UV-Driven Switching of Chain Orientation and Liquid Crystal Alignment in Nanoscale Thin Films of a Novel Polyimide Bearing Stilbene Moieties in the Backbone

UV-Driven Switching of Chain Orientation and Liquid Crystal Alignment in NanoscaleThin 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 VibrationalSpectroscopy, Center for Integrated Molecular Systems, and BK School of Molecular Science, PohangUniVersity of Science and Technology, Pohang 790-784, Republic of Korea, School of Display & ChemicalEngineering, 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) wasnewly synthesized. The most surprising feature of this PSPI is that the PSPI films irradiated with linearpolarized 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 aperpendicular direction to a parallel direction with respect to the polarization direction of LPUVL by simplycontrolling the exposure dose in the irradiation process. These LPUVL-irradiated films were found to providehigh anchoring energy to LCs, always giving very stable, homogeneous cells with unidirectionally alignedLCs regardless of the LC alignment directions. In the films, the PSPI polymer chains were found to undergofavorably unidirectional orientation via a specific orientation sequence of the polymer chain segments led bythe directionally selective trans-cis photoisomerization of the stilbene chromophore units in the backboneinduced by LPUVL exposure. Such unidirectionally oriented polymer chains of the films induce alignmentof the LCs along the orientation direction of the polymer chains via favorable anisotropic molecular interactionsbetween the oriented polymer chain segments and the LC molecules. In addition, the PSPI has an excellentfilm formation processibility; good quality PSPI thin films with a smooth surface are easily produced bysimple spin-coating of the soluble poly(amic acid) precursor and subsequent thermal imidization process. Insummary, this new PSPI is the promising LC alignment layer candidate with rubbing-free processing for theproduction 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 practicalapplicability to produce rubbing-free LC alignment layers thatare key materials in the fabrication of LC display devices.1-6

The photoalignment control of nematic liquid crystals (LCs) isbased on the photochemically induced structural and orienta-tional change of molecules or residues localized at the uppermostsurfaces of substrate plates, which have been provided byappropriate combinations of molecular or polymeric films withsuitable photoactive molecules. Whereas molecular films in-corporating photosensitive moieties have been employed so farto achieve photoalignment control, more extensive studies havebeen carried out on thin films of polymers having photoreactiveunits because of the good availability of thin films by the spin-coating technique. With respect to photosensitive units leadingto LC alignment by linearly polarized light irradiation, threecategories of photochemistry have been investigated. The firstconsists of the photoisomerization of azobenzenes,7-11 spiro-

pyrans,12 and stilbenes.13 The second class displays [2+ 2]photodimerization of cinnamates,1,4,14-16 benzylidenephthalim-idines,17 benzylideneacetophenones (i.e., chalcones),18 and cou-marins.2,5 The third is the photodegradation of the imide groupsof polyimides.19 For the first two approaches, several photo-sensitive polymers have been reported so far, but most of themdeveloped on the basis of the polyvinyl backbone whoseproperties and process conditions are not suitable to thefabrication of LC display devices. The third approach has beendeveloped with conventional PIs, which are currently used orconsidered for the fabrication of LC display devices but werefound to degrade with high exposure doses of ultraviolet (UV)light and to indeed take a long time to process. Therefore, thechallenge remains to deliver high-performance polymers suitablefor 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) (ODPA-Stilbene PSPI) (Figure 1) and characterized in detail itsphotochemistry and photoreaction mechanism, photoinducedmolecular orientation and orientation sequence, LC alignability,and LC anchoring energy in nanoscale thin solid films. Thepresent study demonstrated that the ODPA-Stilbene PSPI in thinfilms exhibits excellent processibility and properties as arubbing-free processing LC alignment layer material suitablefor advanced LC display devices including LC display tele-visions with large display areas as follows. A good quality of

* To whom correspondence should be addressed. Tel:+82-54-279-2120.E-mail: [email protected] (M.R.); Tel:+82-54-279-2106. E-mail: [email protected] (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.

4900 J. Phys. Chem. B2008,112,4900-4912

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

ODPA-Stilbene PSPI thin films with a smooth surface and highthermal stability can easily be produced by simple spin-coatingof the soluble poly(amic acid) (PAA) precursor in solution andsubsequent drying and a thermal imidization process. The PSPIexhibits excellent photoreactivity to UV light via the trans-cisphotoisomerization of the stilbene chromophore units in thebackbone and further excellent photoinduced unidirectionalchain orientation ability via a specific orientation sequence ofthe polymer chain segments led by linearly polarized UV light(LPUVL) exposure. The preferentially oriented PSPI chainsunidirectionally align LC molecules with high anchoring energyalong their orientation direction. The most interesting featureof the ODPA-Stilbene PSPI is that the PSPI films can switcheasily the director of the unidirectionally aligned LC moleculesin contact with the film surface from a perpendicular directionto a parallel direction with respect to the polarization directionof LPUVL by simply controlling the exposure dose in theLPUVL irradiation process of the film without any changes ofits polarization director.

Experimental Section

Material and PSPI Synthesis.4,4′-Oxidiphthalic anhydride(ODPA) was supplied from Chriskev Company and purifiedby recrystallization from acetic anhydride.N,N′-dimethylform-amide (DMF) and N-methyl-2-pyrrolidinone (NMP) werepurchased from Aldrich Company and distilled over calcium

hydride under reduced pressure and under a nitrogen atmo-sphere, respectively. All other chemical compounds weresupplied from Aldrich and used without further purification.4-Nitrobenzyl bromide, 4-nitrobenzaldehyde, ethyl acetate, anddiethyl ether were purchased from Aldrich Company and usedwithout purification.

4-Nitrobenzyl bromide (5.00 g, 23.1 mmol) and triphenyl-phosphine (TPP: 6.67 g, 25.5 mmol) were dissolved in driedDMF (100 mL), and the reaction mixture was gently heated to70 °C under stirring for 24 h. Thereafter, the reaction solutionwas poured into diethyl ether under vigorous stirring, giving(4-nitrobenzyl)triphenylphosphonium bromide as a white pow-der. The precipitate was filtered, washed with diethyl ether, anddried 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, 10.5mmol) with 4-nitrobenzaldehyde (1.58 g, 10.5 mmol) in dryDMF (100 mL) containing sodium ethoxide (0.78 g, 11.5 mmol)were stirred at ambient temperature for 24 h. After stirring, thereaction solution was filtered and the solvent removed by rotaryevaporation to give a pale-yellow powder. The crude productwas recrystallized from hot ethanol to give 4,4′-dinitrostilbenewith 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 reactionmixture was stirred for 1 h at room temperature, followed byrefluxing for 6 h. Thereafter, the reaction solution was pouredinto ice and adjusted to pH 8 with sodium hydroxide solution.After extraction with ethyl acetate four times, the combinedorganic layer was dried over MgSO4, followed by evaporationto obtain 4,4′-diaminostilbene. The diamine was purified topolymerization 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 byadding the equivalent mole of ODPA into the 4,4′-diamino-stilbene dissolved in dried NMP under nitrogen by stirringvigorously (Figure 1). Once the ODPA addition was complete,the reaction flask was sealed tightly, and stirring was continuedfor 24 h to make the polymerization mixture homogeneous andviscous.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 PAAprecursor, an inherent viscosity measurement was performedand found to be 0.95 dL/g at a concentration of 0.1 g/dL inNMP at 25.0°C.

