Kent State University
`Digital Commons @ Kent State University Libraries
`
`Chemical Physics Publications
`
`Department of Chemical Physics
`
`5-11-1998
`
`Mechanism of Unidirectional Liquid-Crystal
`Alignment on Polyimides with Linearly Polarized
`Ultraviolet Light Exposure
`M. Nishikawa
`
`Bahman Taheri
`Kent State University, bahman@alphamicron.com
`
`John L. West
`Kent State University, jlwest@kent.edu
`
`Follow this and additional works at: http://digitalcommons.kent.edu/cpippubs
`Part of the Physics Commons
`
`Recommended Citation
`N sh kawa M.; Taher Bahman; and West John L. (1998). Mechan sm of Un d rect onal L qu d Crystal Al gnment on Poly m des
`w th L nearly Polar zed Ultrav olet L ght Exposure. Applied Physics Letters 72(19) 2403 2405. Retr eved from
`http://d g talcommons.kent.edu/cp ppubs/91
`
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`Page 1 of 4
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`Tianma Exhibit 1026
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`APPLIED PHYSICS LETTERS
`
`VOLUME 72, NUMBER 19
`
`11 MAY 1998
`
`Mechanism of unidirectional liquid-crystal alignment on polyimides
`with linearly polarized ultraviolet light exposure
`M. Nishikawa,a) B. Taheri, and J. L. Westb)
`Liquid Crystal Institute, Kent State University, Kent, Ohio 44242
`共Received 9 February 1998; accepted for publication 5 March 1998兲
`Unidirectional liquid-crystal 共LC兲 alignment by a linearly polarized ultraviolet light 共UV兲 exposure
`was examined using two types of polyimide 共PI兲. PI with a fluorene unit incorporated in a side chain
`showed LC alignment perpendicular to those of conventional PIs, i.e., perpendicular to the rubbing
`direction and parallel
`to the exposure polarization of UV. The results of the dichroic ratio
`measurement of LC cells, UV absorption spectra, birefringence, infrared absorption spectra, and PI
`conformation calculations using molecular mechanics suggest that the photodecomposition of PI by
`UV exposure produces anisotropic van der Waals forces, which align LC along its optical axis.
`© 1998 American Institute of Physics. 关S0003-6951共98兲00919-X兴
`
`Liquid-crystal 共LC兲 displays 共LCDs兲 are widely used be-
`cause of their low-power consumption, thin profile, and full
`color capability. To fabricate LCDs, unidirectional LC align-
`ment with a controlled pretilt angle is typically produced
`using surface rubbing techniques.1 However, rubbing may
`cause static charge, dust, or scratches, which lower the pro-
`duction yield of LCDs. Methods such as Langmuir–Blodgett
`polymers,3 microgrooves,4
`films,2
`stretched
`stamped
`polymers,5 and linearly polarized ultraviolet light 共UV兲 ex-
`posure of polymers,6 have been proposed, which produce
`unidirectional LC alignment without rubbing. Among them,
`LC alignment using linearly polarized UV exposure is a
`strong candidate for overcoming the above problems in ad-
`dition to greatly simplifying production of multidomain dis-
`plays.
`Two main types of material for LC alignment using a
`polarized UV exposure have been proposed. One is based on
`photoisomerization of azocompounds doped in polymers.7–9
`The other utilizes anisotropic photoreactions of poly共vinyl
`cinnamate兲 derivatives6,10–12 and polyimides 共PIs兲13–15 pro-
`duced by linearly polarized UV exposure. Much effort has
`been devoted to develop photoreactive PIs which show high
`heat resistance compared with those of azocompounds and
`poly共vinyl cinnamate兲 derivatives. However, the mechanism
`of unidirectional LC alignment using PIs exposed to linearly
`polarized UV has yet to be clarified. In this letter, we wish to
`explore the mechanism and characteristics of unidirectional
`LC alignment using UV exposed PIs.
`The PI materials used in these experiment are shown in
`Fig. 1. PI films were prepared by heat curing of precursor
`polyamic acid solutions, which were synthesized from the
`reaction between tetracarboxylic dianhydride and diamines.
