METHOD FOR PREPARING LIQUID CRYSTAL ALIGNMENT LAYER

Abstract

The present invention provides a method for preparing a liquid crystal alignment layer having excellent alignment properties and stability as well as enhanced electrical characteristics such as voltage holding ratio. The present invention also provides a liquid crystal alignment layer prepared by the preparation method above and a liquid crystal display device comprising the liquid crystal alignment layer thus prepared.

Claims

1. A method for preparing a liquid crystal alignment layer comprising the steps of: 1) coating a liquid crystal aligning agent composition onto a substrate to form a coating film; 2) drying the coating film; 3) irradiating the coating film with light immediately after the drying step to perform alignment treatment; 4) subjecting the alignment-treated coating film to a low-temperature heat treatment at 200 C. or lower; and 5) curing the heat-treated coating film by heat treatment at a temperature higher than that of the low-temperature heat treatment, wherein the liquid crystal aligning agent composition comprises i) a first polymer for a liquid crystal aligning agent comprising two or more repeating units selected from the group consisting of a repeating unit represented by Chemical Formula 1 below, a repeating unit represented by Chemical Formula 2 below and a repeating unit represented by Chemical Formula 3 below, wherein the repeating unit represented by Chemical Formula 1 below is contained in an amount of 5 to 74 mol % relative to the total repeating units represented by Chemical Formulae 1 to 3 below, ii) a second polymer for a liquid crystal aligning agent comprising a repeating unit represented by Chemical Formula 4 below, and iii) a compound having two or more epoxy groups in a molecule: ##STR00008## in Chemical Formulae 1 to 4, R.sup.1 and R.sup.2 are each independently hydrogen, or C.sub.1-10 alkyl, with the proviso that R.sup.1 and R.sup.2 cannot both be hydrogen, R.sup.3 and R.sup.4 are each independently hydrogen, or C.sub.1-10 alkyl, and X.sup.1 is a tetravalent organic group represented by Chemical Formula 5 below, ##STR00009## in Chemical Formula 5, R.sup.5 to R.sup.8 are each independently hydrogen, or C.sub.1-6 alkyl, X.sup.2, X.sup.3 and X.sup.4 are each independently a tetravalent organic group derived from a hydrocarbon having 4 to 20 carbon atoms, or a tetravalent organic group in which at least one hydrogen in the tetravalent organic group is substituted with a halogen or in which at least one CH.sub.2 is replaced by O, CO, S, SO, SO.sub.2 or CONH such that the oxygen or sulfur atoms are not directly linked, and Y.sup.1, Y.sup.2, Y.sub.3 and Y.sup.4 are each independently a divalent organic group represented by Chemical Formula 6 below, ##STR00010## in Chemical Formula 6, R.sup.9 and R.sup.10 are each independently halogen, cyano, C.sub.1-10 alkyl, C.sub.2-10 alkenyl, C.sub.1-10 alkoxy, C.sub.1-10 fluoroalkyl, or C.sub.1-10 fluoroalkoxy, p and q are each independently an integer between 0 and 4, L.sup.1 is a single bond, O, CO, S, SO.sub.2, C(CH.sub.3).sub.2, C(CF.sub.3).sub.2, CONH, COO, (CH.sub.2).sub.z, O(CH.sub.2).sub.zO, O(CH.sub.2).sub.z, OCH.sub.2C(CH.sub.3).sub.2CH.sub.2O, COO(CH.sub.2).sub.zOCO, or OCO(CH.sub.2).sub.zCOO, wherein z is an integer between 1 and 10, and m is an integer between 0 and 3.

