CN115445453B - A method using solute-solvent co-crystallization to prepare ultra-high-throughput superhydrophobic PVDF membranes - Google Patents

Abstract

The invention discloses a method for preparing an ultra-high flux super-hydrophobic PVDF membrane by utilizing solute solvent co-crystallization, which comprises the following steps: (1) preparation of PVDF with low crystallinity: PVDF is soaked in alkaline methanol solution for reaction, and the reacted mixed solution is added into NaHSO ₃ Standing in an aqueous solution, filtering, washing, and freeze-drying to obtain PVDF with low crystallinity; (2) PVDF super-hydrophobic membrane preparation: and (3) blending PVDF and PVDF with low crystallinity, dissolving in dimethyl sulfoxide to form PVDF solution, standing, spreading on a flat plate to form a coating, immersing in liquid nitrogen, immersing in ice water bath, taking out the PVDF film, and naturally airing. The super-hydrophobic PVDF membrane prepared by the invention has stable performance and high permeation flux, and can be manufactured and applied in large scale.

Description

A method using solute-solvent co-crystallization to prepare ultra-high-flux superhydrophobic PVDF membrane

technical field

The invention relates to the technical field of membranes, in particular to a method for preparing ultra-high-throughput super-hydrophobic PVDF membranes by using a solute-solvent co-crystallization method.

Background technique

Oil-water separation has always been a major challenge in water treatment, and the emergence of superhydrophobic membranes provides a new path for oil-water separation. The water contact angle of the superhydrophobic film is required to be greater than 150°, and the sliding angle is less than 10°. In addition to size screening, its effective separation of oil and water depends more on the selective permeation of oil and water on the membrane surface, that is, water is bounced off the membrane surface, while oil substances can permeate on the membrane surface. Even when treating emulsifier-stabilized oily wastewater, the superhydrophobic membrane still exhibited high separation efficiency. Therefore, it is particularly important to prepare super-hydrophobic membranes efficiently, mainly around two technical points: one is to use low surface energy materials as raw materials; the other is to construct a high-roughness structure. Based on the above two points, many scholars have successfully prepared superhydrophobic membranes. However, the current preparation of superhydrophobic membranes faces problems such as complex preparation process, high production cost, and poor stability. In addition, the preparation method of using polyfluorine groups as low surface energy coating modification may even cause environmental risks. Therefore, the method for the preparation of efficient superhydrophobic membranes still needs to be explored.

Due to its excellent chemical stability and thermodynamic stability, PVDF membranes are widely used in filtration units for wastewater treatment. PVDF also has relatively low surface free energy, so it is often used to prepare superhydrophobic membranes, but currently limited by the preparation process, the permeation flux is relatively low. At present, the superhydrophobic modification of PVDF membranes is mainly blended with inorganic particles or blended with polyfluoropolymers. The disadvantage is that the stability of the material is relatively poor, and the blended substance is easy to fall off, which is easy to cause environmental risks. Other methods, such as electrospinning and template methods, are relatively complicated to operate and difficult to control experimental variables. In view of this, it is necessary to propose a simple and efficient method for preparing superhydrophobic PVDF membranes to address the above problems, so that it has stable superhydrophobic properties and relatively high permeation flux.

Contents of the invention

In order to solve the technical problem of complex preparation process and poor stability of the current super-hydrophobic membrane, the present invention provides a method using solute-solvent co-crystallization to prepare a super-hydrophobic polyvinylidene fluoride (PVDF) membrane with ultra-high flux. PVDF particles with low crystallinity are used to assist in the construction of a graded two-dimensional rough structure to form a rough surface. At the same time, liquid nitrogen freezing is used to rapidly crystallize the solvent (dimethyl sulfoxide) to form open membrane pores to increase the flux of the super-hydrophobic PVDF membrane. The preparation method of this kind of superhydrophobic membrane is relatively simple, the variables that need to be controlled are relatively few, the repeatability is high, and the performance of the membrane is more stable.

