Abstract:
An optically compensated birefringence liquid crystal alignment agent. The liquid crystal alignment agent includes one or more polymerizable monomers polymerized to form a polymer having liquid crystal alignment memory on an alignment layer by irradiating an energy ray. Liquid crystals are rapidly converted from splay state to bend state along the polymer memory direction so that a liquid crystal display reaches steady state immediately by only applying normal voltage, without application of high voltage. The invention also provides a liquid crystal display including the liquid crystal alignment agent and a method for fabricating the liquid crystal display.

Description:
BACKGROUND  
       [0001]     The present invention relates to a liquid crystal display, and more specifically to an optically compensated birefringence liquid crystal display.  
         [0002]     Liquid crystal displays are widely used in various applications due to low power consumption and lightweight for mobile.  
         [0003]     Unfortunately, there are some drawbacks, for example, contrast ratio may be deteriorated by the increased viewing angle, thus, extension of viewing angle is limited. Compared to CRT displays, Liquid crystal displays have slower response speed, resulting in image delay. The National Television Standard Committee (NTSC) dictates that a video frequency signal must be shown once within a 16.7 ms period. Currently, response speed between white and black displays is adequate. Response speed for multiple gray levels is slow, however, particularly for regions having a lower effective applied voltage difference. Thus, a liquid crystal display with wide viewing angle and high response speed is desirable.  
         [0004]     To solve the problems, an optically compensated birefringence (OCB) liquid crystal display has been developed.  FIG. 1  depicts a cross-section of a conventional optically compensated birefringence (OCB) liquid crystal display  10 . The liquid crystal display  10  includes an upper substrate  20  having an upper electrode  22  and an upper alignment layer  24  formed thereon in order, a lower substrate  50  having a lower electrode  52  and a lower alignment layer  54  formed thereon in order, and a liquid crystal layer  40  comprising a plurality of liquid crystals  42  installed between the upper and lower substrates. Referring to  FIGS. 2   a - 2   c , liquid crystal arrangements of the optically compensated birefringence (OCB) liquid crystal display  10  with various applied voltages are illustrated. The liquid crystal layer  40  comprises a first liquid crystal region A contacting with the upper alignment layer  24 , a third liquid crystal region C contacting with the lower alignment layer  54 , and a second liquid crystal region B installed therebetween.  
         [0005]     Referring to  FIG. 2   a , the liquid crystals  42  in the first and third liquid crystal regions (A and C) have small included angles with the upper and lower alignment layers ( 24  and  54  ), respectively, at the initial state of zero applied voltage, and the liquid crystals  42  in the second liquid crystal region B are almost parallel to the alignment layer. These liquid crystals present a splay state arrangement.  
         [0006]     Referring to  FIG. 2   b , the liquid crystals  42  in the first and third liquid crystal regions (A and C) have small included angles with the upper and lower alignment layers ( 24  and  54 ), respectively, with an increased applied voltage from zero to a critical voltage (Vc), and only the central liquid crystal  42  in the second liquid crystal region B is perpendicular to the alignment layer. These liquid crystals present a bend state arrangement, which is a bright state of an optically compensated birefringence (OCB) liquid crystal display.  
         [0007]     Referring to  FIG. 2C , the liquid crystals  42  in the second liquid crystal region B are almost perpendicular to the alignment layer, with an increased applied voltage from the critical voltage (Vc) to a Vd voltage (more greater than Vc), which is a dark state of an optically compensated birefringence (OCB) liquid crystal display. The optically compensated birefringence (OCB) liquid crystal display provides high response speed and wide viewing angle due to the regular arrangement of the liquid crystals cooperated with OCB special optical film.  
         [0008]     The operating voltage of an optically compensated birefringence (OCB) liquid crystal display ranges from Vc (critical voltage) to Vd. A high voltage (usually, more than 20 V) converts the splay state to the bend state, that is to say that the extra driving system needs to use in OCB. As shown in  FIG. 3 , correct retardation ( Δ nd) and compensated film are very important in OCB panel.  
       SUMMARY  
       [0009]     The invention provides an optically compensated birefringence liquid crystal alignment agent to polymerize a polymer having liquid crystal alignment memory on an alignment layer. Liquid crystals are rapidly converted from a splay state to a bend state along the polymer memory direction so that a liquid crystal display immediately reaches steady state at an initial voltage, without requiring application of a high voltage to be driven.  
         [0010]     The optically compensated birefringence liquid crystal agent comprises a polymerizable monomer having formula (I) or (II):  
                         
