Flexible substrate, method for preparing the same, and display device including the same

Abstract

The present disclosure relates to a flexible substrate, a method for preparing the flexible substrate, and a display device including the flexible substrate, and more specifically, to a flexible substrate, a method for preparing the flexible substrate, and a display device including the flexible substrate, wherein the flexible substrate comprises: a first polymer layer, which comprises a first polymer; a second polymer layer, which comprises a second polymer and is located above and below the first polymer layer; and a first interface cross-linking layer, which is located between the first polymer layer and the second polymer layer and comprises a first cross-linking polymer having a mesh structure in which the first polymer is cross-linked with the second polymer, wherein the stiffness of the first polymer layer is greater than the stiffness of the second polymer layer, and the softness of the second polymer layer is greater than the softness of the first polymer layer.

Description

Technical Field

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the priority of Korean Patent Application No. 10-2020-0120379 filed in the Korean Intellectual Property Office on September 18, 2020, the disclosure of which is incorporated into this specification by reference. Technical Field

[0004] The present disclosure relates to a flexible substrate, a method for preparing the flexible substrate, and a display device including the flexible substrate, and more particularly, to a flexible substrate that is easily prepared by a simple method and has improved impact resistance while maintaining high folding characteristics, a method for preparing the flexible substrate, and a display device including the flexible substrate. Background Art

[0005] Recently, with the advent of the information age, the display field of visually expressing electrical information signals has developed rapidly, and for this reason, various display devices having excellent properties such as thin thickness, light weight, and low power consumption have been developed. Specific examples of such display devices may include a liquid crystal display device (LCD), a plasma display panel device (PDP), a field emission display device (FED), an organic light emitting display device (OLED), and the like.

[0006] Meanwhile, efforts are being made to diversify the shapes and sizes of display devices. For example, display devices having various shapes, such as curved display devices having a curved surface, or flexible display devices that maintain their display performance even in a bent or curved state, are being developed. The display panel of the flexible display device uses a base substrate having flexibility, so that a rear substrate is disposed under the display panel to suppress sagging and protect the display panel from external impact. Such a rear substrate is used in the form of a plastic film to maintain the flexible property. Summary of the invention

[0007] Technical issues

[0008] The plastic film used as the rear substrate in the related art has excellent softness, but is composed of a thin single layer to have low rigidity, so there is a problem that the impact resistance of the display device is reduced.

[0009] Therefore, one object to be achieved by the present disclosure is to provide a flexible substrate in which a plurality of layers including different polymers having different mechanical properties are cross-linked to improve stiffness while maintaining high softness.

[0010] Another object to be achieved by the present disclosure is to provide a display device including the above-mentioned flexible substrate to satisfy folding characteristics and improve impact resistance.

[0011] Still another object to be achieved by the present disclosure is to simplify a process of preparing a flexible substrate by cross-linking adhesive layers bonded above and below the flexible substrate to omit a process of laminating adhesive layers.

[0012] Furthermore, an object of the present disclosure is to provide a method for preparing a flexible substrate, which can solve problems in a process caused by a lower modulus of an adhesive layer.

[0013] The objects of the present disclosure are not limited to the above objects, and those skilled in the art can clearly understand other objects not mentioned above from the following description.

[0014] Technical Solution

[0015] According to one aspect of the present disclosure, a flexible substrate includes: a first polymer layer, which includes a first polymer; a second polymer layer, which includes a second polymer and is located above and below the first polymer layer; and a first interface cross-linking layer, which is located between the first polymer layer and the second polymer layer and includes a first cross-linked polymer having a network structure formed by cross-linking the first polymer and the second polymer, the first polymer layer has a higher stiffness than the second polymer layer, and the second polymer layer has a higher softness than the first polymer layer.

[0016] According to one aspect of the present disclosure, a display device includes: a flexible substrate; and a display panel disposed on the flexible substrate, the flexible substrate including: a first polymer layer, which includes a first polymer; a second polymer layer, which is located above and below the first polymer layer and includes a second polymer; and a first interface cross-linking layer, which is located between the first polymer layer and the second polymer layer and includes a first cross-linked polymer having a network structure formed by cross-linking the first polymer and the second polymer, the first polymer layer has a higher stiffness than the second polymer layer, and the second polymer layer has a higher softness than the first polymer layer.

[0017] According to another aspect of the present disclosure, a method for preparing a flexible substrate includes: preparing a first film, the first film including a first layer including a first polymer and a cross-linking agent and a second layer including a second polymer and a cross-linking agent; preparing a second film, the second film including a first layer including a first polymer and a cross-linking agent and a second layer including a second polymer and a cross-linking agent; laminating the first film and the second film so that the first layer of the first film is opposite to the first layer of the second film; and hardening the laminated first and second films, and in the hardening step, the first polymer of the first film and the first polymer of the second film are cross-linked at the interface where the first layer of the first film and the first layer of the second film are in contact with each other to form a network structure so that the first film is bonded to the second film, and the first polymer and the second polymer of the first film are cross-linked at the interface where the first layer and the second layer are in contact with each other to bond the first layer of the first film to the second layer, and the first polymer and the second polymer of the second film are cross-linked at the interface where the first layer and the second layer are in contact with each other to bond the first layer of the second film to the second layer.

[0018] Additional details of exemplary embodiments are included in the detailed description and accompanying drawings.

[0019] Beneficial Effects

[0020] According to the present disclosure, a flexible substrate includes an interfacial cross-linked layer having a network structure formed by cross-linking heterogeneous polymers at an interface where a layer including a polymer having excellent softness contacts a layer including a polymer having excellent stiffness, so as to significantly improve stiffness while maintaining high softness.

[0021] The display device of the present disclosure includes a flexible substrate satisfying both softness and rigidity to significantly improve impact resistance and has excellent folding characteristics.

[0022] In the method for preparing a flexible display device of the present disclosure, polymers contained in each layer are cross-linked so as to be bonded at the interface where layers having different mechanical properties are in contact, thereby omitting the process of forming an adhesive layer between the layers. In addition, the adhesive layer is cross-linked so as to be bonded to the upper and lower parts of the flexible substrate, thereby omitting the process of laminating the adhesive layer. Therefore, the method for preparing the flexible substrate is simplified, thereby improving productivity.

[0023] Furthermore, according to the preparation method of the present disclosure, problems in the process caused by the low modulus of the adhesive layer can be solved.

[0024] The effects according to the present disclosure are not limited to those exemplified above, and more various effects are included in this specification. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The above and other aspects, features and other advantages of the present disclosure can be more clearly understood from the following detailed description in conjunction with the accompanying drawings, in which:

[0026] Figure 1 is a cross-sectional view of a flexible substrate according to an exemplary embodiment of the present disclosure;

[0027] Figure 2 is a flow chart illustrating a method for preparing a flexible substrate according to an exemplary embodiment of the present disclosure;

[0028] Figure 3 is a cross-sectional view illustrating a method for preparing a flexible substrate according to an exemplary embodiment of the present disclosure; and

[0029] Figure 4 is a cross-sectional view of a display device according to an exemplary embodiment of the present disclosure. DETAILED DESCRIPTION

[0030] The advantages and features of the present disclosure and the methods for achieving the advantages and features will become apparent by referring to the exemplary embodiments described in detail below in conjunction with the accompanying drawings. However, the present disclosure is not limited to the exemplary embodiments disclosed herein, but will be implemented in various forms. The exemplary embodiments are provided only as examples to enable those skilled in the art to fully understand the disclosure of the present disclosure and the scope of the present disclosure. Therefore, the present disclosure will be limited only by the scope of the appended claims.

