Thin Solid Films 544 (2013) 74–78

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Soluble polyimide films as alignment layers for bistable chiral-tilted homeotropicnematic liquid crystal display applications

Chi-Jung Chang a,⁎, I-Hsiang Tseng c, Yu-Chi Lin a, Bo-Shiuan Wu a, Bau-Jy Liang b, Mei-Hui Tsai c

a Department of Chemical Engineering, Feng Chia University, 100, Wenhwa Road, Seatwen, Taichung 40724, Taiwan, Republic of Chinab Department of Electrical Engineering, Feng Chia University, 100, Wenhwa Road, Seatwen, Taichung 40724, Taiwan, Republic of Chinac Department of Chemical and Materials Engineering, National Chin-Yi University of Technology, 57, Sec. 2, Zhongshan Rd., Taipin Dist., Taichung, 41170, Taiwan, Republic of China

⁎ Corresponding author. Tel.: +886 4 24517250x367E-mail address: changcj@fcu.edu.tw (C.-J. Chang).

0040-6090/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.tsf.2013.04.123

a b s t r a c t

a r t i c l e i n f o

Available online 2 May 2013

Keywords:Alignment layerPolyimideBistable displayPretilt angleLiquid crystal

Soluble polyimides (PI) with and without aliphatic side chains were synthesized by a one-pot synthesismethod and acted as homogeneous or homeotropic liquid crystal alignment layers. Homeotropic PI was syn-thesized by grafting an alkyl side chain on the backbone of homogeneous PI. The vertical alignment proper-ties can be tuned by adjusting the grafting density and the length of the side chains. In addition, varioushomogeneous and homeotropic PIs were blended by a different ratio to adjust the pretilt angle of the align-ment film. The long-term alignment properties and thermal resistance of the alignment films were investi-gated. Combining the dual frequency liquid crystal with the alignment layers which have an intermediatepretilt angle of 75°, a bistable chiral-tilted homeotropic nematic liquid crystal display device is demonstrated.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

For the traditional liquid crystal (LC) displays, the electric fieldmust be applied constantly on the display to maintain its configura-tion. Once the field is removed, the LC molecules will relax to thefield-free state. Then, there will be no contrast between pixels. Itrequires a lot of power consumption, resulting in short battery life-time. Bistable liquid crystal displays (LCDs) are quite competitivefor the e-paper, e-books, and e-tags applications which do not requirefrequent change of the content. These bistable displays can effectivelyreduce the power consumption by using their two stable states to dis-play information [1]. Several bistable display technologies have beendeveloped, such as bistable cholesteric display, [2] bistable twistednematic display [3], and bistable bend-splay LCD, [4,5] etc. Power isrequired when the cell is going to change from one steady state tothe other.

Typical main-chain polyimides possess the ability of aligning LCmolecules after rubbing treatment. However, they can only generatepretilt angles smaller than 3°. For super-twisted nematic LCD andthin-film transistor LCD, pretilt angles ranging from 5° to 20° areneeded. For multi-domain vertical alignment LCD, the pretilt anglesshould be controlled at around 90°. Polyimides (PI) with side-chainsare synthesized to raise the pretilt angles. Liu et al. [6] prepared PIswith n-(alkyloxy)biphenyl side-chain. To get high pretilt angles, thelength of alkyl side chains should be longer than 6 carbon atoms [7].Yeung et al. [8,9] described an approach for obtaining intermediatepre-tilt angles based on a mixture of homogeneous (horizontally

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align) and homeotropic (vertically align) PIs. After mixing, nano-sizeddomains were formed because of phase separation. This study demon-strates an approach to obtain an arbitrary pre-tilt angle (15° to 85°)by controlling the homogeneous PI to homeotropic PI concentrationratio, the baking temperature and the rubbing strength. Unlike the PImixture reported by Yeung et al. [8,9], polyimides synthesized in thisstudy do not undergo noticeable phase separation as the solvent evap-orates, and the alignment layer does not seem to form nano-sized do-mains. For the alignment layer, electronic interaction between LCmolecules and PI layer surface exhibits larger influences on LC pretiltangles than the surface morphology of the film does [10].

