Liquid crystal display device and method for producing same

Abstract

The present invention provides a liquid crystal display device that is less likely to have display failures and has high reliability and excellent light resistance in aging. The present invention relates to a liquid crystal display device, including: a pair of substrates; a liquid crystal layer containing liquid crystal molecules and interposed between the pair of substrates; and an alignment control layer for perpendicularly aligning the liquid crystal molecules, the alignment control layer being formed by polymerizing a monofunctional monomer and a polyfunctional monomer in a liquid crystal composition containing the liquid crystal molecules, the monofunctional monomer, and the polyfunctional monomer, the polyfunctional monomer generating a radical by annealing and irradiation with light at not less than 340 nm, the monofunctional monomer having a biphenyl skeleton as a core and a polymerizable group bonded to the biphenyl skeleton directly or indirectly via a spacer.

Claims

1. A liquid crystal display device, comprising: a pair of substrates; a liquid crystal layer containing liquid crystal molecules and interposed between the pair of substrates; and an alignment control layer for perpendicularly aligning the liquid crystal molecules, wherein: the liquid crystal display device includes substantially no alignment film; the alignment control layer includes a monofunctional monomer, a polyfunctional monomer, and a polymerization initiator that are polymerized while a liquid crystal composition containing the liquid crystal molecules, the monofunctional monomer, the polyfunctional monomer, and the polymerization initiator, is interposed between the pair of substrates; the polymerization initiator generates radicals by a self-cleavage reaction caused by photoirradiation and includes at least two radical polymerizable groups; the polyfunctional monomer generates a radical by annealing and irradiation with light at not less than 340 nm; the monofunctional monomer is represented by Formula (1): ##STR00011## X represents acrylate, methacrylate, ethacrylate, vinyl, or allyl, m represents an integer of 0 to 12, a and b each independently represent 0 or 1, R represents C1-C20 alkyl, and hydrogen atoms in a ring structure may each independently be substituted with halogen atom, methyl, ethyl, or propyl; the ratio of the molar concentration of the polyfunctional monomer in the liquid crystal composition to the molar concentration of the monofunctional monomer in the liquid crystal composition is not less than 1.5% and not more than 15%; the ratio of the molar concentration of the polyfunctional monomer to the molar concentration of the monofunctional monomer is calculated as (mol % of the polyfunctional monomer in the liquid crystal composition)×100/(mol % of the monofunctional monomer in the liquid crystal composition); the total concentration of the polyfunctional monomer and the polymerization initiator relative to the monofunctional monomer is not more than 18 mol %; the polyfunctional monomer is represented by Formula (2):
P.sup.1-A.sup.1-P.sup.1  (2); P.sup.1s each independently represent acrylate, methacrylate, ethacrylate, vinyl, or allyl, A.sup.1 represents phenanthrylene, and hydrogen atoms in A.sup.1 may each independently be substituted with halogen atom, methyl, ethyl, or propyl; and the polymerization initiator is represented by Formula (5): ##STR00012## wherein R.sup.1 represents C1-C4 linear or branched alkyl or alkenyl, or Sp.sup.3-P.sup.3; R.sup.2 represents C1-C4 linear or branched alkyl or alkenyl, or Sp.sup.4-P.sup.4; P.sup.1, P.sup.2, P.sup.3, and P.sup.4 represent radical polymerizable groups, a total number of P.sup.1, P.sup.2, P.sup.3, and P.sup.4 is at least two; Sp.sup.1 and Sp.sup.2 each represent C1-C6 linear, branched, or cyclic alkylene, alkyleneoxy, or carbonyloxy, or a direct bond; Sp.sup.3 and Sp.sup.4 each represent C1-C6 linear, branched, or cyclic alkylene, alkyleneoxy, or carbonyloxy; L.sup.1 and L.sup.2 each represent —F, —OH, or C1-C12 linear or branched alkyl, alkenyl, or aralkyl; m.sup.1 represents an integer of 1 to 3; m.sup.2 represents an integer of 0 to 3; n.sup.1 and n.sup.2 each represent an integer of 0 to 4; and a total of m.sup.1 and n.sup.1 is an integer of 1 to 5, a total of m.sup.2 and n.sup.2 is an integer of 0 to 5, and a total of m.sup.1 and m.sup.2 is an integer of 1 to 6.

2. The liquid crystal display device according to claim 1, wherein the liquid crystal molecules have negative dielectric anisotropy.

3. The liquid crystal display device according to claim 1, wherein the polyfunctional monomer in the liquid crystal composition has a higher mol concentration than the compound.

4. The liquid crystal display device according to claim 1, wherein the concentration of the polyfunctional monomer is 0.05 to 0.25% by weight of the liquid crystal composition.

5. The liquid crystal display device according to claim 1, wherein the concentration of the polymerization initiator is 0.005 to 0.3% by weight of the liquid crystal composition.

