Chiral ferromagnetic monodomain lyotropic liquid crystal material, preparation method and application thereof

Abstract

The application discloses a preparation method of a chiral ferromagnetic monodomain liquid crystal material, and synthesizes Sc-doped barium ferrite nanosheets through a hydrothermal method, modifies the nanosheets by using citric acid, obtains ferromagnetic barium ferrite nanosheets, and uniformly disperses the ferromagnetic barium ferrite nanosheets into a chiral liquid crystal matrix; under the action of a weak magnetic field (as low as 30 mT), the anisotropic nanosheet orientation is coupled with the chiral liquid crystal orientation, the nanosheets are oriented along the magnetic field direction, and the ordered arrangement of the chiral liquid crystal matrix is guided; the chiral cluster aggregates and liquid crystal domains in the chiral liquid crystal matrix are migrated and fused, and finally, a wireless, planar defect chiral ferromagnetic monodomain structure is formed. The chiral ferromagnetic monodomain liquid crystal material has magnetic response, and the magneto-optic effect under the action of a magnetic field provides a basis for the application of the chiral ferromagnetic monodomain liquid crystal material in the fields of optical equipment and flexible machines.

Description

Technical Field

[0001] This invention relates to the field of liquid crystal materials technology, specifically to a chiral ferromagnetic single-domain lyotropic liquid crystal material, its preparation method, and its applications. Background Technology

[0002] Chiral structures have wide applications in nature, such as the realization of functions in biomolecules and the generation of structural colors. The preparation of chiral materials, especially liquid crystal materials, has become one of the key strategies for developing advanced optoelectronic, sensor, and responsive materials. Traditional chiral liquid crystal materials, such as molecular chiral liquid crystals, have relatively mature technologies, but they typically suffer from low thermal stability, high cost, and poor manufacturability. In recent years, colloidal chiral liquid crystals (such as cellulose nanocrystals (CNC)) have attracted increasing attention due to their superior thermal stability, renewability, and lower cost.

[0003] However, in liquid crystal systems, especially in colloidal chiral liquid crystals, the inherent twisting of chiral liquid crystals often leads to elastic frustration, topological defects, and the formation of multi-domain textures, which limits the formation of large-scale single domains. Controlling the self-assembly process of chiral liquid crystals, especially through enhancing N... * The long-range orientational order of the phase is crucial for transforming liquid crystal materials into materials with robust optical and responsive properties.

[0004] Common colloidal chiral liquid crystal alignment strategies include applying external shear forces, electric fields, or magnetic fields, with magnetic fields offering a particularly flexible approach. However, direct control typically requires extremely high magnetic fields, limiting its practicality. Introducing spherical magnetic nanoparticles can reduce the required magnetic field strength, but it cannot avoid the inherent formation of multiple domains, leaving defects and structural discontinuities (Xiaofang Zhang et al. Nature Communications. 2022, 13:580.).

[0005] A promising alternative is ferromagnetic liquid crystals, in which ferromagnetic nanosheets are uniformly dispersed within a liquid crystal matrix to enhance its response to magnetic fields. Previous studies have struggled to establish stable ferromagnetic liquid crystals due to the magnetic dipole interactions between ferromagnetic particles. The pioneering work on molecular magnetic liquid crystals by Mertelj et al. (Alenka Mertelj et al. Nature. 2013, 504: 237.) demonstrated the use of ferromagnetic nanosheets to control the structure of liquid crystals. Stable dispersion of ferromagnetic nanomaterials in molecular liquid crystals was achieved by controlling the shape anisotropy, magnetism, and surface modification of the nanosheets. However, the application of this method in colloidal liquid crystal systems remains largely unexplored. The unique fluid properties, length scale, and interaction mechanisms of colloidal liquid crystals suggest the potential for unique self-assembly pathways and stable structures that are unattainable in molecular liquid crystals. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method for preparing chiral ferromagnetic single-domain lyotropic liquid crystal materials. By controlling the surface properties, magnetism, and ionic strength of the ferromagnetic barium ferrite, ferromagnetic barium ferrite (BaHF) nanosheets are uniformly dispersed in a chiral liquid crystal matrix. Under a weak magnetic field (as low as 30 mT), a chiral ferromagnetic single-domain structure without surface defects is formed.

[0007] A method for preparing a chiral ferromagnetic single-domain lyotropic liquid crystal material includes the following steps: (1) Sc-doped barium ferrite nanosheets were synthesized by hydrothermal method, and the nanosheets were modified with citric acid to obtain ferromagnetic barium ferrite nanosheets. (2) The ferromagnetic barium ferrite nanosheets obtained in step (1) are uniformly dispersed in a chiral liquid crystal matrix and assembled by a weak magnetic field to obtain a chiral ferromagnetic single-domain lyotropic liquid crystal material. The strength of the magnetic field is ≥30 mT.