Film Preparation. The obtained ODPA-Stilbene PAA solu-tion was diluted to 2% (w/v) with dried NMP and filteredthrough a PTFE membrane of pore size 0.20µm before use,and then the filtered PAA solution was spin-coated onto NaClwindows for transmittance FTIR spectra, silicone substrates forAFM images, and indium tin oxide (ITO) glass substrates foroptical retardations and LC cell assemblies, followed by dryingon a hot plate at 80°C for 1 h. The dried PAA films werethermally imidized in an oven with a dry-nitrogen gas flow bya three-step imidization protocol: 150°C/60 min, 200°C/60min, and 250°C/120 min with a ramping rate of 2.0°C/min.After the thermal imidization, the samples were cooled to roomtemperature at a rate of 10°C/min. The resulting PI films weremeasured to have a thickness of around 100 nm, using a

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

UV-Driven Switching in Nanoscale Thin Films J. Phys. Chem. B, Vol. 112, No. 16, 20084901

spectroscopic ellipsometer (model M2000, J. A. Woollam Inc.)and anR-stepper (model Tektak3, Veeco Company). Some ofthe PI films were exposed to UV light using a high-pressureHg lamp system (1.0 kW, Altech Inc.) equipped with an opticalfilter (Milles Griot Company), which transmits a band beam of260-380 nm wavelength. For LPUVL exposures, a lineardichroic polarizer (Oriel Company) was used. Some other PSPIfilms on substrates were rubbed using a laboratory rubbingmachine (Wande Inc.) with the roller covered by rayon velvetfabrics (model YA-20-R, Yoshigawa Inc.).

The rubbing strength parameterL was varied by changingthe cumulative rubbing time for a constant rubbing depth of0.15 mm,L ) Nl[(2πrn/60V) - 1], whereL is the total lengthof the rubbing cloth which contacts a certain point of thepolymer film (mm),l is the contact length of the circumferenceof the rubbing roller (mm),N is the cumulative number ofrubbings,n and r are the speed (rpm) and the radius (cm) ofthe rubbing roller, respectively, andV is the velocity (cm/s) ofthe substrate stage.20 In addition, for thermal analysis, thick filmsof the polymers were additionally prepared on precleaned glassslides by casting and subsequent drying in a vacuum oven at100 °C for 2 days.

LC Cell Preparation. Some of the LPUVL-irradiated andrubbed films on glass substrates were cut into 2.5× 2.5 cmpieces. Then, two different kinds of LC cells were prepared asfollows. First, paired pieces from a same glass substrate wereassembled together using 50µm thick spacers, aligning thedirection parallel to the polarization of the LPUVL used in theexposure 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 dichroicdye was injected into the cell gap, followed by sealing of theinjection hole with an epoxy glue, giving parallel nematic LCcells with the LPUVL-irradiated film or antiparallel nematicLC cells with the rubbed film. Second, paired pieces from thesame glass substrate were assembled together orthogonally withrespect to the rubbing direction and the polarization of theLPUVL used in the exposure by using silica balls with adiameter of 4.0µm as spacers, injected with LC (5CB), andthen sealed with an epoxy glue, giving 90°-twisted nematic LCcells (TN cells).20 The prepared LC cells were determined tobe homogeneous throughout the cell by optical microscopy.

Measurements.Optical phase retardations were measuredusing an optical setup equipped with either a photoelasticmodulator (model PEM90, Hinds Instruments Company) witha fused silica head or a quarter plate (Oriel). The optical phaseretardation measurements were calibrated with aλ/30 platestandard (Wave plate zero orderλ/30 (λ ) 632.8 nm), AltechnaInc.); λ is the wavelength of a laser light source. Samples wereinstalled perpendicular to the incident beam direction. Opticalphase retardations were measured as a function of the angle ofrotation of the samples. Transmitted FTIR spectroscopic mea-surements were carried out on a Bomem DA8 FTIR spectrom-eter equipped with a polarizer (single diamond polarizer, HarrickScientific Inc.). The samples were installed perpendicular to theincident beam direction. While rotating the polarizer, IR spectrawere recorded at 4 cm-1 resolution with a liquid-nitrogen-cooledmercury cadmium telluride (MCT) detector under vacuum as afunction of the angle of rotation, and 256 interferograms wereaccumulated. Two-dimensional (2D) correlation analyses wereperformed using an algorithm based on the numerical methoddeveloped by Noda.21 The 2D correlation analyses were carriedout after baseline correction of the FTIR spectra. A subroutine

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 measuringthe absorption of the linearly polarized He-Ne laser beam(632.8 nm wavelength) as a function of the rotational angle ofthe cell, allowing the construction of the polar diagrams. In themeasurements, the LC cell was installed perpendicular to theincident laser beam direction. The pretilt angle of the LCmolecules was measured using a crystal rotation apparatus whichwas made in our laboratory.20

For the TN LC cells, the azimuthal anchoring energy wasmeasured by using a Sinco UV-visible spectrophotometerequipped with two Glan-Laser prisms; the analyzer was mountedon a motorized goniometer (model SKIDS-PH, Sigma KokiInc.). Each TN cell was placed between the polarizer and theanalyzer. UV-visible spectra were recorded at 0.8 cm-1

resolution as a function of the angle of rotation of the analyzerin the range of 0-180°. In these measurements, the rotationangles giving a minimum transmittance in the UV-visiblespectra were determined. The azimuthal anchoring energies ofthe LC molecules on the rubbed or LPUVL-irradiated filmsurfaces were estimated from the twist angle using the opticalparameters of the LC.20

Results and Discussion

PSPI Synthesis and Thin Film Formation.4,4′-Diamino-stilbene, a new stilbene diamine monomer, was synthesized,and its polycondensation reaction with ODPA in NMP producedsoluble ODPA-Stilbene PAA (Figure 1). The synthesizedODPA-Stilbene PAA precursor was determined to have aninherent viscosity of 0.95 dL/g at a concentration of 0.1 g/dLin NMP at 25.0°C. Thin films of the obtained ODPA-StilbenePAA were prepared by means of a conventional solution spin-casting and subsequent drying process. The PAA films werefurther 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, produc-ing good-quality thin films of ODPA-Stilbene PSPI.

The thermal properties of the PSPI were measured in anitrogen atmosphere. The PSPI film exhibited a degradationtemperature,Td ) 280 °C. The glass transition temperatureTg

could not be detected in differential scanning calorimetrymeasurements over a temperature range of<Td ()280 °C),suggesting that theTg of ODPA-Stilbene PSPI is higher than280 °C.

Figure 2 shows the AFM images of ODPA-Stilbene PSPIfilms without and with LPUVL exposure and rubbing. Theuntreated film surface apparently reveals small hill structureswhose root-mean-square (rms) surface roughness is 0.74 nmover the area of 1.0× 1.0µm2 (Figure 2a). Similar AFM imagewas obtained for the PSPI film exposed to LPUVL; rms surfaceroughness was determined to be 0.80 nm over the area of 1.0× 1.0µm2 (Figure 2b). Overall, the PSPI films with and withoutLPUVL exposure show a relatively smooth surface. The AFMmeasurements were extended on rubbed PSPI films. A repre-sentative AFM image is shown in Figure 2c, which was takenfrom a PSPI film rubbed at a rubbing strength parameter of129.6 cm. As can be seen in the figure, the rubbed film showsquite different surface morphology from those of the corre-sponding unrubbed and LPUVL-exposed films. The rubbed filmreveals a surface texture consisting of gravel-like grooves onthe tens to hundreds of nanometers scale, which were developedalong the rubbing direction. The rubbed film reveals a 2.55 nmrms roughness along the direction perpendicular to the rubbing

4902 J. Phys. Chem. B, Vol. 112, No. 16, 2008 Hahm et al.

direction and a 1.99 nm rms roughness along the directionparallel to the rubbing direction; the overall rms roughness overthe surface area of 1.0× 1.0 µm2 ranges over 2.46 nm. Insummary, the PSPI films showed a good processibility to theLPUVL exposure and the rubbing process.