6. The liquid crystal display device according to claim 1, wherein the polymerization initiator is represented by Formula (6): ##STR00013## wherein R.sup.3 and R.sup.4 each represent C1-C4 linear or branched alkyl or alkenyl; P.sup.1 and P.sup.2 each represent radical polymerizable groups; Sp.sup.1 and Sp.sup.2 each represent C1-C6 linear, branched or cyclic alkylene, alkyleneoxy, or carbonyloxy, or a direct bond.

7. The liquid crystal display device according to claim 1, wherein the polymerization initiator is represented by Formula (7): ##STR00014## wherein R.sup.5 and R.sup.6 each represent a hydrogen atom or methyl.

8. The liquid crystal display device according to claim 1, wherein, in Formula (1), m represents an integer of 1 to 8.

9. The liquid crystal display device according to claim 1, wherein, in Formula (1), m is zero.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a cross-sectional schematic diagram of a liquid crystal display device according to Embodiment 1 before photoirradiation.

(2) FIG. 2 is a cross-sectional schematic diagram of a liquid crystal display device according to Embodiment 1 after photoirradiation.

(3) FIG. 3 is a cross-sectional schematic diagram of a liquid crystal display device having an alignment film according to a reference example.

(4) FIG. 4 is a cross-sectional schematic diagram of a liquid crystal display device according to Embodiment 2 before photoirradiation.

(5) FIG. 5 is a cross-sectional schematic diagram of a liquid crystal display device according to Embodiment 2 after photoirradiation.

(6) FIG. 6 is a view showing states of a liquid crystal cell of Sample 3 according to Example 1 before and after UV irradiation.

(7) FIG. 7 is a view showing states of a liquid crystal cell of Sample 6 according to Example 2 before and after UV irradiation.

(8) FIG. 8 is a cross-sectional schematic diagram of a liquid crystal display device according to Comparative Example 1 showing states thereof before and after photoirradiation.

(9) FIG. 9 is a flowchart showing an exemplary process of producing a conventional liquid crystal display device.

(10) FIG. 10 is a flowchart showing an exemplary process of producing a first liquid crystal display device of the present invention.

DESCRIPTION OF EMBODIMENTS

(11) The following embodiments are illustrated for specifically describe the present invention with reference to drawings. It should be noted that the present invention is not limited only to these embodiments.

Embodiment 1

(12) FIGS. 1 and 2 are cross-sectional schematic diagrams of a liquid crystal display device according to Embodiment 1. FIG. 1 shows a state before photoirradiation (polymerization) and FIG. 2 shows a state after photoirradiation (polymerization). As shown in FIGS. 1 and 2, a liquid crystal display device according to Embodiment 1 includes an array substrate 1, a color filter substrate 2, and a liquid crystal layer 5 interposed between a pair of substrates consisting of the array substrate 1 and the color filter substrate 2. The array substrate 1 includes an insulating transparent substrate made of glass or the like, various wirings formed on the transparent substrate, a pixel electrode, a TFT (Thin Film Transistor), and the like. The color filter substrate 2 includes an insulating transparent substrate made of glass or the like, a color filter formed on the transparent substrate, a black matrix, a common electrode, and the like.

(13) The liquid crystal layer 5 contains liquid crystal molecules 10, a monofunctional monomer 11, and a polyfunctional monomer 12. Before photoirradiation, the liquid crystal layer 5 contains a prepared liquid crystal composition as it is. The liquid crystal molecules 10 may have positive or negative dielectric anisotropy. From the standpoint of producing a vertical alignment-type liquid crystal display device, the liquid crystal molecules 10 preferably have negative dielectric anisotropy. The monofunctional monomer 11 has a structure represented by Formula (1). Preferably, the polyfunctional monomer 12 generates radicals by annealing and photoirradiation at not less than 340 nm and has a structure represented by Formula (2).

(14) In the entire liquid crystal composition forming the liquid crystal layer 5, namely, in 100% by weight of the liquid crystal composition, the amount of the monofunctional monomer 11 is 0.3 to 4.0% by weight. The amount of the polyfunctional monomer 12 is 0.01 to 0.5% by weight. In addition, in the liquid crystal composition, the ratio of the polyfunctional monomer 12 to the monofunctional monomer 11 is preferably not less than 1.5 mol % but not more than 20 mol %.

(15) A description is given on a method of producing the liquid crystal display device of Embodiment 1 in the following.

(16) The substrates 1 and 2 are attached to each other with a sealing material and the liquid crystal composition is injected to the gap therebetween in vacuo. Then, the inlet was sealed with, for example an UV light-curable resin, thereby forming a liquid crystal cell (enclosing step). Alternatively, after dropwise addition of the liquid crystal composition to one of the substrates 1 and 2 in vacuo, the other substrate may be attached thereto to form a liquid crystal cell.