[0031] The shapes, sizes, ratios, angles, numbers, etc. in the drawings used to describe the exemplary embodiments of the present disclosure are only examples, and the present disclosure is not limited thereto. Throughout the specification, the same reference numerals generally represent the same elements. In addition, in the following description of the present disclosure, the detailed description of known related technologies may be omitted to avoid unnecessary confusion of the subject matter of the present disclosure. Terms such as "comprising", "having" and "consisting of..." used herein are generally intended to allow the addition of other components, unless the term is used together with the term "only". Unless otherwise expressly stated, any singular mentioned may include the plural.

[0032] Even if not explicitly stated, the components are understood to include ordinary error margins.

[0033] When terms such as “on,” “above,” “below,” and “next” are used to describe the positional relationship between two parts, one or more parts may be placed between the two parts unless the terms are used together with the terms “immediately” or “directly.”

[0034] When an element or layer is referred to as being “on” another element or layer, the other layer or element may be directly on the other element or interposed between them.

[0035] Although the terms "first", "second", etc. are used to describe various components, these components are not limited by these terms. These terms are only used to distinguish one component from other components. Therefore, the first component mentioned below can be the second component in the technical concept of the present disclosure.

[0036] Throughout the specification, like reference numerals generally refer to like elements.

[0037] The sizes and thicknesses of various components shown in the drawings are shown for convenience of description, and the present disclosure is not limited to the sizes and thicknesses of the components shown.

[0038] The features of the various embodiments of the present disclosure may be combined or coupled with each other in part or in whole, and may be technically linked in a variety of ways, and these embodiments may be performed independently or in conjunction with each other.

[0039] Hereinafter, a flexible substrate and a display device according to exemplary embodiments of the present disclosure will be described in detail with reference to the accompanying drawings.

[0040] The symbol "*" used in the formulas described in this disclosure refers to a binding site.

[0041] Figure 1 is a cross-sectional view of a flexible substrate according to an exemplary embodiment of the present disclosure. Figure 1 , a flexible substrate 100 according to an exemplary embodiment of the present disclosure includes a first polymer layer 110, first interface cross-linking layers 120a and 120b, second polymer layers 130a and 130b, second interface cross-linking layers 140a and 140b, and adhesive layers 150a and 150b. Here, each layer will be described in detail.

[0042] The first polymer layer 110 is a layer located at the center of the thickness direction of the flexible substrate 100, and imparts high rigidity to the flexible substrate 100. The first polymer layer 110 includes a first polymer. The first polymer may be a polymer including a bulky group in a side chain or two or more side chains bonded to one atom constituting a main chain. The polymer having the above structure has high rigidity because slippage between polymer chains is not likely to occur due to the structural characteristics of the side chain.

[0043] For example, the first polymer layer 110 has a Young's modulus of 2000 MPa or more, or 2000 MPa to 3000 MPa, and thus has high rigidity.

[0044] For example, the first polymer may be a polymer including one or more repeating units selected from the following formula A. That is, the first polymer may be a homopolymer composed of one repeating unit selected from the following formula A, or a copolymer including one or more repeating units selected from the following formula A.

[0045] [Formula A]

[0046]

[0047]

[0048] In Formula A, n may independently be an integer of 500 to 4,000 or 2,000 to 3,000.

[0049] The repeating unit listed in Formula A contains a bulky group such as benzene, pyrrolidone, pyridine and imidazole as a side chain, or contains two side chains bonded to one carbon atom. The side chain hinders the slippage between polymer chains, and thus the rigidity is high.

[0050] For example, the first polymer may be selected from polystyrene, polyparamethylstyrene, polyvinyl imidazole, polyvinyl pyridine, polyvinyl pyrrolidone, polymethyl methacrylate, and polydialkylmethacrylamide, but is not limited thereto.

[0051] The first polymer is cross-linked to form a network structure. When the first polymer is cross-linked to form a network structure, the rigidity of the first polymer layer 110 can be further enhanced.

[0052] For example, the first polymer having a network structure may be selected from compounds represented by Formula D below.

[0053] [Formula D]

[0054]

[0055] In Formula D, z is an integer from 1 to 10 or from 1 to 6, and R1 may be independently selected from hydrogen or methyl.

[0056] In Formula D, A is a polymer comprising one or more repeating units selected from the above Formula A. That is, in Formula D, A may be a homopolymer composed of one repeating unit selected from Formula A, or a copolymer comprising one or more repeating units.

[0057] The second polymer layers 130 a and 130 b are respectively located above and below the first polymer layer 110 , and the first interface cross-linking layers 120 a and 120 b are located between the first polymer layer 110 and the second polymer layers 130 a and 130 b .

[0058] The second polymer layers 130a and 130b impart softness to the flexible substrate 100. The second polymer layers 130a and 130b include a second polymer. The second polymer may be a polymer having no side chains, or a polymer in which a small-volume group such as a hydrogen atom or a hydroxyl group is bonded to a side chain. A polymer having such a structure has no side chains or has a small-volume side chain, so that it is easy to slide between chains. Therefore, the softness is high. As another example, the second polymer may be a copolymer comprising a repeating unit having no side chains, or a copolymer comprising a repeating unit in which a side chain having a smaller volume than the first polymer is bonded to a main chain. In the copolymer, the volume of the side chain bonded to the main chain is smaller than that of the first polymer, and the number of the side chains bonded to the main chain is also small, so that the slippage between chains is easy. Therefore, the softness is higher than that of the first polymer.

[0059] For example, the second polymer layers 130 a and 130 b have a Young's modulus of 1000 MPa or less, or 600 MPa to 800 MPa, thereby being excellent in softness.

[0060] For example, the second polymer may be a polymer including one or more repeating units selected from the following formula B. That is, the second polymer may be a homopolymer composed of one repeating unit selected from the following formula B, or a copolymer including one or more repeating units selected from the following formula B.

[0061] [Formula B]

[0062]

[0063] In Formula B, n is independently an integer of 500 to 4000 or 2500 to 3500, x is independently an integer of 300 to 2500 or 1000 to 1500, and y may be independently an integer of 300 to 2500 or 1000 to 2500.

[0064] The repeating units listed in Formula B do not have a side chain, contain a small-volume group such as chlorine or hydroxyl as a side chain, or have a structure in which a repeating unit having no side chain and a repeating unit containing a small-volume side chain are copolymerized. In addition, one side chain is bonded to one carbon atom constituting the main chain. Therefore, the slippage between chains is easier than the first polymer, and the softness is higher than the first polymer.

[0065] For example, the second polymer may be selected from polyvinyl chloride, polyvinyl alcohol, polyethylene, poly(ethylene-co-vinyl alcohol), and poly(ethylene-co-vinyl acetate), but is not limited thereto.

[0066] The second polymer is cross-linked to form a network structure. For example, the second polymer having a network structure may be selected from compounds represented by the following formula E.

