UV-Driven Switching of Chain Orientation and Liquid Crystal

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J. Phys. Chem. B 2008, 112, 4900-4912

UV-Driven Switching of Chain Orientation and Liquid Crystal Alignment in Nanoscale Thin Films of a Novel Polyimide Bearing Stilbene Moieties in the Backbone Suk Gyu Hahm,‡,† Seung Woo Lee,£,† Taek Joon Lee,‡ Seon Ah Cho,‡ Boknam Chae,‡ Young Mee Jung,§ Seung Bin Kim,*,‡ and Moonhor Ree*,‡ Department of Chemistry, National Research Lab for Polymer Synthesis & Physics, Laboratory for Vibrational Spectroscopy, Center for Integrated Molecular Systems, and BK School of Molecular Science, Pohang UniVersity of Science and Technology, Pohang 790-784, Republic of Korea, School of Display & Chemical Engineering, Yeungnam UniVersity, Gyeongsan 712-749, Republic of Korea, and Department of Chemistry, Kangwon National UniVersity, Chunchon 200-701, Republic of Korea ReceiVed: October 20, 2007; In Final Form: February 17, 2008

A novel photosensitive polyimide, poly(4,4′-stilbenylene 4,4′-oxidiphthalimide) (ODPA-Stilbene PSPI) was newly synthesized. The most surprising feature of this PSPI is that the PSPI films irradiated with linear polarized ultraviolet light (LPUVL) can favorably induce a unidirectional alignment of liquid crystals (LCs) in contact with the film surface and further switch the director of the unidirectionally aligned LCs from a perpendicular direction to a parallel direction with respect to the polarization direction of LPUVL by simply controlling the exposure dose in the irradiation process. These LPUVL-irradiated films were found to provide high anchoring energy to LCs, always giving very stable, homogeneous cells with unidirectionally aligned LCs regardless of the LC alignment directions. In the films, the PSPI polymer chains were found to undergo favorably unidirectional orientation via a specific orientation sequence of the polymer chain segments led by the directionally selective trans-cis photoisomerization of the stilbene chromophore units in the backbone induced by LPUVL exposure. Such unidirectionally oriented polymer chains of the films induce alignment of the LCs along the orientation direction of the polymer chains via favorable anisotropic molecular interactions between the oriented polymer chain segments and the LC molecules. In addition, the PSPI has an excellent film formation processibility; good quality PSPI thin films with a smooth surface are easily produced by simple spin-coating of the soluble poly(amic acid) precursor and subsequent thermal imidization process. In summary, this new PSPI is the promising LC alignment layer candidate with rubbing-free processing for the production of advanced LC display devices, including LC display televisions with large display areas.

Introduction The photoinduced alignment of nematic liquid crystals (LCs) has been attracting increasing interest because of the practical applicability to produce rubbing-free LC alignment layers that are key materials in the fabrication of LC display devices.1-6 The photoalignment control of nematic liquid crystals (LCs) is based on the photochemically induced structural and orientational change of molecules or residues localized at the uppermost surfaces of substrate plates, which have been provided by appropriate combinations of molecular or polymeric films with suitable photoactive molecules. Whereas molecular films incorporating photosensitive moieties have been employed so far to achieve photoalignment control, more extensive studies have been carried out on thin films of polymers having photoreactive units because of the good availability of thin films by the spincoating technique. With respect to photosensitive units leading to LC alignment by linearly polarized light irradiation, three categories of photochemistry have been investigated. The first consists of the photoisomerization of azobenzenes,7-11 spiro* To whom correspondence should be addressed. Tel: +82-54-279-2120. E-mail: [email protected] (M.R.); Tel: +82-54-279-2106. E-mail: sbkim@ postech.edu (S.B.K.); Fax: +82-54-279-3399. ‡ Pohang University of Science and Technology. £ Yeungnam University. § Kangwon National University. † S. G. Hahm and S. W. Lee contributed equally to this work.