(17) Next, the liquid crystal cell is heated in an oven or the like for thermal annealing at a predetermined temperature for a predetermined time (annealing step). In this step, the liquid crystal cell is preferably heated to a temperature not lower than the phase transition temperature (Tni) from the nematic phase to the isotropic liquid phase of the liquid crystal composition. More specifically, heating is preferably performed at not lower than 100° C. but not higher than 140° C. for 1 to 60 minutes. In the present embodiment, the thermal annealing is not essential but is preferably performed before the photoirradiation step from the standpoint of stabilizing the alignment.

(18) The liquid crystal cell, especially the liquid crystal composition, at a temperature exceeding the ambient temperature is irradiated with light at not less than 340 nm (photoirradiation step).

(19) The polyfunctional monomer 12 generates radicals mainly by photoirradiation at not less than 340 nm. The radicals serve as main active species to initiate and progress chain polymerization of polymerizable groups in the monofunctional monomer 11 and the polyfunctional monomer 12 one after another. The resulting polymers are deposited on the substrates 1 and 2 to form an alignment control layer (polymer layer) 7 by phase separation as shown in FIG. 2. Blocking light at less than 340 nm enables to form the alignment control layer 7 excellent in vertical alignment performance while avoiding deterioration of a liquid crystal display device. In FIG. 2, plural spotty alignment control layers 7 are formed on the substrates 1 and 2. Alternatively, the alignment control layer 7 may be formed to cover the substrates 1 and 2 or formed as a network throughout the liquid crystal layer 5.

(20) In the photoirradiation step, light (polarized or unpolarized UV light) at not less than 340 nm is radiated until an alignment control layer is formed. More specifically, photoirradiation of 0.1 to 10 J/cm.sup.2 is performed. The photoirradiation step is carried out at a temperature exceeding the ambient temperature, preferably at least at a temperature that is 30° C. lower than Tni. This achieves the vertical alignment by comparatively small energy of light. More specifically, during the photoirradiation step, the liquid crystal cell, especially the liquid crystal composition, is preferably maintained at a temperature of not lower than 100° C. but not higher than 140° C. Provided that the liquid crystal composition is maintained at a temperature exceeding the ambient temperature during the photoirradiation, heating may or may not be conducted during the photoirradiation. As a case (1), photoirradiation and heating may be simultaneously performed. As a case (2), heating may be performed before the photoirradiation step, and after termination of heating, photoirradiation may be performed at a temperature exceeding the ambient temperature (preferably at least at a temperature that is 30° C. lower than Tni). Exemplary heating means in the case (1) include a hot plate, and exemplary heating means in the case (2) include an oven and a hot plate.

(21) The polyfunctional monomer 12 also generates radicals in the case where photoirradiation at not less than 340 nm is not performed and only annealing (heat treatment) is performed. Accordingly, the alignment control layer 7 is presumably formed only by annealing without the photoirradiation step. In that case, however, the amount of generated radicals is smaller than the case where photoirradiation is performed. Photoirradiation at not less than 340 nm is therefore preferably performed. In the case where only annealing is performed, the liquid crystal composition is preferably heated at least to a temperature that is 30° C. lower than Tni.

(22) Next, the liquid crystal cell is heated in an oven and the like again for thermal annealing at a predetermined temperature for a predetermined time. At this time, the liquid crystal cell is preferably heated to a temperature higher than the phase transition temperature (Tni) from the nematic phase to the isotropic liquid phase of the liquid crystal composition. More specifically, the liquid crystal cell is heated to not lower than 100° C. but not higher than 140° C. for 1 to 60 minutes.

(23) In Embodiment 1, for example, in the case where the photoirradiation step is conducted while a voltage of not lower than the threshold voltage is applied to the liquid crystal layer 5, polymers are formed as to follow liquid crystal molecules aligned under application of a voltage not lower than the threshold voltage. In such a case, the formed alignment control layer has a structure that makes the liquid crystal molecules have an initial pre-tilt angle in a state where no voltage is applied. Here, even in a case where a voltage not lower than the threshold voltage was not applied to the liquid crystal layer 5 in the photoirradiation step, an alignment control layer having an vertical alignment force can be formed, provided that the monofunctional monomer 11 and the polyfunctional monomer 12 in Embodiment 1 are used.

(24) As shown in FIGS. 1 and 2, in Embodiment 1, both the array substrate 1 and the color filter substrate 2 have substantially no alignment film. A sealing material is directly applied to the substrates 1 and 2 along the outlines thereof, and the liquid crystal layer 5 is enclosed between the array substrate 1 and the color filter substrate 2 by the sealing material. Photoirradiation of the liquid crystal layer 5 is performed after the liquid crystal layer 5 is sealed by the sealing material, and therefore, the alignment control layer 7 is formed within a region surrounded by the sealing material.

(25) After these processes, various driving circuits, a backlight unit, and the like are mounted to the liquid crystal cell in which the alignment control layer 7 is formed, thereby producing a liquid crystal display device of Embodiment 1.

(26) Electrical and mechanical defects in the liquid crystal display device after the production thereof can be detected by continuous irradiation (aging) with light emitted from the mounted backlight unit for 100 to 1000 hours.