A method for preparing ultra-high-throughput super-hydrophobic PVDF membranes, specifically comprising the following steps

(1) Preparation of PVDF with low crystallinity: Soak the initial PVDF powder in alkaline methanol solution, stir and react at 50-70°C for 1-5h (such as 1h, 2h, 3h, 4h, 5h), the color of the mixed solution changes from white to red to dark brown. The reacted mixture was transferred to NaHSO ₃ aqueous solution (acidic) and allowed to stand for 1-3h. Afterwards, the modified PVDF in the mixed liquid is filtered out with a vacuum pump, washed with deionized water to neutrality, and PVDF powder with low crystallinity is obtained after freeze-drying.

(2) Preparation of PVDF superhydrophobic membrane: The initial PVDF powder and low crystallinity PVDF powder are blended at a mass ratio of 4-1:1 (for example, 4:1, 3:1, 2:1, 1:1), dissolved in dimethyl sulfoxide (DMSO), heated and stirred at 60-80°C until completely mixed to form a PVDF solution, and then left to remove air bubbles. Scrape-coat the solution on the plate to form a coating, then quickly immerse the plate in liquid nitrogen for 1-5s until complete crystallization, then transfer to an ice-water bath (~0°C) to remove the DMSO template, take out the PVDF film, and dry naturally to obtain a PVDF super-hydrophobic film.

Based on the above technical scheme, preferably, in step (1), the concentration of the basic methanol solution is 4-6%, preferably 5wt%, the alkali in the basic methanol solution is potassium hydroxide, and the preparation method of the basic methanol solution is: potassium hydroxide is dissolved in methanol and prepared.

Based on the above technical solution, preferably, in step (1), the ratio of the PVDF powder to the alkaline methanol solution is 5-15g:200mL, preferably 10g:200mL.

Based on the above technical solution, preferably, in step (1), the concentration of the NaHSO ₃ aqueous solution is 1-2 wt%, preferably 1.2 wt%.

Based on the above technical scheme, preferably, in step (1), the volume ratio of the alkaline methanol solution to the NaHSO _aqueous solution is 1:1.

Based on the above technical solution, preferably, in step (2), the total concentration of PVDF powder and low-crystallinity PVDF powder in the PVDF solution is 5-7wt%, preferably 6wt%.

Based on the above technical solution, preferably, in step (2), the standing time is 12-24h.

Based on the above technical solution, preferably, in step (2), the material of the plate is glass, aluminum or copper.

Based on the above technical solution, preferably, in step (2), the thickness of the coating is 100-250 μm, preferably 200 μm.

Based on the above technical solution, preferably, in step (2), the time for immersing in the ice-water bath is more than 12 hours until the crystal template is completely removed. Based on the above technical scheme, preferably, in step (2), the mass ratio of the initial PVDF powder to the low crystallinity PVDF powder is 3:1.

Beneficial effect

The super-hydrophobic PVDF membrane of the present invention is formed by co-crystallization of solute (initial PVDF powder and low crystallinity PVDF powder after thermal alkali modification) and solvent (DMSO) under freezing of liquid nitrogen. The crystallization of the solute obtains a hierarchical two-dimensional rough structure, and the crystallization of the solvent enables the membrane to obtain ultra-high permeation flux. Compared with the method of adding inorganic particles, this method uses the blending of homologous organic substances, which has more uniform mixing and more stable surface properties. Compared with the method of adding polyfluoropolymers, this method uses PVDF (lower fluorine content) after thermal alkali modification is relatively more environmentally friendly. In general, this preparation method is easier than most of the current researches. There are relatively few variables to be controlled, and the repeatability is strong. The prepared superhydrophobic PVDF membrane has stable performance and high permeation flux. It is a superhydrophobic membrane that can be manufactured and applied on a large scale.

Description of drawings

Fig. 1 is the description of the preparation process involved in the present invention; wherein, a is the preparation process of low crystallinity PVDF powder; b is the description of the process of preparing the film by solute solvent co-crystallization method.

Fig. 2 is the surface contact angle diagram of the low crystallinity PVDF powder prepared in Example 1.

Figure 3 is an electron micrograph of the surface of the low crystallinity PVDF powder prepared in Example 1, wherein PVDF-0 was prepared by heating and stirring at 60°C for 0h, PVDF-1 was prepared by heating and stirring at 60°C for 1h, PVDF-2 was prepared by heating and stirring at 60°C for 2h, PVDF-3 was prepared by heating and stirring at 60°C for 3h, PVDF-4 was prepared by heating and stirring at 60°C for 4h, and PVDF-5 was prepared by heating and stirring at 60°C for 5h.

