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

Abstract

The present application relates to the technical field of displays, and in particular relates to a volume holographic grating raw material system, and a volume holographic grating and a preparation method therefor. The volume holographic grating raw material system comprises a photopolymerizable monomer, a liquid crystal, an organic additive and a photoinitiator, wherein the organic additive has a structural formula of A-B, A being a group showing affinity for the photopolymerizable monomer, and B being a group showing affinity for the liquid crystal. In the present application, a new organic additive is introduced on the basis of the raw material system, and the organic additive facilitates good phase separation of a PDLC system.

Description

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

[0001] This application claims priority to the Chinese patent application filed with the Patent Office of China on December 21, 2023, with application number 2023117761704, and invention name “A volume holographic grating raw material system, volume holographic grating and preparation method thereof”, the entire contents of which are incorporated herein by reference. Technical Field

[0002] The present application relates to the field of display technology, and in particular to a volume holographic grating material system, a volume holographic grating and a preparation method thereof. Background Art

[0003] Volume holographic gratings (VHGs), as coupling optical elements for holographic waveguides, can be used in display technologies such as virtual reality (VR), augmented reality (AR), mixed reality, and head-up displays. The performance of VHGs can be measured by the refractive index modulation (Δn) and diffraction efficiency (DE). The larger these two values, the better the VHG's light modulation performance. Organic VHG material systems are generally divided into two categories: photoinduced second-order polymers and holographic polymer dispersed liquid crystals (HPDLCs). Since the Δn of the HPDLC material system can reach above 0.1, far exceeding that of photoinduced second-order polymers (Δn of 0.04), it is currently the best-performing volume holographic grating material system. However, performance improvements based on this material system have stagnated for several years, and there is an urgent need to develop high-Δn performance improvement methods to meet display applications in the AR / VR field.

[0004] In the HPDLC material system, the main components are photopolymerizable monomers, photoinitiators, and liquid crystal components. These components are evenly mixed and can be coated into a thin film. When two beams of interfering light irradiate the film, a photopolymerization reaction occurs in the bright area, while no reaction occurs in the dark area. 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 toward the bright area, and the liquid crystal component diffusing toward the dark area. Ultimately, polymer-liquid crystal-polymer regions are formed in the order of bright-dark-bright. The greater the difference between the refractive index of the polymer and the refractive index of the liquid crystal, the greater the refractive index modulation of the volume holographic grating. In order to increase Δn, the commonly used methods are to reduce the refractive index of the polymer or increase the refractive index of the liquid crystal.

[0005] Option 1: Lowering the refractive index of the polymer. Shi Mengquan and his colleagues designed and synthesized a low-refractive-index olefin monomer, such as an acrylic acid fragment or a styrene fragment. This low-refractive-index olefin monomer was incorporated into a photopolymer system to produce a holographic storage material (see patent application number CN02149193.3 for details).

[0006] Option 2: Increase the refractive index of liquid crystal. Digilens uses a blend of two liquid crystals with high and low refractive indices to increase the average refractive index of the liquid crystal. The refractive index 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 application number US2020 / 0271973A1 for details).

[0007] However, the two aforementioned solutions simply alter the refractive index of the polymer or the liquid crystal. This alone can significantly change the surface energy, viscosity, and film-forming properties of the corresponding components, thereby altering the compatibility of the polymer and liquid crystal. This means that changing a single component requires readjusting all components of the entire system, severely reducing the efficiency of optimizing the refractive index modulation.

[0008] Therefore, the existing technology still needs to be improved and developed. Summary of the Invention

[0009] In view of the above-mentioned deficiencies in the prior art, the purpose of this application is to provide a volume holographic grating raw material system, a volume holographic grating and a preparation method thereof, aiming to solve the problem that the existing method of improving the refractive index modulation of the volume holographic grating by changing a single component still has insufficient improvement effect.

[0010] The technical solution of this application is as follows:

[0011] In a first aspect of the present application, a volume holographic grating raw material system is provided, wherein the volume holographic grating raw material system includes a photopolymerizable monomer, liquid crystal, an organic additive and a photoinitiator, and the structural formula of the organic additive is AB, wherein A is a photopolymerizable monomer group and B is a liquid crystal group.