(27) In Embodiment 1, the alignment of liquid crystal molecules may also be determined by linear slits provided in a pixel electrode of the array substrate 1 or a common electrode of the color filter substrate 2. In the case where thin linear slits are formed in the pixel electrode and/or common electrode, liquid crystal molecules are all aligned towards the linear slits upon application of the voltage. Accordingly, an alignment control layer that makes liquid crystal molecules have a pre-tilt angle can be formed by polymerization of the monofunctional monomer 11 and the polyfunctional monomer 12 under application of a voltage not smaller than the threshold voltage to the liquid crystal layer 5.

(28) For reference, the configuration of a liquid crystal display device having an alignment film is described based on FIG. 3. In the example shown in FIG. 3, an array substrate 101 and a color filter substrate 102 each have an alignment film 106 formed of a polymeric material (polyimide) having a main chain containing an imide structure. The alignment film 106 with a surface subjected to alignment treatment such as rubbing or photoalignment enables to make the pre-tilt angle of liquid crystal molecules perpendicular to or in parallel with the substrates (make the liquid crystal molecules have an initial slope). The alignment film 106 may be one capable of making the pre-tilt angle of the liquid crystal molecules perpendicular to or in parallel with the substrates without alignment treatment. Between the array substrate 101 and the color filter substrate 102, a sealing material 103 is directly applied to the substrates 101 and 102 along the outlines thereof. The liquid crystal layer 105 is enclosed between the array substrate 101 and the color filter substrate 102 by the sealing material 103. The alignment film 106 needs to be formed prior to the sealing with the sealing material 103 by application of a polyimide solution or the like, and therefore, the alignment film 106 is also formed under the sealing material 103.

(29) The liquid crystal display device according to Embodiment 1 does not have a structure corresponding to the alignment film 106, and liquid crystal molecules are perpendicularly aligned by the alignment control layer 7 as described above.

Embodiment 2

(30) FIGS. 4 and 5 each are a cross-sectional schematic diagram of a liquid crystal display device according to Embodiment 2. FIG. 4 shows a state before photoirradiation (polymerization) and FIG. 5 shows a state after photoirradiation (polymerization). In Embodiment 1, the liquid crystal layer 5 contains the monofunctional monomer 11 and the polyfunctional monomer 12, and the alignment control layer 7 is formed by photoirradiation of the liquid crystal layer 5. In Embodiment 2, the liquid crystal layer 5 before polymerization, namely, a liquid crystal composition contains the monofunctional monomer 11, the polyfunctional monomer 12, and a compound 13. The compound 13 is a compound that generates radicals by a self-cleavage reaction (a polymerization initiator with polymerizable groups). In Embodiment 2, as shown in FIGS. 4 and 5, the alignment control layer 17 is formed in the photoirradiation step by polymerization of the monofunctional monomer 11, the polyfunctional monomer 12, and the compound 13 initiated by photoirradiation of the liquid crystal layer 5.

(31) The compound 13 used in Embodiment 2 may be, for example, a compound represented by Formula (5). The compound represented by Formula (5) may be specifically a compound represented by Formula (6). The compound represented by Formula (6) may be specifically a compound represented by Formula (7).

(32) The compounds represented by Formulae (5) to (7) each have a structure that generates radicals by self-cleavage, and therefore, simply conducting photoirradiation efficiently initiates polymerization. Even with generation of impurities that are easily charged electrically and presumably derived from the polymerization initiator, since formation of an alignment control layer by bonded polymerizable groups causes phase separation, image sticking is less likely to occur compared to a case of forming an alignment control layer using a polymerization initiator that contains no polymerizable group.

(33) In Embodiment 2, in the entire liquid crystal composition forming the liquid crystal layer 5, namely in 100% by weight of the liquid crystal composition, the amount of the monofunctional monomer 11 is 0.3 to 4.0% by weight. The amount of the polyfunctional monomer 12 is 0.01 to 0.5% by weight. The amount of the compound 13 is 0.001 to 0.3% by weight. In the liquid crystal composition, the ratio of the polyfunctional monomer 12 to the monofunctional monomer 11 is not less than 1.5 mol % but less than 20 mol %, the ratio of the polyfunctional monomer 12 and the compound 13 in total to the monofunctional monomer is not more than 20 mol %.

(34) Other component members of the liquid crystal display devices according to Embodiments 1 and 2 are now specifically described.

(35) The liquid crystal display devices according to Embodiments 1 and 2, and liquid crystal display devices produced by the method of producing the liquid crystal display device according to Embodiments 1 and 2 provide excellent display properties when used for display equipment such as TVs, PCs, mobile phones, and information displays.

(36) In the liquid crystal display devices according to Embodiments 1 and 2, the array substrate 1, the liquid crystal layer 5, and the color filter substrate 2 are stacked in the stated order from the rear side toward the screen side of the liquid crystal display device. On the rear side of the array substrate 1, a polarizer is provided. Also on the screen side of the color filter substrate 2, a polarizer is provided. For these polarizers, a retardation plate may be further provided. The polarizers may be circularly polarizing plates.

