Preparation method and application of hollow silicon dioxide microspheres and photonic crystal film

Abstract

The invention provides a preparation method and application of hollow silicon dioxide microspheres and a photonic crystal film. A cationic polymer is used as a template; a weak acid electrolyte and a weak base exist in the reaction system, and the pH value of the reaction system is controlled to be 8.0-9.5; in the reaction system, a silicon source precursor is subjected to a hydrolytic condensation reaction on the surface of the template, and a core-shell structure of a silicon dioxide coated polymer is formed through dual effects of charge shielding and hydrogen bond crosslinking of a weak acid electrolyte; a solvent is adopted to dissolve the polymer in the core-shell structure to obtain the hollow silicon dioxide microspheres, and the hollow silicon dioxide microspheres are low in porosity, high in compactness and good in mechanical property and have good application prospects in the technical fields of drug controlled release, photonic crystals or catalytic carriers and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of inorganic nanomaterial preparation, and specifically relates to a preparation method and application of hollow silica microspheres and a photonic crystal film. Background Art

[0002] Due to its unique structural properties such as high specific surface area, low density, and functionalizable surface, hollow silica microspheres have important applications in the fields of catalysis, biomedicine, photonics, etc.

[0003] However, there are some technical barriers in the existing preparation technologies of hollow silica microspheres. For example, the traditional method mainly relies on strong alkaline conditions (pH>11), which results in a porous shell (porosity>3%), poor mechanical properties (elastic modulus<0.5GPa), and it is difficult to accurately control the hollowness rate; in the traditional hard template method, high temperature or strong corrosive reagents (such as hydrofluoric acid) are required to remove the template during the template removal process, which will cause damage to the shell structure; in addition, when using cationic polymerization spheres as templates to prepare hollow silica spheres, the high surface charge thereof causes excessive aggregation of silicate precursors, resulting in a loose and porous shell.

[0004] Therefore, for hollow silica microspheres, exploring the uniform growth of the silica coating layer on the surface of hollow silica microspheres in a near-neutral environment, reducing the shell porosity and improving its denseness is a key technical problem that needs to be urgently solved by those skilled in the art. This challenge not only concerns the precise control of the microscopic structure of the material, but also determines the core bottleneck for the high-performance application of hollow silica microspheres in frontier fields such as the optical field, fillers for 5G communication equipment, and drug loading in biomedicine. Summary of the Invention

[0005] In view of the above technical status quo, the present invention provides a preparation method of hollow silica microspheres. This method is simple and easy to control, and the silica shell of the hollow silica microspheres prepared by using this method is uniform, has a low porosity, and high denseness.

[0006] The technical solution provided by the present invention is: a preparation method of hollow silica microspheres, comprising the following steps:

[0007] (1) Using a cationic polymer as a template; there is a weak acid electrolyte and a weak base in the reaction system, and the pH of the reaction system is controlled in the range of 8.0-9.5 by the addition amounts of the weak acid electrolyte and the weak base; in this reaction system, the silicon source precursor undergoes a hydrolysis and condensation reaction on the surface of the template to form a core-shell structure of silica-coated polymer;

[0008] (2) Using a solvent to dissolve the polymer in the core-shell structure to obtain hollow silica microspheres.

[0009] The cationic polymer refers to a polymer with a positive charge on its surface. The preparation method of the cationic polymer is not limited. As one implementation, the cationic polymer is prepared by a polymerization reaction. As one implementation, the polymerization reaction system includes a main monomer, a cationic monomer, and an initiator. Under the action of the initiator, the main monomer undergoes a polymerization reaction, and at the same time, the cationic monomer participates in copolymerization to obtain a polymer with cations.

[0010] The main monomer is not limited and can be at least one of methyl methacrylate (MMA), styrene, and acrylic acid.

[0011] The cationic monomer refers to a monomer with a cationic group, preferably a monomer with a quaternary ammonium salt group, such as at least one of acryloyloxyethyl trimethyl ammonium chloride (DMC), methacryloyloxyethyl trimethyl ammonium chloride (METAC), dimethyldiallyl ammonium chloride, vinyl pyridine, and acrylamidomethylpropanesulfonic acid (AMPS).

[0012] The initiator is not limited. For example, it can be at least one of azobis (2 - amidinopropane) hydrochloride (AIBA), azobisisobutyronitrile amidine (AIBN), azobis (2 - isobutylimidazoline) hydrochloride (VA - 044), and azodicyanovaleric acid (ACVA).

[0013] In the polymerization reaction system, the molar content of the main monomer is higher than that of the cationic monomer. By controlling the molar content of the cationic monomer in the polymerization reaction system, the content of positive charges in the cationic polymer can be controlled. Preferably, the molar ratio of the main monomer to the cationic monomer is preferably 4:1 - 10:1, more preferably 4:1 - 6:1.

[0014] In the polymerization reaction system, the masses of the main monomer, the cationic monomer, and the initiator are determined according to the actual mass of the cationic polymer. In some embodiments, the mass of the main monomer accounts for 20% - 90%, the mass of the cationic monomer accounts for 10% - 20%, and the mass of the initiator accounts for 0.5% - 2%.

[0015] The particle size of the cationic polymer can be adjusted by adjusting one or several of the amount of the main monomer used, the amount of the initiator used, the reaction temperature, etc. In the present invention, the particle size of the cationic polymer is preferably 300 nm - 800 nm.

[0016] The weak acid electrolyte is a carboxylic acid compound and can specifically be selected from at least one of acetic acid, propionic acid, citric acid, benzoic acid, carbonic acid, hypochlorous acid, sulfurous acid, phosphoric acid, nitrous acid, hydrosulfuric acid, silicic acid, boric acid, citric acid, lactic acid, oxalic acid, salicylic acid, tartaric acid, malic acid, phenol, amino acids, etc.

[0017] The weak base is not limited, and for example, it can be at least one of dimethylaminoethanol (DMAE), ammonia water, methylamine, ethylamine, propylamine, pyridine, urea, triethylamine, aniline, choline, aluminum hydroxide, magnesium hydroxide, zinc hydroxide, iron hydroxide, copper hydroxide, sodium bicarbonate, etc.