According to our literature survey, homeotropic PIs were usuallysynthesized by the following method. At first, a series of diamineswith long side chains were synthesized. Generally, synthesis ofthese diamines requires three steps [11], including synthesis of asym-metric side chain molecules, grafting asymmetric side chain mole-cules on a compound with dinitro groups, and then a reductionreaction of dinitro groups to make the diamine monomer. Then, thediamine with long side chain was copolymerized with diamine (with-out long side chain) and dianhydride to give poly(amic acid)s, whichwere then converted into homeotropic PIs by thermal imidization[12–14]. In this study, a simple one-pot synthesis method was usedto prepare homeotropic PIs. A series of soluble PIs with two hydroxylgroups on each repeating unit were synthesized by copolymerizationof dianhydride with 2,2-Bis(3-amino-4-hydroxyphenyl) hexafluoro-propane (BisAPAF). Then, long aliphatic side chains were grafted onthe PI chain by the reaction of 1,2-epoxyoctadecane with the hydrox-yl groups on the main chain of PIs. It is a two-step, one-pot synthesis.A series of polyimides with aliphatic side chains were synthesized inthis study. The effects of the rubbing process, together with the

Fig. 1. Synthesis of homogeneous PI (M1H1) and homeotropic PI (PI-ODn or PI-DDn).

Table 1Surface energy and pretilt angle of PI-ODn films with different grafting content.

Sample Surface energy(dyne/cm)

Pretilt angle(°)

PI-OD5 37.4 11.2PI-OD4 37.1 86.9PI-OD3 37.1 86.7PI-OD2 36.7 88.1PI-OD1 35.9 89.9

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grafting density of the side chains, on the pretilt angle of the poly-imide films have been studied. Besides, the feasibility of using thepolyimide for bistable display applications was also investigated.

2. Experimental section

2.1. Synthesis of homogeneous or homeotropic polyimides

1,2-epoxyoctadecane (EOD), 1,2-epoxydodecane (EDD), triphenyl-phosphine (TPP), pyromellitic dianhydride (PMDA) were supplied byTCI. 2,2-Bis(3-amino- 4-hydroxyphenyl) hexafluoropropane (BisAPAF)(M1) was purchased from Chriskev. Acetic anhydride was supplied byAldrich. 2-Ethyl-4-methylimidazole (2E4MI) was purchased fromACROS. Pyridine was supplied by J. T. Baker.

Dianhydride H1 is purified by recrystallization from acetic anhy-dride and then dried in a vacuum oven at 120 °C overnight. Thepolyamic acid (PAA) solutions are made by reacting pyromelliticdianhydride (PMDA, 1 mmol) with 2,2-Bis(3-amino-4-hydroxyphenyl)hexafluoropropane (BisAPAF, 1 mmol) in N,N’-Dimethylacetamide(DMAC) under a nitrogen atmosphere and stirring at 25°C for 12 h.Then, acetic anhydride (2 mmol), triphenyl phosphite (TPP, 0.8 ml)were added into the PAA solution. The solution was heated at 100 °Cunder nitrogen atmosphere for 3 h. After cooling, the reaction mixturewas poured into a large amount of methanol with constant stirring.The precipitate was washed thoroughly with methanol, collected on afilter, and dried at 100 °C under vacuum. The product is soluble PI,M1H1, which acts as homogeneous PI.