[0012] Optionally, the photopolymerizable 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, thienyl, pyrrolyl, cyano-substituted alkyl and halogen-substituted alkyl.

[0013] Further optionally, the structural formula of the organic additive is one or more of the following formulae:

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

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

[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 formulae:

[0018] Optionally, based on the volume holographic grating raw material system, 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%.

[0019] Optionally, the photopolymerizable monomer is at least one of acrylate and acrylate derivatives;

[0020] The liquid crystal is a nematic liquid crystal;

[0021] The photoinitiator is a mixture of dibromofluorescein and phenylglycine, or the photoinitiator is a mixture of acid red 94 and phenylglycine.

[0022] In a second aspect of the present application, a volume holographic grating is provided, wherein the volume holographic grating is prepared by coherent light exposure using the volume holographic grating material system described in the present application.

[0023] A third aspect of the present application provides a method for preparing a volume holographic grating, which comprises the steps of: exposing the volume holographic grating raw material system described in the present application to coherent light to obtain the volume holographic grating.

[0024] Beneficial effects: This application introduces a new type of organic additive based on the raw material system composed of photopolymerizable monomers, liquid crystals and photoinitiators. This organic additive is conducive to good phase separation of 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 of the volume holographic grating to meet the display applications in the AR / VR field. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG1 is a schematic diagram showing a process in which a raw material system containing a photoinitiator, a liquid crystal and a photopolymerizable monomer is coated into a film and then exposed to coherent light to form a periodically distributed grating.

[0026] FIG2 is a schematic diagram showing the principle of organic additives promoting phase separation of PDLC films.

[0027] FIG3 is a graph showing the test results of the refractive index modulation of the volume holographic gratings of Examples 1-3 and Comparative Example 1. DETAILED DESCRIPTION

[0028] This application provides a volume holographic grating material system, a volume holographic grating, and a method for manufacturing the same. To clarify the purpose, technical solution, and effects of this application, the following further describes this application in detail. It should be understood that the specific embodiments described herein are intended only to illustrate this application and are not intended to limit this application.

[0029] Based on the problems that improving the refractive index modulation degree of existing volume holographic gratings is difficult to optimize, the molecular design process is complex, and the performance improvement of the volume holographic grating is limited, this application adopts a new additive that can significantly promote the phase separation between polymer-liquid crystal components and optimize the refractive index modulation degree of the polymer-liquid crystal components.

[0030] Specifically, an embodiment of the present application provides a volume holographic grating raw material system, wherein the volume holographic grating raw material system includes a photopolymerizable monomer, liquid crystal, an organic additive and a photoinitiator, and the structural formula of the organic additive is AB, wherein A is a photopolymerizable monomer group and B is a liquid crystal group.

[0031] It should be noted that the photopolymerizable monomer-affinity 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-affinity group refers to a group that is easily affinity with liquid crystal, or a group that can attract liquid crystal.

[0032] The embodiment of the present application introduces an organic additive based on the raw material system consisting of a photopolymerizable monomer, liquid crystal and photoinitiator. 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, and thus improve the refractive index modulation of the volume holographic grating to meet the display applications in the AR / VR field.

[0033] The organic additives introduced in this embodiment offer advantages such as simple structure, scalable structure, and excellent light and heat stability. Adding these organic additives to PDLC components can significantly improve the Δn value of the corresponding components. Compared to traditional methods, this embodiment eliminates the need for complex molecular design and lengthy multi-component optimization, offering greater universality and further enhancing the optical performance of volume holographic gratings.

[0034] The following describes the phase separation process of the PDLC system in the embodiment of the present application and the basic principle of the organic additives introduced to promote phase separation.

[0035] The phase separation process of the PDLC system can be briefly divided into the following stages: ① As shown in Figure 1, 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 exposed to coherent light, 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 squeeze the liquid crystals into the coherent dark area; no photopolymerization reaction occurs in the coherent dark area, and eventually a periodically distributed grating of polymer-rich phase and liquid crystal-rich phase is formed in the PDLC film.