[0018] The silicon source precursor is not limited and can be selected from at least one of tetraethyl orthosilicate (TEOS), tetramethoxysilane (TMOS), sodium silicate, silicon tetrachloride, methyltrimethoxysilane, phenyltrimethoxysilane, vinyltrimethoxysilane, trichlorosilane, methyltriethoxysilane, dimethyldiethoxysilane.

[0019] In step (1), preferably, the concentration of the weak acid electrolyte in the reaction system is controlled at 0.1 M - 1 M, and more preferably 0.3 M - 0.5 M.

[0020] In step (1), the reaction temperature of the hydrolysis and condensation reaction is not limited. The present invention can carry out the hydrolysis and condensation reaction at a mild reaction temperature, for example, it can be 20°C - 80°C, preferably 30°C - 50°C.

[0021] In step (1), the reaction time of the hydrolysis and condensation reaction is not limited. For example, it can be 6 h - 24 h.

[0022] In step (1), in order to improve the stability of the reaction system, preferably, the Zeta potential of the cationic polymer is in the range of +30 mV to +50 mV.

[0023] In step (1), as a way of implementation, a solvent and a cationic polymer are added to the reaction system, mixed evenly, then a weak acid electrolyte and a weak base are added to control the pH of the reaction system at 8.0 - 9.5, and then a silicon source precursor is added, and the hydrolysis and condensation reaction occurs. The solvent is not limited and can be one or several of ethanol, water, methanol, n-propanol, isopropanol. For example, the solvent is a mixed solvent obtained by mixing ethanol and water, and the volume ratio of ethanol to water is adjusted according to the actual reaction requirements, for example, it can be 3:1, etc.

[0024] In some embodiments, the silicon source precursor is added dropwise to the reaction system, and the dropping rate is preferably 0.1 mL / min - 0.5 mL / min.

[0025] In some embodiments, after the hydrolysis and condensation reaction is completed, the reaction product is separated and collected by centrifugation, and then the template is removed by the solvent dissolution method.

[0026] In step (2), the selected solvent has a solubility parameter similar to that of the cationic polymer, but has a significant difference in solubility from the silica shell, enabling the cationic polymer to dissolve in this solvent while the silica shell does not dissolve. For example, in some embodiments, the selected cationic polymer is PMMA, and tetrahydrofuran, acetone, etc. are selected as solvents; in some embodiments, the selected cationic polymer is polystyrene PS, and toluene, ethylbenzene, etc. are selected as solvents.

[0027] In step (2), after the polymer in the core-shell structure is dissolved, it is preferably subjected to one or more of subsequent treatments such as filtration, washing, and drying to obtain hollow silica microspheres.

[0028] In the present invention, the hollowness rate of the hollow silica microspheres is calculated by the following formula:

[0029] η = v 空 / v 总

[0030] where η is the hollowness rate, V 总 is the total volume of the entire hollow silica microsphere (including the volume of the hollow part and the silica shell), and V 空 is the volume of the hollow part of the hollow silica microsphere.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] (1) The present invention utilizes the cation-electrolyte synergistic action mechanism and combines charge shielding-hydrogen bond crosslinking to prepare hollow silica microspheres with controllable wall thickness.

[0033] (2) The present invention discovers that during the preparation process, when the silane precursor undergoes hydrolysis and condensation reactions on the surface of the cationic polymer, the key to achieving the densification of the silica shell is to synergistically regulate the pH of the reaction system to 8.0 - 9.5 by the weak acid electrolyte and the weak base. Under this condition, the charge shielding and hydrogen bond interactions can effectively balance the hydrolysis and polycondensation rates of the silane, promoting the uniform deposition and directional crosslinking of the silane on the polymer surface to form a dense silica shell. When a strong base is used instead of the weak base, or a metal salt is used instead of the weak acid electrolyte, or a weak acid electrolyte is added to the reaction system without adding a weak base, or a weak base is added to the reaction system without adding a weak acid electrolyte, or the pH of the reaction system is controlled outside the range of 8.0 - 9.5, the balance between the hydrolysis and polycondensation of the silane will be broken, resulting in the transformation of the silica growth mode from monomer addition to aggregation-controlled growth, and finally forming a loose and fragile coating shell.

[0034] (3) The silica shell of the hollow silica microspheres prepared by the present invention has compactness, and its porosity can be less than 0.3%, even less than 0.2%; the elastic modulus can be greater than 1 GPa, preferably greater than 5 GPa, and even greater than 8 GPa, solving the problems of large porosity and poor mechanical strength of the existing hollow silica shell.

[0035] (4) The hollowness rate of the hollow silica microspheres prepared by the present invention can be greater than or equal to 60%, even greater than or equal to 70%; the shell thickness can be controlled within 10 nm - 100 nm, for example, it can be controlled within 15 nm - 50 nm; the average pore diameter can be less than or equal to 3 nm.

[0036] (5) The hollow silica microspheres prepared by the present invention have application prospects in the fields of drug controlled release, photonic crystals, and catalytic carriers.

[0037] (6) The preparation method of the present invention is simple and easy to implement, realizing the uniform deposition of silica under near-neutral conditions, avoiding the high alkali requirement in the traditional method.

[0038] (7) The hollow silica microspheres prepared by the preparation method of the present invention have good compatibility with polymer polymers and can be filled in polymer polymers to prepare photonic crystal films. Preferably, the mass of the hollow silica microspheres accounts for 5% - 20% of the mass of the film. Using a UV spectrophotometer to measure the transmittance of the photonic crystal film filled with the hollow silica microspheres and the film without filling the hollow silica microspheres in the visible light range, it is found that compared with the film without filling the hollow silica microspheres, the transmittance of the photonic crystal film filled with the hollow silica microspheres decreases, but the decrease amplitude is small. For example, in some embodiments, when the hollow silica microspheres are filled in polydimethylsiloxane (PDMS) to prepare a photonic crystal film, where the filling rate of the hollow silica microspheres is 10% and the thickness of the prepared photonic crystal is less than or equal to 100 μm, the decrease in transmittance is less than 10%, and some can reach less than 5%, and even less than 1%. The reason is that on the one hand, the light scattering and absorption ability of the hollow silica microspheres is relatively limited; on the other hand, from the perspective of material compatibility, the small decrease in transmittance indicates that the hollow silica microspheres and polymer polymers have good compatibility. If the compatibility between the two is poor, there are often more defects or scattering centers at the interface, which will lead to a significant increase in light scattering and a significant decrease in transmittance. Therefore, the photonic crystal film can be applied to technical fields such as optical devices, solar cells, and automotive glass coatings, display screen protection coatings. Description of the Drawings

[0039] Figure 1It is the SEM image of the cationic PMMA spheres in Example 1.

