Photoreaction and Molecular Reorientation in Films of Novel

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Photoreaction and Molecular Reorientation in Films of Novel Photosensitive Polyesters Containing n-Alkyl Side Groups and 1,4-Phenylenediacryloyl Units in the Backbone Boknam Chae, Seung Woo Lee, Seung Bin Kim,* Byeongdu Lee, and Moonhor Ree* Department of Chemistry, Polymer Research Institute, BK21 Program, Division of Molecular and Life Sciences, and Center for Integrated Molecular Systems, Pohang University of Science & Technology, San 31, Hyoja-dong, Nam-gu, Pohang 790-784, Republic of Korea Received January 13, 2003. In Final Form: May 12, 2003 Photoreaction and photoinduced molecular reorientation in nanoscaled films of poly[oxy(4-n-alkyl-3,5benzoate)oxy-1,4-phenylenediacryloyl] (PPDA-CnBZ), which contains n-alkyl side groups and photosensitive 1,4-phenylenediacryloyl (PDA) units in the main chain, were in detail investigated by linearly polarized infrared (IR) and ultraviolet (UV) absorption spectroscopy and optical retardation analysis. The PPDACnBZ polymer molecules were found to undergo mainly photodimerization upon UV light irradiation. However, the occurrence of trans-cis photoisomerization in the PPDA-CnBZ film could not be detected. In addition, irradiation with linearly polarized UV light (LPUVL) induced selective photoreaction of the PDA chromophores positioned parallel to the polarization direction of the LPUVL, which caused the molecular reorientation of the polymer chains. In the irradiated films the unreacted PDA chromophores were found to anisotropically distribute in a direction perpendicular to the polarization direction of the LPUVL, whereas the photodimerized PDA units displayed nearly isotropic distribution in the film plane. Thus, the LC alignment on irradiated PPDA-CnBZ films can be deduced from the relationship between the distributions of polymer chains and of liquid crystal (LC) molecules. In particular, the unreacted PDA chromophores remained at the film surface irradiated with LPUVL favorably align the LC molecules along their reorientation direction, which is perpendicular to the polarization direction of the LPUVL via their anisotropic interactions with the LC molecules.

Introduction Organic polymers, in particular polyimides, are widely used as alignment layers of liquid crystal (LC) for LC flat-panel display devices because of their advantageous properties, such as excellent optical transparency, adhesion, heat resistance, dimensional stability, and insulation.1 Such polymer film surfaces need to be treated for producing a uniform alignment of LC molecules.1 At present, a rubbing process using a rayon velvet fabric is the only technique adopted in the LC display industry to treat polymer film surfaces for the mass production of flat-panel LC display devices because its simplicity and the controllability with this method of both the LC anchoring energy and the LC pretilt angle.1,2 The rubbing process is known to induce the reorientation of polymer chains at the film surface along the rubbing direction, which are necessary to align LC molecules unidirectionally.1,2 However, this process has some shortcomings, such * To whom all correspondence should be addressed. Tel: +8254-279-2106 (S.B.K.), 279-2120 (M.R.). Fax: +82-54-279-399. E-mail: [email protected]; [email protected]. (1) (a) Collings, P. J., Ed. Liquid Crystals; IOP Publishing, Ltd.: Bristol, 1990. (b) O’Mara, W. C. Liquid Crystal Flat Panel Displays; van Nostrand Reinhold: New York, 1993. (c) Tannas, E., Jr., Glenn, W. E., Doane, J. W., Eds. Flat-Panel Display Technologies; Noyes: Park Ridge, NJ, 1995. (d) Collings, P. J.; Patel, J. S., Eds. Handbook of Liquid Crystal Research; Oxford University Press: Oxford, 1997. (e) Lee, K.-W.; Paek, S.-H.; Lien, A.; During, C.; Fukuro, H. Macromolecules 1996, 29, 8894. (f) van Aerle, N. A. J.; Tol, J. W. Macromolecules 1994, 27, 6520. (g) Kim, S. I.; Ree, M.; Shin, T. J.; Jung, J. C. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 2909. (h) Kim, S. I.; Pyo, S. M.; Ree, M.; Park, M.; Kim, Y. Mol. Cryst. Liq. Cryst. 1998, 316, 209. (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. (d) Geary, J. M.; Goodby, J. W.; Kmetz, A. R.; Patel, J. S. J. Appl. Phys. 1987, 62, 4100.

as dust generation, electrostatic problems, and poor control of rubbing strength and uniformity.1,2 Thus, alternatives to replace the rubbing process are highly desired in the LC display industry. An alternative approach is the photoinduced molecular reorientation of alignment layers using linearly polarized ultraviolet light (LPUVL) irradiation.3-7 This approach has a capability of the rubbing-free production of LC aligning films so that this method has gained great attention from both academic and industrial fields.3-7 This method however requires photoreactive polymers, which can be reoriented in a proper direction by LPUVL exposure. A representative photoalignment polymer is poly(vinyl cinnamate) (PVCi).4 In fact, this polymer was introduced in 1959 as a negative photoresist.8 Schadt et al.4 first demonstrated that a PVCi film aligns LC molecules perpendicular to the electric vector of the LPUVL when the film is irradiated with LPUVL. Since this demonstration was reported in 1992,4 PVCi and its derivatives have (3) (a) Ichimura, K. Chem. Rev. 2000, 100, 1847. (b) O’Neill, M.; Kelly, S. M. J. Phys. D: Appl. Phys. 2000, 33, R67. (4) (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. (5) (a) Han, K. Y.; Chae, B. H.; Yu, S. H.; Song, J. K.; Park, J. G.; Kim, D. Y. SID 97 DIGEST 1997, 28, 707. (b) Kim, S. I.; Ree, M. Proc. 1st Korea Liquid Crystal Conf. 1998, 1, 55. (c) Iimura, Y.; Kobayashi, S.; Hashimoto, T.; Sugiyama, T.; Katoh, K. HEICE Trans. Electron. 1996, E39 (8), 1040. (d) Lee, S. W.; Ree, M. Mol. Cryst. Liquid Cryst. 2001, 368, 4277. (e) Ree, M.; Lee, S. W.; Kim, J.-H. Mol. Cryst. Liquid Cryst. 2001, 368, 4271. (6) Kim, H.; Lee, J.; Sung, S.; Park, J. Polym. J. 2001, 33, 9. (7) Ichimura, K.; Akita, Y.; Akiyama, H.; Kudo, K.; Hayahsi, Y. Macromolecules 1997, 30, 903. (8) Minsk, L. M.; Smith, J. G.; van Deusen, W. P.; Wright, J. F. J. Appl. Polym. Sci. 1958, 2, 302.

