Liquid crystal polymer film with crosslinking density gradient and preparation method and application thereof

Abstract

The present application belongs to the technical field of liquid crystal polymer materials, and particularly relates to a liquid crystal polymer film with a crosslinking density gradient and a preparation method and application thereof. The present application firstly uniformly mixes bifunctional liquid crystal monomers, small molecule liquid crystal monomers E7 and a free radical photoinitiator, then fills the mixture into a liquid crystal cell, and performs a polymerization reaction under weak ultraviolet light. After the reaction is completed, a liquid crystal polymer film with a crosslinking density gradient is obtained. The film material prepared by the present application can deform under heating conditions, and the film can return to the initial state after the heat source is removed. The deformation size can be adjusted by controlling the temperature, the ultraviolet light intensity and the ultraviolet light irradiation direction, and further processing is not required. The film material can be used for preparing photo-thermal response devices, bionic multifunctional materials and artificial soft robots.

Description

Technical Field

[0001] This invention belongs to the field of liquid crystal polymer materials technology, specifically relating to a liquid crystal polymer film with a crosslinking density gradient, its preparation method, and its application. Background Technology

[0002] Liquid crystal polymers possess the anisotropy of liquid crystals and the viscoelasticity of polymers, making them one of the most promising soft actuation materials. However, while current liquid crystal elastomers can produce extremely high shape changes, their mechanical strength is relatively low, their preparation and synthesis processes are complex and cumbersome, and they often suffer from limited miscibility. This significantly reduces the mechanical properties of composite films and hinders their applications to some extent. Therefore, the preparation of a flexible polymer film is of great significance. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the prior art and provide a liquid crystal polymer film with a crosslinking density gradient, its preparation method, and its application. Specifically, the following technical solution is adopted: According to a first aspect of the present invention, a liquid crystal polymer film having a crosslinking density gradient is provided, comprising the following steps: First, the bifunctional liquid crystal monomer, the small molecule liquid crystal monomer and the free radical photoinitiator are mixed evenly, and then poured into a liquid crystal cell. The polymerization reaction is carried out under weak ultraviolet light. After the reaction is completed, a liquid crystal polymer film with a crosslinking density gradient is obtained.

[0004] In the aforementioned liquid crystal monomer polymerization process, the rapid polymerization of bifunctional liquid crystals under weak ultraviolet light irradiation leads to a high crosslinking density on the upper surface, while small liquid crystal molecules aggregate on the lower surface, resulting in a slower polymerization rate and a difference in crosslinking density, forming a crosslinking density gradient. Under uniform heating conditions, the presence of this crosslinking density gradient causes the material to shrink differentially, resulting in three-dimensional bending deformation. By constructing a crosslinking density gradient across the film thickness, the obtained film possesses diverse and programmable initial shapes, showing broad application prospects in fields such as optical drivers, smart devices, biomimetic multifunctional materials, and artificial soft robots.

[0005] This invention involves adding non-polymerizable small-molecule liquid crystal E7 to a bifunctional liquid crystal monomer to prepare a liquid crystal polymer film with a crosslinking density gradient. The side of the film closest to the UV lamp is defined as the upper surface, and the other side as the lower surface. During polymerization, due to weak UV irradiation, the upper surface has a higher crosslinking density, causing the small-molecule liquid crystal monomers to aggregate on the lower surface, resulting in a difference in crosslinking density and forming a crosslinking density gradient. The film always bends towards the side with the lower crosslinking density. This preparation method is simple to operate, easy to implement, and highly safe.

[0006] The above ultraviolet light intensity is 100 μW / cm 2 —1100 μW / cm 2 When the intensity of ultraviolet light is too weak, the degree of cross-linking of the film is too low, and it cannot deform; when the intensity of light is too strong, the film polymerizes instantaneously, and it cannot generate a cross-linking density gradient, thus it also cannot deform.

