Aerogel film with continuous gradient pore structure as well as preparation method and application of aerogel film

Abstract

The invention provides an aerogel film with a continuous gradient pore structure and a preparation method and application thereof.The preparation method comprises the steps that a pre-crosslinking solution containing a polymer nanofiber suspension, a sodium alginate solution, a polyethyleneimine aqueous solution and a crosslinking agent is sprayed to a temperature-conducting substrate with the initial temperature ranging from-20 DEG C to-15 DEG C at high pressure; and freeze-drying to obtain the aerogel film with the continuous gradient pore structure. The aperture of the aerogel film is gradually increased from bottom to top, and the range of the aperture is mainly 1-110 [mu] m. The aerogel film is placed in a filter membrane clamp, so that a solution to be filtered passes through the aerogel film according to the sequence of'macropore-mesopore-micropore ', the aerogel film is used for intercepting and recycling particles with different sizes, such as micro-plastics, and high water flux and high rejection rate are both considered. The pore size of the aerogel film can be changed by adjusting the temperature of the temperature-conducting substrate; the prepared aerogel film improves the trade-off effect in the existing filtration separation technology, and has a wide market prospect.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of aerogel film preparation, and particularly relates to an aerogel film with a continuous gradient pore structure and a preparation method and application thereof. BACKGROUND

[0002] Ultrafine particles from different sources not only harm the environment and human health, but also require energy-consuming, expensive and complex removal processes. Whether it is urban wastewater / sludge treatment, industrial wastewater treatment, or pure water processes, improving energy efficiency, recycling, low cost, safe and reliable, sustainable, renewable water resource management and development, all represent the basic development ideas of water treatment and wastewater treatment.

[0003] Electrophoresis, microfluidics, magnetic flocculation, centrifugation and filtration have been used to separate ultrafine particles. However, when applied to the purification of suspended contaminated water, these methods face specific challenges. Centrifugation and electrophoresis are heavily dependent on equipment and consume electrical energy to operate, which makes them less practical for widespread application. Microfluidic methods have limitations in the amount of water that can be effectively purified and the high cost associated with device manufacturing. Filtration is a simple, convenient and easy-to-implement separation of suspended particles, mainly based on size exclusion principle.

[0004] Filtration technology has obvious advantages in removing large size microplastics, but lacks the ability to remove ultrafine size particles, and certain filter membranes are prone to clogging during filtration. Although ultrafiltration and nanofiltration technologies can eliminate suspended particles in water, they are very time-consuming due to low permeability, and rely on specialized equipment, limiting their practicality.

[0005] In addition to filtration separation relying on the size exclusion principle of materials, adsorption separation can greatly improve the limitations of filtration separation in removing suspended particles. In addition, three-dimensional aerogels greatly increase the specific surface area due to their porous structure, thereby increasing the adsorption sites and increasing the removal efficiency of suspended particles; and adsorption method has the advantages of simple design and operation, environmental protection, high adsorption efficiency, etc., and has excellent application potential in the field of microplastic removal. However, most of the existing research relies on the static adsorption of aerogels in contaminated solutions to remove suspended particles, which is time-consuming and has limited use.

[0006] The invention patent with the application number CN 202311129450.6 discloses a preparation method of a super-elastic and durable gradient aerogel high-temperature filter. First, a polyamic acid (PAA) solution is synthesized. Then, nanofibers are prepared by electrospinning technology. Next, the nanofiber / microfiber is subjected to high-speed shearing and homogeneous dispersion in an aqueous solution. Then, PAA solid particles are added to the dispersion to form a viscous polyamide acid gel. Finally, through step-by-step freezing technology, freeze-drying and thermal imidization treatment, an aerogel filter for high-temperature environment is prepared. The gradient pore structure filter exhibits a "mesopore-small pore" combined interception effect, realizes selective interception of dispersed particulate matter and makes the particulate matter stratified and accumulated, thereby prolonging the service life of the filter. However, the preparation method uses a step-by-step low-temperature freezing method for freeze-drying pretreatment. The preparation process of the aerogel in this scheme uses a copper block soaked in liquid nitrogen to provide the temperature, and the principle of the gradient structure is to use different concentrations of gel to form a gradient structure by pouring the gel into a mold in steps. In addition, the sample forming requires a vacuum degree of the freeze-drying machine to be as low as 1-10 Pa, and the freeze-drying process takes 48-72 hours. The preparation process is not only costly but also time-consuming.

[0007] Therefore, it is necessary to design an improved preparation method of an aerogel film with a continuous gradient pore structure to solve the above problems. SUMMARY

[0008] The purpose of the present application is to provide an aerogel film with a continuous gradient pore structure and its preparation method and application. The aerogel slurry is high-pressure sprayed on a temperature-conducting substrate with an initial temperature of-20℃ to-15℃ to obtain an aerogel film with a continuous gradient pore structure, which realizes efficient interception of particles of different sizes. Through the process of pre-crosslinking-freeze-drying-re-crosslinking, an aerogel with stable size, good mechanical properties and recyclable use is prepared. The aerogel can be applied to the interception and recovery of particles of different sizes such as microplastics, which improves the "trade-off" effect in the existing filtration and separation technology and has a broad market prospect.