Photochemistry. ODPA-Stilbene PSPI has one stilbenemoiety in the backbone per repeat unit (Figure 1). The ODPA-Stilbene PSPI films with a thickness of around 100 nm wereexposed to unpolarized UV light at varying exposure doses andthen examined by FTIR and UV spectroscopy to investigatethe photoreactions and photoproducts of the polymer in detail.Figure 3 shows the UV absorption spectra of the ODPA-StilbenePSPI films irradiated with UV light at various exposure dosesin the range of 0-5 J/cm2. Before irradiation with unpolarizedUV light, the ODPA-Stilbene PSPI films exhibit absorptionmaxima at 330 nm ()λmax); this band is due to absorption bystilbene units in a trans configuration.23 When the ODPA-Stilbene PSPI film starts to be irradiated, the intensity of theband at 330 nm ()λmax) gradually decreases with increasingexposure dose. At the same time, as the irradiation doseincreases, a band gradually appears at about 280 nm, corre-sponding tocis-stilbene units, and an isobestic point is observedat about 290 nm indicating that the cis-trans isomerization ofthe stilbene units in the polymer film involves a single reactionpath.23

Figure 4 shows the FTIR spectra of the ODPA-Stilbene PSPIfilms irradiated with UV light at exposure doses in the rangeof 0-5 J/cm2. All of the vibrational bands observed in the

spectra can be assigned in accordance with previous results.24-26

The IR spectral data are summarized in Table 1 with assign-ments proposed from the literature and observations in thisstudy. The bands at 1775, 1718, 1515, and 1367 cm-1 areattributed to the symmetric and asymmetric CdO stretchingvibrations of the anhydride ring, the phenyl ring stretchingvibration of the ODPA units, and the N-C stretching vibrationof the imide bond, respectively. The band at 1608 cm-1 isassigned to the phenyl ring stretching vibration of the stilbeneunits. The vinylene CdC stretching vibration of the stilbeneunits is observed at about 1621 cm-1 as a shoulder. Anadditional band at 962 cm-1 corresponds to thetrans-vinyleneC-H deformation in the stilbene chromophores. In the conju-gated olefins such as the stilbene isomers, a difference has beennoted between compounds where the CdC and phenyl ring havea trans or cis configuration.24,25In trans-stilbene, the CdC andphenyl ring bands are further apart than those incis-stilbene.These vibrations give rise to strong Raman bands, while in IRspectra, some bands do not appear with sufficient intensities;therefore, we cannot resolve the characteristic vibrationalfrequencies of vinylene CdC and phenyl ring bands in stilbene

Figure 2. AFM images of ODPA-Stilbene PSPI films with and withoutLPUVL exposure or rubbing: (a) untreated film; (b) film irradiatedwith LPUVL at 0.5 J/cm2; (c) film rubbed at a rubbing strengthparameter of 129.6 cm. The arrow indicates the polarization directionof the used LPUVL or the rubbing direction.

Figure 3. UV absorption spectra of the ODPA-Stilbene PSPI filmsirradiated with unpolarized UV light with varying exposure doses.

Figure 4. FTIR spectra of the ODPA-Stilbene PSPI films irradiatedwith unpolarized UV light with varying exposure doses.

TABLE 1: Characteristic Infrared Bands of theODPA-Stilbene PSPI

frequency (cm-1) assignment descriptiona

1775 νs(CdO)OPDA

1718 νas(CdO)ODPA

1608 ν(C-C)stilbene

1515 ν(C-C)ODPA

1367 ν(C-N)ODPA

962 γ(C-H)stilbene

772 γ(C-H)stilbene

a Symbols used:ν, stretching;γ, out-of-plane bending.


isomers during photoreaction. As shown in Figure 4, theintensity of the band at 962 cm-1 due to thetrans-vinylene C-Hdeformational vibration in the stilbene chromophores decreasesas the exposure energy increases, while the band at 772 cm-1

increases during photoreaction. According to the report ofArenas et al.,26 the cis-vinylene C-H deformational vibrationin cis-stilbene appears at about 771 cm-1. Therefore, we expectthat thecis-vinylene C-H deformation vibration in the ODPA-Stilbene PSPI appears at around 770 cm-1, and the intensitychange of the band at 772 cm-1 due to the UV-exposures mightbe attributed to the trans-cis photoisomerization of the stilbenechromophore moieties.

The UV absorption and FTIR spectroscopic data thus leadto the qualitative conclusion that the irradiation of ODPA-Stilbene PSPI films with UV light causes trans-cis photo-isomerization of the stilbene chromophore moieties in thepolymer backbone.

Dissolution. The viscous solution of the PAA precursor ofODPA-Stilbene PSPI was coated on the glass substrates,followed by drying on a hot plate at 80°C for 1 h. The driedPAA films with a thickness of around 50µm were peeled fromthe glass substrates and then put into NMP solvent. The detachedPAA films turned out to be soluble in NMP solution. Takingthis fact into account, other dried PAA films were exposed tounpolarized UV light up to 5.0 and 10.0 J/cm2. Both of theexposed PAA films were detached and put into NMP solvent.Both of the films were found soluble in NMP.

By a dissolution test of the exposed PAA films at the certainenergy condition of UV light, it was revealed that the stilbeneunits of the ODPA-Stilbene PAA are just isomerized from thetrans to cis form rather than being dimerized.

In addition, the ODPA-Stilbene PAA films were thermallyimidized in an oven with a dry-nitrogen gas flow by a three-step imidization protocol: 150°C/60 min, 200°C/60 min, and250 °C/120 min with a ramping rate of 2.0°C/min. After thethermal imidization, the samples were cooled to room temper-ature at a rate of 10°C/min and detached from the glasssubstrate. The resulting ODPA-Stilbene PSPI films were testedto be insoluble in any solvent.

Photoinduced Molecular Orientation. To determine thephotoinduced molecular segment and chain orientation, wecarried out UV dichroic ratio, IR spectroscopic, and opticalretardation analysis for the ODPA-Stilbene PSPI films irradiatedwith LPUVL with various exposure doses.

The UV dichroic ratio [)(A⊥ - A||)/(A⊥ + A||)] (which is ameasure of the orientation of the unreactedtrans-stilbenechromophore) was determined from the absorbance oftrans-stilbene units at 330 nm measured using a UV-visible lightprobe that was linearly polarized perpendicular to the polariza-tion direction of the LPUVL (A⊥) and the absorbance oftrans-stilbene units at 330 nm measured using a UV-visible lightprobe that was linearly polarized parallel to the polarizationdirection of the LPUVL (A||). The measured dichroic ratio dataare shown in Figure 5. As can be seen in the figure, the dichroicratio sharply increases with positive sign as the exposure doseincreases, reaches its maximum value at an exposure dose of0.5 J/cm2, and then turns to rapidly decrease with furtherincreasing exposure dose up to around 1.4 J/cm2. Surprisingly,above 1.4 J/cm2, the dichroic ratio further turns to have negativesign and increases with the negative sign, reaching its maximumvalue at an exposure dose of 2.0 J/cm2, although the polarizationdirection of the LPUVL in the exposure process is unchanged.Then, the dichroic ratio keeping a negative sign again turns torapidly decrease with increasing exposure dose up to around

5.0 J/cm2 and continues to decrease gradually with furtherincreasing exposure dose.

These UV dichroic ratio studies demonstrate that the LPUVLexposure of the PSPI film can orient the unreactedtrans-stilbeneunits in a direction perpendicular or parallel to the polarizationdirection of the LPUVL via the photoisomerization of thedirectionally selectedtrans-stilbene units (which are locatedparallel to the polarization direction of the LPUVL) withcontrolling of the exposure dose. Namely, the LPUVL exposureof the film with low exposure dose (<1.4 J/cm2) can inducethe unreactedtrans-stilbene units to orient preferentially in adirection perpendicular to the polarization direction of theLPUVL, whereas the LPUVL exposure of the film with highexposure dose (>1.4 J/cm2) can cause the unreactedtrans-stilbene units to orient preferentially in a direction parallel tothe polarization direction of the LPUVL. Interestingly, theperpendicularly orientedtrans-stilbene units, which were achievedby LPUVL exposures with low exposure doses, can furtherswitch their orientation director to the parallel direction byLPUVL exposures with high exposure doses.