Homeotropic PIs were synthesized by grafting alkyl side chains onthe M1H1 chain. M1H1 was dissolved in DMAC (solid content15 wt.%). EOD (or EDD) and catalyst (TPP and 2E4MI) were addedin the M1H1 solution. The solution was heated at 130 °C under nitro-gen atmosphere for 60 h. After cooling, the reaction mixture waspoured into a large amount of deionized water with constant stirring.The precipitate was collected on a filter, and dried under vacuum. Thepowder was dissolved by ethyl acetate, and then poured into a largeamount of hexane. The precipitate was collected on a filter, anddried under vacuum to get homeotropic PIs (PI-ODn or PI-DDn). n isthe ratio of (repeating unit of M1H1)/(moles of EOD or EDD). Thelarger the n value is, the lower the side-chain grafting density is.Fig. 1 shows the reaction scheme for the synthesis of homogeneousPI (M1H1) and homeotropic PI (PI-ODn or PI-DDn).

Homogeneous (M1H1) and homeotropic (PI-ODn) PIswere blendedby different ratio to adjust the pretilt angle of the blended alignmentfilm MPI-ODn-m. The prefix M means blended (or mixed) PIs. m isthe solid weight percent of homeotropic PI in the polymer blendmixture.

2.2. Preparation of alignment layers and cells

PI alignment layers were fabricated by spin-coating PI solutions at1000 rpm for 10 s and at 3500 rpm for 30 s on indium tin oxide-coated glass substrates. The PI films were baked at 80 °C and thenpost-baked at 200 °C for 30 min at each temperature.

The PI-coated substrate was rubbed by a rubbing machine with acloth on the roller. The dual frequency liquid (MLC-2048, Merck)was filled into cells under atmospheric pressure at room temperatureand the cells were sealed. The cell size was 1.8 × 2.4 cm. A bistableLCD device is fabricated by combining a mixed PI film (pretilt anglenear 74°) with a dual frequency liquid (MLC-2048, Merck).

2.3. Characterization

The contact angle was measured using a contact angle meter(CAM-100, Creating-Nanotech Co.). The observed tilt angle betweenLC molecules and the polymer surface is defined as a pretilt angle.

Pretilt angles of LCs in contact with rubbed PI films were measuredby an improved crystal rotation method. [15].

3. Results and discussion

3.1. Changing the side-chain grafting density

Table 1 showed the surface energy of PI-ODn films with differentgrafting content calculated by the Owens-Wendt method [16]. Thesurface energy of the PI-ODn polyimides decreased with increasinggrafting density of the n-octadecyl side chain (decreasing n value).It resulted from a decrease in the surface polarity of the polyimidefilms with the introduction of more n-octadecyl side chain. The pretiltangle increased with a decrease in the surface energy of thepolyimide films or an enrichment of the long alkyl side chains onthe polyimide surface. The pretilt angle which plays an importantrole in fabrication of LCDs was affected by some factors, such asanchoring energy, steric effect and electronic interaction, etc. [17].These factors are relative to the chemical structure of the side chainand main chain of PIs [18]. Smaller n value (the ratio of (repeatingunit of M1H1)/(moles of EOD)) means higher grafting density. Asthe n value became smaller than 4, a high pretilt angle above 89°was achieved. Since the dodecyl chain is not a rigid aromatic chain,high grafting density is necessary for keeping high pretilt angle afterthe rubbing process.

180 190 200 210 220 230

0

20

40

60

80

100

PI-OD41 PI-DD11 PI-OD11 MPI-OD11-35 MPI-OD11-80

Pre

tilt

an

gle

(o)

Temperature(oC)

Fig. 3. The pretilt angles of LC cells using the rubbed alignment layer baked at differentelevated temperatures for 2 h.

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3.2. Effect of chemical structure of side chain on pretilt angle of blendedPI

Intermediate pretilt angles can be obtained based on the com-promised effects of the intermolecular π–π and dipole-dipole interac-tions between LC molecules and aromatic polyimide mainchains,together with the steric repulsion between LC molecules and thealiphatic octadecyl side chains. Fig. 2 shows the prerilt angle of blend-ed PI films versus weight fraction m of homeotropic polyimide(PI-ODn) in the blended PI (MPI-ODn-m). For samples with smallm, the pretilt angle is near zero because the alignment layer is com-posed mostly of the homogeneous (horizontal-aligning) polyimide.The pretilt angle increased as the m value increased. The pretiltangle reached near 90° as the m value was larger than 40 wt.%.