(37) The liquid crystal display devices according to Embodiments 1 and 2 may be of transmission type, reflection type, or transmission/reflection dual-purpose type. In the case of transmission or transmission/reflection dual-purpose type, the liquid crystal display devices of Embodiments 1 and 2 further includes a back light unit. The back light unit is arranged on the further rear side than the array substrate 1 in such a manner that light passes through the array substrate 1, the liquid crystal layer 5, and the color filter substrate 2 in the stated order. In the case of reflection or transmission/reflection dual-purpose type, the array substrate 1 has a reflector for reflecting external light. At least in a region where the reflected light is used for display, the polarizer of the color filter substrate 2 needs to be a circularly polarizing plate having a so-called λ/4 retardation plate.

(38) The liquid crystal display devices according to Embodiments 1 and 2 each may be a color filter-on-array device in which the array substrate has a color filter. The liquid crystal display device according to Embodiments 1 and 2 may also be monochrome display devices. In such a case, a color filter is not needed.

(39) The liquid crystal layer 5 is filled with a liquid crystal composition that aligns in a specific direction by application of a certain voltage. The alignment of liquid crystal molecules in the liquid crystal layer 5 is controlled by application of a voltage not smaller than the threshold voltage. The alignment mode of liquid crystal molecules in Embodiments 1 and 2 may be the VA mode, for example, and is not particularly limited. In the case of using the monofunctional monomer represented by Formula (1), a mode in which the initial alignment is the vertical alignment, such as the VA mode, is preferably employed as an excellent vertical alignment force is achieved.

(40) In the case of the liquid crystal display devices according to Embodiments 1 and 2, the liquid crystal display device (e.g., mobile phone, monitor, liquid crystal TV (television), and information display) may be dismantled and subjected to chemical analysts by nuclear magnetic resonance analysis (NMR), Fourier transform infrared spectroscopy (FT-IR), or Mass spectroscopy (MS). This process allows analysis of monomer components present in the alignment control layer and determination of the ratio of the monomer components present in the alignment control layer and the amount of monomers for forming the alignment control layer contained in the liquid crystal layer.

(41) The liquid crystal display devices of Embodiments 1 and 2 can employ any mode that provides an alignment control structure capable of inclining liquid crystal molecules in a predetermined direction relative to the substrate faces under application of voltage and/or no voltage. Specifically, employable modes include: the MVA (Multi-domain Vertical Alignment) mode that controls the alignment of liquid crystal molecules using wall-like (linear in a plan shape) dielectric protrusions (ribs) provided, as a protrusive member that controls the alignment, on the electrode towards the liquid crystal layer and slits provided on the electrode; the PVA (Patterned Vertical Alignment) mode alignment that controls the alignment of liquid crystal molecules using slits provided, as a protrusive member that controls the alignment, in electrodes of the both substrates; the CPA (Continuous Pinwheel Alignment) mode that controls the alignment of liquid crystal molecules using pillar-shaped (dot-shaped in a plan view) structures (rivets) provided, as a protrusive dielectric, on the electrode or holes formed in the electrode; and the TBA (Transverse Bend Alignment) mode that controls the alignment of liquid crystal molecules, which are perpendicularly aligned under application of no voltage, by generating a transverse electric field using a comb-shaped electrode. Providing these structures stabilizes the alignment of liquid crystal molecules, reducing the possibility of display failures.

Example 1

(42) The following will discuss Example 1 in which a liquid crystal cell of the liquid crystal display device according to Embodiment 1 is actually produced. A pair of substrates was prepared. After cleaning of the substrates, a sealing material was applied to one substrate. Beads were dispersed on the other substrate and the two substrates were then attached to each other. To the gap between the pair of substrates, a liquid crystal composition containing liquid crystal molecules having negative dielectric anisotropy, a monofunctional monomer, and a polyfunctional monomer (bifunctional monomer) was injected. The sealing material may be any of those cured by heat, cured by UV irradiation, and cured by both heat and UV irradiation. A sealing material used in Example 1 was curable by both heat and UV irradiation.

(43) For the liquid crystal composition, 4-acryloyloxybutoxy-4′-octyloxybiphenyl that is a monofunctional monomer represented by Formula (8) and 2,7-dimethacryloxyphenanthrene that is a polyfunctional monomer represented by Formula (9) were used in combination. The monofunctional monomer represented by Formula (8) has biphenyl. Because of this, an alignment control layer that aligns the major axes of liquid crystal molecules with a strong alignment force in a direction along the side chain of the polymer can be formed. The monofunctional monomer represented by Formula (8) has a linear structure from biphenyl to the alkyl chain end. Because of this, an alignment control layer that aligns liquid crystal molecules with a stable alignment force can be formed. The binding power of the alignment control layer is enhanced by the crosslinks derived from the bifunctional monomer represented by Formula (9), providing a more stable vertical alignment force.