For the films irradiated with LPUVL at 0.5 and 2.0 J/cm2

(which reveal maximum dichroic ratio values with a positiveand negative sign, respectively), IR spectroscopic measurementswith a linearly polarized IR light probe source were conductedas a function of the angle of rotation of the film in order todetermine the extent of the orientation of polymer chainsoccurring during photoreaction. The measured peak intensitiesof selected IR bands are plotted in Figures 6 and 7 with respectto the angle of rotation of the film. Figure 6a shows the polardiagrams of the film irradiated with LPUVL at 0.5 J/cm2. Thephenyl ring stretching vibration in the stilbene unit at 1608 cm-1

(which is one of the quadrant stretching vibration pairs for thephenyl CdC bonds, and its dipole-transition moment is alongthe para direction of the phenyl ring) is more intense when theincident IR probe beam is polarized perpendicular to thedirection of the LPUVL. As described earlier, the phenyl ringstretching vibrations intrans- andcis-stilbene units cannot beresolved. However, the dipole-transition moment of the quadrantCdC stretching vibration is aligned more favorably parallel tothe long axis of thetrans-stilbene unit. Taking this fact intoaccount, the IR result indicates that the unreactedtrans-stilbeneunits that remained in the film as parts of the polymer backbonepreferentially oriented perpendicular to the polarization directionof the used LPUVL. Further, a vibrational band was detectedat 1515 cm-1, which is attributed to one of the semicirclestretching vibration pairs for the phenyl CdC bonds in theODPA units, and its dipole-transition moment is along the paradirection of the phenyl ring. This band is more intense when

Figure 5. Dichroic ratios measured at 330 nm for the ODPA-StilbenePSPI films irradiated with LPUVL with varying exposure doses.


the polarization of the incident IR probe beam is also perpen-dicular to the polarization direction of the LPUVL (Figure 6b).The bands at 1775 cm-1 assigned to a symmetric CdOstretching vibration in the ODPA units is also more intense whenthe polarization of the incident IR probe beam is perpendicularto the polarization direction of the LPUVL (Figure 6c). Thesevibrational bands’ dipole-transition moments are aligned favor-ably parallel to the long axis of the ODPA unit. Thus, theorientation directions of the dipole-transition moments of theseIR bands suggest that the OPDA units are relatively morealigned in a direction perpendicular to the polarization directionof the used LPUVL. These results suggest that the photoreaction(i.e., the trans-cis photoisomerization reaction) of the PSPI filmby LPUVL exposure at 0.5 J/cm2 induces the molecularorientation of the unreactedtrans-stilbene units as well as theODPA units, which are perpendicular to the polarizationdirection of the used LPUVL.

On the other hand, the polar diagrams of the PSPI filmirradiated with LPUVL at 2.0 J/cm2 showed the differentmolecular orientation from that of the film irradiated withLPUVL at 0.5 J/cm2. As shown in Figure 7, the phenyl ringstretching in the stilbene units (1608 cm-1) as well as the bondsassociated with the ODPA units (1515 and 1775 cm-1) are moreintense when the incident IR probe beam is polarized along thepolarization direction of the used LPUVL, although the intensitydifferences are small. These indicate that both the unreactedtrans-stilbene units and the OPDA units in the film are alignedalong the polarization direction of the used LPUVL.

These FTIR spectroscopic results again confirm the LPUVL-induced preferential orientations of unreactedtrans-stilbenechromophore units observed in the dichroic ratio measurements.

For the above LPUVL-irradiated films, optical phase retarda-tion analysis was further carried out. In addition, the opticalphase retardation analysis was extended for a film of the PSPIthat was rubbed at a rubbing strength parameter of 129.6 cm.The analysis results are presented in Figures 8a and 9. Figure8a shows the polar diagram of the optical phase retardation[)(in-plane birefringence)× (film thickness)] with the angleof rotation of the rubbed PSPI film. As can be seen in the figure,a maximum retardation arises for the direction 0° T 180°, whichis parallel to the rubbing direction, and a minimum retardationarises for the direction 90° T 270°, which is perpendicular tothe rubbing direction. In general, the rubbing process of polymerthin films is known to have an ability to induce the polymerbackbone as well as the side groups (or side chains) to orientpreferentially along the rubbing direction.20,27 Thus, this polardiagram result indicates that in the rubbed film the PSPI chainsare preferentially oriented along the rubbing direction. The polardiagram result further indicates that the ODPA-Stilbene PSPIis a positive birefringent polymer whose refractive index alongthe polymer backbone chain is higher than that orthogonal tothe polymer backbone. Figure 9 presents polar diagrams of theoptical phase retardation with the angle of rotation of theLPUVL-irradiated PSPI films. As can be seen in Figure 9a, thefilm irradiated with LPUVL at 0.1 J/cm2 shows a maximumretardation along the direction 90° T 270° (i.e., at an angle of90° with respect to the polarization direction (0° T 180°) of

Figure 6. Polar diagrams of the selected vibrational peaks of an ODPA-Stilbene PSPI film irradiated with LPUVL at 0.5 J/cm2, measured bylinearly polarized IR spectroscopy as a function of the angle of therotation of the film: (a) 1608 cm-1, (b) 1515 cm-1, and (c) 1775 cm-1.

Figure 7. Polar diagrams of the selected vibrational peaks of an ODPA-Stilbene PSPI film irradiated with LPUVL at 2.0 J/cm2, measured bylinearly polarized IR spectroscopy as a function of the angle of therotation of the film: (a) 1608 cm-1, (b) 1515 cm-1, and (c) 1775 cm-1.


the LPUVL) and a minimum retardation along the direction 0°T 180°. The films irradiated with LPUVL at 0.5 and 1.0 J/cm2

also reveal anisotropic polar diagrams of retardation (Figure 9band c), whose anisotropic direction is the same as that observedfor the film irradiated at 0.1 J/cm2. However, these anisotropicpolar diagrams are slightly different in shape from one another(Figure 9a-c), which is attributed to some differences betweenthe in-plane birefrigences of the films. Overall, these anisotropicpolar diagrams indicate that in the PSPI films irradiated withLPUVL at <1.4 J/cm2, the polymer chains have been orientedperpendicular to the polarization direction of the used LPUVLin the azimuthal plane.

In contrast, for all of the films irradiated at>1.4 J/cm2, amaximum retardation arises for the direction 0° T 180°, and aminimum retardation arises for the direction 90° T 270° (seeFigure 9d-f for the LPUVL-irradiated films at 2.0, 3.0, and5.0 J/cm2). Thus, these anisotropic polar diagrams collectivelyindicate that in the PSPI films irradiated at higher exposure dosesof >1.4 J/cm2, the polymer chains have been oriented parallelto the polarization direction of the used LPUVL in the azimuthalplane. In conclusion, the LPUVL exposure of the PSPI films athigh exposure doses of>1.4 J/cm2 has switched the perpen-dicular polymer chain orientation achieved by the exposures atlow exposure doses of<1.4 J/cm2 to the parallel polymer chainorientation.

These optical retardation results again confirm that thedirector of the unidirectionally oriented polymer chains coincideswith the orientation directors of the unreactedtrans-stilbene units

and the ODPA units determined for the same film samples inthe above dichroic ratio and IR spectroscopic measurements.