pyrans,12 and stilbenes.13 The second class displays [2 + 2] photodimerization of cinnamates,1,4,14-16 benzylidenephthalimidines,17 benzylideneacetophenones (i.e., chalcones),18 and coumarins.2,5 The third is the photodegradation of the imide groups of polyimides.19 For the first two approaches, several photosensitive polymers have been reported so far, but most of them developed on the basis of the polyvinyl backbone whose properties and process conditions are not suitable to the fabrication of LC display devices. The third approach has been developed with conventional PIs, which are currently used or considered for the fabrication of LC display devices but were found to degrade with high exposure doses of ultraviolet (UV) light and to indeed take a long time to process. Therefore, the challenge remains to deliver high-performance polymers suitable for rubbing-free processing of LC alignment layer films. In this study, we synthesized a novel photosensitive polyimide (PSPI), poly(4,4′-stilbenylene 4,4′-oxidiphthalimide) (ODPAStilbene PSPI) (Figure 1) and characterized in detail its photochemistry and photoreaction mechanism, photoinduced molecular orientation and orientation sequence, LC alignability, and LC anchoring energy in nanoscale thin solid films. The present study demonstrated that the ODPA-Stilbene PSPI in thin films exhibits excellent processibility and properties as a rubbing-free processing LC alignment layer material suitable for advanced LC display devices including LC display televisions with large display areas as follows. A good quality of

10.1021/jp7101868 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/03/2008

UV-Driven Switching in Nanoscale Thin Films

Figure 1. Synthetic scheme and chemical structure of 4,4-diaminostilbene and its photoreactive poly(amic acid) precursor, poly(4,4′stilbenylene 4,4′-oxidiphthalamic acid) (ODPA-Stilbene PAA), and polyimide, poly(4,4′-stilbenylene 4,4′-oxidiphthalimide) (ODPA-Stilbene PSPI).

ODPA-Stilbene PSPI thin films with a smooth surface and high thermal stability can easily be produced by simple spin-coating of the soluble poly(amic acid) (PAA) precursor in solution and subsequent drying and a thermal imidization process. The PSPI exhibits excellent photoreactivity to UV light via the trans-cis photoisomerization of the stilbene chromophore units in the backbone and further excellent photoinduced unidirectional chain orientation ability via a specific orientation sequence of the polymer chain segments led by linearly polarized UV light (LPUVL) exposure. The preferentially oriented PSPI chains unidirectionally align LC molecules with high anchoring energy along their orientation direction. The most interesting feature of the ODPA-Stilbene PSPI is that the PSPI films can switch easily the director of the unidirectionally aligned LC molecules in contact with the film surface from a perpendicular direction to a parallel direction with respect to the polarization direction of LPUVL by simply controlling the exposure dose in the LPUVL irradiation process of the film without any changes of its polarization director. Experimental Section Material and PSPI Synthesis. 4,4′-Oxidiphthalic anhydride (ODPA) was supplied from Chriskev Company and purified by recrystallization from acetic anhydride. N,N′-dimethylformamide (DMF) and N-methyl-2-pyrrolidinone (NMP) were purchased from Aldrich Company and distilled over calcium