[0040] Figure 2 It is the Zeta potential diagram of the cationic PMMA spheres in Example 1.

[0041] Figure 3 It is the SEM image of the hollow silica spheres prepared by the cation - electrolyte synergistic method in Example 1.

[0042] Figure 4 It is the TEM image of the hollow silica spheres prepared by the cation - electrolyte synergistic method in Example 1.

[0043] Figure 5 It is the SEM image of the hollow silica spheres prepared by the cation - electrolyte synergistic method in Comparative Example 1.

[0044] Figure 6 It is the TEM image of the hollow silica spheres prepared by the cation - electrolyte synergistic method in Comparative Example 1.

[0045] Figure 7 It is the nitrogen adsorption - desorption isotherm curve of the hollow silica spheres prepared by the cation - electrolyte synergistic method in Example 1 and Comparative Example 1. Among them, the curve with black hollow circle symbols is the nitrogen adsorption - desorption isotherm curve without adding acetic acid in Comparative Example 1, and the curve with black square symbols is the nitrogen adsorption - desorption curve distribution with adding acetic acid in Example 1.

[0046] Figure 8 It is the pore size distribution diagram of the hollow silica spheres prepared by the cation - electrolyte synergistic method in Example 1 and Comparative Example 1. Among them, the curve with black hollow circle symbols is the pore size distribution diagram without adding acetic acid in Comparative Example 1, and the curve with black square symbols is the pore size distribution diagram with adding acetic acid in Example 1. Detailed implementation mode

[0047] The present invention will be further described in detail below in conjunction with the embodiments. It should be noted that the following embodiments are intended to facilitate the understanding of the present invention, and non - essential improvements and adjustments made by those skilled in the art based on the content of the present invention still fall within the protection scope of the present invention.

[0048] In the present invention, the use of words such as "including" and "comprising" should be interpreted as having an inclusive meaning rather than an exclusive or exhaustive meaning; that is, it means "including but not limited to".

[0049] Example 1:

[0050] (1) Add 300 g of MMA and 2 g of DMC to 1500 g of water, stir at a speed of 30 rpm / min at 70 °C for 1 h. Subsequently, disperse 2 g of AIBA in 20 g of water and slowly drop it into the reaction solution at a speed of 0.5 mL / min. Polymerization reaction occurs at 70 °C for 8 h to obtain a reaction product with a solid content of 14.5%. The solid reaction product is cationic PMMA spheres with a particle size of 400 nm - 500 nm. Its SEM image is as shown in Figure 1 shown, and the Zeta potential image is as shown in Figure 2 shown, showing that its Zeta potential is 45.45 mV.

[0051] (2) Disperse 17.24 g of the cationic PMMA spheres obtained in step (1) in a mixed solution of 1000 g of ethanol and 500 g of water and stir evenly at a speed of 300 rpm / min. Subsequently, add 3 g of acetic acid as a weak electrolyte, control the concentration of acetic acid in the reaction system to be 0.03 M, and add dimethylaminoethanol to make the pH of the reaction system 8.1 and the conductivity 2600 μs / cm. Then, drop 10 g of TEOS at a speed of 0.8 mL / min, and allow the silane to hydrolyze and polycondense at 40 °C for 6 h and coat on the cationic PMMA spheres. After aging overnight, PMMA@SiO2 composite microspheres are obtained.

[0052] (3) At room temperature, disperse the PMMA@SiO2 composite microspheres obtained in step (2) in a certain amount of tetrahydrofuran. Using the principle that the solubility parameters of tetrahydrofuran and PMMA are similar, selectively dissolve PMMA in tetrahydrofuran for 24 h. Subsequently, calcine the dried sample at 300 °C for 2 h to obtain hollow silica microspheres.

[0053] Use SEM and TEM to test the morphology of the obtained hollow silica microspheres. The SEM image is as shown in Figure 3 shown, and the TEM image is as shown in Figure 4 shown. The SEM image shows that in the hollow silica microspheres, the silica shell layer is uniformly coated, forming a dense coating structure. The particle size of the hollow silica microspheres is 446 nm. The TEM image shows that the wall thickness of the hollow silica microspheres is 23 nm.

[0054] The hollowness of the hollow silica microspheres can be calculated by the following formula:

[0055] η = v 空 / v 总

[0056] where η is the hollowness, V 空 is the volume of the hollow part, and V 总 is the total volume of the entire hollow silica microsphere (including the volume of the hollow part and the silica shell layer).

[0057] The hollowness of the hollow silica microspheres was calculated to be 72%.

[0058] The nitrogen adsorption - desorption isotherm and pore size distribution of the hollow silica microspheres were measured, and the results are as Figure 7 、 8 shown, showing that its specific surface area is 328 m 2 / g, the pore volume is 0.34 cm 3 / g, the pore diameter is 2.68 nm, the porosity was calculated to be 0.158%, and the elastic modulus measured by AFM was 10 GPa.

[0059] The hollow silica microspheres were embedded in polydimethylsiloxane (PDMS) to prepare a composite film, where the filling rate of the microspheres was 10%, and the thickness of the prepared composite film was less than or equal to 100 μm. Using a UV - visible spectrophotometer, it was measured that: under the same other preparation conditions, the transmittance of the polydimethylsiloxane (PDMS) film prepared without embedding the hollow silica microspheres in the visible light range was 99.95%, and the transmittance of the composite film prepared by embedding the hollow silica microspheres in the visible light range was 99.89%.

[0060] Example 2:

[0061] (1) 500 g of MMA and 2 g of DMC were added to 1500 g of water, and the mixture was stirred at 70 °C at a speed of 30 rpm / min for 1 h. Subsequently, 2 g of AIBA was dispersed in 20 g of water and slowly added dropwise to the reaction solution at a speed of 0.5 mL / min. The reaction was carried out at 70 °C for 8 h to obtain a reaction product with a solid content of 20%. The solid reaction product was cationic PMMA spheres with a size of 700 nm - 800 nm, and its Zeta potential was 44 mV.

