New Clues to the Factors Governing the Perpendicular Alignment of

Sep 20, 2003 - been found to align LCs in a direction perpendicular to the rubbing direction.7. Unlike the PIs conventionally used in the LC display i...
0 downloads 0 Views 430KB Size
Download PDF
Langmuir 2003, 19, 8735-8743

8735

New Clues to the Factors Governing the Perpendicular Alignment of Liquid Crystals on Rubbed Polystyrene Film Surfaces Seung Woo Lee, Boknam Chae, Hak Chul Kim, Byeongdu Lee, Wooyoung Choi, Seung Bin Kim,* Taihyun Chang, and Moonhor Ree* Department of Chemistry, Division of Molecular and Life Sciences, BK21 Program, Center for Integrated Molecular Systems, and Polymer Research Institute, Pohang University of Science & Technology, San 31, Hyoja-dong, Nam-gu, Pohang 790-784, Republic of Korea Received May 22, 2003. In Final Form: July 30, 2003 We have investigated the surfaces of rubbed polystyrene (PS) films in detail using atomic force microscopy and have discovered a previously unknown surface feature: submicroscale meandering groove-like structures composed of gravel-like grooves in tens of nanometers are present, oriented perpendicular to the rubbing direction. This unusual surface morphology is a significant departure from the surface topographies observed so far for rubbed PS and other polymer films, for which grooves are usually only found parallel to the rubbing direction. We also conclude from retardation analysis and linearly polarized infrared (IR) spectroscopy that the phenyl side groups of the PS chains are reoriented perpendicular to the rubbing direction, with para-directions that are nearly normal to the film plane, whereas the vinyl backbones of the PS chains are reoriented along the rubbing direction. This is the first time IR spectroscopic techniques have been used to determine the three-dimensional reorientation geometry of phenyl rings on the surface of rubbed PS films. Uniform, homogeneous alignment of liquid crystal (LC) molecules is achieved at rubbed PS film surfaces, but the alignment director is usually perpendicular to the rubbing direction. This perpendicular LC alignment was found to have very low anchoring energy (less than 3 × 10-7 J/cm2) and to persist only for a limited time (less than 1 day), indicating that the interaction of LCs with rubbed PS surfaces is very weak. Collectively, the results lead to the conclusion that the unusual well-developed groove topography oriented perpendicular to the rubbing direction plays a critical role in governing the alignment of LC molecules that weakly interact with the perpendicularly reoriented phenyl side groups, overriding the effects of the parallel reoriented vinyl main chains. In addition, the zero degree pretilting behavior of LCs on PS surfaces was discussed, in particular with respect to the rubbing-induced reorientations of the PS polymer segments and their anisotropic interactions with the LC molecules.

Introduction The uniform, unidirectional alignment of liquid crystal (LC) molecules plays an important role in the optical and electrical performance of industrial LC flat-panel display devices.1 The uniform alignment of LC molecules is currently obtained in the LC display industry by rubbing polymer alignment layers with a rayon velvet fabric; this method offers both simplicity and control of LC alignment with regard to both the LC anchoring energy and the pretilt angle.1-3 Polyimides (PIs) are widely used as LC alignment layers because of their advantageous properties, such as excellent optical transparency, adhesion, heat resistance, dimensional stability, and insulation.1-6 Most rubbed polyimides have been reported to align LCs parallel to the rubbing direction, with a pretilt direction that coincides with the rubbing direction.1-6 However, one rubbed PI film, that of poly{p-phenylene 3,6-bis[4-(n-butoxy)phenyloxy]pyromellitimide} (C4-PMDA-PDA PI), has recently been found to align LCs in a direction perpendicular to the rubbing direction.7 * To whom correspondence should be addressed. Tel: +82-54279-2120 (M.R.), 279-2106 (S.B.K.). Fax: +82-54-279-399. E-mail: [email protected], [email protected]. (1) (a) Liquid Crystals; Collings, P. J., Ed.; IOP Publishing Ltd.: Bristol, 1990. (b) Handbook of Liquid Crystal Research; Collings, P. J., Patel, J. S., Eds.; Oxford University Press: Oxford, 1997. (2) (a) Janning, J. L. Appl. Phys. Lett. 1972, 21, 173. (b) Cognard, J. Alignment of Liquid Crystals and Their Mixtures; Gorden & Breach: London, 1982. (c) Mauguin, C. Bull. Soc. Fr. Miner. 1911, 34, 71. (3) Geary, J. M.; Goodby, J. W.; Kmetz, A. R.; Patel, J. S. J. Appl. Phys. 1987, 62, 4100.

