediamin e (18OBD), was synthesized by reduction of 1(octadecyloxy)2,4 -dinitrobenzene, which had been obtained by ether-iŽ cation of 1-chloro-2,4- dinitrobenzene with 1-octadecano l. The 12 copolyimides were synthesized using the two-step method via poly(amic acid) precursors and chemical imidization. Figure 2 shows the structure of the three dianhydrides and four diamines employed. The poly-(amic acid ) precursor was prepared by mixing one dianhydride and one diamine with 18OBD in the molar ratio 5 : 4 : 1, respectively.

2.2.1. 1-(Octadecyloxy)-2,4-dinitrobenzen e

1-Octadecanol (16.9 g, 0.0625 mol ), potassium iodide (0.1 g, 0.0006 mol), potassium hydroxide (3.5 g, 0.0625 mol) and 200 ml of tetrahydrofura n were mixed and heated at re ux for 3 h with stirring. 1Chloro2,4dinitro -benzene (5.0 g, 0.0247 mol) was added to the mixture

Figure 2. Synthesis scheme for three series of copolyimides based on 18OBD.

which was heated under re ux for a further 21 h. After cooling to room temperature , the solvent was evaporated , and the solid obtained was dissolved in ethyl acetate and washed with distilled water until pH5 7. The organic layer was dried with anhydrous MgSO4, the solvent removed in a rotary evaporator, and the crude product recrystallized twice fromn-hexane to yield 9.23 g (86%) of slightly yellowish crystals; m.p. 64.5ß C. 1H NMR (CDCl3, TMS, ppm), d: 0.88 (t, 3H, CH3); 1.26 (m, 28H, CH2(CH2)14CH3); 1.49 (m, 2H, CH2CH2(CH2)14CH3); 1.90 (m, 2H, CH2CH2(CH2)15CH3); 4.23 (t, 2H, O CH2(CH2)16CH3); 7.18 (dd, 1H, ArH of position 6 of 2,4-dinitrobenzene) ; 8.40 (dd, 1H, ArH of position 5 of 2,4-dinitrobenzene) ; 8.74 (d, 1H, ArH of position 3 of Figure 1. Synthesis scheme for the long alkyl-containing

diamine 18OBD. 2,4-dinitrobenzene).

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2.2.2. 4-(Octadecyloxy)-1,3-benzenediamine (18OBD) were determined with a Seiko SSC/5200 diVerential scanning calorimeter (DSC) equipped with a liquid 1-(Octadecyloxy)-2,4-dinitrobenzen e (4 g, 0.0092 mol)

was added to 300 ml ethyl alcohol solution containing nitrogen cooling accessory. Heating and cooling rates were 10ß C minÕ 1. The glass transition temperatures (T

g) 0.4 g (10 wt %) of Pd/C (10% Pd on charcoal); the

solution was stirred and heated to re ux. After adding reported here were observed during the second heating scans at the in ection points. Thermal stability was 4 ml of hydrazine monohydrate slowly, the mixture was

heated at re ux for 24 h. The hot solution was then tested using a Seiko TG/DTA 200 thermal gravimetric analyser under nitrogen at a heating rate of 10ß C minÕ 1. Ž ltered over celite to remove the catalyst and the solvent

removed in a rotary evaporator. The white crystalline product was Ž ltered oV and puriŽ ed by recrystallization

2.3.1. Preparation of coPI alignment layers

twice from ethyl alcohol to yield 2.9 g (84%) of

The synthesized soluble coPIs were used as LC align-crystals; m.p. 76.5ß C.1H NMR (CDCl

3, TMS, ppm), d: ment materials. They were dissolved in selective solvents 0.88 (t, 3H, CH3); 1.26 (m, 28H, CH2(CH2)14CH3);

to form 5 wt % solutions. The coPI solutions were spin-1.46 (m, 2H, CH2CH2(CH2)14CH3); 1.75 (m, 2H,

coated onto indium tin oxide (ITO) glass substrates at CH2CH2(CH2)15CH3); 3.5 (s, broad, 4H, NH2); 3.89

3000 rpm, and the casts were baked at 100ß C for 1 h. (t, 2H, O CH2(CH2)16CH3); 6.04 (dd, 1H, ArH of

The thickness of the coPI layers was measured by a position 6 of 1,3-benzenediamine) ; 6.15 (d, 1H, ArH of

a-stepper to be in the 50–100 nm range. The coPI-coated position 2 of 1,3-benzenediamine) ; 6.6 (dd, 1H, ArH

substrate was buVed 4 times (single direction) by a of position 5 of 1,3-benzenediamine) . IR (KBr, cmÕ 1):

rubbing machine (Sigma Koki RM-50) with a 48 nm 3416, 3450 (NH2), 2850, 2920 (CH ), 1223 ( O ).