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Figure 1. Molecular structure of photosensitive PPDA-CnBZ polymers.

been extensively investigated due to their potential applications in the mass production of LC display devices.3,6,7,9 However, controversial debates have been made for the LC alignment phenomenon at the film surface of PVCi irradiated with LPUVL.4,6,7 According to the report of Schadt et al.,4 three molecular entities of PVCi, namely, the unreacted side chains, the cyclobutane photoderivatives, and the hydrocarbon main chains, contribute synergistically to the linearly polarized photopolymerization induced unidirectional polymer order. The photodimerization of the cinnamoyl chromophores was identified as the dominant photoprocess. Further, the anisotropic dispersive interaction forces of these three entities were shown to exhibit the same uniaxiality, and it was concluded that these entities are responsible for the alignment of the LC molecules. Kim et al.6 recently proposed that the dichroism of the photoaligned LCs does not follow the dichroism of the photoisomer but instead follows that of the photodimer, which suggests that the anisotropic distribution of the photodimer is mainly responsible for the uniform LC alignment. In contrast, Ichimura et al.7 suggested that the homogeneous photoinduced LC alignment at the PVCi film is produced by the polarization photochroism at the uppermost surface of the film that results from repeated photoisomerization of the cinnamoyl unit. As reviewed above, further detailed investigation of the photoproducts and their distribution in films of PVCi and its derivatives is still needed in order to quantitatively determine the contributions of photoisomerization and photodimerization to the LC alignment. Moreover, alignment layer polymers containing photoreactive moieties in the polymer backbone rather than in the side group have been rarely investigated. Recently we synthesized a series of new polyesters, poly[oxy(4-n-alkyl-3,5-benzoate)oxy-1,4-phenylenediacryloyl] (PPDA-CnBZ) (Cn: n-alkyl).10 These polyesters are photoreactive model polymers containing two cinnamoyl moieties per chemical repeat unit of polymer backbone and n-alkyl side groups (see Figure 1). The polymers reveal high dimensional and thermal stability (9) (a) Allen, S. D. M.; Almond, M. J.; Bruneel, J.; Gilbert, A.; Hollins, P.; Mascetti, J. Spectrochim. Acta, Part A 2000, 56, 2423. (b) Creed, D.; Griffin, A. C.; Hoyle, C. E.; Venkataram, K. J. Am. Chem. Soc. 1990, 112, 4049. (c) Ghosh, M.; Chakrabarti, S.; Misra, T. N. J. Raman Spectrosc. 1998, 29, 263. (d) Murase, S.; Kinosita, K.; Horie, K.; Morino, S. Macromolecules 1997, 30, 8088. (10) Lee, S. W.; Choi, W.; Chae, B.; Kim, S. B.; Ree, M. Macromolecules Submitted for publication.

Chae et al.

(>121 °C glass transition temperature (Tg) and 357 °C degradation temperature (Td)),10 which are comparable to those of conventional polyimides used as LC alignment layers in the LC display industry. Further, the polyesters are soluble in common solvents including chloroform, showing excellent processibility for conventional spincoating and drying process. Because of these excellent properties and processibility, we chose some of the novel photoreactive polyesters in this study and investigated their photoreaction mechanisms using ultraviolet (UV) absorption and Fourier transform infrared (FTIR) spectroscopy. To better understand the mechanism of LC alignment by PPDA-CnBZ films irradiated with LPUVL, we measured the orientation distributions of the photoproducts and of both the main chains and of the n-alkyl side groups in the irradiated films, using linearly polarized UV and FTIR spectroscopy and optical phase retardation analysis. In addition, antiparallel LC cells were assembled with the LPUVLirradiated films and injected with a nematic LC, 4-npentyl-4′-cyanobiphenyl (5CB), and the alignment behavior of the LC molecules was characterized. We discuss the observed LC alignment taking into account the interactions between the reoriented polymer chains and the LC molecules. Experimental Section Materials and Film Preparation. All chemicals were purchased from Aldrich Chemical Co. and used without further purification. n-Alkyl 3,5-dihydroxybenzoate (CnBZ) was synthesized as follows. 1-Bromobutane (5.0 g, 36.4 mmol) and 3,5dihydroxybenzoic acid (5.62 g, 36.4 mmol) were added to 50 mL of dimethyl sufoxide containing potassium hydroxide (2.05 g, 36.4 mmol), and the reaction mixture was stirred for 24 h at room temperature. The resulting solution was poured into 300 mL of aqueous hydrochloric acid solution (0.3 N), and the mixture was extracted several times with ethyl acetate. The combined organic layer was dried over magnesium sulfate and then concentrated using a rotary evaporator. The resulting oily product was purified by column chromatography (SiO2, hexane/ethyl acetate (2/3 by volume)) and dried, giving the target compound C4BZ in 92% yield. By the same synthetic procedure, n-octyl 3,5-dihydroxybenzoate (C8BZ) and n-dodecyl 3,5-dihydroxybenzoate (C12BZ) were synthesized in 78-85% yield by reacting 3,5-dihydroxybenzoic acid with 1-bromooctane and 1-bromododecane, respectively. The CnBZ products were identified as solutions in dimethyl-d6 sulfoxide using proton nuclear magnetic resonance (1H NMR) spectroscopy (Bruker AM 300). These monomers were used to prepare photoreactive polyesters. 1,4Phenylenediacrylic acid (PDA, 2.0 g, 9.17 mmol) and triphenylphospine (5.1 g, 19.25 mmol) were dissolved together in 20 mL of chlorobenzene, and then hexachloroethane (4.6 g, 19.25 mmol) was added to the solution. The mixture was heated with stirring at reflux temperature for 7 min and then cooled to room temperature. To the resulting mixture, C4BZ (1.93 g, 9.17 mmol) in 20 mL of chlorobenzene was added dropwise, and then triethylamine (3.9 g, 38.50 mmol) was added with stirring. The reaction mixture was again refluxed with stirring for 5 h. The reaction solution was then poured into hot methanol with vigorous stirring, giving as a precipitate the polymer poly[oxy(4-butyl3,5-benzoate)oxy-1,4-phenylene-diacryloyl] (PPDA-C4BZ). The polymer product was dried at room temperature under vacuum for 2 days. In the same manner, poly[oxy(4-octyl-3,5-benzoate)oxy-1,4-phenylenediacryloyl] (PPDA-C8BZ) and poly[oxy(4-dodecyl-3,5-benzoate)oxy-1,4-phenylenediacryloyl] (PPDA-C12BZ) were synthesized from their respective monomers. The weight-average molecular weights (Mw) of the photoreactive polyesters were determined in chloroform by gel permeation chromatography (Polymer labs model PL-GPC 210) calibrated with polystyrene standards; the Mw was 48 000 for PPDA-C4BZ, 34 000 for PPDA-C8BZ, and 30 000 for PPDA-C12BZ. Their films were obtained by spin casting 1.0 wt % solutions of these polyesters in chloroform onto indium tin oxide (ITO) glasses for the optical