[0036] The basic principle of organic additives promoting phase separation in PDLC films: The ideal volume holographic grating microstructure, as shown in Figure 2 (a), is a periodically ordered distribution of polymer and liquid crystal. However, in actual production, it is difficult to achieve the ideal effect. Figure 2 (b) shows a common defect structure, in which the distribution of polymer and liquid crystal is deviated, with polymer mixed into the liquid crystal phase or liquid crystal mixed into the polymer phase, as shown in the dotted circle. This defect is usually referred to as insufficient phase separation and will cause a decrease in the refractive index modulation. To correct this defect, the embodiment of the present application introduces an organic additive (as shown in the dotted circle in Figure 2 (c)). It has characteristic groups that are pro-photopolymerizable monomers and characteristic groups that are pro-liquid crystals. During the exposure process, it acts as an intermediate component between the liquid crystal phase and the polymer phase, slowing down the insufficient diffusion of the liquid crystal and polymer. The realization of this principle is based on the theory of like dissolving like, that is, liquid crystal-liquid crystal compatibility is greater and photopolymerizable monomer-photopolymerizable monomer compatibility is greater. The introduction of this organic additive reduces the probability of disordered distribution of liquid crystal and polymer, thereby improving the refractive index modulation.

[0037] In one embodiment, the photopolymerizable 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, thienyl, pyrrolyl, cyano-substituted alkyl and halogen-substituted alkyl.

[0038] For example, the structural formula of the organic additive is one or more of the following:

[0039] In one embodiment, the photopolymerizable monomer group may be an olefin-containing group, a carbonyl-containing group, or 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 an ethyl ketone group, an acetaldehyde group, or an acetamide group.

[0040] The above groups have strong polarity, and the carbonyl / oxygen atoms have good compatibility with common photopolymerizable monomers such as acrylates. During the phase separation process, they will become good intermediate fragments between the photopolymerizable monomers and liquid crystals, thereby improving the degree of phase separation.

[0041] For example, the structural formula of the organic additive is one or more of the following:

[0042] In one embodiment, based on the volume holographic grating raw material system, 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 proportions of each component in the volume holographic grating raw material system, the refractive index modulation of the volume holographic grating can be effectively improved.

[0043] In one embodiment, the photopolymerizable monomer may be a monomer commonly found in the art that can undergo photopolymerization, such as at least one of acrylates and acrylate derivatives, but is not limited thereto. The acrylate derivative may be at least one of 2-hydroxy-3-propyl acrylate, methyl acrylate, methyl methacrylate, deuterated methyl methacrylate, fluoromethyl methacrylate, ethyl acrylate, ethyl methacrylate, ethyl ethyl acrylate, and propyl ethyl acrylate.

[0044] In one embodiment, the liquid crystal may be a common nematic liquid crystal, such as 4-cyano-4'-pentylbiphenyl, 5-octylphenyl-2-methoxyphenyl-4-pyridone, Liquid crystal mixture E7 (from Merck), and the like.

[0045] In one embodiment, the photoinitiator can be a common photosensitive system that is sensitive to light in the red / blue wavelength band, etc., and releases free radicals upon photosensitization, thereby initiating polymerization of the photopolymerizable monomer. For example, the photoinitiator can be dibromofluorescein / phenylglycine (a mixture of dibromofluorescein and phenylglycine) or Acid Red 94 / phenylglycine (a mixture of Acid Red 94 and phenylglycine).

[0046] An embodiment of the present application provides a volume holographic grating, which is prepared by using the volume holographic grating material system as described above through coherent light exposure.

[0047] An embodiment of the present application provides a method for preparing a volume holographic grating, which includes the steps of: exposing the volume holographic grating raw material system as described above to coherent light to obtain the volume holographic grating.

[0048] The volume holographic grating raw material system of the embodiment of the present application introduces an organic additive based on the raw material system composed of a photopolymerizable monomer, liquid crystal and photoinitiator. 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, and thus improve the refractive index modulation of the volume holographic grating to meet the display applications in the AR / VR field.

[0049] The present application is described in detail below through specific embodiments.

[0050] Example 1

[0051] The volume holographic grating raw material system of this embodiment totals 10g, of which methyl benzoate with a nuclear magnetic purity of 99% is selected as an organic additive, and the addition amount is 5% (% is mass percentage, the same as in subsequent embodiments); 5-octylphenyl-2-methoxyphenyl-4-pyridone is selected as a liquid crystal component, and the addition amount is 45%; 2-hydroxy-3-propyl acrylate is selected as a photopolymerizable monomer, and the addition amount is 45%; dibromofluorescein / phenylglycine is selected as a photoinitiator, and the addition amount is 1%; and hexanoic acid is added as a plasticizer to maintain coating uniformity, and the addition amount is 4%.