diameter roller covered with a cotton velvet cloth. The Ž bre length was 2 mm. To summarize the rubbing con-2.2.3. Synthesis of copolyimides ditions: roller speed5 200 rpm, speed of substrate state 5

All the copolyimides were synthesized using the con- 7 mm sÕ 1, and pile impression5 0.3 mm. ventional two-step method as illustrated in Ž gure 2. In

a typical example, ODA (0.3227 g, 0.0016 mol ) and

2.3.2. Fabrication of homogeneous L C cells

18OBD (0.1515 g, 0.0004 mol) were placed in a 50 ml

An empty cell was constructed by assembling two 2-neck round bottom  ask Ž tted with a magnetic stirrer.

glass substrates, each covered with a buVed thin coPI The system was purged with nitrogen, and 4 ml of

layer. The two rubbing directions were anti-parallel. The NMP added. After stirring for 10 min, BCDA (0.505 g,

cell gap was controlled at ~30 mm by dispersed spacer 0.002 mol) was added carefully. The neck was rinsed

balls. The cell was then Ž lled with the nematic liquid with NMP and the monomer concentration kept at

crystal, 5CB, at room temperature and sealed with epoxy 15 wt %. The mixture was stirred at room temperature

resin adhesive. Finally, the 5CB cell was annealed at under nitrogen until it attained high viscosity. To this

50ß C (isotropic state) for 10 min to remove  ow marks. solution acetic anhydride (0.76 ml, 0.008 mol) and pyridine

An optical polarizing microscope (Zeiss Axiophot) was (0.65 ml, 0.008 mol ) were added and the mixture was

used to observe the evolving LC alignment in situ. The

stirred at 65ß C for 24 h. The hot solution was then

pretilt angle of the 5CB cell was measured with an poured into vigorously stirred methanol; after Ž ltering

Autronic DMS 101 TBA instrument. and washing with 50 ml methanol, the copolyimide

obtained was further puriŽ ed twice by reprecipitation with methanol from THF solution. The collected product

3. Results and discussion

was dried at 100ß C under reduced pressure for 24 h to

3.1. Synthesis of copolyimides with a long alkyl side

yield 0.65 g (67%) of white solid.

group

Three series of coPIs based on the dianhydrides BCDA, 6FDA and BPDA were synthesized using the 2.3. Characterization and processing

1H NMR spectra (300 MHz) were recorded on a Varian two-step method illustrated in Ž gure 2. The copolyimides were obtained from poly (amic acid ) precursors by VXR-300 spectrometer. FTIR spectra were measured

on a Nicolet 360 FTIR spectrometer. Gel permeation chemical imidization. The poly(amic acid)s were pre-pared by reacting each dianhydride with an equimolar chromatograph y was run using an Applied Biosystem

400LC instrument equipped with a 410 diVerential amount of diamine mixture containing 18OBD and another diamine in a particular mole ratio. For com-refractometer, and samples were prepared as 1 mg mlÕ 1

in THF solution. The inherent viscosities of soluble parison purposes, we chose the mole ratio (18OBD: other diamine5 1 : 4) to be the same as that reported coPIs were measured using Ubbelohde viscometers at a

concentration of 0.5 g dlÕ 1 at 25ß C. Thermal transitions in ref. [4]. Because most of the coPIs are soluble in

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910 W.-C. Lee et al.

polar organic solvents, the mole ratio in the coPIs could 3.2. Solubility, molecular weight and thermal

be checked by NMR measurements. Figure 3 shows the characterization of the soluble coPIs

1H NMR spectrum of 6FDA-ODA-18OBD; the peaks The solubility of the synthesized copolyimides was of aromatic protons (d: 7.55, 7.15 ppm and the other determined in 10 common solvents; the results are listed aromatic proton) of 18OBD are relatively low and in table 1. The three series of coPIs containing the same covered by other peaks. The peak of the methylene amount of 18OBD show a drastic solubility diVerence proton (d: 4.05 ppm) in 18OBD adjacent to the oxygen in various organic solvents. The 6FDA series of coPIs atom was chosen to integrate and was compared with exhibits a better solubility than the other two. In the the signal (d: 7.4 ppm) of the aromatic proton of the BPDA series, only BPDA-DDS-18OBD dissolves in polar other diamine, ODA. The compared integration ratio is solvents, such as NMP, DMAc and DMF. According 1 : 3.6, which is close to the intended 1 : 4 mole ratio. The to ref. [12], the incorporation of a large pendant group 1H NMR spectra of the coPIs can also give information would enhance the solubility of polyimides. Table 1 shows on the imidization ratio of the copolyimides. In Ž gure 3, that incorporation of the long alkyl side chains does the signal of the NH proton (d: 9.3–9.6 ppm) has increase the solubility of coPIs, but the solubility is still vanished, indicating that the imidization ratio is close greatly in uenced by the structure of the dianhydride

