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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 11 — May. 24, 2010
  • pp: 11737–11745
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Pixel-isolation liquid crystals formed by polarization-selective UV-curing of a prepolymer containing cinnamate oligomer

Shi-Joon Sung, Eun Ae Jung, Dae-Hwan Kim, Dae-Ho Son, Jin-Kyu Kang, and Kuk Young Cho  »View Author Affiliations


Optics Express, Vol. 18, Issue 11, pp. 11737-11745 (2010)
http://dx.doi.org/10.1364/OE.18.011737


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Abstract

A pixel isolated liquid crystal display was fabricated by polarization-selective anisotropic photoreaction of a prepolymer containing a cinnamate oligomer. The cinnamate oligomer was mainly distributed on the surface region of a UV-cured polymer wall. Anisotropic photo-dimerization of cinnamate moiety was achieved by polarized UV exposure. It was found that the polymer walls containing cinnamate dimers formed by polarized UV exposure showed ordered orientation of LC molecules at the boundary of the polymer walls resulting in electro-optic performance improvement.

© 2010 OSA

1. Introduction

Due to their important features such as light weight, thin packing, bending capability, and portability, flexible displays are gaining much attention in areas of future application, such as portable mobile displays, wearable computers, and smart cards. In particular, flexible displays based on liquid crystals (LC) have been extensively studied and widely used because of their low-power consumption and potential to produce high-quality images [1

1. J. Jang, “Displays develop a new flexibility,” Mater. Today 9(4), 46–52 (2006). [CrossRef]

,2

2. G. P. Crawford, “Encapsulated liquid crystal materials for flexible display applications,” in Flexible Flat Panel Displays (Wiley, New York, 2005), pp. 313–330.

]. For flexible LC displays, mechanical stability and uniform cell gap between two plastic substrates are critical factors for retaining long-term LC alignment against external perturbations such as pressure, bending distortion, and mechanical shock. In order to solve these problems, several types of polymer walls or networks have been investigated for use as possible support structures. These structures were fabricated using a phase separation method from a LC and polymer mixture by applying a patterned UV exposure [3

3. C. D. Sheraw, L. Zhou, J. R. Huang, D. J. Gundlach, T. N. Jackson, M. G. Kane, I. G. Hill, M. S. Hammond, J. Campi, B. K. Greening, J. Francl, and J. West, “Organic thin-film transistor-driven polymer-dispersed liquid crystal displays on flexible polymeric substrates,” Appl. Phys. Lett. 80(6), 1088–1090 (2002). [CrossRef]

6

6. J. W. Jung, S. K. Park, S. B. Kwon, and J. H. Kim, “Pixel-Isolated Liquid Crystal Mode for Flexible Display Applications,” Jpn. J. Appl. Phys. 43(No. 7A), 4269–4272 (2004). [CrossRef]

].

The pixel-isolated LC (PILC) mode is one of the leading fabrication methods for flexible LC displays. In this structure, LC molecules are isolated in pixels surrounded by inter-pixel vertical polymer walls. This mode shows not only good mechanical stability but also nearly the same optical behavior as that of the normal LC mode. However, the molecular orientation of the LC molecules at the boundary of the polymer walls is disturbed by the interaction between the LC molecules and the polymer walls [7

7. T. Murashige, H. Fujikake, H. Sato, H. Kikuchi, T. Kurita, and F. Sato, “Polymer Wall Formation Using Liquid-Crystal/Polymer Phase Separation Induced on Patterned Polyimide Films,” Jpn. J. Appl. Phys. 43(No. 12B), L1578–L1580 (2004). [CrossRef]

,8

8. J. I. Baek, J. H. Shin, M. C. Oh, J. C. Kim, and T. H. Yoon, “Pixel-isolation walls of liquid crystal display formed by fluorinated UV-curable polymers,” Appl. Phys. Lett. 88(16), 161104 (2006). [CrossRef]

]. In the dark state, this disturbed orientation of LC molecules induces light leakage at the boundary of the polymers wall and the contrast ratio of LC is decreased by the light leakage, which is an important issue for display quality.

