Volume holographic grating raw material system, volume holographic grating and preparation method thereof

Abstract

The invention relates to the technical field of display, in particular to a volume holographic grating raw material system, a volume holographic grating and a preparation method of the volume holographic grating. The volume holographic grating raw material system comprises a photopolymerizable monomer, a liquid crystal, an organic additive and a photoinitiator, the structural formula of the organic additive is A-B, A is a photopolymerizable monomer affinity group, and B is a liquid crystal affinity group. On the basis of a raw material system composed of a photopolymerizable monomer, liquid crystal and a photoinitiator, a novel organic additive is introduced, and the organic additive is beneficial to good phase separation of a PDLC system and increase of the refractive index difference between a bright area and a dark area, so that the refractive index modulation degree of the volume holographic grating is improved, and the display application in the AR / VR field is met.

Description

Technical Field

[0001] The present invention relates to the field of display technologies, and in particular, to a volume holographic grating raw material system, a volume holographic grating, and a preparation method thereof. Background Art

[0002] As a coupling optical element of a holographic optical waveguide, volume holographic gratings (VHG) can be used in display technologies such as virtual reality (VR), augmented reality (AR), mixed reality, and head-up display. The performance of VHG can be measured by the refractive index modulation (Δn) and the diffraction efficiency (DE). The larger these two values are, the better the light modulation performance of VHG. There are usually two types of material systems for organic VHG: photoinduced second-order polymers and holographic polymer dispersed liquid crystals (HPDLC). Since the Δn of the HPDLC material system can reach above 0.1, far exceeding that of photoinduced second-order polymers (Δn is 0.04), it is currently the material system with the best performance for volume holographic gratings. However, the performance improvement based on this material system has stagnated for several years, and it is urgent to develop a method for improving the performance of high Δn to meet the display applications in the AR / VR field.

[0003] In the HPDLC material system, the main components are photopolymerizable monomers, photoinitiators, and liquid crystal components. These components can be uniformly mixed and coated into a film. When two interfering light beams irradiate the film, photopolymerization occurs in the bright regions and does not occur in the dark regions. At this time, the photopolymerizable monomer component and the liquid crystal component gradually change from a uniformly mixed state to the photopolymerizable monomer component diffusing into the bright regions and the liquid crystal component diffusing into the dark regions. Eventually, polymer-liquid crystal-polymer regions are formed in the order of bright-dark-bright. The greater the difference in refractive index between the polymer and the liquid crystal, the greater the refractive index modulation of the volume holographic grating. To increase Δn, the commonly used methods are two schemes: reducing the refractive index of the polymer or increasing the refractive index of the liquid crystal.

[0004] Scheme 1: Reduce the refractive index of the polymer. Shi Mengquan et al. designed and synthesized a low-refractive-index olefin monomer, such as an acrylic acid fragment, a styrene fragment, etc. This low-refractive-index olefin monomer was used in a photoinduced polymer material system to prepare a holographic storage material (see the patent document with the application number CN02149193.3 for details).

[0005] Solution 2: Increase the refractive index of the liquid crystal. The Digilens company blends two liquid crystals with high and low refractive indices to increase the average refractive index of the liquid crystal. The refractive index value of the high-refractive-index liquid crystal can reach 1.7, thereby preparing a volume holographic grating with Δn > 0.1 (see the patent document with the application number US 2020 / 0271973 A1 for details).

[0006] However, the above two solutions simply change the refractive index of the polymer or the liquid crystal. Simply changing the refractive index of the polymer or the liquid crystal will cause significant changes in the surface energy, viscosity, film-forming properties, etc. of the corresponding components, thereby changing the compatibility between the polymer and the liquid crystal. That is to say, the change of a single component will lead to the reallocation of all components in the whole system, seriously reducing the optimization efficiency of the refractive index modulation degree.

[0007] Therefore, the existing technology still needs to be improved and developed. Summary of the Invention

[0008] In view of the above deficiencies of the prior art, the purpose of the present invention is to provide a volume holographic grating raw material system, a volume holographic grating and its preparation method, aiming to solve the problem that the existing method of improving the refractive index modulation degree of the volume holographic grating by changing a single component still has insufficient improvement effect.

