Method for producing nano-oriented porous structure

Abstract

The application discloses a preparation method of a nano-oriented porous structure, which comprises the following steps: adding an organic solvent into a polymer material / water solution to prepare a polymer material / water / organic solvent mixed solution; performing ultrasonic treatment on the polymer material / water / organic solvent mixed solution to form a polymer material / water / organic solvent mixed solution containing nano-level microdroplets; directionally freezing the polymer material / water / organic solvent mixed solution containing nano-level microdroplets to form a block containing nano-level microdroplets; and removing the water and the organic solvent from the polymer material / water / organic solvent block in a low-temperature high-vacuum environment and crosslinking the polymer material at a low temperature to obtain the nano-oriented porous structure. The porous structure has high porosity, high flux and excellent mechanical properties.

Description

Technical Field

[0001] This invention relates to the field of polymer porous material preparation, specifically to a directional porous structure and its preparation method. Background Technology

[0002] In semiconductor manufacturing, wet electronic chemicals require high purity to reduce defects on wafers. Wet electronic chemical manufacturers use batch filtration during production to eliminate most particulate matter or ions in the solution. However, as Moore's Law drives the continuous reduction of critical dimensions, wet chemicals need to undergo high-precision spot filtration before use to eliminate any particulate matter that could potentially become defects on the wafer. For example, in photolithography, photoresist, anti-reflective coatings, or developers need to pass through filters with 10nm or 20nm pore sizes before application to reduce defects and improve yield. In advanced processes, the pore size of the filter membrane can even reach 5nm and 2nm.

[0003] To reduce or eliminate defects on wafers, small-pore membrane filtration technology has been developed.

[0004] Current methods for preparing filter membranes typically include layer-by-layer self-assembly, interfacial polymerization, phase inversion, and freeze-drying.

[0005] Layer-by-layer self-assembly involves adsorbing anionic monomers onto a positively charged base membrane, followed by the adsorption of cationic monomers, and repeating this process layer by layer to form a multilayer self-assembled membrane. This method allows for flexible control of the membrane surface morphology and pore size, but it has low assembly efficiency, often requiring multiple cycles to form a small-pore membrane.

[0006] Interfacial polymerization is one of the most widely used membrane fabrication methods in industry. This method involves the polymerization reaction of two highly reactive monomers at the interface of a porous substrate membrane to form a dense, porous polymer layer. During the polymerization process, high-performance filter membranes can be obtained by adjusting the pore size and hydrophilicity of the substrate membrane, as well as the type, concentration, and diffusion coefficient of the monomers. However, the precision of process control during fabrication has a significant impact on the micro / nano structure and filtration performance of the filter membrane.

[0007] Phase inversion method involves forming a fibrous membrane from a polymer solution using coating or electrospinning, followed by solution evaporation to solidify the fibrous membrane into a porous filter. This method is simple and easy to implement, but the resulting filter membranes generally have a relatively large pore size.

[0008] Freeze-drying is a novel pore-forming method, also known as the ice-templating method. In a high-vacuum environment, the solvent in a frozen polymer solution directly sublimates and dries, leaving the remaining polymer with unchanged physical and chemical properties, forming a porous polymer structure. This method is simple to prepare and can form a dense porous network structure without a porous base membrane. However, the filter membranes prepared by freeze-drying often have pore sizes in the micrometer range, which are relatively large and exhibit disordered structures.

[0009] In summary, current porous filter membrane fabrication processes all have certain limitations, such as low fabrication efficiency, high process control precision, large pore size, and disordered structure. Summary of the Invention

[0010] The purpose of this application is to provide a method and apparatus for preparing porous filter membranes with nano-oriented structures using directional freeze-drying technology.

[0011] Therefore, some embodiments of this application provide a method for preparing a nano-oriented porous structure, which includes the following steps: adding an organic solvent to a polymer / water / organic solvent mixture to form a polymer / water / organic solvent mixture containing nanoscale microdroplets; directionally freezing the polymer / water / organic solvent mixture containing nanoscale microdroplets into a bulk; removing the water and organic solvent from the polymer / water / organic solvent bulk in a low-temperature, high-vacuum environment and crosslinking the polymer at low temperature to obtain the nano-oriented porous structure.

