The present invention relates to material for aligning liquid crystals, and liquid crystal optical elements such as liquid crystal displays (LCDs), liquid crystal devices and liquid crystal optical films.
Current liquid crystal displays (LCD) include a product that utilize a twisted nematic mode, i.e., having a structure wherein the aligning direction of nematic liquid crystal molecules is twisted by 90° between a pair of upper and lower substrates, a product utilizing a supertwisted nematic mode, utilizing a birefringent effect, i.e. having a structure wherein the aligning direction of nematic liquid crystal molecules is twisted by 180° to 300°, a product utilizing optically compensated bend (OCB) mode wherein the aligning direction of nematic liquid crystal molecules is parallel between a pair of upper and lower substrates, an in-plane-switching mode wherein both electrodes controlling the liquid crystal alignment are present on one substrate and the direction of the liquid crystal orientation in the plane of the substrate changes upon application of an electric field, and a product utilizing a ferroelectric liquid crystal substance or an antiferroelectric liquid crystal substance. In addition, there are LCD products that utilize the vertical alignment mode, i.e., the liquid crystal is aligned predominantly normal to the substrates (homeotropic alignment) and upon application of an electric field, the liquid crystal reorients into the plane of the alignment layer.
Many LCDs comprise one or more liquid crystal optical films to broaden the viewing angles of the displays. These optical films are called wide-viewing films, phase compensation films or optical retardation films, wherein reactive liquid crystal mesogens were first aligned on a substrate and then subsequently polymerized to form the optical films.
Common to each of these liquid crystal optical elements is a liquid crystal layer disposed on a substrate or between a pair of substrates coated with a polymeric alignment layer. The polymeric alignment layer controls the direction of alignment of the liquid crystal medium in the absence of an electric field. Usually the direction of alignment of the liquid crystal medium is established in a mechanical rubbing process wherein the polymer layer is rubbed with a cloth or other fibrous material. The liquid crystal medium contacting the rubbed surface typically aligns parallel to the mechanical rubbing direction. For the vertical alignment mode, it is desired that the liquid crystals be aligned slightly off normal to the substrates for better electro-optical performance. Mechanical rubbing, substrate protrusions or electrode geometries are used to give this alignment slightly off normal for the vertical alignment mode. Alternatively, an alignment layer comprising anisotropically absorbing molecules can be exposed to polarized light to align a liquid crystal medium as disclosed in U.S. Pat. Nos. 5,032,009 and 4,974,941 “Process of Aligning and Realigning Liquid Crystal Media.” This non-contact method of alignment is suitable for all the liquid crystal elements such as all the LCD products or modes and liquid crystal optical films mentioned previously.
The process for aligning liquid crystal media with polarized light can be a non-contact method of alignment that has the potential to reduce dust and static charge buildup on alignment layers. Other advantages of the optical alignment process include high resolution control of alignment direction and high quality of alignment.
Requirements of alignment layers for liquid crystal displays include low energy threshold for optical alignment, good mechanical properties for mechanical rubbing alignment, transparency to visible light (no color), good dielectric properties and voltage holding ratios, long-term thermal and optical stability, and in many applications a controlled uniform pre-tilt angle and uniform homeotropic alignment.
Polymers used in forming alignment layers also must have a reasonably broad processing window. Polymers used as alignment layer in commercial liquid crystal displays are generally polyimide-based systems because of their good thermal and electrical properties.
One disadvantage to using polyimides in forming optical alignment layers is that they generally require high doses of polarized light (5-30J/cm2) to induce high quality optical alignment, as disclosed in U.S. Pat. No. 5,958,292. Disadvantages for requiring high doses of polarized light include low throughput in an assembly line due to increased residence time for the substrate in the exposure system, potential damage to the transistors and color filters needed in modern display systems and photodegradation of the alignment layer itself that may impair the long-term stability and performance of the device.
Photoreactive polymers other than polyimides (such as polymethacrylates and polysiloxanes) that provide satisfactory quality alignment with low doses of polarized light (0.05-5 J/cm2) have been described in U.S. Pat. No. 6,224,788, “Liquid Crystal Aligning Agent and Process for Producing Liquid Crystal Alignment Film Using the Same” and U.S. Pat. No. 5,824,377 “Photosensitive Material for Orientation of Liquid Crystal Device and Liquid Crystal Device Thereof.” When irradiated with polarized light, these materials undergo photo-crosslinking to produce optical alignment layers. Advantages of these polymers include higher mobility of the polymer backbone leading to more efficient photo-crosslinking reactions and higher densities of photoreactive groups due to the smaller repeat unit for the polymer. The high density and high mobility of photoreactive groups leads to the requirement of lower doses of polarized light for good alignment. However, the listed physical features that provide advantages in the optical density thresholds can provide for reduced electrical performance and optical stability of devices. In, for example, a thin film transistor TN display, this can result in an inadequate voltage holding ratio (VHR, a measure of the voltage drop in the display after the supplied electrical field has been switched off).