[0009] To achieve the above-mentioned purpose of the application, the present application provides a preparation method of an aerogel film with a continuous gradient pore structure, comprising the following steps:

[0010] S1, uniformly mixing a polymeric nanofiber suspension and a sodium alginate solution, then adding a polyethyleneimine aqueous solution and stirring to obtain an aerogel slurry;

[0011] S2, high-pressure spraying the aerogel slurry obtained in step S1 on a temperature-conducting substrate with an initial temperature of-20℃ to-15℃, freeze-drying it at-40℃ to-50℃ for 8-12 hours to obtain a sodium alginate-based aerogel; during the spraying process, the temperature of the surface of the substrate ranges from-20℃ to-10℃.

[0012] S3, placing the sodium alginate-based aerogel obtained in step S2 in a calcium chloride solution with a mass fraction of 5%-10% for cross-linking reaction for 2-6h, and washing, to obtain the aerogel film with continuous gradient pore structure.

[0013] As a further improvement of the present application, in step S2, the high-pressure spraying process is to control the spraying distance to be 150mm-200mm by using a high-pressure spray gun, and the aerogel slurry obtained in step S1 is continuously sprayed on the temperature-guiding substrate with an initial temperature of-20℃ to-15℃ within 30-50s.

[0014] As a further improvement of the present application, in step S1, the mass ratio of the polymer nanofiber, sodium alginate and polyethyleneimine in the pre-crosslinking solution is 1:(1.1-1.2):(0.2-0.4).

[0015] As a further improvement of the present application, the polymer nanofiber is PVA-co-PE nanofiber, the molecular weight of the polyethyleneimine is 10000-60000, and the crosslinking agent is a polyaldehyde.

[0016] As a further improvement of the present application, the temperature-guiding substrate is a copper block, and it needs to be placed in a-40℃ freezer for 90min-180min before spraying to control the surface temperature of the temperature-guiding substrate.

[0017] To achieve the above-mentioned purposes of the present application, the present application further provides an aerogel film with continuous gradient pore structure, which is prepared by the preparation method of any one of the preceding technical solutions; the aerogel film comprises three layers of upper, middle and lower layers, and the pore size of the aerogel film gradually increases from bottom to top, and the pore size range is 1μm-110μm.

[0018] As a further improvement of the present application, the pore size range of the upper layer of the aerogel film is 60μm-110μm, the pore size range of the middle layer is 20μm-60μm, and the pore size range of the lower layer is 1μm-20μm.

[0019] As a further improvement of the present application, the thickness of the aerogel film with continuous gradient pore structure is 0.8mm-1.2mm, and the grammage is 25g / m 2 -30g / m 2 .

[0020] To achieve the above-mentioned purposes of the present application, the present application further provides the application of the aforementioned aerogel film with continuous gradient pore structure; the aerogel film is placed in a filter clamp, and the solution to be filtered passes through the upper layer, the middle layer and the lower layer of the aerogel film in turn at a speed of 0.1mm / s-1mm / s.

[0021] As a further improvement of the invention, for the field of microplastic filtration.

[0022] The beneficial effects of the present invention are:

[0023] 1. The present application provides a method for preparing aerogel film with continuous gradient pore structure, first, the mixed solution of polymer nanofiber suspension and sodium alginate solution is prepared, then polyethyleneimine aqueous solution is added, stirring to obtain aerogel slurry; then the aerogel slurry is high-pressure sprayed on the substrate with an initial temperature of-20℃ to-15℃, and the temperature range of the substrate surface during the whole spraying process is-20℃ to-10℃; then freeze-drying at-40℃ to-50℃ for 8 to 12 hours to obtain sodium alginate-based aerogel; then the sodium alginate-based aerogel is placed in calcium chloride solution for further crosslinking for 2 to 6 hours to obtain aerogel film with continuous gradient pore structure, which realizes the interception and recovery of different size particles such as microplastics.

[0024] 2. The present application uses temperature-conducting metal as the substrate, controls the placement time of the substrate in the-40℃ freezer to control the initial temperature of the substrate surface, and then forms a temperature gradient in different ranges.

[0025] 3. The method produces aerogel with stable size, good mechanical properties and recyclable use; the aerogel includes a continuous gradient structure with pore size gradually decreasing from top to bottom, the pore size is mainly distributed in 1μm to 110μm, the thickness is about 0.8mm to 1.2mm, and the grammage is about 25g / m 2 ~ 30g / m 2 . Specifically, the gradient structure of the aerogel film is continuous and can be simply divided into an upper layer of pore mode, the pore size is mainly distributed in 60μm to 110μm, the average pore size is about 75μm, the pore wall is smooth, and there are occasional small holes, which not only can improve the water flux but also can multi-directionally distribute the flow to avoid causing pore blockage; the pore size in the middle layer is distributed in about 20 to 60μm, the average pore size is about 38μm, the holes on the pore wall gradually increase until there is no pore wall structure in the lower layer, the hole size is about 1 to 20μm, the average pore size is about 13μm, and they are intertwined with each other, and the pore size is further reduced.

