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`(Continued on inside back cover)
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`Page 2 of 12
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`Proceedings of the
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`17th International
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`Liquid Crystal Conference
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`Part II of V
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`Strasbourg, France
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`July 19-24, 1998
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`Guests Editors
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`Antoine Skoulios
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`Daniel Guillon
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`Page 3 of 12
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`Page 3 of 12
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`Q) I99‘) 0|’/\ (()vcrsr:Is l’iIl»li\lit‘i‘\ /\-.si>\'iiItiim) N,V,
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`1>.,i.1,5im1i.y “mm. "ml" ,1“.
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`ouniim lllltl ”IL'lIt.‘i| H('ii‘II\'C |'tiltll\|It‘I\ Illllllllll.
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`Crys1., I999, Vol. 329. pp. 579-587
`Mol..Cry:!.
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`Reprints available directly from the publisher
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`Photawrrlns Perillittcd by license only
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`Generation of Pretilt Angles on Polyimides with a
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`Single Linearly Polarized UV Exposure
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`MICHINORI NISHIKAWA‘3b and JOHN L. WEST“
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`aKem‘ State University, Liquid Crystal Institute, Kent, Ohio, 44242, USA and
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`bJSR Co., Yokkaichi Research Laboratories. I00, Kawajiri-cho,
`iiikkuiclii. Mir,
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`510-8552. Japan
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`We designed polyimide (Pl) films containing fluorine atoms for liquid crystal (LC) alignment
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`using linearly polarized UV exposure. Using a single oblique polarized UV exposure we
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`obtained unidirectional LC alignment with any desired pretilt angles from 0 ‘’ up to 90 ".
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`Pretilt angles depend on the concentration of fluorine atoms in PI and the UV exposure time
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`and angle. Dichroic ratio and capacitance measurements of the LC cells suggest that ohliquc
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`polarized UV exposure breaks the surface order degeneracy of the LC molecules, generating
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`uniform pretilt angle along the optic axis of Pl film.
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`Keywords:
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`liquid crystal; polyimidc; prctilt angle; polarized UV exposure
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`INTRODUCTION
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`Liquid crystal (LC) alignment using polarized UV exposure” is a promising
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`candidate for overcoming problems such as generation of static chargc. dUSl. 0!
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`scratches caused by rubbing. However. generation of pretilt angle using this
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`technique is not easy because conventional photo-alignment materials. poly(vinyl
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`cinnamate)m and polyimides (PIs)'W. result in LC alignment pcrpcndiculfir 10 lht
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`UV polarization. Some methods have been proposed to generate prctilt angles: (1)
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`double polarized UV exposure with different polarization and irradiation anglcsl".
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`(2) combined polarized UV exposure with both p-wave and s-wave”. (3) °i’“‘l“¢
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`Double UV exposure is
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`polarized UV exposure on poly(coumarin)m-
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`[ll‘)ll/57‘)
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`Page 4 of 12
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`58()l[l 1921
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`MICHINORI NISHIKAWA and JOHN L. wasr
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`complicated for mass production. and combined UV exposure with both p- and 5-
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`wave decreases the polarization efficiency of the polarizer. Poly(coumarin) aligns
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`the LC parallel to the polarized UV direction and results in any desired pretilt angle
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`by adjusting the oblique irradiation angle. However, the long term reliability ofthe
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`LC displays incorporating UV cured poly(coumarin) alignment film is uncertain
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`because of the unreacted coumarin group and lower glass transition temperature.
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`ln our previous papers, we have reported that PI materials containing fluorene
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`units align LC parallel to the UV polarizationm and the introduction of fluorine
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`atoms into Plm generates any desired pretilt angles from 0 ° up to 90 °. However,
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`the mechanism of generation of pretilt angles has not been clarified.
`In this paper,
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`we explore the mechanism of generation of pretilt angles on Pls with a single
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`oblique linearly polarized UV exposure.
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`EXPERIMENTS
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`Pl materials used in these experiments are shown in Fig. 1. These Pl films photo-
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`decompose upon UV exposurem. Pl films were prepared by heat curing of the
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`precursor polyamic acid.
`lt has already been repor1ed“°'"l that PI formed from
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`fluorine containing diamine (F-diamine) causes high pretilt angles.
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`<£ei%l,<i%3:r
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`61en,
`FIGURE l Chemical structure or’ Pls used.