Unlike the PIs conventionally used in the LC display industry, rubbed polystyrene (PS) films have also been (4) (a) Lee, K.-W.; Paek, S.-H.; Lien, A.; During, C.; Fukuro, H. Macromolecules 1996, 29, 8894. (b) van Aerle, N. A. J.; Tol, J. W. Macromolecules 1994, 27, 6520. (5) (a) Kim, S. I.; Ree, M.; Shin, T. J.; Jung, J. C. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 2909. (b) Kim, S. I.; Pyo, S. M.; Ree, M.; Park, M.; Kim, Y. Mol. Cryst. Liq. Cryst. 1998, 316, 209. (c) Ree, M.; Kim, S. I.; Pyo, S. M.; Shin, T. J.; Park, H. K.; Jung, J. C. Macromol. Symp. 1999, 142, 73. (d) Ree, M.; Shin, T. J.; Lee, S. W. Korea Polym. J. 2001, 9, 1. (e) Park, J. H.; Jung, J. C.; Sohn, B. H.; Lee, S. W.; Ree, M. J. Polym. Sci., Polym. Chem. 2001, 39, 3622. (f) Park, J. H.; Sohn, B. H.; Jung, J. C.; Lee, S. W.; Ree, M. J. Polym. Sci., Polym. Chem. 2001, 39, 1800. (g) Jung, J. C.; Lee, K. H.; Sohn, B. H.; Lee, S. W.; Ree, M. Macromol. Symp. 2001, 164, 227. (6) (a) Toney, M. F.; Russell, T. P.; Logan, J. A.; Kikuchi, H.; Sands, J. M.; Kumar, S. K. Nature 1995, 374, 709. (b) Samant, M. G.; Stohr, J.; Brown, H. R.; Russell, T. P.; Sands, J. M.; Kumar, S. K. Macromolecules 1996, 29, 8334. (c) Stohr, J.; Samant, M. G.; Luning, J.; Callegari, A. C.; Chaudhari, P.; Doyle, J. A.; Lacey, J. A.; Lien, S. A.; Purushothaman, S.; Speidll, J. L. Science 2001, 292, 2299. (d) CossyFavre, A.; Diaz, J.; Liu, Y.; Brown, H. R.; Samant, M. G.; Stohr, J.; Hanna, A. J.; Anders, S.; Russell, T. P. Macromolecules 1998, 31, 4957. (e) Lee, E. S.; Vetter, P.; Miyahita, T.; Uchida, T. Jpn. J. Appl. Phys. 1993, 32, L1339. (f) Mori, N.; Morimoto, M.; Nakamura, K. Macromolecules 1999, 32, 1488. (g) Weiss, K.; Woll, C.; Bohm, E.; Fiebranz, B.; Forstmann, G.; Peng, B.; Sheumann, V.; Johannsmann, D. Macromolecules 1998, 31, 1930. (h) Oh-e, M.; Hong, S.; Shen, Y. R. J. Phys. Chem. B 2000, 104, 7455. (i) Hietpas, G. D.; Sands, J. M.; Allara, D. L. J. Phys. Chem. B 1998, 102, 10556. (j) van der Vegt, N. F. A.; MullerPalthe, F.; Gelebus, A.; Johannsman, D. J. Chem. Phys. 2001, 115, 9935. (k) Binger, D. R.; Hanna, S. Liq. Cryst. 1999, 26, 1205. (l) Ge, J. J.; Li, C. Y.; Xue, G.; Mann, I. K.; Zhang, D.; Wang, S.-Y.; Harris, F. W.; Cheng, S. Z. D.; Hong, S.-C.; Zhuang, X.; Shen, Y. R. J. Am. Chem. Soc. 2001, 123, 5768. (m) Ge, J. J.; Xue, G.; Li, F.; McCreight, K. W.; Wang, S.-Y.; Harris, F. W.; Cheng, S. Z. D.; Zhuang, X.; Hong, S.-C.; Shen, Y. R. Macromol. Rapid Commun. 1998, 19, 619. (n) Ban, B. S.; Rim, Y. N.; Kim, Y. B. Liq. Cryst. 2000, 27, 125. (o) Kim, Y. B.; Ban, B. S. Liq. Cryst. 1999, 26, 1579.

10.1021/la034883u CCC: $25.00 © 2003 American Chemical Society Published on Web 09/20/2003

