Pretilt Angle Of Liquid Crystal Molecules Research Articles

Influence of Design Parameters of Liquid Crystal Cell on the Optical Threshold Voltage Value of Electro-Optical Effect

The influence of liquid-crystal (LC) cell design parameters on the voltage of the optical threshold of the interference effect of polarized beams controlled by an electric field has been studied by computer simulation. It is shown that under strict boundary conditions (both symmetrical and antisymmetrical), the voltage value of the optical threshold decreases rapidly depending on the cell thickness in an exponential manner and reaches a certain constant value. Under non-strict boundary conditions (both symmetrical and antisymmetrical), the voltage dependence of the optical threshold on the LC layer thickness follows a hyperbolic pattern. At the same time, under strict symmetric boundary conditions, the voltage magnitude of the optical threshold effect decreases exponentially with an increase in the pretilt angle of LC molecules on the orienting surfaces. If the boundary conditions are rigid and antisymmetric, an increase in the pretilt angle on the orienting surfaces results in a linear decrease of the optical threshold voltage of the effect. The voltage magnitude of the optical threshold effect increases linearly with an increase in the twist angle of the LC structure in the cell, for both symmetric and antisymmetric boundary conditions. A decrease in the anchoring energy of LC molecules on a substrate leads to a decrease in the voltage of the optical threshold of the effect, and this regularity has an exponential character depending on the reciprocal value of the anchoring energy of LC molecules on orienting substrates.

Homogeneous liquid crystal alignment on poly(vinylidene fluoride-trifluoroethylene) films subjected to ion-beam irradiation

We demonstrate homogeneous and uniform liquid crystal (LC) alignment on poly(vinylidene fluoride-trifluoroethylene) [PVDF-TrFE] films using ion-beam (IB) irradiation and a performance improvement of twisted nematic (TN) cells using IB-irradiated PVDF-TrFE films. Spontaneous ferroelectricity of the PVDF-TrFE films was modified by IB irradiation, which affected the LC alignment properties. The variation in the pre-tilt angles of the LC molecules on the IB-irradiated PVDF films is attributed to surface reformation, including defluorination and oxidation because the pre-tilt angles of LC molecules can be controlled by adjusting the fluorine content. The results of contact angle measurements supported this phenomenon. A 58% reduction in the switching voltage was observed for TN cells, indicating that the IB-irradiated PVDF-TrFE films are a promising candidate for use as an alignment layer.

Pretilt angle control of liquid crystal molecules by photoaligned films of azobenzene-containing polyimide with a different content of side-chain

We have investigated the pretilt angle of liquid crystal (LC) molecules induced by photoaligned films of a series of polyimides. The polyimides were random copolymers synthesized from pyromellitic dianhydride and a mixture of 4,4′-diaminoazobenzene and 4-[4′-propylbi(cyclohexan)-4-yl]phenyl 3,5-diaminobenzoate (PBCP-DABA). PBCP-DABA is a diamine to introduce a side-chain structure into polyimide. We found that the pretilt angle of LC molecules can be controlled from 0° to 90° by varying the molar fraction (x) of PBCP-DABA from 0 to 0.5. Defect-free uniform LC alignment was observed for x≤0.125 and x≥0.3, but threadlike textures appeared for 0.15≤x≤0.25. Since the interaction between the polyimide backbone structure and the LC molecule may be blocked by relatively dense side-chains, the appearance of threadlike texture is tentatively attributed to weak azimuthal anchoring strength of the photoaligned polyimide films with x≥0.15.

Pretilt Angle of Liquid Crystals on Polyimide Films Photo-Aligned by Single Oblique Angle Irradiation with Un-polarized Light

We have investigated the pretilt angle of liquid crystal (LC) molecules induced by photo-aligned films of polyimide containing azobenzene in the backbone structure (Azo-PI). The photo-alignment treatment was single oblique-angle irradiation with un-polarized light (UP-L) at an incidence angle of 45°. It was performed on films of polyamic acid (Azo-PAA) using a light source of wavelength 340–500 nm. The photo-treated films of Azo-PAA were converted into Azo-PI by thermal imidization. The pretilt angle of LC molecules was determined by a crystal rotation method. The pretilt angle increased with the UP-L exposure, reaching ∼3° at 880 J/cm2. For UP-L exposures above 440 J/cm2 the LC alignment was uniform and defect-free, while marbled textures were observed below 220 J/cm2. This is due to the small in-plane anisotropy of the photo-aligned Azo-PI films.

Liquid-crystal alignment on rubbed polyimide films with various side chains

A series of polyimides containing various side chains was synthesized in order to investigate the effect of side chains on the alignment of liquid crystals on the rubbed surface. Here, the side chains include short flexible alkyl spacers and isomeric biphenyl mesogens. The pretilting of liquid-crystal (LC) molecules was found to be very sensitive to the isomeric structure of biphenyl mesogen end groups as well as the conformation and length of flexible spacers, in addition to the rubbing process. The pretilt angle of LC molecules in the LC cell was achieved in a wide angle range of 8-27°, depending upon the rubbing density as well as the side chains. The high performance in the pretilt and alignment of LCs might be attributed mainly to a strong interaction between the biphenyl mesogen end group in the side chain and the LC molecule in addition to the microgrooves generated in the rubbing direction.

Analysis of Rubbed Polyimide Films by Polarized Infrared Spectroscopy: Effects of Immersion in Organic Solvents

We applied polarized infrared (IR) spectroscopy to the analysis of rubbed polyimide (PI) films and evaluated the PI orientation induced by rubbing by measuring the dichroic difference. The IR dichroic difference and liquid crystal (LC) alignment were analyzed for rubbed PI films that were immersed in various organic solvents. The solvents which reduced the dichroic difference and pretilt angle of LC molecules could be characterized in terms of their solubility parameters. When the dichroic difference of the PI films was greatly reduced by the immersion in the solvents, a correlation between the LC pretilt angle and the dichroic difference (i.e. the in-plane orientation of the PI molecules) was observed.