An approach to incorporating multiple desired properties (such as improving VHR) of materials for optical alignment layers is described in WO 99/49360 “Liquid Crystal Orientation Layer” and WO 01/72871 A1 “Polymer Blend for Preparing Liquid Crystal Alignment Layer.” Blends of polymeric compounds containing photoreactive polymers (typically non-polyimide) and polyimides are proposed as a method to improve the inadequate VHR of the non-polyimide by blending with material having high VHR (typically a polyimide). The blends have the disadvantage of limited miscibility and, thus, limit the quantity of photoreactive material available for alignment.
An approach to incorporating multiple desired properties into a polyimide for conventional liquid crystal alignment layers has been described in U.S. Pat. No. 5,773,559 “Polyimide Block Copolymer and Liquid Crystal Alignment Layer Forming Agent”. In this process, polyimide block copolymers, wherein a polyimide-type block is copolymerized with a different polyimide-type block, are described which provide multiple properties that are difficult to obtain by conventional polyimide synthesis.
Copolymerization of related monomers is well known in the art. Less well known is the copolymerization of unrelated monomers or polymers to form copolymers, particularly between polyimide-type polymers and addition-type polymers. Curable compositions of polyimides containing reactive double-bonds combined with crosslinking reagents such as tetraethylene glycol diacrylate for use in electronic or optical components have been described in, for example, U.S. Pat. No. 4,778,859. These materials form a cross-linked matrix during cure conditions; however the architecture of the matrix that is formed is not known and cannot be controlled. Hedrick et al (Advances in Polymer Science, Vol 141, 1999, pg 1-43 and references therein) describes the synthesis of ABA triblock and graft copolymers for the preparation of foamed polyimides. In the case of the ABA triblock copolymers, the polyimide block is terminated by amine terminated oligomers of poly(styrenes), poly(methylmethacrylates) and poly(propylene oxides). In the case of the graft copolymers, the oligomers are terminated with diamines. These materials are specifically designed to undergo microphase separation between the thermally stabile polyimide blocks and the thermally labile oligomer blocks. Upon heating, the thermally labile oligomer blocks decompose, leaving nanometer size pores in the structure. U.S. Pat. No. 4,539,342 “Polyimide Foam Prepared from Amino Terminated Butadiene Acrylonitrile Reactant” describes a polyimide foam wherein one of the components is an amine-terminated butadiene-acrylonitrile copolymer. The described materials are known in the art as segmented block copolymers and provide foams that are flexible and resilient and provide high vapor-barrier characteristics. Similarly, U.S. Pat. No. 4,157,430 “Amine Terminated Polymers and the Formation of Block Copolymers” describes the synthesis of amine terminated butadiene polymers for the formation of block copolymers as thermoset rigid foams. The authors describe, but do not teach, the concept of copolymers with polyimides. However, microphase separation and polyimide foam characteristics, common features to the aforementioned papers and patents, are not desirable properties used in materials for liquid crystal alignment layers.
In further developing materials and processes for optical alignment layers, we have invented a novel copolymer comprising units from addition polymers and polyimides, which is described herein. These new materials for alignment layers were invented to remove or reduce the disadvantages of optical alignment layers described previously. We refer to this new class of polymers as hybrid polymers. In addition, we have discovered that the hybrid polymers are suitable as alignment materials for LCDs that utilize mechanical rubbing, substrate protrusions or electrode geometries to align the liquid crystals in either homogeneous or homeotropic alignment. For these applications, it is not a requirement that the hybrid polymers be photoreactive. These hybrid polymers are prepared from at least one component selected from the group consisting of monomer, oligomer and polymer within the class of polyimides, poly(amic acids) and esters thereof and at least one component selected from the group consisting of addition monomer and functionalized addition polymer wherein the two components are covalently bonded to form a copolymer. This novel class of new, hybrid polymers combines the good thermal and electrical properties of polyimides with the high density and high mobility advantages of addition polymers such as polymethacrylates, polyacrylates, polyolefins and polystyrenes. In this way, multiple desirable properties that are difficult to obtain by other materials and processes can be achieved.