Kiyoaki Usami

Alignment of polyamic acid molecules containing azobenzene in the backbone structure: Effects of polarized ultraviolet light irradiation and subsequent thermal imidization

We have investigated the molecular orientation in the films of polyamic acid (PAA) with azobenzene units in the backbone structure. Anisotropic molecular orientation was induced by irradiation of linearly polarized ultraviolet light (LPUVL). The change in the molecular orientation caused by subsequent thermal imidization was also investigated. The orientation of the PAA and polyimide backbone structures was determined by measuring the polarized infrared absorption spectra of the films. When the PAA film was exposed to LPUVL of wavelength 365 to 400 nm at normal incidence, permanent orientational change of the PAA backbone structure occurred through repeated photoisomerization reactions of the azobenzene unit. The PAA backbone structure rotated toward the plane perpendicular to the polarization direction of LPUVL. In subsequent thermal imidization the molecular order increased significantly around the direction perpendicular to both the polarization direction of LPUVL and the surface normal. This enhanceme...

Surface anisotropy of polyimide film irradiated with linearly polarized ultraviolet light

Using polarized infrared (IR) absorption, we have investigated the surface anisotropy of a poly [4, 4′-oxydiphenylene-pyromellitimide] (PMDA-ODA) film that arises from anisotropic decomposition of the polyimide chain during irradiation with linearly polarized ultraviolet (LPUV) light. To monitor the surface anisotropy, we designed the sample structure so that the polyimide films decomposed uniformly over the entire film thickness. The surface anisotropy has a maximum at an irradiation energy of 105 J/cm2. For PMDA-ODA, the maximum surface anisotropy is significantly smaller than the surface anisotropy generated by rubbing. By analyzing the irradiation energy dependence of an IR absorption band, we found that the decomposition rate of the polyimide chain oriented parallel to the polarization direction of the LPUV light is greater only by ∼23% than that oriented perpendicular to it. This is the reason for the small surface anisotropy induced by the LPUV light irradiation.

Photoinduced inclination of polyimide molecules containing azobenzene in the backbone structure

We have investigated the inclined alignment of polyimide molecules (containing azobenzene in the backbone structure) induced by oblique angle irradiation of ultraviolet (UV) light. The UV irradiation was performed for a polyamic acid film, which then was thermally converted into a polyimide film. The orientation of the polyimide backbone structure was determined by measuring the polarized infrared absorption spectra as a function of the angle of incidence. We confirmed that inclined alignment of the polyimide backbone structures is obtained by oblique angle UV irradiation, and that this also induces tilted homogeneous liquid crystal (LC) alignment. For the UV irradiation conditions used in the present study, the average inclination angle of the polyimide backbone structure was about 4° from the surface plane. The pretilt angle of the LC molecules was 0.9°.

Relation between the molecular orientation of a liquid crystal monolayer and the underlying polyimide film exposed to linearly polarized ultraviolet light

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, the sample orientation dependence of the polarized infrared absorption spectrum was measured. The in-plane anisotropy of the LC monolayer was found to be proportional to that of the polyimide film, the proportionality factor being about 75%. This result strongly suggests that the alignment of the LC molecules in contact with the LPUVL-exposed polyimide film is induced by an interaction between the polyimide and LC molecules.

Influence of molecular structure on anisotropic photoinduced decomposition of polyimide molecules

We have investigated the anisotropic decomposition of polyimide molecules induced by irradiation of linearly polarized ultraviolet light (LPUVL). Three polyimides were examined: poly [4,4′-oxydiphenylene-pyromellitimide], poly [4,4′-oxydiphenylene-1,2,3,4-butanetetracarboximide] and poly [4,4′-oxydiphenylene-1,2,3,4-cyclobutanetetracarboximide] (CBDA-ODA). Anisotropic decomposition was monitored by measuring the polarized infrared absorption spectra of ∼10 nm thick films as a function of LPUVL exposure. Among the three polyimides, CBDA-ODA showed the largest anisotropy of decomposition in the initial stage of LPUVL exposure. This result suggests that CBDA-ODA is a suitable polyimide material for photoinduced liquid crystal alignment based on photodecomposition reactions. The details of the LPUVL exposure dependence of the film anisotropy are discussed.

