Optical film having a liquid crystal layer including twisted nematic liquid crystal compounds

Abstract

The present application relates to an optical film and a use thereof. In the present application, through control of an alignment state of a liquid crystal compound in a liquid crystal layer, the liquid crystal layer may exhibit so-called reverse-wavelength dispersion while forming a single thin layer. An optical film including the liquid crystal layer may show optical modulation in a display device such as a liquid crystal display (LCD), organic light emitting device (OLED), or the like, or may be used in various applications, for example, as an optical element capable of improving light utilization efficiency, an element for implementation of a stereoscopic image and quality improvement thereof, and so forth.

Claims

1. An optical film comprising a twisted nematic (TN) liquid crystal layer, the TN liquid crystal layer comprising: nematic liquid crystal compounds twisted along a virtual helical axis aligned in the TN liquid crystal layer and polymerized in a TN aligned state such that the helical axis is parallel to a thickness direction of the TN liquid crystal layer, and of the following Formula 1, and an angle between an optical axis of the nematic liquid crystal compounds present in the lowermost part of the TN layer and an optical axis of the nematic liquid crystal compounds present in the uppermost part of the TN layer is in a range from 50 to 300 degrees, and the layer does not include any cholesteric liquid crystal compounds having a twist angle of 360 degrees; and a chiral agent forming a concentration gradient along a thickness direction of the liquid crystal layer, wherein a rotation angle of the liquid crystal compound variably changes in the thickness direction so that a change of an angle between the optical axis of the liquid crystal compound at the lowermost part of the TN liquid crystal layer and the optical axis of the liquid crystal compound according to a thickness is not constant, wherein: the liquid crystal layer exhibits a reverse-wavelength dispersion and satisfies the following Expression 1: Expression 1 R(650)/R(550)>R(550)/R(550)>R(450)/R(550) where R(650) is an in-plane retardation value of the TN liquid crystal layer with respect to light with a wavelength of 650 nm, R(550) is an in-plane retardation value of the TN liquid crystal layer with respect to light with a wavelength of 550 nm, and R(450) is an in-plane retardation value of the TN liquid crystal layer with respect to light with a wavelength of 450 nm, ##STR00003## wherein in Formula 1, A is a single bond, —COO—, or —OCO—, and R.sub.1 to R.sub.10 are independently hydrogen, a halogen, an alkyl group, an alkoxy group, a cyano group, a nitro group, —O-Q-P, or a substituent of the following Formula 2, respectively, and at least one of R.sub.1 to R.sub.10 is —O-Q-P or a substituent of the following Formula 2, where Q is an alkylene group or an alkylidene group, and P is an alkenyl group, an epoxy group, a cyano group, a carboxyl group, an acryloyl group, a methacryloyl group, an acryloyloxy group, or a methacryloyloxy group, ##STR00004## wherein in Formula 2, B is a single bond, —COO—, or —OCO—, and R.sub.11 to R.sub.15 are each independently hydrogen, a halogen, an alkyl group, an alkoxy group, a cyano group, a nitro group, or —O-Q-P, and at least one of R.sub.11 to R.sub.15 is —O-Q-P, where Q is an alkylene group or an alkylidene group, and P is an alkenyl group, an epoxy group, a cyano group, a carboxyl group, an acryloyl group, a methacryloyl group, an acryloyloxy group, or a methacryloyloxy group.

2. The optical film of claim 1, wherein the R(650)/R(550) is in a range from 1.01 to 1.19, and the R(450)/R(550) is in a range from 0.81 to 0.99.

3. The optical film of claim 1, wherein the in-plane retardation of the TN liquid crystal layer with respect to light with a wavelength of 550 nm is in a range of 110 to 220 nm, or in a range of 240 to 350 nm.

4. The optical film of claim 1, wherein the TN liquid crystal layer has a thickness in a range of 0.1 to 10 μm.

5. An optical laminate comprising a polarizing layer and the optical film of claim 1 arranged on one surface of the polarizing layer.

6. An optical laminate comprising: the optical film of claim 1; and a retardation film positioned at one side of the optical film.

7. The optical laminate of claim 6, wherein an angle between a slow axis of the retardation film and an optical axis of the nematic liquid crystal compound positioned most adjacent to the retardation film in the liquid crystal layer is in a range of 5 to 90 degrees.

