`Part 1, No. 10, October 1998
`©1998 Publication Board, Japanese Journal of Applied Physics
`
`Thermal and Optical Stabilities of Photoisomerizable Polyimide Layers
`
`for Nematic Liquid Crystal Alignments
`
`Byoungchoo PARK”, Youngyi JUNG1, Hyun-Hee CHOI‘, Ha-Keun HWANG1, Youngkyoo KIM1,
`Sooman LEE1, Sei—Hum JANG1, Masa—aki KAKIMOT02 and Hideo TAKEZOE2
`‘Electronic Materials Research Lab., Institute for Advanced Ertgineering, Yortgin, P 0. Box 25, Korea
`2 Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 0-okayama, Meguro—ku, Tokyo 152-8552, Japan
`
`(Received June 24, 1998; accepted for publication July 17, 1998)
`
`We have investigated the thermal and optical stabilities of the photoalignment layers for nematic liquid crystals (LCs).
`For the photoalignment layers, three kinds of photoisomerizable polymer systems were studied: a polyamic acid doped with
`azobenzene molecules, a side-chain-substituted polyamic acid with azobenzene units, and a main-chain-substituted polyamic
`acid with azobenzene units. Photoinduced anisotropy was produced by illumination with linearly polarized near—UV light
`from a mercury lamp, and evaluated by measuring polarized UV absorption spectra and optical birefringence. The nematic LC-
`alignment properties were investigated using the photoalignment layers processed under various thermal and optical conditions.
`It was observed that the main-chain polyimide system exhibited good unidirectional LC alignment and excellent thermal (300°C
`for 1h) and optical stabilities. Moreover, we suggest a new procedure, in which the photoillumination process is carried out
`before the thermal imidization process, to achieve a stable photoalignment layer.
`
`KEYWORDS:
`
`liquid crystal, photoalignment, polarized light, azobenzene, photoisomerlzatlon, polyimide
`
`1.
`
`Introduction
`
`In order to obtain a homogeneous uniaxial alignment of
`liquid crystal (LC) molecules over wide areas on substrates,
`some kind of optical and/or topographical anisotropies have
`to be created on the substrates.” Particularly, to obtain high-
`performance LC displays (LCDs), the appropriate alignment
`of LC molecules is essentially a prerequisite. Convention-
`ally,
`the rubbing technique”) has been generally used for
`LC alignment. However, dust and static electricity are gen-
`erated by the rubbing process, and thus the rubbing treat-
`ment may be a serious drawback for thin—film transistor (TFT)
`technology for producing high—quality LC devices. There-
`fore, alternative processing techniques, without the problems
`of the rubbing method,
`to align LC molecules have been
`presented such as oblique evaporated inorganic materials,“
`Langmuir-Blodgett films,“ mixtures of two surfactants,” and
`topographical anisotropic films by a stamping process.”
`Recently, it has been successfully revealed_ that photosen-
`sitive polymer films align the LC molecules homogeneously
`when the films are exposed to linearly polarized (near) UV
`(LPUV) light. This photoalignment method has great advan-
`tages in applications to LCDs, especially in making high-
`resolution multidomain LC devices.” For example, LPUV
`light has been irradiated onto the derivatives of polyvinyl-
`cinnamate films,8'9) polymer films doped with photoisomer—
`izable azobenzene molecules,1°”13) or polymer film itself”)
`In polyvinylcinnamate (PVCN) derivative films, the cinna-
`mate side—chain groups undergo linear photopolymerization
`through intermolecular photoinduced cycloaddition when the
`film is irradiated by LPUV light. Thus, the PVCN films ho-
`mogeneously align the liquid crystals.8~9) In the photoisomer-
`izable polymer films with azobenzenes, the azobenzenes ori-
`ented parallel to the polarization direction of the illuminated
`light are selectively isomerized due to the selective absorption
`of the linearly polarized light.14‘17) Thus, prolonged irradia-
`tion with an LPUV light creates anisotropic alignment of the
`azobenzenes through multiple trans—cis isomerizations.13r19)
`Therefore, the LC molecules align homogeneously perpen-
`dicular to the polarization direction of illuminated light.