[0007] As a further preferred embodiment, the thickness of the liquid crystal polymer film is 20 μm-200 μm. More preferably, the thickness of the liquid crystal polymer film is 40 μm. Both excessively large and excessively small thicknesses will affect the formation of the crosslinking density gradient, thereby affecting the thermal response speed and deformation magnitude of the film.

[0008] As a further preferred embodiment, the mass ratio of the aforementioned bifunctional liquid crystal monomer to the non-polymerizable small molecule liquid crystal E7 is (70-95):(5-30). When the content of the small molecule liquid crystal monomer E7 is either absent or too high / low, no crosslinking density gradient will occur.

[0009] As a further preferred embodiment, the aforementioned free radical photoinitiator is 2,2-dimethoxy-2-phenylacetophenone; the mass fraction of the free radical photoinitiator is 1%. This free radical photoinitiator can generate free radicals under ultraviolet irradiation and initiate the polymerization of polymer monomers, but will not cause material deformation. The mixed liquid crystal obtained by mixing the aforementioned bifunctional liquid crystal monomer, small molecule liquid crystal monomer E7 and free radical photoinitiator is poured into a liquid crystal cell with two sides rubbed parallel alignment to form a liquid crystal film. The thickness is controlled to be 40 μm, the length is 0.5 cm-6 cm, and the width is 0.2 cm-4 cm by PET spacers.

[0010] As a further preferred embodiment, the above-mentioned bifunctional liquid crystal monomer is selected from at least one of the following compounds: , , ; where R is selected from , , , Where n is any positive integer from 1 to 12. The aforementioned liquid crystal polymer possesses dual functional groups, simultaneously exhibiting both liquid crystal and reactive functional groups. Therefore, it can undergo further cross-linking and curing through chemical reactions to improve its performance and stability. More preferably, the structure of the aforementioned dual-functional liquid crystal monomer is shown in Formula I: Formula I; This structure has good chiral self-assembly properties and high stability.

[0011] As a further preferred embodiment, the above-mentioned small molecule liquid crystal monomer is selected from at least one of the following compounds: , , , , R is selected from , , n is any positive integer from 1 to 12. The liquid crystal molecules mentioned above are all non-polymerizable liquid crystal monomers that aggregate on the lower surface during the polymerization process.

[0012] More preferably, the aforementioned non-polymerizable small-molecule liquid crystal monomers are mixed crystals of 5CB, 7CB, 8OCB, and 5CT, and their molecular structures are shown below: .

[0013] The mass percentages of 5CB, 7CB, 8OCB, and 5CT are 51%:25%:16%:8%. A liquid crystal mixture in this ratio is called E7. E7 liquid crystals have phase transition temperatures of -10°C and 60°C, and remain in the nematic phase state at room temperature.

[0014] As a further preferred embodiment, the polymerization reaction temperature is 25 °C-100 °C, and the reaction time is 30 s-900 s. More preferably, the polymerization time is 900 s. When the polymerization temperature is too high or too low, the liquid crystal film will not be in a liquid crystal state. When the polymerization time is too short, the liquid crystal film cannot be formed; when the polymerization time is too long, the degree of crosslinking of the film will be too high, and the crosslinking density gradient cannot be formed.

[0015] According to a second aspect of the present invention, a liquid crystal polymer film with a crosslinking density gradient is also provided, prepared by the above-described method. The liquid crystal polymer film prepared by the present invention exhibits good deformation at room temperature and can be programmed by controlling the intensity and direction of ultraviolet light irradiation, showing great application potential in the fields of soft actuators and biomimetic devices; furthermore, it can achieve deformation without further processing, playing an important role in the fields of smart materials and biomimetic multifunctional materials.

[0016] According to a third aspect of the present invention, the application of the above-described liquid crystal polymer film in the preparation of programmable optical drivers, smart devices, biomimetic multifunctional materials, and artificial soft robots is also provided.