[0012] In some embodiments, the size of the nanoscale microdroplets is controlled by adjusting the ratio of the polymer / water / organic solvent mixture.

[0013] In some embodiments, the polymer / aqueous solution is an aqueous solution of polyvinyl alcohol. The molecular weight of the polyvinyl alcohol is 10,000 to 20,000 Mw. The mass percentage of the polyvinyl alcohol / aqueous solution is 2% to 15%.

[0014] In some embodiments, the organic solvent includes a solvent miscible with water or a solvent immiscible with water. The solvent miscible with water is ethanol, isopropanol, tert-butanol, acetonitrile, acetone, tetrahydrofuran, or dimethyl sulfoxide. The solvent immiscible with water is cyclohexane, n-hexane, toluene, or chlorobenzene.

[0015] In some embodiments, the ethanol is 5% to 95% by mass.

[0016] In some embodiments, the low-temperature high-vacuum environment is provided by a freeze dryer, with a low temperature range of -30 to -35°C and a vacuum degree range of 0.01 to 0.2 mbar.

[0017] In some embodiments, the drying time of the freeze dryer is 48 hours.

[0018] In some embodiments, the directional freezing includes placing the polymer / water / organic solvent mixture in a mold and performing directional freezing in a directional freezing environment provided by the freezing surface of a cold source, wherein the mold has sidewalls made of insulating material and an end made of thermally conductive material that contacts the cold source.

[0019] In some embodiments, the sidewall portion forms a hollow cylinder, and the end portion is located at the bottom of the hollow cylinder; wherein the bottom is placed on the freezing surface. The thermal insulation material is polyvinylidene fluoride, the thermally conductive material is copper or aluminum sheet, and the cold source is liquid nitrogen.

[0020] In some embodiments, the ultrasonic treatment is performed by a cell disruptor with a power of 10 to 500W and a working time of 1 to 5 minutes; preferably, the power is 350W and the time is 5 minutes.

[0021] The beneficial effects of this application are as follows: Compared with other preparation processes, this invention prepares small-pore nanopores through high-power ultrasound and directional freeze-drying, breaking through the barrier of freeze-drying technology. Its greatest advantages are high pore-forming precision, adjustable pore size, high mechanical strength, and regular and orderly porous structure. It is a new process for preparing nanoporous filter membranes with low cost and simple preparation process, and has great application prospects in semiconductor wet chemical filtration. Attached Figure Description

[0022] Figure 1 This is a flowchart illustrating the principle of the preparation method of the nano-oriented porous structure according to this application;

[0023] Figure 2A This is a structural diagram of a mold used in some embodiments of the method for preparing nano-oriented porous structures according to this application;

[0024] Figure 2B This is a schematic diagram of a placement mold placed on a frozen surface, according to some embodiments of the method for preparing nano-oriented porous structures based on this application.

[0025] Figure 3A This is a cross-sectional scanning electron microscope image of a nano-oriented porous polyvinyl alcohol according to Example 1 of this application;

[0026] Figure 3B yes Figure 3AA magnified view of the area within the black box.

[0027] Figure 3C yes Figure 3A Scanning electron microscope image of the side of the porous polyvinyl alcohol in the image;

[0028] Figures 4 to 9 These are cross-sectional scanning electron microscope images of nano-oriented porous polyvinyl alcohol according to Examples 2 to 7 of this application;

[0029] Figures 10 to 15 The images shown are side scanning electron microscope (SEM) images of the nano-oriented porous polyvinyl alcohols according to Examples 2 to 7 of this application.

[0030] Figures 16A to 16G The figures shown are pore distribution diagrams of nano-oriented porous membranes made according to the nano-oriented porous structures in Examples 1 to 7 of this application.

[0031] Figure 17 The tensile strength curves are shown for the nano-oriented porous membranes in Examples 1 to 7. Detailed Implementation

[0032] The specific embodiments of this application will be described in detail below with reference to the accompanying drawings. These embodiments are merely illustrative of the invention and should not be construed as limiting the scope of the invention to these embodiments.

