A Soluble Photoreactive Polyimide Bearing the Coumarin

A soluble photoreactive polyimide (PSPI) bearing the coumarin (COU) ... Films of polyimides (PIs) are widely used as liquid crystal (LC) alignment lay...
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Langmuir 2003, 19, 10381-10389

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A Soluble Photoreactive Polyimide Bearing the Coumarin Chromophore in the Side Group: Photoreaction, Photoinduced Molecular Reorientation, and Liquid-Crystal Alignability in Thin Films Seung Woo Lee, Sang Il Kim,† Byeongdu Lee, Hak Chul Kim, Taihyun Chang, and Moonhor Ree* Department of Chemistry, Center for Integrated Molecular Systems, BK21 Program, Division of Molecular and Life Sciences, and Polymer Research Institute, Pohang University of Science and Technology, San 31, Hyoja-dong, Nam-gu, Pohang 790-784, The Republic of Korea Received May 13, 2003. In Final Form: September 15, 2003 A soluble photoreactive polyimide (PSPI) bearing the coumarin (COU) chromophore in the side group, 6F-HAB-COU PSPI, was successfully synthesized with a high molecular weight. Good quality films of this PSPI were obtained using a conventional solution spin-casting and drying process. 6F-HAB-COU PSPI was determined to be positively birefringent and thermally stable up to 300 °C. The photochemical reactions and molecular reorientations of the PSPI in films were investigated in detail using ultraviolet-visible and infrared spectroscopy, dissolution testing, and optical retardation measurements. The COU chromophores of the PSPI were found to undergo only photodimerization. Irradiation of the PSPI films with linearly polarized ultraviolet light (LPUVL) induced anisotropic reorientations of the polymer main chains and anisotropic alignments of the unreacted COU chromophores. Moreover, the LPUVL-irradiated films homogeneously aligned nematic liquid crystal (LC) molecules at an angle of 100° with respect to the polarization of the LPUVL, which coincided with the reorientation direction of the PSPI polymer chains. This result indicates that the LC alignment process at the surface of irradiated PSPI films is principally governed by the reorientations of the polymer main chains and the unreacted COU side groups, whose directionally anisotropic interactions with the LC molecules contribute to the LC alignment. In particular, it was found that LPUVL exposure at g0.5 J/cm2 causes the preferential reorientation of the polymer chains at the film surface, which is enough to induce a stable alignment of LC molecules, and their interactions with the LC molecules override interactions with those created by a second LPUVL exposure with a different polarization direction. The LC pretilt angle was measured to be 0.05-0.10°. The 6F-HAB-COU PSPI films retained their LC alignment properties even after heating to 210 °C, which is 78 °C higher than the Tg of the PSPI. Overall, the LC alignment characteristics of the 6F-HAB-COU PSPI make it a promising candidate material for use as a LC alignment layer in advanced LC display devices, in particular those with an in-plane switching mode that require the lowest possible LC pretilt angle.

Introduction Films of polyimides (PIs) are widely used as liquid crystal (LC) alignment layers in LC displays because of their advantageous properties, which include excellent optical transparency, adhesion, heat resistance, dimensional stability, and insulation.1-3 The surfaces of PI films need to be treated if they are to induce a uniform alignment of LC molecules with a well-defined range of pretilt angle values.1 At present, the most widely used film treatment method in the LC display industry is rubbing the film surface with a velvet fabric; this technique has been adopted because it is simple and enables control of the LC alignment.4 However, the rubbing process has several shortcomings, including dust generation, electrostatic * To whom all correspondence should be addressed. Phone: 8254-279-2120. Fax: 82-54-279-3399. E-mail: [email protected]. † Present address: Samsung Electronics Company, LCD Division, Giheung, Gyeonggi-do, Republic of Korea. (1) Collings, P. J., Patel, J. S., Eds. Handbook of Liquid Crystal Research; Oxford University Press: Oxford, 1997. (2) (a) 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. (b) Lee, K.-W.; Paek, S.-H.; Lien, A.; During, C.; Fukuro, H. Macromolecules 1996, 29, 8894. (c) van Aerle, N. A. J.; Tol, J. W. Macromolecules 1994, 27, 6520. (d) Kim, S. I.; Ree, M.; Shin, T. J.; Jung, J. C. J. Polym. Sci.: Part A: Polym. Chem. 1999, 37, 2909. (e) Kim, S. I.; Pyo, S. M.; Ree, M.; Park, M.; Kim, Y. Mol. Cryst. Liq. Cryst. 1998, 316, 209. (f) Uchida, T.; Hirano, M.; Sakai, H. Liq. Cryst. 1989, 5, 1127.