In this letter, we propose a polarization-selective anisotropic photoreaction based UV-curable prepolymer system for a pixel isolation wall to improve the light leakage problem at the boundary of the polymer walls. The cinnamate oligomer, which shows the anisotropic photo-dimerization reaction of cinnamate moiety by polarized UV exposure, was synthesized and applied as an additive to prepare an acrylate prepolymer system that can improve contrast ratio. The distribution of the cinnamate oligomer after UV-curing was confirmed by measuring the water contact angle. The orientation of the LC molecules by polarization-selective distribution of cinnamate dimers in the UV-cured prepolymer was also observed. The prepolymer containing the cinnamate oligomer was used to prepare the PILC and the behavior of LC molecules at the boundary of the polymer walls was investigated.

2. Experiments

2.1. Synthesis and characterization of cinnamate oligomers

Polyethylene glycol (PEG) with cinnamate moiety on both terminals, which was used as the cinnamate oligomer was synthesized using PEG of molecular weight 600 (Aldrich Co.) and cinnamoyl chloride (Aldrich Co.). The synthesized cinnamate oligomer was named poly(ethylene glycol)-dicinnamate (PEG-diCi) (Fig. 1
Fig. 1 Synthetic scheme and molecular structure of PEG-diCi
). The photoreactivity of the synthesized PEG-diCi was investigated by measuring the UV-vis spectra with UV exposure from a 4W filtered lamp with a wavelength of 365 nm (Vilber Lourmat, France). PEG-diCi was dissolved in cyclohexanone to prepare a 1 wt% solution. The solution was spread onto the quartz plate (20 mm x 20 mm) and dried for 10 min at room temperature. UV-vis spectra of coated PEG-diCi with the different UV exposure times were collected using a UV-vis spectrometer (Optizen 2120UV plus, Mecasys).

2.2. Preparation of prepolymers

For the preparation of the prepolymer for PILC, bi-component prepolymer was prepared by blending the PEG-diCi and the ethyl hexyl acrylate (EHA, Aldrich Co.) based acrylate prepolymer. EHA, well known as a component of a prepolymer for polymer dispersed liquid crystals (PDLC), was selected for the monomer due to its good phase separation property with LC. The acrylate prepolymer was prepared by mixing 90 wt% EHA, a 9 wt% crosslinker (polyethyleneglycol diacrylate, PEGDA, Aldrich Co.), and a 1 wt% photoinitiator (Darocur4265, Aldrich Co.). Hydrophilicity of the cured bi-component prepolymer was measured by the sessile drop technique using a contact angle analyzer (Phoenix 300 Plus, SEO Co.).

2.3. LC alignment on bi-component prepolymers

The behavior of the LC molecules on the bi-component prepolymer was confirmed by investigating polarized optical microscope images of the LC cell. The bi-component prepolymer was spin coated onto the ITO glass substrate and polarized UV used for irradiation to induce the anisotropic photoreaction of the cinnamate moiety. LC cells were made by assembling a pair of glass substrates coated with bi-component prepolymer; the cell gap between the two ITO glasses was maintained at 5 μm by using polystyrene spacers. Nematic LC (E7, Merck Co.) was injected into the cell by capillary motion at 65°C, upon which the liquid crystals changed to the isotropic phase. This procedure was followed by cooling down of the cell to room temperature.

2.4. PILC device fabrication

To fabricate LC cells with polymer walls, we prepared a mixture of LC and the bi-component prepolymer with a weight ratio of 50:50. The LC cells were prepared by using indium-tin-oxide (ITO) coated glass substrates. A polyimide solution was spin coated on the ITO glass and rubbed. The cell gap between the two ITO glasses was maintained at 5 μm by using polystyrene spacers. The mixture of the LC and the prepolymer was injected into the empty cell by the capillary effect at 100 °C, above the clearing temperature. In order to cure the prepolymer, UV light at 10 mW/cm2 was irradiated through a photomask while maintaining the sample temperature at 100 °C. The photomask had a pattern of 300 × 300 μm2 dark squares with 30 μm spacing. In order to induce the anisotropic photoreaction of the cinnamate moiety, a UV linear polarizer (20LP-UV, Newport) was used for UV exposure.

