Usami Kiyoaki

We have investigated the effect of alkyl-amine vapor treatment on pretilt angle generation by photo-alignment films of polyimide containing azobenzene in the backbone structure (Azo-PI). By performing alkyl-amine vapor treatment in preparing photo-alignment Azo-PI layers, the controllable range of the pretilt angle was extended at least up to 38° without appearance of threadlike disclination loops. We found that such a high pretilt angle was induced by the photo-alignment layers with a high inclination angle and narrow orientation distribution of the Azo-PI backbone structure in the out-of-plane direction.

<p>Photo-alignment efficiency of polyimides containing azobenzene in the backbone structure (Azo-PIs) is significantly enhanced by performing a simple alkylamine vapor treatment on the precursor (polyamic acid: Azo-PAA) films prior to photo-alignment. This is due to the efficient photo-induced rotation of Azo-PAA backbone structures in the amine-treated films, which are swollen by absorbing alkylamine molecules. In this study, we have examined the relationships between the amine vapor treatmen t time and the thickness of the amine-treated Azo-PAA film, and also between the amine vapor treatment time and the photo-induced in-plane anisotropy of the Azo-PAA and Azo-PI films.</p>

<p>By performing alkyl-amine vapor treatment in preparing photo-alignment layers of polyimides containing azobenzene in the backbone structure (Azo-PIs), the controllable range of the pretilt angle of liquid crystals can be extended at least to 38 without appearance of threadlike disclination loops. In this study, we have investigated the relationship between the average inclination angle of the Azo-PI backbo ne structure and the pretilt angle. We found that the pretilt angle increases with increas ing average inclination angle of the Azo-PI backbone structure.</p>

Recently, we succeeded in improving the controllable range of the pretilt angle of liquid crystal with defect-free alignment by using amine-vapor-treated photoalignment layers of polyimides containing azobenzene in the backbone structure (Azo-PI). The molecular orientation of the Azo-PI photoalignment layers with and without the amine vapor treatment was determined by measuring polarized infrared absorption spectra as a function of the angle of incidence. We found that the average inclination angle of the Azo-PI backbone structures in the amine-vapor-treated film was much higher than that in the untreated film.

We have succeeded in controlling the pretilt angle of liquid crystal (LC) molecules over the whole range from 0 to 90 degrees by using photo-aligned polyimide films. The polyimide contains azobenzene in the backbone structure (Azo-PI) and has side-chains. The photo-alignment treatment was linearly polarized light irradiation at normal incidence followed by oblique angle irradiation of unpolarized light, and it was performed on the corresponding polyamic acid (Azo-PAA) film. By curing the Azo-PAA films, the photo-aligned polyimide films were obtained. The pretilt angle of LC molecules induced by the photo-aligned polyimide films increased as the side-chain content in Azo-PI increased.

We have investigated the orientation of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophene] (F8T2) in the layers formed on photo-aligned polyimide films. The polyimide (Azo-PI) contains azobenzene in the backbone structure, which allow us to optically control the alignment of the backbone structure. The uniaxially-aligned F8T2 layers were obtained by heating the sample to the liquid crystalline phase and then quenching it to room temperature. The ~20 nm-thick F8T2 layer shows a dichroic ratio of ~14 at 490 nm (the absorption maximum of F8T2), indicative of a high degree of alignment. Since the photo-alignment is contact-free and has potential capability for alignment patterning, the photo-aligned Azo-PI film is an attractive alignment film for F8T2.

We have measured the in-plane anisotropy of the liquid crystal (LC) monolayer in contact with a photo-aligned film of polyimide containing azobenzene in the backbone structure (Azo-PI). We found that the photo-aligned film can induce large in-plane anisotropy of the LC monolayer, which was much larger than that induced by rubbed polyimide films. We have also measured the light transmittance of parallel LC cells that were placed between two polarizers in cross-position. The black level of the parallel LC cell was improved by using the photo-aligned Azo-PI films instead of rubbed polyimide films.

We have investigated the orientation of polyimide (Azo-PI) molecules containing azobenzene in the backbone structure induced by single oblique angle irradiation with unpolarized light (UP-light). The UP-light irradiation was performed at the incidence angle of 45 degrees on the 16 nm thick film of polyamic (Azo-PAA) acid that is the precursor of Azo-PI. The UP-light exposure was varied up to 882 J/cm². After thermal imidization of the photo-treated film, the orientational distribution of the Azo-PI backbone structure was determined using infrared absorption spectroscopy. We found that the backbone structure rotates toward the minimum electric field direction of the UP-light in the Azo-PAA film during the UP-light irradiation.

By using polarized infrared absorption spectroscopy, we have determined the average inclination angle of polyimide backbone structure in photo-aligned films. Polyimide used in this study contains azobenzene in the backbone structure. Photo-alignment treatment was performed for the corresponding polyamic acid film, using a light source of wavelength 340 〜 500 nm. The polyamic acid film (〜16 nm thick) was first irradiated at normal incidence with linearly polarized light (LP-light) of 156 J/cm^2, and then oblique angle irradiation of unpolarized light (UP-light) was performed in the plane of incidence perpendicular to the polarization direction of LP-light. We found that the average inclination angle of the polyimide backbone structure, measured from the surface plane, increases with the UP-light exposure.

We have investigated the pretilt angle of liquid crystal (LC) molecules induced by photoaligned films of polyimide containing azobenzene in the backbone structure. The polyimide films with different inclination of the backbone structure were prepared by performing photoalignment treatment for the corresponding polyamic acid films and then imidizing them thermally. The average inclination angle of polyimide backbone structure was determined by measuring the polarized infrared absorption spectra of the films as a function of the angle of incidence. The pretilt angle of LC molecules was determined by a crystal rotation method. A positive correlation (not linear relation) was found between the pretilt angle of LC molecules and the average inclination angle of the polyimide backbone structure.