problems, and poor control of rubbing strength and uniformity. To overcome the shortcomings of the rubbing process, several novel approaches based on photoinduced LC alignment using linearly polarized ultraviolet light (LPUVL) irradiation have recently been proposed.5 One representative class of photoalignment materials that has attracted interest comprises poly(vinyl cinnamate) and its derivatives.5 However, the practical utility of these polymers as LC-aligning materials is limited by several (3) (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) Cossy-Favre, 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. (d) Mori, N.; Morimoto, M.; Nakamura, K. Macromolecules 1999, 32, 1488. (e) Weiss, K.; Woll, C.; Bohm, E.; Fiebranz, B.; Forstmann, G.; Peng, B.; Sheumann, V.; Johannsmann, D. Macromolecules 1998, 31, 1930. (f) van der Vegt, N. F. A.; Muller-Palthe, F.; Gelebus, A.; D. Johannsman, D. J. Chem. Phys. 2001, 115, 9935. (g) Binger, D. R.; Hanna, S. Liq. Cryst. 1999, 26, 1205. (h) 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. (i) 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. (4) (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. (b) Geary, J. M.; Goodby, J. W.; Kmetz, A. R.; Patel, J. S. J. Appl. Phys. 1987, 62, 4100.

10.1021/la0348158 CCC: $25.00 © 2003 American Chemical Society Published on Web 11/04/2003

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factors. First, although the polymer chains in films of poly(vinyl cinnamate) derivatives are oriented preferentially by exposure to LPUVL, the low glass transition temperatures (Tg) of these polymers mean that the LPUVLinduced chain reorientation may not be stable under environmental influences such as temperature changes and aging. Furthermore, films of these polymers have weak anchoring energies for LCs, causing severe reliability problems for LC devices fabricated using these polymers. Moreover, the photoalignment mechanisms of these polymers are not fully understood on account of the complex nature of the photoreaction of the cinnamate chromophore. Several photoreaction pathways (i.e., transcis photoisomerization, photopolymerization, and photocyclization) are available for cinnamate chromophores, making it difficult to predict the effects of LPUVL on particular poly(vinyl cinnamate) derivatives. Thus, the development of an effective alternative for the rubbing process in the fabrication of LC displays remains a formidable challenge requiring the development of new high-performance materials. Given the complexity of the photoreaction of the cinnamate chromophore and the other limitations of poly(vinyl cinnamate) derivatives, it is worth exploring the photoinduced LC alignment properties of other polymers. One promising alternative to the cinnamate chromophore is the coumarin chromophore, which undergoes a simple [2+2] photodimerization because of its fused-ring structure.6 Taking this property of the coumarin chromophore into account, and considering the advantageous properties of PIs mentioned above, it seems possible that highperformance photoalignable materials could be developed by incorporating the coumarin chromophore into PIs. In the present study we tested this hypothesis by synthesizing a new photoreactive polyimide (PSPI) containing the coumarin (COU) chromophore in a side group. The resulting PI, referred to as 6F-HAB-COU PSPI, was stable up to 300 °C. The photoreactivity of this polymer in nanoscaled films was investigated by ultraviolet-visible (UV-vis) and Fourier infrared (FTIR) spectroscopy. The photoalignment characteristics of the PSPI film induced by photoreaction with LPUVL were determined using polarized UV-vis spectroscopy and optical retardation analysis. At the surfaces of PSPI films irradiated with LPUVL, the alignment behavior of LC molecules was investigated. In this LC alignment study, films of 6FHAB-COU PSPI prepared in various ways were investigated, including films exposed to two doses of LPUVL with different polarization directions and films that were irradiated with LPUVL and then subjected to thermal annealing. Experimental Section Materials. 2,2′-Bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6F) and 3,3′-hydroxy-4,4′-diaminobiphenyl (HAB) were obtained from Chriskev Company (Kansas, USA). All other chemicals were purchased from Aldrich Chemical Company. 6F was purified by recrystallization from acetic anhydride, and HAB (5) (a) Schadt, M.; Schmitt, K.; Kozinkov, V.; Chigrinov, V. Jpn. J. Appl. Phys. 1992, 31, 2115. (b) Schadt, M.; Seiberle, M.; Schuster, A.; Kelly, S. M. Jpn. J. Appl. Phys. 1995, 34, L764. (c) Schadt, M.; Seiberle, H.; Schuster, A. Nature 1996, 381, 212. (d) Iimura, Y.; Kobayashi, S.; Hashimoto, T.; Sugiyama, T.; Katoh, K. HEICE Trans. Electron. 1996, E39, 1040. (e) Ichimura, K.; Akita, Y.; Akiyama, H.; Kudo, K.; Hayashi, Y. Macromolecules 1997, 30, 903. (f) Minsk, L. M.; Smith, J. G.; van Deusen, W. P.; Wright, J. F. J. Appl. Polym. Sci. 1958, 2, 302. (6) (a) Ghosh, U.; Misra, T. N. J. Polym. Sci., Part A: Polym. Chem. 1987, 25, 215. (b) Ghosh, M.; Chakrabarti, S.; Misra, T. N. J. Phys. Chem. Solids 1996, 57, 1891. (c) Chen, Y.; Hong, R. T. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 2999. (d) Obi, M.; Morino, S.; Ichimura, K. Chem. Mater. 1999, 11, 656.

