Kent State University
`Digital Commons @ Kent State University Libraries
`
`Physics Publications
`
`5-15-1998
`
`Department of Physics
`
`Alignment of Liquid Crystals on Polyimide Films
`Exposed To Ultraviolet Light
`
`Jae-Hoon Kim
`
`Satyendra Kumar
`Kent State University Kent Campus, skumar@kent.edu
`
`Sin-Doo Lee
`
`Follow this and additional works at: http://digitalcommons.kent.edu/phypubs
`Part of the Physics Commons
`
`Recommended Citation
`K m Jae Hoon; Kumar Satyendra; and Lee S n Doo. "Al gnment of L qu d Crystals on Poly m de F lms Exposed To Ultrav olet
`L ght." Physical Review E 57 no. 5 (1998): 5644 5650. Accessed at http://d g talcommons.kent.edu/phypubs/16
`
`Th s A c e s b oug o you o ee a d ope access by e Depa
`accep ed o c us o
` P ys cs Pub ca o s by a au o zed ad
`o
`a o , p ease co ac ea c a1@ke .edu, k@ke .edu.
`
`o s @ Ke S a e U ve s y L b a es. I as bee
`e o P ys cs a D g a Co
`s a o o D g a Co
`o s @ Ke S a e U ve s y L b a es. Fo o e
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`Page 1 of 8
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`Tianma Exhibit 1028
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`

`
`PHYSICAL REVIEW E
`
`VOLUME 57, NUMBER 5
`
`MAY 1998
`
`Alignment of liquid crystals on polyimide films exposed to ultraviolet light
`
`Jae-Hoon Kim and Satyendra Kumar
`Department of Physics, Kent State University, Kent, Ohio 44242
`
`Sin-Doo Lee
`School of Electrical Engineering, Seoul National University, Kwanak P.O. Box 34, Seoul 151-742, Korea
`共Received 5 January 1998兲
`
`The influence of unpolarized and linearly polarized UV exposure on previously rubbed as well untreated thin
`polyimide 共PI兲 alignment layers was studied. Optical retardation and surface morphology measurements were
`carried out to understand the nature of surface modification as a function of the polarization and the exposure
`time of the UV light under different surface conditions. The exposure of the UV light on the PI layer was found
`to change drastically the morphological anisotropy due to photochemical dissociation. The control of aniso-
`tropic surface forces by the linearly polarized UV 共LPUV兲 exposure combined with the rubbing process is
`important to study the alignment mechanism of liquid crystals on various substrates. A simple model incor-
`porating the effect of the LPUV exposure is presented together with the essential features of the experimental
`results. 关S1063-651X共98兲15005-5兴
`
`PACS number共s兲: 61.30.Gd, 61.16.Ch
`
`I. INTRODUCTION
`
`Substrates with anisotropic surface potential such as ob-
`liquely evaporated SiO layers, Langmuir-Blodgett films, and
`rubbed polymer films have been used to control the align-
`ment of the optic axis of liquid crystals 关1兴. Mechanical rub-
`bing of polyimide 共PI兲 layer is the most common method
`used in mass production of liquid crystal 共LC兲 displays be-
`cause of its simplicity and thermal stability. Although the
`mechanism responsible for the resultant alignment is not yet
`fully understood, it is believed that shearing of the film dur-
`ing the rubbing process orients polymer aggregates or poly-
`mer chains along the rubbing direction, as revealed by pre-
`关2–5兴
`vious
`surface
`studies
`employing atomic
`force
`microscopy 共AFM兲 and optical birefringence measurement.
`The disadvantages of the rubbing method are the generation
`of dust particles, electrostatic charges, physical damage, and
`nonuniformities which are detrimental to the fabrication of
`thin film transistor based devices.
`To eliminate these problems, a nonrubbed photoalignment
`process has been developed. It has been demonstrated that
`poly共vinyl兲4-methoxycinnamate and poly共vinyl兲cinnamate
`films, when exposed to a linearly polarized ultraviolet
`共LPUV兲 light, can be very effective as alignment layers 关6,7兴.