2. The method for preparing a liquid crystal alignment layer of claim 1, wherein the X.sup.2, X.sup.3 and X.sup.4 are each independently a tetravalent organic group described in Chemical Formula 7 below: ##STR00011## in Chemical Formula 7, R.sup.5 to R.sup.8 are each independently hydrogen, or C.sub.1-6 alkyl, L.sup.2 is a single bond, O, CO, S, C(CH.sub.3).sub.2, C(CF.sub.3).sub.2, CONH, COO, (CH.sub.2).sub.z, O(CH.sub.2).sub.zO, or COO(CH.sub.2).sub.zOCO, wherein z is an integer between 1 to 10.

3. The method for preparing a liquid crystal alignment layer of claim 1, wherein the compound having two or more epoxy groups in a molecule has a molecular weight of 100 to 10,000.

4. The method for preparing a liquid crystal alignment layer of claim 1, wherein the compound having two or more epoxy groups in a molecule is a cycloaliphatic-based epoxy, bisphenol-based epoxy, or a novolak-based epoxy.

5. The method for preparing a liquid crystal alignment layer of claim 1, wherein the compound having two or more epoxy groups in a molecule is contained in an amount of 0.1 to 30% by weight based on the weight of the polymer for a liquid crystal aligning agent.

6. The method for preparing a liquid crystal alignment layer of claim 1, wherein the weight ratio of the first polymer for a liquid crystal aligning agent to the second polymer for a liquid crystal aligning agent is 1:9 to 9:1.

7. The method for preparing a liquid crystal alignment layer of claim 1, wherein the liquid crystal aligning agent composition is a composition in which the first polymer for a liquid crystal aligning agent, the second polymer for a liquid crystal aligning agent and the compound having two or more epoxy groups in a molecule are dissolved or dispersed in an organic solvent.

8. The method for preparing a liquid crystal alignment layer of claim 1, wherein the drying of Step 2 is carried out at 50 to 130 C.

9. The method for preparing a liquid crystal alignment layer of claim 1, wherein the alignment treatment of Step 3 is carried out by irradiating polarized ultraviolet rays having a wavelength of 150 to 450 nm.

10. The method for preparing a liquid crystal alignment layer of claim 1, wherein the low-temperature heat treatment of Step 4 is carried out at 110 to 200 C.

11. The method for preparing a liquid crystal alignment layer of claim 1, wherein the heat treatment of Step 5 is carried out at 200 to 250 C.

12. A liquid crystal alignment layer prepared according to claim 1.

13. A liquid crystal display device comprising the liquid crystal alignment layer of claim 12.

14. A method for preparing a liquid crystal alignment layer comprising the steps of: 1) coating a liquid crystal aligning agent composition onto a substrate to form a coating film; 2) drying the coating film; 3) irradiating the coating film with light immediately after the drying step to perform alignment treatment; 4) subjecting the alignment-treated coating film to a low-temperature heat treatment at 200 C. or lower; and 5) curing the heat-treated coating film by heat treatment at a temperature higher than that of the low-temperature heat treatment, wherein the liquid crystal aligning agent composition comprises a polyimide precursor and a compound having two or more epoxy groups in a molecule.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0076] FIG. 1 shows retardation changes according to the temperature for the low-temperature heat treatment in one Example and Comparative Example of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0077] Hereinafter, the present invention will be described in more detail by way of Examples. However, these Examples are given for illustrative purposes only, and the scope of the invention is not intended to be limited by these Examples.

PREPARATION EXAMPLE 1

Synthesis of Diamine

Preparation Example 1-1) Synthesis of Diamine DA-1

[0078] Diamine DA-1 was synthesized according to the following reaction.

##STR00006##

[0079] Specifically, 1,3-dimethylcyclobuthane-1,2,3,4-tetracarboxylic dianhydride (DMCBDA) and 4-nitroaniline were dissolved in DMF (dimethylformamide) to prepare a mixture. Then, the mixture was reacted at about 80 C. for about 12 hours to prepare an auric acid. Subsequently, the amic acid was dissolved in DMF, and acetic anhydride and sodium acetate were added thereto to prepare a mixture. Then, the amic acid contained in the mixture was imidized at about 90 C. for about 4 hours. The thus-obtained imide was dissolved in DMAc (dimethylacetamide), and then Pd/C was added thereto to prepare a mixture. The mixture was reduced at 45 C. under hydrogen pressure of 6 bar for 20 minutes to prepare diamine DA-1.