[0017] The beneficial effects of this invention are as follows: By adjusting the ratio of bifunctional liquid crystal polymers to small molecule liquid crystals and the intensity of ultraviolet light, this invention prepares liquid crystal polymer films with crosslinking density gradients. The preparation method of this invention is simple and easy to operate, and has high safety. The ultraviolet light used is a clean energy source with advantages such as easy availability, non-contact remote control, and ease of control in time and space. The film material prepared by this invention can deform under heating conditions and return to its initial state after the heat source is removed. The magnitude of the deformation can be adjusted by controlling the temperature, ultraviolet light intensity, and ultraviolet light irradiation direction, without further processing. It can be used to prepare photothermal response devices, biomimetic multifunctional materials, and artificial soft robots, etc. Attached Figure Description

[0018] Figure 1 The image shown is a deformation effect diagram of the liquid crystal polymer film with crosslinking density gradient prepared in Example 1. Figure 2 The image shown is a physical diagram of the thermal response of the liquid crystal polymer actuator with crosslinking density gradient prepared in Example 2. Figure 3 The figure shown is a thermal stability test diagram of the liquid crystal polymer actuator with crosslinking density gradient prepared in Example 2. Figure 4 The image shown is a shape-programmable liquid crystal polymer actuator prepared in Example 4. Figure 5 The image shown is a physical picture of the liquid crystal polymer actuator with crosslinking density gradient prepared by adding 6OBA in Supplementary Example 1. Figure 6 The image shows a cholesteric phase liquid crystal polymer film with structural color prepared in Comparative Example 1. Detailed Implementation

[0019] The following will provide a clear and complete description of the concept, specific structure, and technical effects of the present invention in conjunction with the embodiments and accompanying drawings, so as to fully understand the purpose, solution, and effects of the present invention. It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other.

[0020] Example 1 A liquid crystal polymer film with a crosslinking density gradient is prepared by the following steps: This experiment consisted of six experimental groups: Experiment 1, Experiment 2, Experiment 3, Experiment 4, and Experiment 5. Experiment 5 and Experiment 6.

[0021] First, a bifunctional liquid crystal monomer and a small molecule liquid crystal E7 (a mixed crystal of 5CB, 7CB, 8OCB, and 5CT, with a mass percentage of 51%:25%:16%:8%) were mixed at a mass ratio of 70:30. Then, this mixture was combined with 1% (by mass) of the free radical photoinitiator Irg651 and poured into a liquid crystal cell that had undergone parallel alignment treatment. The thickness was controlled to 40 μm using PET spacers. The polymerization temperature was 80℃, and the polymerization time was 15 min. The UV light intensity was 100 μW / cm for group 1, 30 μW / cm for group 2, 500 μW / cm for group 3, 700 μW / cm for group 4, 900 μW / cm for group 5, and 1100 μW / cm for group 6. μW / cm, after polymerization, the liquid crystal cell was opened to obtain liquid crystal polymer films with crosslinking density gradients. The results are as follows: Figure 1 As shown, within a suitable range of light intensity, different crosslinking density gradients are formed in the film due to different light intensities, thus causing different degrees of deformation in the film.

[0022] The polymerizable bifunctional liquid crystal monomer structure used in this embodiment is as follows: ; The molecular structure of the small-molecule liquid crystal E7 used in this embodiment is as follows: .