[0008] In this invention, ferromagnetic barium ferrite (BaHF) nanosheets are uniformly dispersed in a chiral liquid crystal matrix. Under a weak magnetic field (≥30 mT), the anisotropic nanosheet orientation couples with the chiral liquid crystal orientation, causing the nanosheets to align along the magnetic field direction and guiding the orderly arrangement of the chiral liquid crystal matrix. Crystalline aggregates and liquid crystal domains in the chiral liquid crystal matrix migrate and fuse, ultimately forming a chiral ferromagnetic single-domain structure without planar defects. The fabrication process involves designing surface modification of the magnetic disks, controlling the saturation magnetization, and managing the ion strength of the liquid crystal system. The chiral ferromagnetic single-domain liquid crystal material exhibits magnetoresponsiveness, and its magneto-optical effect under a magnetic field provides a foundation for its application in optical devices, flexible machines, and other fields.

[0009] Preferably, in step (1), the temperature of the hydrothermal method is 200~280 ℃ and the time is 0.5~2 h.

[0010] Preferably, in step (1), the size of the Sc-doped barium ferrite nanosheet is 10~100 nm and the thickness is 2~10 nm.

[0011] Preferably, in step (1), the magnetic saturation intensity of the Sc-doped barium ferrite nanosheets is 20~45 emu / g. Further increases in the magnetic field saturation intensity will increase the magnetic dipole force between the nanosheets, leading to aggregation and disrupting the colloidal stability.

[0012] Preferably, in step (1), the mass ratio of citric acid to Sc-doped barium ferrite nanosheets is 1~15:1. Modification within this range yields a stable BaHF suspension.

[0013] Preferably, in step (1), the temperature of the modification treatment is 60~80 ℃ and the time is 1~2 h.

[0014] Preferably, in step (2), the chiral liquid crystal matrix is ​​a chiral modified, doped or chiral nanomaterial, such as one of the following: one-dimensional materials: cellulose nanocrystals (CNC), carbon nanotubes, etc., and two-dimensional materials: graphene oxide, Mxene, etc.

[0015] More preferably, the chiral liquid crystal matrix is ​​cellulose nanocrystals, and the coexistence range of the isotropic phase (I) and chiral nematic phase (N*) in the liquid crystal system is 1.0 ± 0.1 wt%. φ I ) to 3.3 ± 0.2 wt% φ N* ).

[0016] Preferably, in step (2), the mass ratio of the ferromagnetic barium ferrite nanosheets to the chiral liquid crystal matrix is ​​0.01~1:0.1~5.

[0017] The present invention also provides a chiral ferromagnetic single-domain liquid crystal material prepared by the above preparation method.

[0018] This invention also provides the application of the above-mentioned chiral ferromagnetic single-domain liquid crystal materials in the fabrication of optical devices and flexible machines.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: In this invention, ferromagnetic barium ferrite (BaHF) nanosheets are uniformly dispersed in a chiral lyotropic liquid crystal matrix. Under a weak magnetic field (≥30 mT), entropy-induced structural locking, rather than magnetic dipole interaction, is used to couple the orientation of the anisotropic nanosheets with that of the chiral liquid crystal. This causes the nanosheets to align along the magnetic field direction and guides the orderly arrangement of the chiral liquid crystal matrix. The quasi-crystal aggregates and liquid crystal domains in the chiral liquid crystal matrix migrate and fuse, ultimately forming a chiral ferromagnetic single-domain structure without surface defects, which exhibits strong magneto-optical response and thermal stability. Attached Figure Description

[0020] Figure 1 TEM image of the citric acid-modified ferromagnetic BaHF nanosheets prepared in Example 1.

[0021] Figure 2 The hysteresis loops are those of the ferromagnetic barium ferrite nanosheets synthesized in Examples 1 and 2.

[0022] Figure 3 This is a schematic diagram of the assembly process of a large-area chiral ferromagnetic single-domain structure under the action of a weak magnetic field.

[0023] Figure 4 These are characterization and analysis images of the chiral ferromagnetic single-domain liquid crystal material formed under a magnetic field gradient in Example 1, where a is a polarized image of the CNC+BaHF mixed solution when phase separation reaches quasi-equilibrium in Example 1; b is an IN... * c is a microscopic polarized image of the phase interface; d is a polarized image of the chiral ferromagnetic single-domain liquid crystal material; e is a SEM image of the chiral ferromagnetic single-domain liquid crystal material; and e is a polarized image of the chiral liquid crystal formed in Comparative Example 1.