3.3. Thermal stability of LC vertical alignment properties

The thermally stable LC vertical alignment is required, especiallyfor LCDs which are employed at a higher temperature. Fig. 3 showsthe pretilt angles of LC cells using the rubbed alignment layer bakedat different elevated temperatures for 2 hours. The results wereused to investigate the thermal stability of LC vertical alignment ofPI films. The vertical alignment property of PI-OD1 films remainedalmost the same after being baked at 230 °C for 2 h. Grafting densityof PI-OD4 is lower than that of PI-OD1. Side-chain length of PI-DD1 isshorter than that of PI-OD1. Thermal stability of the vertical align-ment property deteriorated when PIs with shorter side-chain orlower side-chain grafting density were used as alignment films. Theorientation of the side chain of PIs induced the vertical alignment ofLC molecules. For PI-OD4 and PI-DD1 alignment films which hadbeen baked at 180 °C for 2 h, the vertical alignment property is stable.The optical micrograph of the LC cell exhibits a good and uniformdarkness when the polarizing microscopy stage rotates. However,the pretilt angles of both PI-OD4 and PI-DD1 films decreased dramat-ically when the baking temperature was 200 °C.

For the blended PI alignment films, MPI-OD1-80 with highhomeotropic PI content exhibited thermally stable LC vertical align-ment properties. Its vertical alignment property remained the sameafter being baked at 230 °C for 2 h. However, the LC alignment prop-erties for MPI-OD1-35 with low homeotropic PI content dropped asthe baking temperature reached °C.

3.4. Shelf life

In order to check the effect of molecule structure on the shelf-lifeof the LC cell, LC cells using homeotropic PIs or mixed PIs as alignment

0 20 40 60 80 1000

20

40

60

80

100

MPI-OD4 MPI-OD1

Pre

tilt

an

gle

(o )

Concentration of the side-chain PI(%)

Fig. 2. The prerilt angles of blended PI films versus weight fraction m of homeotropicpolyimide.

films were stored at room temperature for 6 months. Then, the LCalignment of these LC cells was tested and compared with those mea-sured before storage. Table 2 illustrates the pretilt angles of LCD withdifferent alignment layers measured after being stored for 1 day and6 months. PIs with higher (PI-OD1) and lower (PI-OD4) grafting den-sity of side chains were used as homeotropic alignment films. Afterbeing stored for 6 months, the pretilt angle of the LC cell usingPI-OD1 as the alignment film is almost the same as that measured be-fore storage. However, the pretilt angle of the LC cell droped from86.9° to 47.5° after 6 months when the PI-OD4 was used as the align-ment film. It indicates that the grafting content of side chains haslarge influences on the long term stability of the alignment proper-ties. Increasing the grafting density of side chains helps to increasethe long term stability. Such improvement in long term stabilitymay result from the increased steric hindrance among the side chainsof PI molecules when the grafting content of side chains increases.

Similar trends were observed for the mixed PI alignment film. Thepretilt angle of the LC cell using MPI-OD1-80 as the alignment film isalmost the same as that measured before storage. However, thepretilt angle of the LC cell with MPI-OD4-80 alignment film dropedfrom 89.3° to 6.2° after 6 months. The change in pretilt angle of theLC cell using MPI-OD4-80 was larger than that of the LC cell usingPI-OD4. Mixed PIs were prepared by mixing homeotropic PI withhomogeneous PI. The concentrations of the side chains of mixed PIsare lower than those of the pristine homeotropic PIs. The polymerchain was swelled by the LC solution. It leads to great drop of pretiltangle after 6 months. The results of thermal stability shown inSection 3.3 and the shelf life of the LC cell exhibited similar trend.Therefore, the thermal stability tests can be carried out to evaluatethe life time of LCD.