[0009] The technical solution of the present invention is as follows:

[0010] In the first aspect of the present invention, a volume holographic grating raw material system is provided, wherein the volume holographic grating raw material system includes a photo-polymerizable monomer, a liquid crystal, an organic additive, and a photoinitiator. The structural formula of the organic additive is A-B, where A is a photo-polymerizable monomer group and B is a liquid crystal group.

[0011] Optionally, the photo-polymerizable monomer group is an ester group, and the liquid crystal group is one of phenyl, fluorine-substituted phenyl, cyano-substituted phenyl, trifluoromethyl-substituted phenyl, methyl-substituted phenyl, thiophenyl, pyrrolyl, cyano-substituted alkyl, and halogen-substituted alkyl.

[0012] Further optionally, the structural formula of the organic additive is one or more of the following formulas:

[0013]

[0014] Optionally, the photo-polymerizable monomer group is one of an olefin-containing group, a carbonyl-containing group, and an alkyl ether group, and the liquid crystal group is phenyl.

[0015] Further optionally, the olefin-containing group is allyl.

[0016] The carbonyl-containing group is one of ethyl ketone group, acetaldehyde group, and acetamide group.

[0017] Further optionally, the structural formula of the organic additive is one or more of the following formulas:

[0018]

[0019] Optionally, based on the raw material system of the volume holographic grating, the weight percentage of the photopolymerizable monomer is 30-50%, the weight percentage of the liquid crystal is 30-50%, the weight percentage of the organic additive is 1-10%, and the weight percentage of the photoinitiator is 1%.

[0020] Optionally, the photopolymerizable monomer is at least one of acrylate and acrylate derivatives;

[0021] The liquid crystal is a nematic liquid crystal;

[0022] The photoinitiator is a mixture of dibromofluorescein and phenylglycine, or the photoinitiator is a mixture of acid red 94 and phenylglycine.

[0023] In the second aspect of the present invention, a volume holographic grating is provided, which is prepared by coherent light exposure using the raw material system of the volume holographic grating of the present invention.

[0024] In the third aspect of the present invention, a method for preparing a volume holographic grating is provided, which includes the step of performing coherent light exposure on the raw material system of the volume holographic grating of the present invention to obtain the volume holographic grating.

[0025] Beneficial effects: Based on the raw material system composed of a photopolymerizable monomer, a liquid crystal, and a photoinitiator, the present invention introduces a new type of organic additive. This organic additive is beneficial to the occurrence of good phase separation in the PDLC (polymer-liquid crystal) system, increases the refractive index difference between the bright area and the dark area, and thus improves the refractive index modulation degree of the volume holographic grating to meet the display requirements in the AR / VR field. Description of the Drawings

[0026] Figure 1 It is a schematic diagram of a periodically distributed grating formed after a raw material system containing a photoinitiator, a liquid crystal, and a photopolymerizable monomer is coated into a film and then subjected to coherent light exposure.

[0027] Figure 2 It is a schematic diagram of the principle of the organic additive promoting the phase separation of the PDLC film.

[0028] Figure 3 It is a test result diagram of the refractive index modulation degree of the volume holographic gratings of Examples 1-3 and Comparative Example 1. Detailed Embodiments

[0029] The present invention provides a volume holographic grating raw material system, a volume holographic grating and a preparation method thereof. To make the objectives, technical solutions and effects of the present invention clearer and more definite, the present invention is further described in detail below. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0030] Based on the problems that it is difficult to optimize the refractive index modulation degree of existing volume holographic gratings, the molecular design process is complex, and the performance improvement of volume holographic gratings is limited, the present invention adopts a new type of additive, which can significantly promote the phase separation between polymer-liquid crystal components and optimize the refractive index modulation degree of polymer-liquid crystal components.