to 100%. employed. Because BPDA has a symmetric structure

and has the potential to yield the crystalline polyimide [12], the solubility of the BPDA series is poor in spite of the large alkyl group. In the BCDA series, the four coPIs exhibit the same solubility in test solvents except for a higher solubility of BCDA-DDS-18OBD in MeOH. This could result from the bridging-group , sulfonyl in DDS, as it is known that a large or bulky bridging group enhances solubility [12]. The same phenomenon is observed in the 6FDA-DDS-18OBD and BPDA-DDS-18OBD series. However, the 6FDA series have diVerent solubilities in 2-PTO, DMF, DMSO and CHCl3. The eVect of diamine structures on solubility are especially manifest in the 6FDA series.

The molecular weight of these soluble coPIs was measured using inherent viscosity (ginh) detection and gel permeation chromatograph y (GPC). Table 2 sum-Figure 3. 1H NMR spectrum (CDCl3, TMS) for

6FDA-ODA-18OBD. marizes the inherent viscosity data for nine soluble coPIs

Table 1. Solubilityabehaviour of copolyimides based on 18OBD.

Solventb

Copolyimide NMP DMAc c-BL 2-PTO THF DMF DMSO CHCl3 Acetone MeOH

BCDA-MDA-18OBD 1 1 1 Õ ] 1 1 Õ Õ Õ BCDA-ODA-18OBD 1 1 1 Õ ] 1 1 Õ Õ Õ BCDA-PPD-18OBD 1 1 1 Õ ] 1 1 Õ Õ Õ BCDA-DDS-18OBD 1 1 1 Õ ] 1 1 Õ Õ ] 6FDA-MDA-180OBD 1 1 1 1 1 ] Õ 1 1 Õ 6FDA-ODA-180OBD 1 1 1 ] 1 1 ] 1 1 Õ 6FDA-PPD-180OBD 1 1 1 ] 1 1 ] ] 1 Õ 6FDA-DDS-180OBD 1 1 1 1 1 1 1 1 Õ BPDA-DDS-18OBD 1 1 Õ Õ Õ 1 Õ Õ Õ Õ BPDA-MDA-18OBD Õ Õ Õ Õ Õ Õ Õ Õ Õ Õ BPDA-ODA-18OBD Õ Õ Õ Õ Õ Õ Õ Õ Õ Õ BPDA-PPD-18OBD Õ Õ Õ Õ Õ Õ Õ Õ Õ Õ

a1 , soluble; ] , partially soluble; Õ , insoluble at room temperature.

bNMP, N-methyl-2-pyrrolidone; DMAc, N,N-dimethyl acetamide; c-BL, c-butyrolactone; 2-PTO, 2-pentanone; THF,

tetrahydrofuran; DMF,N,N-dimethyl formamide; DMSO, dimethyl sufoxide.

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Table 2. Inherent viscosity, molecular weight and thermal analysis results of copolyimides based on 18OBD. TGAd

ginha/ Solubility GPC DSC 5% weight loss 10% weight loss Residue wt % Copolyimides dl gÕ 1 in THF Mw Tgb/ß C temperature/ß C temperature/ß C at 850ß C

BCDA-MDA-18OBD 0.14 Insoluble — 216.3 384.1 409.6 0.0 BCDA-ODA-18OBD 0.14 Insoluble — 223.8 379.6 410.0 0.0 BCDA-PPD-18OBD 0.27 Insoluble — —c 332.6 372.9 0.0 BCDA-DDS-18OBD 0.24 Insoluble — 251.8 348.3 375.1 0.0 6FDA-MDA-18OBD 0.47 Soluble 107310 247.4 433.3 480.2 3.4 6FDA-ODA-18OBD 0.38 Soluble 71049 239.2 426.5 466.8 0.0 6FDA-PPD-18OBD 0.21 Soluble 49121 245.3 399.7 443.2 0.0 6FDA-DDS-18OBD 0.10 Soluble 28444 258.2 408.7 462.4 0.0 BPDA-DDS-18OBD 0.20 Insoluble — —c 401.9 444.4 0.8

aMeasured at a concentration of 0.5 g dlÕ 1 in the coating solvent at 30ß C.