Lee et al. was dried at 100 °C under vacuum for 1 day. N-Methyl-2pyrrolidone (NMP) and tetrahydrofuran (THF) were distilled over calcium hydride under reduced pressure, respectively. The other chemicals were used as received. Synthesis of 7-(2-Hydroxyethoxy)-4-methylcoumarin (HEMCOU). 2-Bromoethanol (3.241 g, 25.93 mmol) and 3,4dihydro-2H-pyran (3.81 g, 43.38 mmol) in dry methylene chloride (30 mL) containing pyridinium p-toluenesulfonate (PPTS, 1.960 g, 7.779 mmol) were reacted with stirring for 8 h at room temperature, according to a method described in the literature.7 The reaction mixture was diluted with ether and washed twice with a half-saturated brine solution to remove the PPTS, followed by drying over magnesium sulfate. An oily product (3.278 g) remained after evaporation of the solvent; this product was dissolved in dry dimethyl sulfoxide (DMSO, 50 mL). Then, 7-hydroxy-4-methylcoumarin (3.654 g, 20.74 mmol) and potassium hydroxide (4.87 g, 86.9 mmol) were added into the DMSO solution containing the product. The reaction mixture was stirred at 80 °C for 6 h and then poured into a cold solution of 0.3 N HCl (300 mL), after which it was extracted twice with ethyl acetate. The combined organic layer was dried over magnesium sulfate, followed by evaporation. The resulting product was dissolved in ethanol (50 mL) without any separation or identification, and PPTS (3.27 g, 12.97 mmol) was added to the resulting solution. The reaction mixture was stirred at 80 °C for 6 h, and then the solvent was removed by evaporation. Column chromatography (SiO2, hexane/ethyl acetate (1:1 by volume)) gave a crude product of 7-(2-hydroxyethoxy)-4-methylcoumarin (HEMCOU). The crude product was recrystallized from a hexane/ethyl acetate mixture (1/1 by volume), giving HEMCOU (2.438 g, 11.07 mmol) in 42.7% yield. The HEMCOU product was dissolved in CDCl3 and characterized by 1H NMR spectroscopy (Bruker, model Aspect 300 MHz): 1H NMR (δ, CDCl3), 7.52 (d, 1H, ArH), 6.90 (dd, 1H, ArH), 6.84 (d, 1H, ArH), 6.16 (d, 1H, -C(CH3)dCH-), 4.16 (t, 2H, Ar-O-CH2-), 4.03 (t, 2H, -CH2-OH), 2.41 (s, 3H, -C(CH3)d CH-), 1.27 (s, 1H, -OH). Synthesis of 6F-HAB-COU PSPI. 6F-HAB-COU PSPI was synthesized as follows (Figure 1). To synthesize a polyimide containing hydroxyl side groups (6F-HAB PI), equivalent moles of the monomers 6F and HAB were dissolved together with two molar equivalents of isoquinoline catalyst in dry NMP. The solution was maintained at 70 °C with stirring for 2 h and then refluxed with stirring for 5 h. When refluxing was complete, the reaction solution was poured into a mixture of methanol and water (6:4 by volume) with vigorous stirring, giving 6F-HAB PI in the form of a precipitated powder. This powder was isolated by filtering and drying. The PI product was dissolved in dimethyld6 sulfoxide (DMSO-d6) and identified by 1H NMR spectroscopy: 1H NMR (δ, DMSO-d ), 10.10 (s, 1H, ArOH), 8.24 (d, 1H, ArH), 6 8.03 (d, 1H, ArH), 7.53 (d, 2H, ArH), 7.82 (d, 1H, ArH), 7.43 (d, 2H, ArH), 7.23 (d, 2H, ArH). From the 6F-HAB PI and HEMCOU, 6F-HAB-COU PSPI was synthesized according to the scheme shown in Figure 1. 6F-HAB PI, HEMCOU, and triphenyl phosphine (1:3:3 in moles) were dissolved in dry THF under a nitrogen atmosphere. Three molar equivalents of diisopropyl diazocarboxylate (DIAD) was slowly added into the solution. After the mixture was stirred for 1 h at room temperature, the reaction solution was poured into methanol with vigorous stirring. 6F-HAB-COU PSPI precipitated as a powder, which was isolated by filtering and drying. The PSPI product was dissolved in DMSO-d6 and characterized by 1H NMR spectroscopy: 1H NMR (δ, DMSO-d6), 8.06 (d, 1H, ArH), 7.80-7.51 (m, 6H, ArH), 6.81 (s, 1H, ArH), 6.72 (d, 1H, ArH), 6.13 (s, 1H, -C(CH3)dCH-), 4.53 (s, 2H, Ar-O-CH2-), 4.27 (s, 2H, -CH2-O-Ar), 2.30 (t, 3H, -C(CH3)dCH-). Film Preparation. Varying amounts of the 6F-HAB-COU PSPI were dissolved in a mixture of NMP and cyclohexanone (1:1 by volume), giving solutions with 1-5 wt % polymer. These solutions were filtered through a PTFE membrane of pore size 0.20 µm before use. The PSPI solutions were spin-cast onto glass slides for property measurements, onto quartz substrates for recording UV-vis spectra, onto calcium fluoride (CaF2) windows (25 mm (diameter) × 2 mm (thickness)) for recording FTIR (7) Miyashita, N.; Yoshikoshi, A.; Grieco, P. A. J. Org. Chem. 1977, 42, 3772.

