Liang-Chy Chien

Cholesteric Liquid-Crystal Polymer Dispersion for Haze-Free Light Shutters

A new dispersion involving a polymer in low concentration with a cholesteric liquid crystal is reported. Two types of light modulators from these materials are described as a normal mode shutter which is opaque (light scattering) in the field‐OFF state and transparent in the field‐ON state and a reverse‐mode shutter with the opposite field conditions. The transparent state of both cells is haze‐free for all viewing angles making the material attractive for window applications and direct view displays.

Structure and morphology of polymer-stabilized cholesteric diffraction gratings

We report a study of polymer network morphology in planar diffraction gratings formed from polymer-stabilized cholesteric liquid crystals. Photopolymerizing a mesogenic monomer yields a regular sawtooth morphology, composed of polymer sheets running at an acute angle between thick, uniformly aligned bundles which reside on the opposing substrates. To explain this morphology and evidence of two grating vectors in the diffraction pattern, we propose a model for the liquid-crystal director where out-of-phase twists at the surfaces gear into an interior twist, which has half the period of the surface twist, via splay distortions running between the substrates. The morphology also changes significantly when a nonmesogenic monomer is substituted or when selectively polarized states of UV radiation are used to cure the polymer.

Cholesteric liquid crystal/polymer gel dispersion bistable at zero field

A novel liquid crystal/polymer gel dispersion is reported which exhibits two contrasting optical states which are stable at zero field; one state reflects light at a preselected color and the other state is scattering. In a display cell, the material can be selectively driven to either state by a suitable voltage pulse. A polymer gel can be used to stabilize the optical states of a cholesteric liquid crystal to create a bistable color display. This system has a high potential for multiplexed flat panel displays without the use of an active matrix substrate. Such a display can also be used for long-term information storage and optical signal processing.<<ETX>>

Photo-, and electroluminescence studies of 2,5-bis[2'-(4-(6-hexoxy benzyl))-1'-ethenyl]-3,4-dibutyl thiophenes

Abstract We have synthesized a thiophene-based compound 2,5-bis[2′-(4″-(6-hexoxy benzyl))-1′-ethenyl]-3, 4-dibutyl thiophene (HBDT) and a copolymer poly(2,5-bis(2′-(4″-(6-hexoxy benzyl))-1′-ethenyl)-3,4-dibutyl thiophene-1,6-diisocyanatohexane) (HBDT-PU) consisting of alternating HBDT and urethane spacer units. Absorption and photoluminescence (PL) spectra of the compounds coincide indicating that emission in HBDT-PU occurs from the thiophene containing unit. PL is emitted in the blue–green region with a maximum at 460 nm. Concentration quenching occurs in pure materials: PL efficiency is strongly enhanced when the compounds are dispersed in a polymer matrix like poly( N -vinyl carbozole) (PVK) or poly(methyl metacrylate) (PMMA). Light emitting devices were fabricated using a PVK:2-(4-biphenylyl)-5-(4- tert -butylphenyl)-1,3,4-oxadiazole (PBD) blend doped with HBDT as the emitter material. Efficient energy transfer from PVK:PBD to HBDT molecules takes place in blended films. The electroluminescence (EL) spectra coincided with the PL spectra of HBDT indicating that EL emission comes solely from HBDT molecules. The influence of the doping concentration on the EL efficiency was found to coincide with the concentration dependence of the PL quantum yield.

Current trends in polymer dispersed liquid crystals

A novel cholesteric liquid crystal/polymer gel dispersion has been constructed which can be operated either in normal mode or reverse mode. The normal-mode cell is opaque in a field-OFF condition and clear in a field-ON condition. The reverse-mode cell is clear in a field-OFF condition and opaque in a field-ON condition. In the clear state, both cells are haze-free for all viewing angles. The cholesteric liquid crystal/polymer gel dispersion can be used to make normal-mode and reverse-mode light shutters. The reverse-mode shutter has very good electrooptical characteristics, exhibiting a sharp transition, small hysteresis, and fast dynamic response.<<ETX>>

Formation and Localization of Internal Polymer Columnar Structures in Liquid Crystal Cells

Internal polymer columnar structures in liquid crystal (LC) cells have been produced using the method of thermally-induced phase separation of LC/prepolymer solution and solidified by photopolymerization. These polymer columns adhere to both substrates and enhance control of the LC cell spacing and mechanical stability. The existence and profile of the polymer columns were investigated by optical and scanning electron microscopy techniques. Supertwisted nematic LC cells with polymer columnar structures have been constructed using this method and their electro-optical characteristics measured.

Concentration Dependence of Photoluminescence and Electroluminescence in 2,5 Bis[2'-(4”-Hydroxyhexanol Benzyl)-l'-Ethyl]3,4-Dibutyl Thiophenes

A new thiophene-based compound 2,5 bis[2-(4”-hydroxyhexanol benzyl)-l-ethyl]3,4-dibutyl thiophene (HBDT) as well as a copolymer (HBDT-PU) consisting of alternating HBDT and urethane spacer units were synthesized. Absorption and luminescence properties of both compounds were studied in solution and in different polymer matrices. Absorption and luminescence spectra of HBDT and HBDT-PU coincide indicating that emission in HBDT-PU occurs from the thiopene containing unit. Photoluminescence (PL) is emitted in the blue-green region of the visible spectrum with a maximum at 460 nm. The PL efficiency of both compounds is strongly enhanced when dispersed in a PVK or PMMA matrix indicating that concentration quenching occurs in the pure material. Light emitting devices were fabricated utilizing a PVK/PBD blend doped with HBDT and HBDT-PU at different concentrations as emitter material. The electroluminescence (EL) spectra coincided with the PL spectra of HBDT indicating that EL emission originates from the dopant molecules. The dependence of the EL efficiency on the doping concentration was measured and found to be close to the concentration dependence of the PL quantum yield.