[0067] [Formula E]

[0068]

[0069] In Formula E, z is an integer from 1 to 10 or from 1 to 6, and R1 may be independently selected from hydrogen or methyl.

[0070] In Formula E, B' is a polymer comprising one or more repeating units selected from the above Formula B. That is, in Formula E, B' may be a homopolymer composed of one repeating unit selected from Formula B, or a copolymer comprising one or more repeating units.

[0071] The first interface cross-linked layers 120a and 120b are located between the first polymer layer 110 and the second polymer layers 130a and 130b. The first interface cross-linked layers 120a and 120b are layers formed by the first polymer of the first polymer layer 110 and the second polymer of the second polymer layer, and the first polymer and the second polymer are cross-linked at the interface where the first polymer layer 110 and the second polymer layers 130a and 130b contact each other. This will be described in more detail below.

[0072] The first interface cross-linked layers 120a and 120b include a first cross-linked polymer having a network structure formed by cross-linking the first polymer of the first polymer layer 110 with the second polymer of the second polymer layer. As described above, the first polymer of the first polymer layer 110 and the second polymer of the second polymer layers 130a and 130b are cross-linked at the interface where the first polymer and the second polymer contact each other to form a network structure. Therefore, the first polymer layer 110 and the second polymer layers 130a and 130b can be bonded to each other without an additional bonding element.

[0073] Generally, since in a single-layer film, there is a trade-off between rigidity and softness, when the cross-linking density is increased to increase rigidity, the softness is reduced, so it is difficult to increase rigidity while maintaining high softness. According to the flexible substrate 100 disclosed in the present invention, a first polymer layer 110 having excellent rigidity and a second polymer layer 130a and 130b having excellent softness and a first cross-linked polymer are included. The first cross-linked polymer has a mesh structure formed by cross-linking the first polymer and the second polymer at the interface between the first polymer layer and the second polymer layer. By doing so, the rigidity can be significantly improved while maintaining high softness. In addition, the first polymer layer 110 and the second polymer layer 130a and 130b having different polymers are not in physical contact with each other, but the first polymer is cross-linked with the second polymer to form a chemical bond, so that the peeling strength is excellent. Therefore, when the flexible substrate 100 disclosed in the present invention is used as a substrate of a foldable display device, the folding characteristics are excellent, and the deformation of the substrate, the peeling or slipping between the layers during the folding process can be minimized.

[0074] For example, the first cross-linked polymer may be selected from compounds represented by Formula C below.

[0075] [Formula C]

[0076]

[0077] In Formula C, z is an integer from 1 to 10, and R1 may be independently selected from hydrogen or methyl.

[0078] In Formula C, A may be a homopolymer consisting of one repeating unit selected from the above Formula A, or a copolymer including one or more repeating units. That is, in Formula C, A is the same polymer as the first polymer of the first polymer layer 110 .

[0079] In Formula C, B' may be a homopolymer composed of one repeating unit selected from the above Formula B, or a copolymer including one or more repeating units. That is, in Formula C, B' is the same polymer as the second polymer of the second polymer layers 130a and 130b.

[0080] The adhesive layers 150a and 150b are located at the uppermost portion and the lowermost portion of the flexible substrate 100. That is, the adhesive layers 150a and 150b are located above the second polymer layer 120a (located above the first polymer layer 110) and below the second polymer layer 120b (located below the first polymer layer 110), respectively. However, this is not limited thereto, and as needed, the adhesive layers 150a and 150b may be omitted and may be provided only at the uppermost portion or the lowermost portion.

[0081] The adhesive layers 150a and 150b include a third polymer. The third polymer may be a polymer having adhesive properties and forming a cross-linked bond with the second polymer of the second polymer layers 130a and 130b.

[0082] For example, the third polymer includes a side chain capable of imparting adhesive force, and the side chain may be a group including a hydroxyl group, or a group such as ethyl acrylate or butyl acrylate.

[0083] For example, the third polymer may be a polymer including one or more repeating units selected from the following formula F. That is, the third polymer may be a homopolymer composed of one repeating unit selected from the following formula F, or a copolymer including one or more repeating units.

[0084] [Formula F]

[0085]

[0086] In Formula F, R2 is independently hydrogen or methyl, R3 is an alkyl group having 1 to 10 or 1 to 4 carbon atoms, o is an integer from 1 to 10, p is an integer from 1 to 10 or 1 to 4, and l may each independently be an integer from 500 to 3000 or from 1000 to 2000.

[0087] For example, the third polymer may be selected from poly(2-hydroxyethyl methacrylate), poly(4-hydroxybutyl acrylate), poly(dopamine methacrylamide), polyethyl acrylate, and polybutyl acrylate, but is not limited thereto.

[0088] The third polymer is cross-linked to form a network structure. For example, the third polymer having a network structure may be selected from compounds represented by the following formula G.

[0089] [Formula G]

[0090]

[0091] In Formula G, z is an integer from 1 to 10 or from 1 to 6 and R1 may be independently selected from hydrogen or methyl.

[0092] In formula G, F' may be a homopolymer consisting of one repeating unit selected from the above formula F, or a copolymer comprising one or more repeating units.

[0093] The second interface cross-linked layers 140a and 140b are located between the second polymer layers 130a and 130b and the adhesive layers 150a and 150b. The second interface cross-linked layers 140a and 140b are respectively disposed between the second polymer layer 130a and the adhesive layer 150a located above the first polymer layer 110 and between the second polymer layer 130b and the adhesive layer 150b located below the first polymer layer 110.

[0094] The second interface crosslinked layers 140a and 140b are layers formed by crosslinking the second polymer of the second polymer layers 130a and 130b and the third polymer of the adhesive layers 150a and 150b at interfaces where the second polymer layers 130a and 130b and the adhesive layers 150a and 150b contact each other. This will be described in more detail below.

[0095] The second interface cross-linked layers 140a and 140b include a second cross-linked polymer having a network structure formed by cross-linking the second polymer of the second polymer layers 130a and 130b and the third polymer of the adhesive layers 150a and 150b. As described above, the second polymer and the third polymer are cross-linked at the interface where the second polymer contacts the third polymer to form a network structure. Therefore, the second polymer layers 130a and 130b and the adhesive layers 150a and 150b can be bonded to each other with a higher peel strength. Therefore, when the flexible substrate 100 of the present disclosure is used as a substrate for a foldable display device, the folding characteristics are excellent, and the deformation of the substrate, the peeling or slipping between the layers during the folding process can be minimized.

[0096] For example, the second cross-linked polymer may be selected from compounds represented by Formula H below.

[0097] [Formula H]

[0098]

[0099] In Formula H, z is an integer from 1 to 10 or from 1 to 6, and R1 may be independently selected from hydrogen or methyl.

[0100] In Formula H, B' may be a homopolymer composed of one repeating unit selected from the above Formula B, or a copolymer including one or more repeating units. That is, in Formula H, B' is the same polymer as the second polymer of the second polymer layers 130a and 130b.

[0101] In Formula H, F' may be a homopolymer composed of one repeating unit selected from the above Formula F, or a copolymer including one or more repeating units. That is, in Formula H, F' is the same polymer as the third polymer of adhesive layers 150a and 150b.