A Soluble Photoreactive Polyimide

Figure 1. Synthetic scheme used to prepare the photoreactive polyimide 6F-HAB-COU PSPI. spectra, and onto indium tin oxide (ITO) glasses for optical retardation measurements and LC cell assembly, followed by drying on a hot plate at 80 °C for 1 h. The dried PSPI films were further dried for 2 h in a vacuum oven at 100 °C. The thicknesses of the resulting PSPI films were measured be around 200 nm using a spectroscopic ellipsometer (model M2000, J. A. Woollam, USA) and an alpha-stepper (model Tektak, Veeco, USA). Some of the PSPI films were subjected to UV light irradiation either with or without a linear dichroic polarizer (Oriel, USA) using a high-pressure Hg lamp system (1.0 kW, Altech, Korea) equipped with an optical filter (Milles Griot, USA), which transmits a band beam of wavelength 260-380 nm. In all cases, UV light irradiation was conducted under vacuum. The intensity of the unpolarized UV light was 50 mW/cm2 and the intensity of the linearly polarized UV light (LPUVL) was 10 mW/cm2. The exposure dose was measured using an International Light photometer (model IL-1350, International Light, USA) fitted with a sensor (model SED-240, International Light). Some PSPI films irradiated with LPUVL at 1.5 J/cm2 were thermally annealed in an accumulative step manner from 90 to 210 °C using the regime 90 °C/10 min, 150 °C/10 min, and 210 °C/10 min, and cooled back to room temperature. LC cell assemblies were constructed from the annealed films. For thermal analysis, dissolution testing, and refractive index measurements, 2 to 10 µm thick films of the PSPI were additionally prepared on precleaned glass slides by spin-casting of a concentrated polymer solution and subsequent drying in a vacuum oven at 100 °C for 2 days. LC Cells. Some of the LPUVL-exposed PSPI films on glass substrates were cut into 2.5 × 2.5 cm pieces. Paired pieces from the same glass substrate were assembled together using 50 µm thick spacers, aligning the direction parallel to the polarization

Langmuir, Vol. 19, No. 24, 2003 10383 of the LPUVL. A nematic LC, 4′-pentyl-4-biphenylcarbonitrile (5CB, Aldrich, USA), containing 1.0 wt % Disperse Blue 1 (Aldrich) as a dichroic dye was injected into the cell gap, followed by sealing of the injection hole with epoxy glue. To remove any flow-induced memory possibly induced by the LC injection process, the LC cells were heat treated for 5 min at 40 °C (a temperature slightly higher than the nematic-to-isotropic transition temperature of 5CB). The prepared LC cells were determined to be homogeneous throughout the cell by optical microscopy. Measurements. Molecular weights were measured using a gel permeation chromatography (GPC) system calibrated with polystyrene standards. In the GPC measurements, a flow rate of 1.0 mL/min was employed and THF was used as the eluent. The glass transition temperature (Tg) and degradation temperature (Td) of each film were measured over 25-400 °C using a Seiko differential scanning calorimeter. In these measurements, a ramping rate of 10.0 °C/min was employed and the system was purged with dry nitrogen gas at a flow rate of 80 cm3/min. In each run, a sample of about 5 mg was used. Tg was taken as the onset temperature of the glass transition in the thermogram. Thermal stability was measured over 50-800 °C using a thermogravimeter (model TGA7, Perkin-Elmer, USA). In these measurements, a ramping rate of 5.0 °C/min was employed and the system was purged with dry nitrogen gas at a flow rate of 100 cm3/min. Refractive index measurements were performed on PI films of thickness approximately 5.0 µm using a prism coupler8 equipped with a He-Ne laser source (wavelength 632.8 nm). The refractive index in the film plane (nxy) was measured in the transverse electric mode, and the out-of-plane refractive index (nz) was obtained in the transverse magnetic mode. All these measurements were performed using a cubic zirconia prism of refractive index n ) 2.1677 at a wavelength of 632.8 nm. UVvis spectra were recorded using an HP 8452 Hewlett-Packard spectrometer with and without an Oriel linear dichroic polarizer. FTIR spectroscopic measurements were carried out on a Bomem DA8 FTIR spectrometer equipped either with or without a polarizer (single diamond polarizer, Harrick Scientific, USA). Samples were installed perpendicular to the incident beam direction. While the polarizer was rotated, IR spectra were recorded at 4 cm-1 resolution using a liquid nitrogen cooled mercury cadmium telluride (MCT) detector under vacuum, as a function of the angle of rotation; 256 interferograms were accumulated. Optical phase retardation was measured using a phase retardation analyzer built in our laboratory.2a In these measurements, the laser beam was incident normal to the film surface and the transmitted light intensity ()(in-plane birefringence) × (phase)) was monitored as a function of the angle of rotation of the film with respect to the surface normal. The LC alignment in LC cells containing Disperse Blue 1 dichroic dye was examined using an optical setup,2 which was equipped with a He-Ne laser (wavelength 632.8 nm), a polarizer, a photodiode detector, and a goniometer. In the measurements, the laser beam was incident normal to the surface of the LC cell mounted on the goniometer and these components were placed between the polarizer and the detector. Light absorption of the dichroic dye molecules (which is aligned parallel to the LCs in the cell) was then monitored as a function of the angle of rotation of the cell. The pretilt angle R of the LC molecules was measured using a crystal rotation apparatus.2