[0024] Figure 5 The images show the characterization and magneto-optical responsiveness of chiral ferromagnetic single domains in Example 2, where a is the hysteresis loop of the chiral ferromagnetic single domain, b is the fast Fourier transform of the chiral single domain texture, and c and d are polarized photographs of the topological deformation process of the chiral ferromagnetic single domain liquid crystal material driven by a magnetic field. Detailed Implementation

[0025] The present invention will be further described in detail below with reference to the embodiments, but the implementation of the present invention is not limited to the following embodiments.

[0026] All raw materials used in this invention are commercially available.

[0027] Example 1 (1) Preparation of ferromagnetic barium ferrite nanosheets Barium ferrite nanosheets doped with Sc were synthesized via a hydrothermal method, with a diameter of 53.9 nm and a thickness of 4.5 nm; two-dimensional barium ferrite magnetic nanocrystals (BaFe) were also synthesized via a hydrothermal method.12-x Sc x O 19 First, measure out a certain amount of ferric nitrate, barium nitrate, and scandium nitrate, with a molar ratio of n(Ba):n(Fe):n(Sc) = 1:4.65:0.5, and dissolve them in 50 mL of water. Then, measure out a certain amount of NaOH, making n[OH] - / n[NO3] - =16, dissolved in 50 mL of deionized water. The two solutions were mixed and poured into a 250 mL reaction vessel (with a polytetrafluoroethylene liner). The reaction vessel was then placed in a muffle furnace for reaction, with a heating rate controlled at 3 °C / min. Once the temperature reached the set temperature of 270 °C, it was held at that temperature for 30 minutes and then allowed to cool naturally to room temperature. The resulting reaction product was washed once with 0.032 M nitric acid and twice with deionized water. During centrifugation, the speed was 12000 r / min and the time was 30 min. The resulting product was then directly surface-modified or dried in a drying oven at 60 °C for 24 hours and ground in a mortar and pestle to obtain BaHF powder.

[0028] Subsequently, citric acid was used to modify the Sc-doped barium ferrite nanosheets to obtain ferromagnetic barium ferrite nanosheets. The specific process was as follows: 750 mg of BaHF powder was added to 150 mL of deionized water and ultrasonically dispersed using a cell disruptor at a power of 300 W for 5 min, with a 2-second pause. The resulting stable suspension was then transferred to a 250 mL round-bottom flask, and 6 mL of 0.6 g / mL citric acid was added. The pH was then adjusted to 5.1 using 25% ammonia. The mixture was then heated and stirred at 80 °C for 90 min at a stirring speed of 300 r / min. After cooling to room temperature, 25% ammonia was added to adjust the pH to 10.1, and the mixture was then centrifuged at 3000 r / min for 5 min. The nanosheets were then washed three times with diluted ammonia solution at pH 10.1 and three times with deionized water. The resulting lower sediment was the citric acid-modified BaHF nanosheets. TEM characterization of the nanosheets is shown below. Figure 1 As shown.

[0029] (2) Chiral liquid crystals (CNCs) were processed. The CNCs were purchased from Nanjing Ruiniu Company (China). First, the CNCs were dialyzed using a membrane with a molecular weight cutoff of 7 kDa, with the water changed daily for three days. The dialyzed CNCs were then ultrasonically dispersed and centrifuged at 8000 rpm for 5 minutes. The supernatant was collected as a stable suspension. This suspension was concentrated to approximately 2.0 wt% by continuous stirring and evaporation. Next, the concentrated CNC suspension was transferred to a long, narrow glass tube and allowed to stand for three days to promote phase separation. The resulting chiral liquid crystal phase, located at the bottom of the tube, was separated as the CNC sample, with a mass fraction of approximately 3.03 wt%. Finally, citric acid was added to achieve a final concentration of 0.5 mM to enhance ionic strength and adjust the pH.

[0030] (3) The ferromagnetic barium ferrite nanosheets obtained in step (1) are uniformly dispersed into the chiral liquid crystal CNC obtained in step (2), with concentrations of 0.1 wt% BaHF and 3 wt% CNC, respectively. The mixed solution is added to a transparent glass container, and the magnetic field gradient is provided by an N52 NdFeB permanent magnet. Assembly is carried out under a weak magnetic field (the maximum value of the magnetic field gradient is 53.2 ± 2.0 mT). After phase separation, the chiral ferromagnetic single-domain liquid crystal material can be obtained, such as... Figure 4 As shown in a~c.