Preparation 1-2) Synthesis of Diamine DA-2

[0080] ##STR00007##

[0081] DA-2 having the structure above was prepared in the same manner as in Preparation Example 1, except that cyclobuthane-1,2,3,4-tetracarboxylic dianhydride (CBDA) was used instead of 1,3-dimethylcyclobuthane-1,2,3,4-tetracarboxylic dianhydride.

PREPARATION EXAMPLE 2

Preparation of Polymer for Liquid Crystal Aligning Agent

Preparation Example 2-1) Preparation of Polymer for Liquid Crystal Aligning Agent P-1

[0082] (Step 1)

[0083] 5.0 g (13.3 mmol) of DA-2 prepared in Preparation Example 1-2 was completely dissolved in 71.27 g of anhydrous N-methyl pyrrolidone (NMP). Then, 2.92 g (13.03 mmol) of 1,3-dimethylcyclobuthane-1,2,3,4-tetracarboxylic dianhydride was added to the solution under an ice bath and stirred at room temperature for 16 hours.

[0084] (Step 2)

[0085] The solution obtained in the Step 1 was poured into an excessive amount of distilled water to form a precipitate. Then, the formed precipitate was filtered and washed twice with distilled water and three times with methanol. The thus-obtained solid product was dried in a vacuum oven at 40 C. for 24 hours to obtain 6.9 g of the polymer for a liquid crystal aligning agent P-1.

[0086] As a result of confirming the molecular weight of P-1 through GPC, the number average molecular weight (Mn) was 15,500 g/mol, and the weight average molecular weight (Mw) was 31,000 g/mol. Further, the monomer structure of the polymer P-1 is determined by the equivalent ratio of the monomers used, and the ratio of the imine structure in the molecule was 50.5%, and the ratio of the amic acid structure was 49.5%.

Preparation Example 2-2) Preparation of Polymer for Liquid Crystal Aligning Agent P-2

[0087] 5.0 g of DA-1 prepared in Preparation Example 1-1 and 1.07 g of p-phenylenediamine (PDA) were completely dissolved in 103.8 g of NMP. Then, 2.12 g of cyclobutane-1,2,3,4-tetracarboxylic dianhydride (CBDA) and 3.35 g of 4.4-oxydiphthalic dianhydride (OPDA) were added to the solution under an ice bath and stirred at room temperature for 16 hours. Thereafter, the polymer P-2 was prepared in the same manner as in Step 2 of Preparation Example 2-1.

[0088] As a result of confirming the molecular weight of P-2 through GPC, the number average molecular weight (Mn) was 18,000 g/mol, and the weight average molecular weight (Mw) was 35,000 g/mol. Further, as for the polymer P-2, the ratio of the imine structure in the molecule was 36.4%, and the ratio of the amic acid structure was 63.6%.

Preparation Example 2-3) Preparation of Polymer for Liquid Crystal Aligning Agent P-3

[0089] 6.0 g of DA-2 prepared in Preparation Example 1-2 and 1.37 g of 4,4-oxydianiline (ODA) were completely dissolved in 110.5 g of NMP. Then, 3.47 g of DMCBDA and 1.44 g of pyromellitic dianhydride (PMDA) were added to the solution under an ice bath and stirred at room temperature for 16 hours. Thereafter, the polymer P-3 was prepared in the same manner as in the Step 2 of Preparation Example 2-1.

[0090] As a result of confirming the molecular weight of P-3 through GPC, the number average molecular weight (Mn) was 14,500 g/mol, and the weight average molecular weight (Mw) was 29,000 g/mol. Further, as for the polymer P-3, the ratio of the imine structure in the molecule was 41.9%, and the ratio the amic acid structure was 58.1%.