[0026] 4. The method is by pre-crosslinking-freeze drying-re-crosslinking process, first crosslinking PVA-co-PE nanofiber and sodium alginate, then adding polyethyleneimine to obtain aerogel slurry; wherein the PVA-co-PE nanofiber acts as a skeleton, and the polyethyleneimine can improve the crosslinking degree between sodium alginate and between sodium alginate and PVA-co-PE nanofiber, so as to improve the mechanical properties of the aerogel and also improve the hydrophilicity of the aerogel. Then the aerogel slurry is sprayed onto the temperature guide substrate with an initial temperature of-20℃ to-15℃ by a high-pressure spray gun, and the small droplets atomized by the spray gun crystallize in the instant contact with the low-temperature substrate to obtain the lower microporous layer; at this time, due to the endothermic process of solution freezing, the temperature of the copper block increases, and the higher the temperature, the slower the solution crystallization speed, and more small droplets are aggregated into large droplets to form the middle microporous layer; similarly, the endothermic process of solution crystallization leads to the increase of the temperature of the copper block, so that the upper microporous layer with larger pores is obtained. When the temperature is transmitted from bottom to top and is higher than 0℃, the upper surface of the film cannot be formed. According to this principle, the maximum pore size of the gradient structure can be controlled by whether the upper surface of the film is solidified or not. Then, under the condition that the film crystal exists, the freeze-drying program can be directly entered. The cold trap temperature of the vacuum freeze dryer is required to be about-40℃, and the vacuum degree is about 50Pa. After the program runs for 8h, the continuous gradient pore aerogel film with pore size from large to small is obtained.

[0027] 5. The aerogel film with continuous gradient pore structure is placed in a filter membrane clamp, and a syringe is vertically embedded into a pusher device to perform dynamic filtration. Specifically, the microplastic solution passes through the upper layer, the middle layer and the lower layer of the aerogel film in turn at a speed of 0.5mm / s (in the order of “large pore-mid pore-small pore” through the gradient membrane); compared with static adsorption, the static adsorption time of the same quality is about 2h-5h or even longer; while the dynamic filtration only needs 10s-180s, which is about 40-1800 times higher in time cost. The retention rate is at least guaranteed to be more than 90%, which is better than static adsorption. In addition, the aerogel film not only can filter and separate microplastics according to pore size, but also can adsorb and separate microplastics by electrostatic interaction between various functional groups on the film and microplastics.

[0028] In summary, the continuous gradient structure of the film can balance high water flux and high retention rate, greatly improving the “trade-off” effect in membrane filtration and separation, and has broad market prospects. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 The chemical mechanism diagram of the present application.

[0030] Figure 2 The preparation process diagram of the aerogel film with continuous gradient pore structure of the present application.

[0031] Figure 3 SEM images of the aerogel film with continuous gradient pore structure prepared in Example 1 of the present application; left image is the SEM image of the cross section, right upper image is the SEM image of the upper plane, and right lower image is the SEM image of the lower plane.

[0032] Figure 4 SEM images of the aerogel film prepared using different initial temperatures of the substrate; (a) 0℃-10℃; (b) -10℃-0℃; (c) -30℃--20℃; (d) -40℃--30℃; (e) -50℃--40℃.

[0033] Figure 5 Samples for dynamic filtration experiment; (a) is a uniform large-pore structure film with an average pore size of about 93 μm; (b) is a uniform medium-pore structure film with an average pore size of about 45 μm; (c) is a uniform small-pore structure film with an average pore size of about 18 μm.

[0034] Figure 6 Schematic diagram of the filter membrane clamp device, optical photograph of the gradient film and PP circular ring substrate.

[0035] Figure 7 A syringe device for dynamic filtration of the gradient film.

[0036] Figure 8 Filtering effect of the gradient film at different fluxes.

[0037] Figure 9 Comparison chart of the effect of the gradient film on dynamic continuous filtration of microplastics. DETAILED DESCRIPTION

[0038] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be described in detail below with reference to the drawings and specific embodiments.

[0039] Here, it also needs to be explained that, in order to avoid the present application being obscured by unnecessary details, only the structures and / or processing steps closely related to the scheme of the present application are shown in the drawings, and other details not closely related to the present application are omitted.

[0040] In addition, it also needs to be explained that the term “comprise”, “include” or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such process, method, article or device.