`
`In practical applications of the photoalignment method, the
`thermal and optical stabilities of the photoalignment layer
`are of great importance for the design of various LC de-
`vices. When the anisotropic orientation of the alignment units
`aligned by LPUV light is damaged during the thermal pro-
`cess in manufacturing LC devices, as a consequence, the LC-
`aligning properties of the photoalignment layers will deteri-
`orate too. In this respect, it is important to improve thermal
`stability of the photoalignment layer. However, the thermal
`stabilities of the photoalignment layers so far reported are not
`sufficiently high. Moreover, the photopolymer film is also op-
`tically unstable so that subsequent illumination of LPUV un-
`der different polarization directions easily reorients the align-
`ment direction.1°*1” Recently, a new hybrid linearly pho-
`topolymerizable technique was developed and thermal sta-
`bility was improved to be resistant to thermal processes near
`200°C?) However, the thermal stability of the photoalignment
`layer should be further improved for practical applications.
`In the present paper, we studied the thermal and optical sta-
`bilities of photoisomerizable polymer films used as LC align-
`ment layers. In order to improve the stabilities, we introduce
`a kind of polyamic acid system which can be converted into a
`thermally stable polyimide system by the imidization process.
`To introduce the photosensitive function into the polyamic
`acid, we prepared three polyamic acid systems modified with
`azobenzene molecules. First, we analyzed the anisotropic
`alignment of azobenzene chromophores upon irradiation by
`monitoring the polarized UV—VIS absorption spectral changes
`and birefringence changes in the polymer films before and af-
`ter photoirradiation. Next, for the stability test, the nematic
`LC cells were fabricated using the phototreated polymer films
`processed under various thermal and optical conditions. The
`LC-alignment properties of the alignment films were assessed
`from the viewpoints of their thermal and optical stabilities.
`
`2. Experiments
`
`In this study, we used three kinds of polymers as pho-
`toisomerizable LC alignment layers. As shown in Fig. 1,
`P1 is a commercial polyamic acid (SE150, Nissan) doped
`with azobenzene molecules to a concentration of 10wt%.
`
`Page 1 of 6
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`Jpn. J. Appl. Phys. Vol. 37 (1998) Pt. 1, No. 10
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`B. PARK er al.
`
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`
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`
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`
`Fig. 1. Chemical structures of poly(amic acid) containing an azobenzene
`unit. P1 is a commercial polyamic acid doped with azobenzene molecules.
`P2 is a side—chain-substituted polyamic acid with azobenzene groups and
`P3 is a main-chain-substituted polyamic acid with azobenzene units.
`
`P2 is a side—chain-substituted polyamic acid with azobenzene
`groups?” and P3 is a main-chain—substituted polyamic acid
`with azobenzene units. These photosensitive polyamic acids
`were dissolved in N-methyl-2—pyrollidinone (NMP) used as a
`solvent to a concentration of 1 wt%. The polymer solutions
`were spin coated onto quartz substrates at 3000 rpm After the
`subsequent drying at 80°C for 30 min (soft baking), the thick-
`ness of the coated polymer film was about 40 nm.
`films was
`The photoisomerization of
`these polymer
`achieved by irradiation with LPUV light from a 1kW high-
`pressure mercury lamp for 10 min at room temperature.
`LPUV light with an intensity of 20 mW/cm: was obtained by
`passing the light through a Glan—Tompson prism and a near—
`UV glass filter tuned at 300 — 500nm corresponding to the
`resonance absorptions of the azobenzene units.
`The polyamic acid films were converted into polyimide
`films by the thermal imidization process.
`In order to deter-
`mine the imidization temperature, the IR absorption spectra
`of the polymer films were measured as a function of the cur-
`ing temperature. For example, Fig. 2 shows the IR absorption
`spectra of the P3 main—chain—substituted polyamic acid ob-
`tained at various curing temperatures for 60 min. As shown
`in the figure, one can clearly observe the disappearance of
`polyamic acid peaks (1650, 3000 — 4000 cm”) and the sharp
`increase of polyimide peaks (720, 1720, 1780 cm“) near the
`curing temperature of 250°C. Thus, we can confirm that P3
`polyamic acid is converted into P3 polyimide by thermal cur-
`ing at 250°C for 60 min.
`A pair of polymer alignment layers were stacked and sealed
`to fabricate a vacant cell with a gap of 10 am. Nematic LC
`(ZLI2293, Chisso) was introduced into the prepared cell using
`capillary action at the temperature of the isotropic phase of the
`nematic LC
`
`Page 2 of 6
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`I
`
`l
`
`4000
`
`3500
`
`3000
`
`2000
`
`1500
`
`1000
`
`500
`
`Wave Number (cm'1 )
`
`IR absorption spectra of the P3 main—chain—substituted polyamic
`Fig. 2.