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`LC cells were prepared to measure the dicliroic ratios and pretilt angles of the
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`LCs aligned by polarized UV exposed Pl films. Pl films were deposited on [T0
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`glass substrates and then cured at 250 °C for l hr. The lhickncss ofghe p] mm was
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`controlled at 50 nm. The Pl films were exposed with polarized UV at 3 number of
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`Page 5 of 12
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`Page 5 of 12
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`GENERATION OF l’RETlLT ANGLES ON PIS
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`ll l\)3|l5lll
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`incident angles to the Pl 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 after passing through the
`polarizer was about
`I mW/cm’ at 254 nm. LC cells were fabricated using two
`polarized UV exposed substrates with anti-parallel polarization axis. LC. ZLI-2293
`(Merck), and dichroic LC, ZLl-2293 and 0.5 % M-6l8 (Mltsuitoatsu, Amax-SSO
`nm). were filled into the cells in the isotropic state (I20 °C) and slowly cooled to
`room temperature for measurement of pretilt angles and dichroic ratios of LC cells,
`respectively.
`The dichroic ratios of the LC cells were measured using one polarizer and a UV-
`Vis spectrometer. We used crystal rotation method to measure pretilt angles below
`IS ° and capacitive method to measure pretilt angels higher than l5 °.
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`RESULTS AND DISCUSSION
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`Figure 2 shows the relationship between the chemical structure of the Pls and the
`resulting pretilt angle of the LCs. The pretilt angles of LCs gradually increase with
`the molar fraction of F—diamine in Pl. Furthermore. the direction of the Pmm '"8'¢
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`coincides with that of the p-wave in the polarized UV lightjudging from the signs of
`symmetry points in the crystal rotation charts.
`In our previous papetsm". We
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`UV exposure angle. 45 '
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`UV exposure time‘ 20 mm Pretilt
`angleofLCcelll°) 5
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`10
`40
`60
`I0
`I00
`Molar fraction of F-dtamme X (molt /-l
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`FIGURE 2 Dependence of pretilt angles on molar fraction of F-diamine in Pl.
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`Page 6 of 12
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`532/: I my
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`MICIIINORI NISHIKAWA and JOHN L. WEST
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`to the exposed
`the selective photo-decomposition of Pls parallel
`reported that
`polarization of UV light causes the randomization of PI main chains. The residual
`Pl main chains perpendicular to the exposed UV polarization. which show no photo-
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`decomposition. cause anisotropic van der Waals forces"" which align the LC along
`the optic axis of Pl. This means that the director of the LC is controlled by the
`polarization axis of UV light. We assume that the p-wave in polarized UV light
`generates pretilt angles along its polarization axis. We chose the Pl containing 30
`molar ‘/o of F-diamine (Pl-JOF) to analyze the behaviors of pretilt angles in more
`detail.
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`Figure 3 (a) shows the relationship between polarized UV exposure time and the
`pretilt angles of the LC cells at different exposure angles. Pretilt angles initially
`increase with polarized UV exposure angle and show maximum values for UV
`exposure time. Furthermore. UV exposure time required to produce the maximum
`pretilt angle gradually increases with UV exposure angles. We measured the
`dichroic ratios of the LC cells by changing the polarized UV exposure angles and
`UV exposure time. Figure 3 (b) shows the relationship between polarized UV
`exposure time and the dichroic ratios of the LC cells with different UV exposure
`angles. The dichroic ratios of the LC cells gradually increase with UV exposure
`time and reach a constant value of about I0.S. which is the same as that obtained
`
`using rubbed Pl-30F alignment films. Furthermore. the dichroic ratios of the LC
`
`v-
`
`go
`go
`go
`20
`I0
`60
`40
`20
`Polaared uv uposure mm (min)
`Poltrirtd UV s-vow-c mm mm)
`FIGURE 3 Relationship between polarized UV exposure time and pretilt
`angles (a) and dichroic ratios (b) of LC cells using Pl-30F.
`
`100
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`0
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`.00
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`rlioofLCsell
`Dnchroie
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`Page 7 of 12
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`GENERATION OF PRETILT ANGLES ON PIS
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`ll 1951/533
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`UV exposure angle
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`0:
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`67.5‘
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`I4 2:
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`,, T:
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`3
`
`I2 .8
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`:3
`ll
`no
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`..