[0102] The first interface cross-linking layers 120 a and 120 b , the second polymer layers 130 a and 130 b , the second interface cross-linking layers 140 a and 140 b , and the adhesive layers 150 a and 150 b are laminated to have a symmetrical structure with respect to the first polymer layer 110 .

[0103] In the following, we will refer to Figure 2 and Figure 3 The configuration and effects of the flexible substrate according to the exemplary embodiment of the present disclosure are described in more detail.

[0104] Figure 2 is a flowchart illustrating a method for preparing a flexible substrate according to an exemplary embodiment of the present disclosure. Figure 3 is a schematic cross-sectional view illustrating a method of preparing a flexible substrate according to an exemplary embodiment of the present disclosure.

[0105] The method for preparing the flexible substrate 100 according to the exemplary embodiment of the present disclosure includes: step S110, sequentially laminating a first layer L1a, a second layer L2a, and a third layer L3a on a carrier substrate CS; step S120, preparing a first film FM1 by once hardening the laminated first layer L1a, second layer L2a, and third layer L3a, and then removing the carrier substrate CS; step S130, preparing a second film FM2 by the same method as the first film FM1; step S140, laminating the first film FM1 and the second film FM2; and step S150, preparing the flexible substrate 100 by secondarily hardening the laminated first film FM1 and the second film FM2. Hereinafter, the steps of the method for preparing the flexible substrate will be described in detail.

[0106] First, the first layer L1a, the second layer L2a, and the third layer L3a are sequentially laminated on the carrier substrate CS (S110).

[0107] To form each layer, a first polymer solution including a first monomer, a crosslinker and an initiator, a second polymer solution including a second monomer, a crosslinker and an initiator, and a third polymer solution including a third monomer, a crosslinker and an initiator are prepared.

[0108] The first monomer is polymerized to form a first polymer. For example, the first monomer may be one or more compounds selected from the group consisting of compounds represented by the following Formula A-1.

[0109] [Formula A-1]

[0110]

[0111] The second monomer is polymerized to form a second polymer. For example, the second monomer may be one or more compounds selected from the group consisting of compounds represented by the following formula B-1.

[0112] [Formula B-1]

[0113]

[0114] The third monomer is polymerized to form a third polymer. For example, the third monomer may be one or more compounds selected from the group consisting of compounds represented by the following Formula F-1.

[0115] [Formula F-1]

[0116]

[0117] In Formula F-1, R2 is independently hydrogen or methyl, R3 is an alkyl group having 1 to 10 or 1 to 4 carbon atoms, o is an integer from 1 to 10, and p may be an integer from 1 to 10 or 1 to 4.

[0118] The first monomer, the second monomer, and the third monomer each contain an unsaturated group, and the crosslinking agent may be a compound containing two or more unsaturated groups that can be polymerized with these monomers. Therefore, the polymer formed by the monomers through the reaction of the monomers with the crosslinking agent may be crosslinked to have a network structure.

[0119] For example, the cross-linking agent may be one or more compounds selected from the group consisting of compounds represented by Formula X below.

[0120] [Formula X]

[0121]

[0122] In Formula X, R1 is independently selected from hydrogen or methyl, and z may be an integer from 1 to 10 or from 1 to 6.

[0123] The initiator may be a compound that can generate free radicals when irradiated with light, for example, 2,2-dimethoxy-1,2-diphenylethan-1-one, 2-hydroxy-2-methyl-1-phenylpropanone, etc., but the initiator is not limited thereto.

[0124] After preparing the polymer solution, the first polymer solution is coated on the carrier substrate CS and simply dried to form the first layer L1a.

[0125] Unless otherwise specified, the polymer solution may be coated on the carrier substrate (CS) by a method known in the art such as bar coating or dip coating. After coating the first polymer solution, light is irradiated to briefly harden the first polymer solution to form the first layer L1a.

[0126] The second polymer solution is applied onto the first layer L1a and simply hardened to form the second layer L2a. The second polymer solution can be applied by the same method as the first layer L1a and simply hardened.

[0127] The third polymer solution is applied onto the second layer L2a and simply hardened to form the third layer L3a. The third polymer solution may be applied and simply hardened by the same method as the first layer L1a.

[0128] Since the second layer La2 and the third layer La3 are sequentially laminated on the first layer L1a, the number of times exposed to light gradually decreases from the first layer L1a to the third layer L3a. Therefore, the hardness decreases from the first layer L1a to the third layer L3a, so that a mechanical property gradient can be exhibited. Therefore, the degree of hardening is different from the first polymer layer 110 to the adhesive layers 150a and 150b of the finally prepared flexible substrate 100, so that a modulus gradient and a stiffness gradient can be exhibited.

[0129] Next, the first film FM1 is prepared by once hardening the laminated first layer L1a, second layer L2a, and third layer L3a and then removing the carrier substrate CS (S120).

[0130] The primary hardening is performed by irradiating light on the laminated first layer L1a, the second layer L2a, and the third layer L3a. The first layer L1a, the second layer L2a, and the third layer L3a are hardened at the same time. In the primary hardening step, light having an energy lower than that of the light irradiated in the secondary hardening step described below is irradiated. That is, in the primary hardening step, light having a first energy is irradiated for hardening, and in the secondary hardening step described below, light having a second energy higher than the first energy is irradiated for hardening.

[0131] For example, the wavelength of the light having the first energy may be longer than the wavelength of the light having the second energy. As another example, the wavelength of the light having the first energy and the light having the second energy are the same, but the output power is different.

[0132] When light having a first energy is irradiated on the laminated first layer L1a, the second layer L2a, and the third layer L3a, polymers corresponding to the respective layers are formed by polymerization of the monomers. At this time, the first polymer is cross-linked by the cross-linking agent contained in the first layer L1a to form a network structure. For example, the first polymer having a network structure can be selected from the compound represented by the above formula D. The first polymer imparts rigidity to the flexible substrate 100, and the cross-linking density can be increased by increasing the concentration of the cross-linking agent when preparing the first polymer solution to further improve the rigidity while maintaining high softness.

[0133] In addition, in the second layer L2a, the second polymer is formed by polymerization of the second monomer, and the second polymer is cross-linked by a cross-linking agent to form a network structure. For example, the second polymer having a network structure may be selected from the compounds represented by Formula E above.

[0134] In addition, in the third layer L3a, the third polymer may be formed by polymerization of the third monomer, and the third polymer may be cross-linked by a cross-linking agent to form a network structure. For example, the third polymer having a network structure may be selected from the compound represented by Formula G above.

[0135] In the primary hardening step, light having a lower first energy is irradiated to harden the polymer used to form each layer to have a polymerization conversion rate of 70% to 80%. Therefore, in addition to the polymer, each layer may contain unreacted monomers and crosslinking agents. That is, the first layer L1a contains a first polymer, a first monomer, and a crosslinking agent, the second layer L2a contains a second polymer, a second monomer, and a crosslinking agent, and the third layer L3a may contain a third polymer, a third monomer, and a crosslinking agent.

[0136] After the primary hardening is completed, the carrier substrate CS is removed to prepare the first film FM1.

[0137] Next, a second film FM2 is prepared ( S130 ).