Molecular Orientation of Liquid Crystal Monolayers on Polyimide Films Exposed to Linearly Polarized UV Light

By using optical second harmonic generation (SHG), we have investigated the molecular orientation of liquid crystal (LC) monolayers in contact with polyimide films exposed to linearly polarized ultraviolet light (LPUVL) at the wavelength of 266 nm. For different exposures at 3, 8, 20, and 80 J/cm2, the second harmonic (SH) signal from the LC monolayer was measured as a function of the rotation angle of the sample around the surface normal. The SH signal has no rotation angle dependence, independent of the energy density of exposure. On the other hand, uniform parallel (homogeneous) alignment of bulk LC was observed for a LC cell made with two polyimide-coated substrates exposed to LPUVL at 8 J/cm2. From these results we found that the in-plane anisotropy of the LC monolayer in contact with the polyimide film is very small, if any, even though the polyimide film can induce the homogeneous alignment of bulk LC. The average tilt angle of LC molecules in the monolayers in contact with the polyimide films was also determined. We found that the average tilt angle of LC molecules measured from the surface normal decreases with the increase of UV exposure.

In-plane Molecular Order of a Photo-oriented Polyamic Acid Film: Enhancement upon Thermal Imidization

We have determined the in-plane molecular order of a polyamic acid (PAA) film irradiated with linearly polarized ultraviolet light (LPUVL), as well as that of the polyimide film obtained by thermally imidizing it. The PAA examined in this study contains azobenzene units in the backbone structure. The in-plane molecular order of the PAA and polyimide films was determined from the anisotropy in the polarized IR absorption of the phenyl C-C stretching vibration polarized along the backbone structure. We found that the photoinduced anisotropy in the in-plane molecular orientation was small, but it increased significantly after thermal imidization; i.e. the in-plane molecular order of the polyimide film was much greater than that of the PAA film. The enhancement of the in-plane molecular order was tentatively attributed to the crystallization of the film caused by thermal imidization

CHANGE OF IN-PLANE ANISOTROPY OF UV IRRADIATED POLYIMIDE FILMS CAUSED BY WASHING TREATMENT

We have investigated the change in the in-plane anisotropy caused by a washing treatment of UV irradiated polyimide (poly[4,4′-oxydiphenylene-1,2,3,4-cyclobutanetetracarboximide]) films. The in-plane anisotropy of the films exposed to linearly polarized UV light (LPUVL) was determined by measuring the polarized infrared absorption spectra. We found that part of the polymer fragments produced by photo-induced decomposition are unstable, and that they can be removed by a few minutes of ultrasonic cleaning. In the LPUVL exposure range below 1.5 J/cm2, the film anisotropy slightly increases after the washing treatment and beyond this exposure it decreases. As a result, the film anisotropy becomes maximum at 1.5 J/cm2 for the washed film, while it is maximum at ∼3 J/cm2 for the unwashed film.

Anisotropic Decomposition of Polyimide Molecules Induced by Irradiation of Linearly Polarized UV Light

Abstract We have investigated the anisotropic decomposition of polyimide (poly[bis(4,4′-oxydiphenylene)-dimetylmetyl-pyromellitimide]) molecules induced by exposure to linearly polarized ultraviolet light (LPUVL) of wavelength ∼ 300 nm. The decomposition of the polyimide molecule was monitored by measuring the polarized infrared (IR) absorption spectra as a function of LPUVL exposure. We propose an empirical equation that describes the relation between the orientational distribution of polyimide chains and LPUVL exposure. We consider two decomposition rates β/ and β⊥ for the polyimide chains oriented parallel and perpendicular to the polarization direction of LPUVL, respectively. By taking account of the increase of the decomposition rates around 30 J·cm−2, the IR absorption data could be reproduced with β/β⊥ = 1.23 ± 0.02. The decomposition rate β/ is (4.5 ± 1.0) × 10−3J−1·cm2 for the LPUVL exposure range up to -30 J·cm−2, and (1.6 ± 0.1) × 10−2 J−1·cm2 beyond -30 J·cm−2.

Anisotropic Molecular Orientation of Liquid Crystal Monolayer in Contact with UV-irradiated Polyimide Film

By polarized infrared (IR) absorption spectroscopy we have determined the in-plane anisotropy in the molecular orientation of a liquid crystal (LC) monolayer in contact with a polyimide film exposed to linearly polarized ultraviolet light (LPUVL). The in-plane anisotropy of the underlying polyimide film surface was also determined. The average orientational directions of the LC molecules and the polyimide molecules were the same, both being perpendicular to the polarization direction of LPUVL. The in-plane molecular order of the LC molecules was smaller than that of the polyimide molecules. This result indicates that the anisotropic orientational distribution of the LC molecules in contact with the LPUVL-exposed polyimide film is induced through a short-range interaction between the LC molecules and the polyimide molecules.