Optical member and manufacturing method thereof

Abstract

To provide an optical member and a manufacturing method with which it is possible to stably and reversibly change the diffusion state of transmitted light in succession corresponding to the ambient temperature.SOLUTION: The optical member comprises a nematic liquid crystal elastomer having a laminate structure of unit layers which are composed of a molecular oriented liquid crystal polymer which is given a prescribed angle with respect to an axis on a principal plane and are laminated along the axis, in which structure the liquid crystal polymer is crosslinked. The diffusion state is reversibly changed by changing the angle amplitude relative to a prescribed angle in relation to temperature. A method for manufacturing the same involves uniaxially drawing a sheet body or a planar body composed of a partially crosslinked liquid crystal polymer along the axis on the principal plane and then releasing tension, heating to a prescribed temperature and causing to shrink in the axial direction to give a laminate structure, and further secondarily crosslinking to obtain a nematic liquid crystal elastomer.SELECTED DRAWING: Figure 14

Description

[Technical Field]

[0001] The present invention relates to an optical element that reversibly changes its light diffusion state and a method for manufacturing the same, and more particularly to an optical element that reversibly and continuously changes its light diffusion state in response to ambient temperature and a method for manufacturing the same. [Background technology]

[0002] As an optical element that can reversibly change the light diffusion state, those using liquid crystal materials are widely known, and the light diffusion state is reversibly controlled by changing the orientation of the liquid crystal material by applying a voltage. On the other hand, a method has also been proposed in which the light diffusion state is controlled by reversibly changing the shape of the structural interface that causes light scattering.

[0003] For example, Patent Document 1 discloses an optical element that can reversibly change the light diffusion state by adjusting the strain in the surface direction to change the uneven structure caused by surface buckling. For example, a thin film layer made of an imide resin or vinylidene chloride resin is provided on a substrate made of a polysiloxane polymer, forming a spatially periodic unevenness. The spatial period of the unevenness is set to 0.1 mm or more and 10 mm or less, and the strain is adjusted so that the aspect ratio (groove depth / spatial period of the unevenness) changes within a range of 0 to 0.3, thereby making it possible to reversibly control the light diffusion state. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-153530 Summary of the Invention [Problem to be solved by the invention]

[0005] The above-mentioned Patent Document 1 also describes that the distortion of the surface layer of the light diffusing laminate can be changed depending on the temperature, and that temperature changes can be detected from the light diffusing state, and that the laminate can be applied to a simple temperature change sensor. In such applications, continuous and stable operation against repeated temperature changes is required.

[0006] The present invention has been made in consideration of the above-mentioned circumstances, and its object is to provide an optical element and a method for manufacturing the same that can stably, reversibly, and continuously change the diffusion state of transmitted light in response to the ambient temperature. [Means for solving the problem]

[0007] The optical element according to the present invention is an optical element consisting of a sheet or plate, which reversibly and continuously changes the diffusion state of transmitted light that enters through one of a pair of main surfaces and exits through the other in response to the ambient temperature, and which has a laminated structure in which unit layers made of liquid crystal polymers that are molecularly oriented at a predetermined angle relative to an axis on the main surfaces are stacked along the axis, and the liquid crystal polymer is a crosslinked nematic liquid crystal elastomer, and the diffusion state is reversibly changed by changing the angular amplitude relative to the predetermined angle with temperature.

[0008] According to this feature, the diffusion state of transmitted light can be stably, reversibly and continuously changed in response to the ambient temperature.

[0009] In addition, the manufacturing method of the present invention is a method for manufacturing an optical element consisting of a sheet or plate, which reversibly and continuously changes the diffusion state of transmitted light that enters from one of a pair of main surfaces and exits from the other in response to the ambient temperature, and is characterized in that a sheet or plate made of a partially crosslinked liquid crystal polymer is uniaxially stretched along an axis located on the main surface, and then the tension is released and heated to a predetermined temperature to cause it to shrink along the axis, giving it a laminated structure in which unit layers made of the liquid crystal polymer that have been molecularly oriented at a predetermined angle relative to the axis are stacked along the axis, and further, it is secondarily crosslinked to give it a nematic liquid crystal elastomer, and the angular amplitude relative to the predetermined angle is changed by temperature to reversibly change the diffusion state.

