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`Effect of Chemical Structures of Polyimides on
`
`Unidirectional Liquid Crystal Alignment
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`Produced by a Polarized Ultraviolet-Light
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`Exposure
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`ARTICLE
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`JAPANESE JOURNAL OF APPLIED PHYSICS - MARCH 1999
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`pac Fac o:
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`A 3 DO: 0.
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`43/JJAP.38. 334
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`READS
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`3AUTHORS, INCLUDING:
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`Tamas Kosa
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`AlphaMicron, Inc.
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`SSPUBLCA ONS 746C A ONS
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`SEEPROFLE
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`Page 1 of 5
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`Avaiabe 0 :Ta asKosa
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`Jpn. J. Appl. Phys. Vol. 38 (1999) pp. L 334–L 337
`Part 2, No. 3B, 15 March 1999
`c(cid:176)1999 Publication Board, Japanese Journal of Applied Physics
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`Effect of Chemical Structures of Polyimides on Unidirectional Liquid Crystal
`Alignment Produced by a Polarized Ultraviolet-Light Exposure
`Michinori NISHIKAWA, Tamas KOSA1 and John L. WEST1
`Tsukuba Research Laboratory, JSR Corporation, 25 Miyukigaoka, Tsukuba, Ibaraki 305-0841, Japan
`1Liquid Crystal Institute, Kent State University, Kent, Ohio, 44242, U.S.A.
`
`(Received January 6, 1999; accepted for publication February 5, 1999)
`
`Unidirectional liquid crystal (LC) alignment produced by a polarized ultraviolet-light (UV) exposure was examined using
`various polyimides (PIs) which consist of different tetracarboxylic dianhydrides and diamines. The photosensitivity and an-
`choring energy of the LC alignment as a function of UV dosage are summarized with respect to the chemical structures of
`the PIs. The results suggest that the UV absorption efficiencies, molecular conformations, mechanisms of decomposition, and
`anisotropic van der Waals forces of the PIs largely affect the photoalignment properties of the LC.
`KEYWORDS:
`liquid crystal, photoalignment, polyimide, dichroic ratio, anchoring energy
`
`1.
`
`Introduction
`Most electrooptic applications of liquid crystals (LCs) re-
`quire well-oriented LC molecular conformations. A good ex-
`ample is the twisted nematic LC displays which are fabricated
`using mechanically rubbed polyimide (PI) films as LC align-
`ment films.1) However, this method has several problems,
`such as creation of contaminating particles and production
`of electrostatic charges, which lowers the production yield of
`LC displays. Therefore, an alternative technique to align LC
`without rubbing has been required.
`Langmuir-Blodgett films,2) stretched polymers,3) micro-
`grooves,4) stamped polymers,5) and polarized ultraviolet-light
`(UV) exposure of polymers,6) have been developed to pro-
`duce unidirectional LC alignment. LC alignment produced
`using polarized UV exposure is the most promising nonrub-
`bing technique, overcoming the problems mentioned above
`and greatly simplifying production of multidomain displays.7)
`Photoinduced isomerization of azo compounds doped
`in polymers,6, 8–10)
`isomerization11) or cross-
`cis-trans
`linking12–14) of poly(vinyl cinnamate) derivatives, and pho-
`todecomposition of PIs,15–19) have been shown to produce the
`alignment of LC upon polarized UV exposure. Much effort
`has been concentrated on the development of photoreactive
`PIs, which are more heat resistant than azo compounds and
`poly(vinyl cinnamate) derivatives. One of the key issues re-
`garding development of photoalignment materials is increas-
`ing their photosensitivity and anchoring energies as well as
`improving the quality of LC alignment. However, the param-
`eters of PI materials which affect the photosensitivity and an-
`choring energy have not been clarified yet. In this paper, we
`explore the effect of the PI chemical structures on the pho-
`toalignment properties of the LC.
`
`2. Experimental
`PI materials used in this experiment are summarized in
`Fig. 1. The PIs were prepared by heat curing the precursor
`polyamic acids, which were synthesized from an equimolar
`reaction between tetracarboxylic dianhydrides and diamines.
`LC cells were prepared to measure the dichroic ratios
`and azimuthal anchoring energies of the LCs aligned by
`polarized-UV-exposed PI films. PI films were deposited by
`first spin-coating dilute solutions of the respective polyamic
`acids on indium tin oxide glass substrates and then curing at
`–
`C for 1h to achieve imidization.20) The thickness of the
`250
`
`PI film was controlled at 50 nm.
`The PI films were exposed with polarized UV incident nor-
`mal to the surface. We used a 450 W-xenon lamp (Oriel,
`model 6266) as a UV source, and a surface film polar-
`izer (Oriel, model 27320) whose effective range is between
`230 nm to 770 nm.
`The intensity of UV after passing
`through the polarizer was about 1 mW/cm2 at 254 nm. LC
`cells were fabricated using two polarized-UV-exposed sub-
`strates with antiparallel polarization axis. The cell gaps of
`the LC cells were controlled at 10 „m. Dichroic LC, n-
`pentylcyanobiphenyl (5CB, EM Industries) and 0.5% M-618
`(Mitsuitoatsu, ‚max D 550 nm), and LC, 5CB, were filled into
`the cells in the isotropic state and slowly cooled to room tem-
`perature for measurement of the dichroic ratios and azimuthal
`anchoring energies of the LC cells, respectively.