bCollected during the second heating scans at a heating rate of 10ß C minÕ 1 in nitrogen.

cNoTgwas obtained.

dMeasured at a heating rate of 10ß C minÕ 1.

and GPC data for THF-soluble coPIs. The measured DDS-18OBD during the second heating scans from inherent viscosity of coPI solutions is in the range of 35ß C to 300ß C. Their Tgis probably higher than 300ß C 0.1–0.47 dl gÕ 1, which is relatively low in comparison with [16].

the inherent viscosity of non-branched polyimides. The The thermal stability of soluble coPIs was evaluated incorporation of alky side chains thus appears to reduce from the 5% weight loss temperature in TGA measure-the viscosity of PI solutions; measure-the same phenomenon was ment. The 5% weight loss temperature of these coPIs reported in previous publications [6, 13, 14]. Since the ranges from 330 to 435ß C under nitrogen. The incor-6FDA series of coPIs have good solubility in THF, their poration of long alkyl side groups reduces the thermal molecular weights can also be determined by GPC. The stability of polyimides in comparison with linear poly-molecular weights of 6FDA-MDA-18OBD, 6FDA-ODA- imides without side groups. In the BCDA and 6FDA 18OBD, 6FDA-PPD-18OBD and 6FDA-DDS-18OBD series, the coPIs based on PPD diamine possess a are 107 310, 71 049, 49 121 and 28 444, respectively. It relatively lower thermal stability than other coPIs. It is seems that the molecular weight of a coPI increases as supposed that the symmetric structure of PPD without the basicity of the diamine increases. This suggests that a bridging group gives the coPIs based on PPD a more the acylation rate constant of a diamine is correlated rigid and linear backbone. Thus, there is less space for with its structure (basicity) [15]. In a bridged diamine, alkyl side groups to weave through the packed main the acylation rate constant decreases as the electron- chains. We suggest that the packed main chains help to withdrawing ability of the bridge group increases. In stabilize the interweaved alkyl side groups and retard the case of ODA and DDS, the electron-withdrawing their thermal degradation. Finally, almost all of the ability of sulfonyl group in DDS is higher than that of coPIs leave no residues after heating to 850ß C.

the oxygen atom in ODA. Thus, the molecular weight (acylation rate constant ) of 6FDA-ODA-18OBD is

3.3. L C alignment and pretilt angle

higher than that of 6FDA-DDS-18OBD. However, this

The LC alignment layers were prepared by spin-phenomenon was not found in the BCDA series. It

coating the coPI solutions onto ITO glass substrates. seems that there are factors other than diamine basicity,

To optimize the solubility of the coPIs, various solvents for example, impurity in BCDA dianhydride (purity

such as NMP, DMAc, c-butyrolacetone , 2-pentanone , >95%, from TCI) which predominantl y in uence the

THF, DMF and CHCl3were tested. After considering the reactivity between BCDA and diamines.

Ž lm property and solvent evaporation rate, we selected Thermal analysis results for the soluble coPIs are also

a suitable solvent for each coPI series. Homogeneous summarized in table 2. The Tg of most coPIs is in the

cells were prepared with rubbed coPI layers and Ž lled 210 to 260ß C range. In the BCDA and 6FDA series,

with 5CB nematic liquid crystal at room temperature. the coPI based on DDS diamine exhibits a higher Tg

All the rubbed coPI Ž lms aligned 5CB well before anneal-because DDS has a highly dipolar bridging group. No

Tg was obtained for BCDA-PPD-18OBD or BPDA- ing. Good uniformity of LC alignment was observed

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912 W.-C. Lee et al.