[0010] According to this feature, it is possible to obtain an optical member that can stably, reversibly, and continuously change the diffusion state of transmitted light in response to the ambient temperature. [Brief explanation of the drawings]

[0011] [Figure 1] 1A is a photograph of the appearance of an example of an optical member according to the present invention, and FIG. 1B is a polarized fluorescence microscope image of a cross section thereof. [Figure 2] 1 is a cross-sectional view of an optical element according to the present invention. [Figure 3] 3A to 3C are perspective and enlarged views of a liquid crystal elastomer in each manufacturing step of an optical member according to the present invention, and views showing the orientation of liquid crystal molecules. [Figure 4] 3A to 3C are perspective and enlarged views of a liquid crystal elastomer in each manufacturing step of an optical member according to the present invention, and views showing the orientation of liquid crystal molecules. [Figure 5] 1 is a structural formula showing the raw material of one embodiment of an optical member. [Figure 6] 1 is a schematic diagram showing the structure of a liquid crystal polymer produced in the manufacturing process of an optical member. [Figure 7] 1 is a graph showing the relationship between engineering strain and stress when a primary crosslinked body is stretched. [Figure 8] 1 is a graph showing the relationship between the annealing temperature of a stretched primary crosslinked body and the engineering strain. [Figure 9] 1 shows polarized fluorescence microscope images of the laminated structure obtained depending on the annealing temperature. [Figure 10] Photographs of the scattering patterns of transmitted light as a function of annealing temperature. [Figure 11] 1 is a graph of angle-dependent scattered light intensity of a fabricated optical element. [Figure 12] 10 is a graph showing the relationship between the wavelength of transmitted light and the intensity of scattered light for each scattering angle. [Figure 13] 10 is a photograph of the scattering pattern of a manufactured optical member at each temperature. [Figure 14] 10 is a graph showing the relationship between temperature and shrinkage of a manufactured optical member. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

[0013] As shown in Figure 1(a), the optical element 1 is a rubber-like elastic body made of a nematic liquid crystal elastomer and is manufactured as a sheet or plate. Referring also to Figure 1(b), the optical element 1 has a laminated structure in which layers 2, which are unit layers, are periodically stacked along an axis X on (within) its main surface.

[0014] Referring also to Figure 2, layer 2 has a thickness of about several micrometers as a unit layer, and liquid crystal molecules 3 are oriented in one direction within it to form nematic domains. This molecular orientation is tilted at a predetermined angle with respect to the direction of axis X, which is the lamination direction of layer 2. Furthermore, this molecular orientation is in a different direction between adjacent unit layers. This layered structure is formed automatically during the manufacturing process of the liquid crystal elastomer, and details will be described later.

[0015] The optical element 1 changes the magnitude of the angular amplitude of the liquid crystal molecules 3 aligned at a predetermined angle relative to the axis X of the laminated structure in response to changes in ambient temperature. For example, because thermal vibration and the binding force of the liquid crystal elastomer change with temperature, the angular amplitude of each liquid crystal molecule 3 expands or contracts while aligned at a predetermined angle. That is, as the temperature increases, the angular amplitude increases, decreasing the so-called alignment. Conversely, as the temperature decreases, the angular amplitude decreases, increasing the so-called alignment. As a result, the diffusion state of transmitted light entering one of the pair of principal surfaces and exiting the other can be changed in response to changes in ambient temperature. The angular amplitude of the liquid crystal molecules changes reversibly and continuously with temperature, allowing for stable changes in the diffusion state.

[0016] In particular, when the layer width is equal to or several times the wavelength of visible light, the visible light is diffused well by diffraction interference. As a result, the transmitted light of the optical element 1 is anisotropically scattered in a plane including its optical axis and axis X. The optical element 1 also exhibits chromatic dispersion, exhibiting coloration that is sensitively dependent on the angle of the optical axis of the transmitted light. Furthermore, increasing the temperature of the optical element 1 increases the angular amplitude, reducing the orientation and making the optical element 1 almost transparent to visible light, making it appear as if the layer structure described above has disappeared. The diffusion state of the reflected light can also be changed in a similar manner by attaching a reflector, for example.

[0017] Next, an example of a method for manufacturing such an optical member 1 will be described.