`The photoalignment method allows for an easy control of the
`alignment direction and anchoring strength so that multido-
`main devices, with improved viewing angle characteristics,
`can be readily produced. However, this method yields align-
`ment layers that possess poor thermal and chemical stability
`and reliability compared to the rubbing method.
`Recently, several research groups have reported align-
`ment of LC’s by PI films exposed to the LPUV light 关8,9兴.
`Fourier transform infrared 共FTIR兲 spectroscopy has shown
`that the UV irradiation anisotropically photodissociates pho-
`tosensitive chemical bonds in PI’s including those in the
`imide ring 关10兴. This reduces the polarizability of PI mol-
`ecules 关11兴 and, as we will show here, changes the surface
`morphology. Since the LC alignment is believed 关12兴 to de-
`
`pend on the competition of various physical and chemical
`interactions between the LC molecules and the substrate sur-
`face, it is very important to determine the role of surface
`morphology in LC alignment. To the best of our knowledge,
`there have been no such systematic studies of UV exposed PI
`layers.
`In this paper, we report the results of the influence of
`LPUV light on surface properties of thin PI films. We have
`measured the optical birefringence 共retardation兲 and charac-
`terized surface morphology with AFM for various UV expo-
`sures and surface conditions. We also present a phenomeno-
`logical model to help understand the relationship between
`the measured phase retardation, surface morphology, and LC
`alignment.
`
`II. EXPERIMENT
`
`The PI used in this study is SE610 of Nissan Chemical
`Company. Spin coated films of SE610 were soft baked at
`100 °C for 15 min, then hard baked at 220 °C for 1 h. The
`film was exposed to the LPUV light from a Xe lamp. The
`intensity of the UV light after passing through the polarizer
`is approximately 6 mW/cm2. A metal cylinder wrapped with
`velvet, spun with a linear velocity of 1.1 m/min, was used to
`rub the films.
`We used a photoelastic modulator 共PEM90, Hinds Instru-
`ments兲 with a fused silica head and a He-Ne laser for optical
`phase retardation measurements. The photoelastic modulator
`共PEM兲 was placed between two crossed polarizers with its
`optic axis at 45° to the axes of polarizer and analyzer. The
`LC cell prepared with photoalignment layers was placed be-
`tween the PEM and the analyzer. The signal from the pho-
`todetector was fed to a lock-in amplifier 共EG & G Princeton
`Applied Research, Model 5210兲 for measuring the ac signal
`and a digital multimeter for the dc signal. The lock-in am-
`plifier was tuned to the 50-kHz reference signal from the
`PEM. The laser beam was incident normal to the sample
`cell’s surface. The signal was monitored while rotating the
`
`1063-651X/98/57共5兲/5644共7兲/$15.00
`
`57
`
`5644
`
`© 1998 The American Physical Society
`
`Page 2 of 8
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`
`57
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`ALIGNMENT OF LIQUID CRYSTALS ON POLYIMIDE . . .
`
`5645
`
`FIG. 1. Optical phase retardation as a function of the rotation
`angle of the sample for 共curve a兲 unrubbed, 共curve b兲 rubbed, and
`共curve c兲 LPUV exposed 共30 min兲 PI films.
`
`FIG. 2. Optical phase retardation as a function of the LPUV
`exposure time for an unrubbed PI film. The solid line represents the
`fit of the data to Eq. 共2兲.
`
`sample with respect to the surface normal. The sensitivity of
`this method enables us to measure the phase retardation with
`a precision of ⫾0.01°.
`Surface morphology was measured with a commercial
`atomic force microscope 共Nanoscope III, Digital Instruments
`Inc.兲 operated in a contact mode with a constant force. The
`measurements were made in air at room temperature using a
`microfabricated pyramidal shaped Si3N4 tip integrated into a
`rectangular cantilever with a spring constant of 0.58 N/m.
`
`III. RESULTS AND DISCUSSION
`
`The optical phase retardation was measured as a function
`of the angle of rotation for an unrubbed PI film, a PI film
`exposed to LPUV for 30 min, and a rubbed film 共see Fig. 1兲.