[0052] The volume holographic grating raw material system was placed in a round-bottom flask and stirred using a magnetic stirrer for 10 minutes to ensure uniform mixing. The uniformly mixed volume holographic grating raw material system was then filled into a liquid crystal cell (LCD cell) made of a glass substrate using capillary action. After sealing the edges of the LCD cell, a standard sample was obtained. After exposure to red light for 30 seconds using standard holographic exposure equipment, a volume holographic grating was produced. The refractive index modulation of the prepared volume holographic grating was tested on an optical platform.

[0053] Example 2

[0054] The volume holographic grating raw material system of this embodiment is 10g in total, wherein 1-cyanopropionic acid methyl ester 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 selected as the liquid crystal component, and the addition amount is 45%; 2-hydroxy-3-propyl acrylate is selected as the photopolymerizable monomer, and the addition amount is 45%; dibromofluorescein / phenylglycine is selected as the photoinitiator, and the addition amount is 1%; and hexanoic acid is added as a plasticizer to maintain coating uniformity, and the addition amount is 4%.

[0055] The volume holographic grating raw material system was placed in a round-bottom flask and stirred using a magnetic stirrer for 10 minutes to ensure uniform mixing. The uniformly mixed volume holographic grating raw material system was then filled into a liquid crystal cell (LCD cell) made of a glass substrate using capillary action. After sealing the edges of the LCD cell, a standard sample was obtained. After exposure to red light for 30 seconds using standard holographic exposure equipment, a volume holographic grating was produced. The refractive index modulation of the prepared volume holographic grating was tested on an optical platform.

[0056] Example 3

[0057] The volume holographic grating raw material system of this embodiment is 10 g in total, wherein α-methylstyrene with a nuclear magnetic purity of 99% is selected as an organic additive, and the addition amount is 5%; 5-octylphenyl-2-methoxyphenyl-4-pyridone is selected as a liquid crystal component, and the addition amount is 45%; 2-hydroxy-3-propyl acrylate is selected as a photopolymerizable monomer, and the addition amount is 45%; dibromofluorescein / phenylglycine is selected as a photoinitiator, and the addition amount is 1%; and hexanoic acid is added as a plasticizer to maintain coating uniformity, and the addition amount is 4%.

[0058] The volume holographic grating raw material system was placed in a round-bottom flask and stirred using a magnetic stirrer for 10 minutes to ensure uniform mixing. The uniformly mixed volume holographic grating raw material system was then filled into a liquid crystal cell (LCD cell) made of a glass substrate using capillary action. After sealing the edges of the LCD cell, a standard sample was obtained. After exposure to red light for 30 seconds using standard holographic exposure equipment, a volume holographic grating was produced. The refractive index modulation of the prepared volume holographic grating was tested on an optical platform.

[0059] Comparative Example 1

[0060] The raw material system of the volume holographic grating in this comparative example is 10 g in total, wherein 5-octylphenyl-2-methoxyphenyl-4-pyridone is used as the liquid crystal component, and the addition amount is 45%; 2-hydroxy-3-propyl 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%; and hexanoic acid is added as a plasticizer to maintain coating uniformity, and the addition amount is 9%.

[0061] The volume holographic grating raw material system was placed in a round-bottom flask and stirred using a magnetic stirrer for 10 minutes to ensure uniform mixing. The uniformly mixed volume holographic grating raw material system was then filled into a liquid crystal cell (LCD cell) made of a glass substrate using capillary action. After sealing the edges of the LCD cell, a standard sample was obtained. After exposure to red light for 30 seconds using standard holographic exposure equipment, a volume holographic grating was produced. The refractive index modulation of the prepared volume holographic grating was tested on an optical platform.