[0062] (2) 12.5 g of the cationic PMMA spheres obtained in step (1) were dispersed in a mixed solution of 1000 g of ethanol and 500 g of water and stirred evenly at a speed of 300 rpm / min. Subsequently, 2 g of acetic acid was added as a weak electrolyte to control the concentration of acetic acid in the reaction system to be 0.03 M, and dimethylaminoethanol was added to make the pH of the reaction system 8.1 and the conductivity 2600 μs / cm. Then, 10 g of TEOS was added dropwise at a speed of 0.8 mL / min, and the silane was hydrolyzed and polycondensed at 40 °C for 6 h and coated on the cationic PMMA spheres. After aging overnight, PMMA@SiO2 composite microspheres were obtained.

[0063] (3) At room temperature, the PMMA@SiO2 composite microspheres obtained in step (2) were dispersed in a certain amount of tetrahydrofuran. Based on the principle that the solubility parameters of tetrahydrofuran and PMMA are similar, PMMA was selectively dissolved in tetrahydrofuran for 24 h. Subsequently, the dried sample was calcined at 300 °C for 2 h to obtain hollow silica microspheres.

[0064] The morphology of the obtained hollow silica microspheres was tested by SEM and TEM. The SEM images showed that, similar to Example 1, the silica shell layer in the hollow silica microspheres was uniformly coated to form a dense coating structure. The particle size of the hollow silica microspheres was 768 nm. The TEM images showed that the wall thickness of the hollow silica was 14 nm.

[0065] The hollowness of the hollow silica microspheres was calculated to be 83%.

[0066] The nitrogen adsorption-desorption isotherms and pore size distribution curves of the hollow silica microspheres were tested, showing that its specific surface area was 368 g / m 2 , the pore volume was 0.42 cm 3 / g, the pore size was 2.72 nm, and its porosity was calculated to be 0.170%. The elastic modulus measured by AFM was 8.6 GPa.

[0067] The hollow silica microspheres were embedded in polydimethylsiloxane (PDMS) to prepare a photonic crystal film, where the filling rate of the microspheres was 10%, and the thickness of the prepared photonic crystal film was less than or equal to 100 μm. Using a UV spectrophotometer to test, it was obtained that: under the same other preparation conditions, the transmittance of the polydimethylsiloxane (PDMS) film prepared without embedding the hollow silica microspheres in the visible light range was 99.95%, and the transmittance of the photonic crystal film prepared by embedding the hollow silica microspheres in the visible light range was 99.65%.

[0068] Example 3:

[0069] (1) 300 g of MMA and 2 g of DMC were added to 1500 g of water, and stirred at 70 °C at a speed of 30 rpm / min for 1 h. Subsequently, 2 g of AIBA was dispersed in 20 g of water and slowly added dropwise to the reaction solution at a speed of 0.5 mL / min, and reacted at 70 °C for 8 h to obtain a reaction product with a solid content of 14.5%. The solid reaction product was cationic PMMA spheres with a particle size of 400 nm - 500 nm, and its Zeta potential was 45.45 mV.

[0070] (2) Disperse 17.24 g of the cationic PMMA obtained in step (1) in a mixed solution of 1000 g of ethanol and 500 g of water, stir evenly at a speed of 300 rpm / min, then add 4 g of propionic acid as a weak electrolyte, control the concentration of acetic acid in the reaction system to be 0.03 M, and add dimethylaminoethanol to make the pH of the reaction system 8.1 and the conductivity 2598 μs / cm. Then, dropwise add 10 g of TEOS at a speed of 0.8 mL / min, and allow the silane to hydrolyze and polycondense at 40 °C for 6 h and coat on the cationic PMMA spheres. After aging overnight, PMMA@SiO2 composite microspheres are obtained.

[0071] (3) At room temperature, disperse the PMMA@SiO2 composite microspheres obtained in step (2) in a certain amount of tetrahydrofuran. Using the principle that the solubility parameters of tetrahydrofuran and PMMA are similar, selectively dissolve PMMA in tetrahydrofuran for 24 h. Then, calcine the dried sample at 300 °C for 2 h to obtain hollow silica microspheres.

[0072] Use SEM and TEM to test the morphology of the obtained hollow silica microspheres. The SEM image shows that, similar to Example 1, the silica shell layer in the hollow silica microspheres is uniformly coated, forming a dense coating structure. The particle size of the hollow silica microspheres is 450 nm, and the wall thickness is 25 nm.

[0073] Calculate that the hollowness rate of the hollow silica microspheres is 70.2%.

[0074] Test the nitrogen adsorption-desorption isotherm and pore size distribution map of the hollow silica microspheres, showing that its specific surface area is 348 g / m 2 , the pore volume is 0.43 cm 3 / g, the pore diameter is 2.86 nm, calculate its porosity to be 0.173%, and the elastic modulus measured by AFM is 8.4 GPa.

[0075] Embed the hollow silica microspheres into polydimethylsiloxane (PDMS) to prepare a photonic crystal film, where the filling rate of the microspheres is 10%, and the thickness of the prepared photonic crystal film is less than or equal to 100 μm. Using a UV spectrophotometer to test, it is obtained that: when other preparation conditions are the same, the transmittance of the polydimethylsiloxane (PDMS) film prepared without embedding the hollow silica microspheres in the visible light range is 99.95%, and the transmittance of the photonic crystal film prepared by embedding the hollow silica microspheres in the visible light range is 99.75%.

[0076] Comparative Example 1:

[0077] (1) 300 g of MMA and 2 g of DMC were added to 1500 g of water, and the mixture was stirred at 70 °C at a speed of 30 rpm / min for 1 h. Subsequently, 2 g of AIBA was dispersed in 20 g of water and slowly added dropwise to the reaction solution at a speed of 0.5 mL / min. The reaction was carried out at 70 °C for 8 h to obtain a reaction product with a solid content of 14.5%. The solid reaction product was cationic PMMA spheres with a particle size of 400 nm - 500 nm and a Zeta potential of 45.45 mV.