8. The optical laminate of claim 6, further comprising a polarizing layer.

9. The optical laminate of claim 6, sequentially comprising the polarizing layer, the retardation film, and the optical film, wherein an angle between a light absorbance axis of the polarizing layer and a slow axis of the retardation film is in a range of 10 to 20 degrees, and an angle between a slow axis of the retardation film and an optical axis of the nematic liquid crystal compound positioned most adjacent to the retardation film in the liquid crystal layer of the optical film is in a range of 8 to 16 degrees.

10. The optical laminate of claim 9, wherein a twist angle of the liquid crystal layer is in a range of 36 to 50 degrees.

11. The optical laminate of claim 6, sequentially comprising the polarizing layer, the retardation film, and the optical film, wherein a slow axis of the retardation film and a light absorbance axis of the polarizing layer are perpendicular to each other, and an optical axis of the liquid crystal compound most adjacent to the retardation film in the liquid crystal layer of the optical film is in a range of 50 to 70 degrees.

12. The optical laminate of claim 11, wherein a twist angle of the liquid crystal layer is in a range of 10 to 30 degrees.

13. The optical laminate of claim 6, sequentially comprising the polarizing layer, the retardation film, and the optical film, wherein a slow axis of the retardation film and a light absorbance axis of the polarizing layer are parallel to each other, and an angle between an optical axis of the liquid crystal compound most adjacent to the retardation film in the liquid crystal layer of the optical film and a slow axis of the retardation film is in a range of 15 to 35 degrees.

14. The optical laminate of claim 13, wherein a twist angle of the liquid crystal layer is in a range of 60 to 85 degrees.

15. A method of producing the optical film of claim 1, comprising: inducing a variable change in concentration according to a thickness of a coating layer of the chiral agent with respect to a liquid crystal coating layer comprising a nematic liquid crystal compound, a chiral agent, and a polymerization initiator; and polymerizing the nematic liquid crystal compound while the change in concentration of the chiral agent is induced resulting in a non-linear change of angle between the optical axis of the liquid crystal compound at the lowermost part of the liquid crystal coating layer and the optical axis of the liquid crystal compound according to a thickness.

16. The method of producing the optical film of claim 15, wherein steps of inducing a change in concentration and polymerizing the liquid crystal compound comprise a process of irradiating a liquid crystal coating layer with ultraviolet rays of an ultraviolet ray A area at an intensity of radiation of 10 to 500 mJ/cm.sup.2 at 40 to 80° C.; and a process of irradiating a liquid crystal coating layer having a change in concentration of the chiral agent with ultraviolet rays.

17. A display device comprising the optical film of claim 1.

18. The optical film of claim 1, wherein a graph in which an x-axis is a thickness of the TN liquid crystal layer, and a y-axis is an angle between an optical axis of the TN liquid crystal compound present in the corresponding thickness and an optical axis of the liquid crystal compound present in the lowermost part of the TN liquid crystal layer (the position where the x is 0) is nonlinear as shown in FIG. 2B.

19. The optical film of claim 18, wherein the graph comprises a part in which a slope of the graph increases as the thickness of the TN liquid crystal layer increases.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a conceptual diagram for describing an alignment of a liquid crystal compound.

(2) FIG. 2A is a conceptual graph for describing an alignment of the liquid crystal compound. FIG. 2B is a graph showing a rotation angle of the liquid crystal compound that variably changes in a non-linear manner, where the x-axis is a thickness change in a direction from the lower part to the upper part of the TN layer, and the y-axis is an angle between an optical axis of the TN liquid crystal compound present in the corresponding thickness and an optical axis of the liquid crystal compound present in the lowermost part of the TN liquid crystal layer.

(3) FIG. 3 is an illustrative cross-sectional view of an optical film.

(4) FIGS. 4 to 7 are views showing various structures of an optical laminate.

(5) FIG. 8 is a view comparing a reverse-wavelength dispersion of examples and comparative examples.

EXPLANATION OF THE MARKS IN THE DRAWINGS

(6) 101: the base material layer

(7) 102: the alignment layer

(8) 103: the TN layer

(9) 201: the linear graph

(10) 202: the non-linear graph

(11) 401, 601: the polarizing layer

(12) 402: the retardation film

(13) 403, 602: the optical film

(14) 6011: the absorptive polarizing layer

(15) 6022: the reflective polarizing layer

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(16) Hereinafter, an optical film will be described in detail in conjunction with examples and comparative examples, but the scope of the film is not limited to the following examples.