`to §
`
`9 =
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`'5
`8
`
`7
`"3
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`6 E
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`3 “
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`
`0
`:3
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`I00
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`0
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`530
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`A 520
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`‘E
`g 510
`8
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`
`4 500
`‘S
`5,
`3 490
`
`t:
`
`
`
`Q 480
`%
`
`
`U 470
`450
`450
`
`so
`so
`40
`2o
`
`
`
`
`Polarized UV exposure time (min)
`
`
`
`
`
`FIGURE 4 Relationship between polarized UV exposure time and capacitanees
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`of LC cells using Pl-30F for various UV exposure angles.
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`cells increase with decrease of UV exposure angles at lower UV dosage. These
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`results suggest that the alignment order ofthe LC on PI films with lower UV dosage
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`or larger UV exposure angle is smaller than those with higher UV dosage or smaller
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`UV exposure angle.
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`Figure 4 shows the capacitances of the LC cells as a function of UV exposure
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`time. The estimated pretilt angle from the capacitance is also shown on right
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`vertical axis. The capacitances of the LC cells show no change even thought
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`changing UV exposure time and UV exposure angles, and the estimated pretilt
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`angles are about 8 ". These results suggest that the surface pretilt angle on the Pl
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`film does not change before and after polarized UV exposure. Funhennore, the
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`decrease of the pretilt angles observed in the LC cells with higher UV dosage, Fig. 3
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`(a), probably result not from the decomposition of F-diarnine which generates high
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`pretilt angle, but from breaking the surface order degeneracy ofthe LC molecules on
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`Pl films induced by polarized UV exposure as the same phenomenon observed in
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`LC cell with rubbed Pl film using a second harmonic generation technique"°‘"'.
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`To analyze the reason why pretilt angle shows the maximum VBIUC TOY UV
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`exposure time, we measured the reflected UV light intensity (Fig. 5 (a)) from Pl-
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`ITO surface and the absorption spectrum ofthe Pl-30F film (Fig. 5 (b)). The result
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`of the reflected UV measurement at the incident UV angle of 67.5‘ to the surface
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`Page 8 of 12
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`Page 8 of 12
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`

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`584/I I I96]
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`MICIIINORI NISHIKAWA and JOHN L. WEST
`
`(I)
`Refleeled UV
`through 1'1
`
`Polarized UV
`
` AbsorbaneeofPl-JOF O D
`
`0.5
`o 2
`o I
`on
`zoo
`
`240
`
`no
`no
`wavelenguu (nm)
`
`360
`
`ton
`
`FIGURE 5 Reflected UV thought Pl film (a) and absorption spectrum of
`Pl-30F (b).
`
`normal suggests that about 5 % of reflected UV light to the incident UV light can
`pass through Pl film because of the low UV absorbance of Pl-30F (Fig. 5 (b)).
`Furthermore. it should be noted that the direction of the p-wave in the reflected UV
`light is opposite to that in the incident UV light as shown in Fig. 5 (a).
`To elucidate the mechanism of generation of pretilt angles in more detail. we
`measured the pretilt angles of the LC cells using Pl films with double polarized UV
`exposure. First the Pl film was exposed to polarized UV at an incident angle of 67.5
`
`UV exposure nngie
`—o—su.nm 3-)
`-o—Dmblc(I1.!'o 411-;
`
`.
`
`5 N
`
`ratiooiLCcell
`L
`
`Pu-tiltAngleo!l.Ccell(') oh0
`Doehroie
`
`
`-6
`0
`10
`40
`60
`I0
`I00 I20
`I40 I60
`Poll‘!!! UV GWOWR lit“! (mill)
`
`0
`
`o
`
`UV exposure angle
`...o_ Single (61 5 '
`—-o— Double (67.1
`
`I00 no H0160
`to
`so
`40
`20
`Polarised UV exposure time (min)
`
`FIGURE 6 Relationship between polarized UV exposure time and (a) ptetilt
`angles and (b) dichroic ratios ol'LC cells with single and double
`UV exposure.
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`Page 9 of 12
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`

`
`GENERATION OF PRETILT ANGLES ON PIS
`
`I l I97]/585
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`° to the surface nonnal. Then film was exposed to polarized UV at the opposite
`incident angle of -67.5 °. Figure 6 (a) shows the relationship between polarized UV
`exposure time and the pretilt angles of the LC cells compared with single and double
`UV exposure.