`The rubbing direction and the LPUV light’s polarization axis
`were kept collinear but perpendicular to the PEM’s optical
`axis. Under these conditions, the sample birefringence causes
`the optical phase retardation to vary at twice the rate of the
`rotation frequency of the sample. The sign of the retardation
`was determined from the phase difference. The fast and slow
`axes of the sample can be determined from the amplitude
`and phase of the optical retardation.
`
`A. Alignment on unrubbed PI films exposed to LPUV light
`An unrubbed PI film normally shows a negligible optical
`retardation, typically less than 0.05°, due to the flow-induced
`ordering of the PI chains during the film deposition process
`and/or the strain-induced birefringence of the glass substrate.
`The dashed curve (a) in Fig. 1 represents the optical retar-
`dation of such unrubbed sample.
`The optical retardation of a PI film increases drastically
`upon rubbing, as evident from curve (b) in Fig. 1. It is be-
`lieved 关2,3兴 that the alignment of polymer chains caused by
`rubbing is the origin of such a large optical anisotropy. Op-
`tical retardation of PI films also exhibits rapid increase upon
`LPUV exposure. The result of the film exposed for 30 min is
`represented by the dotted curve (c) in Fig. 1. Clearly, the
`magnitude of the retardation is comparable, but has an oppo-
`
`site sign to that of the rubbed film. This indicates that the
`LPUV exposure causes anisotropic photochemical dissocia-
`tion of imide bonds parallel to its polarization, leaving the
`polymer chains in the perpendicular direction relatively un-
`perturbed. This is consistent with the conclusion drawn by a
`previous FTIR study 关10兴. The broken bonds reduce the po-
`larizability of the PI molecules. In contrast to the previous
`report that the LC alignment is mainly achieved via the in-
`teractions of LC molecules with the polar functional group in
`PI produced by LPUV light 关13兴, our results show that an-
`isotropic irreversible depolymerization is primarily respon-
`sible for LC alignment on SE610 PI films exposed to LPUV
`light.
`Within the framework of the previous model 关11兴, the
`photoreaction rate can be described by a coupling term
`mជ(cid:149)pជ, where mជ and pជ are the transition moment of the pho-
`tosensitive bonds and the polarization direction of LPUV
`light, respectively. The time dependent angular distribution
`of photosensitive bonds can be written as
`
`共1兲
`
`N共␪,␾,t 兲⫽N 0共␪,␾兲exp共⫺␣t cos2␾ cos2␪兲,
`where N 0(␪,␾) is the initial angular distribution of photo-
`sensitive bonds, and ␣is a constant. Here ␾and ␪denote the
`azimuthal and polar angles of mជ with respect to pជ, respec-
`tively. The initial distribution of photosensitive bonds with-
`out any surface treatment is assumed to be azimuthally iso-
`i.e., N 0(␪,␾)!N 0(␪). Considering
`tropic,
`a
`two-
`dimensional system in which all molecules lie in a plane
`parallel to the substrate, the total optical retardation of such a
`film depolymerized by the LPUV light can be written as
`
`共2 cos2␾⫺1 兲
`
`R共t 兲⫽A冕
`
`␲/2
`
`0
`
`⫻兵1⫺exp共⫺␣t cos2␾ cos2␪兲其d␾,
`
`共2兲
`
`where A is a constant. The optical phase retardation as a
`function of the UV exposure time is shown in Fig. 2. The
`optical anisotropy rapidly increases with the exposure time,
`and becomes saturated in 1 h, after which it begins to de-
`
`Page 3 of 8
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`5646
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`JAE-HOON KIM, SATYENDRA KUMAR, AND SIN-DOO LEE
`
`57
`
`FIG. 3. The time evolution of polymer chain alignment with
`increasing the LPUV exposure time. The arrows indicate the in-
`crease in the exposure time. Rods represent molecular units of the
`polymer.