Results and Discussion Synthesis and Properties. As shown in Figure 1, 6FHAB-COU PSPI was synthesized in three major steps: functionalization of coumarin (HEMCOU), synthesis of soluble 6F-HAB PI, and incorporation of HEMCOU into 6F-HAB PI as side groups. (8) (a) Kim, S. I.; Shin, T. J.; Pyo, S. M.; Moon, J. M.; Ree, M. Polymer 1999, 40, 1603. (b) Pyo, S. M.; Kim, S. I.; Shin, T. J.; Park, Y. H.; Ree, M. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 937. (c) Ree, M.; Shin, T. J.; Park, Y.-H.; Kim, S. I.; Woo, S. H.; Cho, C. K.; Park, C. E. J. Polym. Sci, Part B: Polym. Phys. 1998, 36, 1261. (d) Ree, M.; Kim, K.; Woo, S. H.; Chang, H. J. Appl. Phys. 1997, 81, 698. (e) Ree, M.; Chu, C. W.; Goldberg, M. J. J. Appl. Phys. 1994, 75, 1410. (f) Ree, M.; Shin, T. J.; Lee, S. W. Korea Polym. J. 2001, 9, 1. (g) Goh, W. H.; Kim, K.; Ree, M. Korea Polym. J. 1998, 6, 241.

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6F-HAB PI was synthesized directly by polycondensation of the respective monomers using isoquinoline as a catalyst. The obtained polymer was characterized by 1H NMR spectroscopy. The 1H NMR spectrum of 6F-HAB PI contains a proton peak due to the hydroxyl side groups at 10.0 ppm and features in the range of 6.9-8.3 ppm due to the protons of the aromatic rings on the polymer backbone (see NMR data in Experimental Section). No spectral features characteristic of amino protons are observed in the spectrum, suggesting that the product contained negligible amounts of partially imidized 6FHAB poly(amic acid) and therefore that the 6F-HAB PI had a high molecular weight. By GPC analysis calibrated with polystyrene standards, the PI polymer was found to have a weight-averaged molecular weight (Mw) of 53 400 and a polydispersity of 1.87. In conclusion, a soluble 6FHAB PI was synthesized with a reasonably high molecular weight. To the soluble 6F-HAB PI, the functionalized coumarin (HEMCOU) was incorporated as side groups, giving 6FHAB-COU PSPI. The 1H NMR spectrum of 6F-HAB-COU PSPI does not show the proton peak originating from the hydroxyl groups of 6F-HAB PI (see NMR data in Experimental Section), indicating that those groups reacted completely with HEMCOU to give 6F-HAB-COU PSPI bearing two COU side groups per repeat unit of the polymer backbone. Good quality thin films of the obtained PSPI were prepared by means of a conventional solution spin-casting and subsequent drying process. The 6F-HAB-COU PSPI film exhibited a Tg of 132 °C and a Td of 300 °C, whereas the 6F-HAB PI showed a Td of 440 °C and no glass transition in the range 50-500 °C. Thus, incorporation of the COU side groups into the 6FHAB PI lowered the Tg and Td values of the polymer. The average refractive index (nav ) (2nxy + nz)/3) and out-ofplane birefringence (∆xy-z ) nxy - nz) were 1.599 and 0.038 for 6F-HAB PI and 1.632 and 0.011 for 6F-HAB-COU PSPI, respectively. The higher refractive index of 6FHAB-COU PSPI compared to 6F-HAB PI may be due to a decrease in the fluorine fraction of the polymer. Both 6F-HAB-COU PSPI and 6F-HAB PI in thin films showed positive out-of-plane birefringence. Given that long-chain polymer molecules in thin films tend to lie in the film plane, resulting in an out-of-plane birefringence,8 we interpret the measured ∆xy-z values to be evidence that the polymer chains in both the PI and the PSPI films are preferentially oriented in the plane of the film. Furthermore, the fact that the ∆xy-z values are positive indicates that both 6F-HAB PI and 6F-HAB-COU PSPI are positive birefringent polymers whose polarizations are larger along the polymer chain axis than along the direction normal to the polymer chain axis. Photoreactivity. Figure 2a shows UV absorption spectra of a film of 6F-HAB-COU PSPI irradiated with UV light at various exposure energies. Before UV light irradiation, the PSPI film exhibits an absorption maximum at 306 nm ()λmax), which is due to the COU chromophores in the side groups. As the energy exposure is increased, the absorption peak at 306 nm decreases in intensity due to the photoreaction of the COU chromophores. Furthermore, irradiation of the PSPI films with UV light at 1.01.5 J/cm2 caused the polymer film to become insoluble in a mixture of NMP and cyclohexanone (1:1 by volume), suggesting that UV light irradiation induces cross-linking between the PSPI molecules. Figure 2b shows FTIR absorption spectra of a film of 6F-HAB-COU PSPI irradiated with UV light at various exposure energies. As the UV dose increases, both the

Lee et al.