[0023] The mass percentages of 5CB, 7CB, 8OCB, and 5CT are: 51%, 25%, 16%, and 8%, respectively. Example 2 A liquid crystal polymer film with a crosslinking density gradient (compared to Example 1, only the mass ratio of the bifunctional liquid crystal monomer and the small molecule liquid crystal monomer E7 is changed to 75:25, the ultraviolet light intensity is selected as 700 μW / cm, and other conditions remain unchanged), is prepared by the following steps: First, a bifunctional liquid crystal monomer and a small molecule liquid crystal monomer E7 (a mixed crystal of 5CB, 7CB, 8OCB, and 5CT, with a mass percentage of 51%:25%:16%:8%) were mixed at a mass ratio of 75:25. Then, this mixture was mixed with 1% by mass of the free radical photoinitiator Irg651 and poured into a liquid crystal cell that had undergone parallel alignment treatment. The thickness was controlled at 40 μm using PET spacers. The polymerization temperature was 80 °C, the polymerization time was 15 min, and the UV light intensity was 700 μW / cm². 2 After polymerization, due to the faster polymerization rate of bifunctional groups and the slower polymerization rate of monofunctional groups, a gradient liquid crystal polymer film is formed.

[0024] Example 3 The liquid crystal thin film material obtained in Example 2 was tested, and the testing method included the following steps: This embodiment tests the temperature response and stability of the flexible actuator with crosslinking density gradient prepared in Example 2. Figure 2 As shown, when the obtained film is placed under different temperature conditions, the degree of deformation of the film increases with increasing temperature. Figure 3 As shown, when the film was placed at 25 °C and 100 °C for 20 cycles, there was no significant difference in the deformation of the film, indicating that the film has good thermal stability.

[0025] Example 4 A shape-programmable liquid crystal polymer film with a crosslinking density gradient is prepared by the following steps: First, a bifunctional liquid crystal monomer and a small-molecule liquid crystal E7 (a mixed crystal of 5CB, 7CB, 8OCB, and 5CT, with a mass percentage of 51%:25%:16%:8%) were mixed at a mass ratio of 70:30. Then, this mixture was combined with 1% (by mass) of the free radical photoinitiator Irg651 and poured into a liquid crystal cell that had undergone parallel alignment treatment. The thickness was controlled to 40 μm using PET spacers. After covering half of the liquid crystal cell with a mask, a 700 μW / cm² photoinitiator was used. 2 The polymerization was initiated by ultraviolet light at a temperature of 80 °C for 15 min. The polymerized portion was then masked, and the mask was removed. A 700 μW / cm² UV light source was then used to initiate the polymerization. 2 Ultraviolet light was used to initiate polymerization at a temperature of 80 °C for 15 min, thereby preparing a thin film with two irradiation directions.

[0026] Due to the different directions of ultraviolet light irradiation, the crosslinking gradients of the two parts of the film will be completely opposite, resulting in two completely opposite deformations in the two parts of the film, with the effect as follows: Figure 4 As shown.

[0027] The bifunctional liquid crystal monomer structure used in this embodiment is as follows: ; The molecular structure of the small molecule liquid crystal monomer E7 used in this embodiment is as follows: .

[0028] Example 5 A liquid crystal polymer film with a crosslinking density gradient (the difference between this embodiment and Example 2 is the use of two different bifunctional liquid crystal molecules) is prepared by the following steps: First, a bifunctional liquid crystal monomer (the mass ratio of C6M to 6OBA in the bifunctional liquid crystal monomer is 75:25) and a nonpolymerizable small molecule liquid crystal E7 (a mixed crystal of 5CB, 7CB, 8OCB and 5CT, with the mass percentages of 5CB, 7CB, 8OCB and 5CT being 51%:25%:16%:8%) are mixed at a mass ratio of 80:20. Then, this mixture is mixed with 1% by mass of the free radical photoinitiator Irg651 in the above system. The mixture is then poured into a liquid crystal cell that has undergone parallel alignment treatment. The thickness is controlled at 40 μm using PET spacers. The polymerization temperature is 80℃, the polymerization time is 15 min, and the ultraviolet light intensity is 800 μW / cm. 2 After polymerization, opening the liquid crystal cell yields a liquid crystal polymer film with a crosslinking density gradient. The results are as follows... Figure 5 As shown, the film exhibits good bending deformation and good thermal response properties.

[0029] The bifunctional liquid crystal monomer structure used in this embodiment is as follows: C6M; 6OBA.