[0033] Throughout this text, unless otherwise defined, the unit "g" represents "gram"; the unit "mbar" represents "millibar"; the unit "cm" represents "centimeter"; and the unit "cm" represents "millibar". 3 "" represents "cubic centimeter"; "nm" represents "nanometer"; "℃" represents "degrees Celsius"; "Mw" represents "molecular weight"; "W" represents "watt"; "min" represents "minute"; "h" represents "hour".

[0034] Throughout the text, the symbol " / " is used to separate two different materials to represent "and"; the symbol " / " is used in calculations to represent "÷"; the symbol "%" represents "percentage"; and the symbol "wt%" represents "weight percentage".

[0035] Throughout the text, nanometers refer to the range of 1 to 999 nm.

[0036] This application provides a method for preparing nano-oriented porous structures, which involves introducing an organic solvent into a polymer material / aqueous solution to form micron or nano-droplets inside, followed by directional freezing, sublimation, and pore formation.

[0037] The polymeric material / aqueous solution includes polyvinyl alcohol / aqueous solution.

[0038] The organic solvent includes solvents that are miscible with water or solvents that are immiscible with water. The solvents that are miscible with water are ethanol, isopropanol, tert-butanol, acetonitrile, acetone, tetrahydrofuran, or dimethyl sulfoxide. The solvents that are immiscible with water are cyclohexane, n-hexane, toluene, or chlorobenzene.

[0039] Specifically, this invention provides a method for preparing nano-oriented porous structures, such as... Figure 1 As shown, the main steps include:

[0040] Step S1: Dissolve the polymer material in water to prepare a polymer material / aqueous solution; alternatively, a pre-prepared polymer material / aqueous solution can be used directly.

[0041] Step S2: Add one of the above-mentioned organic solvents to the polymer material / aqueous solution and perform ultrasonic treatment to prepare a homogeneous polymer material / water / organic solvent mixture with nano-sized microdroplets inside;

[0042] Step S3: Place the obtained polymer material / water / organic solvent mixture of nano-sized microdroplets in a directional freezing environment to form a directionally frozen polymer material / water / organic solvent block of nano-sized microdroplets;

[0043] Step S4: The polymer / water / organic solvent block of the fully oriented solidified nano-sized microdroplets is subjected to low-temperature high-vacuum environment to allow the solvent to fully sublimate and dry in order to remove the water and organic solvent, and to allow the polymer to undergo low-temperature cross-linking, thereby obtaining the nano-oriented porous structure.

[0044] The volume and content of the nanoscale microdroplets can be adjusted by the power and time of the cell disruptor, and the volume and content of micro-nano droplets in the liquid can be directly detected, for example, by using an infrared spectrometer to detect suspended micro-nano droplets.

[0045] According to embodiments of this application, the molecular weight of the polyvinyl alcohol is preferably between 10,000 and 20,000 Mw, i.e., a commonly used and easily soluble molecular weight; the mass percentage of the polyvinyl alcohol / water solution is preferably between 4% and 15%. The advantage of polyvinyl alcohol within the above parameter range is that it exhibits good solubility in water, allowing for control of the polymer concentration in the solution. Different molecular weights of polyvinyl alcohol used in this application have little impact on the results in some embodiments because the cell disruptor breaks the molecular chains in these embodiments. 15% is the highest concentration of polyvinyl alcohol / water solution; concentrations greater than 15% are generally insoluble. Examples 1 to 7 show results with different mass percentages of polyvinyl alcohol / water solution, resulting in different pore sizes and strengths.

[0046] According to embodiments of this application, the preferred mass percentage of ethanol is 5% to 95%. The advantage of ethanol within this parameter range is that it has good solubility in water, allowing for control of its concentration in the solution. Different ethanol concentrations can produce oriented porous structures, and the ethanol concentration has a certain influence on pore size and morphology. The ethanol concentration used in this application is the optimal concentration.