[0018] As shown in Figures 3(a1) and 3(a2), a sheet or plate made of a partially crosslinked liquid crystal polymer is first prepared. The liquid crystal polymer can be obtained, for example, by mixing the liquid crystal molecules, spacer molecules, and crosslinking molecule monomers as raw materials in a solvent and then thermally polymerizing the mixture. The liquid crystal molecules used are main-chain liquid crystal molecules, also known as mesogens, which are rigid monomer molecules that exhibit liquid crystallinity.

[0019] Referring also to Figure 5, for example, the following monomers can be used as raw materials. As shown in Figure 5(a), the liquid crystal molecules are bifunctional RM257 with a main chain and acrylic groups at both ends. As shown in Figure 5(b), the spacer molecules are bifunctional EDDET with thiol groups at both ends. Furthermore, as shown in Figure 5(c), the crosslinking molecules are tetrafunctional PETMP with thiol groups at four ends. At this time, a photoradical agent and a stabilizer are added for secondary crosslinking by photopolymerization, as described below.

[0020] These materials are blended in a predetermined ratio, leaving unreacted acrylic groups. For example, in this example, the molar fraction is RM257:EDDET:PETMP=55:44:3, and the total number of acrylic groups relative to the total number of thiol groups is approximately 110%. These raw materials are then mixed with 30% by mass of toluene as a solvent, and thermally polymerized by a thiol-ene reaction in the presence of an amine catalyst (tetraethylamine) at the isotropic phase temperature before crosslinking to obtain a primary crosslinked product. At this time, a mold of predetermined dimensions is used to define the shape of the molded product, which is a plate or sheet of the optical component 1.

[0021] This produces a liquid crystal polymer, which is a primary crosslinked polymer with a unit structure as shown in Figure 6(a). The liquid crystal polymer then transitions to a nematic phase by lowering its temperature to room temperature. At this time, as shown in Figure 6(b), the produced liquid crystal polymer microscopically forms domains 4 in which the liquid crystal molecules 3 are aligned in the same direction.

[0022] Referring again to Figure 3(a1), the particle size of these domains 4 is about several microns, and a polydomain structure is formed with a large number of domains 4. Furthermore, since each domain 4 has birefringence in its orientation direction, the liquid crystal polymer as a whole strongly scatters light and appears cloudy.

[0023] Next, as shown in Figure 3(b1), the obtained liquid crystal polymer is uniaxially stretched along the axis X on the main surface. Here, stretching is performed at room temperature. This causes the liquid crystal molecules 3 to be preferentially aligned in the direction of the axis X, which is the stretching direction, as shown in Figure 3(b2).

[0024] Referring to Figure 7, when a liquid crystal polymer is stretched until the engineering strain e (hereinafter simply referred to as "strain") reaches approximately 1.5, the relationship between strain e and stress is not linear. For example, there is a region where the stress does not increase linearly until the strain e reaches approximately 1. This is known as the soft elastic region specific to nematic liquid crystal elastomers, and in this region, the liquid crystal molecules 3 are aligned in one direction, forming a monodomain for the entire liquid crystal polymer. When this occurs, scattering from the domain boundaries of the polydomain structure disappears, and the liquid crystal polymer becomes transparent, becoming a nematic liquid crystal elastomer.

[0025] Furthermore, as shown in Figure 4(a1), when the load applied during stretching is removed and the stress is released, the strain in the nematic liquid crystal elastomer with main-chain liquid crystal molecules does not fully return to its original state, but rather returns to a strain e of approximately 1, leaving a residual strain greater than this. In other words, the residual strain causes the elastomer to become approximately twice as long as it was before stretching. This is because the nematic order suppresses shape recovery.

[0026] Next, as shown in Figure 4(b1), the liquid crystal elastomer is annealed by heating at a predetermined temperature below the nematic-isotropic phase transition temperature. The annealing is preferably performed for, for example, about one minute, and the liquid crystal elastomer shrinks along the X axis as it is heated. In a nematic liquid crystal elastomer, increasing the temperature reduces its orientation, destabilizing the orientation of the liquid crystal molecules. This releases some of the entropy elasticity that had been suppressed by the nematic order. Therefore, the liquid crystal elastomer irreversibly shrinks along the X axis in an attempt to return to the state it had before being uniaxially stretched. After annealing, the liquid crystal elastomer is cooled to room temperature.

