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`Citation:
`
`18. M. Nishikawa, B. Taheri andJ.W. Doane, "Effect of
`Chemical Structures of Polyimides on Unidirectional
`Liquid Crystal Alignment Using a Single Linearly Polarized
`UV Exposure," Mol. Cryst. Liq. Cryst. 325, 63-78 (1998).
`
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`MOLECULAR CRYSTALS .
`J AND
`
`LIQUID CRYSTALS
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`CONTENTS Announcement
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`~
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`Liquid Crystals
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`tr’
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`K. NEGITA: Anomalous Viscosity in the B1
`.
`in
`:TIA and Y. XING:
`»:
`X. M. ZHAO, X. CHEN, L. B. CHENG, N.
`Synthesis, Crystal Growth and Structure of lV-'If3l—Cyano-2-n.itro-phenyl)—L-serine
`and Investigation of SHG Activity
`K. R. HA, J. L. WEST and G. R. MAGYAK' Polarized Infrared Spectroscopic
`Studies of Polarized UV-exposed Polyimide Films for Liquid Crystal Alignment
`C. V. YELAMAGGAD, V. PRASAD, M. MANICKAM and S. KUMAR: New Chiral
`Discotic Liquid Crystals
`K. Y. HAN, J. K. SONG and J. S. LEE: Optically controlled Alignment of Liquid Crystals
`on Alignment Layer (PWD) Containing Azo-group
`R. LU, K. XU, S. XIAO, Y. LE, X. ZHOU, H. WU, J. GU and Z. LU: Liquid Crystal
`Alignment on Polymer Films
`A
`M. NISHIKAWA. B. TAHERI and J. L. WEST: Effect of Chemical Structures of
`Polyimides on Unidirectional Liquid Crystal Alignment using a Single Linearly
`Polarized UV Exposure
`A. P. DAVEY, R. G. HOWARD, B. LAHR, W. J. BLAU and H. J. BYRNE: Measurement
`of Degree of Order in Mixed Polarised Fluorescent Polymer Liquid Crystal Films
`P. SARKAR, P. SARKAR, P. MANDAL and T. MANISEKARAN: Molecular Structure
`and Packing in the Crystalline State of 4-n—Ethyl—4’—Cyanobiphenyl (ZCB) by Single Crystal
`X-ray Diffractometry
`G.-H. CHEN, J. SPRINGER, W. THYEN and P. ZUGENMAIER: Surface Phenomena
`of Liquid Crystalline Substances
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`(continued on back cover)
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`Manuscripts should be submitted to the following editors. See inside back cover for addresses. Notes
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`Liquid Crystals: D. W. Bruce, (UK); S. Chandrasekhar, (India); (USA); G, Heppke,
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`Page 3 of 19
`
`( Cuntinued on inside back cover)
`
`Page 3 of 19
`
`

`
`Effect of Chemical Structures of Polyimides
`on Unidirectional Liquid Crystal
`Alignment using a Single Linearly
`Polarized UV Exposure
`
`M. N|SHIKAWA*, B. TAHERI and J. L.WEST
`
`Liquid Crystal Institute, Kent State University, Kent, Ohio, 44242, USA
`
`(Received 18 February 1998; In final form 13 April 1998)
`
`Unidirectional liquid crystal (LC) alignment by a linearly polarized UV exposure was examined
`using various polyimides (PIS) which consist of different tetracarboxylie dianhydrides and
`diamines. Measurement of dichroic ratios of LC cells suggests that sensitivity of unidirectional
`LC alignment for UV dosage is largely affected by the chemical structures of Pls used. The
`results of dichroie ratio measurement of LC cells, UV absorption spectra, birefringences, FT-IR
`spectra of PI films, and molecular conformations of diamines in PIs calculations using mole-
`cular mechanics suggest that the selective photo—decomposition of PI caused the anisotropic Van
`der Waals forces which aligns the LC along its optic axis in residual PI chain.
`
`Keywords: Liquid crystal; poly-imide; alignment; polarized UV exposure
`
`1. INTRODUCTION
`
`Almost all liquid crystal displays (LCDs) are fabricated using LC alignment
`techniques to obtain well-oriented LC molecular conformations. A good
`example is the twisted nematic LCDs which are fabricated using mechani-
`cally rubbed polyimide (PI) films as LC alignment fihns [1]. However,
`rubbing may cause static charge, dust, or scratches which lowers the
`
`*Corresponding author. Tel: 330-672-3999 X306, Fax: 330-672-2796, e-mail: mnishi@lci.~
`kenL.edu On leave from Yokkaichi Research Laboratories, JSR Co., Yokkaichi, Mie, 510-8552,
`Japan.
`
`63
`
`Page 4 of 19
`
`Page 4 of 19
`
`