Photoinduced Molecular Orientation retardation measurement and the assembly of LC cells, onto quartz for the UV spectral measurement, and onto sodium chloride (NaCl) windows (25 mm diameter × 2 mm thick) for the FTIR spectral measurement. The films were then dried at room temperature for 12 h under vacuum. The resulting polyester films were measured as having a thickness of around 400 nm, using a spectroscopic ellipsometer (J. A. Woollam, model M-44) and an R-stepper (Veeco, model Tektak3). LC Cells. Some of the photoirradiated polyester films on ITO glass substrates were cut into 2.5 cm × 2.5 cm pieces. Paired pieces from the same glass substrate were assembled together in the antiparallel direction of linearly polarized photoirradiation by using 50 µm thick spacers. A nematic LC, 5CB (Aldrich) containing a dichroic dye (Disperse Blue 1, Aldrich) of 1.0 wt % was injected into the cell gap, followed by sealing of the injection hole with an epoxy glue. The LC cells were then heat treated for 5 min at 40 °C, which is slightly higher than the nematic-toisotropic transition temperature of 5CB, to remove any flowinduced memory that might have been induced by the LC injection process. Measurements. The polyester films were irradiated with ultraviolet (UV) light of wavelength 260-380 nm using a highpressure 1.0 kW Hg lamp system (Altech, model ALHg-1000) with an optical filter (Milles Griot, model 03-FCG-179). For LPUVL exposures, a linear dichroic polarizer (Oriel, model 27320) was used. The exposure dose was measured using a photometer (International Light Photometer, model IL1350) with a sensor (model SED 240). UV-visible absorption spectra were obtained as a function of the exposure dose using a Hewlett-Packard 8453 spectrophotometer with or without a dichroic polarizer (Oriel, model 27320). Optical phase retardation was measured using an optical setup with a photoelastic modulator (PEM) (Hinds Instruments, model PEM90) with a fused silica head, a He-Ne laser of 632.8 nm wavelength (Spectra-Physics, model 106-1), a pair of polarizers (Oriel, model 27300), a photodiode detector (UDT Sensors, model PIN-10DL), and a pair of lock-in amplifiers (Stanford Research Systems, model SR510), as described elsewhere.11 FTIR spectroscopic measurements were carried out on a Bomem DA8 FTIR spectrometer equipped with a polarizer (Single diamond polarizer, Harrick Scientific) for transmission FTIR spectra and with a Seagull attachment (Harric Scientific) for external reflection FTIR spectra. For all transmission FTIR spectra, the samples were installed perpendicular to the incident beam direction. While the polarizer was rotated, IR spectra were recorded at 4 cm-1 resolution with a liquid nitrogen cooled mercury cadmium telluride (MCT) detector under vacuum, as a function of the angle of rotation, and interferograms were accumulated 256 times. The external reflection IR spectra were obtained with p-polarized radiation at an angle of incidence of 82°. The reflection IR spectra are 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. The LC alignment in the cell was examined by measuring the absorption of the linearly polarized He-Ne laser beam (632.8 nm wavelength) as a function of the rotational angle of the cell, allowing the construction of polar diagrams. For these measurements, the LC cell was installed perpendicular to the incident laser beam direction.

Results and Discussion Photoreaction in the Films. Nanoscaled films of PPDA-CnBZ polymers having two cinnamoyl moieties per chemical repeat unit in the backbone were exposed to unpolarized UV light and then examined by UV and FTIR spectroscopy in order to investigate in detail the photoreactions of the polymers. Parts a and b of Figure 2 show UV absorption spectra of the PPDA-C4BZ and PPDA-C12BZ films irradiated with UV light at various exposure energies, respectively. Since the UV absorption spectra of the PPDA-C8BZ film are very similar to those of the PPDA-C12BZ film, its spectra (11) 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.

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Figure 2. UV-visible absorption spectra of nanoscaled films of PPDA-CnBZ irradiated with unpolarized UV light with varying exposure energy: (a) PPDA-C4BZ; (b) PPDA-C12BZ.

are not shown here. Before UV light irradiation, the PPDAC4BZ and PPDA-C12BZ films exhibit absorption maxima at 332 nm ()λmax) and 334 nm ()λmax), respectively; these bands are due to the PDA chromophore units in the polymer backbones. For the PPDA-C4BZ film, the intensity of the peak at 332 nm ()λmax) decreases abruptly during the early stages of photoreaction and thereafter decreases gradually with increasing exposure energy (Figure 2a). Furthermore, this peak is blue shifted to about 292 nm upon irradiation. The reduction in spectral intensity can be attributed to a loss of PDA chromophores due to photoreaction, and the shift in peak position can be attributed to a reduction in the π-electron conjugation length of the higher conjugated main-chain chromophores due to their photoreactions. As seen in Figure 2a, one isobestic point, rather than multiple isobestic points, is observed at about 247 nm. This result suggests that the photoreactions of the conjugated chromophores in the polymer film mainly follow a single reaction path. In principle, several photoreaction pathways are available for the acryloyl units of the PDA chromophores in PPDA-C4BZ: (i) trans-cis photoisomerization, (ii) photopolymerization (which produces open-type chain products), and (iii) photocyclization (so-called photodimerization). To understand these possible reactions, consider the chemical structure of the polymer chain shown in Figure 1. As seen in the figure, the reactive acryloyl units form part of the polymer main chain, so their mobility is highly restricted in the solid film. Taking this into account, photoisomerization should significantly disrupt the conformation and packing of the polymer chains even though the reaction is a local process.