`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 on indium tin oxide glass sub-
`strates and then curing at 250 °C for an hour. The thickness
`
`a兲On leave from Yokkaichi Research Laboratories, JSR Co., Yokkaichi, Mie,
`510-0871, Japan. Electronic mail: mnishi@lci.kent.edu
`b兲Electronic mail: johnwest@scorpio.kent.edu
`
`FIG. 1. Chemical structures of PIs used.
`
`of the PI film was controlled at 50 nm. The PI films were
`exposed with linearly polarized UV incident normal to the
`surface. We used a 450 W xenon lamp 共Oriel, model 6266兲
`as the UV source, and a surface film polarizer 共Oriel, model
`27320兲 whose effective range is between 230 and 770 nm.
`The intensity of UV after passing through the polarizer was
`about 6 mW/cm2. LC cells for dichroic ratio measurement
`were fabricated using two polarized UV-exposed substrates
`with parallel polarization axes. Dichroic LC, ZLI-2293
`共Merck兲 and 0.5% M-618 共Mitsuitoatsu, ␭max⫽550 nm兲,
`was filled into the cells in the isotropic state 共120 °C兲. Di-
`chroic ratios of the LC cells were measured using one polar-
`izer and a UV–visible spectrometer, Perkin Elmer Lambda
`19. PI film birefringences were measured using an instru-
`ment described previously.16 Infrared spectra of the PI films
`were measured using an infrared spectrometer, Nicolet 550.
`The LC alignments produced by either rubbing or expo-
`sure to normally incident polarized UV are summarized in
`Table I. PI-1 shows LC alignment parallel to the rubbing
`direction and perpendicular to the UV polarization. This is
`consistent with results reported previously.13,14 PI-2, how-
`ever, aligns LC perpendicular to the rubbing direction and
`
`TABLE I. LC alignment on PIs.
`
`LC alignment direction
`
`PI
`
`PI-1
`PI-2
`
`Rubbing
`
`Parallel
`Perpendicular
`
`Polarized UV
`
`Perpendicular
`Parallel
`
`0003-6951/98/72(19)/2403/3/$15.00
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`2403
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`© 1998 American Institute of Physics
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`Page 2 of 4
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`2404
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`Appl. Phys. Lett., Vol. 72, No. 19, 11 May 1998
`
`Nishikawa, Taheri, and West
`
`FIG. 2. Relationship between UV exposure time and dichroic ratios of LC
`cells on PI films.
`
`parallel to the UV polarization. Figure 2 shows the relation-
`ship between the polarized UV exposure time and dichroic
`ratios of LC cells. Dichroic ratios of LC cells initially in-
`crease logarithmically with UV dosage, and approach a con-
`stant value at higher dosage. Furthermore, PI-2 shows low
`sensitivity to UV exposure compared with that of PI-1.
`To elucidate the LC alignment mechanism, we con-
`firmed the effective axis for LC alignment. Rubbed PI films
`were exposed normally with polarized UV parallel and per-
`pendicular to the rubbing axis for 0–40 min. Figure 3 shows
`the relationship between polarized UV irradiation time and
`dichroic ratios of LC cells. In both PIs, dichroic ratios of LC
`cells exposed with polarized UV parallel to the rubbing di-
`rection gradually decreased with UV irradiation time. On the
`other hand, those with polarized UV exposure perpendicular
`to the rubbing direction showed no change of their dichroic
`ratios. It is well known that the rubbing treatment of the PI
`surface results in the alignment of PI chains parallel to the
`rubbing direction.17 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 the dichroic ratios of the LC cells.
`Changes in the UV absorption spectra of the PI films
`
`FIG. 4. UV absorption spectra of PI films.