8736

Langmuir, Vol. 19, No. 21, 2003

found to align LCs perpendicular to the rubbing direction, with zero pretilt.8 As expected, the rubbing of PS films generates microgrooves in the films and reorients the polymer main chains parallel to the rubbing direction, as is also always observed for PI films, while the phenyl side groups are reoriented perpendicular to the rubbing direction.3,8-10 These results for rubbed PS films have widely been cited as a clue that the microgrooves generated by the rubbing process are not necessarily involved in the alignment of LCs on surfaces. However, the above statements as to the reorientations of the polymer main chains and the phenyl side groups on rubbed PS film surfaces were derived qualitatively from limited and independent measurements, as is briefly reviewed in the following. The reorientation of PS main chains has only been estimated qualitatively from either the dichroic ratio of the methylene asymmetric stretching vibrations (2924 cm-1) obtained from spectra measured parallel and perpendicular to the rubbing direction by Fourier transform infrared (FTIR) absorption spectroscopy8 or from optical retardation measurements.3,9,10 Recently, detailed near-edge X-ray absorption fine structure (NEXAFS) studies were conducted on rubbed PS films, but only information about the reorientations of the phenyl rings was obtained.11 The zero pretilt behavior of LCs on the rubbed PS films was also discussed, but no clear picture of the zero pretilt phenomenon was obtained from these NEXAFS results.11 The rubbed PS film surface has also been examined by scanning electron microscopy (SEM)8 and atomic force microscopy (AFM),9,10 but the conclusions were only qualitative. If the exact mechanism behind the unusual LC alignment with zero pretilt is to be understood, the rubbing-induced reorientations of all PS chain segments and the rubbed film surface morphology must be quantitatively and comprehensively investigated. In the present study, we quantitatively investigated the orientational distributions of the main chains and the side groups of PS film surfaces before and after rubbing, using linearly polarized FTIR spectroscopy and optical retardation analysis. We also examined the film surface topography using high spatial resolution AFM. For rubbed PS films coated with a nematic LC [4-n-pentyl-4′-cyanobiphenyl (5CB) or a Merck nematic LC (MLK-6424)], the alignment behavior of the LC molecules on the surface was examined. Further, antiparallel and 90°-twisted cells were assembled from the rubbed films and injected with LC, and the pretilt angles and the anchoring energies of the LC molecules were measured. We discovered welldeveloped submicroscale grooves in the rubbed film surface that lie in a direction perpendicular to the rubbing direction, which we suggest contribute significantly to the observed LC alignment. The observed LC alignment is discussed, taking into account the interactions of the LC molecules with the reoriented polymer segments and with the grooves. (7) Chae, B.; Kim, S. B.; Lee, S. W.; Kim, S. I.; Choi, W.; Lee, B.; Ree, M.; Lee, K. H.; Jung, J. C. Macromolecules 2002, 35, 10119. (8) Ishihara, S.; Wakemoto, H.; Nakazima, K.; Mastuo, Y. Liq. Cryst. 1989, 4, 669. (9) (a) Seo, D.-S.; Muroi, K.-I.; Isogami, T.-R.; Matsuda, H.; Kobayashi, S. Jpn. J. Appl. Phys. 1992, 31, 2165. (b) Seo, D.-S.; Oh-Ide, T.; Matsuda, H.; Isogami, T.-R.; Muroi, K.-I.; Yabe, Y.; Kobayashi, S. Mol. Cryst. Liq. Cryst. 1993, 231, 95. (10) Seo, D.-S.; Yoshida, N.; Kobayashi, S.; Nishikawa, M.; Yabe, Y. Jpn. J. Appl. Phys. 1995, 34, 4876. (11) (a) Stohr, J.; Samant, M. G.; Cossy-Favre, A.; Diaz, J.; Momoi, Y.; Odahara, S.; Nagata, T. Macromolecules 1998, 31, 1942. (b) Sto¨hr, J.; Samant, M. G. J. Electron Spectrosc. Relat. Phenom. 1999, 98-99, 189.

Lee et al.

Experimental Section Materials and PS Film Preparation. PS [68 000 weightaverage molecular weight (M h w), 1.008 polydispersity (PDI), 104 °C glass transition temperature (Tg)] was synthesized by anionic polymerization, as described elsewhere.12 Tetrahydrofuran (THF) and the nematic LC 5CB [ne (extraordinary refractive index) ) 1.717 and no (ordinary refractive index) ) 1.530]13 were purchased from Aldrich Chemical Co., and another nematic LC (MLK-6424, ne ) 1.5697 and no ) 1.4769) was received from Merck Co.; all were used without further purification. A solution of PS in THF (2 wt % solid) was prepared. The solution was spin-coated onto calcium fluoride windows for the FTIR spectra, onto gold-coated silicon wafers for the AFM measurements, and onto indium-tin oxide glass substrates for the optical retardation measurements and LC cell assembly. The films were dried at 120 °C for 12 h. The resulting films were measured to have a thickness of around 200 nm, using a spectroscopic ellipsometer (J. A. Woollam Co., model M2000) and an alpha-stepper (Veeco Co., model Tektak3). The PS films coated onto substrates were rubbed using a laboratory rubbing machine (Wande Co.) with a roller covered by a rayon velvet fabric (Yoshikawa Co., model YA-20-R). The rubbing density (L/l) was varied by changing the cumulative rubbing time for a constant rubbing depth of 0.15 mm: L/l ) N[(2πrn/60v) - 1] where L is the total length of the rubbing cloth which contacts a certain point of the polymer film (mm), l is the contact length of the circumference of the rubbing roller (mm), N is the cumulative number of rubbings, n and r are the speed (rpm) and the radius (cm) of the rubbing roller, respectively, and v is the velocity (cm/s) of the substrate stage.5,7 LC Cells. Some of the rubbed PS films on glass substrates were cut into 2.5 cm × 2.5 cm pieces and then used for assembling two different LC cells as follows. First, paired pieces cut from each glass substrate were assembled together antiparallel with respect to the rubbing direction by using 50 µm thick spacers, injected with LC (5CB or MLK-6424), and then sealed with an epoxy glue, giving antiparallel nematic LC cells. Second, paired pieces from each glass substrate were assembled together orthogonally with respect to the rubbing direction by using silica balls of 4.0 µm diameter as spacers, injected with LC (5CB or MLK-6424), and then sealed with an epoxy glue, giving 90°twisted nematic LC cells (TN cells). In addition, a solution of the LC in ethyl ether (10 wt % LC) was directly spin-coated (at 4500 rpm for 60 s) onto some of the rubbed PS films, which were then dried at room temperature for 3 h; the typical thickness of the coated LC layers was around 400 nm, as measured using the spectroscopic ellipsometer. Measurements. Surface images were obtained using a tapping mode atomic force microscope (Digital Instruments, model Multimode AFM Nanoscope IIIa). An ultralever cantilever (with a 26 N/m spring constant and 268 kHz resonance frequency) was used for scanning. Optical phase retardations were measured using an optical setup described elsewhere;7 the laser beam was incident normal to the surface of the film, and the transmitted light intensity was monitored as a function of the angle of rotation of the film sample with respect to the surface normal. FTIR spectroscopic measurements were carried out on a Bomem DA8 FTIR spectrometer equipped with a polarizer (Single diamond polarizer, Harrick Scientific Co.) for transmission FTIR spectra and a Seagull attachment (Harrick Scientific Co.) for external reflection FTIR spectra. For all transmission FTIR spectra, the film planes of the samples were installed perpendicular to the incident beam direction. IR spectra were recorded at 4 cm-1 resolution with a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector under a vacuum as a function of the angle of rotation of the polarizer, and interferograms were accumulated 256 times. The external reflection IR spectra were obtained with p-polarized radiation at an incidence angle of 82°. The reflection IR spectra were also recorded at 4 cm-1 resolution as a function of the angle of rotation of the film sample with respect to the surface normal, and interferograms were accumulated 512 times. (12) (a) Kwon, K.; Lee, W.; Cho, D.; Chang, T. Korea Polym. J. 1999, 7, 321. (b) Lee, W.; Cho, D.; Chang, T.; Hanley, K. J.; Lodge, T. P. Macromolecules 2001, 34, 2353. (13) Nastishin, Yu. A.; Polak, R. D.; Shiyanovskii, S. V.; Lavrentovich, O. D. Appl. Phys. Lett. 1999, 75, 202.