[0078] (2) 17.24 g of the cationic PMMA spheres obtained in step (1) were dispersed in a mixed solution of 1000 g of ethanol and 500 g of water and stirred evenly at a speed of 300 rpm / min. Subsequently, dimethylaminoethanol was added to make the pH of the reaction system 8.1 and the conductivity 900 μs / cm. Then, 10 g of TEOS was added dropwise at a speed of 0.8 mL / min, and the silane was hydrolyzed and polycondensed at 40 °C for 6 h and coated on the cationic PMMA spheres. After aging overnight, PMMA@SiO2 composite microspheres were obtained.

[0079] (3) At room temperature, the PMMA@SiO2 composite microspheres obtained in step (2) were dispersed in a certain amount of tetrahydrofuran. Using the principle that the solubility parameters of tetrahydrofuran and PMMA are similar, PMMA was selectively dissolved in tetrahydrofuran for 24 h. Subsequently, the dried sample was calcined at 300 °C for 2 h to obtain hollow silica microspheres.

[0080] The morphology of the obtained hollow silica microspheres was tested by SEM and TEM. The SEM image is as Figure 5 shown, and the TEM image is as Figure 6 shown. The SEM image shows that compared with Example 1, the uniformity of the silica shell layer in the hollow silica microspheres is reduced, and the compactness of the coating structure is reduced. The particle size of the hollow silica microspheres is 454 nm, and the TEM shows that its wall thickness is 27 nm.

[0081] The hollowness of the hollow silica microspheres was calculated to be 68.3%.

[0082] The nitrogen adsorption - desorption isotherm and pore size distribution of the hollow silica microspheres were tested, showing that its specific surface area is 618 g / m 2 , the pore volume is 1.29 cm 3 / g, the pore diameter is 2.99 nm, the porosity was calculated to be 0.283%, and the elastic modulus measured by AFM is 0.4 GPa.

[0083] The hollow silica microspheres are embedded in polydimethylsiloxane (PDMS) to prepare a photonic crystal film, where the filling rate of the microspheres is 10%, and the thickness of the prepared photonic crystal film is less than or equal to 100 μm. It is measured by an ultraviolet spectrophotometer that: when other preparation conditions are the same, the transmittance of the polydimethylsiloxane (PDMS) film prepared without embedding the hollow silica microspheres in the visible light range is 99.95%, and the transmittance of the photonic crystal film prepared by embedding the hollow silica microspheres in the visible light range is 92.2%.

[0084] Comparative Example 2:

[0085] (1) 300 g of MMA and 2 g of DMC are added to 1500 g of water, and stirred at 70 °C at a speed of 30 rpm / min for 1 h. Subsequently, 2 g of AIBA is dispersed in 20 g of water and slowly added dropwise to the reaction solution at a speed of 0.5 mL / min. Polymerization reaction occurs at 70 °C for 8 h to obtain a reaction product with a solid content of 14.5%. The solid reaction product is cationic PMMA spheres with a particle size of 400 nm - 500 nm, and its Zeta potential is 45.45 mV.

[0086] (2) 17.24 g of the cationic PMMA spheres obtained in step (1) are dispersed in a mixed solution of 1000 g of ethanol and 500 g of water and stirred evenly at a speed of 300 rpm / min. Subsequently, 3 g of acetic acid is added as a weak electrolyte, and the concentration of acetic acid in the reaction system is controlled to be 0.03 M, so that the pH of the reaction system is 4.56 and the conductivity is 2600 μs / cm. Subsequently, 10 g of TEOS is added dropwise at a speed of 0.8 mL / min, and the silane is hydrolyzed and polycondensed at 40 °C for 6 h and coated on the cationic PMMA spheres. After aging overnight, PMMA@SiO2 composite microspheres are obtained.

[0087] (3) At room temperature, the PMMA@SiO2 composite microspheres obtained in step (2) are dispersed in a certain amount of tetrahydrofuran. Using the principle that the solubility parameters of tetrahydrofuran and PMMA are similar, PMMA is selectively dissolved in tetrahydrofuran for 24 h. Subsequently, the dried sample is calcined at 300 °C for 2 h to obtain hollow silica microspheres.

[0088] The morphology of the obtained hollow silica microspheres is tested by SEM and TEM. The SEM image shows that, similar to Comparative Example 1, the silica shell layer in the hollow silica microspheres is loose, and the uniformity and density of the coating structure are low. The particle size of the hollow silica microspheres is 454 nm, and the wall thickness is 27 nm.

[0089] It is calculated that the hollowness rate of the hollow silica microspheres is 68.3%.

[0090] The nitrogen adsorption-desorption isotherm and pore size distribution of the hollow silica microspheres were tested, showing a specific surface area of 356 m 2 / g, a pore volume of 0.56 cm 3 / g, a pore diameter of 2.90 nm, a calculated porosity of 0.169%, and an elastic modulus of 1.0 GPa measured by AFM.

[0091] The hollow silica microspheres were embedded in polydimethylsiloxane (PDMS) to prepare a photonic crystal film, with a filling rate of 10% of the microspheres. The thickness of the prepared photonic crystal film was less than or equal to 100 μm. Using a UV spectrophotometer, it was measured that when other preparation conditions were the same, the transmittance of the polydimethylsiloxane (PDMS) film prepared without embedding the hollow silica microspheres in the visible light range was 99.95%, and the transmittance of the photonic crystal film prepared by embedding the hollow silica microspheres in the visible light range was 99.65%.

[0092] Comparative Example 3:

[0093] (1) 300 g of MMA and 2 g of DMC were added to 1500 g of water, and stirred at 70 °C at a speed of 30 rpm / min for 1 h. Subsequently, 2 g of AIBA was dispersed in 20 g of water and slowly added dropwise to the reaction solution at a speed of 0.5 mL / min. The reaction was carried out at 70 °C for 8 h to obtain a reaction product with a solid content of 14.5%. The solid reaction product was cationic PMMA spheres with a particle size of 400 nm - 500 nm and a Zeta potential of 45.45 mV.