`
`micro-grooves [4], stamped polymers [5] and linearly polarized ultraviolet
`light (UV) exposure of polymers [6], have been used to produce uni-
`directional LC alignment. LC alignment using a linearly polarized UV
`exposure is the most promising candidate to overcome the problems with
`rubbing in addition to greatly simplifying production of multi—domain
`displays.
`Three main types of mateirals for LC alignment using polarized UV
`exposure have been proposed. First is based on photo-isonierization of azo
`compounds doped in polymers [6—9]. Second utilizes anisotropic cz's— trans
`isomerization [10] or cross-linking [1 1 — 15] of poly(vinyl cinnamate)
`derivatives. Third is based on photo-decomposition of Pls produced by
`linearly polarized UV exposure [l6—20]. Much eifort has been concentrated
`on development of photoreactive Pls which are more heat resistant than azo
`compounds and poly(vinyl cinnamate) derivatives. However, the mechanism
`of unidirectional LC alignment using linearly polarized UV exposure has yet
`to be clarified. In this paper, we explore the mechanism and characteristics
`of the unidirectional LC alignment using UV exposed Pls with respect to
`their chemical structures.
`
`2. EXPERIMENTS
`
`2.1. Synthesis of Pls
`
`PI materials used in this experiments are summarized in Figure 1. PI-1—PI—7
`were prepared by heat curing of precursor polyamic acids which were
`synthesized from the equal molar
`reaction between tetracarboxylic
`dianhydrides and diamines. PI—8 is an organic-solvent-soluble polyimide
`synthesized from the reaction reported previously [21].
`
`2.2. Preparation of LC Cells
`
`LC cells were prepared to measure dichroic ratios of LCs aligned by linearly
`polarized UV exposed PI films. PI films were deposited by first spin-coating
`dilute solutions of the respective polyamic acids and polyirnide on ITO glass
`substrates and then cured at 250°C for 1 hr to accomplish imidization [22].
`The thickness of the PI film was controlled at 50 nm. The PI films were
`
`Page 5 of 19
`
`Page 5 of 19
`
`

`
`0
`
`O
`
`
`
`
`
`O O
`00/
`
`fig
`
`9°
`
`FIGURE 1 Chemical structures of Pls used.
`
`exposed with linearly polarized UV incident normal to the surface. We used
`a 450 W-Xe lamp (Oriel, model 6266) as a UV source, and a surface film
`polarizer (Oriel, model 27320) whose effective range is between 230 nm to
`770 nm. The intensity of UV in the UV B region after passing through the
`polarizer was about 6 mW/cmz. The intensity of Xe lamp monotonically
`increases with wavelength between 200 to 400 nm; relative intensities of
`UV light for the light intensity at 250 nm are about 2.2, 2.9 and 3.2 at 300,
`350 and 400 nm, respectively [23]. LC cells for dichroic ratio measurement
`were fabricated using two polarized UV exposed substrates with paral-
`lel polarization axis. Dichroic LC, ZLI-2293 (Merck) and 0.5% M-618
`(Mitsuitoatsu, azo derivative, /\ma,,= 550 um) [24], was filled into the cells in
`the isotropic state (l20°C) and slowly cooled to room temperature to
`remove flow alignment. LC cells with LC injection in the nematic state
`
`Page 6 of 19
`
`Page 6 of 19
`
`