J. Phys. Chem. B, Vol. 112, No. 16, 2008 4901 hydride under reduced pressure and under a nitrogen atmosphere, respectively. All other chemical compounds were supplied from Aldrich and used without further purification. 4-Nitrobenzyl bromide, 4-nitrobenzaldehyde, ethyl acetate, and diethyl ether were purchased from Aldrich Company and used without purification. 4-Nitrobenzyl bromide (5.00 g, 23.1 mmol) and triphenylphosphine (TPP: 6.67 g, 25.5 mmol) were dissolved in dried DMF (100 mL), and the reaction mixture was gently heated to 70 °C under stirring for 24 h. Thereafter, the reaction solution was poured into diethyl ether under vigorous stirring, giving (4-nitrobenzyl)triphenylphosphonium bromide as a white powder. The precipitate was filtered, washed with diethyl ether, and dried under vacuum (9.17 g, 83% yield). 1H NMR (CDCl3, δ): 7.81 (m, 11H, ArH), 7.61 (m, 6H, ArH), 7.50 (d, 2H, ArH), 6.05 (d, 2H, -CH2-P). (4-Nitrobenzyl)triphenylphosphonium bromide (5.00 g, 10.5 mmol) with 4-nitrobenzaldehyde (1.58 g, 10.5 mmol) in dry DMF (100 mL) containing sodium ethoxide (0.78 g, 11.5 mmol) were stirred at ambient temperature for 24 h. After stirring, the reaction solution was filtered and the solvent removed by rotary evaporation to give a pale-yellow powder. The crude product was recrystallized from hot ethanol to give 4,4′-dinitrostilbene with 54% yield. 1H NMR (CDCl3, δ): 8.21 (d, 4H, ArH), 7.63 (d, 4H, ArH), 7.69 (s, 2H, dCH-). 4,4′-Dinitrostilbene (3.00 g, 11.1 mmol) and SnCl2 (10.5 g, 55.5 mmol) were dissolved in a mixture of ethanol (80 mL) and hydrochloric acid (36% concentration, 24 mL). The reaction mixture was stirred for 1 h at room temperature, followed by refluxing for 6 h. Thereafter, the reaction solution was poured into ice and adjusted to pH 8 with sodium hydroxide solution. After extraction with ethyl acetate four times, the combined organic layer was dried over MgSO4, followed by evaporation to obtain 4,4′-diaminostilbene. The diamine was purified to polymerization grade by recrystallization from ethanol (1.21 g, 5.7 mmol, 52% yield). 1H NMR (DMSO-d6, δ): 7.18 (d, 4H, ArH), 6.71 (2H, dCH-), 6.54 (d, 4H, ArH), 5.14 (s, 2H, -NH2). Soluble ODPA-Stilbene PAA precursor was prepared by adding the equivalent mole of ODPA into the 4,4′-diaminostilbene dissolved in dried NMP under nitrogen by stirring vigorously (Figure 1). Once the ODPA addition was complete, the reaction flask was sealed tightly, and stirring was continued for 24 h to make the polymerization mixture homogeneous and viscous. 1H NMR (DMSO-d6, δ): 10.45 (s, 2H, Ar-NH-), 7.98 (2H ArH), 7.66 (m, 4H, ArH), 7.54 (m, 6H, ArH), 7.25 (m, 2H, ArH), 7.14 (s, 2H, dCH-). For this synthesized PAA precursor, an inherent viscosity measurement was performed and found to be 0.95 dL/g at a concentration of 0.1 g/dL in NMP at 25.0 °C. Film Preparation. The obtained ODPA-Stilbene PAA solution was diluted to 2% (w/v) with dried NMP and filtered through a PTFE membrane of pore size 0.20 µm before use, and then the filtered PAA solution was spin-coated onto NaCl windows for transmittance FTIR spectra, silicone substrates for AFM images, and indium tin oxide (ITO) glass substrates for optical retardations and LC cell assemblies, followed by drying on a hot plate at 80 °C for 1 h. The dried PAA films were thermally imidized in an oven with a dry-nitrogen gas flow by a three-step imidization protocol: 150 °C/60 min, 200 °C/60 min, and 250 °C/120 min with a ramping rate of 2.0 °C/min. After the thermal imidization, the samples were cooled to room temperature at a rate of 10 °C/min. The resulting PI films were measured to have a thickness of around 100 nm, using a

