Charged pvdf and copolymer thereof separation membrane, preparation method and use

Abstract

The application discloses a novel charged PVDF and copolymer membrane preparation based on piezoelectric effect, which imparts inherent dipole moment to the membrane in the polarization process, forms a double electron layer at the solid-liquid interface when liquid filtration is performed, charges the solid surface, and achieves the purpose of resisting the same charge pollutants. The P(VDF-TrFE) powder is mixed with a pore forming agent (such as PVP, PEG, etc.) solution by using a solution blending method, the PVDF powder is mixed with a pore forming agent (such as PVP, PEG, etc.) and a water-soluble ionic liquid solution, a phase separation method is used to prepare a membrane, residual solvents in the membrane are removed, and the membrane is dried to obtain a pure beta phase PVDF and copolymer porous membrane. The membrane is polarized by using corona polarization or thermal polarization. The polarization temperature is 30-110 DEG C, and the polarization time is 0.5-2 hours. Finally, the electric field is removed, and the membrane is stored at room temperature.

Description

Technical Field

[0001] This invention relates to a method for preparing a separation membrane of charged PVDF and its copolymers, and more specifically to a method for improving the chargeability of PVDF and its copolymer separation membranes. Background Technology

[0002] PVDF and its copolymers are semi-crystalline and partially fluorinated polymers. They possess not only the excellent thermal stability, oxidation resistance, and radiation resistance of fluorinated polymers, but also certain chemical stability, excellent mechanical properties, dielectric properties, good processability, and good solubility in polar solvents. These favorable properties make PVDF-TrFE an important material for preparing porous membranes. In the preparation of PVDF-TrFE membrane materials, such as through non-solvent-induced phase separation (NIPS) or thermally induced phase separation methods, polar solvents such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP) are added as solvents for PVDF-TrFE. Generally, membranes prepared through phase separation have poor charge, which limits the antifouling and retention properties of PVDF and its copolymer membranes. In charged ultrafiltration, if molecules carry the same type of charge as the charged membrane, even if the molecules are smaller than the membrane pore size, they cannot pass through the membrane due to electrostatic repulsion. This separates molecules of similar size but different charges, achieving selective separation in ultrafiltration. Due to the electrostatic repulsion of like charges, the membrane's antifouling properties are also enhanced. Charged membranes offer advantages and applications that neutral membranes lack in improving retention rates, enhancing antifouling properties, and selective permeability. Traditional methods for preparing charged PVDF membranes typically include impregnation, surface treatment, and blending. However, these methods produce membranes with poor charge properties and uneven charge distribution, significantly impacting the use of charged PVDF and its copolymer separation membranes. Summary of the Invention

[0003] This invention addresses the low chargeability of PVDF and its copolymers in current production processes, aiming to find a novel method to improve the chargeability of PVDF and its copolymer membranes, thereby enhancing their antifouling and retention performance. A method for polarizing P(VDF-TrFE) separation membranes to obtain chargeability is proposed. This method involves polarizing the β-phase PVDF and its copolymer membranes in an electronegative gaseous medium or high-dielectric-constant silicone oil or mineral oil. Even after the electric field is removed, the β-phase PVDF and its copolymers retain their dipole moment reversal, resulting in a structure with more H atoms at one end and more F atoms at the other. At the liquid-solid interface, the membrane surface selectively adsorbs certain ions, becoming positively charged by adsorbing positive ions and negatively charged by adsorbing negative ions. The end in contact with the negative electrode is enriched with more H atoms, allowing for the adsorption of more negative ions and resulting in a negative charge. Based on electrostatic interactions, this provides better repulsion of negatively charged pollutants during water treatment, thus improving antifouling performance.

[0004] A polymer separation membrane is made of PVDF or a PVDF-based copolymer, with the main body being the β phase, and the zeta potential difference between one side and the other side of the membrane is greater than 10 mV.

[0005] In one implementation, the zeta potential difference is above 20mV.

[0006] In one embodiment, "mainly β phase" means that more than 90% of the material is β phase, preferably 100%.

[0007] In one embodiment, the polymer separation membrane has a porosity of 50-90% and a tensile strength of 5-50 MPa.

[0008] In one embodiment, the PVDF-based copolymer is P(VDF-TrFE).