Preparation Example 2-4) Preparation of Polymer for Liquid Crystal Aligning Agent Q-1

[0091] 5.00 g of 4,4-methylenedianiline and 5.05 g of 4,4-oxydianiline were completely dissolved in 221.4 g of NMP. Then, 14.55 g of 4,4-biphthalic anhydride was added to the solution under an ice bath and stirred at room temperature for 16 hours. Thereafter, the polymer Q-1 was prepared in the same manner as in the Step 2 of Preparation Example 2-1.

[0092] As a result of confirming the molecular weight of Q-1 through GPC, the number average molecular weight (Mn) was 25,000 g/mol, and the weight average molecular weight (Mw) was 40,000 g/mol.

PREPARATION EXAMPLE 3

Preparation of Liquid Crystal Aligning Agent Composition

Preparation Examples 3-1

[0093] 5 parts by weight of P-1 prepared in Preparation Example 2-1, 5 parts by weight of Q-1 prepared in Preparation Example 2-4 and 0.5 part by weight of (3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (Celloxide 2021P manufactured by Daicel) were completely dissolved in a mixed solvent of NMP and n-butoxyethanol in a weight ratio of 8:2. Then, the resultant was subjected to pressure filtration with a filter made of poly(tetrafluoroethylene) having a pore size of 0.2 m to prepare a liquid crystal aligning agent composition.

Preparation Example 3-2

[0094] A liquid crystal aligning agent composition was prepared in the same manner as in Preparation Example 3-1, except that P-2 prepared in Preparation Example 2-2 was used instead of P-1 prepared in Preparation Example 2-1.

Preparation Example 3-3

[0095] A liquid crystal aligning agent composition was prepared in the same manner as in Preparation Example 3-1, except that P-3 prepared in Preparation Example 2-3 was used instead of P-1 prepared in Preparation Example 2-1.

Example 1

Preparation of Liquid Crystal Alignment Layer and Liquid Crystal Cell

[0096] A liquid crystal cell was prepared by the following method using the liquid crystal aligning agent composition prepared above.

[0097] First, the liquid crystal aligning agent composition prepared in Preparation Example 3-1 was coated onto a substrate (lower plate) in which comb-shaped IPS mode ITO electrode patterns having a thickness of 60 nm, an electrode width of 3 m and a spacing between the electrodes of 6 m are formed on a rectangular glass substrate having a size of 2.5 cm2.7 cm and to a glass substrate (upper plate) having no electrode pattern each using a spin coating method.

[0098] Then, the substrates to which the liquid crystal aligning agent composition was coated were placed on a hot plate at about 80 C. for one minute to evaporate the solvent. In order to align the thus-obtained coating film, the ultraviolet rays of 254 nm were irradiated with an intensity of 0.3 J/cm.sup.2 using an exposure apparatus in which a linear polarizer was adhered to the coating film of each upper and lower plates.

[0099] Then, the coating film was placed on a hot plate at 130 C. for 500 seconds, thereby subjecting it to a low-temperature heat treatment. Thereafter, the coating film was calcinated (cured) in an oven at about 230 C. for 20 minutes to obtain a coating film having a thickness of 0.1 m. Then, a sealing agent impregnated with a ball spacer having a size of 3 m was applied to the edge of the upper plate except the liquid crystal injection hole. Subsequently, the alignment layers formed on the upper plate and the lower plate were aligned such that they face each other and that the alignment directions are aligned with each other, and then the upper and lower plates were bonded together and the sealing agent was cured to prepare an empty space. Then, a liquid crystal was injected into the empty cells to produce an IPS mode liquid crystal cell.

Examples 2 to 6

[0100] Each liquid crystal cell was prepared in the same manner as in Example 1, except that the temperature for the low-temperature heat treatment was raised to 160 C. (Example 2), 180 C. (Example 3), 200 C. (Example 4), 210 C. (Example 5) and 220 C. (Example 6), respectively.