[0094] (2) 17.24 g of the cationic PMMA spheres obtained in step (1) were dispersed in a mixed solution of 1000 g of ethanol and 500 g of water and stirred evenly at a speed of 300 rpm / min. Subsequently, 3 g of acetic acid was added as a weak electrolyte to control the concentration of acetic acid in the reaction system to 0.03 M, and sodium hydroxide was added to make the pH of the reaction system 8.1 and the conductivity 3200 μs / cm. Then, 10 g of TEOS was added dropwise at a speed of 0.8 mL / min, and the silane was hydrolyzed and polycondensed at 40 °C for 6 h and coated on the cationic PMMA spheres. After aging overnight, PMMA@SiO2 composite microspheres were obtained.

[0095] (3) At room temperature, the PMMA@SiO2 composite microspheres obtained in step (2) were dispersed in a certain amount of tetrahydrofuran. Using the principle that the solubility parameters of tetrahydrofuran and PMMA are similar, PMMA was selectively dissolved in tetrahydrofuran for 24 h. Subsequently, the dried sample was calcined at 300 °C for 2 h to obtain hollow silica microspheres.

[0096] The morphology of the obtained hollow silica microspheres was tested using SEM and TEM. The SEM images showed that, similar to Comparative Example 1, the silica shell layer in the hollow silica microspheres was loose, and the uniformity and compactness of the coating structure were low. The particle size of the hollow silica microspheres was 452 nm, and the wall thickness was 26 nm.

[0097] The hollowness of the hollow silica microspheres was calculated to be 69.3%.

[0098] The nitrogen adsorption-desorption isotherms and pore size distribution curves of the hollow silica microspheres were tested, showing that their specific surface area was 578 g / m 2 , the pore volume was 0.75 cm 3 / g, the pore size was 2.93 nm, the porosity was calculated to be 0.176%, and the elastic modulus measured by AFM was 1.1 GPa.

[0099] The hollow silica microspheres were embedded in polydimethylsiloxane (PDMS) to prepare a photonic crystal film, where the filling rate of the microspheres was 10%, and the thickness of the prepared photonic crystal film was less than or equal to 100 μm. Using a UV spectrophotometer, it was measured that when other preparation conditions were the same, the transmittance of the polydimethylsiloxane (PDMS) film prepared without embedding the hollow silica microspheres in the visible light range was 99.95%, and the transmittance of the photonic crystal film prepared by embedding the hollow silica microspheres in the visible light range was 96.32%.

[0100] Comparative Example 4:

[0101] (1) 300 g of MMA and 2 g of DMC were added to 1500 g of water, and the mixture was stirred at 70 °C at a speed of 30 rpm / min for 1 h. Subsequently, 2 g of AIBA was dispersed in 20 g of water and slowly added dropwise to the reaction solution at a speed of 0.5 mL / min. The reaction was carried out at 70 °C for 8 h to obtain a reaction product with a solid content of 14.5%. The solid reaction product was cationic PMMA spheres with a particle size of 400 nm - 500 nm, and its Zeta potential was 45.45 mV.

[0102] (2) 17.24 g of the cationic PMMA spheres obtained in step (1) were dispersed in a mixed solution of 1000 g of ethanol and 500 g of water and stirred evenly at a speed of 300 rpm / min. Subsequently, NaCl was added as a weak electrolyte, and the concentration of NaCl in the reaction system was controlled to be 0.2 M. Dimethylaminoethanol was added to make the pH of the reaction system 8.1 and the conductivity 3340 μs / cm. Then, 10 g of TEOS was added dropwise at a speed of 0.8 mL / min, and the silane was hydrolyzed and polycondensed at 40 °C for 6 h and coated on the cationic PMMA spheres. After aging overnight, PMMA@SiO2 composite microspheres were obtained.

[0103] (3) Disperse the obtained PMMA@SiO2 composite microspheres in a certain amount of tetrahydrofuran. Utilize the principle that the solubility parameters of tetrahydrofuran and PMMA are similar to selectively dissolve PMMA in tetrahydrofuran for 24 h. Subsequently, calcine the dried sample at 300 °C for 2 h to obtain hollow silica microspheres.

[0104] Use SEM and TEM to test the morphology of the obtained hollow silica microspheres. The SEM image shows that, similar to Comparative Example 1, the silica shell layer in the hollow silica microspheres is loose, and the uniformity and density of the coating structure are low. The particle size of the hollow silica microspheres is 468 nm, and the wall thickness is 34 nm.

[0105] The calculated hollowness of the hollow silica microspheres is 62.4%.

[0106] Test the nitrogen adsorption-desorption isotherm and pore size distribution diagram of the hollow silica microspheres, showing that its specific surface area is 623 g / m 2 , the pore volume is 1.34 cm 3 / g, the pore diameter is 2.99 nm, calculate its porosity to be 0.295%, and the elastic modulus measured by AFM is 0.8 GPa.

[0107] Embed the hollow silica microspheres into polydimethylsiloxane (PDMS) to prepare a photonic crystal film, where the filling rate of the microspheres is 10%, and the thickness of the prepared photonic crystal film is less than or equal to 100 μm. Test with a UV spectrophotometer to obtain: when other preparation conditions are the same, the transmittance of the polydimethylsiloxane (PDMS) film prepared without embedding the hollow silica microspheres in the visible light range is 99.95%, and the transmittance of the photonic crystal film prepared by embedding the hollow silica microspheres in the visible light range is 91.8%.

[0108] Comparative Example 5:

[0109] (1) Add 300 g of MMA and 2 g of DMC to 1500 g of water, stir at a speed of 30 rpm / min at 70 °C for 1 h. Subsequently, disperse 2 g of AIBA in 20 g of water and slowly drop it into the reaction solution at a speed of 0.5 mL / min, and carry out a polymerization reaction at 70 °C for 8 h to obtain a reaction product with a solid content of 14.5%. The solid reaction product is cationic PMMA spheres with a particle size of 400 nm - 500 nm, and its Zeta potential is 45.45 mV.

[0110] (2) Disperse 17.24 g of the cationic PMMA spheres obtained in step (1) in a mixed solution of 1000 g of ethanol and 500 g of water, stir evenly at a speed of 300 rpm / min, then add 3 g of acetic acid as a weak electrolyte, control the concentration of acetic acid in the reaction system to be 0.03 M, and add dimethylaminoethanol to make the pH of the reaction system 10.5 and the conductivity 2600 μs / cm. Then, dropwise add 10 g of TEOS at a speed of 0.8 mL / min, and hydrolyze and polycondense the silane at 40 °C for 6 h to coat it on the cationic PMMA spheres. After aging overnight, PMMA@SiO2 composite microspheres are obtained.