using an polarizing optical microscopy. The LC cells of alkyl side groups; however, their pretilt angles vary were then annealed at 50ß C for 10 min to eliminate  ow from 0.16ß to 15.54ß . Therefore, diVerent main chain marks. Unexpectedly, the LC alignment of the cells structures should make important contributions to the prepared with 6FDA-MDA-18OBD and 6FDA-ODA- observed pretilt angle variations. In ref. [18], the authors 18OBD deteriorated after annealing. proposed that semi-crystalline polyimides would lead To understand the alignment degradation , we observed to a higher pretilt angle than amorphous ones. Similar the LC alignment evolution by microscope during the results were found in our experiments. As listed in annealing process. We found that domains were formed table 3, the two coPIs based on a linear and symmetric locally at the beginning and gradually expanded until structure (PPD) exhibit large pretilt angles. Other coPIs the whole LC alignment was disordered. In a parallel based on bent structures (ODA and MDA) result in experiment, another LC cell was prepared with coPI a small pretilt angle. The coPIs based on DDS show layers baked at 50ß C for 1 h after rubbing. These 5CB mixed results, i.e. high pretilt angle in the BCDA series LC cells showed uniform alignment. This implies that (4th row in table 3) but low pretilt angle in the 6FDA the alignment deterioration results from molecular inter- and BPDA series (the last two rows in table 3). actions between coPI layers and 5CB. Judging from the The large pretilt angle of BCDA-DDS-18OBD could good solubility of the 6FDA series of coPIs, we believe result from the more rigid bridge group in DDS than that 5CB molecules in the isotropic state (Tc~35.3ß C) in ODA and MDA. However, the other two DDS-can act as a solvent to swell the surface of coPI layers. based coPIs (6FDA and BPDA) are exceptional: their The 5CB alignment was thus degraded by the swelling. pretilt angle is rather small. Thus, BCDA must have a The pretilt angle of each 5CB cell was measured using diVerent eVect from the other two dianhydrides (6FDA the crystal rotation method [17]. An Autronic DMS and BPDA) on pretilt angles. To investigate this, we used 101 TBA instrument was used for these measurements; molecular modelling to simulate the structures of these results are summarized in table 3. For each cell, the three dianhydrides. We chose the ‘Equilibrium Geometry pretilt angle is reduced somewhat after annealing. But with Semi-Empirical’ method to calculate the dipole of after eliminating the  ow marks, the pretilt angle remains each structure. From the Titan simulation program, we stable. For those cells employing BCDA coPI alignment found that the BDCA dipole is clearly diVerent from layers, the pretilt angle varies from 2.56ß to 14.32ß . On that of 6FDA and BPDA. The calculated dipole of the other hand, most of 6FDA and BPDA cells produce 6FDA, BPDA and BCDA dipoles are 5.634, 5.596, and less than 1ß pretilt angle, except for 6FDA-PPD-18OBD.

3.818 D, respectively. Its pretilt angle is the highest (15.54ß ) among all the

Based on previous studies, [4, 5] increasing the coPIs studied.

electronic attraction between LC and alignment layer Table 3 demonstrates the main chain structural eVect

molecules tends to decrease the LC pretilt angle. In our on the pretilt angle. These coPIs have the same amount

studies, the stronger dipoles of 6FDA and BPDA should experience higher electronic interactions than BCDA under the same rubbing conditions. Therefore, the coPIs Table 3. Measured LC pretilt angles on rubbed copolyimide based on 6FDA and BPDA, in conjunction with DDS Ž lms. (7.725 D), exhibit small pretilt angles as listed in table 3. On the other hand, the large pretilt angle obtained Pretilt

from a long alkyl-branched PI Ž lm could result from angle/degree

the steric interaction between LCs and long alkyl side Coating Before After groups [4, 5]. Our studies indicate that the PI main Copolyimides solventa annealing annealingb

chain structure greatly in uences the LC pretilt angle on the alkyl-branched coPI Ž lms. An asymmetric and BCDA-MDA-18OBD DMAc 4.54 2.56

 exible main chain structure would lead to a small pretilt BCDA-ODA-18OBD DMAc 5.49 5.11

BCDA-PPD-18OBD DMAc 15.26 14.32 angle while a symmetric and rigid main chain structure BCDA-DDS-18OBD DMAc 14.79 13.68 would generate a high LC pretilt angle. The steric inter-6FDA-MDA-18OBD c-BL 0.04 —c

action of long alkyl side groups seems to be stronger 6FDA-ODA-18OBD c-BL 0.47 —c

when a linear and rigid PI main chain is employed. 6FDA-PPD-18OBD c-BL 20.88 15.54

6FDA-DDS-18OBD c-BL 0.36 0.16 Another factor in uencing the LC pretilt angle on alkyl-BPDA-DDS-18OBD DMAc 0.83 0.53 branched coPI Ž lms is the dipole interaction of the main chain structure. A strong main chain dipole may enhance

aDMAc,N,N-dimethyl acetamide; c-BL, c-butyrolactone.

the electrical interaction between LC molecules and

bAnnealing at 50ß C for 10 min.

cLC alignment deteriorated after annealing. coPI Ž lms and thus reduce the pretilt angle.