3. Results and discussion

In order to endow the prepolymer with polarization-selective characteristics, the cinnamate moiety, well known for its anisotropic photo-dimerization reaction by polarized UV, was attached to the flexible PEG backbone. In order to confirm the photoreactivity of the cinnamate moiety of PEG-diCi, the UV absorption spectrum of PEG-diCi as a function of UV exposure time was investigated (Fig. 2
Fig. 2 UV absorption spectrum of PEG-diCi as a function of UV exposure time.
). UV light of wavelength 365 nm was used to irradiate the coated PEG-diCi film. From the UV absorption spectra, maximum peak intensity was observed at 280 nm, which is the characteristic peak of the cinnamate moiety; the peak diminished as the UV exposure dose increased. The decrease of the peak originated from the photo-dimerization reaction of the attached cinnamate moiety of PEG-diCi. From the UV absorption spectrum, it was found that the photo-dimerization reaction of the cinnamate moiety of PEG-diCi successfully occurred due to the UV exposure.

Due to the hydrophobic alkyl chain of EHA, the water contact angle of the polymer film without PEG-diCi was about 50°. However, the water contact angle of the polymer film containing PEG-diCi dropped below 30°, which illustrates the change in the surface character from hydrophobic to hydrophilic. This change is attributed to the hydrophilicity of the incorporated PEG-diCi. In spite of the small amount of PEG-diCi (10 wt%), water contact angle on the polymer film showed an abrupt decrease; a further increase of the amount of PEG-diCi showed little effect on the change of water contact angle. From these results, it was determined that the PEG-diCi was well located in the surface region of the polymer film regardless of the amount of PEG-diCi. Consequently, the surface property of the bi-component prepolymer film could be said to be mainly dominated by the cinnamate dimers even if only a small amount of PEG-diCi was used.

In order to compare the orientation of LC molecules more precisely, the azimuthal anchoring energy was measured, calculated by measuring the width of the Neel Wall [10

10. X. T. Li, D. H. Pei, S. Kobayashi, and Y. Iimura, “Measurement of Azimuthal Anchoring Energy at Liquid Crystal/Photopolymer Interface,” Jpn. J. Appl. Phys. 36(Part 2, No. 4A), L432–L434 (1997). [CrossRef]

] of the LC cells prepared by using polarized UV irradiated bi-component polymer film. Figure 5
Fig. 5 Azimuthal anchoring energy of LC cell prepared by using bi-component prepolymer film exposed by polarized UV (4 J/cm2) as a function of PEG-diCi weight ratio.
shows the azimuthal anchoring energy of the polarized UV irradiated prepolymer films containing different amounts of PEG-diCi. In the case of prepolymer containing PEG-diCi, the azimuthal anchoring energy was the same order of magnitude irrespective of the amount of PEG-diCi, which was in accordance with the surface coverage of the cinnamate dimers. However, polymer film without PEG-diCi showed an abrupt decrease of the azimuthal anchoring energy despite the polarized UV exposure. The cinnamate dimers located in the surface region of the UV-cured bi-component prepolymer film had a dominant effect on the orientation of LC molecules on the polymer film.

The state of LC molecules near the polymer walls was investigated by preparing a PILC using a prepolymer containing PEG-diCi. In order to confirm the effect of the anisotropic photoreaction of the cinnamate moiety on the orientation of LC molecules at the boundary of the polymer walls, a linear UV-polarizer was used selectively for the formation of polymer walls. Figure 6
Fig. 6 Polarized optical microscope images of PILC using bi-component prepolymers: (a) PEG-diCi 5 wt%, 8 J/cm2, (b) PEG-diCi 10 wt%, 8 J/cm2, (c) PEG-diCi 5 wt%, 12 J/cm2, (d) PEG-diCi 10 wt%, 12 J/ cm2. The blue arrow represent rubbing direction and the red arrow represents the polarization direction of UV exposure.
shows the polarized optical microscope images of the PILC structure prepared using a bi-component prepolymer containing PEG-diCi.

All of the samples showed typical pixel isolated polymer walls regardless of UV exposure time or polarization. In the case of unpolarized UV exposure, light leakage at the boundary of the polymer walls was observed in the dark state (45° rotated to the axis of the cross-polarizers of the microscope), which was caused by the disordered orientation of LC molecules at the boundary of the polymer walls [8

8. J. I. Baek, J. H. Shin, M. C. Oh, J. C. Kim, and T. H. Yoon, “Pixel-isolation walls of liquid crystal display formed by fluorinated UV-curable polymers,” Appl. Phys. Lett. 88(16), 161104 (2006). [CrossRef]