[0041] Please refer to Figures 1 to 2 the drawings, the present application provides a preparation method of an aerogel film with a continuous gradient pore structure, which comprises the following steps:

[0042] S1, adding a crosslinking agent to a mixed solution of a polymer nanofiber suspension and a sodium alginate solution, then adding a polyethyleneimine aqueous solution, stirring to obtain an aerogel slurry;

[0043] Specifically, the mass ratio of the polymer nanofiber, the sodium alginate and the polyethyleneimine is 1: (1.1-1.2): (0.2-0.4), and the mass ratio is preferably 1:1:0.3. By limiting the content of each component of the polymer nanofiber, the sodium alginate and the polyethyleneimine, the aerogel with a stable crosslinked network structure can be obtained without wasting raw materials and saving costs.

[0044] The polymer nanofiber is PVA-co-PE nanofiber, the molecular weight of the polyethyleneimine is 10000-60000, and the crosslinking agent is a polyaldehyde.

[0045] The addition amount of the crosslinking agent is 0.25%-0.6% of the volume of the PVA-co-PE nanofiber suspension. Among them, the PVA-co-PE nanofiber acts as a skeleton in the prepared aerogel, and the polyethyleneimine can improve the crosslinking degree between the sodium alginate and between the sodium alginate and the PVA-co-PE nanofiber, so as to improve the mechanical properties of the aerogel, and also improve the hydrophilicity of the aerogel and the liquid absorption performance when applied to wound dressings.

[0046] S2, high-pressure spraying the aerogel slurry obtained in step S1 on a substrate with an initial temperature of -20℃ to -15℃, and then freeze-drying at -40℃ to -50℃ for 8-12h to obtain a sodium alginate-based aerogel; during the spraying process, the temperature range of the surface of the substrate is -20℃ to -10℃;

[0047] Among them, the high-pressure spraying process is to spray the aerogel slurry on the temperature-guiding substrate with an initial temperature of -20℃ to -15℃ through a high-pressure spray gun, and the small droplets atomized by the spray gun crystallize in the instant of contacting with the low-temperature substrate to obtain the lower layer micropores; at this time, due to the endothermic process of solution freezing, the temperature of the copper block increases, and the higher the temperature, the slower the solution crystallization speed, and more small droplets gather into large droplets to form the middle layer micropores; similarly, the endothermic process of solution crystallization leads to the increase of the temperature of the copper block, so that the upper layer micropores with larger pores are obtained.

[0048] The temperature-guiding substrate is a copper block, and it needs to be placed in a -40℃ freezer for 90min-180min before spraying to control the surface temperature of the temperature-guiding substrate. For example, the temperature-guiding substrate can be a 70mm*70mm*2mm copper block, and it needs to be placed in a -40℃ freezer for 120min before spraying, at which time the surface temperature of the copper block is about -20℃ to -15℃.

[0049] S3, the sodium alginate-based aerogel obtained in step S2 is placed in a 5%-10% by mass calcium chloride solution for cross-linking reaction for 2-6 hours, and then washed to obtain the aerogel film with continuous gradient pore structure.

[0050] The present application adopts a process mode of pre-crosslinking-freeze drying-re-crosslinking, first crosslinks PVA-co-PE nanofibers and sodium alginate, and then adds polyethyleneimine to obtain an aerogel slurry; then the aerogel slurry is sprayed onto a substrate with an initial temperature of-20℃ to-15℃ by a high-pressure spray gun, and when the spray gun sprays small droplets uniformly, the droplets crystallize in contact with the low-temperature substrate to obtain lower micropores; at this time, due to the endothermic process of solution freezing, the temperature of the copper block increases, and the higher the temperature, the slower the solution crystallization speed, and more small droplets are aggregated into large droplets to form the middle micropores; similarly, the endothermic process of solution crystallization leads to the increase of the temperature of the copper block, so that the upper micropores with larger pores are obtained. When the temperature is transmitted from bottom to top and is higher than 0℃, the film surface cannot be formed, at this time, it is transferred to the condition of subzero, the volume of the crystal gradually decreases from top to bottom, and the gradient pore aerogel film with pore size from large to small is obtained by freeze drying program. In addition, according to the temperature condition of solution crystallization, the maximum pore size of the gradient structure can be controlled by whether the upper surface of the film is solidified or not.

[0051] The aerogel film prepared according to the foregoing method includes three layers with pore size gradually decreasing from top to bottom, the pore size is mainly distributed in 1μm-110μm, the thickness is about 0.8mm-1.2mm, and the grammage is about 25g / m 2 -30g / m 2 . Specifically, the pore size distribution range of the upper layer pores is 60μm-110μm, the pore size range of the middle layer pores is 20μm-60μm, and the pore size range of the lower layer pores is 1μm-20μm.

[0052] The preparation method of the aerogel film with continuous gradient pore structure provided by the present application will be described below in combination with specific examples.

[0053] Example 1

[0054] The present application provides a preparation method of an aerogel film with continuous gradient pore structure, comprising the following steps:

[0055] S1, 1g of dry PVA-co-PE nanofiber is uniformly dispersed in 100mL of water:isopropanol=1 / 2 solution, and then sheared in a blender for 15min to become a uniform suspension. The impurities are filtered out using a 200-mesh filter screen, and the suspension is placed in a 50ml centrifuge tube and put into a high-speed centrifuge (25℃, 8000rpm) to remove the upper layer of water and isopropanol mixed solution;

[0056] The obtained nanofiber was washed with deionized water for three times, and centrifuged and dried to obtain nanofiber powder particles. The treated nanofiber was dispersed in deionized water and emulsified for 15 min by using an emulsifier to obtain a nanofiber suspension.