[0138] The second film FM2 may be prepared by sequentially laminating the first layer L1b, the second layer L2b, and the third layer L3b on the carrier substrate CS and then removing the carrier substrate CS. The second film FM2 is prepared by the same method as the above-described first film FM1, and thus redundant description is omitted.

[0139] As necessary, the step of applying the third polymer solution to form the third layers L3a and L3b may be omitted selectively in the step of preparing the first film FM1, in the step of preparing the second film FM2, or in both steps.

[0140] Next, the first film FM1 and the second film FM2 are laminated ( S140 ).

[0141] The first film FM1 and the second film FM2 are laminated such that the first layer L1a of the first film FM1 and the first layer L1b of the second film FM2 are opposed to each other.

[0142] Next, the laminated first film and second film FM2 are secondarily hardened to prepare the flexible substrate 100 ( S150 ).

[0143] Secondary hardening is performed by irradiating light on the laminated first film FM1 and the second film FM2. The first film FM1 and the second film FM2 are hardened simultaneously. As described above, in the secondary hardening step, light having a second energy higher than the first energy is irradiated for additional hardening. In the secondary hardening step, interfacial polymerization is performed. For example, at the interface where the first layer L1a of the first film FM1 and the first layer L1b of the second film FM2 contact each other, the first film FM1 and the second film FM2 are bonded by a cross-linking reaction between the first polymer of the first film FM1 and the first polymer of the second film FM2.

[0144] In addition, at the interface where the first layer L1a and the second layer L2a of the first film FM1 contact each other, a first interface cross-linked layer 120a including a first cross-linked polymer is formed by a cross-linking reaction of the first polymer of the first layer L1a and the second polymer of the second layer L2a. For example, the first cross-linked polymer may be selected from the compound represented by the above formula C. Since the first polymer of the first layer L1a is cross-linked with the second polymer of the second layer L2a, the first layer L1a and the second layer L2a are bonded to each other.

[0145] In addition, at the interface where the second layer L2a and the third layer L3a of the first film FM1 contact each other, a second interface cross-linked layer 140a including a second cross-linked polymer is formed by a cross-linking reaction of the second polymer of the second layer L2a and the third polymer of the third layer L3a. For example, the second cross-linked polymer can be selected from the compound represented by the above formula H. Since the second polymer of the second layer L2a is cross-linked with the third polymer of the third layer L3a, the second layer L2a and the third layer L3a are bonded to each other.

[0146] In the first layer L1b and the second layer L2b of the second film FM2, similar to the first film FM1, a first interface cross-linked layer 120b is formed at the interface between the first layer L1b and the second layer L2b by a cross-linking reaction. In addition, similar to the first film FM1, a second interface cross-linked layer 140b is formed at the interface between the second layer L2b and the third layer L3b of the second film FM2, so redundant description will be omitted.

[0147] As in the secondary hardening step, light having a second energy is irradiated for additional hardening, the polymerization conversion rate of the polymer contained in each layer is higher than that in the primary hardening step, for example, may be 90% to 99.9%.

[0148] In the secondary hardening step, light of a second energy having relatively high energy is irradiated so that a polymerization reaction occurs at the interface of each layer, and the first film FM1 and the second film FM2 are bonded together by crosslinking between the polymers contained in different layers. In addition, first interface crosslinking layers 120a and 120b are formed at the interface between the first layer L1a and L1b and the second layer L2a and L2b of each of the first film FM1 and the second film FM2, and second interface crosslinking layers 140a and 140b are formed between the second layer L2a and L2b and the third layer L3a and L3b. Finally, a flexible substrate 100 having the following structure can be obtained, in which the first interface crosslinking layers 120a and 120b, the second polymer layers 130a and 130b, the second interface crosslinking layers 140a and 140b, and the adhesive layers 150a and 150b are symmetrically laminated above and below the first polymer layer 110 and integrate the layers.

[0149] The first layers L1a and L1b include a polymer having excellent rigidity, the second layers L2a and L2b include a polymer having excellent softness, and the third layers L3a and L3b include a polymer providing adhesion. Therefore, the flexible substrate 100 obtained by simultaneously secondary hardening the first layers L1a and L1b, the second layers L2a and L2b, and the third layers L3a and L3b can satisfy both softness and rigidity.

[0150] In addition, in the flexible substrate of the related art, it is difficult to process the adhesive layer in the process of laminating the adhesive layer on the substrate. Specifically, the adhesive layer has a low modulus to provide adhesion, and due to its high adhesive performance, residual adhesive is easily generated in the process of punching the adhesive layer to have a desired shape and size. However, the flexible substrate 100 of the present disclosure does not require the process of punching and laminating the adhesive layer, and thus has a process advantage.

[0151] The flexible substrate 100 of the present disclosure has adhesiveness and excellent rigidity and softness, so that the flexible substrate can be used as a substrate for supporting a display panel. In addition, the flexible substrate 100 of the present disclosure is used as a substrate of a foldable display device to provide a display device with improved impact resistance while maintaining high folding characteristics.

[0152] In the following, reference will be made to Figure 4 A display device according to an exemplary embodiment of the present disclosure is described. Figure 4 is a schematic cross-sectional view of a display device according to an exemplary embodiment of the present disclosure. Figure 4 In the present invention, for the convenience of description, the display device is shown as a foldable display device, but is not limited thereto. The display device of the present disclosure can be implemented not only as a flat display device, but also as a display device having various shapes such as a curved or multi-folded display device.

[0153] Reference Figure 4 , the display device 1000 according to the exemplary embodiment of the present disclosure includes a support plate 300, a flexible substrate 100, and a display panel 200. Figure 4 In the display device, the flexible substrate 100 and Figure 1 The flexible substrate 100 shown in FIG. 1 is the same as that in FIG. 1 , and thus redundant description will be omitted.

[0154] The display panel 200 is a panel that displays an image. For example, the display panel 200 may be an organic light emitting display panel including an organic light emitting diode, but is not limited thereto.

[0155] For example, an organic light emitting diode may include an anode, a cathode, and an organic light emitting layer disposed therebetween. In the organic light emitting diode, holes injected from the anode and electrons injected from the cathode combine on the organic light emitting layer to emit light. The light emitted as described above may be used to display an image.

[0156] Reference Figure 1 and Figure 4, the flexible substrate 100 is disposed on the rear surface of the display panel 200. The flexible substrate 100 supports various elements constituting the display panel 200 and is called a backplane. The flexible substrate 100 includes a first polymer layer 110 having excellent stiffness, second polymer layers 130a and 130b having excellent softness, and first interface crosslinking layers 120a and 120b having a mesh structure formed by crosslinking the first polymer and the second polymer between the first polymer layer and the second polymer layer. Therefore, stiffness and softness are satisfied at the same time. The flexible substrate is disposed on the rear surface of the display panel 200 to ensure impact resistance while maintaining high folding characteristics of the display device 1000. By doing so, the display panel 200 can be easily folded and can be protected from external impacts.

[0157] Adhesive layers 150a and 150b are located above and below the flexible substrate 100. The display panel 200 may be bonded to the flexible substrate 100 through the adhesive layer 150a located above the flexible substrate 100.

[0158] The display device implemented as a foldable device is made of a material having high softness so as to be easily folded. Therefore, when the display device is folded, it is difficult to maintain a predetermined shape and is easily affected by external impact.