4902 J. Phys. Chem. B, Vol. 112, No. 16, 2008 spectroscopic ellipsometer (model M2000, J. A. Woollam Inc.) and an R-stepper (model Tektak3, Veeco Company). Some of the PI films were exposed to UV light using a high-pressure Hg lamp system (1.0 kW, Altech Inc.) equipped with an optical filter (Milles Griot Company), which transmits a band beam of 260-380 nm wavelength. For LPUVL exposures, a linear dichroic polarizer (Oriel Company) was used. Some other PSPI films on substrates were rubbed using a laboratory rubbing machine (Wande Inc.) with the roller covered by rayon velvet fabrics (model YA-20-R, Yoshigawa Inc.). The rubbing strength parameter L was varied by changing the cumulative rubbing time for a constant rubbing depth of 0.15 mm, L ) Nl[(2πrn/60V) - 1], where L is the total length of the rubbing cloth which contacts a certain point of the polymer film (mm), l is the contact length of the circumference of the rubbing roller (mm), N is the cumulative number of rubbings, n and r are the speed (rpm) and the radius (cm) of the rubbing roller, respectively, and V is the velocity (cm/s) of the substrate stage.20 In addition, for thermal analysis, thick films of the polymers were additionally prepared on precleaned glass slides by casting and subsequent drying in a vacuum oven at 100 °C for 2 days. LC Cell Preparation. Some of the LPUVL-irradiated and rubbed films on glass substrates were cut into 2.5 × 2.5 cm pieces. Then, two different kinds of LC cells were prepared as follows. First, paired pieces from a same glass substrate were assembled together using 50 µm thick spacers, aligning the direction parallel to the polarization of the LPUVL used in the exposure and the direction antiparallel to the rubbing direction. A nematic LC, 4′-pentyl-4-biphenylcarbonitrile (5CB, Aldrich) 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 an epoxy glue, giving parallel nematic LC cells with the LPUVL-irradiated film or antiparallel nematic LC cells with the rubbed film. Second, paired pieces from the same glass substrate were assembled together orthogonally with respect to the rubbing direction and the polarization of the LPUVL used in the exposure by using silica balls with a diameter of 4.0 µm as spacers, injected with LC (5CB), and then sealed with an epoxy glue, giving 90°-twisted nematic LC cells (TN cells).20 The prepared LC cells were determined to be homogeneous throughout the cell by optical microscopy. Measurements. Optical phase retardations were measured using an optical setup equipped with either a photoelastic modulator (model PEM90, Hinds Instruments Company) with a fused silica head or a quarter plate (Oriel). The optical phase retardation measurements were calibrated with a λ/30 plate standard (Wave plate zero order λ/30 (λ ) 632.8 nm), Altechna Inc.); λ is the wavelength of a laser light source. Samples were installed perpendicular to the incident beam direction. Optical phase retardations were measured as a function of the angle of rotation of the samples. Transmitted FTIR spectroscopic measurements were carried out on a Bomem DA8 FTIR spectrometer equipped with a polarizer (single diamond polarizer, Harrick Scientific Inc.). The samples were installed perpendicular to the incident beam direction. While rotating the polarizer, 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 256 interferograms were accumulated. Two-dimensional (2D) correlation analyses were performed using an algorithm based on the numerical method developed by Noda.21 The 2D correlation analyses were carried out after baseline correction of the FTIR spectra. A subroutine

Hahm et al. KG2D22 composed in Array Basic language (GRAMS/386; Galactic Inc.) was employed in the 2D correlation analyses. 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 the polar diagrams. In the measurements, the LC cell was installed perpendicular to the incident laser beam direction. The pretilt angle of the LC molecules was measured using a crystal rotation apparatus which was made in our laboratory.20 For the TN LC cells, the azimuthal anchoring energy was measured by using a Sinco UV-visible spectrophotometer equipped with two Glan-Laser prisms; the analyzer was mounted on a motorized goniometer (model SKIDS-PH, Sigma Koki Inc.). Each TN cell was placed between the polarizer and the analyzer. UV-visible spectra were recorded at 0.8 cm-1 resolution as a function of the angle of rotation of the analyzer in the range of 0-180°. In these measurements, the rotation angles giving a minimum transmittance in the UV-visible spectra were determined. The azimuthal anchoring energies of the LC molecules on the rubbed or LPUVL-irradiated film surfaces were estimated from the twist angle using the optical parameters of the LC.20 Results and Discussion PSPI Synthesis and Thin Film Formation. 4,4′-Diaminostilbene, a new stilbene diamine monomer, was synthesized, and its polycondensation reaction with ODPA in NMP produced soluble ODPA-Stilbene PAA (Figure 1). The synthesized ODPA-Stilbene PAA precursor was determined to have an inherent viscosity of 0.95 dL/g at a concentration of 0.1 g/dL in NMP at 25.0 °C. Thin films of the obtained ODPA-Stilbene PAA were prepared by means of a conventional solution spincasting and subsequent drying process. The PAA films were further imidized through a thermal process (150 °C/60 min, 200 °C/60 min, and 250 °C/120 min with a ramping rate of 2.0 °C/ min) in a nitrogen atmosphere and subsequent cooling, producing good-quality thin films of ODPA-Stilbene PSPI. The thermal properties of the PSPI were measured in a nitrogen atmosphere. The PSPI film exhibited a degradation temperature, Td ) 280 °C. The glass transition temperature Tg could not be detected in differential scanning calorimetry measurements over a temperature range of 1.4 J/cm2, the polymer chains have been oriented parallel to the polarization direction of the used LPUVL in the azimuthal plane. In conclusion, the LPUVL exposure of the PSPI films at high exposure doses of >1.4 J/cm2 has switched the perpendicular polymer chain orientation achieved by the exposures at low exposure doses of 1.4 J/cm2, the film showed a parallel chain orientation rather than the perpendicular or random chain orientation (Figures 5 and 10). These results suggest that in the PSPI film, multisite photoisomerizations take place significantly at higher exposure doses of >1.4 J/cm2, causing the paralleloriented polymer chains (or segments) (Figure 11) to override the perpendicular-oriented polymer chains (or segments) in