[0031] Specifically, an embodiment of the present invention provides a volume holographic grating raw material system, wherein the volume holographic grating raw material system includes a photopolymerizable monomer, a liquid crystal, an organic additive and a photoinitiator, and the structural formula of the organic additive is A-B, where A is a photopolymerizable monomer-philic group and B is a liquid crystal-philic group.

[0032] It should be noted that the photopolymerizable monomer-philic group refers to a group that is easily affinity with the photopolymerizable monomer, or a group that can attract the photopolymerizable monomer; the liquid crystal-philic group refers to a group that is easily affinity with the liquid crystal, or a group that can attract the liquid crystal.

[0033] Based on the raw material system composed of a photopolymerizable monomer, a liquid crystal and a photoinitiator, an organic additive is introduced in an embodiment of the present invention. This organic additive can promote the phase separation of the PDLC film during exposure, increase the refractive index difference between the bright area and the dark area, thereby improving the refractive index modulation degree of the volume holographic grating to meet the display requirements in the AR / VR field.

[0034] The organic additive introduced in the embodiment of the present invention has the characteristics of simple structure, expandable structure, good light / thermal stability, etc. Adding this type of organic additive to the PDLC component can significantly increase the Δn value of the corresponding component. Compared with the traditional method, the embodiment of the present invention does not require complex molecular design and lengthy multi-component optimization, has better universality, and can further improve the optical performance of the volume holographic grating.

[0035] Next, the phase separation process of the PDLC system in the embodiment of the present invention and the basic principle of the introduced organic additive to promote phase separation are introduced.

[0036] The phase separation process of the PDLC system can be briefly divided into the following stages: ① As Figure 1As shown, the raw material system will first be coated in the middle of the support. At this time, the photoinitiator, liquid crystal, photopolymerizable monomer, etc. in the raw material system are evenly mixed and randomly distributed in the film; ② When coherent light is used for exposure, different physical and chemical processes occur in the light and dark stripes. In the coherent bright area, the photoinitiator absorbs photons to generate active centers, which trigger the polymerization of photopolymerizable monomers to form polymers (polymer networks), and at the same time squeeze the liquid crystal into the coherent dark area; no photopolymerization reaction occurs in the coherent dark area, and finally a periodically distributed grating of polymer-rich phase and liquid crystal-rich phase is formed in the PDLC film.

[0037] The basic principle of organic additives promoting phase separation in PDLC films: ideal volume holographic grating microstructures such as Figure 2 As shown in (a), the polymer and liquid crystal are distributed in a periodic and orderly manner. However, it is difficult to achieve the ideal effect in actual production. Figure 2 (b) shows a common defect structure, in which the distribution of polymer and liquid crystal is deviated, and the polymer is mixed into the liquid crystal phase or the liquid crystal is mixed into the polymer phase, as shown in the dotted circle. This defect is usually called insufficient phase separation, which will cause a decrease in the refractive index modulation. In order to change this defect, the embodiment of the present invention introduces an organic additive (such as Figure 2 (c) The dotted circle shows that it has characteristic groups that are affinity to photopolymerizable monomers and characteristic groups that are affinity to liquid crystals. During the exposure process, it acts as an intermediate component between the liquid crystal phase and the polymer phase, slowing down the diffusion of liquid crystal and polymer. The realization of this principle is based on the similarity theory that liquid crystal is more compatible with liquid crystal and photopolymerizable monomer is more compatible with photopolymerizable monomer. The introduction of this organic additive reduces the probability of chaotic distribution of liquid crystal and polymer, thereby improving the refractive index modulation.

[0038] In one embodiment, the photopolymerizable monomer group is an ester group, and the liquid crystal-philic group is one of phenyl, fluorine-substituted phenyl, cyano-substituted phenyl, trifluoromethyl-substituted phenyl, methyl-substituted phenyl, thienyl, pyrrolyl, cyano-substituted alkyl and halogen-substituted alkyl.

[0039] As an example, the structural formula of the organic additive is one or more of the following:

[0040]

[0041] In one embodiment, the photopolymerizable monomer group may be one of an olefin-containing group, a carbonyl-containing group, and an alkyl ether group, and the liquid crystal-affinity group may be a phenyl group. The olefin-containing group may be an allyl group, and the carbonyl-containing group may be one of an ethyl ketone group, an acetaldehyde group, and an acetamide group.