`
`2.3. Measurement Methods
`
`The -dichroic ratios of the LC cells were measured using one polarizer and
`a UV-Vis spectrometer. PI film birefringences were measured using an
`instrument described previously [26]. Infrared spectra of PI films were
`measured using an FT-IR spectrometer, Nicolet 550.
`
`3. RESULTS AND DISCUSSION
`
`3.1. LC Alignment on Various Pl Films
`
`The LC alignment produced by various Pls cured at 250°C for 1 hr and
`either rubbed or normally exposed with linearly polarized UV are sum-
`marized in Table I. In most cases, PI shows LC alignment parallel to the
`rubbing direction and LC alignment perpendicular to the UV polarization.
`This is consistent with results reported previously [16, 17]. PI-6 containing
`fluorene diamine, however, aligns LC perpendicular to the rubbing direction
`and parallel to the exposed UV polarization.
`
`3.2. Effect of Diamine Structures in Pls on LG Alignment
`
`Figure 2 shows the relationship between polarized UV exposure time and
`the dichroic ratios of LC cells with respect to the number of benzene rings
`in Pls. The dichroic ratios of LC cells initially increase logarithmically with
`UV dosage, and approach a constant value at higher dosage. The order of
`
`PI
`
`Pl-1
`PI—2
`PI-3
`PI-4
`PI—5
`Pl—6
`PI-7
`PI-8
`
`.
`
`TABLE I LC aligiment on Pls
`LC alignment direction
`Polarized UV
`Perpendicular
`Perpendicular
`Perpendicular
`Perpendicular
`Perpendicular
`Parallel
`Perpendicular
`Perpendicular
`
`Rubbing
`Parallel
`Parallel
`Parallel
`Parallel
`Parallel
`Perpendicular
`Parallel
`Parallel
`
`Page 7 of 19
`
`Page 7 of 19
`
`

`
`on
`
`
`
`DichroicratioofLCcell
`
`J».O
`
`.1
`
`.2
`
`.5
`
`1
`
`2
`
`5
`
`I0
`
`20
`
`SD
`
`100
`
`Polarized UV exposure time (min)
`
`FIGURE 2 Relationship between UV exposure time and dichroic ratios of LC cells on various
`PI films.
`
`dichroic ratios at lower UV dosage using PI-l — PI—3 is as below:
`
`PI-3 > PI-1 > PI—2
`
`The order of dichroic ratios of LC cells is not consistent with benzene ring
`number in PIS. Figure 3 shows the molecular conformations of diamines
`used in these PIS calculated using molecular mechanics method. Diamine in
`PI-l shows the flat molecular conformation with respect to its side view. On
`the other hand, those of PI—2 and H-3 are bent because of the ether linkage
`between benzene rings. The optic axes of benzenes are also shown in Figure
`3 as arrows. It is thought that the anisotropic van der Waals forces [27]
`which would be a main force to align LC increases with the benzene ring
`number in PI. In the case of PI-2 and H-3, however, the interplay of optic
`axes with diflerent vector directions in Pls decreases the magnitude of the
`anisotropic van der Waals force in UV exposed PI films and results in the
`decrease of dichroic ratios of LC cells.
`
`Comparison of the dichroic ratios of LC cells using P1 with and without
`aromatic benzene ring suggests that PI without benzene ring (PI-4) shows
`much lower sensitivity for UV dosage than that with benzene ring (PI-2) as
`reported previously [28]. UV absorption spectra of these PI films coated on
`quartz glass are shown in Figure 4. PI-2 shows the absorption around 250
`nm, which can be attributed to it-—7r* transitions of benzene ring, and PI-4
`
`Page 8 of 19
`
`Page 8 of 19
`
`

`
`Side View )-coo»-{ mag
`Benzene ring
`1
`2
`number
`
`3
`
`Optical axis 4-> V \/V‘
`FIGURE 3 Molecular conformations of diarnines in PIs calculated using molecular mecha—
`nics method.
`
`Absorbancc
`
`(a.u.)
`
`200 220 240 260 280 300 320 340 360 380 400
`
`Wavelength (rim)
`
`FIGURE 4 UV absorption spectra of PI films.
`
`has very small absorption around 250 nm. Results suggest that PI-4 has the
`low sensitivity for UV dosage because of no UV absorption in the range
`over 230 nm Where polarizer works effectively.
`Figure 5 shows the dependence of the dichroic ratios of LC cells on
`polarized UV exposure time with respect to PI with fluorene unit located
`in main chain andiside chain. It is found that PI—5 shows higher sensitivity
`for UV dosage than that of PI-6. In addition, PI-5 shows LC alignment
`perpendicular to the exposed UV polarization, and that of PI-6 is parallel to
`the UV polarization. Figure 6 shows the molecular conformations of
`diamjnes used in these PIS. Diamine in PI-5 has fiuorene unit parallel to its
`
`Page 9 of 19
`
`Page 9 of 19
`
`