[0057] 100 ml of 1.5 wt% sodium alginate (SA) aqueous solution was mixed with 100 ml of 1 wt% nanofiber suspension, and emulsified by using an emulsifier for 5 min. 20 ml of diluted glutaraldehyde solution (GA) was added and emulsified for 5 min, and then magnetically stirred for 2 h. Then, 20 ml of 5 wt% polyethyleneimine aqueous solution (PEI) was added to the obtained solution, and magnetically stirred for 2 h to obtain an aerogel slurry, which was denoted as ENSA.

[0058] S2, the ENSA slurry was sprayed on a copper block with an initial temperature of -20°C by using a high-pressure spray gun at a constant liquid flow rate of 100 ml / min. The temperature of the copper block surface during the whole spraying process was in the range of -20°C to -10°C, and then freeze-dried at -40°C for 8 h to obtain a sodium alginate-based aerogel.

[0059] S3, the sodium alginate-based aerogel obtained in step S2 was placed in a 5 wt% calcium chloride (CaCl2) solution for cross-linking reaction for 2 h. The residual CaCl2 solution on the film was washed with deionized water to obtain an aerogel film sample with a continuous gradient pore structure, which could be used for dynamic filtration experiments.

[0060] Please refer to Figure 3 The cross-sectional and planar electron microscope images of the aerogel film with a continuous gradient pore structure prepared in Example 1 are shown. It can be seen that the gradient pore size of the film can be simply divided into three layers, i.e., upper, middle and lower layers. The lower layer is the smallest pore formed by the instantaneous crystallization of the atomized droplets contacting the low-temperature substrate, and the pore size is distributed in the range of 1 μm to 20 μm. Due to the endothermic effect of the crystallization of the droplets in the lower layer, the temperature transferred from the substrate to the middle layer increases, which slows down the crystallization speed of the droplets, and the droplets aggregate to form larger crystals, and the pore size increases, which is distributed in the range of 20 μm to 60 μm. When the temperature gradually increases, the droplets converge and form longitudinal crystals along the temperature direction, i.e., the radial pore channel morphology after freeze-drying. The pore size of this channel is in the range of 60 μm to 110 μm. The SEM image of the large holes on the upper plane of the gradient film has an average pore size of about 75 μm, and the average pore size of the small holes on the lower plane is about 13 μm.

[0061] When the initial temperature of the copper block is -20℃, the droplets in the slurry reach the crystallization condition. Uniformly sized small droplets are sprayed from the spray gun. These droplets crystallize instantly upon contact with the low-temperature copper block, forming the lower layer of micropores (average 13μm). At this point, due to the endothermic process of solution freezing, the temperature of the copper block increases. The higher the temperature, the slower the crystallization rate, and more small droplets aggregate into larger droplets, forming the middle layer of micropores (average 38μm). Similarly, the endothermic process of solution crystallization leads to an increase in the temperature of the copper block, resulting in an upper layer of micropores with even larger pores (average 75μm). Throughout the process, the temperature range of the copper block surface is -20℃ to -10℃. Then, a freeze-drying process yields a gradient-pore aerogel film with decreasing pore size.

[0062] As shown in the figure, the prepared aerogel film consists of three layers: upper, middle, and lower. Specifically, the pore size of the upper layer of the aerogel film ranges from 60 μm to 110 μm, with smooth pore walls and occasional small holes. This not only increases water flux but also allows for multi-directional flow diversion, preventing pore blockage. The pore size of the middle layer is approximately 20 μm to 60 μm, with the number of pores on the pore walls gradually increasing until the lower layer has no pore wall structure. The pores have a diameter of approximately 1 μm to 20 μm and are intertwined with each other, further reducing the pore size.

[0063] Comparative Examples 1-5

[0064] The difference between Comparative Examples 1-5 and Example 1 is that the initial temperature of the copper block surface is different in step S2, and the temperature range of the copper block surface during the spraying process is different; otherwise, they are roughly the same as Example 1, and will not be repeated here. See the table below for details.

[0065]

[0066] Figure 4 The following are cross-sectional SEM images of the aerogel films obtained in Comparative Examples 1-5: (a) 0℃~10℃; (b) -10℃~0℃; (c) -30℃~-20℃; (d) -40℃~-30℃; (e) -50℃~-40℃.

[0067] As can be seen, Figure 4 As shown in (a), the pore size of the thin film under normal temperature conditions is mainly concentrated in the range of 100 μm to 120 μm. Because the substrate surface temperature cannot cause the atomized droplets to crystallize in time, the droplets freely aggregate like ENSA slurry. When transferred to sub-zero conditions, the crystal size is the same as the size of the aggregated droplets.