4908 J. Phys. Chem. B, Vol. 112, No. 16, 2008

Figure 11. Schematic diagram of molecular segmental orientations and overall orientation directors of a single ODPA-Stilbene PSPI chain induced by the trans-cis photoisomerization reactions of stilbene chromophore moieties at two different LPUVL exposure doses, 0.5 and 2.0 J/cm2.

population. The maximum population of such parallel-oriented polymer chains (or segments) was achieved in the film irradiated with LPUVL at 2.0 J/cm2 (Figures 5 and 10). Sequence of Photoinduced Molecular Orientation. In the previous section, we explored the molecular orientation of the ODPA-Stilbene PSPI film induced by LPUVL irradiation by use of the results of the UV-visible and IR dichroism, optical retardation, and birefringence analyses. However, we could not determine the sequence of orientation of the polymer segments in the PSPI film by the irradiating process with LPUVL. To examine the orientation of the segments of the PSPI during LPUVL irradiation, FTIR spectra in Figure 4 were further analyzed by the 2D correlation spectroscopy. Figure 12 shows the synchronous and asynchronous 2D FTIR correlation spectra of the ODPA-Stilbene PSPI film with the exposure dose varying in the range of 0-5 J/cm2 in the region of 1800-700 cm-1. A power spectrum extracted along the diagonal line of the synchronous 2D correlation spectrum is also shown at the top of the Figure 12a. As shown in the figure, autopeaks at 1608 cm-1 and those at 1718, 1515, and 1367 cm-1 originate from the bands assigned to the vibrational motion of the stilbene chromophores and the ODPA units, respectively. These autopeaks suggest that the photoreaction induces the local orientation motion of both the stilbene chromophores and the OPDA units. Further, at (1718, 1515), (1718, 1367), and (1515, 1367) cm-1,

Hahm et al.

Figure 12. (a) Synchronous and (b) asynchronous 2D correlation spectra in the region of 1800-700 cm-1 generated from the IR spectra of an ODPA-Stilbene PSPI film irradiated with unpolarized UV light with varying exposure energy. Solid (blue) and dashed (red) lines indicate positive and negative cross peaks, respectively.

there are positive cross peaks between the bands of the OPDA units, while the band at 1608 cm-1 assigned to the phenyl ring vibration in the stilbene chromophores has negative cross peaks between the bands of the OPDA units (1515 and 1367 cm-1). These observations propose that the segmental motions of the OPDA units are directly correlated with one another, whereas the band of the stilbene chromophores is not correlated with the OPDA units. The analysis of the asynchronous 2D correlation spectra in Figure 12b shows the following sequence of orientations of the molecular segments in the PSPI backbone: 962 cm-1 (transvinylene C-H deformation in the stilbene units) f 1608 cm-1 (phenyl ring stretching in the stilbene units) f 1718 cm-1 (asymmetric CdO stretching in the OPDA units) f 1515 cm-1 (phenyl ring stretching in the OPDA units) f 1367 cm-1 (N-C stretching in the ODPA units) f 772 cm-1 (cis-vinylene C-H deformation in the stilbene units). These correlation analyses inform one that the stilbene chromophores change more rapidly than the ODPA units and trans-cis photoisomerization processes take place in the stilbene chromophores irradiated by UV light. Moreover, this result confirms that the photoreaction induces the molecular orientation of photosensitive stilbene chromophores as well as their adjacent units in the ODPAStilbene PSPI film.

UV-Driven Switching in Nanoscale Thin Films

J. Phys. Chem. B, Vol. 112, No. 16, 2008 4909

Figure 13. Polar diagrams of the absorbance measured from parallel LC cells assembled with LPUVL-irradiated ODPA-Stilbene PSPI films at various exposure doses, as a function of the angle of the rotation of the LC cells.