`
`crease slowly. This may be due to the fact that the prolonged
`UV exposure eventually dissociates even the bonds oriented
`perpendicularly to the polarization direction. The solid line
`in Fig. 2 represents the fit of the data to Eq. 共2兲 with ␣
`⫽0.026. Assume that the spin coated PI film can be treated
`as a two-dimensional system composed of interconnected
`rods forming the polymer chains. If the absorbed photon en-
`ergy exceeds the photosensitive bond energy, the polymer
`chains are photochemically dissociated 共i.e., the link between
`the rods is broken兲. In Fig. 3, the time evolution of the poly-
`mer chain density and orientation is shown during LPUV
`exposure. The polymer chains are modeled as consisting of
`1000 rods. Initially, the rods are distributed isotropically.
`With increasing LPUV exposure time, an increasing number
`of polymer chains, oriented with their constituting rods par-
`allel to the polarization direction, is dissociated, thus leaving
`intact those chains which are oriented perpendicularly. Since
`the majority of remaining polymer chains are aligned nearly
`perpendicular to the LPUV light’s polarization, the stacks of
`polymer segments are observed by AFM as macroscopic do-
`mains elongated in that direction.
`Since the LC alignment occurs at the substrate surface, it
`is very important to understand the role of surface morphol-
`ogy in LC alignment. Figure 4 shows the surface morpholo-
`gies of PI films determined by AFM for different LPUV
`exposure times. The unexposed PI film shown in 共a兲 consists
`of randomly distributed circular domains with diameter of
`100–200 nm and root-mean-square 共rms兲 vertical roughness
`of 0.4 nm. After 1 h of LPUV exposure, as shown in Fig.
`4共b兲, elongated polymer aggregates 100–200 nm long and
`50–100 nm wide, were formed in the direction perpendicular
`to the LPUV polarization. The roughness of the surface be-
`comes 0.2 nm. The resultant two-dimensional power spec-
`trum shown in Fig. 4共c兲, obtained by Fourier analysis of Fig.
`4共b兲, clearly reveals a new component in the direction per-
`
`FIG. 4. Surface morphologies of the unrubbed PI film deter-
`mined by AFM for various LPUV exposure times: 共a兲 0 h,共 b兲1 h,
`共d兲 2 h, and共 e兲4 h. The two-dimensional power spectrum of
`共b兲
`obtained by a fast Fourier transformation is shown in 共c兲.
`
`pendicular to the polarization of UV light 共shown by the
`arrow兲. The degree of anisotropy is smaller than in the
`rubbed film. The anisotropy of surface morphology has also
`been confirmed by x-ray reflectivity experiments 关14兴. We
`believe that the formation of polymer aggregates caused by
`the photochemical dissociation and associated morphological
`anisotropy plays a very important role in LC alignment. With
`increasing exposure time,
`the surface roughness also in-
`creases to 0.28 and 0.38 nm for 2- and 4-h 关Figs. 4共d兲 and
`4共e兲兴 exposures, respectively. In all cases, the polymer ag-
`gregates are preferentially oriented perpendicular to the po-
`larization direction of the UV light.
`To understand the effect of surface morphology on LC
`alignment, we observe a microscopic texture with a nematic
`LC, Merck-E48, in a 6-␮m-thick cell. The best alignment
`was obtained with 30–60 min of UV exposure of both sur-
`faces. The alignment began to degrade with longer exposure.
`Sample cells with 4-h exposure exhibited microscopic do-
`mains, and were less uniform than cells with 30-min expo-
`sure. This may depend on two factors. One is the possibility
`of reverse tilt at the surface. Since we exposed LPUV light
`normal to the surface, it is possible to create pretilt angles in
`two opposite directions with equal probability. The other is
`the effect of surface roughness. On the basis of AFM results,
`it was found that the degree of alignment decreases with
`
`Page 4 of 8
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`

`
`57
`
`ALIGNMENT OF LIQUID CRYSTALS ON POLYIMIDE . . .