 
         [0011]     wherein R is the same or different and comprises H or methyl, n is 1-5, and m and 1 are 0-12. The invention also provides a liquid crystal display comprising a first substrate having a first surface and a second substrate having a second surface, an alignment layer formed on the first and second surfaces, respectively, a liquid crystal layer, and a polymer having liquid crystal alignment memory, wherein the first and second substrates are parallel and the first surface is opposite to the second surface. The method for fabricating the polymer having liquid crystal alignment memory and the liquid crystal layer is provided, comprising the following steps. A liquid crystal composition comprising a liquid crystal compound and the disclosed optically compensated birefringence liquid crystal alignment agent is prepared. The liquid crystal composition is injected into a space between the first and second substrates. A first voltage is applied to convert the liquid crystal compound of the liquid crystal layer from a splay state to a bend state. After the liquid crystal compound reaches steady state, a second voltage is applied to leave the liquid crystal compound in the splay state or the bend state. An energy ray is then applied to polymerize the polymerizable monomer having formula (I) or (II) to form the polymer having liquid crystal alignment memory with continuous application of the second voltage. The invention provides a rapidly drivable optically compensated birefringence liquid crystal display.  
         [0012]     The invention further provides a method for fabricating a liquid crystal display, comprising the following steps. A liquid crystal composition comprising a liquid crystal compound and the disclosed optically compensated birefringence liquid crystal alignment agent is prepared. Next, a first substrate having a first surface and a second substrate having a second substrate are provided, wherein the first and second substrates are parallel and the first surface is opposite to the second surface. Next, an alignment layer is formed on the first and second surfaces, respectively. The liquid crystal composition is then injected into a space between the first and second substrates. Next, a first voltage is applied to the electrodes of the first and second substrates to convert the liquid crystal compound from a splay state to a bend state. After the liquid crystal compound reaches steady state, a second voltage is applied to leave the liquid crystal compound in the splay state or the bend state. An energy ray is then applied to polymerize the polymerizable monomer having formula (I) or (II) to form polymer layers on both sides of substrates having liquid crystal alignment memory with continuous application of the second voltage.  
         [0013]     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:  
         [0015]      FIG. 1  is a cross section of a related optically compensated birefringence liquid crystal display.  
         [0016]      FIGS. 2   a - 2   c  show liquid crystal arrangements of optically compensated birefringence liquid crystal display under various applied voltages.  
         [0017]      FIG. 3  shows a relationship between applied voltage and light transmittance of conventional optically compensated birefringence liquid crystal displays having various pretilt angles.  
         [0018]      FIGS. 4   a - 4   c  are cross sections of an optically compensated birefringence liquid crystal display of the invention.  
         [0019]      FIGS. 5   a - 5   b  show shift directions of polymerizable monomers and liquid crystals are the same in the invention, without being influenced by electric field, if no UV irradiation.  
         [0020]      FIGS. 6   a - 6   b  show shift directions of polar monomers are influenced by electric field.  
         [0021]      FIG. 7  shows a relationship between voltage and brightness of the invention fabricated under various voltage applications.  
         [0022]      FIG. 8  shows one case of a relationship between voltage and brightness of a liquid crystal display of the invention.  
         [0023]      FIG. 9  shows a scanning electron microscope spectrogram of the lower substrate of the optically compensated birefringence liquid crystal display in example 1 of the invention.  
         [0024]      FIG. 10  shows a view angle diagram of an optically compensated birefringence liquid crystal display of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0025]     The invention provides a liquid crystal composition comprising an optically compensated birefringence liquid crystal alignment agent to form a pair of polymer layers by phase separation method, which nano-functional layers having liquid crystal alignment memory on an alignment layer to substantially reduce the warm-up time without extra driving circuit.  
         [0026]     The invention provides a method for fabricating a liquid crystal display, comprising the following steps. A liquid crystal composition comprising a liquid crystal compound, such as an optically compensated birefringence (OCB) liquid crystal compound, and an optically compensated birefringence liquid crystal alignment agent is prepared. Preferably, the liquid crystal compound has a positive dielectric constant difference. The optically compensated birefringence liquid crystal alignment agent comprises a polymerizable monomer having formula (I) or (II).  
                         
 
         [0027]     In formula (I) or (II), R is the same or different and may comprise H or methyl, n may be 1-5, and m and 1 may be 0-12, preferably 1-11. The polymerizable monomer having formula (I) or (II) has a weight percentage of about 0.01-30 wt%, preferably 0.1-5 wt%, in the liquid crystal composition. Specifically, the polymerizable monomer having formula (I) or (II) can be polymerized by applying ray or energy, without addition of any initiator. Residual image or response delay resulting from remaining initiators can thus be avoided. Nevertheless, low quantities of initiators, such as light initiators or heat initiators, less than 0.05 wt% can be used to increase the rate of polymerization. The polymerizable monomers having formula (I) or (II) provided by the invention are shown in Table 1. Specifically, 80% purity or more of the polymerizable monomer having formula (I) or (II) is required to avoid non-uniform optical performance deteriorating image quality.  
                   TABLE 1                       Number   Polymerizable monomer structure                               1                                             2                                             3                                             4                                             5                                             6                                             7                                             8                                             9                                                
 