[0042] The above groups have strong polarity. The carbonyl group / oxygen atom has good compatibility with common photopolymerizable monomers such as acrylates. During the phase separation process, it will become a good intermediate segment between the photopolymerizable monomer and the liquid crystal, thereby enhancing the degree of phase separation.

[0043] As an example, the structural formula of the organic additive is one or more of the following:

[0044]

[0045] In one embodiment, based on the raw material system of the volume holographic grating, the weight percentage of the photopolymerizable monomer is 30 - 50%, the weight percentage of the liquid crystal is 30 - 50%, the weight percentage of the organic additive is 1 - 10%, and the weight percentage of the photoinitiator is 1%. By controlling the proportion of each component in the raw material system of the volume holographic grating, it is possible to effectively improve the refractive index modulation degree of the volume holographic grating.

[0046] In one embodiment, the photopolymerizable monomer can be a common monomer that can undergo photopolymerization reaction in the art, such as at least one of acrylates, acrylate derivatives, etc., but not limited thereto. Among them, the acrylate derivatives can be at least one of 2 - hydroxy - 3 - acryloxypropyl acrylate, methyl acrylate, methyl methacrylate, deuterated methyl methacrylate, fluorinated methyl methacrylate, ethyl acrylate, ethyl methacrylate, ethyl ethyl acrylate, propyl ethyl acrylate, etc.

[0047] In one embodiment, the liquid crystal can be a common nematic liquid crystal, such as 4 - cyano - 4'-pentylbiphenyl, 5 - octylphenyl - 2 - methoxyphenyl - 4 - pyridone, Liquid crystal mixture E7 (from Merck & Co., Inc.), etc.

[0048] In one embodiment, the photoinitiator can be a common photosensitive system, which is sensitive to light in bands such as red / blue, and releases free radicals after photosensitization to initiate the polymerization reaction of the photopolymerizable monomer. For example, the photoinitiator can be eosin dibromide / phenylglycine (referring to a mixture of eosin dibromide and phenylglycine) or acid red 94 / phenylglycine (referring to a mixture of acid red 94 and phenylglycine), etc.

[0049] An embodiment of the present invention provides a volume holographic grating, which is prepared by coherent light exposure using the above - mentioned raw material system of the volume holographic grating.

[0050] An embodiment of the present invention provides a method for preparing a volume holographic grating, which includes the step of performing coherent light exposure on the above - mentioned raw material system of the volume holographic grating to obtain the volume holographic grating.

[0051] Based on the raw material system of volume holographic grating in the embodiments of the present invention, which consists of a photo-polymerizable monomer, a liquid crystal, and a photoinitiator, an organic additive is introduced. This organic additive can promote the phase separation of the PDLC film during exposure, increase the refractive index difference between the bright and dark regions, thereby enhancing the refractive index modulation of the volume holographic grating to meet the display requirements in the AR / VR field.

[0052] The present invention will be described in detail below through specific embodiments.

[0053] Example 1

[0054] The total amount of the raw material system of the volume holographic grating in this example is 10 g. Among them, methyl benzoate with a nuclear magnetic purity of 99% is selected as the organic additive, and the addition amount is 5% (the percentage is the mass percentage, the same in the subsequent examples); 5-octylphenyl-2-methoxyphenyl-4-pyridone is used as the liquid crystal component, and the addition amount is 45%; 2-hydroxypropyl acrylate is used as the photo-polymerizable monomer, and the addition amount is 45%; dibromofluorescein / phenylglycine is used as the photoinitiator, and the addition amount is 1%; in addition, hexanoic acid is added as a plasticizer to maintain the coating uniformity, and the addition amount is 4%.