[0047] According to an embodiment of the present invention, the freezing environment can be achieved by providing a fixed structure in which the resulting polymer / water / organic material solution is placed. The fixed structure can be, for example... Figure 2A The mold 100 shown includes a hollow body, such as a hollow cylinder 10, formed by an insulating sidewall, and a bottom 20, such as a sheet-like thermally conductive material, located at the end of the hollow cylinder 10. The bottom of the insulating hollow body 10 contacts the upper surface of the sheet-like thermally conductive material 20. The interior of the insulating hollow cylinder 10 is used to contain a solution. Specifically, the insulating material may be polyvinylidene fluoride (PVDF), and the thermally conductive material may be copper, aluminum, or a composite material. The shaped structure is placed on a cooling surface, with the sheet-like thermally conductive material in contact with the cooling surface. The cooling surface can be provided by a liquid nitrogen refrigeration device 200, thereby creating a temperature gradient from the bottom to the top of the shaped structure. This allows the polymer / water / organic solvent mixture containing nanoscale microdroplets in the shaped structure to rapidly and directionally solidify into a solid state.

[0048] The polymer / water / organic solvent mixture is in contact with liquid nitrogen at -196°C at the bottom and with air at 25°C at the top. This temperature gradient from bottom to top causes ice crystals to grow upwards from the low-temperature bottom, forming oriented ice crystals. These ice crystals then sublimate to form oriented channels. The easiest temperature gradient to achieve is bottom-up; while left-to-right, right-to-left, and top-to-bottom temperature gradients can also create oriented structures, they are relatively more difficult to achieve.

[0049] According to the present invention, the ultrasonic treatment conditions are: power 10 to 500 W; working time 1 to 5 min. Its advantage lies in the generation of suspended nanodroplets within the solution. As described above, ultrasonic treatment using a cell disruptor also has the additional effect of breaking the molecular chains of polymer materials.

[0050] According to the present invention, the preferred vacuum conditions are: a temperature of -30 to -35°C and a vacuum level of 0.01 to 0.2 mbar. The advantage is that polyvinyl alcohol undergoes structural recombination and crosslinking at low temperatures, improving mechanical strength. The lower the temperature in the above steps, the better; the reason for the temperature limitation is that the minimum temperature of a refrigerated dryer is generally -30 to -35°C. Similarly, the lower the vacuum level in the above steps, the better; the reason for the vacuum level limitation is that the vacuum level of a refrigerated dryer is generally 0.01 to 0.2 mbar.

[0051] The directional porous structure prepared by the method described above in this invention has advantages such as small pore size, controllable pore size, ordered structure, and high mechanical strength. It can be used in fields such as filtration, separation, adsorption, catalysis, evaporation, and liquid transport.

[0052] The above method can be implemented in a device for preparing a nano-oriented porous structure, comprising: a mixing unit configured to introduce an organic solvent into a polymer solution to form a polymer / water / organic solvent mixture; a dispersion unit configured to disperse the polymer / water / organic solvent mixture to form a polymer / water / organic solvent mixture containing nanoscale microdroplets; a directional freezing unit configured to directionally freeze the polymer / water / organic solvent containing nanoscale microdroplets into a polymer / water / organic solvent mass in a temperature gradient freezing environment; and a low-temperature crosslinking unit configured to remove the solvent from the polymer / water / organic solvent mass in a low-temperature high-vacuum environment and perform low-temperature crosslinking of polyvinyl alcohol to obtain a nano-sized oriented porous structure.

[0053] The directional freezing unit can be the aforementioned freezing surface and a mold 100 placed on the freezing surface. After injecting a polyvinyl alcohol / water / ethanol solution into the mold 100, as... Figure 2B As shown; then directional freezing is performed on the surface of the liquid nitrogen refrigeration device 200; the freezing time is usually about 5 minutes. The top surface of the liquid nitrogen refrigeration device can be a refrigeration surface, which is related to the mold. Its advantage is that the polymer / water / organic material solution can be directionally solidified in an environment with a temperature gradient from the bottom to the top of the mold, and the directional solidification speed is fast.

[0054] According to an embodiment of the present invention, the dispersion unit may be a cell disruptor, the power of which is 10 to 500W and the working time is 1 to 5 minutes.

[0055] The low-temperature crosslinking unit can be a freeze dryer, the low temperature range of which is -30 to -35°C; the vacuum degree of which is 0.01 to 0.2 mbar.