Factors Governing LC Alignment at Rubbed PS Films For the antiparallel LC cells, the polar anchoring energies and the pretilt angles were measured using a polar anchoring energy apparatus14 and a crystal rotation apparatus,5,7 respectively. For the TN LC cells, the azimuthal anchoring energy was measured by using an ultraviolet-visible (UV-vis) spectrophotometer (model S-300, Scinco, Korea) equipped with two Glan-Laser prisms (one a polarizer and the other an analyzer, model PGL5015, Casix, China); the analyzer was mounted on a motorized goniometer (model SKIDS-PH, Sigma Koki, Tokyo, Japan). The TN cell was placed between the polarizer and the analyzer. UV-vis spectra were recorded at 0.8 cm-1 resolution as a function of the angle of rotation of the analyzer in the range 0-180° with an interval of 1.0°. The rotation angles with minimum transmittance in the UV-vis spectra were determined from these measurements. These angles were used for determining the twist angles at which the easy axes of the upper and lower substrates of the TN cell occur.15a From the twist angles, the azimuthal anchoring energies of the LCs on the rubbed PS film surfaces were estimated using the optical parameters of each LC.15b In addition, the cell gap was determined from the UV-vis spectra.

Results and Discussion Surface Morphology. Using the AFM technique, we examined the surfaces of the PS films in detail before and after they had been rubbed with a rubbing density of 50. Figure 1a shows the surface morphology of a PS film before rubbing. The unrubbed PS film surface is apparently covered with spikes; the root-mean-square (rms) surface roughness is only 0.32 nm. This surface morphology derives mainly from the characteristics of the polymer chains, which govern the aggregation that occurs after spin-casting during the drying process. Of course, the observed surface morphology might also partly reflect the surface roughness of the substrates. Overall, the film surface is very smooth. Figure 1b shows an AFM image of the rubbed surface of the film from above. As seen in this figure, typical scratch lines have developed along the rubbing direction. The distances between the scratch lines are in the range 1.82.2 µm, which corresponds to the width of the surface contacted by a fiber filament of the velvet fabric, as is always observed for polymer layers, including those of PIs.3-10 Each scratch line is created at the surface area met by adjacent fiber pairs during the rubbing process. The height difference between the valley and the hill of each scratch line is approximately 13 nm. In addition to the typical scratch lines, a new type of meandering groove structure is clearly visible in Figure 1b; these grooves lie in a direction perpendicular to the rubbing direction. Panels c and d of Figure 1 show surface profiles taken across and along the rubbing direction (along the X1-X2 and Y1-Y2 lines in Figure 1b, respectively). The rms surface roughnesses across and along the rubbing direction are 3.0 and 2.7 nm, respectively, which are much larger than that of the unrubbed film surface. The surface profile oscillates with a periodicity of approximately 700 nm across the rubbing direction and with a periodicity of approximately 250 nm along the rubbing direction; the height of these profiles is approximately 12 nm. These data indicate that each meandering groove-like structure is approximately 700 nm long, 250 nm wide, and 12 nm high. For further insight into these meandering groove-like structures, the area marked by a circle in Figure 1b was (14) (a) Yokoyama, H.; van Sprang, H. A. J. Appl. Phys. 1985, 57, 4520. (b) Nastishin, Yu. A.; Polak, R. D.; Shiyanovskii, S. V.; Bodnar, V. H.; Lavrentovich, O. D. J. Appl. Phys. 1999, 86, 4199. (15) (a) Yoon, K. H.; Ahn, S. H.; Kim, J. H.; Kim, W. Y.; Kwon, S. B. Asia Display 1998, 98, 1131. (b) Sato, Y.; Sato, K.; Uchida, T. Jpn. J. Appl. Phys. 1992, 31, L579.