`
`5647
`
`FIG. 5. The LC alignment texture 共a兲 and surface morphology
`共b兲 for a rubbed PI film; 共c兲 and 共d兲 show the texture and surface
`morphology, respectively, of the rubbed sample subsequently ex-
`posed to RPUV light.
`
`increasing surface roughness. This is consistent with the fact
`that the surface ordering decreases with increasing isotropic
`surface roughness 关15兴. However, it should be pointed out
`that the roughness anisotropy, which appears to govern the
`LC alignment, cannot be reliably measured from AFM im-
`ages.
`
`B. Alignment on rubbed PI film exposed to unpolarized
`UV light
`Since the azimuthal angle of the easy axis of a LC is
`strongly influenced by the distribution of photosensitive
`bonds 关11兴, it is important to determine the dependence of
`the anisotropic surface interactions, induced by LPUV light,
`on the initial distribution of the photosensitive chemical
`bonds produced by rubbing.
`Figure 5 shows microscopic LC texture in a 6-␮m-thick
`cell and surface morphology of the rubbed PI film before 关共a兲
`and 共b兲兴 and after 关共c兲 and 共d兲兴 its exposure to randomly
`polarized UV 共RPUV兲 light. Very uniform LC alignment
`was achieved before UV exposure. Surface morphology of
`the rubbed surface clearly shows microscratches of different
`widths and depths, and elongated polymer clusters along the
`rubbing direction. The alignment and hence the texture of
`this cell changed drastically after 15-min exposure to RPUV
`light, as shown in Fig. 5共c兲. This kind of the Schlieren tex-
`ture is usually obtained for unrubbed substrates. Since the
`polymer chains in the unrubbed sample aligned randomly
`共and hence isotropically兲,
`it may be concluded that
`the
`RPUV light dissociates the polymer chain in all directions
`rendering it isotropic. The properties of the RPUV exposed
`surface are expected to be similar to an unrubbed substrate.
`As is clear from Fig. 5共d兲, many of the scratch lines, espe-
`cially finer ones, become obscured to differing degrees.
`However, the large scratch lines still remain. It appears that
`microscopic scratches, at a length scale smaller than a critical
`dimension, determine LC alignment in much the same way
`
`FIG. 6. A homogeneously aligned cell between crossed polariz-
`ers. The cell rubbing direction R coincides with the axis of one of
`the crossed polarizers. Dark 共bright兲 regions marked as I 共II兲 are
`without 共with兲 LPUV exposure.
`
`as relief gratings 关16兴. This suggests that macroscopic
`scratches 共mechanical grooves兲 which are parallel to the rub-
`bing direction are inconsequential for the LC alignment. This
`conclusion is consistent with the inferences drawn on the
`basis of x-ray reflectivity measurements of surface morpho-
`logical anisotropy as those are over a length scale of x-ray
`coherence length (⬍0.5 ␮m) 关14兴.
`
`C. Alignment on rubbed PI film exposed to LPUV light
`When a rubbed PI film is exposed to LPUV light, the
`competition between the effects of rubbing and LPUV light
`determines the direction and the degree of alignment 关17兴.
`Figure 6 shows the microscopic textures of a sample that was
`rubbed twice and then exposed to LPUV light for 20 min.