[0159] Therefore, the support plate 300 may be disposed under the flexible substrate 100. The support plate 300 may be referred to as a board assembly. The support plate 300 is bonded to the rear surface of the flexible substrate 100 through the adhesive layer 150b located under the flexible substrate 100.

[0160] The support plate 300 may include a plate top and a plate bottom. As required, the plate top and the plate bottom may be selectively formed in one piece, and as required, the plate top or the plate bottom may be omitted.

[0161] The bottom of the board may include an opening pattern in a portion corresponding to the folding area of ​​the display device. By doing so, the folding stress can be effectively alleviated while maintaining a high rigidity of the display device.

[0162] For example, the bottom of the board may be a metal material such as stainless steel (SUS) or invar. However, the bottom of the board is not limited thereto and may be a plastic material such as polymethyl methacrylate (PMMA), polycarbonate (PC), polyvinyl alcohol (PVA), acrylonitrile butadiene styrene (ABS), polyethylene terephthalate (PET), silicone or polyurethane (PU).

[0163] The top of the board is disposed between the flexible substrate 100 and the bottom of the board. The top of the board is formed of a material having high rigidity to enhance the rigidity of the display device. In addition, the top of the board can suppress the phenomenon that the opening pattern of the bottom of the board is visible through the display panel 200. For example, the top of the board can be formed of a metal material such as stainless steel (SUS), invar alloy, aluminum-based material or magnesium. However, the top of the board is not limited thereto and can be formed of a plastic material such as polymethyl methacrylate (PMMA) or polycarbonate (PC).

[0164] A display device according to an exemplary embodiment of the present disclosure includes a flexible substrate including an interface cross-linked layer having a mesh structure formed by cross-linking a heterogeneous polymer at an interface where a layer including a polymer having excellent softness contacts a layer including a polymer having excellent stiffness, so as to significantly improve stiffness while maintaining high softness. The flexible substrate is disposed on a rear surface of a display panel to make the display device easy to fold, and improves impact resistance to protect the display device from external impact.

[0165] Exemplary embodiments of the present disclosure may also be described as follows:

[0166] According to one aspect of the present disclosure, a flexible substrate includes: a first polymer layer, which includes a first polymer; a second polymer layer, which includes a second polymer and is located above and below the first polymer layer; and a first interface cross-linking layer, which is located between the first polymer layer and the second polymer layer and includes a first cross-linked polymer having a network structure formed by cross-linking the first polymer and the second polymer, wherein the stiffness of the first polymer layer is higher than the stiffness of the second polymer layer, and the softness of the second polymer layer is greater than the softness of the first polymer layer.

[0167] The first polymer may include one or more repeating units selected from the following formula A, and the second polymer may include one or more repeating units selected from the following formula B.

[0168] [Formula A]

[0169]

[0170] [Formula B]

[0171]

[0172] (In Formula A and Formula B, n may independently be an integer from 500 to 4000, x may independently be an integer from 300 to 2500, and y may independently be an integer from 300 to 2500)

[0173] The first cross-linked polymer may be selected from compounds represented by Formula C below.

[0174] [Formula C]

[0175]

[0176] (In Formula C, z may be an integer from 1 to 10, R1 may be independently selected from hydrogen or methyl, A may include one or more repeating units selected from Formula A, and B' may include one or more repeating units selected from Formula B)

[0177] The first polymer of the first polymer layer may be cross-linked to form a network structure, and the second polymer of the second polymer layer may be cross-linked to form a network structure.

[0178] The first polymer having a network structure may be selected from compounds represented by Formula D below.

[0179] [Formula D]

[0180]

[0181] (In formula D, A may include one or more repeating units selected from formula A, z may be an integer from 1 to 10, and R1 may be independently selected from hydrogen or methyl)

[0182] [Formula A]

[0183]

[0184] (In formula A, n may independently be an integer from 500 to 4000)

[0185] The second polymer having a network structure may be selected from compounds represented by Formula E below.

[0186] [Formula E]

[0187]

[0188] (In Formula E, B' may include one or more repeating units selected from Formula B, z may be an integer from 1 to 10, and R1 may be independently selected from hydrogen or methyl)

[0189] [Formula B]

[0190]

[0191] (In Formula B, n may independently be an integer from 500 to 4000, x may independently be an integer from 300 to 2500, and y may independently be an integer from 300 to 2500)

[0192] The flexible substrate may further include an adhesive layer located above the second polymer layer above the first polymer layer and below the second polymer layer below the first polymer layer and including a third polymer.

[0193] The third polymer may include one or more repeating units selected from Formula F below.

[0194] [Formula F]

[0195]

[0196] (In Formula F, R2 may independently be hydrogen or methyl, R3 may be an alkyl group having 1 to 10 carbon atoms, o may be an integer from 1 to 10, p may be an integer from 1 to 10, and l may independently be an integer from 500 to 3000)

[0197] The third polymers of the adhesive layer may be cross-linked with each other to form a network structure, and the third polymer having the network structure may be selected from compounds represented by Formula G below.

[0198] [Formula G]

[0199]

[0200] (In formula G, F' may include one or more repeating units selected from formula F, z may be an integer from 1 to 10, and R1 may be independently selected from hydrogen or methyl)

[0201] The flexible substrate may further include a second interface cross-linking layer located between the adhesive layer and the second polymer layer and including a second cross-linking polymer having a network structure formed by cross-linking the third polymer and the second polymer.

[0202] The second cross-linked polymer may be selected from compounds represented by Formula H below.

[0203] [Formula H]

[0204]

[0205] (In formula H, B' may include one or more repeating units selected from formula B, F' may include one or more repeating units selected from formula F, z may be an integer from 1 to 10, and R1 may be independently selected from hydrogen or methyl)

[0206] [Formula B]

[0207]

[0208] (In Formula B, n may independently be an integer from 500 to 4000, x may independently be an integer from 300 to 2500, and y may independently be an integer from 300 to 2500)

[0209] [Formula F]

[0210]

[0211] (In Formula F, R2 may independently be hydrogen or methyl, R3 may be an alkyl group having 1 to 10 carbon atoms, o may be an integer from 1 to 10, p may be an integer from 1 to 10, and l may independently be an integer from 500 to 3000)

[0212] According to another aspect of the present disclosure, a display device includes a flexible substrate and a display panel disposed on the flexible substrate, wherein the flexible substrate includes: a first polymer layer, which includes a first polymer; a second polymer layer, which is located above and below the first polymer layer and includes a second polymer; and a first interface cross-linking layer, which is located between the first polymer layer and the second polymer layer and includes a first cross-linked polymer having a network structure formed by cross-linking the first polymer and the second polymer, wherein the stiffness of the first polymer layer is greater than the stiffness of the second polymer layer, and the softness of the second polymer layer is greater than the softness of the first polymer layer.

[0213] The first cross-linked polymer may be selected from compounds represented by Formula C below.