Langmuir, Vol. 19, No. 21, 2003 8737

scanned with AFM at a higher magnification, and the resulting AFM image is shown in Figure 1e. This AFM image clearly shows that the whole film surface has a gravelly appearance that on a large scale is composed of the meandering groove-like structures described above. Panels f and g of Figure 1 show surface profiles taken across and along the rubbing direction (along the x1-x2 and y1-y2 lines in Figure 1e, respectively). The surface profile across the rubbing direction oscillates with a periodicity of approximately 50 nm and a height of 6 nm, indicating that the gravel-like grooves are approximately 50 nm in diameter and 6 nm in height. The gravel-like grooves are detected in the surface profile of Figure 1g as oscillations with a short period. The long-period oscillations in Figure 1g are due to the large-scale meandering groove-like structures described above. This is the first report that the rubbing of a PS film generates in a direction perpendicular to the rubbing direction meandering groove-like structures (approximately 700 nm long × 250 nm wide × 12 nm high), which are composed of gravel-like grooves in tens of nanometers (approximately 50 nm in diameter and 6 nm in height). This surface morphology is a significant departure from the previously reported AFM and SEM surface topographies of rubbed PS films8-10 and also from the surface morphologies observed for PI alignment layers.1-7 The observed surface morphology might be due to the characteristic deformation response of PS, which is a flexible polymer with a relatively low Tg (104 °C) compared to those of PIs used in the LC display industry, in this case to the shear force caused by contact of fibers with the surface during the rubbing process. Optical Retardation. To obtain information about the reorientation of polymer chains produced by the rubbing process, PS films of approximately 200 nm in thickness were rubbed and their optical phase retardations were measured. Figure 2a displays a polar diagram of the transmitted light intensity of a rubbed PS film with respect to the angle of rotation of the film, as measured in the optical retardation measurements; the film was rubbed at a rubbing density of 120, and the transmitted light intensity is equal to [(in-plane birefringence) × (phase)]. As seen in the figure, the polar diagram reveals a maximum intensity of transmitted light along the 270° T 90° direction, which lies perpendicular to the rubbing direction, but a minimum light intensity along the 180° T 0° direction, which is parallel to the rubbing direction. PS is well-known as a negatively birefringent polymer,3,9,10 as was confirmed by spectroscopic ellipsometry in the present study (data not shown). The negative birefringence of the PS polymer chain is due to the phenyl side groups, which have a larger polarizability than the vinyl chain backbone. Further, the favored conformation of the phenyl side groups is orthogonal to the vinyl backbone. Taking these facts into account, the anisotropic polar diagram in Figure 2a indicates that on rubbed PS film surfaces the phenyl rings are preferentially reoriented in a direction perpendicular to the rubbing direction but the vinyl backbones are reoriented parallel to the rubbing direction. In contrast, the polar diagrams of transmitted light intensity for unrubbed PS films are isotropic with respect to the angle of rotation of the film (data not shown), confirming that the PS polymer chains in the unrubbed films are positioned randomly in the film plane. Figure 2b shows the variation of the optical retardation [)(in-plane birefringence) × (film thickness)] with rubbing density. The retardation (i.e., degree of reorientation of PS polymer chains) of the rubbed PS films rapidly decreases with rubbing density for rubbing densities up

8738

Langmuir, Vol. 19, No. 21, 2003

Lee et al.

Figure 1. AFM images and surface profiles of a PS film without and with rubbing at a rubbing density of 50: (a) image (scan area of 1000 × 1000 nm2) of an unrubbed film; (b) image (scan area of 5 × 5 µm2) of a rubbed film; (c) surface profile along the X1-X2 line in image b (perpendicular to the rubbing direction); (d) surface profile along the Y1-Y2 line in image b (parallel to the rubbing direction); (e) higher magnification image of the elliptical area in image b; (f) surface profile along the x1-x2 line in image e (perpendicular to the rubbing direction); (g) surface profile along the y1-y2 line in image e (parallel to the rubbing direction). The arrow indicates the rubbing direction.

Factors Governing LC Alignment at Rubbed PS Films

Figure 2. (a) Polar diagram of the light transmittance [)(inplane birefringence) × (phase)] as a function of the angle of rotation of the film taken from the optical phase retardation measurements of a PS film rubbed with a rubbing density of 120. (b) Variation of the optical retardation [)(in-plane birefringence) × (film thickness)] of PS films rubbed with varying rubbing density.