[0030] The molecular structure of the small molecule liquid crystal monomer E7 used in this embodiment is as follows: .

[0031] Comparative Example 1 A cholesteric phase liquid crystal polymer film with crosslinking density gradient (compared to Example 1, only 1.65% more chiral molecules were added, while other conditions remained unchanged) was prepared by the following steps: This experiment was set up with 4 experimental groups: Experiment 1, Experiment 2, Experiment 3 and Experiment 4.

[0032] First, a bifunctional liquid crystal monomer and a small molecule liquid crystal monomer E7 (a mixed crystal of 5CB, 7CB, 8OCB and 5CT, with a mass percentage of 51%:25%:16%:8%) were mixed at a mass ratio of 70:30. Then, the mixture was mixed with 1% of the free radical photoinitiator 651 and 1.65% of the chiral molecule by mass. The mixture was then poured into a liquid crystal cell that had undergone parallel orientation treatment. The thickness was controlled at 40 μm using PET spacers. The polymerization temperature was 80 ℃. The UV light intensity was 500 μW / cm for group 1, 700 μW / cm for group 2, 900 μW / cm for group 3, and 1100 μW / cm for group 4. The polymerization time was 15 min. After polymerization, the liquid crystal cell was opened to obtain a liquid crystal polymer film with a crosslinking density gradient.

[0033] The cholesteric phase liquid crystal polymer film with crosslinked density gradient prepared in this comparative example was subjected to thermal response deformation testing. The film was placed on a hot stage at different temperatures, and the change in its bending angle was observed. The results at room temperature and 90℃ are as follows: Figure 6 As shown, the thin film did not exhibit significant thermal response deformation.

[0034] Although the description of the invention has been quite detailed and particularly of several described embodiments, it is not intended to limit it to any of these details or embodiments or any particular embodiment, but should be considered as providing a broad possible interpretation of the claims by referring to the appended claims and taking into account the prior art, thereby effectively covering the intended scope of the invention. Furthermore, the invention has been described above with respect to embodiments foreseeable by the inventors in order to provide a useful description, and non-substantial modifications to the invention that have not yet been foreseen may still represent equivalent modifications.

Claims

1. A method for preparing a liquid crystal polymer film with a crosslinking density gradient, characterized in that, Includes the following steps: First, the bifunctional liquid crystal monomer, the small molecule liquid crystal monomer and the free radical photoinitiator are mixed evenly, and then poured into a liquid crystal cell. The polymerization reaction is carried out under weak ultraviolet light. After the reaction is completed, a liquid crystal polymer film with a crosslinking density gradient is obtained. The bifunctional liquid crystal monomer is a polymerizable bifunctional liquid crystal monomer, and the structural formula of the polymerizable bifunctional liquid crystal monomer is as follows: ； The small-molecule liquid crystal monomer is a mixed crystal of 5CB, 7CB, 8OCB and 5CT, and its molecular structure is shown below: ； The mass ratio of the 5CB, 7CB, 8OCB, and 5CT is 51%:25%:16%:8%; The mass ratio of the bifunctional liquid crystal monomer to the small molecule liquid crystal molecule is (70-95):(5-30).

2. The preparation method according to claim 1, characterized in that, The thickness of the liquid crystal polymer film is 20 μm-200 μm.

3. The preparation method according to claim 1, characterized in that, The free radical photoinitiator is 2,2-dimethoxy-2-phenylacetophenone.

4. The preparation method according to claim 1, characterized in that, The polymerization reaction is carried out at a temperature of 25℃-100℃ and a reaction time of 30 s-900 s.

5. A liquid crystal polymer film with a crosslinking density gradient, characterized in that, It is prepared by the preparation method according to any one of claims 1-4.

6. The application of the liquid crystal polymer film with crosslinking density gradient as described in claim 5 in the preparation of programmable optical drivers, smart devices, biomimetic multifunctional materials, and artificial soft robots.

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