`
`ratioofLCcell
`Dichroic
`
`5
`
`10
`
`20
`
`50
`
`100
`
`.1
`
`.2
`
`.5
`
`1
`
`2
`
`Polarized UV exposure time (min)
`
`FIGURE 5 Relationship between UV exposure time and dichroic ratios of LC cells on various
`PI films containing fluorene unit.
`
`PI-5
`
`PI-6
`
`‘
`TopView
`SideView.9-<osc1oeo(>0-o: Vi
`
`Optical Axis V \
`
`FIGURE 6 Molecular conformations of diamiues in PIs containing fluorene unit calculated
`using molecular mechanics method.
`
`main chain, and that in PI-6 has fluorene unit perpendicular to its main
`chain. Perpendicular location of fluorene unit in PI-6 to the main chain leads
`LC molecule to align perpendicular to the rubbing axis as sa.me result
`
`Page 10 of 19
`
`Page 10 of 19
`
`

`
`3.3. Effect of Tetracarboxylic Dianhydride Structures
`in Pls an LC Alignment
`
`Figure 7 shows the relationship between polarized UV exposure time and
`the dichroic ratios of LC cells. Results show that the chemical structures of
`tetracarboxylic dianhydrides also considerably affect the sensitivities of LC
`alignment for UV exposure. In the case of PI—7, the previous paper reported
`that decomposition of PI chain is restrained due to the charge transfer in
`P1 observed as UV absorption over 300 nm [30]. This_ charge transfer
`phenomenon is peculiarly observed in Pls synthesized from aromatic
`tetracarboxylic dianhydrides and aromatic diarnjnes. However, the differ-
`ence of sensitivities for UV dosage between PI-2 and PI-8 can not be
`explained by charge transfer phenomenon, because H-2 and PI-8 have
`almost same UV absorption spectra without absorption over 300 nm. To
`0OI1fiITI1 the difference between two samples, dichroic ratios of LC cells were
`measured using PI films UV exposed in air and nitrogen atmosphere (Fig. 8).
`In this case of PI-8, the sensitivity for UV exposure largely decreased in
`
`ratioofLCcell
`Dicliroic
`
`.1
`
`.2
`
`.5
`
`1
`
`2
`
`5
`
`10
`
`20
`
`50
`
`100
`
`Polarized UV exposure time (min)
`
`FIGURE 7 Relationship between UV exposure time and dichroic ratios of LC cells on various
`PI films containing different tetracarboxylic dianhydrides.
`
`Page 11 of 19
`
`_..a..
`
`Page 11 of 19
`
`

`
`—A— PI-8 (in N7)
`
`—o—1>1.2
`—o—1>1—2(mN,)
`—A— PI-8
`
`Eo
`_,
`"5
`.2
`:5
`.2
`
`2 §
`
`4o
`
`s
`
`6
`
`2
`
`r
`
`I
`
`.1
`
`.2
`
`.5
`
`1
`
`2
`
`5
`
`10
`
`20
`
`50
`
`100
`
`Polarized UV exposure time
`
`FIGURE 8 Relationship between UV exposure time and dichroic ratios of LC oells on various
`PI films UV exposed in air and nitrogen atmosphere.
`
`nitrogen atmosphere compared with those in air. In contrast, those of PI-2
`showed no diiference at lower UV dosage. These results suggest that PI-2
`shows two kinds of decomposition, imido ring cleavage [31] which require
`oxygen and cyclobutane ring cleavage [32] which does not require oxygen.
`Furthermore, the PI decomposition by cyclobutane ring cleavage is do-
`minant at lower UV dosage. PI-8 shows_ no cyclopentane ring cleavage in
`nitrogen atmosphere, so this leads PI—8 into the lower sensitivity for UV
`exposure compared with that of PI-2.
`
`3.4. Analysis of Mechanism of Unidirectional LC Alignment
`using Polarized UV Exposure
`
`To elucidate the LC alignment mechanism in more detail, we chose two
`types of PI which show LC alignment perpendicular (PI-2) and LC align-
`ment parallel fl’I-6) to the exposure polarization of UV. We first confirmed
`the effective axis for LC alignment. Rubbed PI films were normally ex-
`posed with polarized UV parallel and perpendicular to the rubbing axis for 0
`min to 40 min. Figure 9 shows the relationship between polarized UV
`irradiation time and dichroic ratios of LC cells. In both Pls, dichroic ratios
`of LC cells exposed with polarized UV exposure parallel to the rubbing
`direction gradually decreased with UV irradiation time. On the other hand,
`
`Page 12 of 19
`
`Page 12 of 19
`
`