3.5. Bistable chiral-tilted homeotropic nematic (BHN)

Hsu [19] proposed a bistable chiral tilted-homeotropic nematic(BHN) LC device, which can be switched between the tilted homeo-tropic state (TH state) and the twisted state (T state) by using dual

Table 2Pretilt angles of LCD with alignment layers prepared by PI and blended PI measuredafter being stored for 1 day and 6 months.

Sample Pretilt angle(°)-Initial-

Pretilt angle(°)After 6 months

PI-OD1 89.9 89.9MPI-OD1-80 89.7 89.7PI-OD4 86.9 47.5MPI-OD4-80 89.3 6.2

Fig. 4. Director configurations of the BH, BT, TH and T states, and the transmissionmicrographs of transition among BH, BT, T and TH states of BHN LCD usingMPI-OD1-35 alignment film.

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frequency liquid crystals (DFLC). The dielectric anisotropy (Δε) ofDFLC changes with the frequency. The dielectric anisotropy is positiveupon biasing at a low frequency and negative at a high frequency.

Fig. 4 shows the director configurations of the BH, BT, TH and Tstates, together with the transition processes of the BHN LC device.The transmission micrographs of transition among BH, BT, T and THstates of BHN LCD using MPI-OD1-35 alignment film were illustratedat the four corners of the figure.

As shown in Fig. 4, the transmittance of a BHN LC cell was mea-sured by placing a LC cell between two crossed polarizers. It wasrubbed parallel to one of the polarizer axes. The cell is filled withMLC-2048 (pitch = −10 μm) and the pretilt angle is about 75°.When the cell is in the twisted state (T state), a vertical electricfield pulse with a low frequency f1, which corresponds to a positivedielectric anisotropy of MLC-2048, pulls the LC molecules verticallytoward the biased homeotropic state (BH state). This director config-uration causes the BH state to be darker than the TH state. When thepulse ends, the LC molecules relax to the TH state or back to T state.Since a free energy barrier [20] exists between the two stable states,an electric field that acts on the molecules is necessary for theswitching from the T state to the TH state.

In order to switch back to T state, a pulse with frequency f1 isapplied on the cell and the cell is back to BH state. Then, the cell isswitched to the biased twisted state (BT state) after applying anotherpulse with a high frequency of f2 (the liquid crystals possess a nega-tive dielectric anisotropy Δε2 at the frequency f2). Then, the directorsrelax to T state when the voltage is off. The transmittance of the BHNLCD using MPI-OD1-35 alignment film can be achieved betweencrossed polarizers. The observed transmission micrographs of transi-tion of BHN LCD using MPI-OD1-35 alignment film reveal the feasibil-ity of transition among BH, BT, T, and TH states.

4. Conclusion

Various soluble polyimides (PIs) which can act as homogeneousor homeotropic alignment films were synthesized. Homeotropic PI

was synthesized by grafting alkyl side chain on the backbone of ho-mogeneous PI. The vertical alignment property is stable when thetemperature is lower than 180 °C. The pretilt angle increased withan enrichment of the long alkyl side chains. Increasing the graftingdensity of n-octadecyl side chains not only improve the thermal sta-bility at high temperature, but also improve the long-term stabilityof the alignment properties. After being stored for 6 months, thepretilt angle of the LC cell using PI-OD1 as the alignment filmremained the same as that measured before storage. Thermal stabilityof the vertical alignment property deteriorated when PIs with shorterside-chain or lower side-chain grafting density were used as align-ment films. The thermal stability tests can be carried out to evaluatethe life time of LCD. The observed transmission of BHN LCD amongBH, BT, T, and TH states using blended MPI-OD1-35 alignment filmsreveal the feasibility for bistable display applications.

Acknowledgement

The authors would like to thank the financial support from NationalScience Council under the contract of NSC-98-2221-E-035-003. Theauthors appreciate the Precision Instrument Support Center of FengChia University in providing the measurement facilities.

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