[0056] Example 1:

[0057] A method for preparing a nano-oriented porous structure includes the following steps:

[0058] Take 15g of polyvinyl alcohol and 85g of deionized water, heat and stir in a water bath at 50℃ to dissolve them, and prepare a 15% polyvinyl alcohol / water solution by mass.

[0059] 20g of ethanol was added to the polyvinyl alcohol / water solution, and the mixture was sonicated in a cell disruptor at a power of 350W for 5 minutes to prepare a homogeneous polyvinyl alcohol / water / ethanol solution containing microdroplets. Microdroplets are droplets with a size ranging from nanometers to millimeters, whose surface tension allows them to maintain a shape, for example, spherical or near-spherical. In Example 1, the ethanol mass percentage was 20g / (15g+85g+20g) = 16.67%. Commonly used ethanol reagent is analytical grade with a concentration of 95%, so the ethanol mass percentage in Example 1 could also be 20*0.95 / (15+85+20) = 15.83%. Similarly, in Examples 2 to 7, the ethanol mass percentage can be recorded as 16.67% or 15.83%.

[0060] The obtained polyvinyl alcohol / water / ethanol solution is placed in mold 100, and mold 100 is placed on the surface of liquid nitrogen, such as the surface of liquid nitrogen refrigeration device 200, and directionally frozen for 5 minutes to produce directionally frozen polyvinyl alcohol / water / ethanol block.

[0061] The fully directional solidified polyvinyl alcohol / water / ethanol block was dried in a low-temperature, high-vacuum environment of -35℃ and 0.01mbar for 48 hours to allow the solvent to fully sublimate and dry, and to crosslink the polyvinyl alcohol at low temperature, thus obtaining a nano-sized directional porous structure.

[0062] The cross-sectional scanning electron microscope image of the nano-oriented porous polyvinyl alcohol prepared in this embodiment is shown below. Figure 3A See the magnified scanning electron microscope image of the cross-section. Figure 3B It is evident that it possesses a nanoporous structure with a pore size of approximately 2 nm; the side scanning electron microscope image is shown below. Figure 3C It is evident that it has a directional channel structure.

[0063] Examples 2 to 7 present similar methods for preparing nano-oriented porous structures. The only difference between these methods and the method in Example 1 is that:

[0064] Example 2: 14g of polyvinyl alcohol and 86g of deionized water were used to prepare a 14wt% polyvinyl alcohol / water solution; its cross-sectional scanning electron microscope image is shown below. Figure 4 As shown, the side scanning electron microscope image is as follows. Figure 10 As shown, the pore size is approximately 5 nm.

[0065] Example 3: 13g of polyvinyl alcohol and 87g of deionized water were used to prepare a 13wt% polyvinyl alcohol / water solution; its cross-sectional scanning electron microscope image is shown below. Figure 5 As shown, the side scanning electron microscope image is as follows. Figure 11 As shown, the pore size is approximately 10 nm.

[0066] Example 4: 10g of polyvinyl alcohol and 90g of deionized water were used to prepare a 10wt% polyvinyl alcohol / water solution; its cross-sectional scanning electron microscope image is shown below. Figure 6 As shown, the side scanning electron microscope image is as follows. Figure 12 As shown, the pore size is approximately 100 nm.

[0067] Example 5: 8g of polyvinyl alcohol and 92g of deionized water were used to prepare an 8wt% polyvinyl alcohol / water solution; its cross-sectional scanning electron microscope image is shown below. Figure 7 As shown, the side scanning electron microscope image is as follows. Figure 13 As shown, the pore size is approximately 250 nm.

[0068] Example 6: 6g of polyvinyl alcohol and 94g of deionized water were used to prepare a 6wt% polyvinyl alcohol / water solution; its cross-sectional scanning electron microscope image is shown below. Figure 8 As shown, the side scanning electron microscope image is as follows. Figure 14 As shown, the pore size is approximately 500 nm.

[0069] Example 7: 4g of polyvinyl alcohol and 96g of deionized water were used to prepare a 4wt% polyvinyl alcohol / water solution; its cross-sectional scanning electron microscope image is shown below. Figure 9 As shown, the side scanning electron microscope image is as follows. Figure 15 As shown, it is a directional pore structure with an aperture of approximately 750 nm.

