Qingbing Wang

Shape-persistent V-shaped mesogens—formation of nematic phases with biaxial order

A homologous series of shape-persistent V-shaped molecules has been designed to form the biaxial nematic phase. Phenyleneethynylene moieties are attached to a bent fluorenone unit to create an apex angle of about 90°, which is determined from the single crystal structure. Two mesogens, one symmetric and another unsymmetric, have been synthesized by attaching a cyano group to one or both of the peripheral phenyl units, respectively. These groups introduce local dipoles essential for the formation of the nematic phases. The tendency to form a crystalline phase is reduced by laterally substituted hexyloxy chains which allow the nematic phase to be supercooled to a glassy state. Two of the three fluorenone derivatives exhibit a transition from the uniaxial nematic to the biaxial nematic phase. This transition has an undetectably small transition enthalpy, but the X-ray diffraction, polarizing optical microscopy, and conoscopy reveal the presence of the biaxial order in the low temperature nematic phase.

Control of Polymer Structures in Phase-Separated Liquid Crystal-Polymer Composite Systems

The formation of different structures in phase-separated composite films (PSCOFs) from formulations of liquid crystals and photocurable monomers has been investigated using polarizing optical and scanning electron microscopies. Two processes, spatially nonuniform polymerization and diffusion of small molecules, play important roles in determining a specific PSCOF polymer structure. The variations in UV irradiation intensity and temperature, at which phase separation is carried out, strongly influence these two processes and can change the resultant structure from a homogeneous PSCOF bilayer structure to a heterogeneous polymer dispersed liquid crystal (PDLC) structure. A variation in cell thickness changes the distance through which small molecules have to diffuse during phase separation and, thus, affects the structure obtained in thick and thin cells.

Submillisecond switching of nematic liquid crystal in cells fabricated by anisotropic phase-separation of liquid crystal and polymer mixture

Liquid crystal (LC) cells consisting of a very thin (submicrometer) nematic LC layer switch with time response smaller than 1ms. The total response time, i.e., sum of the turn-on and turn-off times, can be made as small as 1.3ms. Such devices were prepared by a photoinduced anisotropic phase separation of mixtures of the LC and prepolymer. The formation of the LC–polymer bilayer in these devices was confirmed by polarized light microscopy and scanning electron microscopy. The phase separation method permits one to fine-tune the LC film thickness by varying the LC concentration in the mixture. This technique can be used to fabricate fast LC devices for TV and video applications.

Flexible Plastic Displays Fabricated Using Phase-Separated Composite Films of Liquid Crystals

A method of fabricating flexible plastic liquid crystal displays with a polymer-embedded spacers has been developed using phase-separated composite films. Scanning electron microscopy confirms the formation of a solidified polymer layer that strongly holds the spacers at their initial sites in these liquid crystal cells. The electrooptical properties of plastic liquid crystal cells prepared by this method were measured and analyzed under flat and bent conditions. Optics simulations suggested that the use of polymer materials with large permittivity would help decrease the driving voltage of phase-separated composite films (PSCOF) liquid crystal cells.

Method for fabrication of liquid‐crystal cells with narrow gap, fast switching, and flexible plastic substrates

— A phase-separation method for the construction of devices with specific internal architecture of LC/polymer composite system is presented. The method results in adjacent uniform polymer and LC films parallel to the substrates. Scanning electron microscopy was employed to investigate the internal structures. The results show that the thicknesses of the LC and the polymer films in cells constructed with fixed-size spacers directly depends on the LC/polymer in the initial mixture. This can be effectively used for precision cell-gap control and to fine-tune the LC optical path length. Cells with a submicrometer gap, prepared with this method and with 35 wt.% of LC in the mixture, exhibited a total response time approaching 1 msec. This method has also been used to fabricate devices with plastic substrates.

Different limits of phase separation and their applications

The process of phase separation leads to several well known display technologies such as Polymer Dispersed liquid crystals and Polymer Stabilized cholesteric and ferroelectric devices. Several new limits of the general phenomena of phase separation have been discovered in recent years. In one of the limits, a very simple and powerful process known as the phase separated composite structures method permits the construction of conventional devices; such as, TN, STN, and FLC devices with great ease and with flexible substrates. It has also been employed in the fabrication of one- and two-dimensional optical gratings and flys eye lenses (micro-lens array) with electrically controllable focal length. In the second limit, one obtains microscopic polymer columns perpendicular to the substrates. These structures have been used to fabricate large area homeotropic nematic devices having very high-contrast.

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.

Liquid crystal photonic microdevices

We have utilized the process of three dimensional anisotropic phase separation of mixtures of liquid crystal (LC) and polymer to fabricate electro-optical and photonic devices such as one and two dimensional diffraction gratings and microlens arrays. The operation of optical gratings and microlens arrays (or, flys eye lenses) is controllable with an external electric field. This method has also been applied to one obtain internal microscopic polymer columns perpendicular to the substrates in optical devices. These column structures have been used to fabricate large area flexible substrate homeotropic nematic devices having high-contrast. This method has also been used to construct conventional devices; such as, TN, STN, and FLC devices with great ease and with flexible substrates.