`The dichroic ratios of the LC cells were measured using
`one polarizer and a UV-visible spectrometer.18) The azimuthal
`anchoring energies of the LC cells were measured utilizing
`the Neel wall method reported previously.21) The fluorescence
`spectra of PI films were measured in a front-face arrange-
`ment and the band passes were 1 nm for both the excitation
`and emission monochromators. The molecular conformations
`of the diamines used in the PIs were calculated using the
`MOPAC Ver. 6 program with AM1 parameters.22) The PI film
`birefringences were measured using an instrument described
`elsewhere.23)
`
`3. Results and Discussion
`3.1 Photosensitivity of LC alignment
`Figure 2 shows the dichroic ratios of the LC cells as a func-
`tion of polarized UV exposure time. The dichroic ratios of the
`LC cells initially increase logarithmically with UV dosage,
`and then approach a constant value of about 7.0, which is
`comparable to that of the LC cell with rubbed PI alignment
`films. The photosensitivity of the LC alignment is largely
`affected by the chemical structures of the PIs used. Further-
`more, in most cases, PI shows LC alignment perpendicular to
`the UV polarization. PI-5, however, aligns LC parallel to the
`UV polarization.
`The LC cells (PI-3, 4) with a cyclobutane tetracarboxylic
`dianhydride moiety in PIs result in higher photosensitivity of
`LC alignment than those with pyromellitic dianhydride (PI-6,
`7). The difference in the photosensitivity can be explained
`by the main mechanism of the decomposition of the PIs: cy-
`clobutane ring cleavage24) and imido ring cleavage25) which
`
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`Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 2, No. 3B
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`M. NISHIKAWA et al.
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`L 335
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`Fig. 1. Chemical structures of PIs used.
`
`Fig. 3. UV absorption spectra of PI films.
`
`Fig. 4. Fluorescence spectra of PI films.
`
`Fig. 2. Relationship between polarized UV exposure time and dichroic ra-
`tios of LC cells.
`
`requires a higher dosage of UV.
`To clarify the dependence of the photosensitivity of the LC
`alignment on the diamine moieties in the PIs (PI-1-5), we
`measured the UV absorption and fluorescent spectra of the
`PI films and calculated their molecular conformations using
`molecular mechanics methods. The UV absorption spectra of
`PI films (Fig. 3) suggest that PI-1 has lower photosensitivity
`because of less UV absorption at wavelengths greater than
`230 nm, where the polarizer used works effectively. Mea-
`surement of fluorescence spectra (Fig. 4) suggests that PI-4
`shows relatively strong fluorescence compared with the other
`PI films competing with photodecomposition due to the intra-
`or intermolecular charge-transfer.26) This could be one rea-
`son why PI-4 has lower photosensitivity. Figure 5 shows the
`molecular conformation of PI-5 compared with that of PI-
`
`Fig. 5. Molecular conformations of PIs.
`
`3. PI-5 contains a fluorene unit, which produces large bire-
`fringence, perpendicular to the main chain. The perpendicu-
`lar orientation of the fluorene unit in PI-5 to the main chain
`causes LC molecules to align parallel to the UV polarization
`because of the selective photodecomposition of the PI chain
`parallel to the UV polarization.18) Before exposure to UV, PI
`chains are randomly aligned on the substrate. PI chains paral-
`lel to the UV polarization are selectively decomposed by UV
`exposure, and photoproducts after UV exposure are randomly
`relocated in PI films. The residual PI chains perpendicular
`to the UV polarization, which show no photodecomposition,
`cause the anisotropic van der Waals force21, 27) to align the LC
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`Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 2, No. 3B
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`M. NISHIKAWA et al.
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`LC cells with the same dichroic ratio suggests that an increase
`in the number of benzene rings in the PI results in higher az-
`imuthal anchoring energy. This can probably be explained
`by the difference in the anisotropic van der Waals forces21, 27)
`caused by polarized-UV-exposed PI materials. Furthermore,
`the optic axis in the PI also affects the azimuthal anchoring
`energy. Although PI-5 has four benzene rings in its chemi-
`cal structure, it shows lower azimuthal anchoring energy than
`that of PI-3.
`
`4. Conclusion
`In this paper we reported LC alignment properties of LC
`cells with different polarized-UV-exposed PI films as an
`alignment film. The photosensitivity of the LC alignment for
`UV dosage and the azimuthal anchoring energies of the LC
`cell were measured as a function of polarized UV exposure
`time. The results suggest that the photosensitivity of the LC
`alignment is largely affected by the parameters of PIs, such as
`the UV absorption efficiencies defined by the UV absorbance
`and fluorescence, the mechanisms of decomposition, and the
`molecular conformations of the PIs. Furthermore, the az-
`imuthal anchoring energy of the LC cell is controlled by the
`parameters of PIs, such as the number of benzene rings in the
`PIs and the molecular conformations of the PIs. Further con-
`sideration of the anchoring energy dependence is now under
`way. We will publish the results elsewhere.
`
`Acknowledgements
`We acknowledge Dr. L. C. Chien of Kent State Univer-
`sity and Dr. Yu. Reznikov of the Institute of Physics of the
`Ukrainian National Academy of Science for their useful dis-
`cussion. We also thank Dr. N. Bessho and Dr. Y. Matsuki of
`JSR Co. for their support in this research. The research was
`supported in part by the NSF Science and Technology Center
`ALCOM, DMR 89-20147.
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`
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`Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 2, No. 3B
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