`
`
`
`DicllroicratioofLCcell
`
`ks)
`
` UV exposure to rubbing
`
`
`—C% PI-2 (parallel)
`—."' PI-2
`erpendicular)
`
`
`-1’-‘am PI-6
`arallel)
`PI-6 (perpendicular)
`
`
`50
`
` 0
`
`40
`30
`20
`10
`Polarized UV exposure time (min)
`
`FIGURE 9 Relationship between polarized UV exposure time after rubbing and dichroic
`ratios of LC cells.
`
`those with polarized UV exposure perpendicular to the rubbing direction
`showed no change of their dichroic ratios. It is well known that rubbing
`treatment of PI surface results in the alignment of PI chains parallel to the
`rubbing direction [21]. These results suggest that polarized UV absorbed
`parallel to the aligned PI main chains selectively causes the photochemical
`reaction of PI, and results in the decreases of dichroic ratios of LC cells.
`Dianline used in PI—6 has two optical axis attributed to fluorene unit and
`diphenyl methane unit as shown in Figure 6. Electron transfer between
`fluorene unit and diphenyl methane unit is inihibited because their 7r-elec-
`tron orbitals are perpendicular to each other [33]. This means that fluorene
`unit and diphenyl methane unit in diamine used in Pl-6 can individually
`absorb UV light and diphenyl methane unit directly bonded with
`cyclobutane tetracarboxylic dianhydride probably causes the decomposition
`of PI by UV exposure.
`
`UV absorption spectral changes in P1 films before and after polarized UV
`exposure were monitored to measure UV absorption spectra. PI films on
`quartz substrates were prepared and exposed for 2 hrs with measured
`normal to the surface. Figure 10 shows UV absorption spectra of PI films
`before and after polarized UV exposure. After UV exposure, both PI films
`showed decreases of absorption of 250 nm and increases in the broad
`absorption above 290 nm. I have not yet determined the photochemical
`
`Page 13 of 19
`
`Page 13 of 19
`
`

`
`Absorbance
`
`(a.u.)
`
`200 220 240 260 280 300 320 340 360 380 400
`
`Wavelength (run)
`
`FIGURE 10 UV absorption spectra of PI films before and after UV exposure.
`
`changes in P1 films, but it is clear that the broad absorption over 290 nm are
`generated by the decomposition of PI which corresponds to the absorption
`of 250 nm. These phenomena are also previously reported on P1 material
`with different chemical structure [16, 17].
`Figures 11 and 12 show the dichroic UV absorption spectra of rubbed PI
`films and polarized UV exposed PI films measured parallel (Apm) and
`perpendicular (Aper)
`to the rubbing direction and the exposed UV po-
`larization, respectively. Dichroic UV absorbance was measured using a
`surface film polarizer. In the case of rubbing (Fig. 11), PI—2 shows a positive
`dichroic spectrum, and PI-6 shows a negative dichroic spectrum. The
`behaviors using polarized UV exposure is opposite to those produced by
`rubbing (Fig. 12). Furthermore, it should be noted that the subtraction
`spectra of UV absorption spectra over 290 nm show no dichroism (Fig. 12).
`Birefringence measurement of PI films after linearly polarized UV exposure
`also showed that PI-2 had the optical axis perpendicular to the exposed UV
`polarization, and PI-6 had the parallel optic axis.
`Changes in PI films before and after polarized UV exposure were also
`monitored using FT—IR (Fig. 13). PI films on silicone wafers were normally
`exposed to polarized UV for 2 hr. Attribution of IR absorption peaks [34]
`and relative peak intensity [35] of each peak for the peak at 1500 cm‘1
`attributed to zx(l,4-benzene) are summarized in Table II. In both PI films,
`peak intensities of most of the peaks decrease by UV exposure in air. This
`
`Page 14 of 19
`
`Page 14 of 19
`
`

`
`
`
`
`
`DichroicUVabsorbance(a.u.)
`
`0_0o .................................
`
`-0.02
`
`-0.04
`
`‘
`-0.06
`200 220 240 260 280 300 320 340 360 380 400
`Wavelength (nm)
`
`FIGURE 11 Dichroic UV absorption spectra (Apm—Ape,) of rubbed PI films measured re-
`lative to the rubbing direction.
`
`0.04
`
`PcN
`
`0.00
`
`
`
`DichroicUVabsorbance(a.u.) .'=4?
`
`-0.02
`
`'
`-0.06
`200 220 240 260 280 300 320 340 360 380 400
`Wavelength (nm)
`
`FIGURE 12 Dichroic UV absorption spectra (APm—Ape,) of UV exposed PI films measured
`relative to the UV polarization.
`
`result shows that surface etching of PI film is caused by UV exposure [36].
`Furthermore,
`relative peak intensities at
`1380 cm" attributed to
`z/(imideC—N —C) decrease and those at 1720 cm” attributed to
`
`Page 15 of 19
`
`Page 15 of 19
`
`