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Thus, by using diVerent main chain structures, we References

have achieved a relatively broad range of LC pretilt [1] Cognard, J., 1982,Mol. Cryst. liq. Cryst., S1, 1. [2] Xu, M., Yang, D. K., and Bos, P. J., 1998,SID Dig., 139. angles. Low LC pretilt angle is particularly attractive

[3] Fukuro, H., and Kobayashi, S., 1988, Mol. Cryst. liq. for In-Plane-Switching or twisted-nematic LCD modes

Cryst., 163, 157.

while high pretilt angle is necessary for the super twisted [4] Lee, K. W., Paek, S. H., Lien, A., Durning, C., and nematic [19] or surface-stabil ized ferroelectric LCDs [20]. Fukuro, H., 1996, Macromolecules, 29, 8894.

The synthesized coPIs are soluble in organic solvents [5] Lee, K. W., Lien, A., Stathis, J. H., and Paek, S. H., 1997,Jpn. J. appl. Phys., 36, 3591.

and their processing temperature is quite low. Such

[6] Jung, J. T., Yi, M. H., Kwon, S. K., and Choi, K. Y., features are particularly important for low temperature

1999,Mol. Cryst. liq. Cryst., 333, 1.

poly-silicon TFT-LCD processes. [7] Lee, S. W., Kim, S. I., Park, Y. H., Ree, M., Rim, Y. N., Yoon, H. J., Kim, H. C., and Kim, Y. B., 2000, Mol. Cryst. liq. Cryst., 349, 279.

4. Conclusion

[8] Ban, B. S., Rim, Y. N., and Kim, Y. B., 2000,L iq. Cryst., We have synthesized and characterized three series 27, 125.

of copolyimides based on 4-(octadecyloxy)-1,3-benzene - [9] Seo, D. S., Kobayashi, S., and Nishikawa, M., 1992, Appl. Phys. L ett., 61, 2392.

diamine (18OBD) which has a long linear alkyl group

[10] Seo, D. S., Nishikawa, M., and Kobayashi, S., 1997, with 18 carbon atoms. Most of these copolyimides show

L iq. Cryst., 22, 515. excellent solubility in selected polar solvents. Homogeneous

[11] Nishikawa, M., 2000,Polym. Adv. T echnol., 11, 404. 5CB cells were prepared for examination of the LC align- [12] Clair, T. L. St., 1990,Polyimides, edited by D. Wilson, ment performance of soluble copolyimide layers. The H. D. Stenzenberger and P. M. Hergenrother (New York: LC alignment was generally good except for a swelling Chapman and Hall ), pp. 58–78.

[13] Tsuda, Y., Kawauchi, T., Hiyoshi, N., and Mataka, S., phenomenon due to the excellent solubility of

6FDA-2000,Polym. J., 32, 594. based copolyimides. The LC pretilt angles vary wildly

[14] Tsuda, Y., Kanegae, K., and Yasukouchi, S., 2000, from 0.16ß to 15.54ß , even though the copolyimides are Polym. J., 32, 941.

branched with the same amount of alkyl group. It was [15] Harris, F. W., 1990, Polyimides, edited by D. Wilson, found that both main chain structures and dipole inter- H. D., Stenzenberger and P. M. Hergenrother (New

York: Chapman and Hall ), pp. 1–37. actions play key roles in determining the pretilt angle.

[16] Chun, B. W., 1994, Polymer, 35, 4203. The main chain copolyimides with long alkyl side group,

[17] Witter, V., Baur, G., and Berreman, D. W., 1976, small dipole, linear, symmetric and rigid core structures

Phys. L ett., 56A, 142.

are favourable for the generation of large LC pretilt [18] Yokokura, H., Myrvold, B., Kondo, K., and

angles. Oh-hara, S., 1994,J. mater. Chem., 4, 1667.

[19] Scheffer, T. J., and Nehring, J., 1984,Appl. Phys. L ett.,

45, 1021.

The UCF group is indebted to AFOSR for Ž nancial [20] Clark, N. A., and Lagerwall, S. T., 1980, Appl. Phys. L ett., 36, 899.

support under contract number F49620-01-1-0377 .

數據

Figure 2. Synthesis scheme for three series of copolyimides based on 18OBD.
Table 1. Solubility a behaviour of copolyimides based on 18OBD.
Table 2. Inherent viscosity, molecular weight and thermal analysis results of copolyimides based on 18OBD

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