`acid at various curing temperatures for 60 min.
`
`3. Results and Discussion
`
`3.1 Polarized absorption spectra
`In order to study the photoreorientation of azobenzene units
`in the alignment layers, we measured the polarized absorption
`spectra for the polyamic acid films before and after irradiation
`with LPUV light. As shown in Fig. 3, the spectra were mea-
`sured using polarization directions parallel (Aparallel) and per-
`pendicular (Aperpen) to the polarization direction of the irradi-
`ated LPUV light. Prior to irradiation, we confirmed the pres-
`ence of a random orientation; Apmuel coincided with Apemen
`(= A0). After irradiation with LPUV light, anisotropy ap-
`peared and the absorbance for Aperpen was larger than that for
`Apmnel. We introduce the anisotropic order parameter (P3)
`[= (3(cos2 9) — 1)/2] for the photoaligned layer. Here 6 is the
`angle between the azobenzene molecular axis and the direc-
`tion perpendicular to the polarization of the irradiated light,
`and the bracket denotes the average over molecular orienta-
`tions. The value of (P2) indicates the degree of axial ordering
`along the direction perpendicular to the polarization of the
`LPUV light. From the polarized UV absorption spectra of P1
`film after irradiation, we obtained a value of 0.07 for (P3).
`Similarly, the anisotropic alignment of azobenzene units was
`generated by the LPUV light irradiation of the P2 film, as
`shown in Fig. 3(b). From the spectra, we obtained a value of
`0.19 for (P3) after the irradiation. This behavior is consistent
`with the photoreorientation of azobenzene units in previous
`repo1ts.21‘23)
`For the P3 film, the anisotropy due to the reorientation of
`azobenzene units was very small, as shown in Fig. 3(0). The
`measured value of (P2 ) for the photoinduced anisotropy of P3
`film was 0.02, which was the smallest value among the poly~
`
`Page 2 of 6
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`Jpn. J. Appl. Phys. Vol. 37 (1998) Pt. 1, No. 10
`
`B. PARK er al.
`
`5665
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`0.10
`
`0.020
`
`.0Coo
`
`0oo\
`
`0.02
`
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`
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`
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`
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`
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`
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`
`Absorbance(arb.units) 0oo
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`Absorbance(arb.
`
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`
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`
`0.05
`
`300
`
`400
`
`500
`
`600
`
`700
`
`800
`
`Wavelength (nm)
`
`Fig. 3. The polarized UV absorption spectra for P] (a), P2 (b), and P3 (c)
`films before (A0) and after the irradiation with LPUV light; Apmlm and
`Apefpen are absorption spectra for the light polarized parallel and perpen-
`dicular to the polarization direction of LPUV light, respectively.
`
`mer films used. This is because both ends of the azobenzene
`
`units in P3 film are bound by the polymer backbones. Thus it
`is clear that the photoreorientation of azobenzene units in the
`P3 film is less effective than those in P1 and P2 films.
`
`To detect the photoinduced anisotropy of the P3 film more
`clearly, the birefringence of the P3 film was measured as a
`function of exposure time, as shown in Fig. 4. In this experi-
`ment, we used a photoelastic modulator (PEM) configuration
`with a 632.8 nm probing light from a He—Ne laser. As shown
`in the figure, the birefringence rapidly increased during the
`first 500 s, and reached a maximum value near 2000 s. There-
`
`fore, the azobenzene units in the main—chain polymer rapidly
`reorient during the first 10 min due to the irradiation of LPUV
`light.
`
`3.2 Nematic liquid crystal alignments
`We studied the LC—alignment properties of the polymer
`
`Page 3 of 6
`
`.0o ._. us
`
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`.0o ._. o Birefringence(nm) 0.005 *
`
`0.000 *8I
`0
`
`u
`
`v
`
`I
`1000
`
`I
`2000
`
`I
`
`l
`3000
`
`4000
`
`Time (s)
`
`Fig. 4. The birefringence of P3 film as a function of exposure time.
`
`films from direct observation of the textures of nematic LC
`cells fabricated with phototreated polymer films. The LC-
`alignment properties of the polymer layers were examined as
`follows; first, we investigated the LC alignments on the poly-
`mer films which were irradiated by LPUV light before the
`imidization (polyamic acid => irradiation, step 1). Next, we
`investigated the LC alignment on the polymer films which
`were imidized after the photoirradiation (polyamic acid =>
`irradiation =:» polyimide, step 2). In this step, we could de-
`termine whether or not the photoinduced anisotropies of the
`polyamic acid films were maintained during the irrridization
`process.