[0068] Figure 4As shown in (b), when the initial temperature of the copper block is -10℃ (subzero), the droplets in the slurry reach the crystallization condition. During the whole process of spraying, the temperature of the copper block gradually rises due to the endothermic process of solution freezing, and the temperature range during the whole process is -10℃-0℃. It can be seen that the obtained aerogel film has two layers. The lower layer is a small pore structure formed by the crystallization and freeze-drying of the subzero temperature-conducting matrix crystallized droplets. Due to the initial temperature of the matrix being -10℃ and the endothermic effect of the lower layer solution crystallization, the temperature transmitted to the upper layer increases, the crystallization speed slows down, the droplets converge, and the temperature conduction distance is limited, so that the upper plane is disordered crystallization to form large pores.

[0069] As shown in (b), when the initial temperature of the copper block is -10℃ (subzero), the droplets in the slurry reach the crystallization condition. During the whole process of spraying, the temperature of the copper block gradually rises due to the endothermic process of solution freezing, and the temperature range during the whole process is -10℃-0℃. It can be seen that the obtained aerogel film has two layers. The lower layer is a small pore structure formed by the crystallization and freeze-drying of the subzero temperature-conducting matrix crystallized droplets. Due to the initial temperature of the matrix being -10℃ and the endothermic effect of the lower layer solution crystallization, the temperature transmitted to the upper layer increases, the crystallization speed slows down, the droplets converge, and the temperature conduction distance is limited, so that the upper plane is disordered crystallization to form large pores. Figure 4 As shown in (c), (d), and (e), no gradient pores are formed. This is because the initial temperature of the matrix is too low, and the sprayed droplets are crystallized instantaneously. Moreover, the temperature conduction time is long and the distance is far from the bottom to the top during the spraying process, so that the duration of the crystallization of the atomized droplets is longer, and the thickness of the micro-nano pores formed thereby is increased. If the endothermic effect during the continuous spraying process can increase the temperature to an appropriate -20℃-10℃, the gradient structure will also appear at this position, and radial pores formed along the temperature transmission direction can be obviously observed.

[0070] Filtering separation and adsorption separation experiment

[0071] As shown in (b), when the initial temperature of the copper block is -10℃ (subzero), the droplets in the slurry reach the crystallization condition. During the whole process of spraying, the temperature of the copper block gradually rises due to the endothermic process of solution freezing, and the temperature range during the whole process is -10℃-0℃. It can be seen that the obtained aerogel film has two layers. The lower layer is a small pore structure formed by the crystallization and freeze-drying of the subzero temperature-conducting matrix crystallized droplets. Due to the initial temperature of the matrix being -10℃ and the endothermic effect of the lower layer solution crystallization, the temperature transmitted to the upper layer increases, the crystallization speed slows down, the droplets converge, and the temperature conduction distance is limited, so that the upper plane is disordered crystallization to form large pores. Figure 6 As shown in (b), when the initial temperature of the copper block is -10℃ (subzero), the droplets in the slurry reach the crystallization condition. During the whole process of spraying, the temperature of the copper block gradually rises due to the endothermic process of solution freezing, and the temperature range during the whole process is -10℃-0℃. It can be seen that the obtained aerogel film has two layers. The lower layer is a small pore structure formed by the crystallization and freeze-drying of the subzero temperature-conducting matrix crystallized droplets. Due to the initial temperature of the matrix being -10℃ and the endothermic effect of the lower layer solution crystallization, the temperature transmitted to the upper layer increases, the crystallization speed slows down, the droplets converge, and the temperature conduction distance is limited, so that the upper plane is disordered crystallization to form large pores.

[0072] Figure 7 It is a pusher device diagram for dynamic filtration. The device is to fix the syringe vertically and set a certain speed (0.1mm / s-2mm / s) to realize the uniform speed of the micro-plastic solution through the gradient pore structure of the aerogel film.

[0073] Figure 8 It is to set different moving speeds of the pusher to explore the interception capacity of the aerogel gradient film under different fluxes.

[0074] Dynamic experiment sample preparation:

[0075] Three kinds of uniform pore membranes and the aerogel film with gradient pore structure prepared in Example 1 are used as samples for dynamic filtration of micro-plastics to explore the influence of different pore diameters on the filtration effect of micro-plastics.

[0076] The cross-sectional SEM images of the three kinds of uniform pore membranes are as follows:Figure 5 The average pore size of the macroporous uniform membrane is about 93 μm, as shown in (a) of FIG. 1. Figure 5 The average pore size of the mesoporous uniform membrane is about 45 μm, as shown in (b) of FIG. 1. Figure 5 The average pore size of the microporous uniform membrane is about 18 μm, as shown in (c) of FIG. 1. Figure 5 The average pore size of the microporous uniform membrane is about 18 μm, as shown in (c) of FIG. 1.

[0077] A 1 g / L polyethylene terephthalate plastic solution (PET) was prepared, the microplastics had a diameter of 5 μm, and the solvent was 0.5 g / L Tween 80. The solution was uniformly dispersed by ultrasonic dispersion for use.