]. However, unlike the case of unpolarized UV exposure, the polymer walls formed by polarized UV exposure showed little light leakage at their boundary. The polarization direction of the UV exposure was adjusted to the direction parallel to the rubbing direction of the polyimide alignment layer. Because of the polarized UV exposure, the cinnamate moiety distributed near the surface of the polymer wall experienced anisotropic photo-dimerization and the cinnamate dimers had anisotropic orientation as expected from the measurement of the water contact angle and azimuthal anchoring energy. The cinnamate dimers oriented at the boundary of the polymer walls interacted with LC molecules and the LC molecules were aligned parallel to the rubbing direction. Due to the orientation of the LC molecules, light leakage at the boundary of the polymer walls was considerably improved. A small amount of PEG-diCi was sufficient to improve the level of light leakage. This might be attributable to the surface-oriented anisotropic distribution of PEG-diCi in the bi-component prepolymer. From this result, it was determined that the anisotropic photoreaction of a photo-dimerizable prepolymer is a very effective way to improve the interface between LC molecules and the polymer walls of PILC.

4. Conclusions

In summary, we have proposed a polarization-selective anisotropic photoreaction-based polymer wall formation to improve the electro-optical performance of PILC. A synthesized cinnamate oligomer was used as an additive for the acrylate prepolymer system and the surface-oriented anisotropic distribution of the cinnamate oligomers was demonstrated. Anisotropic photo-dimerization of the cinnamate moiety was achieved by polarized UV exposure; the orientation of LC molecules at the boundary of the polymer walls was controlled. It is expected that polymer wall formation by anisotropic photoreaction can be applied to various types of LC/polymer composite systems to enhance electro-optical performance.

Acknowledgments

This work was supported by the DGIST Basic Research Program of the MEST (Ministry of Education, Science & Technology).

References and links

1.

J. Jang, “Displays develop a new flexibility,” Mater. Today 9(4), 46–52 (2006). [CrossRef]

2.

G. P. Crawford, “Encapsulated liquid crystal materials for flexible display applications,” in Flexible Flat Panel Displays (Wiley, New York, 2005), pp. 313–330.

3.

C. D. Sheraw, L. Zhou, J. R. Huang, D. J. Gundlach, T. N. Jackson, M. G. Kane, I. G. Hill, M. S. Hammond, J. Campi, B. K. Greening, J. Francl, and J. West, “Organic thin-film transistor-driven polymer-dispersed liquid crystal displays on flexible polymeric substrates,” Appl. Phys. Lett. 80(6), 1088–1090 (2002). [CrossRef]

4.

V. Vorflusev V and S. Kumar, “Phase-separated composite films for liquid crystal displays,” Science 283(5409), 1903–1905 (1999). [CrossRef] [PubMed]

5.

Y. Kim, J. Francl, B. Taheri, and J. L. West, “A method for the formation of polymer walls in liquid crystal/polymer mixtures,” Appl. Phys. Lett. 72(18), 2253–2255 (1998). [CrossRef]

6.

J. W. Jung, S. K. Park, S. B. Kwon, and J. H. Kim, “Pixel-Isolated Liquid Crystal Mode for Flexible Display Applications,” Jpn. J. Appl. Phys. 43(No. 7A), 4269–4272 (2004). [CrossRef]

7.

T. Murashige, H. Fujikake, H. Sato, H. Kikuchi, T. Kurita, and F. Sato, “Polymer Wall Formation Using Liquid-Crystal/Polymer Phase Separation Induced on Patterned Polyimide Films,” Jpn. J. Appl. Phys. 43(No. 12B), L1578–L1580 (2004). [CrossRef]

8.

J. I. Baek, J. H. Shin, M. C. Oh, J. C. Kim, and T. H. Yoon, “Pixel-isolation walls of liquid crystal display formed by fluorinated UV-curable polymers,” Appl. Phys. Lett. 88(16), 161104 (2006). [CrossRef]

9.

K. Ichimura, “Photoalignment of Liquid-Crystal Systems,” Chem. Rev. 100(5), 1847–1874 (2000). [CrossRef]

10.

X. T. Li, D. H. Pei, S. Kobayashi, and Y. Iimura, “Measurement of Azimuthal Anchoring Energy at Liquid Crystal/Photopolymer Interface,” Jpn. J. Appl. Phys. 36(Part 2, No. 4A), L432–L434 (1997). [CrossRef]

11.