`If the anisotropy was maintained after the second
`step, we then investigated the LC alignments on the films
`which were illuminated by LPUV light after the irrridization
`process (polyamic acid => polyimide =:» irradiation, step
`3).
`In this step,
`the optical stability could be tested, ie,
`whether the polymer surface after imidization is disturbed by
`stray UV light or not.
`Now we investigated the LC—alignment properties of our
`polymer films to clarify the stabilities of the alignment layers.
`Microphotographs of nematic LC cells with the phototreated
`alignment layers between crossed polarizers are shown in
`Fig. 5 at four azimuthal rotation angles (qb) between the di-
`rection of one of the crossed polarizers and the polarization
`direction of irradiated LPUV light. The polarized rnicropho—
`tographs of an LC cell with P1 polyamic acid layers which
`were irradiated with LPUV light are shown in Fig. 5(a) (step
`1). It was observed that the LC molecules were aligned ho-
`mogeneously on the phototreated P1 polyarrric acid film. This
`indicates that the Pl polyamic acid film is a good candi-
`date for the photoalignment layer. Next, we observed the
`LC—alignment of another LC cell with P1 layers which were
`irrridized after illurrrination with LPUV light. The n1icropho—
`tographs of the cell are shown in Fig. 5(b) (step 2).
`In the
`figure, it is clearly seen that the LC texture is inhomogeneous
`and contains many defects. This shows that the photoinduced
`surface anisotropy of azobenzene molecules was partially de-
`stroyed during the thermal imidization process. Thus, it is
`revealed that Pl polyamic acid film is thermally unstable.
`Therefore, it is clear that the polymer film P1 is not suitable
`as a photoalignment layer.
`the side—chain—
`layer,
`For
`the P2 polymer alignment
`substituted polyarnic acid with azobenzene units, it was found
`that the LC molecules were aligned homogeneously on the
`
`Page 3 of 6
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`Jpn. J. Appl. Phys. Vol. 37 (1998) Pt. 1, N0. 10
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`B. PARK el al,
`
`(a) 2
`
`(b)
`
`
`
`
`_
`
`(d)
`
`(a) The polarized microphotographs of a nematic LC (ZLI 2293) cell with Pl polyamic acid layers which were irradiated with
`Fig. 5.
`LPUV light. (top left—hand side for ¢ = 0°, top right—hand side for ¢ = 45°, bottom left-hand side for 90° and bottom right-hand
`side for 135°).
`(b) The polarized microphotographs of an LC cell with P1 polyamic acid layers which were imidized after LPUV
`irradiation. (c) The polarized microphotographs of an LC cell with P2 polyamic acid layers which were irradiated with LPUV light.
`(cl) The polarized microphotographs of an LC cell with P2 polyamic acid layers which were imidized after LPUV irradiation.
`
`
`
`
`3 (b)
`
`(d)
`
`(a) The polarized microphotographs of an LC cell with P3 polyamic acid layers which were irradiated with LPUV light. (b) The
`Fig. 6.
`polarized microphotographs of an LC cell with P3 polyimide layers which were imidized after LPUV irradiation. (c) The polarized
`microphotographs of an LC cell with P3 polyimide layers which were illuminated with LPUV light after thermal imidization. (cl) The
`polarized microphotographs of an LC cell with phototreated P3 polyimide layers which were baked at 300°C for 60 min.
`
`Page 4 of 6
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`B: PARK er al.
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`5667
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`films which were illuminated by LPUV light, as shown in
`Fig. 5(c) (step 1). However, for the P2 layer irnidized after
`irradiation, it was observed that the texture of the LC was not
`homogeneous and contained many defects, as shown in Fig.
`5(d) (step 2). Thus, photoinduced anisotropy of the azoben—
`zene side—chains of the P2 film was also destroyed during the
`imidization process; and P2 film is not suitable for use as a
`photoalignment layer.
`Now, we discuss the thermal stability of the P3 film,
`the main-chain—substituted polyamic acid with azobenzene
`units. The polarized rnicrophotographs of the LC cell with P3
`polyamic acid film illuminated by LPUV light are shown in
`Fig. 6(a) (step 1). The nematic LC cell with the film exhibited
`a good homogeneous LC alignment, although the photoreori—
`entation of azobenzene units in the P3 film was less effective
`
`than those in P1 and P2 films. Moreover, the homogeneous
`alignment of LC molecules was also observed on the P3 film
`which was irnidized after the illumination, as shown in Fig.