[0111] (3) At room temperature, disperse the PMMA@SiO2 composite microspheres obtained in step (2) in a certain amount of tetrahydrofuran. Using the principle that the solubility parameters of tetrahydrofuran and PMMA are similar, selectively dissolve PMMA in tetrahydrofuran for 24 h. Then, calcine the dried sample at 300 °C for 2 h to obtain hollow silica microspheres.

[0112] Use SEM and TEM to test the morphology of the obtained hollow silica microspheres. The SEM image shows that, similar to Comparative Example 1, the silica shell layer in the hollow silica microspheres is loose, and the uniformity and compactness of the coating structure are low. The particle size of the hollow silica microspheres is 472 nm, and the wall thickness is 36 nm.

[0113] The hollowness of the obtained hollow silica microspheres is calculated to be 60.8%.

[0114] Test the nitrogen adsorption-desorption isotherm and pore size distribution map of the hollow silica microspheres, and it shows that its specific surface area is 421 m 2 / g, the pore volume is 0.89 cm 3 / g, the pore diameter is 2.99 nm, calculate its porosity to be 0.171%, and the elastic modulus measured by AFM is 0.6 GPa.

[0115] Embed the hollow silica microspheres into polydimethylsiloxane (PDMS) to prepare a photonic crystal film, where the filling rate of the microspheres is 10%, and the thickness of the prepared photonic crystal film is less than or equal to 100 μm. Using a UV spectrophotometer to test, it is obtained that: under the same other preparation conditions, the transmittance of the polydimethylsiloxane (PDMS) film prepared without embedding the hollow silica microspheres in the visible light range is 99.95%, and the transmittance of the photonic crystal film prepared by embedding the hollow silica microspheres in the visible light range is 93.5%.

[0116] The reaction conditions and the silica properties on the surface of PMMA spheres in Examples 1-3 and Comparative Examples 1-5 are shown in Table 1.

[0117]

[0118] The silica coating on the surface of PMMA spheres in Examples 1-3 and Comparative Examples 1-5 is as follows:

[0119] (1) Examples 1-3 show that under the synergistic action of dimethylaminoethanol and weak acid electrolytes such as acetic acid or propionic acid, the hydrogen bond interaction force and charge shielding effect between the methyl methacrylate chain segments on the PMMA surface and carboxylate ions or propionate ions can significantly improve the stability of the silane hydrolysis rate and condensation rate, thereby enabling uniform coating of silica, improving the denseness of the silica shell layer and reducing the porosity, obtaining hollow silica microspheres with a dense shell layer and extremely low porosity, and significantly improving their mechanical properties.

[0120] (2) Comparing Example 1 with Example 2, it can be seen that by adjusting the content of the main monomer in the polymerization reaction system, the particle size of the cationic polymer can be adjusted, and silica coating on the surface of cationic PMMA spheres with different particle sizes can be achieved under the synergistic action of dimethylaminoethanol and acetic acid.

[0121] (3) Comparing Example 1 and Example 3, it can be seen that by adjusting the weak acid electrolyte, the coating effect of silica can be adjusted. When propionic acid or acetic acid is used as the weak acid electrolyte, a similar mechanism of action is shown, and both contribute to improving the denseness of the coating shell layer.

[0122] (4) Different from the reaction system with the synergistic action of weak acid electrolyte and weak base in Example 1, only weak base is introduced in Comparative Example 1 without adding weak acid electrolyte. The experimental results show that the silica shell layer prepared by this system presents a loose structure, with problems such as low denseness, high porosity and poor mechanical properties. This confirms that only when the weak acid electrolyte participates in the reaction system, the charge shielding and hydrogen bond synergistic effects jointly generated by it and the weak base can effectively promote the uniform deposition and directional crosslinking of silane hydrolysis oligomers, and then form a dense silica shell layer. On the contrary, if the weak acid electrolyte is lacking, the dynamic balance of silane hydrolysis and polycondensation reactions will be broken, resulting in the growth mode of silica changing from ordered monomer addition to disordered aggregation-controlled growth, and finally leading to a loose structure and fragile texture of the coating shell layer.

[0123] (5) Compared with the weak acid electrolyte and weak base coexisting in the reaction system of Example 1, the reaction system of Comparative Example 2 adds a weak acid electrolyte but does not add a weak base, and the hollow silica shell obtained is loose, indicating that a single weak acid electrolyte cannot effectively regulate the hydrolysis-condensation equilibrium of silane: lacking the synergy of a weak base, the system is difficult to form a charge shielding effect of sufficient strength, and the strength and directionality of hydrogen bonding are insufficient, resulting in the inability of silane hydrolysis product oligomers to be uniformly dispersed and directionally cross-linked. Due to the imbalance in reaction rate regulation, the silica growth process changes from an ordered monomer addition mode to a disordered aggregation growth, resulting in a large number of pores in the shell during the deposition process, and the structural density is significantly reduced, which cannot meet the performance requirements for the hollow silica shell.

[0124] (6) Compared with the weak base dimethylaminoethanol used in Example 1, the reaction system in Comparative Example 3 uses strong base sodium hydroxide, and the degree of ionization of hydroxide ions is higher. When the concentration of hydroxide ions in the system is high, its strong alkalinity significantly accelerates the hydrolysis and condensation reaction rates of silane. This rate imbalance breaks the dynamic equilibrium of the hydrolysis and condensation process of silane, prompting the hydrolyzed silane precursor to accumulate rapidly in the solution and reach an oversaturated state. Under this condition, the silicon oxide oligomer no longer follows the orderly growth mode of monomer addition, but instead rapidly self-assembles in an aggregation-controlled manner, resulting in a decrease in the uniformity of the shell coating, thereby reducing the density of the shell, increasing the porosity, and weakening its mechanical properties.

