A holographic polymer-dispersed liquid crystal grating material and its preparation method

Abstract

This invention relates to the field of optical functional materials technology, specifically disclosing a holographic polymer-dispersed liquid crystal grating material and its preparation method. The holographic polymer-dispersed liquid crystal (HPDLC) grating material comprises a photoinitiator, a co-initiator, liquid crystal, a monofunctional monomer, a bifunctional monomer, a multifunctional monomer, and a solvent, and also includes a small molecule additive; the small molecule additive is selected from one or more of 2-phenylbenzothiazole, 2-(4-methoxyphenyl)benzothiazole, 2,2′-(azobis(p-phenylene)bis(6-methylbenzothiazole), and N-phenylcarbazole. This material achieves a balance between low haze, high diffraction efficiency, and consistent performance by regulating the polymerization reaction and phase separation behavior during photocuring through the small molecule additive, thus solving the technical problems of high haze, difficult phase separation control, and poor process compatibility of existing HPDLC materials.

Description

Technical Field

[0001] This invention relates to the field of optical functional materials technology, specifically to a holographic polymer-dispersed liquid crystal grating material and its preparation method. Background Technology

[0002] Holographic polymer-dispersed liquid crystals (HPDLCs) are functional optical materials based on the combination of liquid crystal materials and photopolymerization technology. They record holograms through the principle of polymerization-induced phase separation, exhibiting unique photoelectric modulation properties and finding wide application in holographic storage, 3D display, and optical sensing. The grating formation process involves coherent laser light interfering with a two-beam array to create a sinusoidal intensity distribution. The bright areas trigger the generation of active free radicals in the system, initiating monomer polymerization. Phase separation occurs between the liquid crystal and the polymer, forming a periodic refractive index-modulated structure (alternating polymer-rich and liquid crystal-rich phases).

[0003] Haze is a core indicator of the optical performance of HPDLCs, originating from incomplete phase separation between the liquid crystal and polymer during photopolymerization, non-uniform liquid crystal droplet size, abrupt changes in refractive index at the polymer-liquid crystal interface, and scattering effects from residual unreacted components. Current haze reduction techniques primarily focus on optimizing the liquid crystal composition ratio (e.g., terphenyl or pyrimidine liquid crystal blends) or adjusting the structure of polymerizable monomers, thereby improving liquid crystal droplet distribution by controlling the phase separation rate. However, these approaches have inherent limitations: single-component optimization struggles to balance phase separation uniformity and interfacial compatibility, easily leading to decreased diffraction efficiency; reliance on specially structured liquid crystal monomers results in high costs and poor process compatibility; and insufficient control over the refractive index matching at the polymer-liquid crystal interface means that interfacial scattering remains a key bottleneck in haze reduction. Therefore, developing HPDLC materials that balance low haze, high diffraction efficiency, and controllable processes is an urgent problem to be solved. Summary of the Invention

[0004] The present invention aims to overcome the shortcomings of at least one of the above-mentioned prior art and provide a holographic polymer-dispersed liquid crystal grating material and its preparation method. By controlling the polymerization reaction and phase separation behavior in the photocuring process through small molecule additives, the invention achieves a balance between low haze, high diffraction efficiency and performance consistency, thus solving the technical problems of high haze, difficult phase separation control and poor process compatibility of existing HPDLC materials.

[0005] The technical solution adopted in this invention is to provide a holographic polymer-dispersed liquid crystal grating material, the components of which include a photoinitiator, a co-initiator, liquid crystal, a monofunctional monomer, a difunctional monomer, a polyfunctional monomer, and a solvent, and further include a small molecule auxiliaries; the small molecule auxiliaries are selected from one or more of 2-phenylbenzothiazole, 2-(4-methoxyphenyl)benzothiazole, 2,2′-(azobis(p-phenylene)bis(6-methylbenzothiazole), and N-phenylcarbazole. The aforementioned small molecule additives have heterocyclic structures, such as benzothiazole rings or carbazole rings, which can respond to the incident light field under illumination, regulate the local light intensity distribution, light energy transfer path and active free radical generation rate, delay local excessively fast polymerization reaction, reduce reaction rate inhomogeneity, extend the molecular diffusion and phase separation regulation time window before gelation, so that phase separation can be carried out under controlled conditions, avoid the formation of large-scale phase regions, and finally the cured material forms a uniform microstructure with characteristic size smaller than the critical scale of visible light scattering, suppressing light scattering, reducing haze from a mechanistic level, and ensuring that the diffraction efficiency remains unchanged in optical performance.

[0006] Furthermore, the mass fractions of the components are as follows: Photoinitiator 0.5-2 parts; 5-10 parts of co-initiator; 0.5-1 part of small molecule additives; 35-50 LCD screens; Monofunctional monomers, 5-30 parts; 5-30 parts of bifunctional monomers; 5-10 parts of multifunctional monomers; Solvent 5-15 parts.