[0027] Referring to Figure 8, the shrinkage increases continuously with increasing heating temperature during annealing, and the phase transition temperature T NI When the temperature reaches 100°C, the liquid crystal will shrink until the strain is zero. Therefore, as mentioned above, annealing is performed at a temperature below the phase transition temperature, i.e., at a temperature in the nematic temperature range. Heating within this temperature range maintains the nematic order. However, because macroscopic shrinkage occurs, the monodomain aligned along the axis X is compressed in the same direction, and in order to absorb this compression, the liquid crystal molecules aligned along the axis X rotate, tilting alternately relative to the axis X, like the buckling of a rod.

[0028] As a result, as shown in Figure 9, unit layers of domains of liquid crystal molecules rotated in different directions in a staggered pattern are observed stacked in the direction of axis X in a striped pattern. In this way, self-organization occurs due to thermal contraction, and a nematic liquid crystal elastomer with a layered structure observed in a striped pattern is obtained. Note that, as shown in Figure 4(b2), the orientation of the liquid crystal molecules is not limited to the xy plane. In other words, the staggered structure of liquid crystal molecules can exist in all planes including axis X.

[0029] Furthermore, when unpolarized red laser light (wavelength 633 nm) was irradiated perpendicularly onto the main surface of the liquid crystal elastomer annealed at each temperature, a first-order scattering band corresponding to the domain layer structure was observed in the scattering pattern of the transmitted light, as shown in Figure 10. Note that here, the longitudinal end (direction of axis X) of the strip-shaped liquid crystal elastomer in the middle of production was fixed and suspended so that only its own weight was applied.

[0030] The striped laminated structure obtained here is thermally and mechanically unstable and is irreversibly disrupted by temperature rise or strain. Therefore, the acrylic groups remaining unreacted when obtaining the primary crosslinked structure are subjected to photocrosslinking to cause an acrylic-acrylic reaction. This stabilizes the structure by forming a network through further crosslinking in the primary crosslinked structure. To achieve this, as described above, a photoradical agent and stabilizer for photopolymerization are added in advance. For example, 0.2% by mass of Irgacure 2959 can be used as the photoradical agent, and 0.5% by mass of butylated hydroxytoluene (BHT) can be used as the stabilizer. UV light is then irradiated, and the generated radicals drive the reaction. As a result, the laminated structure self-assembled by the thermal shrinkage described above can be stabilized by secondary crosslinking. This allows for the production of an optical component 1 having a stabilized laminated structure that responds reversibly to heating.

[0031] In this way, an optical element 1 can be obtained in which the diffusion state of transmitted light, which enters one of a pair of principal surfaces and exits the other, changes continuously and reversibly in response to the ambient temperature. In particular, in each unit layer constituting the above-described laminated structure, the axial direction of birefringence due to uniaxial nematics alternates between adjacent layers. Furthermore, with the optical element 1, while maintaining a predetermined angle with respect to the axis X of the molecular orientation of the unit layer, the angular amplitude of the liquid crystal molecules with respect to this predetermined angle can be changed by temperature changes, thereby continuously and stably changing the diffusion state of transmitted light in a reversible manner.

[0032] Next, the characteristics of the optical member 1 manufactured as described above will be described.

[0033] The resulting optical component 1 has a layered structure in which the birefringence directions of uniaxial nematics are arranged alternately, forming a transmission grating. In other words, interference occurs in the transmitted light depending on the layer width Λ (see Figure 4(b1)) and the wavelength of the transmitted light, causing scattering in a specific direction.

[0034] Therefore, as shown in Figure 11, we investigated the angle-dependent scattered light intensity in the x and y directions along the principal surface of the optical component 1. Here, the x direction is the same direction as the axis X (approximately perpendicular to the direction in which the layers extend), and the y direction is perpendicular to the axis X (approximately parallel to the direction in which the layers extend). As a result, we found that the scattered light intensity in the x direction varies significantly with angle (see Figure 11(a)), whereas the angle dependence of the scattered light intensity in the y direction is small (see Figure 11(b)). In other words, we found that the angle-dependent scattered light intensity on the principal surface of the optical component 1 has strong anisotropy. We also found that the angle dependence in the x direction varies significantly with temperature. When white light is incident on such an optical component 1 at room temperature, it appears to have a pearlescent luster (see Figure 11(a)).