         [0028]     Next, referring to  FIG. 4   a , a first substrate  120  having a first surface  121  and a second substrate  150  having a second surface  151  are provided. The first substrate  120  and the second substrate  150  are parallel and the first surface  121  is opposite to the second surface  151 . The first substrate  120  and the second substrate  150  are separated by a plurality of spherical particles or spacers (nor shown) to form a space  160 . Next, a color filter  122 , a first electrode  124 , and a first alignment layer  126  are formed on the first surface  121 , and a second electrode  152  and a second alignment layer  154  are formed on the second surface  151  in order. The alignment layers ( 126  and  154 ) may comprise polyimide. Furthermore, a first polarizer  130  is installed outside the first substrate  120  and a second polarizer  170  is installed outside the second substrate  150 . Specifically, the first and second alignment layers ( 126  and  154 ) are rubbed after being formed and their alignment directions are approximately the same.  
         [0029]     Next, the liquid crystal composition is injected into the space  160  between the first substrate  120  and the second substrate  150  by a capillarity fill process or a one-drop fill (ODF) process. A first voltage of about 0-40 V is then applied to produce a potential difference between the first and second electrodes to convert the liquid crystal compound from a splay state to a bend state. When the potential difference is formed, a capacitor comprising the first electrode  124 , the second electrode  152 , and the liquid crystal compound is formed, simultaneously. A second voltage of about 0-10 V is applied to leave the liquid crystal compound in the splay state or the bend state, preferably the bend state, after reaching steady state, as shown in  FIG. 4   b . The polymerizable monomer  141  having formula (I) or (II) is also shifted to the bend state as the liquid crystal compound shifts, without being influenced by electric field, symmetrical molecules, as shown in  FIGS. 5   a  and  5   b . On the contrary, the asymmetric molecules under electric field easily become a shift direction different from the liquid crystal compound in electric field, as shown in  FIGS. 6   a  and  6   b . Various electrical performances among a pure liquid crystal compound, a polar-doped liquid crystal compound, and a non-polar-doped liquid crystal compound are compared in Table 2.  
                                                     TABLE 2                                           Specific           Doping amount   Leakage   resistance           (%)   current (pA)   (ρ)                                        Pure liquid   —   2.4   1.10E+14           crystal           compound           Asymmetric   5%   440   6.90E+11           molecule           Symmetrical   5%   2   1.67E+14           molecules                      
 
         [0030]     The results indicate that the asymmetric liquid crystal mixtures have 100 times the leakage current of the symmetric liquid crystal mixtures.  
         [0031]     Next, referring to  FIG. 4   c , after a second voltage is applied to leave the liquid crystal compound in the splay state or the bend state, an energy ray, preferably ultraviolet, is applied to polymerize the shifted polymerizable monomer to form a polymer  143  with continuous application of the second voltage. The shifted polymerizable monomer is polymerized on the alignment layer so that the formed polymer  143  remains in the original shift direction of the monomer, having liquid crystal alignment memory. The alignment layer having the polymer formed thereon has a pretilt angle of about 1-25°. Clearly, the polymer  143  is formed on the alignment layers ( 126  and  154 ) on both sides of a liquid crystal layer  140  comprising the liquid crystal compound  142 , that is, formation of a functional nano-surface structure, not a network structure in all cell. Referring to  FIG. 7 , a relationship between voltage and brightness of optically compensated birefringence liquid crystal displays fabricated under various second voltage applications is illustrated. The results indicate that the applied second voltage affects electrical performance of liquid crystal displays.  
         [0032]     To remove the remaining polymerizable monomer  141 , a thermal process or a visible light irradiation is performed to consume the polymerizable monomer  141  completely. The thermal process is performed at a temperature of about 50-250° C.  
         [0033]     The fabrication method of the invention is also suitable for fabricating color filter on array (COA), transflective, twisted nematic (TN), multi-domain vertical alignment (MVA), or patterned multi-domain vertical alignment (PMVA) liquid crystal displays.  
         [0034]     Referring to  FIG. 8 , a relationship between applied voltage and brightness of a preferable liquid crystal display of the invention is illustrated. The doping amount of the polymerizable monomer having formula (I) or (II) is 3.5 wt%. The liquid crystal approaches the bend state at a lower initial voltage, avoiding a non-continuous arrangement from the splay state to the bend state.  
         [0035]     Compared to related art optically compensated birefringence liquid crystal displays, the invention provides a liquid crystal display having faster response time.  
       EXAMPLES  
     Comparative Example 1  
       [0036]     The response time of various liquid crystal displays was measured. The liquid crystal layers thereof comprise non-doped optically compensated bend liquid crystal compound A (OCB LC-A, manufactured and sold by Merck, Δ n=0.171, Δ ε=11.4, γ=166.0 mPa·s), non-doped optically compensated bend liquid crystal compound B (OCB LC-B, manufactured and sold by Chisso, Δ n=0.169, Δ ε=10.1, γ=213.0 mPa·s), and non-doped optically compensated bend liquid crystal compound C (OCB LC-C, manufactured and sold by DIC, Δ n=0.180, Δ ε=12.5, γ=157.0 mPa·s), respectively. Each of the polyimide alignment layers thereof has various rubbing depth of 0.4, 0.6, and 0.8 mm. The voltages of 2.5 V, 6.5 V, and 2.5 V are applied in order. The measurement results are shown in Table 3.  
                                                                                 TABLE 3                                       Rubbing depth                0.4 mm   0.6 mm   0.8 mm            Liquid   Response time            crystal   Tr/Tf   Sum   Tr/Tf   Sum   Tr/Tf   Sum               OCB   0.39/2.35   2.74   0.39/2.35   2.74   0.39/2.75   3.14       LC-A   0.59/2.35   2.94   0.59/2.35   2.94   0.39/2.75   3.14           0.59/2.35   2.94   0.39/2.35   2.74   0.59/2.75   3.34           Average   2.87   Average   2.81   Average   3.21       OCB   0.59/2.75   3.34           0.59/2.94   3.53       LC-B   0.59/2.94   3.53           0.59/2.95   3.54           0.59/2.95   3.54           0.59/2.96   3.55           Average   3.47           Average   3.54       OCB   0.39/2.75   3.14   0.39/2.35   2.74   0.59/2.16   2.75       LC-C   0.39/2.76   3.14   0.39/2.35   2.75   0.59/2.16   2.75           0.39/2.77   3.14   0.39/2.35   2.74   0.39/2.35   2.74           Average   3.14   Average   2.74   Average   2.75                  
 