Figure 3. FTIR dichroic spectra of a PS film rubbed with a rubbing density of 50. Solid and dashed lines represent the FTIR spectra with IR light polarized parallel and perpendicular to the rubbing direction, respectively.

to 80 and then more slowly decreases with further increase of the rubbing density. The measured retardation for rubbed PS films is in good agreement with those reported previously.3,9,10 Molecular Reorientation. Only modes with in-plane components could be directly observed in our application of transmission FTIR spectroscopy because only normal incidence spectra were measured. The in-plane orientations of the PS polymer chain segments were determined using polarized IR spectroscopy in transmission mode. Figure 3 presents two polarized IR spectra of a PS film rubbed at a rubbing density of 50: one (solid line) measured with the IR light polarized parallel to the rubbing direction and the other (dotted line) measured with the IR light polarized perpendicular to the rubbing direction. The bands at 1602 and 1583 cm-1 are due to the quadrant stretching of the phenyl CdC bonds, and those at 1492 and 1452 cm-1 are associated with the semicircle stretch-

Langmuir, Vol. 19, No. 21, 2003 8739

ing of the phenyl CdC bonds,16,17 while the bands at 2925 and 2852 cm-1 correspond to the asymmetric and symmetric stretching vibrational modes of CH2 in the vinyl backbone, respectively.17 As shown by the contrast between the two spectra, the rubbed film exhibits anisotropy; the IR bands at 1602, 1583, 1492, 1452, 2925, and 2852 cm-1 are more intense when the incident beam is polarized perpendicular to the rubbing direction. The intensity anisotropies of these bands in the dichroic IR spectra indicate that the PS polymer chain segments are preferentially reoriented by the rubbing of the film surface. In contrast, the unrubbed PS films exhibit isotropic spectra in dichroic IR spectral measurements (data not shown), indicating that the unrubbed films are isotropic in the film plane. In addition to the dichroic IR spectroscopic measurements described above, IR spectroscopic measurements with a linearly polarized IR light source were quantitatively conducted on the rubbed PS film as a function of the angle of rotation of the polarizer. These measurements were carried out in order to determine the extent of the reorientation of polymer chains that occurs as a result of the rubbing process. The measured peak intensities of selected IR bands are plotted in Figure 4 with respect to the angle of rotation as polar diagrams. Figure 4a,b shows that the bands at 1602 cm-1 for quadrant CdC stretching of the phenyl ring and at 1492 cm-1 for semicircle CdC stretching of the phenyl ring are more intense when the polarization of the incident beam is perpendicular to the rubbing direction. Both these aromatic CdC vibrations occur in the phenyl side groups, so the orientations of the dipole transition moments for these absorption bands coincide with the para-direction of the phenyl ring and are parallel to the C-C bond that connects the phenyl ring to the polymer backbone. As shown in Figure 4c,d, the bands at 1583 cm-1 for quadrant CdC stretching of the phenyl ring and at 1452 cm-1 for semicircle CdC stretching of the phenyl ring are also more intense when the polarization of the incident beam is perpendicular to the rubbing direction. The dipole transition moments for these absorption bands are oriented in the plane of the aromatic ring but perpendicular to the dipole transition moments for the 1602 and 1492 cm-1 bands. The directions of the dipole transition moments of the bands at (1602 and 1492) cm-1 and (1583 and 1452) cm-1 indicate that the reoriented phenyl rings of the PS polymer chains lie in planes perpendicular to the rubbing direction. Figure 4e,f additionally indicates that the bands for the asymmetric CH2 stretching vibration at 2925 cm-1 and for the symmetric CH2 stretching vibration at 2852 cm-1 are more intense when the polarization of the incident beam is perpendicular to the rubbing direction. The dipole transition moment for the asymmetric CH2 stretching vibration is orthogonal to the dipole transition moment for the symmetric CH2 stretching vibration, and both these dipole transition moments are oriented perpendicular to the polymer backbone. The orientations of the dipole transition moments of these CH2 stretching bands indicate that the PS polymer backbones are aligned parallel to the rubbing direction. These IR results lead to the conclusion that on rubbed PS film surfaces the phenyl rings are preferentially reoriented into planes perpendicular to the rubbing direction but the polymer backbones are aligned parallel (16) Colthup, N. B.; Daly, L. H.; Wiberly, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic: San Diego, 1990; Chapter 9. (17) Ren, Y.; Murakami, T.; Nishioka, T.; Nakashima, K.; Noda, I.; Ozaki, Y. Macromolecules 1999, 32, 6307.

8740

Langmuir, Vol. 19, No. 21, 2003

Lee et al.

Figure 4. Polar diagrams of some specific vibrational peaks of a PS film rubbed with a rubbing density of 50 as a function of the angle of rotation of the film, obtained using linearly polarized IR spectroscopy: (a) 1602 cm-1; (b) 1492 cm-1; (c) 1583 cm-1; (d) 1452 cm-1; (e) 2925 cm-1; (f) 2852 cm-1.

to the rubbing direction, which is consistent with the result derived from the optical retardation measurements described above. To determine the orientations of the phenyl rings in rubbed PS films in more detail, external reflection IR spectra were also obtained with p-polarized radiation at an incidence angle of 82°. Figure 5a shows an external reflection IR spectrum of the rubbed PS film used above, which was measured with the IR light polarized parallel to the rubbing direction and propagated toward the rubbing direction. This reflection IR spectrum is compared in Figure 5b with the transmission IR spectrum measured from the same PS film with the IR light polarized parallel to the rubbing direction. The phenyl CdC vibrations at