`In this figure. a negative sign of pretilt angle afler second UV
`exposure means that the direction of the pretilt angle caused by second UV exposure
`is opposite to that obtained by first UV exposure,
`In the case of single UV
`exposure. pretilt angles initially increase and show a maximum value for polarized
`
`UV exposure time. On the other hand, those after second UV exposure show steep
`decrease of pretilt angles. then show a negative maximum value. and gradually
`decrease for polarized UV exposure time.
`
`Figure 6 (b) shows the relationship between polarized UV exposure time and
`diehroic ratios of LC cells compared with single and double UV exposure. The
`dichroic ratios of the LC cells monotonically increase with polarized UV exposure
`time, and there is no difference in the tlichroic ratios of the LC cells using single or
`
`double UV exposure. These results suggest that the tilt angle direction of the LC
`can be easily controlled by the p-wave direction in the UV light.
`Using these results. we can develop a hypothesis of the mechanism of
`generation of pretilt angles. Figures 7 show the in-plane configurations of the LC
`molecules on UV exposed Pl during UV exposure.
`In Fig. 7 (3). "rows $h°W ll"
`tilt angle directions of the LC molecules on Pl
`film. Numbers which are
`
`'
`
`v
`
`(a)
`(I)
`(2)
`0)
`p-wave]
`
`5)
`Q‘ 314 l¢\/\l/\\d>ll lung
`°~.;e«= “git
`//95‘:
`llt_l.l)"
`l/D’ all
`(6
`.
`(5)
`,>lnml;
`t'tt,m‘
`rlu l‘
`¢r'n\t >l:ll.“'~r9l'llllW
`
`(hi
`
`6)
`Incident I
`|lV
`Polamed IIV exposure time
`
`ln~plnne configurations of LC molecules on l’l films (-1) dull"!
`FIGURE 7
`polarized UV exposure (h): arrows show the tilt angle direction of LC.
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`Page 10 of 12
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`

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`586/[I I98]
`
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`MICHINORI NISHIKAWA and JOHN L. WEST
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`corresponding to the in-plane configurations of the LC molecules shown in Fig. 7
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`(a) are also written in Fig. 7 (b). Before UV exposure the in-plane configuration of
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`the LC molecules on Pl film is random (Number 1). LC molecules tend to align
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`with tilt angle direction along the direction of the p-wave in the UV polarization due
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`to the decomposition of PI chains”) (Number 2, 3). The resulting measured pretilt
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`angles gradually increase with the degree of PI decomposition until its saturation
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`(Number 4). Further UV exposure, however, gradually causes the decrease of the
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`pretilt angle due to the reflected UV light from Pl-ITO surface, whose p-wave
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`direction is opposite to that in the incident UV light (Number 5, 6).
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`CONCLUSION
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`
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`In this paper, we reported the mechanism of generation of the pretilt angles on P]
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`films with a single linearly polarized UV exposure. Any desired pretilt angles from
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`0 ° up to 90 ° were obtained using Pl films with different fluorine concentration.
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`Prctilt angles also depended on the UV exposure time and UV exposure angles to
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`the Pl surface. We proposed that the pretilt angles on UV exposed Pl films depend
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`on the selective photo-decomposition of PI along the UV polarization which
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`produces anisotropic van der Waals forces to align the LC along the optic axis of the
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`PI. The dependence of pretilt angles ofthe LC on UV exposure conditions can be
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`explained by breaking the degeneracy of the in—plane configurations of LC
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`molecules during polarized UV exposure.
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`Acknowledgments
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`
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`We acknowledge Drs. L. C. Chien, B. Taheri. and X. D. Wang of Kent State
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`University and Dr. Yu. Reznikov of Institute of Physics of the Ukrainian National
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`Academy of Science for their useful discussion, we a|so thank [);5_ N, Bessho and
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`Y. Matsuki ofJSR Co. for their support in this research. Research was supported in
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`part by NSF Science and Technology Center ALCOM. DMR 89-20147.
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`
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`
`lll
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`K. Schmitt. V. Kozinknv, and V. Chigrinov, Jpn. J. Appl. Pl1y:., 3|. 2155
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`Page 11 of 12
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`GENERATION OF PRETILT ANGLES ON ms
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`[1190]/SR7
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`M. Hasegawa and Y. Tairil, J. Phatapnlym. Sci. T(‘(‘/L. 8, 703 (I995).
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