[0070] Pore ​​distribution test: The pore distribution of the nano-oriented porous membranes prepared by the nano-oriented porous structures in Examples 1 to 7 is shown in the figure below. Figures 16A to 16G As can be seen, the pore size of the filter membrane decreases with the increase of polyvinyl alcohol content.

[0071] Tensile strength test: The tensile strength of the nano-oriented porous membranes in Examples 1 to 7 is shown in the test results. Figure 17 As can be seen, the tensile strength increases with the increase of polyvinyl alcohol content, while the tensile strain decreases with the increase of polyvinyl alcohol content.

[0072] The applicant discovered a positive correlation between freezing and drying time and mold volume. The freezing and drying times described in the above embodiments of this application can cover a mold with an inner diameter of 5cm, a height of 10cm, and a volume of 196.25cm³. 3 The freezing and drying times can be adjusted depending on the mold used.

[0073] It should be noted that the terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0074] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a nano-oriented porous structure, characterized in that, Including the following steps: A polymer / water / organic solvent mixture is prepared by adding an organic solvent to a polymer / water / organic solvent mixture; wherein the organic solvent is ethanol, and the ethanol mass percentage is 5% to 95%. The polymer / water / organic solvent mixture is ultrasonically treated to form a polymer / water / organic solvent mixture containing nano-sized microdroplets; Directional freezing of a polymer material / water / organic solvent mixture containing nano-sized microdroplets into a bulk material; The polymer / water / organic solvent bulk material is subjected to a low-temperature, high-vacuum environment to remove the water and the organic solvent, and the polymer material is cross-linked at low temperature to obtain the nano-oriented porous structure. The polymeric material / aqueous solution is a polyvinyl alcohol / aqueous solution; the polyvinyl alcohol / aqueous solution has a mass percentage of 2% to 15%. The low-temperature, high-vacuum environment is provided by a freeze dryer, with a low temperature range of -30 to -35°C and a vacuum degree range of 0.01 to 0.2 mbar.

2. The method for preparing the nano-oriented porous structure according to claim 1, characterized in that, The size of the nanoscale microdroplets can be controlled by adjusting the ratio of the polymer material / water / organic solvent mixture.

3. The method for preparing the nano-oriented porous structure according to claim 1, characterized in that, The molecular weight of the polyvinyl alcohol is 10,000 to 20,000 Mw.

4. The method for preparing the nano-oriented porous structure according to claim 1, characterized in that, The polyvinyl alcohol / water solution has a mass percentage of 4% to 15%.

5. The method for preparing the nano-oriented porous structure according to claim 4, characterized in that, The polyvinyl alcohol / water solution has a mass percentage of 15%.

6. The method for preparing the nano-oriented porous structure according to claim 1, characterized in that, The ethanol content is 16.67% or 15.83% by mass.

7. The method for preparing the nano-oriented porous structure according to claim 1, characterized in that, The low temperature is -35°C and the vacuum degree is 0.01 mbar.

8. The method for preparing the nano-oriented porous structure according to claim 1, characterized in that, The directional freezing includes placing the polymer / water / organic solvent mixture in a mold and performing directional freezing in a directional freezing environment provided by the freezing surface of a cold source, wherein the mold has sidewalls made of insulating material and an end made of thermally conductive material that contacts the cold source.

9. The method for preparing the nano-oriented porous structure according to claim 8, characterized in that, The sidewall portion forms a hollow cylinder, and the end portion is located at the bottom of the hollow cylinder; wherein the bottom portion is placed on the freezing surface.

10. The method for preparing the nano-oriented porous structure according to claim 8, characterized in that, The insulation material is polyvinylidene fluoride, the thermally conductive material is copper or aluminum sheet, and the cold source is liquid nitrogen.

11. The method for preparing a nano-oriented porous structure according to claim 1, characterized in that, The ultrasonic treatment is performed by a cell disruptor with a power of 10 to 500W and a working time of 1 to 5 minutes.

12. The method for preparing a nano-oriented porous structure according to claim 11, characterized in that, The cell disruptor has a power of 350W and a time of 5 minutes.

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