Daeseung Kang

Continuous control of liquid crystal pretilt angle from homeotropic to planar

A mixture of two polyamic acids, one having an alkyl side chain and ordinarily used for vertical liquid crystal alignment and the other without a side chain and ordinarily used for planar alignment, is deposited on two substrates and baked at high temperatures. When the resulting cell is filled with the liquid crystal pentyl cyanobiphenyl, it is found that the pretilt angle θ0 is a function of the baking temperature, and can be controlled continuously over the range of 0°⩽θ0≲90°.

No bias pi cell using a dual alignment layer with an intermediate pretilt angle

The authors fabricated a no-bias pi cell using a dual alignment layer with an intermediate pretilt angle via a rubbing. In the dual alignment layer system, the competition between crest region favoring the vertical alignment and trough region favoring planar alignment made it possible to achieve various pretilt angles, and adjusted pretilt angle from 90° to 20° with rubbing. In addition, as the intermediate pretilt angle plays a role in eliminating the activation energy and thus allowing formation of the initial bend state in pi cell fabrication, this approach achieved a no-bias pi cell for a liquid crystal display with both low power consumption and fast response.

Full control of nematic pretilt angle using spatially homogeneous mixtures of two polyimide alignment materials

Two substrates were coated with a mixture containing two polyamic acids. One of the polyamic acids has an alkyl side chain and ordinarily is used for vertical liquid crystal alignment. The other polyamic acid does not have a side chain and ordinarily is used for planar liquid crystal alignment. The substrates were baked in order to promote imidization. It was observed that over a wide concentration range of mixtures the two polymer species comprising the alignment layer do not phase separate. A cell then was constructed and filled with the liquid crystal pentylcyanobiphenyl. It was found that the nematic pretilt angle θ0 is a function of the relative polymer concentration and can be controlled over the range from near 0° to ∼70° with respect to the vertical orientation. A model is proposed in which quartic terms in the surface energy are responsible for the variation of θ0 with concentration.

Liquid crystal pretilt angle control using adjustable wetting properties of alignment layers

The authors demonstrate the production of amorphous fluorinated carbon (a-C:F) thin film with adjustable wetting properties, inducing variable liquid crystal (LC) pretilt angles. To control the surface wetting properties, they apply a dual radio frequency magnetron system with a controlled power ratio of targets. In this manner we obtain various compositional surfaces with fluorine and carbon components and adjust the surface energy with regard to the various compositions. Whereas the fluorine-rich a-C:F layer shows a preference for homeotropic (vertical) LC alignment, the carbon-rich a-C:F layer shows a planar LC alignment. To achieve uniform LC alignment with a proper pretilt angle, an accelerated Ar+ ion beam irradiates the films after the deposition process. The ion beam selectively destroys the surface bonding of the a-C:F films, yielding an intermediate pretilt angle.

An Oblique Orientation of Nematic Liquid Crystals on a Photosensitive Aligning Polymer

Abstract The problem of an oblique orientation of a liquid crystal in cells with light induced easy orientation axis is considered. Theoretical and experimental studies of the director distribution in a cell with a photosensitive orientant, fluorinated polyvinylcinnamate polymer film, are performed. The two exposure technique for liquid crystal oblique orientation and the condition for a new anchoring transition in a cell with a photosensitive orientant are also presented.

Fabrication of Electro-Optic Devices Using Liquid Crystals With a Single Glass Substrate

A recently developed phase separated composite film method has been employed to fabricate a liquid crystal (LC) based electro-optical device using a single glass substrate. The resultant device is made of adjacent parallel layers of LC and polymer created by phase separation. The LC layer is confined between a film of solidified polymer layer on one side and the glass substrate on the other. Electro-optical properties of these devices demonstrate their technological potential in light weight and hand-held electronic products.

Photoalignment behaviour on polystyrene films containing chalcone moieties

A series of polystyrene derivatives containing chalcone (one of the biological compounds) side groups, poly(4-(4-vinylbenzyloxy)chalcone) (PCHAL#), where # is the molar content of chalcone using polymer modification reactions, were prepared in order to study the effect of the chalcone side groups on the liquid crystal (LC) alignment properties. The LC alignment behaviour of the polymer was investigated using photoalignment method. The LC cells made from photoirradiated PCHAL# films exhibited homogeneous planar LC alignment with an extremely low pretilt angle of approximately 0°. We found that LC aligning ability of the LC cells made from photoirradiated PCHAL# films was affected by the molar content of chalcone side groups. For example, the azimuthal anchoring energy of a PCHAL100 (9 × 10−6 J/m2) was found to be much larger than that of PCHAL18 (5 × 10−7 J/m2). In addition the electro-optical characteristics of the LC cells fabricated with PCHAL100 films such as threshold voltage, driving voltage, and response time were as good as those fabricated from rubbed polyimide films, the most commonly used LC alignment layers.

Liquid crystal alignment properties of n-alkylsulphonylmethyl-substituted polyoxyethylenes

The liquid crystal (LC) alignment properties of LC cells fabricated with films of n-alkylsulphonylmethyl-substituted polyoxyethylenes (#S-PEO, # = 4, 6, 7, 8 and 10), where # is the number of carbon atoms in the n-alkyl side groups having different n-alkyl chain length, were investigated as a function of the rubbing density. The LC cells made from unrubbed #S-PEO (# ≥8) films having more than eight carbon atoms in the n-alkyl side groups showed homeotropic LC alignment. The homeotropic LC alignment behaviour correlated well with the surface energy values of the unrubbed #S-PEO films; homeotropic LC alignment was observed when the surface energy values of the unrubbed #S-PEO films were smaller than about 21.62 mJ m−2. The LC cells made from rubbed #S-PEO (# ≥7) films having more than seven carbon atoms with a rubbing density of 150 showed homeotropic LC alignment. It was also found that the tilt angle of the LCs on the rubbed #S-PEO films was affected not only by the n-alkyl chain length of the polymers, but also by the rubbing density, regardless of the surface energy value of the #S-PEO film.

Liquid crystal alignment properties of polystyrene derivatives containing coumarin side groups

The liquid crystal (LC) alignment properties of LC cells fabricated with films of polystyrene derivatives containing coumarin side groups (P7COU#, where # is the molar content of coumarin side groups in %) were investigated. Good optical transparency of the P7COU# films was observed in the visible light region. LC cells fabricated with rubbed P7COU# films with more than 51 mol. % of coumarin‐containing monomeric units exhibited LC alignment perpendicular with respect to the rubbing direction with a high azimuthal anchoring energy (1×10−5–7×10−5 J m−2). LC cells made from rubbed P7COU# films with more than 65 mol. % of coumarin‐containing monomeric units exhibited a shift of the LC alignment direction by approximately 10° compared with those made from a polystyrene film. The electro‐optical performance of the LC cells fabricated with the rubbed P7COU82 film was found to be as good as that of the LC cells fabricated with a rubbed polyimide film.

Fabrication of a Multidomain and Ultrafast‐Switching Liquid Crystal Alignment Layer Using Contact Printing with a Poly(dimethylsiloxane) Stamp

A compartmentalized multidomain alignment state of a layer of liquid crystal display is achieved using an ultrathin, highly transparent, and ultrafast-responsive alignment layer fabricated by a simple method. The ultrathin alignment layer consists of a self-assembled oligomer layer of poly(dimethylsiloxane) (PDMS) formed by utilizing the oligomers that diffuse out from a PDMS elastomer stamp during a contact printing process.