Linearly Polarized Ultraviolet Light Research Articles

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.

Morphological investigation of polyvinyl-4-methoxy cinnamate photopolymer thin and ultrathin films under linear photopolymerization

The characteristics of the surface of polyvinyl-4-methoxy cinnamate (PVMC) photopolymer film which promotes the uniform alignment in adjacent liquid crystals upon photopolymerization by linear polarized UV light have been investigated. We have found that under photopolymerization the surface morphology of the polymer film changes and the polymer orients normal to the polarization direction of the UV light. Our results confirm the microscopic model of the linear photo polymerized induced unidirectional order as reported by Schadt et al. (M. Schadt, K. Schmitt, V. Koznikov, V. Chignirov, Jpn. J. Appl. Phys. 31 (1992) 2155).

Photoalignment Films of Polyesters with Photoreactive Main Chain

The photoreactive thin films of poly(oxyalkyleneoxy-1,4-phenylenediacryloyl), a PDA-n polyester (n=2–5), were prepared. Their photoreactions and the liquid crystal (LC) photoalignment by irradiation of linearly polarized UV light (LPL) were investigated. The PDA-n polyesters have photoreactive sites in the polymer main chain and not in the side chain which is different from poly(vinyl cinnamate) (PVCi), a well-known photoalignment polymer. These PDA-n thin films show different photoreactivities depending on the number of methylene units, n. Odd-numbered PDA-n polyesters with higher photosensitivity than even-numbered PDA-n polyesters show a dichroism due to irradiation with LPL which results in a homogeneous alignment of the nematic LC.

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.

Mechanism of Photo-Induced Liquid Crystal Alignment on Poly(vinyl cinnamate) Surface.

The photopolymer material of poly(vinyl cinnamate) (PVCi) has been used to align liquid crystal molecules using linearly-polarized UV light, and the LC alignment mechanism on the films has been investigated. Comparing the LC alignment on rubbed PVCi films with that on UV-exposed PVCi film, it is shown that the side chains of PVCi molecules strongly interact with the LC molecules on the PVCi films and thus the alignment mechanism on linearly-polarized UV-exposed PVCi films is due to the anisotropic van der Waals interaction between LC molecules and optically-anisotropic PVCi films arising from the photo-induced anisotropic side chain distribution on the films. We also study the thermal stability of UV- exposed PVCi films, and clarify the improvement of the thermal stability with increasing UV exposure energy.

Liquid crystal alignment on photopolymer surfaces exposed by linearly polarized uv light.

A photo-alignment of nematic liquid crystals (NLC) on a photopolymer film of polyvinyl cinnamate (PVCi) has been achieved by using linearly-polarized UV light. The mechanism of the LC alignment is clarified and good unidirectional LC alignment with pretilt angle is obtained by appropriate combination of normal and oblique exposure of linearly-polarized UV light to the PVCi films. This photo-alignment method is applied to fabricate super-multidomain (SMD) TN-LCDs making a feature of quartered pixels, and we demonstrate that reverse tilt disclination (RTD)-free SMD TN-LCDs is successfully fabricated by using the photo-alignment method.