LC Alignment. Parallel LC cells with LPUVL-irradiated films and antiparallel LC cells with rubbed films were prepared. All LC cells were found to be very stable and homogeneous through the whole cell. These LC cells were used to measure LC alignment in the cell. The results are presented in Figures 8b and 13. Figure 8b shows the polar diagram of variations of the absorbance with the angle of rotation of antiparallel LC cells fabricated with the PSPI films rubbed at a rubbing strength parameter of 129.6 cm. As is clear from the figure, the LC cell exhibits a maximum absorbance along the direction 0° T 180°, which lies parallel to the rubbing direction. This result indicates that the LC molecules in contact with the rubbed film surfaces are induced homogeneously to align parallel to the rubbing direction. In combination with the polymer chain orientation results described above, this polar diagram shows that the LC molecules are induced to align parallel to the polymer chain orientation. Figure 13a displays shows the polar diagram of the absorbance with the angle of rotation of parallel LC cells fabricated with the PSPI films exposed to LPUVL at an exposure dose of 0.1 J/cm2. As can be seen in the figure, the LC cell exhibits a maximum absorbance along a direction at an angle of 90° T 270°, which is quite different from that observed in the LC cell fabricated with rubbed films. The LC cells prepared with the irradiated films at 0.5 and 1.0 J/cm2 also reveal polar diagrams of the absorbance (Figure 13b and c), whose anisotropic directors are coincident with that of the LC cell fabricated with the irradiated film at 0.1 J/cm2. For these polar diagrams,

the direction of the anisotropy is perpendicular to the polarization direction of the used LPUVL but coincident with the orientation direction of the polymer chains (Figure 9a-c). Therefore, these polar diagram results indicate that the polymer chains preferentially oriented in the films by the LPUVL exposures of 0.1-1.0 J/cm2 successfully induce the homogeneous LC alignment along their orientation direction in contact with the film surface. In contrast, the LC cells, which were fabricated with PSPI films exposed to LPUVL at 2.0-5.0 J/cm2, exhibit a maximum absorbance along a direction at an angle of 0° T 180° (Figure 13d-f), which is parallel to the polarization direction of the used LPUVL. Similar polar diagram of the absorbance were observed for the LC cells fabricated with the irradiated films at 7.0 and 10.0 J/cm2. For these polar diagrams, the director of the anisotropy is further coincident with the orientation direction of the polymer chains induced in the films by LPUVL irradiations at the same exposure doses (Figure 9d-f). Taking these results into account, the measured polar diagrams inform one that the polymer chains preferentially oriented in the films by the LPUVL exposures at g2.0 J/cm2 also successfully induce the homogeneous LC alignment along their orientation direction in contact with the film surface. Taking the observed LC alignments into account, the pretilt angle of the LCs in the LC cells was measured by using the crystal-rotation method. The measured LC pretilt angle is 0.05° for the rubbed PSPI film. On the LPUVL-exposed film surfaces, the LC pretilt angle is 0.03°, regardless of the exposure doses. Overall, all of the PSPI films induce LC molecules to align