       EXAMPLE 1  
       [0037]     2.0 wt% polymerizable monomer  9  disclosed in Table 1 was doped into the optically compensated bend liquid crystal compound (manufactured and sold by Chisso, Δ n=0.142, Δ ε=10.5, γ=35.7 mPa·s). The liquid crystal compound is then injected into a liquid crystal display. Next, a voltage is applied to convert the liquid crystal compound from a splay state to a bend state. The polymerizable monomer is polymerized by ultraviolet irradiation under 5 V. The response time thereof is measured in the same way as comparative example 1, as recited in Table 4.  
         [0038]     Referring to  FIG. 9 , a scanning electron microscope spectrogram of the lower substrate of the optically compensated birefringence liquid crystal display disclosed in example 1 is shown. A lower electrode  210 , an alignment layer  220 , and a polymer  230  having liquid crystal alignment memory are formed on the lower substrate  200  in order. The polymer  230  heightens a pretilt angle and reduces surface free energy of the alignment layer, to rapidly convert the liquid crystal compound from splay state to bend state.  
       EXAMPLE 2  
       [0039]     2.05 wt% polymerizable monomer  9  disclosed in Table 1 was doped into the optically compensated bend liquid crystal compound (manufactured and sold by Chisso,Δ n=0.142, Δ ε=10.5, γ=135.7 mPa·s). The liquid crystal compound is then injected into a liquid crystal display. Next, a voltage is applied to convert the liquid crystal compound from splay state to bend state. The polymerizable monomer is polymerized by ultraviolet irradiation under 5 V. The response time thereof is measured in the same way as comparative example 1, as recited in Table 4.  
                                                                                       TABLE 4                                   Example 1       Example 2                                        Doping   2 wt %       2.5 wt %               amount           Polymerizing   5 V       2.5 V           voltage                Response   0.39/1.96   2.35   0.39/2.75   3.14           time   0.39/1.76   2.15   0.39/2.76   3.14               0.39/1.77   2.16   0.39/2.35   2.74               Average   2.22   Average   3.01                      
 
         [0040]     Referring to  FIG. 10 , a viewing angle diagram of an optically compensated birefringence liquid crystal display of the invention is shown. The maximum contrast ratio (CR) thereof is about 810 and its full viewing angle contrast ratio is almost greater than 10.  
         [0041]     The invention provides a liquid crystal composition comprising an optically compensated birefringence liquid crystal alignment agent to form a polymer having liquid crystal alignment memory on an alignment layer to substantially reduce consumption time and applied voltage requirements for converting a splay state to a bend state in an optically compensated birefringence liquid crystal display in the initial operation and acquire faster response time, as shown in Tables 3 and 4.  
         [0042]     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.