[0031] Example 2 (1) Preparation of ferromagnetic barium ferrite nanosheets The synthesis process was the same as in Example 1, but the reactant ratios were changed: a certain amount of ferric nitrate, barium nitrate, and scandium nitrate were measured, with a molar ratio of n(Ba):n(Fe):n(Sc) = 1:4.5:0.35. The reaction temperature and time were maintained at 270 °C for 30 minutes to obtain the desired nanosheets. Then, 6 mL of 0.6 g / mL citric acid was used for modification by heating at 80 °C for 90 minutes to obtain a suspension that was uniformly dispersed in an aqueous solution.

[0032] (2) The ferromagnetic barium ferrite nanosheets obtained in step (1) were uniformly dispersed into the chiral liquid crystal CNC obtained in step (2) of Example 1, with concentrations of 0.1 wt% BaHF and 4 wt% CNC, respectively. The mixed solution was added to a transparent glass container, and the magnetic field gradient was provided by an N52 NdFeB permanent magnet. Assembly was carried out under a weak magnetic field (the maximum value of the magnetic field gradient was 53.2 ± 2.0 mT). After phase separation, the chiral ferromagnetic single-domain liquid crystal material was obtained, such as... Figure 5 As shown in c in the figure.

[0033] Comparative Example 1 20 nm Fe3O4 spherical particles were dispersed in chiral liquid crystal CNC prepared in step (2) of Example 1, with concentrations of 0.1 wt% Fe3O4 and 4 wt% CNC. The mixture was then placed in a weak magnetic field (maximum magnetic field gradient of 53.2 ± 2.0 mT) for assembly. The results showed that single domains could not be formed. Figure 4 As shown in e, it should be noted that chiral liquid crystals doped with iron oxide do not possess ferromagnetism, and therefore cannot form single domains, nor can they form ferromagnetic single domains.

[0034] Sample Analysis I. Characterization of Ferromagnetic Barium Ferrite Nanosheets Figure 1 The image shows a TEM image of the citric acid-modified ferromagnetic barium ferrite nanosheets prepared in Example 1. As shown, the sample is a hexagonal sheet with a statistically determined lateral particle size of 53.9 ± 14.7 nm (statistical count of 125). The hysteresis loop of the ferromagnetic barium ferrite nanosheets synthesized in Examples 1 and 2 was measured using a vibrating sample magnetometer. Figure 2 As shown, the nanosheets synthesized by the reaction are hard ferromagnets with high remanence and coercivity. Among them, the nanosheets of Example 1 have better magnetic properties due to their superior n(Fe):n(Sc) ratio, specifically in terms of saturation magnetization (~38.3 emu / g), coercivity (~114.4 mT), and remanence (17.1 emu / g).

[0035] II. Formation of Chiral Ferromagnetic Single-Domain Liquid Crystal Materials Figure 3 The figure illustrates the assembly process of a large-area chiral ferromagnetic single-domain structure under a weak magnetic field. Nucleation and crystallization occur in the mixed suspension of CNC and BaHF, forming spherical or ellipsoidal crystalline aggregates. BaHF nanosheets are uniformly dispersed within the helical structure formed by the CNC. When an external magnetic field is applied, the normals of the magnetic nanosheets are parallel to the magnetic field. Due to the anisotropic shape of the magnetic nanosheets, when dispersed in the pseudo-interlayer of the CNC helix, their normals couple with the N* normal of the CNC, causing the entire crystalline aggregate to reorient. The oriented crystalline aggregates fuse together to form large-sized single domains.

[0036] The samples were characterized by polarized light microscopy (POM), and the optical spectra of liquid crystal samples with different concentrations of BaHF were measured by FFT analysis to evaluate their order and stability of single-domain structures.