Figure 4 is the contact angle diagram of the PVDF film prepared in different proportions in Example 1, where PVDF@PVDF-4 is a blend of PVDF and PVDF powder modified for 4h (low crystallinity PVDF obtained by heating and stirring at 60°C for 4h).

Fig. 5 is an electron microscope image of PVDF films prepared in different proportions in Example 1, wherein the PVDF with low crystallinity is PVDF with low crystallinity obtained by heating and stirring at 60° C. for 4 hours.

Figure 6 is a dynamic process diagram of the sliding angle test of the PVDF film surface prepared in Example 1, wherein a is a pure PVDF film without rolling; b is a superhydrophobic film surface obtained by blending PVDF and PVDF powder modified for 4 hours (low crystallinity PVDF obtained by heating and stirring at 60°C for 4 hours) in a mass ratio of 3:1, and rolling occurred.

Fig. 7 is a characterization graph of acid-base resistance and thermodynamic stability of the PVDF membrane prepared in Example 1 (the mass ratio of PVDF to PVDF powder modified for 4h is 3:1).

Figure 8 is a diagram of a graduated cylinder filter.

Figure 9 shows the permeation flux of PVDF membrane with self-gravity as the driving force.

Detailed ways

The following non-limiting examples can enable those skilled in the art to understand the present invention more fully, but do not limit the present invention in any way.

The test methods described in the following examples, unless otherwise specified, are conventional methods; the reagents and materials, unless otherwise specified, can be obtained from commercial sources.

Example 1

The preparation method of the solute-solvent co-crystallized superhydrophobic PVDF membrane is as follows:

Step 1: Put 10g of PVDF in 200mL alkaline methanol solution (5wt% KOH dissolved in methanol), heat and stir at 60°C (0, 1, 2, 3, 4, 5h respectively), soak the mixed solution in 200mL NaHSO ₃ aqueous solution (1.2wt%) for 2h, filter out the modified PVDF in the mixed solution with a vacuum pump, then clean the modified PVDF powder with deionized water, and obtain low crystallinity after freeze-drying of PVDF powder.

Step 2: Blend PVDF and PVDF with low crystallinity (4:1, 3:1, 2:1, 1:1 respectively) in DMSO (6wt%), heat and stir at 80°C for 12h until uniform, then let stand for 12h to remove air bubbles. Scrape-coat the solution on a glass sheet (length×width: 10cm×10cm, thickness 1mm) to form a coating. The thickness of the coating is 200 μm, then quickly immerse the glass sheet in liquid nitrogen for about 3 seconds, then immerse it in an ice-water bath and let it stand for 12 hours until the crystal template is completely removed. Take out the PVDF membrane and let it dry naturally.

Example 2

The modified PVDF powder was scratch-coated on a glass plate according to the preparation method of the PVDF film in Example 1, and after freezing and crystallizing with liquid nitrogen, it was placed in an ice-water bath to remove the template to obtain the surface of low-crystallinity PVDF to investigate the possibility of preparing low-crystallinity PVDF. The contact angle of the material surface is shown in Figure 2. The water contact angle of the unmodified initial PVDF is about 130°. With the increase of the modification time, the surface contact angle of the modified material at 1h and 2h decreased, but when the modification time reached 3h, the contact angle increased significantly. This is because the crystallinity of PVDF decreased significantly, and the small spherical crystals increased the roughness of the material surface. In summary, it shows that PVDF particles with low crystallinity can be obtained by thermal alkali modification, forming a rough superhydrophobic structure.

Example 3

In order to construct a stable super-hydrophobic PVDF membrane, here, we consider blending PVDF with modified PVDF for 4 hours (low crystallinity PVDF obtained by heating and stirring at 60°C for 4 hours) to construct a super-hydrophobic PVDF membrane with a micro-nano layered structure, blending at a ratio of (4:1, 3:1, 2:1, 1:1, respectively). The contact angle of the film surface is used to measure its superhydrophobic performance, and a superhydrophobic surface is obtained when the ratio is 3:1, as shown in Figure 4. The electron micrograph is shown in Figure 5. The large spherical crystal is formed by the initial PVDF, and the small spherical crystals around it are formed by the modified 4h PVDF. The electron micrograph clearly reflects the surface morphology of the membrane. In order to ensure high mechanical properties, 3:1 was selected as the optimal film-making ratio. The sliding angle was measured on a simple self-made slant plate, and the dynamic process was captured by a high-speed camera. The film began to slide on an inclined plane of about 4°. The surface of the film has good superhydrophobic properties, as shown in Figure 6.