PREPARATION EXAMPLE

Preparation of Liquid Crystal Compositions

(17) Liquid crystal compositions were prepared by the following method. RM1230 (compositions including a chiral dopant) and RM1231 (compositions not including a chiral dopant), which are reactive mesogens (RMs) available from Merck & Co., Inc. that are well known for use in preparation of a cholesteric liquid crystal (CLC) were mixed in a ratio of 1:1, and then mixed with a mixing solvent of toluene and cyclohexanone (mixing weight ratio=7:3 (toluene:cyclohexanone)) to form about 40 wt % of solid fractions. Subsequently, a photo-polymerization initiator (Irgacure 907) having a maximum absorption wavelength in a range of 280 to 320 nm as a radical initiator was mixed at 3 wt % with respect to the solid fractions of RMs, and a photo-polymerization initiator (Darocure TPO) having a maximum absorption wavelength in a range of 360 to 400 nm was also mixed at 0.4 wt % with respect to the solid fractions of RMs. Thereafter, the mixed solution was heated at a temperature of about 60° C. for 1 hour, and then sufficiently cooled down to prepare a homogeneous solution.

EXAMPLE 1

(18) After a well-known rubbing alignment layer was formed on one surface of a poly(ethylene terephthalate) (PET) film, the alignment layer was coated with the prepared compositions using a wire bar No. 6, and then dried at 100° C. for about two minutes. Then, a concentration gradient of a chiral agent was induced by irradiating the coating layer with ultraviolet rays having a wavelength in a range of 350 to 400 nm using an ultraviolet irradiation device (TLK40W/10R; available from Royal Philips Electronics N.V.) at a temperature of about 60° C. (intensity of irradiation: about 100 mJ/cm.sup.2). Thereafter, the coating layer on which the concentration gradient was induced was irradiated with the strong ultraviolet rays at an intensity of 1 mJ/cm.sup.2 or more using the ultraviolet irradiation device (Fusion UV, 400 W) to sufficiently polymerize RMs to form a TN layer having a thickness of about 3 μm. In the result of the determination using an optical polarizing microscope (available from Leica Microsystems Ltd.), a twist angle of the TN layer was about 90 degrees, and showed non-linear properties as shown in 202 of FIG. 2A and in FIG. 2B.

COMPARATIVE EXAMPLE 1

(19) Liquid crystal layers were formed in a same manner as in Example 1 except that the coating layer was irradiated with the strong ultraviolet rays at an intensity of 1 mJ/cm.sup.2 or more using the ultraviolet irradiation device (Fusion UV, 400 W) to sufficiently polymerize RMs to form a TN layer having a thickness of about 3 μm, without a process of irradiating the coating layer with ultraviolet rays having a wavelength in a range of 350 to 400 nm at an intensity of about 100 mJ/cm.sup.2 using an ultraviolet irradiation device (TLK40W/10R; available from Royal Philips Electronics N.V.) at a temperature of about 60° C. In the result of the determination using an optical polarizing microscope (available from Leica Microsystems Ltd.), a twist angle of the TN layer was about 90 degrees, and showed linear properties as shown in 201 of FIG. 2A, and in FIG. 2B.

(20) Physical Property Evaluation

(21) Reverse-wavelength dispersion of liquid crystal films prepared in each of the examples and comparative examples was determined using an Axoscan (available from Axometrics, Inc.) device, and the results are specified in FIG. 8. As shown in FIG. 8, although the optical films of the examples and comparative examples have the same twist angles, the optical film of Example 1 showing a feature of a non-linear graph through an induced gradient of concentration may be determined to implement the so-called reverse-wavelength dispersion, and the optical film of Comparative Example 1 may be determined to implement a normal wavelength dispersion. In FIG. 8, the results of the examples are cases in which R(650)/R(550) is more than 1. Further, the optical film of Example 1 may be determined to be applied to various uses requiring a ¼ wavelength plate or ½ wavelength plate due to its reverse-wavelength dispersion and exhibit excellent properties.