[0214] [Formula C]

[0215]

[0216] (In Formula C, z may be an integer from 1 to 10, R1 may be independently selected from hydrogen or methyl, A may include one or more repeating units selected from Formula A, and B' may include one or more repeating units selected from Formula B)

[0217] [Formula A]

[0218]

[0219] [Formula B]

[0220]

[0221] (In Formula A and Formula B, n may independently be an integer from 500 to 4000, x may independently be an integer from 300 to 2500, and y may independently be an integer from 300 to 2500)

[0222] The first polymer of the first polymer layer may be cross-linked to form a network structure, and the first polymer having the network structure may be selected from compounds represented by Formula D below.

[0223] [Formula D]

[0224]

[0225] (In formula D, A may include one or more repeating units selected from formula A, z may be an integer from 1 to 10, and R1 may be independently selected from hydrogen or methyl)

[0226] [Formula A]

[0227]

[0228] (In formula A, n may independently be an integer from 500 to 4000)

[0229] The second polymer of the second polymer layer may be cross-linked to form a network structure, and the second polymer having the network structure may be selected from compounds represented by Formula E below.

[0230] [Formula E]

[0231]

[0232] (In Formula E, B' may include one or more repeating units selected from Formula B, z may be an integer from 1 to 10, and R1 may be independently selected from hydrogen or methyl)

[0233] [Formula B]

[0234]

[0235] (In Formula B, n may independently be an integer from 500 to 4000, x may independently be an integer from 300 to 2500, and y may independently be an integer from 300 to 2500)

[0236] The display device also includes: an adhesive layer, which is located above the second polymer layer above the first polymer layer and below the second polymer layer below the first polymer layer, and contains a third polymer; and a second interface cross-linking layer, which is located between the adhesive layer and the second polymer layer and contains a second cross-linked polymer having a network structure formed by cross-linking the third polymer and the second polymer.

[0237] The second cross-linked polymer may be selected from compounds represented by Formula H below.

[0238] [Formula H]

[0239]

[0240] (In formula H, B' may include one or more repeating units selected from formula B, F' may include one or more repeating units selected from formula F, z may be an integer from 1 to 10, and R1 may be independently selected from hydrogen or methyl)

[0241] [Formula B]

[0242]

[0243] (In Formula B, n may independently be an integer from 500 to 4000, x may independently be an integer from 300 to 2500, and y may independently be an integer from 300 to 2500)

[0244] [Formula F]

[0245]

[0246] (In Formula F, R2 may independently be hydrogen or methyl, R3 may be an alkyl group having 1 to 10 carbon atoms, o may be an integer from 1 to 10, p may be an integer from 1 to 10, and l may independently be an integer from 500 to 3000)

[0247] The third polymers of the adhesive layer may be cross-linked with each other to form a network structure, and the third polymer having the network structure may be selected from compounds represented by Formula G below.

[0248] [Formula G]

[0249]

[0250] (In Formula G, F' may include one or more repeating units selected from the following Formula F, z may be an integer from 1 to 10, and R1 may be independently selected from hydrogen or methyl)

[0251] [Formula F]

[0252]

[0253] (In Formula F, R2 may independently be hydrogen or methyl, R3 may be an alkyl group having 1 to 10 carbon atoms, o may be an integer from 1 to 10, p may be an integer from 1 to 10, and l may independently be an integer from 500 to 3000)

[0254] The adhesive layer located above the second polymer layer can bond the display panel to the flexible substrate.

[0255] The display device may further include a support plate disposed below the flexible substrate, wherein the adhesive layer below the second polymer layer may adhere the support plate to the flexible substrate.

[0256] According to another aspect of the present disclosure, a method for preparing a flexible substrate includes: preparing a first film, the first film including a first layer including a first polymer and a cross-linking agent, and a second layer including a second polymer and a cross-linking agent; preparing a second film, the second film including a first layer including a first polymer and a cross-linking agent, and a second layer including a second polymer and a cross-linking agent; laminating the first film and the second film so that the first layer of the first film is opposite to the first layer of the second film; and hardening the laminated first and second films, wherein, during the hardening, the first polymer of the first film and the first polymer of the second film are cross-linked at the interface where the first layer of the first film and the first layer of the second film are in contact with each other to form a network structure so that the first film is bonded to the second film, and the first polymer of the first film and the second polymer are cross-linked at the interface where the first layer and the second layer are in contact with each other to bond the first layer and the second layer of the first film, and the first polymer of the second film and the second polymer are cross-linked at the interface where the first layer and the second layer are in contact with each other to bond the first layer and the second layer of the second film.

[0257] The preparation of the first film may further include laminating a third layer comprising a third polymer and a cross-linking agent on the second layer, and the preparation of the second film may further include laminating a third layer comprising a third polymer on the second layer, and in the hardening step, the first layer, the second layer and the third layer may be hardened simultaneously.

[0258] In the step of hardening, the second polymer and the third polymer may be cross-linked at interfaces where the second layer and the third layer may respectively contact each other to bond the second layer and the third layer.

[0259] Although the exemplary embodiments of the present disclosure have been described in detail with reference to the accompanying drawings, the present disclosure is not limited thereto and can be implemented in a variety of different forms without departing from the technical concept of the present disclosure. Therefore, the exemplary embodiments of the present disclosure are provided for illustrative purposes only and are not intended to limit the technical concept of the present disclosure. The scope of the technical concept of the present disclosure is not limited thereto. Therefore, it should be understood that the above exemplary embodiments are illustrative in all aspects and do not limit the present disclosure. The scope of protection of the present disclosure should be based on the following claims, and all technical concepts within the equivalent scope thereof should be understood to fall within the scope of the present disclosure.

Claims

1. A flexible substrate, comprising: a first polymer layer, the first polymer layer comprising a first polymer; a second polymer layer comprising a second polymer and disposed above and below the first polymer layer; as well as a first interface cross-linked layer, the first interface cross-linked layer being located between the first polymer layer and the second polymer layer and comprising a first cross-linked polymer having a network structure formed by cross-linking the first polymer and the second polymer, wherein the stiffness of the first polymer layer is greater than that of the second polymer layer, and the softness of the second polymer layer is greater than that of the first polymer layer, Wherein, the first polymer comprises one or more repeating units selected from the following formula A, and the second polymer comprises one or more repeating units selected from the following formula B: [Formula A] [Formula B] wherein, in Formula A and Formula B, n is independently an integer from 500 to 4000, x is independently an integer from 300 to 2500, and y is independently an integer from 300 to 2500, Wherein, the first cross-linked polymer is selected from the compound represented by the following formula C: [Formula C] Wherein, in Formula C, A includes one or more repeating units selected from Formula A, and B' includes one or more repeating units selected from Formula B.

2. The flexible substrate according to claim 1, wherein: The first polymer of the first polymer layer is cross-linked to form a network structure, and the second polymer of the second polymer layer is cross-linked to form a network structure.

3. The flexible substrate according to claim 2, wherein: The first polymer having the network structure is selected from the compounds represented by the following formula D: [Formula D] Wherein, in formula D, A comprises one or more repeating units selected from formula A, [Formula A] Wherein, in formula A, n is independently an integer of 500 to 4000.

4. The flexible substrate according to claim 2, wherein: The second polymer having the network structure is selected from the compounds represented by the following formula E: [Formula E] Wherein, in formula E, B' comprises one or more repeating units selected from formula B, [Formula B] Wherein, in Formula B, n is independently an integer from 500 to 4000, x is independently an integer from 300 to 2500, and y is independently an integer from 300 to 2500.

