Peter Van De Witte

Modification of the pitch of chiral nematic liquid crystals by means of photoisomerization of chiral dopants

New photoisomerizable chiral dopants have been studied. The dopants used were menthone derivatives, a chiral stilbene derivative and a nematic copolymer of a menthone derivative and a benzoyloxybenzonitrile derivative. NMR, HPLC and UV results showed that the E-Z-isomerization of all the compounds indicated proceeded rapidly upon UV exposure, without the formation of undesired by-products. Isomerization of the menthone derivatives, including the copolymer, induced a substantial decrease in the helical twisting power. The changes in helical twisting power induced by the isomerization of the chiral stilbene derivative were limited to a factor of 2. In mixtures of the photoisomerizable dopants with commercial dopants of opposite twisting senses and a nematic host mixture, the sign of the twisting sense could be reversed by illuminating the mixture with UV light. The viewing angle dependence of irradiated regions of a 90 -twisted nematic cell was rotated 90 with respect to the viewing angle dependence of the ...

LCD components obtained by patterning of chiral nematic polymer layers

An overview is given of the methods for preparation of birefringent films with patterned optical properties and their applications in liquid crystal displays. A versatile method of obtaining patterned films is based on manipulation of the helical twisting power of chiral dopants by photoisomerization reactions. Because of their transparency to visible light, high helical twisting power and large changes of the helical twisting power during UV irradiation, menthone derivatives are very suitable dopants for this process. A number of potential applications of the principles are discussed. Patterning of chiral nematic films with regions with different twisting angles is a suitable method for the preparation of patterned polarization rotators. Patterning of chiral nematic liquid crystal layers can also be used to control the reflection color and state of polarization of the reflected light. Such layers are useful for the preparation of reflective color filter arrays and broad band reflective polarizers.

New functional polymers for liquid crystal displays review of some recent developments

Polymeric materials play an important role in the construction and performance of liquid crystal displays. New functional polymers are developed to improve the displays on brightness, power efficiency and viewing angle performance. Polymer films with a controlled molecular structure and architecture provide new polarization optics to convert non-polarized light into polarized light with higher efficiency than the traditional polarizers based on stretched iodine doped poly(vinyl alcohol). Other films provide a polarization compensation function to maintain an angular invariant net optical retardation of the display device and therewith a better contrast to wider viewing angles. Color generation by non-absorbing methods is believed to improve on the display brightness. Special control of the liquid crystal alignment by photo-sensitive orientation layers, polymer protrusions or photo-formed polymer walls provide multiple director patterns within a single pixel to average out angular LC effects.

Optical components from a new vitrifying liquid crystal

New low molar mass liquid crystalline vitrifying materials have been synthesized and tested for application in optical films. The molecules were based on spiro compounds derived from pentaerythritol and mesogenic groups derived from cyanobiphenylyl moieties. The resulting materials showed glass transition temperatures as high as 85 degrees C and nematic to isotropic phase transition temperatures up to 222 degrees C. Crystallization from the melt was strongly suppressed. Well-aligned, solid, birefringent layers were obtained from the materials by spincoating. Uniaxially oriented layers with an optic axis tilted with respect to the substrate were obtained by spincoating the liquid crystals on pretilt amplification layers. When an anisotropic dye was incorporated in the liquid crystals, polarizers with a tilted absorption axis were obtained. In addition, the compounds were found to be suitable as hosts for photo-induced reorientation of photo-isomerizable dyes.

Camphor and nopinone derivatives as new photosensitive chiral dopants

E-Z photoisomerizable chiral dopants are a class of materials that can be used to prepare birefringent optical components with patterned optical properties. Two new photoisomerizable chiral dopants have been synthesized and analysed. The materials were derived from nopinone and camphor. The properties of these compounds were compared with the properties of the methone derivative described in earlier publications. The E-isomers of the nopinone and camphor derivatives had helical twisting powers of 2.7 and 1.7 μm-1, respectively. This is a factor of about 10 lower than the value obtained for the menthone derivative (-19 μm-1). Due to the high absorption of the Z-isomers relative to the E-isomers of the nopinone and camphor derivatives, isomerization during 365 nm UV exposure proceeded to a much lesser extent than the isomerization of the menthone derivative. At shorter wavelengths, the absorption of the Z-isomer is much lower than that of the E-isomer and much higher degrees of conversion could be achieved.

Preparation of Retarders with a Tilted Optic Axis

Foils with a tilted optic axis can be used as compensation foils for active matrix twisted nematic displays. A method is described for preparing these foils. The method makes use of the differences in alignment for liquid crystals at the liquid crystal-air interface and the liquid crystal-substrate interface. The retardation as a function of the angle of incidence was used to analyze the director profile. Layers with tilt angles close to 0° were obtained from a low molecular weight glass-forming liquid crystal and a side-chain liquid crystalline polymer. Layers with high average tilt angles could be obtained by photopolymerizing thin layers of cyanobiphenyl derivatives and a liquid crystalline diacrylate. The tilt angle of the liquid crystal at the liquid crystal-air interface could be varied between 90° in the case of mixtures with high contents of cyanobiphenyl derivatives and to 20° in the case of mixtures with high contents of diacrylates.

P-76: Economic Wide-Viewing Angle AMLCD Using Dual-Domain Photoalignment and a Single Discotic Compensation Foil

A wide viewing angle AMLCD has been made in which the pixels are divided into two oppositely twisted domains using photoalignment. This wide viewing angle display uses only one discotic compensation film. The system was optimised by 2×2 Jones matrix modelling. The optical properties are evaluated showing improved viewing angles properties, reduced grey scale inversion, a better colour performance and symmetric isocontrast contours.