(44) ##STR00008##

(45) Sample 1 prepared was a liquid crystal composition containing 1.2% by weight of a monofunctional monomer and 0.01% by weight of a polyfunctional monomer. Sample 2 prepared was a liquid crystal composition containing 1.2% by weight of a monofunctional monomer and 0.05% by weight of a polyfunctional monomer. Sample 3 prepared was a liquid crystal composition containing 1.2% by weight of a monofunctional monomer and 0.1% by weight of a polyfunctional monomer. Sample 4 prepared was a liquid crystal composition containing 1.2% by weight of a monofunctional monomer and 0.2% by weight of a polyfunctional monomer. Table 1 shows formulations of Samples 1 to 4. In all the samples, the amount of each monomer is based on 100% by weight of the liquid crystal composition.

(46) TABLE-US-00001 TABLE 1 Sample 1 Sample 2 Sample 3 Sample 4 Monofunctional 1.2 1.2 1.2 1.2 monomer (% by weight) Polyfunctional 0.01 0.05 0.1 0.2 monomer (% by weight)

(47) After injection of the liquid crystal composition, annealing was performed at 100° C. for an hour. The liquid crystal composition has a Tni of lower than 100° C. Next, while the temperature was kept at 100° C., the substrate was irradiated with unpolarized UV light (0.25 mW/cm.sup.2) in a normal direction relative to the substrate until the vertical alignment was achieved. The light source was a black light lamp (FHF-32BLB, TOSHIBA Lighting & Technology Corporation) having a peak wavelength of about 350 nm. The electrodes were plane electrodes without slits. During the polymerization reaction, no voltage was applied to the liquid crystal cell.

(48) After the photoirradiation, annealing was performed at 100° C. for one hour.

(49) The VHR of the liquid crystal cell produced by the above process was measured. The VHR was measured using a LC material characteristics measurement system Model 6254 (product of TOYO Corporation). The liquid crystal cell was placed in an oven at 70° C. and an electric charge was applied between electrodes under a voltage of 1 V for 60 μs. Then, the electric potential between the electrodes was measured in the opening period (period during which no voltage was applied) for 16.67 ms, thereby determining the percentage of the retained charge.

(50) The VHR was also measured before UV irradiation of the liquid crystal composition. After UV irradiation of the liquid crystal composition, the liquid crystal cell was aged on the LED back light unit for 1000 hours. Then, the VHR was again measured.

(51) Table 2 shows energies of UV light required for achievement of the vertical alignment and the VHR of Samples 1 to 4. Table 3 shows the mol concentrations of the polyfunctional monomer relative to the monofunctional monomer in Samples 1 to 4.

(52) TABLE-US-00002 TABLE 2 Sample 1 Sample 2 Sample 3 Sample 4 Energy of UV light required — 3000 2300 — for achievement of vertical (vertical alignment (vertical alignment alignment (mJ/cm.sup.2) not achieved) not achieved) VHR before irradiation (%) 99.9 99.9 99.9 — VHR after irradiation (%) 99.1 97.3 94.6 — VHR after aging (%) 99.1 98.6 97.7 —

(53) TABLE-US-00003 TABLE 3 Mol concentration of polymfunctional monomer relative to monofunctional monomer (mol %) 1.4 5.4 10.3 20.5 Alignment Bad Good Good Bad (vertical (vertical (vertical (vertical alignment alignment alignment alignment not achieved) achieved) achieved) not achieved)

(54) FIG. 6 is a view showing states of a liquid crystal cell of Sample 3 according to Example 1 before and after UV irradiation. The liquid crystal cell was provided with polarizers arranged in the crossed Nicols. As shown in FIG. 6, significantly favorable vertical alignment was achieved without generation of bright lines.

(55) As disclosed in Patent Literatures 1 and 2, in the case of using a bifunctional monomer having phenyl, biphenyl, or a steroid skeleton as a polyfunctional monomer, light at 310 nm is needed for polymerization and the required energy of light is not less than 9000 mJ/cm.sup.2. In contrast, in the case of using a bifunctional monomer having a phenanthrene skeleton, polymerization is initiated by light at not less than 340 nm and the vertical alignment is achieved by photoirradiation of about 2000 to 3000 mJ/cm.sup.2 as in the case of Samples 2 and 3 shown in Table 2. Moreover, aging on the back light unit is likely to improve the VHR, so that the light resistance is enhanced. Consequently, a highly reliable liquid crystal display device can be produced.

(56) As shown in Tables 2 and 3, the vertical alignment was not achieved in the case where the concentration of the polyfunctional monomer relative to the monofunctional monomer was 1.4 mol % (Sample 1), and in the case where the concentration of the polyfunctional monomer relative to the monofunctional monomer was 20.5 mol % (Sample 4). If the mol concentration of the polyfunctional monomer relative to the monofunctional monomer is too low, the vertical alignment is not achieved because polymerization does not occur. If the mol concentration of the polyfunctional monomer relative to the monofunctional monomer is too high, polyfunctional monomers are problematically polymerized with each other to immobilize liquid crystal molecules randomly, failing to achieve the vertical alignment. Accordingly, the concentration of the polyfunctional monomer relative to the monofunctional monomer is preferably not less than 1.5 mol % but less than 20 mol %.