From the above polar diagram data of retardation, weextracted in-plane birefringence (∆) values for the LPUVL-irradiated PSPI films and plotted them with exposure doses inFigure 10: ∆ ) n⊥ - n||, wheren⊥ and n|| are the in-planerefractive indices in the perpendicular and parallel direction withrespect to the polarization direction of the used LPUVL,respectively. As can be seen in the figure, in the low exposuredose region of<1.4 J/cm2, the∆ value always reveals a positivesign and steeply increases with increasing exposure dose andthen turns to sharply decrease with further increasing exposuredose. However, in the high exposure dose region of>1.4 J/cm2,the ∆ value changes its sign negative and always keeps thenegative sign. Then, the negative∆ value increases steeply withincreasing exposure dose and turns to decrease and then levelsoff. As described above, the ODPA-Stilbene PSPI chain ispositively birefringent. Thus, the determined in-plane birefrin-gence∆ is a measure of the orientation of the PSPI polymerchains in the film. Taking these facts into account, thedetermined∆ value and its variations with exposure dosesclearly indicate that the LPUVL exposure of the ODPA-StilbenePSPI film induces favorably the unidirectional orientation ofthe polymer chains; however, the degree of such unidirectionalpolymer chain orientation and the chain orientation directionare strongly dependent on the exposure dose. Here, it noteworthythat the trend of the∆ variation with exposure dose is verymuch comparable with that observed in the dichroic ratiovariation with exposure dose (Figure 5). In fact, the stilbeneunits are parts of a single chain of the PSPI polymer. Indeed,the dichroic ratio, which monitors the preferential orientationof the unreactedtrans-stilbene units in the irradiated PSPI film,can give information about the orientation of the PSPI polymerchains.

Collectively, the above dichroic ratio, IR spectroscopy, opticalphase retardation, and birefringence analyses confirm that theLPUVL exposure of the PSPI films favorably induce aunidirectional orientation of polymer chains including theunreactedtrans-stilbene units in the film. Moreover, theseanalyses demonstrates for the first time that the LPUVLexposure can easily switch the director of the unidirectionallyoriented polymer chains in the PSPI film from a perpendiculardirection to a parallel direction with respect to the used LPUVLby only controlling the exposure dose without any changes ofits polarization director, which has never been reported so farfor other photoreactive polymer films.

The observed LPUVL-induced polymer orientation and itsdirectional switching behavior can be understood as follows.Consider a single chain of the ODPA-Stilbene PSPI havingmultiple stilbene units in the backbone (i.e., one stilbene unitper repeat unit of the polymer backbone) (Figure 1). Takinginto account the trans-cis photoisomerization characteristic ofthe PSPI film discussed earlier, thetrans-stilbene chromophoreunit can favorably undergo cis photoisomerization throughLPUVL exposure when the chromophore unit is located parallelto the polarization of the LPUVL and then causes the formationof a kink point at the corresponding cis photoisomerized stilbeneunit in the polymer backbone. Thus, the photoreaction of onetrans-stilbene chromophore moiety leads to orientation of thepolymer chain in a certain direction. This oriented polymer chaincan change its orientation direction continuously when anotherunreactedtrans-stilbene choromophore moiety undergoes photo-reaction. This kind of polymer chain orientation change keepsgoing as the other unreactedtrans-stilbene chromophore moi-

Figure 8. (a) Polar diagram of optical phase retardation [)(in-planebirefringence)× (film thickness)] measured from an ODPA-StilbenePSPI film rubbed with a rubbing strength parameter of 129.6 cm, as afunction of the angle of the rotation of the film. (b) Polar diagram ofabsorbance measured from an antiparallel LC cell assembled with therubbed PSPI film in (a), as a function of the angle of the rotation ofthe LC cell.


eties undergo photoreaction, depending on the exposure doseof LPUVL. Consequently, photoisomerizaton of these multisites(trans-stilbene chromophore moieties) in a single PSPI chainleads to orient the whole polymer chain and gives rise to thechange of the optical and spectroscopic anisotropy.

Taking these photoisomerization characteristics into account,LPUVL-induced photoisomerization of one to twotrans-stilbeneunits in a single ODPA-Stilbene PSPI chain may cause thepolymer chain to orient perpendicular to the polarization of theLPUVL (Figure 11). This perpendicular chain orientation canbe increased in population when ODPA-Stilbene PSPI chains

are involved more in such one- to two-site photoisomerizationswith increasing of the exposure dose of LPUVL. The one-siteto two-site photoisomerizations may take place favorably at lowexposure doses of LPUVL. Therefore, the observations that theperpendicular chain orientation in the PSPI film was caused toincrease initially with increasing of the exposure dose of LPUVLand reached its maximum value at 0.5 J/cm2 (Figures 5, 6, and10) might result from such one-site to two-site photoisomer-izations of the PSPI polymer chains. As described earlier, themaximum perpendicular chain orientation in the film turned todecrease with a further increase of the exposure dose of LPUVLup to around 1.4 J/cm2 (Figures 5 and 10). This result may bean indication that multisite photoisomerizations occurred on thePSPI polymer chains in the LPUVL-irradiated film (Figure 11)as an early stage of the photoreaction process; in the early stageof multisite photoisomerizations, a few oftrans-stilbene units(perhaps two to five units) of the polymer chain may participate.In particular, the film irradiated with LPUVL at 1.4 J/cm2

revealed random polymer chain orientation (Figures 5 and 10),which might be attributed to comparable amounts of theperpendicular- and parallel-oriented polymer chains (or polymersegments) formed in the irradiated film as a result of the earlystage of multisite photoisomerizations. When the film wasirradiated with LPUVL at>1.4 J/cm2, the film showed a parallelchain orientation rather than the perpendicular or random chainorientation (Figures 5 and 10). These results suggest that in thePSPI film, multisite photoisomerizations take place significantlyat higher exposure doses of>1.4 J/cm2, causing the parallel-oriented polymer chains (or segments) (Figure 11) to overridethe perpendicular-oriented polymer chains (or segments) in

Figure 9. Polar diagrams of optical phase retardation [)(in-plane birefringence)× (film thickness)] taken from optical phase retardation measurementsof ODPA-Stilbene PSPI films irradiated with LPUVL at various exposure doses as a function of the angle of the rotation of the films.

Figure 10. Variations of the in-plane birefringence∆ with exposuredose of LPUVL-irradiated ODPA-Stilbene PSPI films. Here∆ ) n⊥- n||, where n⊥ and n|| are the in-plane refractive indices in theperpendicular and parallel direction with respect to the polarizationdirection of the used LPUVL, respectively.


population. The maximum population of such parallel-orientedpolymer chains (or segments) was achieved in the film irradiatedwith LPUVL at 2.0 J/cm2 (Figures 5 and 10).

Sequence of Photoinduced Molecular Orientation.In theprevious section, we explored the molecular orientation of theODPA-Stilbene PSPI film induced by LPUVL irradiation byuse of the results of the UV-visible and IR dichroism, opticalretardation, and birefringence analyses. However, we could notdetermine the sequence of orientation of the polymer segmentsin the PSPI film by the irradiating process with LPUVL. Toexamine the orientation of the segments of the PSPI duringLPUVL irradiation, FTIR spectra in Figure 4 were furtheranalyzed by the 2D correlation spectroscopy. Figure 12 showsthe synchronous and asynchronous 2D FTIR correlation spectraof the ODPA-Stilbene PSPI film with the exposure dose varyingin the range of 0-5 J/cm2 in the region of 1800-700 cm-1.

A power spectrum extracted along the diagonal line of thesynchronous 2D correlation spectrum is also shown at the topof the Figure 12a. As shown in the figure, autopeaks at 1608cm-1 and those at 1718, 1515, and 1367 cm-1 originate fromthe bands assigned to the vibrational motion of the stilbenechromophores and the ODPA units, respectively. These auto-peaks suggest that the photoreaction induces the local orientationmotion of both the stilbene chromophores and the OPDA units.Further, at (1718, 1515), (1718, 1367), and (1515, 1367) cm-1,

there are positive cross peaks between the bands of the OPDAunits, while the band at 1608 cm-1 assigned to the phenyl ringvibration in the stilbene chromophores has negative cross peaksbetween the bands of the OPDA units (1515 and 1367 cm-1).These observations propose that the segmental motions of theOPDA units are directly correlated with one another, whereasthe band of the stilbene chromophores is not correlated withthe OPDA units.