4910 J. Phys. Chem. B, Vol. 112, No. 16, 2008 with low pretilt angles in the rubbed film surfaces as well as in the LPUVL-exposed film surfaces. In particular, this low pretilt angle of LC molecules is suitable for the production of advanced LC display devices with an in-plane switching mode that requires as low as possible LC pretilt angles. LC Anchoring Energy. With the observed LC alignment results, 90°-twisted nematic (TN) LC cells were prepared. All of the prepared TN LC cells were very stable and homogeneous through the whole cell. These TN LC cells were used in measurements of the twist angle of the LC molecules using a UV-visible spectroscopic technique reported previously.20 The twist angle was measured to be 80° for the cells with the PSPI films irradiated with LPUVL at 0.5 J/cm2 and 75° for the cells with the films irradiated with LPUVL at 2.0 J/cm2. From the measured twist angles, the azimuthal anchoring energy of LC molecules was determined to be 1.18 × 10-5 J/m2 for the film irradiated with LPUVL at 0.5 J/cm2 and 0.83 × 10-5 J/m2 for the film irradiated with LPUVL at 2.0 J/cm2. In the same way, twist angle measurements were conducted for the TN cells fabricated with PSPI films rubbed at a rubbing strength of 129.6 cm. The twist angle was measured to be 84° for the cell with the ODPA-Stilbene PSPI film. The determined azimuthal anchoring energy of LC molecules was 2.80 × 10-5 J/m2 for the rubbed film. As compared above, the film exposed to 0.5 J/cm2 exhibits a relatively larger LC anchoring energy than that of the film exposed to 2.0 J/cm2. Taking into account the dichroic ratio and in-plane birefringence data (Figures 5 and 10), the LPUVLirradiated film revealing a higher dichroic ratio and in-plane birefringence (i.e., higher magnitude, regardless of its sign) has a larger LC anchoring energy. As discussed above, the in-plane birefringence, as well as the dichroic ratio in the present study, is a measure of the preferential orientation of the PSPI polymer chains in the film. Taking these facts into account, the measured LC anchoring energy data indicate that more preferentially oriented polymer chains interact more strongly with LC molecules in contact with the film surface. In particular, the trans-stilbene unit is a mesogen group, which has a more similar configuration to the LC molecule in the most stable energy state. Indeed, this configuration of the unreacted trans-stilbene units preferentially oriented at the film surface can make favorable interactions with LC molecules’ mesogen groups in contact with the PSPI film via π-π interactions. On the other hand, the LC anchoring energy is relatively higher at the rubbed film surface than that at the LPUVLexposed film surface, as listed above. This result suggests that the rubbing process is more effective in orienting the polymer chains at the film surface than the photoalignment process; of course, the microgrooves, which were generated parallel to the orientation direction of the polymer chains, positively contribute in part to the high anchoring energy of the LCs. Surprisingly, the anchoring energy difference, however, is very small when it is considered that the LPUVL-irradiated PSPI films do not have a microgroove texture at the surface, compared to that of the rubbed film. Thus, it is worth examining the chemical structures of the PSPI and the surface morphology and then considering their possible interactions with LC molecules. The used LC molecule, 4′-pentyl-4-biphenylcarbonitrile (5CB), is ∼1.8 nm in length and ∼0.25 nm in diameter. This molecular dimension is comparable to that of the chemical repeat unit of the PSPI polymer backbone including the stilbene chromophore groups (Figure 1). In contrast, the microgrooves generated by the rubbing process have much larger dimensions than that of the LC molecules. The polymer chain components are reason-

Hahm et al. ably well matched with the LCs in size and thus can more effectively interact with the LC molecules, compared to the microgrooves that are mismatched significantly with the LC molecules in size. The observed alignment and relatively high anchoring energy of LC molecules in the LC cells fabricated with the LPUVL-irradiated PSPI films collectively support that the LC molecules are more likely to be aligned by favorable anisotropic interactions with the oriented polymer main chains and their trans-stilbene chromophore groups rather than by the microgrooves. The 5CB LC molecule is composed of biphenyl mesogen, an n-pentyl tail at one end, and a cyano group at the other end. Therefore, the LC molecules in contact with the rubbed and LPUVL-exposed PSPI film surfaces undergo favorable anisotropic interactions with the oriented polymer chains as follows. The aromatic mesogen might favorably interact with the phenyl and trans-stilbene components of the PSPI via π-π interactions, the polar cyano end group might interact with the imide rings in the backbone via polar-polar interactions, and the nonpolar n-pentyl tail may interact with the other components of the PSPI via van der Waals type interactions. Only in the case of a weak interaction between LC molecules and a polymer film may the LC alignment be dominantly governed by the microgrooves generated by the rubbing process. Conclusions The soluble PAA precursor of a novel photosensitive polyimide, ODPA-Stilbene PSPI was newly synthesized and then easily fabricated by simple spin-coating on substrates and subsequent drying and thermal imidization process, producing good-quality PSPI thin films with a smooth surface. The PSPI thin films with and without LPUVL (or unpolarized UV light) and rubbing were characterized in detail by UV-visible spectroscopy, FTIR spectroscopy, and 2D correlation analysis, dissolution analysis, and optical retardation analysis. Using the films, LC cells were fabricated and analyzed. These analyses provided important features about the novel ODPA-Stilbene PSPI material as follows. First, the ODPA-Stilbene PSPI is positively birefringent and reveals Td ) 280 °C but no Tg over the temperature range of