[0009] The preparation method of the polymer separation membrane includes the following steps:

[0010] PVDF or PVDF-based copolymers and additives are dissolved in an organic solvent to form a casting solution;

[0011] A porous separation membrane was prepared by phase inversion of the casting solution;

[0012] The porous separation membrane is polarized in a medium.

[0013] The mass fraction of PVDF or PVDF-based copolymers in organic solvents is 5-35%.

[0014] The additive has a mass fraction of 0.5%-10% in the organic solvent.

[0015] In one embodiment, the additive is a porogen or a water-soluble ionic liquid.

[0016] In one embodiment, the pore-forming agent is one or more of polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), and glycerin.

[0017] In one embodiment, the polyvinylpyrrolidone is PVP K30 or PVP K90.

[0018] In one embodiment, the water-soluble ionic liquid is one or more of 1-ethyl-3-methylimidazolium chloride ([Emim]Cl) and 1-ethyl-3-methylimidazolium hydrogen sulfate ([Emim]HSO4).

[0019] In one embodiment, the organic solvent is one or more of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), or cyclohexanone.

[0020] In one embodiment, deionized water is used as the coagulation bath in the phase inversion method.

[0021] In one embodiment, after obtaining the base film through the casting solution in the phase inversion method, it needs to pass through an air bath before entering the coagulation bath.

[0022] In one embodiment, the air humidity in the air bath is 40-70% RH.

[0023] In one embodiment, the medium is liquid.

[0024] In one embodiment, the medium is mineral oil or silicone oil.

[0025] In one embodiment, the polarization temperature is 30°C to 110°C.

[0026] In one implementation, the polarization time is 0.5h to 2h.

[0027] In one embodiment, the phase inversion method employs non-solvent-induced phase separation (NIPS), thermally induced phase separation (TIPS), direct drying, and a combined thermally / non-solvent-induced phase separation method (N-TIPS).

[0028] Applications of polymer separation membranes in liquid filtration.

[0029] In one embodiment, the liquid filtration refers to the filtration of a negatively charged substance.

[0030] In one embodiment, the negatively charged substance is selected from proteins, cells, colloidal particles, or natural organic matter.

[0031] Beneficial effects

[0032] This invention prepares a PVDF and its copolymer polymer separation membrane with a high β-phase composition, exhibiting high charge characteristics after polarization and resistance to membrane fouling. In the preparation method of this invention, the PVDF and its copolymer separation membrane is polarized in a strongly electronegative gas medium and in silicone oil or mineral oil, avoiding the damage to the separation membrane structure caused by air polarization and improving the breakdown field strength of the separation membrane. The charged PVDF and its copolymer membrane prepared by this invention has broad application prospects in membrane separation, flexible transducers, smart wearables, and human tissue engineering. Attached Figure Description

[0033] Figure 1 The preparation mechanism and application mechanism of piezoelectric PVDF film.

[0034] Figure 2 XRD characterization of β-phase PVDF and its copolymer films

[0035] Figure 3 Infrared characterization of β-phase PVDF and its copolymer films Figure 4 EDS characterization of the front and back sides of the polarized PVDF film

[0036] Figure 5 It is a characterization of the zeta potential of the polarized PVDF film and the zeta potential of the unpolarized film.

[0037] Figure 6 These are the DSC test results of β-phase PVDF and its copolymer films. Detailed Implementation

[0038] Example 1

[0039] Preparation of PVDF membrane

[0040] 9 g of PVDF, 1.5 g of additive PVPK30, and 0.1 g of water-soluble ionic liquid (1-ethyl-3-methylimidazolium chloride ([Emim]Cl)) were dissolved in 39.5 g of DMAc at 60 °C and stirred for 16 hours to obtain a homogeneous casting solution. The solution was then degassed at 60 °C until a clear, bubble-free solution was obtained. A 250 μm gap height cutter was used at a speed of 1.0 m·min. -1 The membrane was cast at a high speed. After exposure to the atmosphere for 15 seconds, the membrane was immersed in a coagulation bath with a concentration of 30% DMAc at 60°C. The relative humidity of the air was approximately 60%. The prepared membrane was washed several times with deionized water to remove residual solvent and then stored in deionized water.