[0007] Furthermore, the photoinitiator is selected from one or more of rose red, eosin Y, and erythrosine B. These dye-based photoinitiators possess broad-spectrum absorption characteristics and high initiation activity, making them suitable for the visible / near-infrared exposure system of HPDLC, ensuring efficient initiation of the polymerization reaction.

[0008] Furthermore, the co-initiator is selected from one or more of N,N-dimethylaniline, N-phenylglycine, and triethanolamine. It is used in synergy with the aforementioned photoinitiator to promote the generation of active free radicals, increase the polymerization rate and conversion rate, and reduce residual monomer scattering.

[0009] Furthermore, the liquid crystal is selected from one or more of E7, MLC6608, BL087, and E63. The aforementioned commercial liquid crystal has suitable dielectric anisotropy, birefringence, and phase transition temperature, adapting to the refractive index modulation requirements of volumetric holographic gratings and ensuring stable grating performance.

[0010] Furthermore, the monofunctional monomer is selected from one or more of isobornyl acrylate, benzyl acrylate, and 2-phenoxyethyl methacrylate. The monofunctional monomer acts as a diluent and network modifier to increase the flexibility of the polymer.

[0011] Furthermore, the bifunctional monomer is selected from one or more of tricyclodecanediethanol dimethacrylate, 1,6-hexanediol diacrylate ethoxylate, and dipropylene glycol diacrylate. The bifunctional monomer serves as the main crosslinking agent for constructing the three-dimensional network framework.

[0012] Furthermore, the multifunctional monomer is selected from one or more of pentaerythritol hexaacrylate and pentaerythritol triacrylate. As a high-crosslinking-density monomer, the multifunctional monomer enhances network stability. By mixing monomers with different functionalities, the crosslinking density, flexibility, and mechanical strength of the final polymer network can be precisely controlled, thereby synergistically promoting liquid crystal phase separation and obtaining HPDLC materials with low haze and consistent performance.

[0013] Furthermore, the solvent is selected from one or more of N-methylpyrrolidone, N,N-dimethylformamide, and N-ethylpyrrolidone. These solvents improve the solubility and mixing uniformity of the components, facilitate vacuum filling, and do not affect the final material properties after evaporation.

[0014] Another object of the present invention is to provide a method for preparing the above-mentioned holographic polymer-dispersed liquid crystal grating material, comprising the following steps: S1. Mix all components, sonicate, and fill into a liquid crystal cell under vacuum, then place in a dark room to mix evenly. S2. Expose the liquid crystal cell using a dual-beam light source to obtain the holographic polymer dispersed liquid crystal grating material.

[0015] In the above preparation method, ultrasonic dispersion ensures uniformity of components, vacuum filling eliminates air bubbles, darkroom mixing avoids prepolymerization, and dual-beam exposure forms a bulk holographic grating structure.

[0016] Furthermore, the thickness of the liquid crystal cell described in step S1 is 5-10 μm. This thickness is beneficial for balancing the grating diffraction efficiency and fabrication feasibility, avoiding excessive light scattering due to excessive thickness or insufficient grating intensity due to insufficient thickness.

[0017] Furthermore, in step S2, the included angle of the dual-beam light source is 40°-80°, the exposure time is 1-5 min, and the radiation intensity is 4-6 mW / cm². 2 The included angle controls the grating period, while the exposure time and light intensity control the degree of polymerization and the phase separation process. Within this parameter range, a uniform grating with high diffraction efficiency and low haze can be obtained.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention provides a holographic polymer-dispersed liquid crystal grating material and its preparation method. By introducing a small-molecule additive containing a heterocyclic structure, the polymerization reaction and phase separation behavior during photocuring are controlled, achieving a balance between low haze, high diffraction efficiency, and consistent performance. This additive responds to the light field, delaying locally rapid polymerization and significantly extending the time window for molecular diffusion and phase separation regulation before gelation. This allows phase separation to occur under controlled conditions, avoiding the formation of large-scale phase regions. The resulting cured material forms a uniform microstructure with characteristic dimensions smaller than the critical scale for visible light scattering, mechanistically suppressing light scattering to reduce haze while ensuring optical transparency and performance stability. Based on this, combined with a specific component ratio and a dual-beam exposure process, a HPDLC material exhibiting both low haze and high diffraction efficiency was successfully prepared, solving the technical problems of high haze, difficult phase separation control, and poor process compatibility in existing materials. Detailed Implementation

[0019] The present invention will now be further illustrated with specific examples. The following embodiments are merely illustrative and do not constitute a limitation thereof. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0020] In the following examples, unless otherwise specified, the experimental conditions are understood as conventional conditions or conditions recommended by the reagent company; the reagents and consumables used are commercially available unless otherwise specified.