1602 and 1492 cm-1 have dipole transition moments that lie in the para-direction of the phenyl ring; the bands of these modes are more intense in the reflection IR spectrum than in the transmission spectrum. The phenyl bands at 1583 and 1452 cm-1 have dipole transition moments that lie in the phenyl ring plane but perpendicular to those of the vibrations at 1602 and 1492 cm-1; these bands are in contrast much weaker in intensity in the reflection spectrum than in the transmission spectrum. These IR spectral results indicate that the para-directions of the phenyl side groups are on average aligned nearly normal to the rubbed film surface. This is the first time IR spectroscopic techniques have been used to determine the three-dimensional reorientation geometry of phenyl rings

Factors Governing LC Alignment at Rubbed PS Films

Figure 5. FTIR spectra of a PS film rubbed with a rubbing density of 50: (a) external reflection mode and (b) transmission mode.

on the surface of rubbed PS films. This IR result is in agreement with that derived from the EXAFS spectroscopic measurements of Stohr et al.11 Combining these results for the in-plane and out-ofplane orientations of the polymer chain segments leads to the conclusion that on rubbed PS film surfaces the vinyl main chains are reoriented parallel to the rubbing direction while the planes of the phenyl side groups are reoriented perpendicular to the rubbing direction with para-directions that are positioned nearly normal to the film plane. We can thus draw a schematic configuration model of the reoriented PS chain on the rubbed film surface, as shown in Figure 6. Alignment and Anchoring Energy of LCs. Figure 7 displays a polar diagram of the transmitted light intensity [)(in-plane birefringence) × (phase)] with respect to the angle of rotation of a rubbed PS film coated with the Merck nematic LC; the film was rubbed at a rubbing density of 150. Similar polar diagrams were obtained for LC-coated PS films rubbed with rubbing densities in the range 50-150. Similar polar diagrams were also observed for rubbed PS films coated with 5CB. As shown in Figure 7, the LC-coated PS film exhibits a maximum transmitted light intensity along the direction 270° T 90°, which is perpendicular to the rubbing direction. This result indicates that the LC molecules in contact with the rubbed film surface are induced homogeneously to align perpendicular to the rubbing direction. Taking into account the surface morphology and the reorientations of the polymer chain segments described in earlier sections, these LC alignment data show that the LC molecules align parallel to the grooves and the phenyl side groups (both oriented perpendicular to the rubbing direction), indicating that the alignment of LCs is directly induced by the cooperation of the grooves and the reoriented phenyl side groups, not by the reoriented polymer main chains. TN cells of the Merck LC were prepared using PS films rubbed with rubbing densities in the range 50-150 and then used for measuring the twist angles of the LC molecules in the LC cells. Immediately after the LC cells had been prepared, their twist angles were found to be 19°, regardless of the rubbing density employed. From this twist angle, the azimuthal anchoring energy of the LC molecules on the rubbed PS surfaces is estimated to be 3 × 10-7 J/cm2. After 1 day, however, the twist angles of the TN cells were found to be significantly reduced to 0-1°, implying an azimuthal anchoring energy of nearly zero. On the other hand, the TN cells of 5CB were found to have a twist angle of 0° both immediately after preparation and after 1 day, suggesting that the 5CB molecules also have an azimuthal anchoring energy at

Langmuir, Vol. 19, No. 21, 2003 8741

the rubbed PS film surface that is nearly zero. Thus the azimuthal anchoring energies of LC molecules on rubbed PS films are approximately 0 to 3 × 10-7 J/cm2, depending on the type of LC and on the aging time of the LC cell. These anchoring energy values are much smaller than those (greater than 1 × 10-3 J/cm2) observed for the rubbed films of the PIs currently used in the LC display industry; this azimuthal anchoring energy value for LC cells using PIs was measured in the present study. We also attempted to measure the polar anchoring energies of LC molecules in antiparallel LC cells fabricated with rubbed PS films. However, the uniform, homogeneous LC alignments in the cells were easily destroyed even at the very low electric voltage applied in the polar anchoring energy measurements, regardless of the type of LC and of the rubbing conditions. Even when no electric voltage was applied, the homogeneity of the LC cells was lost after aging for 1 day at room temperature. These results suggests that on rubbed PS film surfaces both 5CB and the Merck LC have very low polar anchoring energies, which are too small to be measured. Using the same measurement technique, Seo et al.9b have previously measured a polar anchoring energy of 6 × 10-5 J/cm2 for antiparallel 5CB LC cells using PS films rubbed with nylon velvet fabric rather than rayon velvet fabric, a value that is much smaller than those (>1 × 10-3 J/cm2) observed for rubbed PI films. Taking these polar anchoring energy data into account, both types of LC molecules considered in the present study are suspected to have polar anchoring energies on rubbed PS surfaces that are much smaller than 6 × 10-5 J/cm2. In conclusion, LC molecules on rubbed PS film surfaces have a very low anchoring energy that further decreases with time. These results indicate that perpendicular LC alignments on rubbed PS films are very unstable. To understand the low anchoring energies of the observed LC alignments, we need to consider all the factors possibly involved in the interactions of the LC molecules with the rubbed PS film surfaces. First, the LC molecules are generally composed of aromatic mesogens and aliphatic tails. The aromatic mesogens might interact anisotropically with the perpendicularly reoriented phenyl rings of PS via a π-π interaction, positively contributing to the observed LC alignments, but the aliphatic tails might interact anisotropically via a van der Waals type interaction with the parallel reoriented vinyl PS backbones, negatively contributing to the observed LC alignments. Second, as described above, the rubbed film surface has well-developed meandering groove-like structures that are composed of gravel-like grooves in tens of nanometers, lying perpendicular to the rubbing direction, and seem too large to interact effectively with the LC molecules, which are approximately 2 nm in length and 0.3 nm in diameter. However, taking into account the nature of LC molecules, which exhibit high short-range as well as longrange order, the submicroscale grooves and the fine grooves (about 50 nm in size) might interact anisotropically with the LC molecules, positively contributing to the observed LC alignment. Therefore, the observed LC alignments are due to the anisotropic interactions of the LC molecules with the perpendicularly reoriented phenyl side groups and with the grooves developed perpendicular to the rubbing direction, which override the anisotropic interactions with the polymer main chains reoriented along the rubbing direction. However, the low anchoring energies and stabilities observed for this LC alignment suggest that the interactions of the LC molecules with the reoriented phenyl side groups and with the grooves are only slightly