We have investigated the molecular orientation of the polyimide film irradiated with p-polarized ultraviolet (UV) light at the incidence angle of 60°. The molecular orientation was determined from the incident angle dependence of the polarized infrared (IR) absorption spectra. The undecomposed polyimide molecules were, on average, inclined in the plane of incidence of the UV light. The inclination angle measured from the surface plane increased up to 〜10° with UV exposure.

We have determined the relation between the in-plane anisotropy of the molecular orientation of a liquid crystal (LC) monolayer and the underlying polyimide film exposed to linearly polarized ultraviolet light (LPUVL). To evaluate the anisotropy of the LC monolayer and the polyimide film surface, the sample orientation dependence of the polarized infrared absorption spectra was measured. The in-plane anisotropy of the LC monolayer was proportional to that of the polyimide film surface. This result suggests that the alignment of the LC molecules in contact with the LPUVL-exposed polyimide film is induced by a shortrange interaction between the polyimide and LC molecules.

By using polarized infrared absorption spectroscopy, we have investigated the molecular alignment of thin films of polyamic acid (PAA) containing azobenzene units in the backbone structure. The molecular alignment was induced by irradiation of linearly polarized ultraviolet light (LPUVL). We found that the alignment of the PAA molecule through repeated photo-isomerization reactions of the azobenzene unit occurred with a low efficiency. When the LPUVL-irradiated PAA film was thermally imidized, we observed significant enhancement in the molecular order of the film. We found that the enhancement of molecular order plays an important role in the realization of polyimide films with large in-plane anisotropy.

We have investigated the change in molecular orientational distribution caused by a washing treatment in UV irradiated polyimide (poly [4,4'-oxydiphenylene-l,2,3,4-cyclobutanetetracarboximide]) films. The polyimide films were exposed to linearly polarized UV light (LPUVL) at different energy densities. The molecular orientation of the LPUVL-exposed films was determined by measuring the polarized IR absorption spectra. In the UV exposure range below 1.5 J/cm^2, the film anisotropy slightly increases by the washing treatment and beyond this exposure it decreases. This result shows that the contribution of the polymer fragments with low molecular weights to the film anisotropy increases with the LPUVL exposure above 1.5 J/cm^2. Thus the film anisotropy becomes maximum at 1.5 J/cm^2 for the washed film, while 〜3 J/cm^2 for the unwashed film.

We have investigated the surface anisotropy of polyimide films induced by irradiation with linearly polarized ultraviolet light (LPUVL). The surface anisotropy was monitored by measuring the polarized infrared absorption spectra of 〜11 nm-thick films as a function of LPUVL exposure. The measurement was performed for) three kinds of polyimides: poly [4,4'-oxydiphenylene-pyromellitimide] (PMDA-ODA), poly [4,4'-oxydiphenylene-1,2,3,4-butanetetracarboximideI (BDA-ODA), and poly [4,4'-oxydiphenylene-1,2,3,4-cyclobutanetetracarboximide] (CBDA-ODA). We found that the LPUVL exposure dependence of the surface anisotropy is strongly influenced by the molecular structure of polyirnides.

We have measured the azimuthal anchoring strength of liquid crystal molecules in contact with poly[4,4'-oxydiphenylen-1,2,3,4-cyclobutanetetracarboxyimide] (CBDA-ODA) films. The measurement was performed for) a 10 nm-thick film exposed to linearly polarized ultraviolet light (LPUVL) of wavelength 〜250 nm at I J/cm^2 as well as for a rubbed film with the same film thickness. Tile infrared (IR) dichroic difference of the phenyl C-C stretching vibration of CBDA-ODA was almost the same for both films. However, the azimuthal anchoring strength for the LPUVI-exposed film was about 5 times smaller than that for the rubbed film. We believe that the small anchoring strength of the LPUVL-exposed film is due to the decrease of the molecular density and/or the decrease of the polyimide chain length that arise from the photo-induced decomposition. In our presentation. we will also report the relation between the IR dichroism of LPUVL-exposed films and the azimuthal anchoring strength.

We have measured the polarized infrared (IR) absorption spectra of poly[4,4'-oxydiphenylen-l,2,3,4- cyclobutanetetracarboxyimide] (CBDA-ODA) films irradiated with linearly polarized ultraviolet light (LPUVL) of wavelength 〜 250 nm. The measurement was carried out for a very thin film (10 nm thick) as a function of the LPUVL exposure. The dichroic difference, A_⊥- A_//, of the phenyl C-C stretching vibration of CBDA-ODA, which is polarized along the polyimide chain, has a maximum around 3 J/cm^2. Here A_//and A_⊥ are the absorbance for the IR light polarized parallel and perpendicular to the polarization direction of LPUVL, respectively. The dichroic difference per unit film thickness at 3 J/cm^2 was greater than that of a rubbed CBDA-ODA film with the same film thickness. Although the average IR absorbance, (A_⊥+ A_//)/2, decreases monotonically over the entire measurement range of LPUVL exposure, A_⊥ increases up to 1 J/cm^2. This result shows that the number of polyimide molecules oriented perpendicular to the polarization direction of LPUVL increases. We conclude that the photo-induced orientation contributes to the large surface anisotropy of LPUVL-exposed CBDA-ODA films in addition to the anisotropic photo-induced decomposition.