Figure 2. (a) UV and (b) FTIR spectra measured from 6FHAB-COU PSPI films exposed to UV light (260-380 nm) with varying exposure energy.

stretching vibration of CdC (1613 cm-1) and out-of-plane bending vibration of C-H (984 cm-1) in the vinylene linkage of the coumarin group decrease in intensity. In addition, the stretching vibration of CdO in the pyrone moiety, which appears at 1735 cm-1 as a shoulder, also decreases in intensity and shifts to high frequency with increasing UV dose. The coumarin chromophore is known to undergo only [2+2] photodimerization because of its fused-ring structure.6 Taking this into account, the intensity drops of the conjugated CdC and CdO stretching bands can be primarily attributed to the loss of COU chromophores as a result of UV-light-induced photodimerization. Therefore, the effects of increasing UV exposure energys diminishing intensity in the UV-vis absorption spectrum and transformation of the polymer film from soluble to insolublesare attributed to the formation of cross-links of the COU side groups via [2+2] photodimerization. UV Dichroic Ratio. For the PSPI films irradiated with LPUVL, the dichroic ratio ()(A⊥ - A|)/(A⊥ + A|)) was determined from the absorbance at λmax (maximum absorption wavelength) measured with a UV-visible light probe that was linearly polarized perpendicular to the polarization direction of the LPUVL (A⊥) and the absorbance at λmax measured with a UV-visible light probe that was linearly polarized parallel to the polarization direction of the LPUVL (A|). The dichroic ratios determined using this approach are displayed in Figure 3. All of the measured dichroic ratios are positive for the exposure energy range considered (0.25 J/cm2), however, the COU chromophores in the PSPI films undergo photoreaction with increasingly less directional selectivity. This reduction in directional selectivity may be due to a disturbance in the distribution of the remained COU chromophores, which is caused by the LPUVLinduced photoreactions of COU chromophores in the early stages, and also to a decrease in the mobility of the polymer chains resulting from the cross-linking of COU chromophores during the early stages of photoreaction. Optical Retardation. Figure 4a shows a polar diagram of the transmitted light intensity ()(in-plane birefringence) × (phase)) constructed from optical phase retardation measurements of a film of 6F-HAB-COU PSPI irradiated with LPUVL at 1.5 J/cm2 as a function of the angle of rotation of the film. The irradiated film shows maximum light intensity along the direction 280° T 100° (i.e., at an angle of 100° with respect to the polarization direction (0° T 180°) of the LPUVL), and minimum light intensity along the direction 190° T 10° (i.e., an angle of 10° with respect to the polarization direction of the LPUVL). In contrast to the anisotropic polar diagrams of the LPUVL-irradiated films, the PSPI films that were not subjected to LPUVL irradiation showed isotropic transmitted light intensity. Given that the PSPI chain is positively birefringent, as established above in the section Synthesis and Properties, the anisotropic form of the polar diagram indicates that the PSPI polymers are preferentially reoriented such that their chains lie at an angle of 100° with respect to the polarization direction of the LPUVL to which they were exposed.

Figure 4. (a) Polar diagram of transmitted light intensity ()(in-plane birefringence) × (phase)) taken from the optical phase retardation measurement of a 6F-HAB-COU PSPI film irradiated with linearly polarized UV light (LPUVL) (260-380 nm) at 1.5 J/cm2 as a function of the angle of rotation of the film. (b) Variation of the optical retardation ()(in-plane birefringence) × (film thickness)) of a 6F-HAB-COU PSPI film irradiated to LPUVL (260-380 nm) with varying exposure energy.

The preferred orientation of the polymer main chains after irradiation with LPUVL does not exactly coincide with the alignment director of the unreacted COU chromophores remaining in the PSPI film, which was established above in the section Dichroic Ratio. This difference in the directions can be reconciled by considering the geometrical structure of the PSPI polymer. As seen in Figure 1, the polymer chain contains a kink in each repeat unit due to the 6F moiety. As a result, the long chain axis of the polymer chain is not parallel to the long axis of the HAB unit linked to two imide N-C bonds. Furthermore, the COU side groups, which include an oxyethylenyloxy spacer, are linked at an angle of approximately 60° or 120° to the polymer main chain, and therefore their long axes are not parallel to that of the polymer chain. Thus, due to the geometry of the COU side groups and the polymer main chain, the directionally selective photoreactions of the COU side groups could lead to the reorientation of the polymer chains to the direction shown in Figure 4a. The optical retardation values of the PSPI films irradiated with LPUVL at various exposure energies were determined from their maximum light intensity values; for example, the optical retardation for the film irradiated with LPUVL at 1.5 J/cm2 (Figure 4a) was determined from the light intensity along the direction 280° T 100°. The resulting retardation values, plotted in Figure 4b, rapidly increase with exposure energy up to around 0.25 J/cm2 and then slowly decrease with further increase of the exposure energy. This result suggests that an LPUVL exposure energy of only 0.25 J/cm2 is sufficient to induce preferential reorientation of the PSPI polymer chains,