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Table 1. Characteristic Infrared Bands of the Photosensitive PPDA-CnBZ Polymers frequency (cm-1) PPDA-C4BZ

PPDA-C8BZ

PPDA-C12BZ

2958 2927

2922

2856 1756 1724 1631 1595 1560 1514 1378 1295 1126 979

2852 1756 1724 1631 1593 1560 1512 1378 1296 1126 979

2871 1756 1724 1631 1595 1560 1513 1379 1297 1126 979

Consequently, photoisomerization in the PPDA-C4BZ film is likely to be a high-energy process and is expected to occur only rarely in the film. Although photopolymerization is possible, it will be limited only to acryloyl units that are sufficiently close to each other to react and hence is likely to produce only open-type dimers. In this case, the photoreaction may still need an additional step to terminate the radicals on the dimer products. In contrast to photopolymerization, photocyclization requires only a pair of acryloyl units, making this a more favorable photoreaction for the acryloyl units of the PDA chromophores. This reaction process does not need an additional termination step. Moreover, cinnamate, an analogue of the acryloyl unit of the PDA chromophore, is known to undergo photocyclization rather than photopolymerization when in the solid state.12 This observation, combined with the other factors mentioned above, suggests that photocyclization (i.e., photodimerization) is likely to be favored over photopolymerization in PPDA-C4BZ polymers in films. As seen in Figure 2b, the PPDA-C12BZ film displays analogous characteristics upon irradiation, suggesting that the length of the n-alkyl side groups on the polyester does not significantly influence the photoreactions in PPDA-CnBZ films. The photoreactions of the PPDA-CnBZ films were further investigated by FTIR spectroscopy. Parts a and b of Figure 3 display FTIR spectra of the PPDA-C4BZ and PPDA-C12BZ films irradiated with UV light at various exposure energies, respectively. All the observed vibrational bands in the spectra could be assigned in accordance with results reported previously.13-15 The bands at 1631 and 979 cm-1 correspond to the vinylene CdC stretching vibration and the trans-vinylene C-H deformation in the PDA chromophores, respectively.13,14 Additional bands at 1593 and 1513 cm-1 correspond to the vibrational modes of the benzene ring (12) (a) Egerton, P. L.; Pitts, E.; Reiser, A. Macromolecules 1981, 14, 95. (b) Egerton, P. L.; Trigg, J.; Hyde, E. M.; Reiser, A. Macromoelcules 1981, 14, 100. (13) (a) Kawatsuki, N.; Ono, H.; Takatsuka, H.; Yamamoto, T.; Sangen, O. Macromolecules 1997, 30, 6680. (b) Kawatsuki, N.; Matuyoshi, K.; Hayashl, M.; Takatsuka, H.; Yamamoto, T. Chem. Mater. 2000, 12, 1549. (c) Kawatsuki, N.; Suehiro, C.; Shindo, H.; Yamamoto, T.; Ono, H. Macromol. Rapid Commun. 1998, 19, 201. (14) (a) Perny, S.; Le Barny, P.; Delaire, J.; Buffetean, T.; Sourisseau, C.; Dozov, I.; Forget, S.; Martinot-Lagrade, P. Liq. Cryst. 2000, 27, 329. (b) Perny, S.; Le Barny, P.; Delaire, J.; Buffetean, T.; Sourissean, C. Liq. Cryst. 2000, 27, 341. (15) (a) Colthup, N. B.; Daly, L. H.; Wiberly, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic: San Diego, CA, 1990. (b) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic: San Diego, CA, 1991.

assignment description CH3 asymmetric stretching vibration CH2 asymmetric stretching vibration CH3 symmetric stretching vibration CH2 symmetric stretching vibration nonconjugated CdO stretching vibration conjugated CdO stretching vibration vinylene CdC stretching vibration ν(CdC) in benzene ring ν(CdC) in benzene ring ν(CdO) in benzene ring CH3 deformation C-O stretching vibration C-O stretching vibration trans-vinylene C-H deformation

in the backbone.15 There are two kinds of substituted benzene ring in the PPDA-CnBZ polymer, one in PDA and the other in the CnBZ unit. When a 3-fold axis of symmetry is present, as in the CnBZ unit, then the vibrations with the two components 1588 and 1486 cm-1 are doubly degenerate, but for less symmetrical substitution of the benzene ring, the degeneracy is broken and the two components are found in the region 1600-1500 cm-1.15 Thus, the bands at 1593 and 1513 cm-1 can be assigned to the quadrant stretching and semicircle stretching vibrations of the para-substituted benzene rings in the PDA chromophores. The stretching vibrations of the CH3 and CH2 units in the n-alkyl side groups appear in the region 3000-2800 cm-1, as listed in Table 1. The asymmetric and symmetric CH3 stretching vibrations in the PPDA-C4BZ film are located at 2958 and 2871 cm-1, respectively. The asymmetric and symmetric CH3 stretching vibrations in the PPDA-C8BZ film are located at 2927 and 2856 cm-1 (data not shown) and in the PPDA-C12BZ film at 2922 and 2852 cm-1, respectively. CH3 deformations in the n-alkyl side groups are detected at 1378 cm-1. It is well documented that the wavenumbers of the methylene stretching vibration are conformationally sensitive,16 where its lower wavenumbers are characteristic of highly ordered conformations. It can therefore be concluded that the n-alkyl side groups in the PPDA-C12BZ film (νa(CH2) about 2922 cm-1) exhibit more conformational ordering than those in the PPDA-C8BZ film (νa(CH2) about 2926 cm-1) and in the PPDA-C4BZ film (νa(CH2) about 2958 cm-1). However, the wavenumber of νa(CH2) in the PPDA-C12BZ film is still high, compared to that (2918 cm-1) for the full trans-conformation, suggesting that the n-alkyl side groups in the PPDAC12BZ film are disordered in a certain level. Further, in the FTIR spectra there are a number of sharp bands in the 1450-950 cm-1 region, many of which involve in-plane C-H bending vibrations in the benzene ring and C-O stretching vibrations in the ester linkage. The bands at about 1724, 1295, and 1126 cm-1 are due to the conjugated CdO vibration, C-O-C asymmetric and symmetric stretching vibrations in the ester linkages, respectively. It was not possible to discern in the present experiments whether these bands arise from the ester linkages in the PDA chromophores or from those in the CnBZ units, but these bands are much influenced by photoreaction. As seen in Figure 3a, the intensities of the bands at 1631 and 979 cm-1 (due to the vinylene CdC stretching vibration and the trans-vinylene C-H deformation vibra(16) Sun, L.; Crooks, R. M. Langmuir 1993, 9, 1775.

