What is probed into the invention is a liquid crystal device with stratified phase-separated composite and method for forming the same. Detailed descriptions of the composite composition and device structure will be provided in the following in order to make the invention thoroughly understood. Obviously, the application of the invention is not confined to specific details familiar to those who are skilled in the art. On the other hand, the common structures and fabricating steps that are known to everyone are not described in details to avoid unnecessary limits of the invention. Some preferred embodiments of the present invention will now be described in greater details in the following. However, it should be recognized that the present invention can be practiced in a wide range of other embodiments besides those explicitly described, that is, this invention can also be applied extensively to other embodiments, and the scope of the present invention is expressly not limited except as specified in the accompanying claims.
In the first embodiment of the present invention, a liquid crystal device is disclosed. The liquid crystal device comprises a first substrate covered with an alignment layer and a composite material, wherein the composite material is phase-separated by a first polymerization into a polymer layer (continuous polymer layer is preferred) and a liquid crystal layer, wherein the liquid crystal layer is disposed adjacent to the alignment layer, and the polymer layer is disposed adjacent to the liquid crystal layer. Furthermore, the liquid crystal layer comprises polymer formed in situ by a second polymerization. In a preferred example of this embodiment, the mentioned liquid crystal device further comprises a second substrate located atop said polymer layer, wherein the second substrate is in planer contact with the polymer layer. Furthermore, the material of the substrates is selected from a group consisting of: polycarbonate (PC), poly ethylene terephthalate (PET), poly urethane (PU), poly-methylmethacrylate (PMMA), metallocene catalyzed cyclic olefin copolymer (mCOC), indium tin oxide (ITO) coated membrane and derivatives thereof.
In this embodiment, the composite material is formed into the layers in substantially planar form by anisotropic phase separation in the vertical direction from a solution of polymer precursor and liquid crystal. The content of liquid crystal ranges from 10% to 90% of the total weight of the solution. The liquid crystal is selected from the calamitic group consisting of nematic liquid crystal, smectic liquid crystal, cholesteric liquid crystal and (anti-)ferroelectric liquid crystal. Moreover, the first polymerization is performed by photo illumination on the covered side of the first substrate, so that the liquid crystal layer is formed adjacent to the alignment layer, and the polymer layer is formed adjacent to the liquid crystal layer when the solution is phase separated.
Afterwards, the second polymerization in the liquid crystal layer is performed by photo illumination on the uncovered side of the first substrate. The subsequently formed liquid crystal layer, which contains polymer inside, is classified into one of the three categories: polymer-dispersed liquid crystal (PDLC), polymer-network liquid crystal (PNLC) and polymer-stabilized liquid crystal (PSLC). In another preferred example of this embodiment, the mentioned polymer is cross-linked, and so does the polymer in the liquid crystal layer. These cross-linked structures are stronger and more durable than uncross-linked structures. Especially in the flexible display field, the variation of the cell gap in the bending condition is greatly reduced by the aids of cross-linked polymer structure in the thin display. In liquid crystal layer, the cross-linked polymer comprises “bridges” in horizontal direction and vertical direction, wherein the vertical bridges, connecting the polymer layer with the alignment layer, primarily provide the ability to bend or roll a display into any desired shape.
The mentioned liquid crystal device is selected from the group consisting of a display device, a spatial light modulator, a wavelength filter, a variable optical attenuator (VOA), an optical switch, a light valve, a color shutter, a lens and lens with tunable focus. Moreover, when the liquid crystal device is a display device, which is a direct addressing, a multiplexed, or an active-matrix addressing TN (twisted nematic), HAN (hybrid-aligned nematic), VA (vertical alignment), planar nematic, STN (super-TN), optically compensated bend (OCB), IPS (in plane switching) or FFS (fringe field switching) mode liquid crystal display.
In the second embodiment of the present invention, a method for fabricating a liquid crystal device with stratified phase-separated composite is disclosed. First, a solution of polymer precursor and liquid crystal is provided, wherein the content of liquid crystal ranges from 10% to 90% of the total weight of the solution. The selection of the liquid crystal is described in the first embodiment. Furthermore, the solution can further comprise cross-linking agent. Next, a substrate covered with an alignment layer is provided. The solution is then coated onto the alignment layer. Afterwards, a first polymerization of the solution is performed by a first photo illumination applied directly on said solution, so as to induce anisotropic phase separation of the solution to form a polymer layer (continuous polymer layer is preferred) and a liquid crystal layer, wherein the liquid crystal layer is formed adjacent to the alignment layer, and the polymer layer is formed adjacent to the liquid crystal layer, whereby an intermediate device is formed. Finally, a second polymerization is performed in the liquid crystal layer by a second photo illumination on the uncovered side of said substrate, so as to produce polymer fibrils in the liquid crystal layer.
In this embodiment, the temperature of the solution in the first polymerization is equal to or more than 70° C., and the intensity of the first photo illumination ranges from 0.05 mW/cm2 to 0.5 mW/cm2. After the first polymerization and before the second polymerization, the content of liquid crystal in the liquid crystal layer ranges from 30% to 99% of the total weight of the liquid crystal layer. According to different polymer content in the liquid crystal, the subsequently formed liquid crystal layer comprising polymer is classified into one of the group consisting of polymer-dispersed liquid crystal (PDLC), polymer-network liquid crystal (PNLC) and polymer-stabilized liquid crystal (PSLC).