`The polarization direction of LPUV 共marked as ‘‘UV’’ in
`Fig. 6兲 light makes an angle of 40° with respect to the rub-
`bing direction. Only half of the surface is exposed to LPUV
`light, so that the exposed and unexposed areas could be com-
`pared. The rubbing direction R coincides with the axis of one
`of the polarizers, and minimum transmittance is obtained in
`the unexposed region 共marked as I兲, as expected. In the ex-
`posed region 共region II兲, a dark state can be obtained by
`rotating the LC cell by 50° with respect to the axis of the
`same polarizer. To further understand the effect of LPUV
`light, we determine the changes in surface morphology with
`AFM. Prior to LPUV exposure, microscratches and PI clus-
`ters extending in the direction of rubbing are clearly visible
`as shown in Fig. 5共b兲. After LPUV exposure, the prominence
`of these scratches and hence the anisotropy was diminished
`as evident from Fig. 7. Interestingly, the PI microclusters
`now appear to be elongated in the direction perpendicular to
`the polarization. It is believed that the photoreaction process
`is responsible for their formation. The power spectrum
`shows a new branch at an angle of ⬃50° with respect to the
`rubbing direction. This subtle change in surface morphology
`profoundly changes the LC alignment direction, as shown in
`Fig. 6. Since the easy axis of LC alignment coincides with
`the fast optic axis of the PI films, the angle ⌬␰, which rep-
`resents the deviation of the alignment direction from the di-
`rection normal to the polarization of LPUV, can be deter-
`mined from the measured angular dependence of film
`birefringence. Figure 8 shows the angular dependence of the
`
`Page 5 of 8
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`

`
`5648
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`JAE-HOON KIM, SATYENDRA KUMAR, AND SIN-DOO LEE
`
`57
`
`FIG. 7. Surface morphologies obtained by AFM and its power
`spectra for the rubbed PI film with LPUV exposure at an angle of
`40°.
`
`film birefringence in polar coordinates for the twice rubbed
`PI film with LPUV exposures of 0 min 共open circles兲, 5 min
`共filled squares兲, and 20 min 共open triangles兲. After approxi-
`mately 20 min, the birefringence and LC alignment direction
`is effectively rotated by 50°. The dependence of ⌬␰ on the
`exposure time is shown in Fig. 9 for films rubbed two and
`four times. Clearly, with increasing exposure time, ⌬␰!0. It
`should be possible to utilize these observations to control and
`fine tune the LC alignment on a rubbed PI surface by adjust-
`ing the LPUV exposure time.
`To understand the dependence of ⌬␰ upon LPUV expo-
`sure, one needs to consider the initial distribution of polymer
`chains and photosensitive bonds in the rubbed film. The ori-
`entational distribution that polymer chains acquire during
`rubbing can be assumed to be a Gaussian peaked in the di-
`rection of rubbing. However, LPUV causes a selective dis-
`sociation of the C共O兲vN bonds, which are nearly parallel to
`the polymer chain. Assuming that the azimuthal and polar
`distributions are independent of each other, Eq. 共1兲 can be
`modified as
`
`w
`⫻exp关⫺␣t cos2共␾0⫺␾兲cos2␪兴,
`
`共3兲
`
`w being the width of the distribution. Here ␾b and ␾0 denote
`the azimuthal angles for the rubbing and LPUV directions,
`respectively.
`Figure 10 shows the distribution of photosensitive bonds
`according to Eq. 共3兲 for different LPUV exposure times and
`for ␣⫽0.026, w⫽5, ␪⫽0, and ␾b⫽0°. Before UV expo-
`sure, the polymer chains possess a Gaussian distribution ac-
`quired during rubbing 关Fig. 10共a兲兴. When it is exposed to
`
`FIG. 8. The angular dependence of the optical anisotropy for the
`twice rubbed PI film subsequently exposed to LPUV light with a
`polarization at 40° to the rubbing direction. The open circles, filled
`squares, and open triangles represent exposures for 0, 5, and 20
`min, respectively.
`
`FIG. 10. Orientational distribution of photosensitive bonds cal-
`culated from Eq. 共3兲 for different LPUV exposures on the rubbed PI
`film. 共a兲 is the distribution with no exposure; 共b兲 and 共c兲 are the
`distributions after 10 min and 30 min exposures, respectively, with
`␾0⫽40°; and 共d兲 and 共e兲 are the distributions after 30- and 60-min
`exposures, respectively, with ␾0⫽0°. For all curves, ␣⫽0.026, w
`⫽5, ␪⫽0°, and ␾b⫽0°.
`
`FIG. 9. Dependence of ⌬␰ on the LPUV exposure time. The
`circles and triangles denote the experimental data for the PI films
`rubbed two and four times, respectively. The solid lines represent
`the best fits of the data to Eq. 共4兲. The dashed and dotted lines are
`the calculated curves for A⫽0.1 and 0.5 min/deg, respectively.