(57) As shown in Table 2, within the range that allows the vertical alignment, as the concentration of the polyfunctional monomer decreases, the energy of UV light required for the vertical alignment tends to increase. In contrast, as the concentration of the polyfunctional monomer increases, the VHR tends to decrease. Accordingly, the optimal concentration may be set from the standpoint of alignment properties and the VHR.

Comparative Example 1

(58) In Comparative Example 1, a liquid crystal cell was prepared in the same manner as in Example 1, except that IRGACURE 651 that is a polymerization initiator was added to the liquid crystal composition in an amount of 0.03% by weight, instead of 2,7-dimethacryloxyphenanthrene that is a polyfunctional monomer represented by Formula (9). Specifically, in the liquid crystal cell of Comparative Example 1, as shown in FIG. 8, an alignment control layer 207 formed of a monofunctional monomer 211 (4-acryloyloxybutoxy-4′-octyloxybiphenyl) and a polymerization initiator 213 (IRGACURE 651) was formed by UV irradiation. Table 4 shows difference between Sample of Comparative Example 1 and Sample 2 of Example 1.

(59) TABLE-US-00004 TABLE 4 Comparative Example 1 Example 1 (Sample 2) Polyfunctional monomer not used used (Phenenthrene monomer) (0.05% by weight) Polymerization initiator Irgacure 651 not used (0.03% by weight)

(60) Table 5 shows the energy of UV light required for achievement of the vertical alignment and the VHR before and after photoirradiation and after aging of the liquid crystal cells of Comparative Example 1 and Sample 2 of Example 1. As shown in Table 5, in the case of using the polymerization initiator of Comparative Example 1, UV irradiation until the vertical alignment was achieved lowered the VHR to 69.3%, and aging further lowered the VHR. In contrast, in the case of using the bifunctional monomer of Example 1, the VHR was kept as high as 97.3% even after UV irradiation until the vertical alignment was achieved. In addition, after aging, the VHR was improved to 98.6%.

(61) TABLE-US-00005 TABLE 5 Comparative Example 1 Example 1 (Sample 2) Energy of UV light required 700 3000 for achievement of vertical alignment (mJ/cm.sup.2) VHR before irradiation (%) 99.9 99.9 VHR after irradiation (%) 69.3 97.3 VHR after aging (%) 69.3 or less 98.6

Example 2

(62) The following will discuss Example 2 in which a liquid crystal cell of the liquid crystal display device according to Embodiment 2 was actually produced. To the liquid crystal composition were added 4-acryloyloxybutoxy-4′-octyloxybiphenyl that is a monofunctional monomer represented by Formula (8) and 2,7-dimethacryloxyphenanthrene that is a polyfunctional monomer represented by Formula (9). Moreover, further added was a compound that has a radical polymerizable group and generates radicals by a self-cleavage reaction represented by Formula (10) (a polymerization initiator with a polymerizable group) was added. In this manner, a liquid crystal cell of Example 2 was produced.

(63) ##STR00009##

(64) Sample 5 prepared was a liquid crystal composition containing 1.2% by weight of a monofunctional monomer, 0.1% by weight of a polyfunctional monomer, and 0.1% by weight of a polymerization initiator with a polymerizable group. Sample 6 prepared was a liquid crystal composition containing 1.2% by weight of a monofunctional monomer, 0.05% by weight of a polyfunctional monomer, and 0.02% by weight of a polymerization initiator with a polymerizable group. Table 6 shows formulations of Samples 5 and 6. In any of the samples, the amounts of respective monomers and the polymerization initiator were based on 100% by weight of the liquid crystal composition.

(65) TABLE-US-00006 TABLE 6 Sample 5 Sample 6 Monofunctional monomer 1.2 1.2 (% by weight) Polyfunctional monomer 0.1 0.05 (% by weight) Polymerization initiator 0.1 0.02 with polymerizable group (% by weight)

(66) Other conditions and processes were the same as those in Example 1.

(67) Table 7 shows energies of UV light required for achievement of the vertical alignment and the VHR of Samples 5 and 6.

(68) TABLE-US-00007 TABLE 7 Sample 5 Sample 6 Energy of UV light required — 700 for achievement of vertical (vertical alignment alignment (mJ/cm.sup.2) not achieved) VHR before irradiation (%) 99.9 99.9 VHR after irradiation (%) 92.6 97.9 VHR after aging (%) 95.7 98.9

(69) FIG. 7 is a view showing states of a liquid crystal cell of Sample 6 according to Example 2 before and after UV irradiation. The liquid crystal cell was provided with polarizers arranged in the crossed Nicols. As shown in FIG. 7, significantly favorable vertical alignment was achieved without generation of bright lines.