In a preferred example of this embodiment, after the first polymerization, the intermediate device is cooled to be equal to or less than 35° C. Moreover, the interval between the first polymerization and the second polymerization can be equal to or more than 12 hours, and more preferred, equal to or more than 24 hours. Additionally, in the second polymerization, the temperature of the liquid crystal layer is equal to or less than 70° C., and the intensity of the second photo illumination is equal to or more than 1 mW/cm2.
In the third embodiment of the present invention, a method for fabricating a liquid crystal device with stratified phase-separated composite is disclosed. First, a solution of polymer precursor and liquid crystal is provided, wherein the content of liquid crystal ranges from 10% to 90% of the total weight of the solution. The selection of the liquid crystal is described in the first embodiment. Furthermore, the solution can further comprise cross-linking agent. Next, a first substrate and a second substrate with a cell gap there between are provided, wherein the first substrate is covered with an alignment layer facing the second substrate. The solution is then introduced into the cell gap. Afterwards, a first polymerization of the solution is performed by a first photo illumination on the second substrate, so as to induce anisotropic phase separation of the solution to form a polymer layer (continuous polymer layer is preferred) and a liquid crystal layer, wherein the liquid crystal layer is formed adjacent to the alignment layer, and the polymer layer is formed adjacent to the liquid crystal layer, whereby an intermediate device is formed. Finally, a second polymerization is performed in the liquid crystal layer by a second photo illumination on the first substrate, so as to produce polymer fibrils in the liquid crystal layer.
In this embodiment, the temperature of the solution in the first polymerization is equal to or more than 70° C., and the intensity of the first photo illumination ranges from 0.05 mW/cm2 to 0.5 mW/cm2. After the first polymerization and before the second polymerization, the content of liquid crystal in the liquid crystal layer ranges from 30% to 99% of the total weight of the liquid crystal layer. According to different polymer content in the liquid crystal, the subsequently formed liquid crystal layer comprising polymer is classified into one of the group consisting of polymer dispersed liquid crystal (PDLC), polymer network liquid crystal (PNLC) and polymer-stabilized liquid crystal (PSLC).
In a preferred example of this embodiment, after the first polymerization, the intermediate device is cooled to be equal to or less than 35° C. Moreover, the interval between the first polymerization and the second polymerization can be equal to or more than 12 hours, and more preferred, equal to or more than 24 hours. Additionally, in the second polymerization, the temperature of the liquid crystal layer is equal to or less than 70° C., and the intensity of the second photo illumination is equal to or more than 1 mW/cm2.
To fabricate a PSCOF for comparison, we mixed 50 wt. % poly(mercaptoesters) NOA-65 (Nordland Optical Adhesives Co.) as a photopolymerizable monomer and 50 wt. % cyano-based nematic liquid-crystal mixture E7 (Merck Co.) which exhibits a positive dielectric anisotropy. By capillary action at a temperature well above the nematic-isotropic phase transition, the blend was introduced into an empty cell consisting of a pair of transparent, electrically conductive glass substrates. Only one of the substrates was spin-coated with a thin film of polyimide as the alignment layer. The cell gap was controlled by ˜5.4-μm ball spacers. In order to obtain a smoothly layered structure, phase separation was initiated by shining the cell with a collimated beam of ultraviolet (UV) light through the untreated substrate at a very low UV intensity (˜0.1 mW/cm2). The sample was kept at 90° C. using a hot stage during the 30-min exposure. After photocuring, the cell was cooled to the room temperature slowly. Polarizing optical microscopy and scanning electron microscopy were exploited to characterize the internal configuration, confirming the double-layer structure.
The production of our stratified polymer-stabilized liquid crystal (SPSLC), which requires an extra UV exposure at a higher intensity of 3 mW/cm2 on the other side of the sample for 30 min at the room temperature, is depicted in
In order to understand the electro-optical properties of a LC device with the SPSLC structure, a typical setup was constructed, enabling one to acquire the transmittance as a function of the ac voltage and to obtain the associated response curve (see
Comparisons of the electro-optical (EO) characteristics between a 5.4-μm SPSLC with a 2.7-μm-thick PSLC layer and a 5.4-μm PSCOF device with a LC thickness of 2.7 μm are illustrated in
In summary, we have demonstrated that nematic LC cells prepared with two-step photopolymerization-induced phase separation of LC and polymer can easily attain a polymer-PSLC bilayer structure, taking advantages of a PSCOF as well as a PSLC. The photoinduced phase separation method allows one to fine-tune the LC film thickness and the polymer content in the LC bulk. In comparison with the Paintable liquid-crystal display technology or the pixel-isolated liquid-crystal mode, this invention requires only a very simple processing procedure for manufacturing and is demonstrated to be fast-switching, exhibiting a response time of the order of 1 ms.
Obviously many modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the present invention can be practiced otherwise than as specifically described herein. Although specific embodiments have been illustrated and described herein, it is obvious to those skilled in the art that many modifications of the present invention may be made without departing from what is intended to be limited solely by the appended claims.