8742

Langmuir, Vol. 19, No. 21, 2003

Lee et al.

Figure 6. A schematic configuration model of a representative PS chain on the rubbed film surface.

Conclusion

Figure 7. Polar diagram of the light transmittance [)(inplane birefringence) × (phase)] taken from the optical phase retardation measurements of a rubbed PS film coated with a Merck nematic LC (MLK-6424, ne ) 1.5697 and no ) 1.4769); the film was rubbed at a rubbing density of 150, and the thickness of the coated LC layer was 300 nm.

stronger than the interactions with the reoriented main chains. Further, the interactions of the LC molecules with the reoriented polymer segments and with the grooves (including the fine grooves) are very weak. For the antiparallel LC cells fabricated with rubbed PS films, we measured the pretilt angle of the LCs along the director of LC alignment, that is, along the direction perpendicular to the rubbing direction, using the crystalrotation technique. For both 5CB and the Merck LC, the pretilt angles were 0°. This result might be due to the directionally anisotropic interactions of the LC segments with the reoriented polymer segments, as follows. First, the aromatic mesogen unit of the LC molecule interacts anisotropically with the perpendicularly reoriented phenyl side group whose para-direction is positioned nearly normal to the film plane (Figure 6) via a π-π interaction, resulting in the anchoring of the LC molecule in parallel with the plane of the phenyl ring. For this anchoring of the LC molecule, the aliphatic tail of the LC molecule interacts with the parallel reoriented polymer main chains, causing the LC molecule to be oriented parallel to the film plane. These interactions thus cause the LC molecule to align with zero pretilt angle.

The surface morphology of and molecular orientation within PS films before and after rubbing were investigated in detail by AFM microscopy, optical retardation analysis, and linearly polarized IR spectroscopy. The alignments, pretilt angles, and anchoring energies of nematic LC molecules on the rubbed PS films were measured. This is the first time that submicroscale meandering groove-like structures composed of gravel-like grooves in tens of nanometers oriented in a direction perpendicular to the rubbing direction have been observed on rubbed PS film surfaces; this morphology is a significant departure from the surface topographies previously reported for rubbed PS and PI polymer films. In this unusual surface morphology, the phenyl side groups of PS are reoriented perpendicular to the rubbing direction with their paradirections nearly normal to the film plane, whereas the vinyl backbones of PS are reoriented along the rubbing direction. Rubbed PS films were found to align LCs perpendicular to the rubbing direction. This LC alignment is governed by the favorable anisotropic interactions of LCs with the reoriented phenyl side groups and with the submicroscale and fine grooves, which override interactions with the reoriented vinyl main chains. However, the perpendicular alignments of the LCs were found to have very low anchoring energies and limited lifetimes. These results reveal some important features of the alignment of LCs on rubbed PS film surfaces. First, the anchoring of LCs on rubbed PS films is inherently very weak. Second, the anisotropic interactions of LCs with the perpendicularly reoriented phenyl side groups seem to be comparable in strength to the interactions with the parallel reoriented vinyl main chains. Finally, the effectiveness of the submicroscale and fine grooves in aligning LCs is likely to be limited because their dimensions are much larger than those of LC molecules (a few nanometers in length and around 0.3 nm in diameter). Despite the limited effectiveness of the submicroscale and fine grooves in aligning LCs, these grooves govern the alignment of the LC molecules that weakly interact with

Factors Governing LC Alignment at Rubbed PS Films

the perpendicularly reoriented phenyl side groups, overriding the effects of the parallel reoriented vinyl main chains. The pretilting angle of LCs on rubbed PS films was measured to be 0°, as has been observed previously. This LC pretilting behavior was discussed, with consideration of the geometric structures of the reoriented PS polymer

Langmuir, Vol. 19, No. 21, 2003 8743

segments and their anisotropic interactions with the LC molecules. Acknowledgment. This study was supported by the KOSEF via the Center for Integrated Molecular Systems and by the Ministry of Education (BK21 Project). LA034883U