[0062] Result analysis:

[0063] The test results are shown in Figure 3. It can be seen from Figure 3 that the organic additive methyl benzoate has the best optimization effect. The refractive index modulation of the prepared volume holographic grating can reach 0.08, which is much higher than that of the comparative example 1 without adding organic additives. This may be because methyl benzoate has a strong polar ester group at one end and a weak polar aromatic group at the other end. The other two groups of organic additives methyl 1-cyanopropionate and α-methylstyrene also have a certain optimization effect on the refractive index modulation of the volume holographic grating. In general, the introduced organic additives have a good optimization effect on the volume holographic grating, have practical effects, and are beneficial for the preparation of AR / VR display devices with high optical performance.

[0064] In summary, the present application provides a volume holographic grating raw material system, 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 application introduces an organic additive. The 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, and thus improve the refractive index modulation of the volume holographic grating to meet the display applications in the AR / VR field. The organic additive introduced in the present application 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 improve the Δn value of the corresponding component. Compared with traditional methods, the present application 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.

[0065] It should be understood that the application of this application is not limited to the above examples. For ordinary technicians in this field, they can make improvements or changes based on the above description. All these improvements and changes should fall within the scope of protection of the claims attached to this application.

Claims

1. A volume holographic grating raw material system, comprising a photopolymerizable monomer, liquid crystal, an organic additive and a photoinitiator, wherein the organic additive has a structural formula of AB, wherein A is a photopolymerizable monomer group, and B is a liquid crystal group.

2. The volume holographic grating raw material system according to claim 1, wherein: The photopolymerizable monomer group is an ester group.

3. The volume holographic grating raw material system according to claim 1, wherein: 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.

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

5. The volume holographic grating raw material system according to claim 1, wherein: The photopolymerizable monomer group is one of an olefin-containing group, a carbonyl-containing group, and an alkyl ether group.

6. The volume holographic grating raw material system according to claim 5, wherein: The olefin-containing group is an allyl group.

7. The volume holographic grating raw material system according to claim 5, wherein: The carbonyl-containing group is one of an ethyl ketone group, an acetaldehyde group and an acetamide group.

8. The volume holographic grating raw material system according to claim 1, wherein: The liquid crystal-philic group is a phenyl group.

9. The volume holographic grating raw material system according to any one of claims 5 to 8, wherein: The structural formula of the organic additive is one or more of the following:

10. The volume holographic grating raw material system according to claim 1, wherein: Based on the volume holographic grating raw material system, 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%.

11. The volume holographic grating raw material system according to claim 1, wherein: The photopolymerizable monomer is at least one of acrylate and acrylate derivatives.

12. The volume holographic grating raw material system according to claim 11, wherein: The acrylate derivative is at least one of 2-hydroxy-3-propyl acrylate, methyl acrylate, methyl methacrylate, deuterated methyl methacrylate, fluoromethyl methacrylate, ethyl acrylate, ethyl methacrylate, ethyl ethyl acrylate, and propyl ethyl acrylate.

13. The volume holographic grating raw material system according to claim 1, wherein: The liquid crystal is a nematic liquid crystal.

14. The volume holographic grating raw material system according to claim 13, wherein: The nematic liquid crystal is at least one of 4-cyano-4'-pentylbiphenyl, 5-octylphenyl-2-methoxyphenyl-4-pyridone, and Liquid crystal mixture E7.

15. The volume holographic grating raw material system according to claim 1, wherein: The photoinitiator is a mixture of dibromofluorescein and phenylglycine or a mixture of acid red 94 and phenylglycine.

16. A volume holographic grating, prepared by using the volume holographic grating material system according to any one of claims 1 to 15 through coherent light exposure.

17. A method for preparing a volume holographic grating, comprising the steps of: exposing the volume holographic grating material system according to any one of claims 1 to 15 to coherent light to obtain the volume holographic grating.

18. The method for preparing a volume holographic grating according to claim 17, wherein: The method comprises: The volume holographic grating raw material system is placed in a round-bottom flask and stirred evenly; Filling the uniformly mixed volume holographic grating raw material system into a liquid crystal box by using capillary action; Sealing the edge of the liquid crystal box to obtain a standard holographic exposure device; The volume holographic grating is prepared by exposure using a standard holographic exposure device.

19. The method for preparing a volume holographic grating according to claim 18, wherein: The liquid crystal cell is composed of a glass substrate.

20. The method for preparing a volume holographic grating according to claim 18, wherein: The exposure is red light exposure.

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