The analysis of the asynchronous 2D correlation spectra inFigure 12b shows the following sequence of orientations of themolecular segments in the PSPI backbone: 962 cm-1 (trans-vinylene 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-Cstretching in the ODPA units)f 772 cm-1 (cis-vinylene C-Hdeformation in the stilbene units). These correlation analysesinform one that the stilbene chromophores change more rapidlythan the ODPA units and trans-cis photoisomerization pro-cesses take place in the stilbene chromophores irradiated byUV light. Moreover, this result confirms that the photoreactioninduces the molecular orientation of photosensitive stilbenechromophores as well as their adjacent units in the ODPA-Stilbene PSPI film.

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

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


LC Alignment. Parallel LC cells with LPUVL-irradiatedfilms and antiparallel LC cells with rubbed films were prepared.All LC cells were found to be very stable and homogeneousthrough the whole cell. These LC cells were used to measureLC alignment in the cell. The results are presented in Figures8b and 13.

Figure 8b shows the polar diagram of variations of theabsorbance with the angle of rotation of antiparallel LC cellsfabricated with the PSPI films rubbed at a rubbing strengthparameter of 129.6 cm. As is clear from the figure, the LC cellexhibits a maximum absorbance along the direction 0° T 180°,which lies parallel to the rubbing direction. This result indicatesthat the LC molecules in contact with the rubbed film surfacesare induced homogeneously to align parallel to the rubbingdirection. In combination with the polymer chain orientationresults described above, this polar diagram shows that the LCmolecules are induced to align parallel to the polymer chainorientation.

Figure 13a displays shows the polar diagram of the ab-sorbance with the angle of rotation of parallel LC cells fabricatedwith the PSPI films exposed to LPUVL at an exposure dose of0.1 J/cm2. As can be seen in the figure, the LC cell exhibits amaximum absorbance along a direction at an angle of90° T 270°, which is quite different from that observed in theLC cell fabricated with rubbed films. The LC cells preparedwith the irradiated films at 0.5 and 1.0 J/cm2 also reveal polardiagrams of the absorbance (Figure 13b and c), whose aniso-tropic directors are coincident with that of the LC cell fabricatedwith the irradiated film at 0.1 J/cm2. For these polar diagrams,

the direction of the anisotropy is perpendicular to the polariza-tion direction of the used LPUVL but coincident with theorientation direction of the polymer chains (Figure 9a-c).Therefore, these polar diagram results indicate that the polymerchains preferentially oriented in the films by the LPUVLexposures of 0.1-1.0 J/cm2 successfully induce the homoge-neous LC alignment along their orientation direction in contactwith the film surface.

In contrast, the LC cells, which were fabricated with PSPIfilms exposed to LPUVL at 2.0-5.0 J/cm2, exhibit a maximumabsorbance along a direction at an angle of 0° T 180° (Figure13d-f), which is parallel to the polarization direction of theused LPUVL. Similar polar diagram of the absorbance wereobserved for the LC cells fabricated with the irradiated films at7.0 and 10.0 J/cm2. For these polar diagrams, the director ofthe anisotropy is further coincident with the orientation directionof the polymer chains induced in the films by LPUVLirradiations at the same exposure doses (Figure 9d-f). Takingthese results into account, the measured polar diagrams informone that the polymer chains preferentially oriented in the filmsby the LPUVL exposures atg2.0 J/cm2 also successfully inducethe homogeneous LC alignment along their orientation directionin contact with the film surface.

Taking the observed LC alignments into account, the pretiltangle of the LCs in the LC cells was measured by using thecrystal-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

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


with low pretilt angles in the rubbed film surfaces as well as inthe LPUVL-exposed film surfaces. In particular, this low pretiltangle of LC molecules is suitable for the production of advancedLC display devices with an in-plane switching mode thatrequires as low as possible LC pretilt angles.

LC Anchoring Energy. With the observed LC alignmentresults, 90°-twisted nematic (TN) LC cells were prepared. Allof the prepared TN LC cells were very stable and homogeneousthrough the whole cell. These TN LC cells were used inmeasurements of the twist angle of the LC molecules using aUV-visible spectroscopic technique reported previously.20

The twist angle was measured to be 80° for the cells withthe PSPI films irradiated with LPUVL at 0.5 J/cm2 and 75° forthe cells with the films irradiated with LPUVL at 2.0 J/cm2.From the measured twist angles, the azimuthal anchoring energyof LC molecules was determined to be 1.18× 10-5 J/m2 forthe 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 rubbingstrength of 129.6 cm. The twist angle was measured to be 84°for the cell with the ODPA-Stilbene PSPI film. The determinedazimuthal 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 filmexposed to 2.0 J/cm2. Taking into account the dichroic ratioand in-plane birefringence data (Figures 5 and 10), the LPUVL-irradiated film revealing a higher dichroic ratio and in-planebirefringence (i.e., higher magnitude, regardless of its sign) hasa larger LC anchoring energy. As discussed above, the in-planebirefringence, as well as the dichroic ratio in the present study,is a measure of the preferential orientation of the PSPI polymerchains in the film. Taking these facts into account, the measuredLC anchoring energy data indicate that more preferentiallyoriented polymer chains interact more strongly with LCmolecules in contact with the film surface. In particular, thetrans-stilbene unit is a mesogen group, which has a more similarconfiguration to the LC molecule in the most stable energy state.Indeed, this configuration of the unreactedtrans-stilbene unitspreferentially oriented at the film surface can make favorableinteractions with LC molecules’ mesogen groups in contact withthe PSPI film viaπ-π interactions.

On the other hand, the LC anchoring energy is relativelyhigher at the rubbed film surface than that at the LPUVL-exposed film surface, as listed above. This result suggests thatthe rubbing process is more effective in orienting the polymerchains at the film surface than the photoalignment process; ofcourse, the microgrooves, which were generated parallel to theorientation direction of the polymer chains, positively contributein part to the high anchoring energy of the LCs. Surprisingly,the anchoring energy difference, however, is very small whenit is considered that the LPUVL-irradiated PSPI films do nothave a microgroove texture at the surface, compared to that ofthe rubbed film. Thus, it is worth examining the chemicalstructures of the PSPI and the surface morphology and thenconsidering their possible interactions with LC molecules. Theused LC molecule, 4′-pentyl-4-biphenylcarbonitrile (5CB), is∼1.8 nm in length and∼0.25 nm in diameter. This moleculardimension is comparable to that of the chemical repeat unit ofthe PSPI polymer backbone including the stilbene chromophoregroups (Figure 1). In contrast, the microgrooves generated bythe rubbing process have much larger dimensions than that ofthe LC molecules. The polymer chain components are reason-

ably well matched with the LCs in size and thus can moreeffectively interact with the LC molecules, compared to themicrogrooves that are mismatched significantly with the LCmolecules in size. The observed alignment and relatively highanchoring energy of LC molecules in the LC cells fabricatedwith the LPUVL-irradiated PSPI films collectively support thatthe LC molecules are more likely to be aligned by favorableanisotropic interactions with the oriented polymer main chainsand theirtrans-stilbene chromophore groups rather than by themicrogrooves. The 5CB LC molecule is composed of biphenylmesogen, ann-pentyl tail at one end, and a cyano group at theother end. Therefore, the LC molecules in contact with therubbed and LPUVL-exposed PSPI film surfaces undergofavorable anisotropic interactions with the oriented polymerchains as follows. The aromatic mesogen might favorablyinteract with the phenyl andtrans-stilbene components of thePSPI viaπ-π interactions, the polar cyano end group mightinteract with the imide rings in the backbone via polar-polarinteractions, and the nonpolarn-pentyl tail may interact withthe other components of the PSPI via van der Waals typeinteractions. Only in the case of a weak interaction betweenLC molecules and a polymer film may the LC alignment bedominantly governed by the microgrooves generated by therubbing process.