[0078] The uniform membranes with the three aforementioned pore sizes and the aerogel film with the shear-thinning pore structure prepared in Example 1 were each cut into a circle with a diameter of 25 mm to be placed in a filter clamp device. In order to protect the film from being damaged when the device is mechanically twisted, a PP base ring with an outer diameter of 25 mm and an inner diameter of 23 mm was prepared, so that the inner part of the filter clamp device was a sealing ring-gradient film-PP base. At this time, the preparation of the filtration device was completed, as shown in FIG. 2. Figure 6

[0079] A syringe device was used to achieve uniform passage of the plastic solution through the gradient structure. The device was obtained by vertically embedding and fixing a syringe and uniformly pushing the upper part of the syringe with a flat plate. The speed can be set in the range of 0.1 mm / s to 1 mm / s.

[0080] As shown in FIG. 4, the syringe was filled with 20 ml of the microplastic solution and placed in the syringe device. Then, the inlet of the filtration device was connected to the outlet of the syringe, and a transparent sample bottle was placed below to store the filtered plastic solution. Figure 7 The syringe device was started, and the plastic solution passed through the gradient film at a uniform speed of 0.5 mm / s. The clarity of the sample bottle could be quickly observed to determine the retention effect of the gradient film on microplastics of a certain particle size. The flux of the microplastic solution on the gradient film could be calculated using the total volume of the filtrate, the required time for the entire experiment, and the area of the gradient film.

[0081] The retention rate was calculated as the percentage of the ratio of the mass difference of the microplastics in the solution before and after the filtration of the aerogel film to the initial mass. The mass of the microplastics in the solution was obtained by calculating the dry weight difference before and after the filtration of the nanofiltration membrane using the filtration device.

[0082] The water flux is the volume of water that can pass through a unit area of the membrane per unit time, and the unit is usually liters per square meter per hour (L / m 2 / h). The calculation formula is:

[0083] F = V / A*t

[0084] F = V / A*t

[0085] ​Where F is the water flux, V is the volume of water passed through the membrane (unit: L), A is the area of the membrane (unit: m 2 ), and t is the time (unit: h). Since the membrane area and the volume of solution passed through are constant, the water flux calculated by this method is inversely proportional to time and positively correlated with speed. Under the premise of ensuring the integrity of the membrane, the faster the setting speed of the syringe, the greater the water flux.

[0086] The mass of microplastics in the microplastic solution is measured by drying the microplastics in a 60°C oven after being filtered by a nanofiltration membrane until the mass no longer changes, and the weight is measured for quantification, and the calculation formula is:

[0087] M = M2- M1

[0088] Where M1 is the mass of the dried filter membrane, and M2 is the dried mass of the filter membrane after filtering microplastics.

[0089] The retention rate is the ability of the membrane to prevent or retain microplastics in the feed liquid, and the commonly used formula is:

[0090] R = (1- M / M0) * 100%,

[0091] Where R is the retention rate, M is the mass of the filtered microplastics, and M0 is the initial mass of the microplastics in the original solution.

[0092] The test results are shown in the following table.

[0093]

[0094] As shown in the above table, when the moving speed of the syringe is set to 0.5 mm / s, the flux is 1257 (L / m 2 *h). At this time, the retention rate of the 93 pm large-pore membrane film to 5 pm PET is as low as 23%, because the large-pore structure is conducive to the rapid passage of the microplastic solution, but the pores much larger than the diameter of the microplastics are not conducive to the filtration effect; the retention rate of the homogeneous medium-pore membrane with an average pore size of 45 pm to microplastics is 63%, which is higher than that of the large-pore membrane, which may be because the internal pores of the membrane are narrowed and interlaced, making the path of the microplastics inside the membrane longer. And in the entire filtration process, the retention rate will be further improved as the pores are blocked by the interception of microplastics; the retention rate of the homogeneous small-pore membrane with an average pore size of 18 pm is 94%, which is Figure 5(C) can be seen that the pores of the film are further reduced and interlaced into a more compact three-dimensional network structure, and the 18 μm uniform pore feature significantly improves the filtration capacity of the film for 5 μm PET, but this feature also makes the film more prone to clogging; the prepared aerogel film with a continuous gradient pore structure in the application has a retention rate of up to 98% for 5 μm PET, and the 100 μm large pore structure in the middle and upper layers can accommodate more microplastics and thus reduce clogging.

[0095] In addition, the gradient membrane area of a single filter (such as Figure 6 shown) is about 415 mm 2 At a flux of 1257 (L / m 2 *h), the film can continuously filter 20 ml of 1 g / L 5 μm plastic solution for 15 times, which is equivalent to 1 m 2 The gradient membrane can continuously filter about 722 L of 1 g / L plastic solution, and ensure a stable retention rate of 98% throughout the filtration process (as shown in Figure 9 ).