S. R. Lee, J. H. Shin, J. I. Baek, M. C. Oh, T. H. Yoon, and J. C. Kim, “Initially π−twisted nematic liquid crystal cell stabilized by a fluorinated polymer wall,” Appl. Phys. Lett. 90(16), 163513 (2007). [CrossRef]

12.

F. Roussel and B. M. Fung, “Anchoring behavior, orientational order, and reorientation dynamics of nematic liquid crystal droplets dispersed in cross-linked polymer networks,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 67(4), 041709 (2003). [CrossRef] [PubMed]

OCIS Codes
(160.3710) Materials : Liquid crystals
(230.3720) Optical devices : Liquid-crystal devices
(240.6670) Optics at surfaces : Surface photochemistry

ToC Category:
Optical Devices

History
Original Manuscript: January 20, 2010
Revised Manuscript: May 3, 2010
Manuscript Accepted: May 13, 2010
Published: May 19, 2010

Citation
Shi-Joon Sung, Eun Ae Jung, Dae-Hwan Kim, Dae-Ho Son, Jin-Kyu Kang, and Kuk Young Cho, "Pixel-isolation liquid crystals formed by polarization-selective UV-curing of a prepolymer containing cinnamate oligomer," Opt. Express 18, 11737-11745 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-11-11737


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References

  1. J. Jang, “Displays develop a new flexibility,” Mater. Today 9(4), 46–52 (2006). [CrossRef]
  2. G. P. Crawford, “Encapsulated liquid crystal materials for flexible display applications,” in Flexible Flat Panel Displays (Wiley, New York, 2005), pp. 313–330.
  3. C. D. Sheraw, L. Zhou, J. R. Huang, D. J. Gundlach, T. N. Jackson, M. G. Kane, I. G. Hill, M. S. Hammond, J. Campi, B. K. Greening, J. Francl, and J. West, “Organic thin-film transistor-driven polymer-dispersed liquid crystal displays on flexible polymeric substrates,” Appl. Phys. Lett. 80(6), 1088–1090 (2002). [CrossRef]
  4. V. Vorflusev and S. Kumar, “Phase-separated composite films for liquid crystal displays,” Science 283(5409), 1903–1905 (1999). [CrossRef] [PubMed]
  5. Y. Kim, J. Francl, B. Taheri, and J. L. West, “A method for the formation of polymer walls in liquid crystal/polymer mixtures,” Appl. Phys. Lett. 72(18), 2253–2255 (1998). [CrossRef]
  6. J. W. Jung, S. K. Park, S. B. Kwon, and J. H. Kim, “Pixel-Isolated Liquid Crystal Mode for Flexible Display Applications,” Jpn. J. Appl. Phys. 43(No. 7A), 4269–4272 (2004). [CrossRef]
  7. T. Murashige, H. Fujikake, H. Sato, H. Kikuchi, T. Kurita, and F. Sato, “Polymer Wall Formation Using Liquid-Crystal/Polymer Phase Separation Induced on Patterned Polyimide Films,” Jpn. J. Appl. Phys. 43(No. 12B), L1578–L1580 (2004). [CrossRef]
  8. J. I. Baek, J. H. Shin, M. C. Oh, J. C. Kim, and T. H. Yoon, “Pixel-isolation walls of liquid crystal display formed by fluorinated UV-curable polymers,” Appl. Phys. Lett. 88(16), 161104 (2006). [CrossRef]
  9. K. Ichimura, “Photoalignment of Liquid-Crystal Systems,” Chem. Rev. 100(5), 1847–1874 (2000). [CrossRef]
  10. X. T. Li, D. H. Pei, S. Kobayashi, and Y. Iimura, “Measurement of Azimuthal Anchoring Energy at Liquid Crystal/Photopolymer Interface,” Jpn. J. Appl. Phys. 36(Part 2, No. 4A), L432–L434 (1997). [CrossRef]
  11. S. R. Lee, J. H. Shin, J. I. Baek, M. C. Oh, T. H. Yoon, and J. C. Kim, “Initially π−twisted nematic liquid crystal cell stabilized by a fluorinated polymer wall,” Appl. Phys. Lett. 90(16), 163513 (2007). [CrossRef]
  12. F. Roussel and B. M. Fung, “Anchoring behavior, orientational order, and reorientation dynamics of nematic liquid crystal droplets dispersed in cross-linked polymer networks,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 67(4), 041709 (2003). [CrossRef] [PubMed]

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