Conclusions

The soluble PAA precursor of a novel photosensitive poly-imide, ODPA-Stilbene PSPI was newly synthesized and theneasily fabricated by simple spin-coating on substrates andsubsequent drying and thermal imidization process, producinggood-quality PSPI thin films with a smooth surface. The PSPIthin films with and without LPUVL (or unpolarized UV light)and rubbing were characterized in detail by UV-visiblespectroscopy, FTIR spectroscopy, and 2D correlation analysis,dissolution analysis, and optical retardation analysis. Using thefilms, LC cells were fabricated and analyzed. These analysesprovided important features about the novel ODPA-StilbenePSPI material as follows.

First, the ODPA-Stilbene PSPI is positively birefringent andrevealsTd ) 280 °C but noTg over the temperature range of<280 °C, which are higher than those of the conventionalrubbing-type polyimides currently used in the LC displayindustry.

Second, the PSPI undergoes trans-cis photoisomerizationrather than photodimerization or a cross-link reaction whenirradiated by UV light, namely, thetrans-stilbene chromophoresin the polymer backbone are induced to favorably undergo cisphotoisomerization.

Third, the LPUVL exposure selectively induces the cisphotoisomerization of thetrans-stilbene moieties whose longoptical axis is positioned parallel to the polarization directionof the LPUVL, causing the unidirectional orientation of thepolymer chains. Surprisingly, the direction of the unidirectionallyoriented polymer chains can switch from perpendicular toparallel to the polarization direction of the used LPUVL,depending on the exposure doses. This interesting orientationalswitching behavior was found to originate from the participationof the trans-stilbene chromophore moieties of a single PSPIpolymer backbone in the cis photoisomerization as multiplephotoreaction sites where the degree of their participation inthe cis photoisomerization depends on the exposure dose. TheLPUVL exposure of the PSPI film over the exposure dose rangeof <1.4 J/cm2 causes the overall polymer chain orientationperpendicular to the polarization of the LPUVL. Conversely,


the LPUVL exposure of>1.4 J/cm2 induces the overall polymerchain orientation parallel to the polarization of the LPUVL. Inparticular, a maximum degree of the perpendicular polymerchain orientation is achieved by the LPUVL exposure at 0.5J/cm2, whereas a maximum degree of the parallel polymer chainorientation is achieved by the LPUVL exposure at 2.0 J/cm2.

Fourth, the unidirectional orientation of the polymer chainsinduced by LPUVL exposure was found to occur through thepolymer segmental orientations in the following sequence: 962cm-1 (trans-vinylene C-H deformation in the stilbene units)f 1608 cm-1 (phenyl ring stretching in the stilbene units)f1718 cm-1 (asymmetric CdO stretching in the OPDA units)f 1515 cm-1 (phenyl ring stretching in the OPDA units)f1367 cm-1 (N-C stretching in the ODPA units)f 772 cm-1

(cis-vinylene C-H deformation in the stilbene units). Theseresults confirm that in the PSPI film, the photoinduced cisisomerization of thetrans-stilbene chromophore moieties in thepolymer backbones can successfully orient the rest of the parts(namely, ODPA units, imide bonds, and unreacted stilbene units)of the polymer chain in a certain direction.

Fifth, the rubbing process of the PSPI film causes polymerchain orientation along the rubbing direction and furthergenerates microgrooves along the rubbing direction, which havebeen observed in most common rubbing-type polyimides.

Sixth, LC molecules were found to align along the orientationdirection of the PSPI polymer chains, induced by either theLPUVL exposure process or the rubbing process. The LCanchoring energies of the photoaligned PSPI films are in therange of e1.18 × 10-5 J/m2, depending on the degree ofunidirectional orientation of the polymer chains; higher uni-directional orientation of the polymer chains gives larger LCanchoring energy. The highest LC anchoring energy for thephotoaligned PSPI is comparable to that (2.80× 10-5 J/m2)for the rubbed film. Further, all of the LC cells fabricated withthe photoaligned and rubbed PSPI films were found to be verystable. These results indicate that the anchoring and alignmentof LC molecules by PSPI films are primarily governed by theiranisotropic interactions with the segments of the preferentiallyoriented polymer chains and are, in part, positively supportedby their interactions with the directionally developed micro-grooves.

In summary, in the present study the ODPA-Stilbene PSPIin thin films has demonstrated the homogeneous, uniaxial LC-aligning ability as well as the switching ability of the uniaxialLC alignment from the perpendicular direction to the paralleldirection. The properties of this PSPI make it a promisingcandidate material for use as rubbing-free processing LCalignment layers in advanced LC display devices including LCdisplay televisions with large display areas, in particular, indevices with an in-plane switching mode that requires as lowas possible LC pretilt angles.

Acknowledgment. This study was supported by the KoreaScience & Engineering Foundation (National Research LabProgram and Center for Integrated Molecular Systems) and bythe Ministry of Education (BK21 Program).

References and Notes

(1) (a) Schadt, M.; Seiberle, H.; Schuster, A.Nature1996, 381, 212.(b) Iimura, Y.; Kobayashi, S.; Hashimoto, T.; Sugiyama, T.; Katoh, K.Inst.Electron., Inf. Commun. Eng., Trans. Electron.1996, E39, 1040. (c) Minsk,L.; Smith, J. G.; van Deusen, W. P.; Wright, J. F.J. Appl. Polym. Sci.1958, 2, 302.

(2) Ichimura, K., Ed.Polymers as Electrooptical and PhotoopticalActiVe Media; Springer: Berlin, Germany, 1996.

(3) (a) Gibbons, W. M.; Shannon, P. J.; Sun, S.-T.Mol. Cryst. Liq.Cryst.1994, 251, 191. (b) Chen, J.; Boss, P. J.; Bryant, D. R.; Johnson, D.L.; Jamal, S. H.; Kelly, J. R.Appl. Phys. Lett.1995, 67, 1990. (c) Seo, D.S.; Kobayashi, S.Mol. Cryst. Liq. Cryst.1997, 301, 57. (d) Seo, D. S.;Hwang, L. Y.; Kobayashi, S.Liq. Cryst.1997, 23, 923. (e) Yamamoto, T.;Hasegawa, M.; Hatoh, H. Proc.Soc. Inf. Disp. Dig.1996, 642.

(4) (a) Chae, B.; Lee, S. W.; Jung, Y. M.; Ree, M.; Kim, S. B.Langmuir2003, 19, 687. (b) Lee, S. W.; Kim, S. I.; Lee, B.; Choi, W.; Chae, B.;Kim, S. B.; Ree, M.Macromolecules2003, 36, 6527.

(5) Lee, S. W.; Kim, S. I.; Lee, B.; Kim, H. C.; Chang, T.; Ree, M.Langmuir2003, 19, 10381.

(6) (a) Chae, B.; Lee, S. W.; Kim, S. B.; Lee, B.; Ree, M.Langmuir2003, 19, 6039. (b) Chae, B.; Lee, S. W.; Ree, M.; Kim, S. B.Vib. Spectrosc.2002, 29, 69. (c) Ree, M.; Kim, S. I.; Lee, S. W.Synth. Met.2001, 117,273. (d) Ree, M.Macromol. Res.2006, 14, 1. (e) Shin, S. J.; Chi, J. H.;Zin, W. C.; Chang, T.; Ree, M.; Jung, J. C.Polymer (Korea)2006, 30, 97.(f) Ree, M.; Shin, T. J.; Lee, S. W.Korea Polymer J.2001, 9, 1. (g) Han,H.; Ree, M.Korea Polymer J.1997, 5, 152.

(7) Ichimura, K.; Akiyama, H.; Ishizuki, N.; Kawanishi, Y.Makromol.Chem., Rapid Commun.1993, 14, 813.