[0041] membrane polarization

[0042] PVDF films were polarized using thermal polarization with silicone oil as the polarizing medium to improve their residual polarization intensity. The polarization conditions were: polarization electric field strength of 50 MV / m, polarization temperature of 100℃, and polarization time of 30 min. Finally, the polarization field strength was maintained and the film was cooled to room temperature.

[0043] The preparation process of this invention is as follows: Figure 1 As shown, the piezoelectricity of β-phase PVDF originates from the polarity of the β-phase PVDF crystal, specifically the unbalanced positive and negative charges within the crystal, which causes spontaneous polarization. Under natural conditions, β-phase PVDF undergoes spontaneous polarization, producing irregularly stacked dipoles. With the application of a high DC electric field, the spontaneously polarized dipoles align in the same direction. Even after the electric field is removed, the polar axis does not disappear; that is, the dipoles mostly tend to align in the same direction. The PVDF film at this point is a piezoelectric PVDF film.

[0044] During polarization, a positively bound charge layer forms on the film surface near the positive electrode. When in contact with the liquid, the polar surface attracts ions with opposite charges from the solution through electrostatic interactions, thus forming an electric double layer at the solid-liquid interface, including a stern layer and a diffusion layer. The electric double layer at the interface between the piezoelectric PVDF film and the liquid has a high charge density, which allows for the modulation of the surface potential.

[0045] The interaction between the surface charge of the membrane and the charge of the contaminants has a certain impact on membrane fouling. Since proteins, cells, colloidal particles, and natural organic matter all carry a negative charge in aqueous solution, the presence of negatively charged groups on the membrane surface can enhance the electrostatic repulsion between the membrane and the contaminants. Generally speaking, for membrane surfaces, the more negative the zeta potential, the better the antifouling performance.

[0046] Compare with Example 1

[0047] The difference from Example 1 is that no water-soluble ionic liquid was added during the preparation of the PVDF membrane.

[0048] Preparation of PVDF membrane

[0049] 9 g of PVDF and 1.5 g of additive PVPK30 were dissolved in 39.5 g of DMAc at 60 °C and stirred for 16 hours to obtain a homogeneous casting solution. The solution was then degassed at 60 °C until a clear, bubble-free solution was obtained. A 250 μm gap height knife was used at a speed of 1.0 m·min. -1The membrane was cast at a high speed. After exposure to the atmosphere for 15 seconds, the membrane was immersed in a coagulation bath with a concentration of 30% DMAc at 60°C. The relative humidity of the air was approximately 60%. The prepared membrane was washed several times with deionized water to remove residual solvent and then stored in deionized water.

[0050] membrane polarization

[0051] The polarization process employs contact polarization, where electrode plates are placed on the surface of the separation membrane, and the PVDF film is polarized using thermal polarization. Silicone oil is used as the polarization medium to enhance its residual polarization intensity. The polarization conditions are: polarization electric field strength of 50 MV / m, polarization temperature of 100℃, and polarization time of 30 min. Finally, the polarization field strength is maintained, and the film is cooled to room temperature.

[0052] Compare with Example 2

[0053] The difference from Example 1 is that no polarization treatment was performed after the PVDF film was prepared.

[0054] Preparation of PVDF membrane

[0055] 9 g of PVDF, 1.5 g of additive PVPK30, and 0.1 g of water-soluble ionic liquid (1-ethyl-3-methylimidazolium chloride ([Emim]Cl)) were dissolved in 39.5 g of DMAc at 60 °C and stirred for 16 hours to obtain a homogeneous casting solution. The solution was then degassed at 60 °C until a clear, bubble-free solution was obtained. A 250 μm gap height cutter was used at a speed of 1.0 m·min. -1 The membrane was cast at a high speed. After exposure to the atmosphere for 15 seconds, the membrane was immersed in a coagulation bath with a concentration of 30% DMAc at 60°C. The relative humidity of the air was approximately 60%. The prepared membrane was washed several times with deionized water to remove residual solvent and then stored in deionized water.

[0056] Compare with Example 3

[0057] The difference from Example 1 is that air is used as the medium during the polarization process.

[0058] Characterization of crystallinity, phase composition and zeta potential

[0059] The crystallinity of the P(VDF-TrFE) film was characterized using X-ray diffraction (XRD, miniflex 600, RIGAKU, Japan). A copper target was used, and the test parameters were: accelerating voltage 40 kV, tube current 15 mA. The scanning speed and angle range were set to 5°·min. -1 and 5-60°.