[0021] Example 1 A holographic polymer-dispersed liquid crystal grating material, with the following composition and mass percentage:

[0022] Example 2 A holographic polymer-dispersed liquid crystal grating material, with the following composition and mass percentage:

[0023] Example 3 A holographic polymer-dispersed liquid crystal grating material, with the following composition and mass percentage:

[0024] Example 4 A holographic polymer-dispersed liquid crystal grating material, with the following composition and mass percentage:

[0025] Comparative Example 1 A holographic polymer-dispersed liquid crystal grating material, with the following composition and mass percentage:

[0026] Comparative Example 2 A holographic polymer-dispersed liquid crystal grating material, with the following composition and mass percentage:

[0027] Comparative Example 3 A holographic polymer-dispersed liquid crystal grating material, with the following composition and mass percentage:

[0028] Comparative Example 4 A holographic polymer-dispersed liquid crystal grating material, with the following composition and mass percentage:

[0029] The preparation methods of the materials described in the above embodiments and comparative examples are as follows: All components are mixed, ultrasonically treated, and then filled into a liquid crystal cell under a vacuum atmosphere. The liquid crystal cell thickness is 5 μm. The cell is placed in a dark room for 60 min to allow the materials to be further mixed and homogenized. Subsequently, the materials are exposed using a dual-beam light source with the following parameters: a dual-beam angle of 65°, an exposure time of 2 min, and an optical power of 5 mW / cm². 2 This will yield well-formed HPDLC grating materials.

[0030] Optical performance testing: Diffraction efficiency was tested using a grating diffraction efficiency meter to measure the diffraction efficiency of the above-mentioned holographic polymer-dispersed liquid crystal material grating device, where diffraction efficiency = diffracted light intensity / incident light intensity × 100%. Haze testing was conducted using a haze meter according to GB 2410-2008 "Test Methods for Transmittance and Haze of Transparent Plastics", at 23±2℃ to measure the haze of the above-mentioned holographic polymer-dispersed liquid crystal material grating device.

[0031] The test results are shown in the table below:

[0032] The only difference between the above embodiments and the comparative examples with the same serial number is the addition of a small molecule additive. Pairwise comparisons show that the haze of the embodiments is significantly lower than that of the comparative examples with the same serial number, while the diffraction efficiency is not significantly different.

[0033] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the technical solutions of the present invention, and are not intended to limit the specific implementation of the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the claims of the present invention should be included within the protection scope of the claims of the present invention.

Claims

1. A holographic polymer-dispersed liquid crystal grating material, comprising a photoinitiator, a co-initiator, liquid crystal, a monofunctional monomer, a difunctional monomer, a multifunctional monomer, and a solvent, characterized in that, It also includes small molecule adjuvants; said small molecule adjuvants are selected from one or more of 2-phenylbenzothiazole, 2-(4-methoxyphenyl)benzothiazole, 2,2′-(azobis(p-phenylene)bis(6-methylbenzothiazole), and N-phenylcarbazole; The mass fractions of the components are as follows: Photoinitiator 0.5-2 parts; 5-10 parts of co-initiator; 0.5-1 part of small molecule additives; 35-50 LCD screens; Monofunctional monomers, 5-30 parts; 5-30 parts of bifunctional monomers; 5-10 parts of multifunctional monomers; Solvent 5-15 parts; The monofunctional monomer is selected from one or more of isobornyl acrylate, benzyl acrylate, and 2-phenoxyethyl methacrylate; The bifunctional monomer is selected from one or more of tricyclodecanediethanol dimethacrylate, ethoxylated 1,6-hexanediol diacrylate, and dipropylene glycol diacrylate; The multifunctional monomer is selected from one or more of pentaerythritol hexaacrylate and pentaerythritol triacrylate.

2. The holographic polymer-dispersed liquid crystal grating material according to claim 1, characterized in that, The co-initiator is selected from one or more of N,N-dimethylaniline, N-phenylglycine, and triethanolamine.

3. The holographic polymer-dispersed liquid crystal grating material according to claim 1, characterized in that, The solvent is selected from one or more of N-methylpyrrolidone, N,N-dimethylformamide, and N-ethylpyrrolidone.

4. A method for preparing a holographic polymer-dispersed liquid crystal grating material according to any one of claims 1 to 3, characterized in that, Includes the following steps: S1. Mix all components, sonicate, and fill into a liquid crystal cell under vacuum, then place in a dark room to mix evenly. S2. Expose the liquid crystal cell using a dual-beam light source to obtain the holographic polymer dispersed liquid crystal grating material.

5. The preparation method according to claim 4, characterized in that, The thickness of the liquid crystal cell in step S1 is 5-10 μm.

6. The preparation method according to claim 4, characterized in that, The included angle of the dual-beam light source in step S2 is 40°-80°, the exposure time is 1-5 min, and the radiation intensity is 4-6 mW / cm². 2 .

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