`
`before and after polarized UV exposure were monitored. PI
`films on quartz substrates were prepared after polarized UV
`exposure for 2 h at surface normal. Figure 4 shows UV ab-
`sorption spectra of the PI films before and after polarized UV
`exposure. After UV exposure, both PI films showed de-
`creases in the broad absorption around 250 nm, which can be
`attributed to ␲– ␲* transitions of benzene rings in PI, and
`increases in the broad absorption above 290 nm. We have
`not yet determined the photochemical changes in the PI
`films, but it is clear that the broad absorption above 290 nm
`is generated by the decomposition of PI, which corresponds
`to the broad absorption around 250 nm. These phenomena
`have been previously reported on PI material with different
`chemical structure.13
`Figures 5 and 6 show the dichroic UV absorption spectra
`obtained by subtracting absorptions measured parallel (A para)
`and perpendicular (A per) to the rubbing direction and the
`exposed UV polarization, respectively. Dichroic UV absor-
`bance was measured using a surface film polarizer. In the
`case of rubbing 共Fig. 5兲, PI-1 shows a positive dichroic spec-
`trum, and PI-2 shows a negative dichroic spectrum. The be-
`haviors using polarized UV exposure are opposite to those
`produced by rubbing 共Fig. 6兲. Furthermore, it should be
`noted that the subtraction spectra of UV absorption spectra
`
`FIG. 3. Relationship between polarized UV exposure time after rubbing and
`dichroic ratios of LC cells.
`
`FIG. 5. Dichroic UV absorption spectra (A para⫺A per) of rubbed PI films
`measured relative to the rubbing direction.
`
`Page 3 of 4
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`

`
`Appl. Phys. Lett., Vol. 72, No. 19, 11 May 1998
`
`Nishikawa, Taheri, and West
`
`2405
`
`we conclude that before UV exposure, PI chains in the film
`are randomly aligned. PI chains parallel to the exposed UV
`polarization direction are selectively decomposed by UV ex-
`posure, and photoproducts after UV exposure are randomly
`relocated in the PI films. The residual PI chains perpendicu-
`lar to the exposed UV polarization, which show no photode-
`composition, cause the anisotropic van der Waal forces to
`align the LC along its optical axis. Also, the interplay be-
`tween the main chain and the side fluorene unit leads PI-2 to
`have low sensitivity to polarized UV compared with that of
`PI-1 as shown in Fig. 2.
`
`The authors acknowledge Dr. L. C. Chien, Dr. T. Kosa,
`and Dr. X. D. Wang of Kent State University for material
`synthesis, birefringence measurement, and useful discus-
`sions. They also acknowledge Dr. N. Bessho and Dr. Y.
`Matsuki of JSR Co., Ltd. for their support in this research.
`Research was supported in part by NSF Science and Tech-
`nology Center ALCOM, DMR 89-201747.
`
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`FIG. 6. Dichroic UV absorption spectra (A para⫺A per) of UV exposed PI
`films measured relative to the UV polarization.
`
`above 290 nm show no dichroism 共Fig. 6兲. The birefringence
`measurement of PI films after linearly polarized UV expo-
`sure also showed that PI-1 had the optic axis perpendicular
`to the UV polarization, and PI-2 had the optic axis parallel.
`Changes in the PI films before and after polarized UV
`exposure were also monitored using infrared absorption
`spectra. PI films on silicon wafers were normally exposed to
`polarized UV for 2 h. In both PI films, peak intensities at
`1380 cm⫺1 attributed to the ␯(imide C–N–C) decrease and
`those at 1720 cm⫺1 attributed to the ␯(C⫽0) increase after
`UV exposure. These results suggest that imide rings in PIs
`were decomposed by UV exposure, and a new band with
`infrared absorption at around 1720 cm⫺1 was generated by
`the by-product after the decomposition of PI.
`We calculated the conformations of PI-1 and PI-2 using
`the molecular mechanics method 共Fig. 7兲.18 The optical axis
`of PI-1 is aligned along the polymer backbone. On the other
`hand, the fluorene unit in PI-2, which has a large birefrin-
`gence, is aligned perpendicular to the axis of the main chain.
`Through the experiments and simulation in this letter,
`
`FIG. 7. PI conformations calculated using the molecular mechanics method.
`
`Page 4 of 4