Photoinduced Molecular Orientation

Figure 3. FTIR spectra of nanoscaled films of PPDA-CnBZ irradiated with unpolarized UV light with varying exposure energy: (a) PPDA-C4BZ; (b) PPDA-C12BZ.

tion in the PDA chromophores, respectively) decrease rapidly as the UV exposure energy increases. Moreover, as the exposure energy increases, the intensity of the band at 1724 cm-1 due to the conjugated CdO stretching vibration decreases and a new band appears at higher wavenumbers. These intensity drops of the conjugated CdC and CdO stretching bands, as well as the appearance of a new band could be due to two possible factors: the loss of π-conjugation due to photodimerization of the PDA chromophores and/or the photoisomerization of the PDA chromophores from transisomers to cis-isomers. According to the report of Chakrabartk et al.,17 the position shift of the conjugated CdO stretching band due to trans-cis photoisomerization is small. In fact, such a small position shift cannot be resolved in the present study because of the extensive overlapping at higher exposure energy of the band at 1724 cm-1 with the new band, which presumably corresponds to the nonconjugated CdO stretching vibration and is discerned at around 1756 cm-1; its peak intensity increases with increasing exposure energy. This result supports the suggestion that the intensity drop and the position shift of the conjugated CdO stretching band originate principally from the photodimerization of the PDA chromophores and possibly in part from the trans-cis photoisomerization of the PDA chromophores. However, the UV absorption results as described above imply that the intensity drops of the conjugated CdC and (17) Chakrabartk, S.; Maity, A. K.; Misra, T. N. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 1625.

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Figure 4. (a) Linearly polarized UV-visible absorption spectra of PPDA-C4BZ films irradiated with LPUVL at 1.5 J/cm2: (- - -) measured with a probing UV-visible light linearly polarized perpendicular to the polarization direction of LPUVL; (s) measured with a probing UV-visible light linearly polarized parallel to the polarization direction of LPUVL. (b) Dichroic ratios measured from PPDA-CnBZ films irradiated by LPUVL with various exposure energies. Here, dichroic ratio [)(A⊥ A|)/(A⊥ + A|)] was determined from the absorbance at λmax (maximum absorption wavelength) (A⊥) measured with a probing UV-visible light linearly polarized perpendicular to the polarization direction of LPUVL and that (A|) measured with a probing UV-visible light linearly polarized parallel to the polarization direction of LPUVL.

CdO stretching bands result from the loss of PDA chromophores by photodimerization. In particular, it was observed that the band at 1593 cm-1 is influenced by the photoreaction: as the photoreaction proceeds, a new band appears at about 1606 cm-1 as a shoulder. This appearance of a new band at the higher wavenumber could be attributed to the loss of the π-conjugation of the benzene rings with vinylene CdC bonds in the PDA chromophores due to photodimerization of the PDA chromophores. Therefore, the UV absorption and FTIR spectroscopic results lead to the conclusion that the PDA chromophores in PPDA-CnBZ films undergo mainly photodimerization when the films are irradiated with UV light. UV Dichroic Ratio. Figure 4a shows linearly polarized UV-visible absorption spectra of a PPDA-C4BZ film irradiated with LPUVL at 1.5 J/cm2. The spectrum measured with the UV-visible light polarized perpendicular to the polarization direction of the LPUVL reveals slightly stronger absorption over the range of 250-365 nm than that measured with the UV-visible light polarized parallel to the polarization direction of the LPUVL. Similar dichroic absorption spectra were measured for the other PPDA-CnBZ films irradiated with LPUVL at various exposure energies (data not shown). For each irradiated PPDA-CnBZ film, dichroic ratio [)(A⊥ - A|)/(A⊥ + A|)] was determined from the absorbance at λmax (maximum absorption wavelength) (A⊥) measured

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Figure 6. FTIR dichroic spectra of a film of PPDA-C12BZ irradiated with LPUVL at 1.5 J/cm2. Solid and dashed lines indicate the FTIR spectra with the IR light polarized parallel and perpendicular to the LPUVL direction, respectively.

Figure 5. (a) Polar diagram of transmitted light intensity [)(in-plane birefringence) × (phase)] taken from the optical phase retardation measurement of a PPDA-C4BZ film irradiated with LPUVL at 1.5 J/cm2 as a function of the angle of rotation of the film. (b) Birefringence (i.e., in-plane birefringence) variations of PPDA-CnBZ films irradiated with LPUVL at various exposure energies.

with a probing UV-visible light linearly polarized perpendicular to the polarization direction of LPUVL and that (A|) measured with a probing UV-visible light linearly polarized parallel to the polarization direction of LPUVL. The determined dichroic ratios are displayed in Figure 4b. As seen in the figure all the measured dichroic ratio values are positive over the exposure energy of e3.0 J/cm2. The dichroic ratio increases rapidly with increasing exposure energy up to 1.5 J/cm2 and then turns to slowly increase with further increasing exposure energy; all the polymer films show very similar variations of dichroic ratio with exposure energy, suggesting that the photoreactions of these polymers in films are independent of the lengths of the n-alkyl side groups. These results indicate that by LPUVL exposure, the PDA chromophores located parallel to the polarization direction of the LPUVL are consumed more rapidly than those positioned perpendicular to the polarization direction of the LPUVL. Optical Retardation. Figure 5a displays a polar diagram of the transmitted light intensity [ )(in-plane birefringence) × (phase)] taken from the optical phase retardation measurement of a PPDA-C4BZ film irradiated with LPUVL at 1.5 J/cm2 as a function of the angle of rotation of the film. As seen in the figure, the irradiated film reveals a maximum light intensity value along the direction of 270° T 90°, which lies perpendicular to the polarization direction of the LPUVL used in the UV exposure, but a minimum light intensity value along the direction of 0° T 180°, which is parallel to the polarization direction of the LPUVL. Similar polar diagrams were measured for the other PPDA-CnBZ films irradiated with LPUVL at various exposure energies (data not shown).