[0035] Furthermore, as shown in Figure 12, the scattering angle also varies depending on the color (wavelength) of light. Therefore, even a slight difference in angle causes the color tone of the appearance to change, resulting in a dazzling design known as iridescence. Note that while scattering occurs at 20°C, as shown in the figure, at 100°C it becomes transparent to light with wavelengths of 400 to 700 nm.

[0036] As shown in Figure 13, when red laser light (wavelength 633 nm) was irradiated perpendicularly onto the main surface of the optical member 1, it exhibited a scattering pattern at 22°C, and as the temperature increased, it gradually became more transparent, becoming almost transparent at 80°C.

[0037] 14, the optical member 1 contracted as the temperature increased, and reversibly changed its shape at least up to 120° C. In this example, the phase transition temperature T NI is 78°C.

[0038] Here, the optical element 1 aligns liquid crystal molecules at a predetermined angle relative to the axis X in each layer of the laminated structure. As the temperature rises, the angular amplitude of each liquid crystal molecule relative to this predetermined angle increases, and the entire element contracts along the axis X. This increase in the angular amplitude of the liquid crystal molecules with increasing temperature is thought to reduce the alignment of the liquid crystal molecules and reduce the anisotropy of birefringence in each layer. This results in a transparent element, as if the laminated structure had disappeared. However, because this behavior is reversible, cooling reduces the angular amplitude of each liquid crystal molecule, increasing the alignment of the liquid crystal molecules and causing scattering of transmitted light, as if the laminated structure had been restored.

[0039] As described above, the optical element 1 can continuously change the angular amplitude of each liquid crystal molecule depending on the temperature, thereby changing the diffusion state of transmitted and reflected light. Furthermore, because it has color dispersion, it exhibits coloration that is sensitively dependent on the angle. Meanwhile, it becomes transparent upon heating, but the intermediate state can also be controlled by temperature. Furthermore, because the optical element 1 has a laminated structure inside, it is less susceptible to the effects of surface abrasion or contact with liquids with specific refractive indices.

[0040] In the above, the laminated structure is obtained by self-forming, but a precise laminated structure may be formed by lithography.

[0041] Although the embodiments of the present invention and modifications based thereon have been described above, the present invention is not necessarily limited to these examples. Furthermore, those skilled in the art will be able to find various alternative embodiments and modifications without departing from the spirit of the present invention or the scope of the appended claims. [Explanation of symbols]

[0042] 1 Optical components 2 layers 3 Liquid crystal molecules 4. Domains

Claims

1. An optical element that is made of a sheet or plate and that reversibly and continuously changes the diffusion state of transmitted light that enters through one of a pair of main surfaces and exits through the other in response to the ambient temperature, An optical element having a laminated structure in which unit layers made of liquid crystal polymers molecularly oriented at a predetermined angle relative to an axis on the main surface are stacked along the axis so that the predetermined angle is in a different direction between adjacent unit layers, the liquid crystal polymers being a crosslinked nematic liquid crystal elastomer, and the diffusion state is reversibly changed by changing the angular amplitude relative to the predetermined angle with temperature.

2. 2. The optical element according to claim 1, wherein the angular amplitude increases with increasing temperature.

3. A method for manufacturing an optical element comprising a sheet or plate, in which a diffusion state of transmitted light incident on one of a pair of main surfaces and exiting from the other is reversibly and continuously changed in response to ambient temperature, the method comprising: A method for manufacturing an optical element, characterized in that a sheet or plate made of a partially crosslinked liquid crystal polymer is uniaxially stretched along an axis on the main surface, and then the tension is released and the plate is heated to a predetermined temperature to cause it to shrink along the axis, and unit layers made of the liquid crystal polymer that have been molecularly oriented at a predetermined angle relative to the axis are stacked along the axis so that the predetermined angle is in a different direction between adjacent unit layers to form a laminated structure, and further secondary crosslinked to form a nematic liquid crystal elastomer, and the angular amplitude relative to the predetermined angle is changed by temperature to reversibly change the diffusion state.

4. 4. The method for manufacturing an optical member according to claim 3, wherein the angular amplitude increases with increasing temperature.

5. 5. The method for manufacturing an optical member according to claim 3, wherein the predetermined temperature is a temperature equal to or lower than a phase transition temperature.

Images