`
`LIQUID CRYSTAL OF POLYIMIDES
`
`75
`
`Absorbance
`
`(a.u.)
`
`Pl—2 (UV 2 hr)
`PI-2 '
`
`2000
`
`1800
`
`1600
`
`1400
`
`1200
`
`1000
`
`Wavenumber (em-1)
`
`FIGURE 13 FT-IR spectra of PI films before and after UV exposure in air.
`
`TABLE II Attribution of IR absorption peaks and relative intensity
`
`Wavermmber
`(cm‘')
`
`Arrrflnuion
`
`1720
`1500
`I380
`
`u(C=0]
`I/(l,4-benzene)
`1/(C —N —C)
`
`Relative peak imem-izy'
`
`H-6
`H-2
`Before UV UVin air UVin N;Bef0re UVUVDI air I/Vin N1
`L30
`1.44
`1.24
`2.26
`2.91
`2.13
`1.00
`1.00
`1.00
`1.00
`1.00
`1.00
`0.40
`0.30
`0.38
`0.81
`0.74
`0.80
`
`1/(C = 0) increase after UV exposure. In the case of UV exposure in air,
`imide rings in Pls were decomposed by UV exposure, and a new band with
`IR absorption at around 1720 cm“ was generated by the by-product after
`the decomposition of PI. In contrast, PI films UV exposed in nitrogen
`atmosphere show less imide ring cleavage compared with those UV exposed
`in air. These results also support two kinds of PI decomposition, imide ring
`cleavage and cyclobutane ring cleavage, by UV exposure.
`Taking into account results obtained in this paper we can conclude the
`mechanism of unidirectional LC alignment by a linearly polarized UV
`exposure as follows. Figure 14 shows the schematic mechanism of LC
`alignment by a polarized UV exposure. Before UV exposure, PI chains are
`randomly aligned. P1 chains parallel to the exposed UV polarization are
`selectively decomposed by UV exposure, and photo-products after UV
`exposure are randomly relocated in PI films. The residual PI chains
`
`Page 16 of 19
`
`

`
`M. NISHIKAWA er :11.
`
`Before UV exposure
`PI Ehflin ¢0nfi8u1‘8£i0n (fimdflm)
`
`After UV exposure
`Randornization ofphoto-product
`
`FIGURE 14 Schunatic mechanism of LC alignment by a linearly polarized UV exposure;
`dashed lines shows the by-products of Pl after polarized UV exposure.
`
`perpendicular to the exposed UV polarization, which show no photo-
`decomposition, cause the anisotropic van der Waals force to align LC along
`its slow axis.
`
`4. CONCLUSION
`
`In this paper, we report unidirectional LC alignment by a linearly polarized
`UV exposure using various PIs. PI containing fluorene unit in its side chain
`shows LC alignment parallel to the exposed UV polarization, whose be-
`havior is perpendicular to those of conventional PI materials. Measurement
`of dichroic ratios of LC cells suggests that sensitivity of unidirectional LC
`alignment for UV dosage is largely affected by the chemical structures of PIs
`used. Especially, PI containing diamine with large van der Waals force and
`PI containing cyclobutane tetracarboxylic dianhydrides show high sensitiv-
`ity of LC alignment for UV dosage. The results of dichoric ratios of LC
`cells, UV absorption spectra, birefzingences, FT—IR spectra of PI films, and
`molecular conformations of diamines in PIs calculated using molecular
`mechanics suggest that the selective photo-decomposition of PI caused the
`randomization of orientation of photo-products. The residual PI chains
`perpendicular to the exposed UV polarization, which show no photo-
`decomposition, cause the anisotropic van der Waals force to align LC along
`its slow axis.
`
`Acknowledgements
`
`We acknowledge Drs. L. C. Chien, T. Kosa, and X. D. Wang of Kent State
`
`University. for material synthesis, birefringence measurement, and useful
`
`Page 17 of 19
`
`

`
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