`
`冉 ␾⫺␾b
`
`冊 2册
`
`1 2
`
`N共␪,␾,t 兲⫽N 0共␪兲exp冋 ⫺
`
`Page 6 of 8
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`57
`
`ALIGNMENT OF LIQUID CRYSTALS ON POLYIMIDE . . .
`
`5649
`
`FIG. 12. The angular dependence of optical anisotropy for the
`twice rubbed PI film for different LPUV exposure times with ␾0
`⫽␾b⫽0. The filled circles, open circles, open squares, and filled
`squares represent exposures for 0, 10, 30, and 60 min, respectively.
`
`dence of ␣and w on the microscopic properties of LC and
`polymer material parameters is crucial for the effective use
`of this method.
`Figure 11 shows the measured optical anisotropy and the
`phase difference as a function of the exposure time when the
`polarization direction of LPUV light coincides with the rub-
`bing direction 共i.e., ␾0⫽0兲. The optical anisotropy created
`by rubbing rapidly decreases and vanishes after approxi-
`mately 20 min. The phase angle changes abruptly from 0° to
`180° at that time. For longer exposures, the anisotropy dra-
`matically recovers and saturates within 1 h. The angular de-
`pendence of optical anisotropy on the exposure time are
`shown in Fig. 12. Interestingly, each branch points in the
`same direction, which is in contrast to the results shown in
`Fig. 8. However, from the phase change, we note that the
`branches are discontinuously rotated by 90° for exposure of
`30 and 60 min. In the case of Fig. 8, the directions of R and
`UV light were different. Both results show that the popula-
`tions of polymer chains parallel and perpendicular to the
`rubbing direction change abruptly due to chemical dissocia-
`tion. Eventually, more chains remain intact in the direction
`perpendicular to the direction of polarization. Figures 10共d兲
`and 10共e兲 show distributions of photosensitive bonds calcu-
`lated using Eq. 共3兲 for different exposures with ␾b⫽␾0⫽0.
`It is very interesting that there are two symmetric peaks at
`␾⫽␾s and ⫺␾s , different from the asymmetric peak in case
`of ␾0⫽40° 关Figs. 10共b兲 and 10共c兲兴. This suggests that there
`are two easy axes. These peaks move toward ␾⫽⫾90°,
`which becomes the final alignment direction dictated by
`LPUV exposure. For intermediate exposures, the two easy
`axes compete with each other, and the resultant axis does not
`v⫽0兲 for ⫺45°⬍␾s⬍45° because polymer
`rotate 共i.e., ␾
`chains are distributed with equal probability in both direc-
`v abruptly becomes ⫾90° for ␾s⬎45° or
`tions. However, ␾
`␾s⬍⫺45°. At ␾s⫽⫾45°, since both component 共i.e., par-
`allel and perpendicular to the rubbing direction兲 have equal
`probabilities, the average optical anisotropy is reduced to
`zero. The discussion and the model presented here provides a
`satisfactory explanation of the measured optical retardation.
`
`sa
`
`sa
`
`FIG. 11. Optical phase retardation and phase difference as a
`function of the LPUV exposure time for the rubbed PI film for
`which polarization and rubbing directions were parallel.
`
`LPUV light with polarization oriented at 40° with respect to
`the rubbing direction, the peak shifts in the direction perpen-
`dicular to the polarization of LPUV light, and the distribu-
`tion height decreases with increasing the exposure time
`关Figs. 10共b兲 and 10共c兲兴. However, the optical phase retarda-
`tion 共Fig. 8兲 shows different behavior, i.e., the maximum
`value first decreases and then increases with increasing the
`exposure time. This appears to be due to photodissociation of
`bonds in the bulk of the film. Let ␾b⫽0 in Eq. 共3兲; then the
`LC alignment determined by the LPUV exposure ␾s satisfies
`the equation
`
`t sin 2共␾0⫺␾s兲⫹A␾s⫽0,
`
`共4兲
`
`where A⫽(1/w)/(2 ␣ cos2␪). The solid lines in Fig. 9 repre-
`sent fits of the experimental data to Eq. 共3兲 for ␾0⫽40° and
`⌬␰⫽␾s⫹50°. From these fits, we obtain A⫽0.24⫾0.01 and
`0.36⫾0.01 min/deg for films rubbed two and four times, re-
`spectively. If the exposure time t is comparable to the value
`of A, then the angle of rotation lies between 0° and 50°, and
`the magnitude of birefringence is lower than its initial value.