[0037] Figure 4The images show the characterization and analysis of the chiral ferromagnetic single-domain liquid crystal material formed under a magnetic field gradient in Example 1. The CNC concentration was 3 wt%, and the BaHF concentration was 0.1 wt%. Image a is a polarized image of the CNC+BaHF mixed solution in Example 1 when phase separation reaches quasi-equilibrium. The upper bright portion represents the N* phase, and the lower portion represents the quasi-nematic phase (the unstable phase formed by the I phase under the influence of a magnetic field; removing the magnetic field will restore it to the I phase). Image b represents the IN phase. * Microscopic polarized images of the phase interface show distinct serrated phase boundaries. These serrated geometries represent the soliton-like response of the interface, allowing the magnetization of the ferrofluid to align with the external magnetic field. This alignment minimizes the generation of magnetic charges, while the remaining opposite magnetic charges are localized at the ends of the serrated peaks. In this system, both the PN and N* phases exhibit magnetic responsiveness, leading to the formation of serrated protrusions at the interface. c is a polarized image of the chiral ferromagnetic single-domain liquid crystal material, showing a distinct striped texture. The center-to-center distance between adjacent stripes is half the spiral period of the CNC, i.e., d / 2. d is a SEM image of the chiral ferromagnetic single-domain liquid crystal material, showing a distinct striped structure, indicating the formation of distinct chiral single domains. e is the liquid crystal phase formed after three days of standing in a CNC suspension with 0.1 wt% spherical paramagnetic particles (Fe3O4). The sample shows crystalline aggregates dispersed in the I phase, without fusing to form a single-domain structure. Some scholars have reported on the mixed system of Fe3O4 and CNC, in which the fused liquid crystal domains have a large number of surface defects and grain boundaries, and do not have obvious single domain characteristics.

[0038] Figure 5 The chiral ferromagnetic domains obtained in Example 2 were analyzed and characterized. Figure a shows the hysteresis loop of the chiral ferromagnetic domains tested using a magnetic measurement system (MPMS). The results show that it has remanence (intersection with the Y-axis, approximately 0.032 emu / g) and magnetic coercivity (intersection with the X-axis, approximately 130 Oe), indicating that the obtained domains have significant ferromagnetism and are chiral ferromagnetic domains. The structure of this domain was also formalized by Fourier rapid change (FFT) analysis of its polarized texture, as shown below. Figure 5 As shown in b, it exhibits a distinct zero-frequency peak (central bright spot) and multiple high-frequency scattering peaks, indicating excellent periodicity and order. A magnetic field (approximately 40 mT) is applied to this ferromagnetic single-domain liquid crystal, as... Figure 5 As shown in c~d, it exhibits obvious topological deformation and has magneto-optical response characteristics, which lay the foundation for its application in fields such as intelligent soft machines and optical devices.

[0039] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a chiral ferromagnetic single-domain lyotropic liquid crystal material, characterized in that, Includes the following steps: (1) Sc-doped barium ferrite nanosheets were synthesized by hydrothermal method, and the nanosheets were modified with citric acid to obtain ferromagnetic barium ferrite nanosheets. (2) The ferromagnetic barium ferrite nanosheets obtained in step (1) are uniformly dispersed in a chiral liquid crystal matrix and assembled by a weak magnetic field to obtain a chiral ferromagnetic single-domain lyotropic liquid crystal material. The strength of the magnetic field is ≥30 mT.

2. The method for preparing the chiral ferromagnetic single-domain lyotropic liquid crystal material according to claim 1, characterized in that, In step (1), the temperature of the hydrothermal method is 200~280 ℃ and the time is 0.5~2 h.

3. The method for preparing the chiral ferromagnetic single-domain lyotropic liquid crystal material according to claim 1, characterized in that, In step (1), the size of the Sc-doped barium ferrite nanosheet is 10~100 nm and the thickness is 2~10 nm.

4. The method for preparing the chiral ferromagnetic single-domain lyotropic liquid crystal material according to claim 1, characterized in that, In step (1), the magnetic saturation intensity of the Sc-doped barium ferrite nanosheet is 20~45 emu / g.

5. The method for preparing the chiral ferromagnetic single-domain lyotropic liquid crystal material according to claim 1, characterized in that, In step (1), the mass ratio of citric acid to Sc-doped barium ferrite nanosheets is 1~15:

1.

6. The method for preparing the chiral ferromagnetic single-domain lyotropic liquid crystal material according to claim 1, characterized in that, In step (2), the chiral liquid crystal matrix is ​​a chiral modified, doped, or inherently chiral nanomaterial.

7. The method for preparing the chiral ferromagnetic single-domain lyotropic liquid crystal material according to claim 6, characterized in that, The chiral liquid crystal matrix is ​​at least one of cellulose nanocrystals, carbon nanotubes, graphene oxide, and Mxene.

8. The method for preparing chiral ferromagnetic single-domain lyotropic liquid crystal materials according to claim 1, characterized in that, In step (2), the mass ratio of the ferromagnetic barium ferrite nanosheets to the chiral liquid crystal matrix is ​​0.01~1:0.1~5.

9. The chiral ferromagnetic single-domain lyotropic liquid crystal material prepared by the preparation method according to any one of claims 1 to 8.

10. The application of the chiral ferromagnetic single-domain lyotropic liquid crystal material according to claim 9 in the preparation of optical devices and flexible machines.

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