Example 4

Using the superhydrophobic membrane with the optimal ratio (3:1) prepared in Example 3, its superhydrophobic stability was investigated. Soak in ethanol solutions with pH values (respectively 2, 4, 6, 8, 10, 12) for 24 hours, and measure the contact angle, as shown in Figure 7, it is found that it only slightly decreases when the pH is at 10 and 12 (due to dehydrogenation and defluorination), but it is still above 140°. The membrane exhibits extremely high tolerance and stability in acidic environments. Then, we conducted a thermodynamic test on the film, and placed it in an environment (respectively at -196, -80, -25, 25, 50, 75, 100, 125, 150, and 175°C) for 2 hours, and found that the contact angle of the film hardly changed, indicating that the superhydrophobic film can still maintain good superhydrophobic properties under fluctuating temperature environments. In summary, it is shown that the superhydrophobic membrane has good acid environment resistance and thermodynamic stability.

Example 5

The self-gravity separation process is carried out by using the graduated cylinder filter in Figure 8. The membrane is clamped in the middle, the upper part is used to hold the organic reagent, and the lower part is used to hold the leachate. The driving force is the gravity of the liquid itself. To investigate the permeability of the membrane, filter several common organic reagents (dichloromethane (CH ₂ Cl ₂ ), toluene (Toluene), petroleum ether (PE) and isooctane (Isooctane) respectively) to examine the filtration performance of the membrane. It is found that the membrane (3:1) can complete the permeation process driven by only 6.5cm liquid gravity, and the permeation flux of dichloromethane is as high as 88000L/m ² ·h (the effective filtration area of the membrane is: 0.64cm ² ). The fluxes of toluene, petroleum ether and isooctane are all higher than 45000L/m ² ·h, see Figure 9. This shows that the PVDF membrane prepared by the invention has extremely high permeation flux.

Claims (10)

1. The method for preparing the superhigh flux superhydrophobic PVDF membrane is characterized by comprising the following steps:

(1) Low crystallinity PVDF preparation: soaking PVDF in alkaline methanol solution, stirring at 50-70deg.C for 1-5 hr, and adding the reacted mixture into NaHSO ₃ Standing in water solution for 1-3 hr, filtering, washing, and freeze drying to obtain low crystalPVDF in degrees;

(2) Preparing a PVDF super-hydrophobic membrane: blending PVDF and PVDF with low crystallinity, dissolving in dimethyl sulfoxide, heating and stirring at 60-80deg.C to uniformity to form PVDF solution, and standing to remove bubbles; then scraping the PVDF solution on a flat plate to form a coating, immersing the flat plate in liquid nitrogen for 1-5s, immersing the flat plate in ice water bath to remove dimethyl sulfoxide, taking out the PVDF film, and naturally airing to obtain the PVDF super-hydrophobic film; wherein the mass ratio of PVDF to PVDF with low crystallinity is 4-1:1.

2. The method according to claim 1, wherein in the step (1), the concentration of the basic methanol solution is 4 to 6%, and the alkali in the basic methanol solution is potassium hydroxide.

3. The method of claim 1, wherein in step (1), the ratio of PVDF powder to alkaline methanol solution is 5-15g:200mL.

4. The method of claim 1, wherein in step (1), the NaHSO ₃ The concentration of the aqueous solution is 1-2wt%.

5. The method according to claim 1, wherein in step (1), the basic methanol solution is mixed with NaHSO ₃ The volume ratio of the aqueous solution is 1:1.

6. The method of claim 1, wherein in step (2), the PVDF solution is at a concentration of 5-7wt%.

7. The method of claim 1, wherein in step (2), the time of resting is 12-24 hours.

8. The method of claim 1, wherein in step (2), the plate is made of glass, aluminum or copper.

9. The method according to claim 1, wherein in step (2), the thickness of the coating is 100-250 μm.

10. The method according to claim 1, wherein in the step (2), the time for immersing in the ice water bath is 12 hours or longer.