On the contrary, all PPDA-CnBZ films without LPUVL irradiation revealed isotropic polar diagrams of the transmitted light intensity in the optical retardation measurements. In general, polymer chains in thin films are known to have a strong tendency to favorably orient in the film plane rather than randomly; such in-plane orientation of polymer chains takes place more strongly not only as the film becomes thinner but also as the polymer chain rigidity becomes greater.18 As seen in Figure 1, PPDA-CnBZ polymers are somewhat rigid, so that in the films around 400 nm thick the polymer chains are favorably aligned in the film plane. The in-plane orientations of the PPDACnBZ polymers were confirmed in ellipsometric measurements; for the PPDA-CnBZ films with around 400 nm, the in-plane refractive index always was slightly larger than the out-of-plane refractive index.19 This refractive index anisotropy indicates that the PPDA-CnBZ polymer chains are positively birefringent. Taking these results into account, the in-plane birefringence was determined from the maximum light intensity values along the direction of 270° T 90°, which lies perpendicular to the polarization of the LPUVL in Figure 5a for the PPDA-C4BZ film. This in-plane birefringence determination was carried out for all other PPDA-CnBZ films irradiated with LPUVL. The determined in-plane birefringence values, which are always positive, are plotted in Figure 5b as a function of exposure energy. As seen in the figure, the in-plane birefringence rapidly increases with exposure energy up to around 0.5 J/cm2 and then slowly increases with further increasing exposure energy. Above exposure energy of 1.5 J/cm2, the in-plane birefringence turns to slightly decrease. These results collectively lead the conclusion that by LPUVL exposure, positively birefringent PPDA-CnBZ polymer chains in the nanoscaled films are induced to preferentially reorient along the direction perpendicular to the polarization of the LPUVL, and the extent of such photoinduced reorientation of the polymer chains is (18) (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., 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. (19) Lee, S. W.; Ree, M. To be submitted for publication in Langmuir.


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Figure 7. Polar diagrams of some specific vibrational peaks of a PPDA-C4BZ film irradiated with LPUVL at 1.5 J/cm2, measured by linearly polarized IR spectroscopy as a function of the angle of rotation of the film: (a) 1631 cm-1 (vinylene CdC stretching); (b) 1724 cm-1 (conjugated CdO stretching); (c) 1126 cm-1 (C-O stretching); (d) 1595 cm-1 (quadrant phenyl stretching); (e) 1606 cm-1 (a vibrational band due to photodimerization of PDA units); (f) 1756 cm-1 (nonconjugated CdO stretching).

dependent upon the exposure energy. As discussed in the dichroic UV spectroscopy section above, the directionally selective photoreaction of PDA chromophores in the PPDACnBZ films by LPUVL exposure induces a preferential reorientation of the unreacted PDA chromophores along the direction perpendicular to the polarization of the LPUVL. Therefore, the preferentially reoriented PDA chromophores might play as the major component for the polymer chains reoriented perpendicular to the polarization of the LPUVL, which were determined in the optical retardation measurements. As seen in Figure 5b, the birefringence values of the films irradiated with LPUVL show a dependence on the lengths of the n-alkyl side groups over the exposure energy range considered; at a given exposure energy, the film of a polymer bearing a longer n-alkyl side group always

reveals a larger birefringence value. As mentioned earlier, all the polymers are positively birefringent. Taking this fact into account, a longer n-alkyl side group gives lower positive birefringence of the polymer chain. Thus, one can predict that the in-plane film birefringence generated by LPUVL exposure increases in the order PPDA-C12BZ < PPDA-C8BZ < PPDA-C4BZ. However, the measured results are opposed to the prediction. In general, the chain mobility of a polymer is directly reflected to Tg; a polymer having higher chain mobility reveals lower Tg. Tg was measured to be 121 °C for PPDAC12BZ, 125 °C for PPDA-C8BZ, and >125 °C for PPDAC4BZ.10 These results indicate that a PPDA-CnBZ polymer bearing longer n-alkyl side groups has higher chain mobility. Therefore, the larger in-plane birefringences observed in the LPUVL-irradiated films of a PPDA-CnBZ

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Figure 8. Polar diagrams of some specific vibrational peaks of a PPDA-C12BZ film irradiated with LPUVL at 1.5 J/cm2, measured by linearly polarized IR spectroscopy as a function of the angle of rotation of the film: (a) 1631 cm-1 (vinylene CdC stretching); (b) 1724 cm-1 (conjugated CdO stretching); (c) 1126 cm-1 (C-O stretching); (d) 1593 cm-1 (quadrant phenyl stretching); (e) 1606 cm-1 (a vibrational band due to photodimerization of PDA units); (f) 1756 cm-1 (nonconjugated CdO stretching); (g) 2922 cm-1 (asymmetric CH2 stretching); (h) 2852 cm-1 (symmetric CH2 stretching).

polymer having longer n-alkyl side groups might result from the better anisotropic reorientations of the polymer chains which take place favorably due to the higher chain mobility caused by the longer n-alkyl side group. In conclusion, the photoreaction-induced molecular reorientation in the PPDA-CnBZ film is dependent on the length of the n-alkyl side group which contributes to the whole polymer chain mobility. Molecular Reorientation. Only modes with in-plane components can be directly observed in the transmission FTIR spectroscopy because only normal incidence spectra were measured. Thus, transmission FTIR spectroscopy with a linearly polarized IR light source was used to analyze the in-plane molecular orientations within PPDACnBZ films after the films were irradiated with LPUVL.