[0125] (7) Example 1 uses a weak acid electrolyte, while Comparative Example 4 uses a metal salt electrolyte. Since the metal salt ionizes quickly in the solution, it cannot form hydrogen bonds with the silane hydrolysis product like a weak acid. This difference causes the silane polycondensation rate to be much higher than the hydrolysis rate, and the ratio of the two is unbalanced by several times. The unbalanced reaction rate makes it difficult for the silane hydrolysis product to deposit uniformly, and local overpolymerization occurs during the coating process, resulting in uneven thickness of the shell layer formed in the end. This uneven coating not only reduces the density of the shell layer, significantly increases the porosity, but also seriously weakens its mechanical properties.

[0126] (8) Compared with the pH value of 8.1 in the control reaction system in Example 1, the pH value of 10.5 in the control reaction system in Comparative Example 5 shows that the shell layer of the obtained hollow silica sphere is formed by the accumulation of small particles, the shell structure is loose, the porosity is significantly increased, and the mechanical strength is significantly reduced. This shows that too high a pH value will significantly accelerate the hydrolysis and polycondensation rate of silane, breaking the dynamic balance between hydrolysis and polycondensation. In a strong alkaline environment, the oligomers generated by the hydrolysis of silane tend to grow rapidly in aggregate, rather than forming a dense structure through orderly monomer addition and directional cross-linking, resulting in the formation of a shell layer in the form of disordered accumulation of small particles, making it difficult to achieve a uniform and dense coating effect, affecting the overall performance of the hollow silica sphere.

[0127] The above-described embodiments have elaborated on the technical solutions of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, supplements, or substitutions in a similar manner made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing hollow silica microspheres, characterized in that: It includes the following steps: (1) Using a cationic polymer as a template; there is a weak acid electrolyte and a weak base in the reaction system, and the pH of the reaction system is controlled in the range of 8.0 - 9.5 by the addition amounts of the weak acid electrolyte and the weak base. In this reaction system, the silicon source precursor undergoes hydrolysis and condensation reaction on the surface of the template to form a core-shell structure of silica-coated polymer; (2) Dissolving the polymer in the core-shell structure with a solvent to obtain hollow silica microspheres.

2. The preparation method according to claim 1, characterized in that: The cationic polymer is prepared by a polymerization reaction, and the polymerization reaction system includes a main monomer, a cationic monomer, and an initiator.

3. The preparation method according to claim 2, characterized in that: It satisfies at least one of the following (Ⅰ) to (Ⅳ): (Ⅰ) The main monomer is selected from at least one of methyl methacrylate, styrene, and acrylic acid; (Ⅱ) The cationic monomer is a monomer with a quaternary ammonium salt group; (Ⅲ) The cationic monomer is at least one of acryloyloxyethyl trimethyl ammonium chloride, methacryloyloxyethyl trimethyl ammonium chloride, dimethyldiallyl ammonium chloride, vinyl pyridine, and acrylamidomethylpropanesulfonic acid; (Ⅳ) The initiator is at least one of azodiisobutyramidine hydrochloride, azodiisobutyronitrile amidine, azodiisobutylimidazoline hydrochloride, and azodicyanovaleric acid.

4. The preparation method according to claim 1, characterized in that: The particle size of the cationic polymer is 300nm - 800nm, and / or the Zeta potential of the cationic polymer is in the range of +30mV to +50mV.

5. The preparation method according to claim 1, characterized in that: It satisfies at least one of the following (Ⅰ) to (Ⅳ): (Ⅰ) The weak acid electrolyte is a carboxylic acid compound; (Ⅱ) The weak acid electrolyte is selected from at least one of acetic acid, propionic acid, citric acid, benzoic acid, carbonic acid, hypochlorous acid, sulfurous acid, phosphoric acid, nitrous acid, hydrosulfuric acid, silicic acid, boric acid, citric acid, lactic acid, oxalic acid, salicylic acid, tartaric acid, malic acid, phenol, and amino acids; (Ⅲ) The weak base is selected from at least one of dimethylaminoethanol (DMAE), ammonia water, methylamine, ethylamine, propylamine, pyridine, urea, triethylamine, aniline, choline, aluminum hydroxide, magnesium hydroxide, zinc hydroxide, iron hydroxide, copper hydroxide, sodium bicarbonate, etc.; (Ⅳ) The silicon source precursor is selected from at least one of tetraethyl orthosilicate, tetramethoxysilane, sodium silicate, silicon tetrachloride, methyltrimethoxysilane, phenyltrimethoxysilane, vinyltrimethoxysilane, trichlorosilane, methyltriethoxysilane, and dimethyldiethoxysilane.

6. The preparation method according to claim 1, characterized in that: In the step (1), the concentration of the weak acid electrolyte in the reaction system is controlled at 0.1M - 1M, preferably 0.3M - 0.5M.

7. The preparation method according to claim 1, characterized in that: It satisfies at least one of the following (Ⅰ) to (Ⅱ): (Ⅰ) The reaction temperature of the hydrolysis and condensation reaction is 20℃ - 80℃, preferably 30℃ - 50℃; (Ⅱ) The reaction time of the hydrolysis and condensation reaction is 3h - 24h.

8. The preparation method according to claim 1, characterized in that: When the selected cationic polymer is PMMA, tetrahydrofuran and / or acetone are selected as the solvent in the step (2); when the selected cationic polymer is polystyrene PS, toluene and / or ethylbenzene are selected as the solvent in the step (2).

9. The preparation method according to claim 1, characterized in that: The obtained hollow silica microspheres satisfy at least one of the following (Ⅰ) to (Ⅱ): (Ⅰ) The porosity is less than 0.3%, preferably less than 0.2%; (Ⅱ) The elastic modulus is greater than 1 GPa, preferably greater than 5 GPa, more preferably greater than 8 GPa.

10. The preparation method according to claim 1, characterized in that: The prepared hollow silica microspheres satisfy at least one of the following (Ⅰ) to (Ⅲ): (Ⅰ) The hollowness is greater than or equal to 60%, preferably greater than or equal to 70%; (Ⅱ) The shell thickness ranges from 10 nm to 100 nm, preferably from 15 nm to 50 nm; (Ⅲ) The pore diameter is less than or equal to 3 nm.

11. The hollow silica microspheres prepared by using the preparation method according to any one of claims 1 to 10 are applied to drug controlled release, photonic crystals or catalytic carriers.

12. A photonic crystal thin film, comprising a high molecular polymer and the hollow silica microspheres prepared by using the preparation method according to any one of claims 1 to 10.

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