(70) As disclosed in Patent Literatures 1 and 2, in the case of using a polyfunctional monomer such as a bifunctional monomer having phenyl, biphenyl, or a steroid skeleton, light at 310 nm is needed for polymerization and photoirradiation of not less than 9000 mJ/cm.sup.2 is needed. In contrast, in the case of using a bifunctional monomer having a phenanthrene skeleton and a polymerization initiator with a polymerizable group, polymerization is initiated by light at not less than 340 nm and the vertical alignment is achieved by photoirradiation of about 700 mJ/cm.sup.2 as in the case of Sample 6 in Table 7. Moreover, aging on the back light unit is likely to improve the VHR, so that the light resistance is enhanced. Consequently, a highly reliable liquid crystal display device can be produced. As described above, radicals derived from a polymerization initiator with a polymerizable group promotes a polymerization reaction to further reduce the required energy of light compared to the case of Samples 2 and 3 of Example 1.

(71) In the case where the concentration of a polyfunctional monomer and a polymerization initiator with a polymerizable group relative to a monofunctional monomer was 20 mol % (Sample 5), the vertical alignment was not achieved. In conjunction with the results of Example 1, the total concentration of a polyfunctional monomer and a polymerization initiator with a polymerizable group relative to a monofunctional monomer is preferably not less than 1.5 mol % but not more than 20 mol %.

(72) If the mol concentration of a polymerization initiator with a polymerizable group is too high, the polymerization initiator with a polymerizable group is not completely reacted so that a large portion thereof remains in the liquid crystal layer. In such a case, the display quality may be adversely affected. Accordingly, the mol concentration of a polyfunctional monomer is preferably higher than that of a polymerization initiator with a polymerizable group.

Example 3

(73) A liquid crystal cell of Example 3 was prepared by adding to a liquid crystal composition a polyfunctional monomer represented by Formula (11) and 2,7-dimethacryloxyphenanthrene that is a polyfunctional monomer represented by Formula (9).

(74) ##STR00010##

(75) To a liquid crystal composition were added 1.2% by weight of a monofunctional monomer and 0.05% by weight of a polyfunctional monomer. Sample 7 was one containing the monofunctional monomer of Formula (11) in which m is 0, Sample 8 was one containing the monofunctional monomer of Formula (11) in which m is 2, Sample 9 was one containing the monofunctional monomer of Formula (11) in which m is 4, and Sample 10 was one containing the monofunctional monomer of Formula (11) in which m is 8. Table 8 shows formulations of Samples 7 to 10. In any of the samples, the amounts of respective monomers are based on 100% by weight of the liquid crystal composition.

(76) TABLE-US-00008 TABLE 8 Sample 7 Sample 8 Sample 9 Sample 10 Monofunctional monomer 1.2 — — — (m = 0) (% by weight) Monofunctional monomer — 1.2 — — (m = 2) (% by weight) Monofunctional monomer — — 1.2 — (m = 4) (% by weight) Monofunctional monomer — — — 1.2 (m = 8) (% by weight) Polyfunctional monomer 0.05 0.05 0.05 0.05 (% by weight)

(77) Other conditions and processes were the same as those in Example 1.

(78) Table 9 shows energies of UV light required for achievement of the vertical alignment and the VHR of Samples 7 to 10.

(79) TABLE-US-00009 TABLE 9 Sam- Sam- Sam- Sam- ple 7 ple 8 ple 9 ple 10 Energy of UV light required 700 3000 3000 3000 for achievement of vertical alignment (mJ/cm.sup.2) VHR before irradiation (%) 99.9 99.9 99.9 99.9 VHR after irradiation (%) 96.1 97.0 97.3 97.5 VHR after aging (%) 96.5 98.1 98.6 98.9

(80) As a result of evaluations of materials containing the monofunctional monomer different in the number of spacers between the polymerizable group and the core (m=0, 2, 4, 8) shown in Table 9, the vertical alignment was achieved and the VHR was kept high even after achievement of the vertical alignment in all the materials. Moreover, aging on the back light unit is likely to improve the VHR as the materials contain a polyfunctional monomer, so that the light resistance was enhanced. Consequently, a highly reliable liquid crystal display device can be produced.

(81) From the standpoint of further improving the VHR, m is not preferably 0.

(82) The present application claims priority to Patent Application No. 2011-211558 filed in Japan on Sep. 27, 2011 under the Paris Convention and provisions of national law in a designated State, the entire contents of which are hereby incorporated by reference.

REFERENCE SIGNS LIST

(83) 1, 101, 201: Array substrate 2, 102, 202: Color filter substrate 103: Sealing material 4: Monomer 5, 105, 205: Liquid crystal layer 7, 17, 207: Alignment control layer (polymer layer) 10, 210: Liquid crystal molecule 11, 211: Monofunctional monomer 12: Polyfunctional monomer 13: Compound (polymerization initiator with a polymerizable group) 106: Alignment film 213: Polymerization initiator