`6(b) (step 2). This shows that the photoreoriented azoben—
`zene units retain their orientation in the polymer backbone
`during the thermal irnidization process. Compared with the
`P1 and P2 films, the thermal stability of P3 was considerably
`improved (250° C). Next, we estimated the optical stability of
`the P3 film. As shown in the Fig. 6(c) (step 3), it is clear that
`LCs do not align on the P3 film which was irradiated ‘with
`LPUV light after the thermal imidization process. This shows
`that stray light cannot reorient the azobenzene units in the
`polyimide film, i.e. the P3 polyimide layer is indeed highly
`photostable. Therefore, it is clear that P3 film is shown to ex-
`hibit excellent thermal and optical stabilities for a LC align-
`ment layer. To examine the thermal stability of the P3 film,
`we baked the P3 alignment layers, which were irnidized af-
`ter the illumination of LPUV light, at 300°C for 60 min. The
`LC—alignment of the cell with the baked layers is shown in
`Fig. 6(d). As shown in the figure, it is clear that defect—free
`homogeneous LC alignment is preserved after the severe bak-
`ing process mentioned above. Moreover, it was confirmed
`that the P3 film in contact with the LC was also thermally
`stable over the wide temperature range of isotropic (~120°C)
`and nematic phases of the used LC. Therefore, it is concluded
`
`that P3 film irnidized after LPUV irradiation is thermally and
`optically stable as a practical photoalignment layer. In order
`to obtain the thermally stable photoalignment polymer layer,
`we suggest a new procedure, in which the photoaligning pro-
`cess should be performed before the irnidization process. This
`is quite different from the conventional rubbing process, i.e.
`the rubbing process usually is canied out after the imidization
`process.
`Finally, we confirm the utility of the P3 alignment film by
`constructing a monodomain twisted nematic (TN) LC cell.
`We used a similar irradiation configuration as used in ref. 24
`to generate the pretilt angle. The generated pretilt angle was
`0.l7° at an incident angle of 60° for the irradiation. Figure 7
`shows the transmission as a function of applied voltage for the
`TN cell under normally white condition. The P3 TN—LC cell
`shows good electrooptic characteristics with a high contrast
`ratio of over 100 and a bright transmittance of around 35%.
`
`4. Conclusions
`
`In summary, we have investigated the thermal and op~
`tical stabilities of photoinduced LC alignment layers made
`of three kinds of photoisomerizable polyamic acid systems;
`a polyamic acid doped with azobenzene molecules, a side-
`chain—substituted polyamic acid with azobenzene units, and a
`main~chain—substituted polyamic acid with azobenzene units.
`The polarized absorption spectra in these polymer films were
`measured before and after LPUV irradiation. We obtained
`
`the axial order parameter (P3), which gives a measure of
`the anisotropic orientation of the polymer films induced by
`the irradiation.
`It was found that the main—chain system is
`less effective for creating photoinduced anisotropy than the
`doped system and the side—chain system. We also investigated
`the LC—a1ignment properties of these films processed by var-
`ious thermal and optical treatments.
`It was shown that the
`doped system and the side—chain system are thermally unsta-
`ble for use as a liquid crystal alignment layer. On the other
`hand, it was found that a homogeneous LC alignment with
`excellent optical and thermal stabilities was achieved using a
`phototreated main—chain system. Moreover, we proposed a
`new method to obtain a stable photoalignment layer and suc-
`cessfully demonstrated thermal stability up to 300°C for 1 h.
`These results show that azobenzene main-chain polyimides
`such as P3 are interesting materials which provide us with im-
`
`portant information regarding the stability of the photoalign-
`ment mechanism of LC molecules. An investigation of the
`use of these polymers as alignment layers for electrooptic LC
`devices such as a twisted nematic (TN)—LCD is now under-
`way, and details will be published elsewhere.
`
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`
`2)
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`J. Cognard: Alignment of Nemaric Liquid Crystals and Their Mixtures
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`
`
`TransmittedIntensity(arb.units)
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`
`10
`
`Applied Voltage (Volt)
`
`Fig. 7. The characteristics of transmission as a function of applied voltage
`for the TN cell with P3 alignment layer under normally white condition
`(cell gap = 8 um).
`
`Page 5 of 6
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