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Figure 5. (a) Polar diagram of transmitted light intensity taken from the optical phase retardation measurement of a 6F-HABCOU PSPI film irradiated with linearly polarized UV light (LPUVL) (260-380 nm) at 1.5 J/cm2 as a function of the angle of rotation of the film. (b) Polar diagram of absorbances measured from a parallel LC cell fabricated from the film in (a) as a function of the angle of rotation of the LC cell. (c) Polar diagram of transmitted light intensity taken from the optical phase retardation measurement of a 6F-HAB-COU PSPI film irradiated with LPUVL (260-380 nm) at 1.5 J/cm2 by a double exposure process (i.e., 0°(θ) and 90°(θ) exposure in a sequential manner) as a function of the angle of rotation of the film. (d) Polar diagram of absorbances measured from a parallel LC cell fabricated from the film in (c) as a function of the angle of rotation of the LC cell.

which is consistent with the conclusion drawn from the dichroic UV-vis spectroscopy measurements presented above. LC Alignment Properties of Films Irradiated by a Single Exposure of LPUVL. The LC cells fabricated from parallel PSPI films prepared by a single exposure of LPUVL at various exposure energies were all found to be uniform, homogeneous throughout the cell by optical microscopy. Figure 5b shows a representative polar diagram of the absorption of a linearly polarized light probe (wavelength 632.8 nm), which was obtained from measurements on a LC cell fabricated from a PSPI film irradiated with LPUVL at 1.5 J/cm2. In this polar diagram, the main director of the LC molecules lies along the direction 280° T 100°, which is at an angle of 100° with respect to the polarization direction (0° T 180°) of the LPUVL used in the UV exposure. This result indicates that the LC molecules in contact with the film surface are homogeneously aligned at an angle of 100° with respect to the polarization of the LPUVL. The director of the LC alignment was the same for other PSPI films irradiated at exposure energies in the range 0.1-3.5 J/cm2. We now consider the LC (5CB) molecules and how their interactions with the PSPI polymer chains might affect their alignment when in contact with a PSPI film. First, the 5CB molecule has dimensions of approximately 1.8 nm (length) by 0.25 nm (diameter), which are comparable with those of the polymer main chain segments and of the COU side group. Therefore, both the polymer

main chains and the COU side groups in the PSPI film may involve in the alignment of LC molecules in contact with the film surface. Second, it has been previously suggested for conventional, rubbed PI alignment layer materials that the major intermolecular interactions between the PI and the LC molecule are π-π interactions between the phenyl rings of the polymer and those of the LC molecule.9 Taking this into account, the main chain backbone of the 6F-HABCOU PSPI, which bears four phenyl rings per chemical repeat unit, is likely to have stronger intermolecular interactions between the phenyl rings of the polymer and the biphenyl ring of the LC molecule, compared to the two side groups per chemical repeat unit, which have only two phenyl rings (see Figure 1). Moreover, the main chain backbone has a biphenyl unit, which is the same as the mesogen unit of the LC molecule. Thus, the biphenyl units in the polymer main chain may play a significant role in the interactions between the polymer surface and the LC molecules. Third, we consider the unreacted COU side groups left from the directionally selective photoreaction induced by LPUVL-induced photoreaction and their contribution to (9) (a) Stohr, J.; Samant, M. G. J. Electron Spectrosc. Relat. Phenom. 1999, 98, 189. (b) Stohr, J.; Samant, M. G.; Cossy-Favre, A.; Diaz, J. Macromoleules 1998, 31, 1942. (c) Sakamoto, K.; Arafune, R.; Ito, N.; Ushioda, S.; Suzuki, Y.; Morokawa, S. J. Appl. Phys. 1996, 80, 431. (d) Wei, X.; Hong, S.; Zhuang, X.; Goto, T.; Shen, Y. R. Phys. Rev. E 2000, 62, 5160.

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Figure 6. Polar diagrams of absorbances measured from parallel LC cells fabricated from PSPI films irradiated with LPUVL at 1.5 J/cm2 and subsequently annealed at various temperatures, as a function of the angle of rotation of the LC cell: (a), unannealed; (b), 90 °C/10 min; (c), 90 °C/10 min + 150 °C/10 min; (d) 90 °C/10 min + 150 °C/10 min + 210 °C/10 min.