5. The flexible substrate according to claim 1, further comprising: An adhesive layer is located above the second polymer layer above the first polymer layer and below the second polymer layer below the first polymer layer, and includes a third polymer.

6. The flexible substrate according to claim 5, wherein: The third polymer comprises one or more repeating units selected from the following formula F: [Formula F] Wherein, in Formula F, R2 is independently hydrogen or methyl, R3 is an alkyl group having 1 to 10 carbon atoms, o is an integer from 1 to 10, p is an integer from 1 to 10, and l is independently an integer from 500 to 3000.

7. The flexible substrate according to claim 6, wherein: The third polymers of the adhesive layer are cross-linked with each other to form a network structure, and the third polymer having the network structure is selected from compounds represented by the following Formula G: [Formula G] Wherein, in formula G, F' comprises one or more repeating units selected from formula F.

8. The flexible substrate according to claim 5, further comprising: A second interface cross-linked layer is located between the adhesive layer and the second polymer layer and includes a second cross-linked polymer having a network structure formed by cross-linking the third polymer and the second polymer.

9. The flexible substrate according to claim 8, wherein: The second cross-linked polymer is selected from the compounds represented by the following formula H: [Formula H] Wherein, in formula H, B' comprises one or more repeating units selected from formula B, and F' comprises one or more repeating units selected from formula F, [Formula B] wherein, in Formula B, n is independently an integer from 500 to 4000, x is independently an integer from 300 to 2500, and y is independently an integer from 300 to 2500, [Formula F] Wherein, in Formula F, R2 is independently hydrogen or methyl, R3 is an alkyl group having 1 to 10 carbon atoms, o is an integer from 1 to 10, p is an integer from 1 to 10, and l is independently an integer from 500 to 3000.

10. A display device, comprising: The flexible substrate according to claim 1; as well as A display panel is arranged on the flexible substrate. Wherein, the flexible substrate comprises: a first polymer layer, the first polymer layer comprising a first polymer; a second polymer layer located above and below the first polymer layer and comprising a second polymer; and a first interface cross-linked layer, the first interface cross-linked layer being located between the first polymer layer and the second polymer layer and comprising a first cross-linked polymer having a network structure formed by cross-linking the first polymer and the second polymer, The stiffness of the first polymer layer is greater than that of the second polymer layer, and the softness of the second polymer layer is greater than that of the first polymer layer.

11. The display device according to claim 10, wherein: The first cross-linked polymer is selected from the compounds represented by the following formula C: [Formula C] Wherein, in formula C, A comprises one or more repeating units selected from formula A, and B' comprises one or more repeating units selected from formula B, [Formula A] [Formula B] Wherein, in Formula A and Formula B, n is independently an integer from 500 to 4000, x is independently an integer from 300 to 2500, and y is independently an integer from 300 to 2500.

12. The display device according to claim 10, wherein: The first polymer of the first polymer layer is cross-linked to form a network structure, and the first polymer having the network structure is selected from compounds represented by the following formula D: [Formula D] Wherein, in formula D, A comprises one or more repeating units selected from formula A, [Formula A] Wherein, in formula A, n is independently an integer of 500 to 4000.

13. The display device according to claim 10, wherein: The second polymer of the second polymer layer is cross-linked to form a network structure, and the second polymer having the network structure is selected from compounds represented by the following Formula E: [Formula E] Wherein, in formula E, B' comprises one or more repeating units selected from formula B, [Formula B] Wherein, in Formula B, n is independently an integer from 500 to 4000, x is independently an integer from 300 to 2500, and y is independently an integer from 300 to 2500.

14. The display device according to claim 10, further comprising: an adhesive layer located above the second polymer layer above the first polymer layer and below the second polymer layer below the first polymer layer, and comprising a third polymer; as well as A second interface cross-linked layer is located between the adhesive layer and the second polymer layer and includes a second cross-linked polymer having a network structure formed by cross-linking the third polymer and the second polymer.

15. The display device according to claim 14, wherein: The second cross-linked polymer is selected from the compounds represented by the following formula H: [Formula H] Wherein, in formula H, B' comprises one or more repeating units selected from formula B, and F' comprises one or more repeating units selected from formula F, [Formula B] wherein, in Formula B, n is independently an integer from 500 to 4000, x is independently an integer from 300 to 2500, and y is independently an integer from 300 to 2500, [Formula F] Wherein, in Formula F, R2 is independently hydrogen or methyl, R3 is an alkyl group having 1 to 10 carbon atoms, o is an integer from 1 to 10, p is an integer from 1 to 10, and l is independently an integer from 500 to 3000.

16. The display device according to claim 14, wherein: The third polymers of the adhesive layer are cross-linked with each other to form a network structure, and the third polymer having the network structure is selected from compounds represented by the following Formula G: [Formula G] Wherein, in formula G, F' comprises one or more repeating units selected from the following formula F, [Formula F] Wherein, in Formula F, R2 is independently hydrogen or methyl, R3 is an alkyl group having 1 to 10 carbon atoms, o is an integer from 1 to 10, p is an integer from 1 to 10, and l is independently an integer from 500 to 3000.

17. The display device according to claim 14, wherein: The adhesive layer located over the second polymer layer bonds the display panel to the flexible substrate.

18. The display device according to claim 14, further comprising: A support plate disposed below the flexible substrate, The adhesive layer located below the second polymer layer bonds the support plate to the flexible substrate.

19. A method for preparing a flexible substrate, comprising: preparing a first film comprising a first layer comprising a first polymer and a cross-linking agent and a second layer comprising a second polymer and a cross-linking agent; preparing a second film comprising a first layer comprising a first polymer and a cross-linking agent and a second layer comprising a second polymer and a cross-linking agent; laminating the first film and the second film such that the first layer of the first film is opposite to the first layer of the second film; as well as hardening the laminated first film and second film, wherein, in the step of hardening, the first polymer of the first film and the first polymer of the second film are cross-linked at the interface where the first layer of the first film and the first layer of the second film contact each other to form a network structure, so that the first film and the second film are bonded, The first polymer and the second polymer of the first film are cross-linked at an interface where the first layer and the second layer contact each other to bond the first layer and the second layer of the first film, and The first polymer and the second polymer of the second film are cross-linked at an interface where the first layer and the second layer contact each other to bond the first layer and the second layer of the second film, Wherein, the first polymer comprises one or more repeating units selected from the following formula A, and the second polymer comprises one or more repeating units selected from the following formula B: [Formula A] [Formula B] wherein, in Formula A and Formula B, n is independently an integer from 500 to 4000, x is independently an integer from 300 to 2500, and y is independently an integer from 300 to 2500, Wherein, the cross-linking agent is represented by the following formula X: [Formula X] 20. The preparation method according to claim 19, wherein: The preparation of the first film further includes laminating a third layer comprising a third polymer and a cross-linking agent on the second layer; The preparation of the second film further comprises laminating a third layer comprising a third polymer on the second layer; and In the step of hardening, the first layer, the second layer, and the third layer are hardened simultaneously.

21. The preparation method according to claim 20, wherein: In the hardening step, the second polymer and the third polymer are cross-linked at interfaces where the second layer and the third layer are in contact with each other, so as to bond the second layer and the third layer.

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