`With increasing LPUV exposure, the easy axis rotates to-
`wards the alignment direction ‘‘preferred’’ by LPUV light,
`i.e., ⌬␰!0. The width of the Gaussian distribution, w, is
`related to the surface anchoring energy in such a way that w
`decreases with increasing rubbing strength. Therefore, it will
`take longer UV exposures to rotate the easy axis. The dashed
`and dotted lines in Fig. 9 are the numerically calculated re-
`sults for A⫽0.5 and 0.1 min/deg, respectively. These results
`suggest that it should be possible to fine tune the alignment
`direction of the LC molecules by adjusting the LPUV expo-
`sure and/or the rubbing strength. Understanding the depen-
`
`Page 7 of 8
`
`

`
`5650
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`JAE-HOON KIM, SATYENDRA KUMAR, AND SIN-DOO LEE
`
`57
`
`samples exposed for 4 and 8 h, the depth is only 60% and
`30% of the original value, respectively 关Figs. 13共c兲 and
`13共d兲兴. It indicates that the surface anisotropy induced by the
`rubbing process is diminished by LPUV light. One can also
`see the development of PI clusters elongated in the direction
`perpendicular to the rubbing direction, showing that morpho-
`logical anisotropy has changed.
`
`IV. CONCLUDING REMARKS
`
`We have described the physical origin of surface modifi-
`cation of the PI layer caused by exposure to UV light. The
`photomodified unrubbed PI surface shows weak anisotropy
`between the directions parallel and perpendicular to LPUV
`light’s polarization direction, which suggests that the mor-
`phological anisotropy plays a very important role in LC
`alignment. In the case of rubbed PI films subsequently ex-
`posed to RPUV light, it was found that macroscopic scratch
`lines are inconsequential, and that the morphological anisot-
`ropy at a submicrometer length scale is important for LC
`alignment. The fine tuning of LC alignment can be achieved
`by combining rubbing and controlled LPUV exposure of PI
`films. The surface morphological anisotropy produced by
`rubbing is reduced by UV exposure through the photodisso-
`ciation process. Details of the competition between the ef-
`fects of rubbing and LPUV exposure deserve further inves-
`tigations. A simple model presented here can be used to
`describe physical phenomena associated with photochemical
`processes in polymer system.
`
`ACKNOWLEDGMENTS
`
`We gratefully acknowledge the help, in various forms, of
`K. Ha and J. West during this work. This work was sup-
`ported in part by NSF Science and Technology Center,
`ALCOM Grant No. DMR-89-20147, and by the Korea Sci-
`ence and Engineering Foundation.
`
`FIG. 13. Surface morphologies of the rubbed and subsequently
`LPUV exposed PI films determined by AFM for different UV ex-
`posure times with ␾0⫽␾b⫽0; 共a兲 0 h, 共b兲1 h, 共c兲4 h, and 共d兲8 h.
`
`Figure 13 shows surface morphologies of the rubbed
`samples exposed to LPUV with polarization parallel to the
`rubbing direction for different exposure times. Without UV
`exposure, scratch lines caused by rubbing are clearly defined,
`and LC’s are aligned along the rubbing direction 关Fig. 13共a兲兴.
`The visible lines are about 1 nm deep and 50–200 nm wide.
`With increasing exposure, the surface structure gradually be-
`come obscure. Though the wider of the scratch lines remain
`qualitatively the same, their depth decreases. For exposures
`of 1 h, the depth is reduced to 0.7–0.8 nm 关Fig. 13共b兲兴. For
`
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