Figure 6 shows two FTIR spectra of a PPDA-C12BZ film irradiated with an exposure energy of 1.0 J/cm2, one measured with IR light polarized parallel to the polarization direction of the LPUVL and the other with IR light polarized perpendicular to the polarization direction of the LPUVL. Before irradiation with the LPUVL, the PPDA-CnBZ films show no IR dichroism dependence (i.e., polarization angle dependence) (data not shown). However, as seen in the figure, the irradiated films show an anisotropy between the IR spectrum measured with the IR light polarized parallel to the polarization direction of the LPUVL and that measured with the IR light polarized perpendicular to the polarization direction of the LPUVL direction. Most bands show no IR dichroism dependence with respect to the LPUVL direction, except for the IR


bands at 1724, 1631, and 1126 cm-1. These bands are more intense when the incident IR beam is polarized perpendicular to the polarization direction of the LPUVL, although the intensity differences are small. In addition to the dichroic IR spectroscopic measurements described above, IR spectroscopic measurements with a linearly polarized IR light source were conducted for the PPDA-CnBZ films irradiated with LPUVL at 1.5 J J/cm2 as a function of the angle of rotation of the films. These measurements were made in order to determine the extent of the reorientation of polymer chains that occurs during photoreaction. The measured peak intensities of selected IR bands are plotted as polar diagrams with respect to the angle of rotation of the film, in Figure 7 for PPDA-C4BZ and in Figure 8 for PPDA-C12BZ. This procedure enables us to deduce the extent of the reorientations of the unreacted PDA chromophores and the photodimerized PDA units in the polymer chains. For the PPDA-C4BZ film, the vinylene CdC band at 1631 cm-1 is more intense when the polarization of the incident IR beam is nearly perpendicular (i.e., at an angle of 96°) to the polarization direction of the LPUVL (see Figure 7a); the conjugated CdO stretching band at 1724 cm-1 and the benzene ring quadrant stretching band at 1593 cm-1 are more intense when the polarization of the incident IR beam is perpendicular to the polarization direction of the LPUVL, although the differences in intensity are small (see Figure 7b,d). As seen in Figure 7c, the C-O stretching band at 1126 cm-1 also is more intense when the polarization of the incident IR beam is at an angle of 98° to the polarization direction of the LPUVL. The photoreactive vinylene CdC bond is a part of the unreacted PDA chromophore, so its IR result in Figure 7a indicates that the PDA chromophores positioned in the polarization direction of the LPUVL are consumed more rapidly by photoreaction than those positioned perpendicular to the polarization direction of the LPUVL, which is consistent with the conclusion deduced from the dichroic UV spectroscopic measurements described in the earlier section; this selective photoreaction induces a preferentially molecular reorientation of the unreacted PDA chromophores. The dipole-transition moments for the conjugated CdO stretching vibration at 1724 cm-1 and for the benzene ring quadrant stretching vibration at 1595 cm-1 are perpendicular to the polarization direction of the LPUVL. All the chemical bond components considered here are parts of the PDA unit in the polymer backbone, so that their IR results (Figure 7a-d) collectively lead to the conclusion that the PDA chromophores remained (i.e., unreacted PDA chromophores) in the film irradiated with LPUVL are preferentially reoriented perpendicular to the polarization direction of the LPUVL. The direction of the preferentially reoriented PDA chromophores coincides with the major director of the in-plane birefringence of the film measured in the “optical retardation” section. This fact suggests that the preferential reoriented PDA units make the major contribution to the in-plane birefringence and its director determination. As seen in parts e and f of Figure 7, the new band at 1606 cm-1 (due to the vibrational mode of the benzene ring arising from photodimerization) and the nonconjugated CdO band at 1756 cm-1, by contrary, show little difference in intensity when the incident IR beam is polarized perpendicular to the direction polarization of the LPUVL: the benzene ring quadrant stretching and nonconjugated CdO stretching vibrations of the photodimer show no preferential anisotropy with respect to the polarization direction of the LPUVL. These results indicate

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Figure 9. FTIR spectra of a PPDA-C12BZ film irradiated with LPUVL at 2.0 J/cm2: (a) transmission spectrum; (b) external reflection spectrum.

that the molecular reorientation of the photodimer produced by photoreaction happens randomly rather than preferentially along a specific direction in the film plane, so the dichroic ratio of these bands is almost zero. Figure 8 shows polar diagrams of the vibrations of the chemical bond components in the PDA chromophores of the PPDA-C12BZ film irradiated with LPUVL. These polar diagrams are similar to those of the PPDA-C4BZ film described above, except for slight differences on the major directors of some vibrational bands. The preferential reorientation director of the vinylene CdC bond (1631 cm-1), as seen in Figure 8a, is perpendicular to the polarization direction of the LPUVL. The conjugated CdO (1724 cm-1) and C-O bonds (1126 cm-1) as well as the benzene ring (1593 cm-1) are reoriented preferentially along the direction in an angle of 110° to the polarization of the LPUVL (see Figure 8b-d). However, this PPDAC12BZ film reveals UV dichroic ratio and optical retardation very similar to those of the PPDA-C4BZ film irradiated with LPUVL. On combination of the dichroic IR results with the dichroic UV and optical retardation results, it is suggested that the unreacted PDA chromophores in the PPDA-C12BZ film also are preferentially reoriented perpendicular to the polarization of the LPUVL as observed for the PPDA-C4BZ film. Parts e and f of Figure 8 show the benzene ring quadrant stretching and nonconjugated CdO stretching vibrations of the photodimer, respectively, which reveal no anisotropy. These results indicate that the molecular reorientation of the photodimer produced in the PPDA-C12BZ film by photoreaction is nearly isotropic in the film plane as observed for the PPDA-C4BZ film. For the PPDA-C12BZ film, it moreover was possible to determine the extent of the molecular reorientation of the n-alkyl side groups. The dipole-transition moments for the asymmetric and symmetric CH2 stretching vibrations of the n-alkyl side group are orthogonal to each other and furthermore perpendicular to the polymer backbone. The asymmetric CH2 stretching vibration at 2922 cm-1 and the symmetric CH2 stretching vibration at 2852 cm-1 show no anisotropy, respectively, as shown in parts g and h of Figure 8. Similar results were obtained for the other PPDA-CnBZ films irradiated with LPUVL. These results collectively suggest that the n-alkyl side groups in the PPDA-CnBZ films irradiated with LPUVL are distributed isotropically. In addition to the transmission FTIR spectroscopic study above, external reflection IR spectroscopic measurements were conducted with p-polarized radiation at an incidence angle of 82°, to get information on the out-of-plane