Comparative Examples 1 and 2

[0101] A liquid crystal cell was prepared in the same manner as in Example 1, except that the low-temperature heat treatment was omitted (Comparative Example 1). Further, a liquid crystal cell was prepared in the same manner as in Example 1, except that the low-temperature heat treatment was omitted and the calcination (curing) temperature was set to 240 C.

Experimental Example

[0102] The characteristics of the liquid crystal cells prepared in Examples and Comparative Examples were evaluated as follows.

a. Measurement of Retardation (R)

[0103] In Examples, the retardation was measured after carrying out the low-temperature heat treatment process, and the retardation was measured after carrying out the high-temperature heat treatment process. In the case of Comparative Examples, the retardation was measured after carrying out the high-temperature heat treatment process. Each retardation was measured using AxoStep manufactured by Axomertics, Inc., and the results are shown in FIG. 1.

[0104] As shown in FIG. 1, the increase in the retardation was significant when the low-temperature heat treatment was carried out at 130 to 130 C. and then the high-temperature heat treatment was carried out. Particularly, when the low-temperature heat treatment was carried out at 130 C., followed by carrying out the high-temperature heat treatment, the retardation value was about 25% higher than that of Comparative Example 1.

b. Measurement of AC Residual Image

[0105] The AC residual image was measured using the liquid crystal cell of Example 1 and the liquid crystal cell of Comparative Example 1.

[0106] Specifically, polarizing plates were adhered to the upper plate and lower plate of the liquid crystal cell so as to be perpendicular to each other. The liquid crystal cell to which the polarizing plates were adhered was adhered onto a backlight of 7,000 cd/m.sup.2, and the brightness in a black mode was measured using a PR-880 equipment which is a device for measuring the brightness. Then, the liquid crystal cell was driven at room temperature for 24 hours with an AC voltage of 5V. Thereafter, the brightness in a black mode was measured in the same manner as described above in a state in which the voltage of the liquid crystal cell was turned off. The difference between the initial brightness (L.sub.0) measured before driving the liquid crystal cell and the final brightness (L.sub.1) measured after driving the liquid crystal cell was divided by the value of the initial brightness (L.sub.0) and multiplied by 100, thereby calculating the brightness fluctuation rate. As the calculated brightness fluctuation rate is closer to 0%, it means that the alignment stability is excellent.

[0107] As a result of the measurement, the liquid crystal cell of Example 1 had a brightness fluctuation rate of 2.29% (1.32%), while the liquid crystal cell of Comparative Example 1 had a brightness fluctuation rate of 5.08% (1.26%).

c. Evaluation of VHR (Voltage Holding Ratio) High-Temperature Long-Term Reliability

[0108] Using the liquid crystal cell of Example 1 and the liquid crystal cell of Comparative Example 1, the VHR (voltage holding ratio) high-temperature long-term reliability was evaluated.

[0109] Specifically, the voltage holding ratio (VHR) was measured, using a TOYO 6254 equipment, before applying harsh conditions (VHR.sub.Initial), and then measured once again after allowing the liquid crystal cells to stand under harsh conditions of 5V, 60 Hz, 60 C. for 120 hours (VHR.sub.Stress). The measurement results thereof were calculated by the following Equation 1.

VHR high-temperature long-term reliability=(VHR.sub.InitialVHR.sub.Stress)/VHR.sub.Initial [Equation 1]

[0110] In this regard, the VHR high-temperature long-term reliability is superior as the value thereof decreases, and the VHR high-temperature long-term reliability for the liquid crystal cell of Example 1 was 13%, while the VHR high-temperature long-term reliability for the liquid crystal cell of Comparative Example 1 was 25%. Therefore, it can be confirmed that when the liquid crystal cells are prepared by the preparation method according to the present invention, the VHR high-temperature long-term reliability is superior.