(8) Akiyama, H.; Kudo, K.; Ichimura, K.; Yokoyama, S.; Kakimoto,M.; Imai, Y. Langmuir1995, 11, 1033.

(9) Akiyama, H.; Kudo, K.; Ichimura, K.Macromol. Chem., RapidCommun.1995, 16, 35.

(10) Akiyama, H.; Kudo, K.; Hayashi, Y.; Ichimura, K.J. Photopolym.Sci. Technol.1996, 9, 49.

(11) (a) Ichimura, K.; Morino, S.; Huxur, T.; Furumi, S.; Akiyama, H.Proc. Int. Disp. Workshop1997, 359. (b) Ichimura, K.; Morino, S.; Akiyama,H. Appl. Phys. Lett.1998, 73, 921.

(12) Ichimura, K.; Hayashi, Y.; Goto, K.; Ishizuki, N.Thin Solid Films1993, 235, 101.

(13) (a) Ichimura, K.; Tomita, H.; Kudo, K.Liq. Cryst.1996, 20, 161.(b) Park, J. H.; Jung, J. C.; Sohn, B.-H.; Lee, S. W.; Ree, M.J. Polym.Sci., Part A: Polym. Chem.2001, 39, 3622. (c) Park, J. H.; Sohn, B.-H.;Jung, J. C.; Lee, S. W.; Ree, M.J. Polym. Sci., Part A: Polym. Chem.2001, 39, 1800.

(14) (a) Schadt, M.; Schmitt, K.; Kozinkov, K. V.; Chigrinov, V.Jpn.J. Appl. Phys.1992, 31, 2155. (b) Schadt, M.; Seiberle, H.; Schuster, A.;Kelly, S. M. Jpn. J. Appl. Phys.1995, 34, L764.

(15) Marusii, T. Y.; Reznikov, Y. A.Mol. Mater. 1993, 3, 161.(16) (a) Ichimura, K.; Akita, Y.; Akiyama, H.; Kudo, K.; Hayashi, Y.

Macromolecules1997, 30, 903. (b) Schadt, M.Proc. Rad. Technol. Asia1997, 96. (c) Li, X. D.; Zhong, Z. X.; Jin, G.; Lee, S. H.; Lee, M. H.Macromol. Res. 2006, 14, 257.

(17) Suh, D.-H.; Ichimura, K.; Kudo, K.Macromol. Chem. Phys. 1998,199, 375.

(18) Makita, Y.; Natsui, T.; Kimura, S.; Nakata, S.; Kimura, M.; Matsuki,Y.; Takeuchi, Y.J. Photopolym. Sci. Technol.1998, 11, 187.

(19) Hasegawa, M.; Taira, Y.J. Photopolym. Sci. Technol.1995, 8, 241.(20) (a) Lee, K.-W.; Paek, S.-H.; Lien, A.; During, C.; Fukuro, H.

Macromolecules1996, 29, 8894. (b) van Aerle, N. A. J.; Tol, J. W.Macromolecules1994, 27, 6520. (c) Lee, S. W.; Lee, S. J.; Hahm, S. G.;Lee, T. J.; Lee, B.; Chae, B.; Kim, S. B.; Jung, J.; Zin, W. C.; Sohn, B. H.;Ree, M.Macromolecules2005, 39, 4331. (d) Lee, S. W.; Chae, B.; Lee,B.; Choi, W.; Kim, S. B.; Kim, S. I.; Park, S.-M.; Jung, J. C.; Lee, K. H.;Ree, M.Chem. Mater.2003, 15, 3105. (e) Lee, S. J.; Jung, J. C.; Lee, S.W.; Ree, M.J. Polm. Sci., Part A: Polym. Chem.2004, 42, 3130. (f) Chae,B.; Lee, S. W.; Lee, B.; Choi, W.; Kim, S. B.; Jung, Y. M.; Jung, J. C.;Lee, K. H.; Ree, M.Langmuir 2003, 19, 9459. (g) Kim, S. I.; Ree, M.;Shin, T. J.; Jung, J. C.J. Polym. Sci., Part A: Polym. Chem.1999, 37,2909. (h) Lee, S. W.; Kim, H. C.; Lee, B.; Chang, T.; Ree, M.Macromolecules2003, 36, 9905. (i) Lee, S. W.; Chae, B.; Kim, H. C.;Lee, B.; Choi, W.; Kim, S. B.; Chang, T.; Ree, M.Langmuir 2003, 19,8735.

(21) (a) Noda, IAppl. Spectrosc.1993, 47, 1329. (b) Noda, I.; Dowrey,A. E.; Marcott, C.; Story, G. M.Appl. Spectrosc.2000, 54, 236A. (c) Chae,B.; Lee, S. W.; Lee, B.; Choi, W.; Kim, S. B.; Jung, Y. M.; Jung, J. C.;Lee, K. Y.; Ree, M.J. Phys. Chem. B2003, 107, 11911. (d) Noda, I.;Ozaki, Y. Two-Dmensional Corrrelation Spectroscopy: Applictions inVibrational Spectroscopy; John Wiley & Sons, Inc.: New York, 2004. (e)Jung, Y. M.; Noda, I.Appl. Spectrosc. ReV. 2006, 41, 515.

(22) The program can be obtained from Prof. Yukihiro Ozaki of KwanseiGakuin University, Japan. The program is available from the homepage ofProfessor Yukihiro Ozaki of Kwansei Gakuin University, Sanda, Japan(http://science.kwansei.ac.jp/∼ozaki/).

(23) (a) Barbet, F.; Bormann, D.; Warenghem, M.; Khelkfa, B.; Kurios,Y.; Reznikov, Y. Mater. Chem. Phys.1998, 55, 202. (b) Miljanic, S.;Frkanec, L.; Meic, Z.; Zinic, M.Langmuir 2005, 21, 2754. (c) Momotak,A.; Uda, M.; Arai, T.J. Photochem. Photobiol., A2003, 158, 7.

(24) (a) Joseph, R.Photogr. Sci. Eng.1971, 15, 60. (b) Sebastien, P.;Pierre, L. B.Liq. Cryst.2000, 27, 329. (c) Beal, R. N.; Roe, E. M.J. Chem.Soc.1955, 2755. (d) Orchin, MJ. Chem. Educ.1957, 34, 496. (e) Colthup,


N. B.; Daly, L. H.; Wiberly, S. E.Introduction to Infrared and RamanSpectroscopy; Academic: San Diego, CA, 1990

(25) (a) Meic, A.; Suste, T.; Baranovic, G.; Smrecki, V.; Holly, S.;Keresztury, G.J. Mol. Struct.1995, 348, 229. (b) Breton, H.; Letard, J. F.;Lapouyade, R.; Calvez, A.; Rassoul, R. M. A.; Freysz, E.; Ducasse, A.;Belin, C.; Morand, J. P.Chem. Phys. Lett. 1995, 242, 604. (c) Okamoto,H.; Tasumi, M.Chem. Phys. Lett. 1996, 256, 502. (d) Breton, H.; Bennetau,B.; Dunogues, J.; Letard, J. F.; Lapouyade, R.; Vignau, L.; Morand, J. P.

Langmuir1995, 11, 1353. (e) Yan, J.; Liu, L.; Ji, L.; Ye, M.; Xu, L.; Wang,W. J. Phys. D: Appl. Phys. 2004, 37, 1597. (f) Dimitrakopoulos, C. D.;Kowalczyk, S. P.; Lee, K. W.Polymer1995, 36, 4983.

(26) Arenas, J. F.; Tocon, I. L.; Otero, J. C.; Marcos, J. I.J. Phys. Chem.1995, 99, 11392.

(27) Kikuchi, H.; Logan, J. A.; Yoon, D. Y.J. Appl. Phys.1996, 79,6811.


Documents

UV-Driven Switching of Chain Orientation and Liquid Crystal Alignment in Nanoscale Thin Films of a Novel Polyimide Bearing Stilbene Moieties in the Backbone