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Figure 10. Polar diagrams of absorptions of a linearly polarized visible light (632.8 nm wavelength) measured from LC cells fabricated with (a) a PPDA-C4BZ film, (b) a PPDA-C8BZ film, and (c) a PPDA-C12BZ film irradiated with LPUVL at 1.5 J/cm2, as a function of the angle of rotation of the LC cell.

molecular orientations of the polymer chain segments. Figure 9b shows an external reflection IR spectrum of a PPDA-C12BZ film irradiated with LPUVL at 2.0 J/cm2, which was measured with IR light polarized parallel to the LPUVL. This reflection IR spectrum is compared with the transmission IR spectrum (Figure 9a) which was measured from the same PPDA-C12BZ film with IR light polarized parallel to the LPUVL. The external reflection spectrum is on the whole only slightly different from the transmission spectrum of the rubbed film. However, some bands show a difference in intensity. On comparison of the external reflection IR spectrum with the transmission spectrum, the intensities of the bands at 1756 and 1724

cm-1 and 1606 and 1593 cm-1 are different in the two spectra. The bands in the transmission spectrum for the nonconjugated CdO stretching vibration and the benzene ring vibration of the photodimer are more intense than the bands for the conjugated CdO stretching vibration and the benzene ring vibration of unreacted PDA chromophores in the external reflection IR spectrum, indicating that the nonconjugated CdO bond and benzene ring of photodimer are on average oriented mostly normal to the film surface. The irradiated PPDA-C4BZ and PPDA-C8BZ films also exhibit similar results as described above. LC Alignment. Figure 10 displays the polar diagrams constructed for the LC cells fabricated with PPDA-CnBZ


films irradiated with LPUVL at 1.5 J/cm2. As seen in the figure, the main director lies along a direction perpendicular to the direction of the LPUVL, regardless of the length of the n-alkyl moiety in the side group. These results indicate that the LC molecules in contact with the film surface are induced to align in a direction perpendicular to that of the LPUVL. We now consider the intermolecular interactions between the PPDA-CnBZ polymer chains and LC molecules that might affect the alignment of the LC molecules. First, the intermolecular interactions of the polymer chains with the LC molecules are considered. In the films irradiated with LPUVL, the polymer chains are preferentially reoriented perpendicular to the polarization of the LPUVL, which were measured in the optical retardation analysis. These preferentially reoriented polymer chains favor to anisotropically interact with LCs, leading LC alignment along their reorientation direction, which is perpendicular to the polarization of the LPUVL. Second, the contribution of unreacted PDA chromophores to the intermolecular interactions of the polymer chains with the LC molecules is further considered. The PDA chromophores were found to reorient perpendicular to the polarization of the LPUVL and further its constitutional components (vinylene CdC bond, conjugated CdO bond, CsO bond, and benzene ring) to more favorably reorient nearly perpendicular to the direction of the LPUVL, as described above. Thus, the vinylene CdC bonds and the benzene rings in the PDA chromophores may favorably interact with LC molecules (in particular, with the biphenyl group of 5CB); these interactions favor LC alignment perpendicular to the direction of the LPUVL. In addition, conjugated CdO bonds may contribute to an LC alignment that is perpendicular to the direction of the LPUVL. Third, we consider the reorientation of photodimers and their role in LC alignment. The nonconjugated CdO and benzene ring of photodimer formed by the LPUVP irradiation are not shown in any preferential reorientation in the film plane. Thus, the isotropy of the photodimer may not contribute to LC alignment. Finally, the contribution of the n-alkyl side groups to the intermolecular interactions of the polymer chains with LC molecules is taken into account. The n-alkyl groups of PPDA-CnBZ can interact with the n-pentyl tail of 5CB. However, for PPDA-CnBZ, the n-alkyl moiety in the side

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group is also not shown any preferential orientation in the film plane, which may not contribute to the LC alignment. Thus, taking the molecular reorientations of the polymer chains and their photoproducts into account, we suggest that the unreacted PDA chromophores in the main chain and the photodimerized PDA units may synergistically contribute to the unidirectional reorientation of polymer chains induced by the linearly polarized photoreaction; they induce LC alignment, although the contribution of the photodimerized PDA units to LC alignment is very small. We conclude that the unreacted PDA chromophores, which lie in the LPUVL irradiated film along the direction perpendicular to the polarization direction of the LPUVL, make the dominant contribution to LC alignment. Conclusion The photochemical reactions and molecular reorientation of novel photosensitive polyesters containing PDA chromophores in the main chain and n-alkyl side chains, PPDA-CnBZ, were in detail investigated by UV absorption spectroscopy, linearly polarized IR spectroscopy, and optical retardation analysis. The ability of PPDA-CnBZ films irradiated with LPUVL to align LCs was examined. The PDA chromophores in PPDA-CnBZ films were found to favorably undergo mainly photocyclization upon UV irradiation. Irradiated PPDA-CnBZ films align LCs perpendicular to the polarization direction of the LPUVL. This LC alignment result, along with our conclusions as to the reorientation of polymer chains in irradiated films, shows that the reoriented polymer chains in the irradiated films interact anisotropically with LC molecules and align the LC molecules when their reorientation directions coincide. This LC alignment process is governed in the irradiated PPDA-CnBZ films principally by the reorientation of segments of unreacted PDA chromophores, whose directionally anisotropic interactions contribute to the alignment of the LC molecules. Acknowledgment. This study was supported by the Korea Research Foundation (project contracted with Postech Polymer Research Institute in 2002) and by the Ministry of Education (BK21 Program). LA0340596

Photoreaction and Molecular Reorientation in Films of Novel

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