[0096] In order to explore the retention effect of the gradient membrane at different fluxes, the moving speed of the syringe pump was set to 0.1 mm / s, 0.5 mm / s, 1 mm / s, 1.5 mm / s and 2 mm / s, thereby controlling the flux size of the filtration experiment, and by the same test method as 0.5 mm / s, the retention rates were measured as shown in the table. It can be seen that the gradient aerogel film prepared in the application can still achieve a high retention rate of 98% (as shown in Figure 8 ) at a high flux of 4956 (L / m 2 *h).

[0097]

[0098] In summary, the application provides a preparation method of an aerogel film with a continuous gradient pore structure, which comprises the following steps: spraying an aerogel slurry containing a polymer nanofiber suspension, a sodium alginate solution, a polyethyleneimine aqueous solution and a crosslinking agent on a substrate with an initial temperature of-20℃ to-15℃ at a high pressure, using the temperature gradient formed on the surface of the substrate within a certain time to complete the low-temperature freezing process in one step, and then performing freeze-drying treatment, thereby obtaining an aerogel film with a continuous gradient pore structure. Moreover, the obtained aerogel film can not only rely on the pore size to filter and separate microplastics, but also rely on the electrostatic interaction between various functional groups on the film and microplastics for adsorption and separation. Moreover, the gradient structure of the film can greatly improve the "trade-off" effect in membrane filtration and separation, and can balance high water flux and high retention rate, thereby providing a new strategy for aerogel in the membrane field and showing the potential for practical application.

[0099] The above examples are only used to illustrate the technical solutions of the present application but not limit the present application. Although the present application is described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or equivalent replaced without departing from the spirit and scope of the technical solutions of the present application.

Claims

1. A method for preparing an aerogel film having a continuous gradient pore structure, characterized by, The method comprises the following steps: S1, mixing the polymer nanofiber suspension with a sodium alginate solution, adding a crosslinking agent, then adding a polyethyleneimine aqueous solution, and stirring to obtain an aerogel slurry; S2, high-pressure spraying the aerogel slurry obtained in step S1 on a temperature-conducting substrate with an initial temperature of -20℃ to -15℃, and freeze-drying the aerogel slurry at -40℃ to -50℃ for 8-12 hours to obtain a sodium alginate-based aerogel; during the spraying process, the temperature range of the surface of the substrate is -20℃ to -10℃; S3, crosslinking the sodium alginate-based aerogel obtained in step S2 in a calcium chloride solution with a mass fraction of 5%-10% for 2-6 hours, and washing to obtain an aerogel film with a continuous gradient pore structure.

2. The method for preparing an aerogel film with a continuous gradient pore structure according to claim 1, characterized in that, In step S2, the high-pressure spraying process is to control the spraying distance to be 150-200 mm by using a high-pressure spray gun, and the aerogel slurry obtained in step S1 is continuously sprayed on the temperature-conducting substrate with an initial temperature of -20℃ to -15℃ within 30-50 seconds.

3. The method of claim 1, wherein the method further comprises: In step S1, in the pre-crosslinking solution, the mass ratio of the polymer nanofiber, sodium alginate and polyethyleneimine is 1:(1.1-1.2):(0.2-0.4).

4. The method of claim 1, wherein the method further comprises: The polymer nanofiber is a PVA-co-PE nanofiber, the molecular weight of the polyethyleneimine is 10000-60000, and the crosslinking agent is a polyaldehyde.

5. The method of claim 1, wherein the aerogel film having a continuous gradient pore structure is prepared by the steps of: The temperature-conducting substrate is a copper block, and it needs to be placed in a -40℃ freezer for 90-180 minutes before spraying to control the surface temperature of the temperature-conducting substrate.

6. An aerogel film having a continuous gradient pore structure, characterized by, The aerogel film with a continuous gradient pore structure is prepared by the preparation method of any one of claims 1-5; the aerogel film comprises three layers from top to bottom, and the pore size of the aerogel film gradually increases from bottom to top, and the pore size distribution range is 1μm-110μm.

7. The aerogel film with a continuous gradient pore structure according to claim 6, characterized in that: The pore size range of the pores in the upper layer of the aerogel film is 60μm-110μm, the pore size range of the pores in the middle layer is 20μm-60μm, and the pore size range of the pores in the lower layer is 1μm-20μm.

8. The aerogel film with a continuous gradient pore structure according to claim 7, characterized in that: The aerogel film with the continuous gradient pore structure has a thickness of 0.8-1.2 mm and a gram weight of 25 g / m 2 ~ 30 g / m 2 .

9. Use of an aerogel film having a continuous gradient pore structure, characterized in that The aerogel film with a continuous gradient pore structure is prepared by the preparation method of any one of claims 1-5 or is the aerogel film of any one of claims 6-8; the aerogel film is placed in a filter membrane clamp, and the solution to be filtered passes through the upper layer, the middle layer and the lower layer of the aerogel film in turn at a speed of 0.1mm / s-1mm / s.

10. Use of an aerogel film with a continuous gradient pore structure according to claim 9, characterized in that It is used in the field of microplastic filtration.

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