[0055] Place the raw material system of the volume holographic grating in a round-bottom flask and stir it with a magnetic stirrer for 10 minutes to ensure uniform mixing. Subsequently, use capillary action to fill the uniformly mixed raw material system of the volume holographic grating into the liquid crystal cell, which is composed of glass substrates. After sealing the edges of the liquid crystal cell, a standard sample is obtained. After exposure with red light for 30 seconds using a standard holographic exposure device, a volume holographic grating is prepared, and the refractive index modulation of the prepared volume holographic grating is measured through an optical platform.

[0056] Example 2

[0057] The total amount of the raw material system of the volume holographic grating in this example is 10 g. Among them, methyl 1-cyanopropionate with a nuclear magnetic purity of 99% is selected as the organic additive, and the addition amount is 5%; 5-octylphenyl-2-methoxyphenyl-4-pyridone is used as the liquid crystal component, and the addition amount is 45%; 2-hydroxypropyl acrylate is used as the photo-polymerizable monomer, and the addition amount is 45%; dibromofluorescein / phenylglycine is used as the photoinitiator, and the addition amount is 1%; in addition, hexanoic acid is added as a plasticizer to maintain the coating uniformity, and the addition amount is 4%.

[0058] Place the volume holographic grating raw material system in a round-bottom flask and stir it with a magnetic stirrer for 10 minutes to ensure uniform mixing. Subsequently, fill the uniformly mixed volume holographic grating raw material system into a liquid crystal cell by capillary action. The liquid crystal cell is composed of glass substrates. After sealing the edges of the liquid crystal cell, a standard sample is obtained. Expose it through a standard holographic exposure device. After exposing with red light for 30 seconds, a volume holographic grating is prepared. The refractive index modulation of the prepared volume holographic grating is tested by an optical bench.

[0059] Example 3

[0060] The total amount of the volume holographic grating raw material system in this example is 10 g. Among them, α-methylstyrene with a nuclear magnetic purity of 99% is selected as the organic additive, and the addition amount is 5%; 5-octylphenyl-2-methoxyphenyl-4-pyridone is used as the liquid crystal component, and the addition amount is 45%; 2-hydroxypropyl acrylate is used as the photopolymerizable monomer, and the addition amount is 45%; dibromofluorescein / phenylglycine is used as the photoinitiator, and the addition amount is 1%; in addition, hexanoic acid is added as a plasticizer to maintain the coating uniformity, and the addition amount is 4%.

[0061] Place the volume holographic grating raw material system in a round-bottom flask and stir it with a magnetic stirrer for 10 minutes to ensure uniform mixing. Subsequently, fill the uniformly mixed volume holographic grating raw material system into a liquid crystal cell by capillary action. The liquid crystal cell is composed of glass substrates. After sealing the edges of the liquid crystal cell, a standard sample is obtained. Expose it through a standard holographic exposure device. After exposing with red light for 30 seconds, a volume holographic grating is prepared. The refractive index modulation of the prepared volume holographic grating is tested by an optical bench.

[0062] Comparative Example 1

[0063] The total amount of the volume holographic grating raw material system in this comparative example is 10 g. Among them, 5-octylphenyl-2-methoxyphenyl-4-pyridone is used as the liquid crystal component, and the addition amount is 45%; 2-hydroxypropyl acrylate is used as the photopolymerizable monomer, and the addition amount is 45%; dibromofluorescein / phenylglycine is used as the photoinitiator, and the addition amount is 1%; in addition, hexanoic acid is added as a plasticizer to maintain the coating uniformity, and the addition amount is 9%.

[0064] Place the volume holographic grating raw material system in a round-bottom flask and stir it with a magnetic stirrer for 10 minutes to ensure uniform mixing. Subsequently, fill the uniformly mixed volume holographic grating raw material system into a liquid crystal cell by capillary action. The liquid crystal cell is composed of glass substrates. After sealing the edges of the liquid crystal cell, a standard sample is obtained. Expose it through a standard holographic exposure device. After exposing with red light for 30 seconds, a volume holographic grating is prepared. The refractive index modulation of the prepared volume holographic grating is tested by an optical bench.