the intermolecular interactions between the polymer chains and the LC molecules. As mentioned above, the unreacted COU chromophores are present in a direction perpendicular to the polarization direction of the LPUVL. Thus, this anisotropic alignment of COU chromophores might positively contribute to the observed LC alignment via π-π interactions between the coumarin rings and the biphenyl mesogen of the LC molecule. Finally, we consider the LPUVL-induced preferential reorientation of the polymer main chains and its contribution to the interactions between the polymer chains and the LC molecules. As seen in parts a and b of Figure 5, the LC alignment director is parallel to that of the polar diagram of the transmitted light intensity taken from the optical phase retardation measurements of the PSPI films. This indicates that the main director of the LC alignment exactly coincides with the main director of the reoriented polymer chains. This further suggests that the LPUVLinduced anisotropy in the polymer chain orientation, which gives rise to anisotropic interactions with the LC molecules, leads to LC alignment along the polymer reorientation direction. These findings represent strong evidence that the anisotropic PSPI polymer chain orientation resulting from LPUVL exposure plays a major role in the alignment of LC molecules in contact with the irradiated film. Taking into consideration the above-mentioned molecular reorientations and their implications for polymerLC molecule interactions, we conclude that the anisotropically reoriented polymer main chains and COU side groups cooperatively induce a homogeneous uniaxial alignment of LC molecules. In these interactions, the

reoriented polymer main chains are expected to make the dominant contribution to the LC alignment. The pretilt angle of the LCs in each LC cell was determined along the director of the LC alignment in the cell by using the crystal-rotation technique. The pretilt angle R was measured to be 0.05-0.10°, which was insensitive to the variation of exposure energy in the LPUVL irradiation of the polymer films. LC Alignment Properties of Films Irradiated with a Double Exposure of LPUVL. Films of 6F-HAB-COU PSPI were sequentially irradiated with two doses of LPUVL, one at θ ) 0° and the other at θ ) 90°, where θ is defined as the angle between a reference axis of the film sample and the polarization direction of the LPUVL. After this double exposure to LPUVL, optical retardation measurements of the films were carried out. Then, LC cells were constructed from the films and the LC alignment behavior was characterized. Figure 5c shows the polar diagram of the transmitted light intensity as a function of the angle of rotation of the film taken from the optical phase retardation measurements of a PSPI film that had been irradiated by the double exposure process with LPUVL at 1.5 J/cm2. This film shows maximum light intensity along the direction 200° T 20°, that is, at an angle of 110° with respect to the polarization direction (270° T 90°) of the LPUVL used in the second exposure (i.e., 90°(θ) exposure). As described above, a single exposure of LPUVL (i.e., 0°(θ) exposure) caused the preferential reorientation of the polymer chains such that they were oriented at an angle of 100° with respect to the polarization direction of the LPUVL (see Figure 5a). Taking this result into account, the polar diagram of Figure

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5c indicates that the reorientation direction of the polymer chains induced by the first 0°(θ) exposure is rotated clockwise by 110° by the second 90°(θ) exposure. Similar results were obtained for PSPI films irradiated with double exposures of LPUVL at 0.25-1.0 J/cm2 (data not shown). Collectively these results suggest that the ultimate orientation of the polymer chains in irradiated PSPI films is governed by the final LPUVL exposure. Figure 5d displays a polar diagram of the absorption of a linearly polarized light probe measured using a parallel LC cell fabricated from the double exposed film in Figure 5c as a function of the angle of rotation of the LC cell. The LC cell was found to be homogeneous throughout the cell by optical microscopy. This polar diagram shows a very slight disturbance along the direction 190° T 10° that was not observed in the polar diagram of the LC cell fabricated from a film irradiated with a single exposure of LPUVL (Figure 5b). However, the principal director of the polar diagram lies along the direction 280° T 100°, which coincides with that of the LC cell fabricated from a film irradiated with a single exposure of LPUVL (Figure 5b). Overall, the polar diagram of Figure 5d very much resembles that of the LC cell fabricated from a film irradiated with a single exposure of LPUVL (Figure 5b). Similar LC alignment behavior was observed for the films irradiated with double exposures of LPUVL at 0.5 and 1.0 J/cm2. These LC alignment results lead to the conclusion that at the surfaces of PSPI films irradiated with a double exposure of LPUVL at 0.5-1.5 J/cm2, LC alignment is primarily governed by the first LPUVL exposure (0°(θ) exposure) and is only slightly disturbed by the second LPUVL exposure (90°(θ) exposure). The observed LC alignments are a significant departure from that which one can simply expect from the overall reorientation direction of the polymer chains varied very sensitively with changing the polarization direction of LPUVL at each exposure step of the double exposure process as described above. Collectively, the LC alignment results and optical retardation data suggest that a single LPUVL exposure at g0.5 J/cm2 causes a preferential reorientation of the polymer chains at the film surface that is sufficient to induce a stable alignment of LC molecules and that the interactions of these reoriented polymer chains with the LC molecules override the interactions between the LC molecules and polymer chains reoriented by the second LPUVL exposure with a different polarization direction. However, LC cells constructed from a film irradiated with a double exposure of LPUVL at 0.25 J/cm2 induced LC alignment with a director along the direction 310° T 130° (data not shown), which is different from the LC alignment behavior observed at the surfaces of films irradiated with a double exposure of LPUVL at 0.5-1.5 J/cm2. The most likely explanation for this discrepancy is that an energy dose of 0.25 J/cm2 per exposure is insufficient to induce the preferential reorientation and cross-linking of the polymer chains that are required for stable alignment of LC molecules at the film surface. If that were the case, the polymer chains reoriented by the first exposure would compete with those reoriented by the second exposure to align the LC molecules in contact with the film surface, causing a departure from the LC alignment established at the film surface by the first exposure. Conclusively, for PSPI films irradiated with LPUVL at