[0065] Result analysis:

[0066] The test results are shown inFigure 3 , from Figure 3 it can be found that the organic additive methyl benzoate has the best optimization effect, and the refractive index modulation of the prepared volume holographic grating can reach 0.08, which is much higher than that of Comparative Example 1 without organic additives. This may be because one end of methyl benzoate has a strongly polar ester group and the other end has a weakly polar aryl group. The other two organic additives, methyl 1-cyanopropionate and α-methylstyrene, also have a certain optimization effect on the refractive index modulation of the volume holographic grating. Generally speaking, the introduced organic additives have a good optimization effect on the volume holographic grating and have practical effects, which is beneficial for the preparation of AR / VR display devices with high optical performance.

[0067] In summary, the present invention provides a raw material system for a volume holographic grating, a volume holographic grating and a display element. Based on the raw material system composed of a photopolymerizable monomer, a liquid crystal and a photoinitiator, the present invention introduces an organic additive, which can promote the phase separation of the PDLC film during exposure and increase the refractive index difference between the bright area and the dark area, thereby improving the refractive index modulation of the volume holographic grating to meet the display requirements in the AR / VR field. The organic additive introduced in the present invention has the characteristics of simple structure, expandable structure, good light / thermal stability, etc. Adding this type of organic additive to the PDLC component can significantly increase the Δn value of the corresponding component. Compared with the traditional method, the present invention does not require complex molecular design and lengthy multi-component optimization, has better universality, and can further improve the optical performance of the volume holographic grating.

[0068] It should be understood that the application of the present invention is not limited to the above examples. For those of ordinary skill in the art, modifications or variations can be made according to the above description, and all such modifications and variations should fall within the protection scope of the appended claims of the present invention.

Claims

1. A volume holographic grating raw material system, characterized in that The raw material system of the volume holographic grating includes a photopolymerizable monomer, a liquid crystal, an organic additive, and a photoinitiator. The structural formula of the organic additive is A-B, where A is a photopolymerizable monomerophilic group and B is a liquid crystalophilic group.

2. The raw material system of the volume holographic grating according to claim 1, wherein The photopolymerizable monomerophilic group is an ester group, and the liquid crystalophilic group is one of phenyl, fluorine-substituted phenyl, cyano-substituted phenyl, trifluoromethyl-substituted phenyl, methyl-substituted phenyl, thiophenyl, pyrrolyl, cyano-substituted alkyl, and halogen-substituted alkyl.

3. The volume holographic grating raw material system according to claim 2, characterized in that, The structural formula of the organic additive is one or more of the following formulas:

4. The raw material system of the volume holographic grating according to claim 1, characterized in that The photopolymerizable monomerophilic group is one of an olefin-containing group, a carbonyl-containing group, and an alkyl ether group, and the liquid crystalophilic group is phenyl.

5. The bulk holographic grating raw material system according to claim 4, wherein The olefin-containing group is allyl. The carbonyl-containing group is one of ethyl ketone group, acetaldehyde group, and acetamide group.

6. The volume holographic grating raw material system according to claim 5, characterized in that, The structural formula of the organic additive is one or more of the following formulas:

7. The raw material system of the volume holographic grating according to claim 1, characterized in that Based on the raw material system of the volume holographic grating, the weight percentage of the photopolymerizable monomer is 30-50%, the weight percentage of the liquid crystal is 30-50%, the weight percentage of the organic additive is 1-10%, and the weight percentage of the photoinitiator is 1%.

8. The volume holographic grating raw material system according to claim 1, wherein The photopolymerizable monomer is at least one of acrylate and acrylate derivatives; The liquid crystal is a nematic liquid crystal; The photoinitiator is a mixture of dibromofluorescein and phenylglycine, or the photoinitiator is a mixture of acid red 94 and phenylglycine.

9. A volume holographic grating, characterized in that, Prepared by coherent light exposure using the raw material system of the volume holographic grating according to any one of claims 1-8.

10. A method for preparing a volume holographic grating, characterized in that, It includes the steps of: performing coherent light exposure on the raw material system of the volume holographic grating according to any one of claims 1-8 to obtain the volume holographic grating.

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