[0060] like Figure 2As shown, by using ionic liquid treatment during the preparation of the PVDF membrane, the final membrane body is composed of the β phase; at the same time, the copolymer P(VDF-TrFe) itself is the β phase.

[0061] The functional groups within the membrane were analyzed using Fourier transform infrared spectroscopy (FTIR, Nicolet 8700, Thermo, USA), and the content of β polymorphisms was analyzed through specific characteristic peaks. The sample was scanned 32 times in attenuated total reflectance mode with a resolution of 4.

[0062] like Figure 3 It can be seen that after treatment with the ionic liquid, the infrared spectrum shows that the characteristic peak of the α-phase PVDF at 766 is absent, and instead exhibits the characteristic peak of the entire β-phase. Before the addition of the ionic liquid, the surface of the membrane exhibits a state in which the α-phase and β-phase coexist.

[0063] Figure 4 These are the elemental analysis results for the front and back sides of the polarized PVDF film prepared in Example 1. During polarization, the side in contact with the positive electrode has a lower proportion of fluorine and a higher proportion of hydrogen; during polarization, the side in contact with the negative electrode has a higher proportion of fluorine and a lower proportion of hydrogen.

[0064] The surface charge of the membrane was determined using a power analyzer (SurPASS 3, Anton Paar, Austria). The membrane was air-dried and the machine was rinsed with deionized water before analysis. 0.015 g of KCl was dissolved in 250 mL of deionized water using a KCl electrolyte solution. The pH was adjusted using 0.1 M NaOH and HCl to measure the flow potential.

[0065] like Figure 5 As shown, the polarized PVDF membrane has a zeta potential of -60 mV at pH=7; while the PVDF membrane prepared in Control Example 2 has a zeta potential of -28 mV at pH=7. It can be seen that polarization treatment creates a potential difference between the two surfaces of the separation membrane. This property can be used to apply the separation membrane to specific processes for the retention or separation of negatively / positively charged substances.

[0066] The melting and crystallization processes of P(VDF-TrFE) films were investigated using differential scanning calorimetry (DSC, Taq-20, UK). DSC can be used not only to determine the melting point of samples but also to calculate the enthalpy of fusion, thereby comparing the crystallization percentage of different samples. All samples were tested within a temperature range of 20–200 °C.

[0067] like Figure 6As can be seen, after treatment with ionic liquid, the DSC spectrum shows only one melting peak, indicating a crystallinity of 50%; while the PVDF film without ionic liquid treatment shows two melting peaks, which is due to the imperfect crystallization of PVDF. Therefore, the addition of ionic liquid not only induces the B-phase PVDF but also has a beneficial effect on polymer crystallization, which is conducive to improving piezoelectric properties.

[0068] In Example 1 and Comparative Example 3, polarization treatment was performed under different media conditions. Table 1 shows the breakdown voltage of the film under ordinary polarization and the breakdown voltage of the film under silicone oil polarization.

[0069] polarization medium Membrane pore size (nm) Film thickness (μm) Polarization temperature (°C) Breakdown voltage (kV) Air 100 125 90 3 silicone oil 100 125 90 8

[0070] Under normal polarization conditions, the breakdown field strength of the membrane is much smaller than that of PVDF and its copolymers (60 MV / m), making it impossible to obtain piezoelectric PVDF membranes. Under oil bath conditions, the silicone oil fills the internal cavities of the membrane pores, greatly increasing the breakdown field strength.

Claims

1. Use of a polymeric separation membrane in the retention or separation process of negatively or positively charged substances, the material of which is PVDF, the main body of which is in the β phase, characterized in that, The polymer separation membrane has a zeta potential of -60 mV at pH=7; "main body β phase" means that the material is 100% β phase; the preparation method of the polymer separation membrane includes the following steps: 9 g PVDF, 1.5 g additive PVPK30, and 0.1 g water-soluble ionic liquid 1-ethyl-3-methylimidazolium chloride are dissolved in 39.5 g DMAc at 60℃ and stirred for 16 hours to obtain a uniform casting solution. The solution is then degassed at 60℃ until a clear, bubble-free solution is obtained. A 250 μm gap height knife is used at a speed of 1.0 m∙min⁻¹. -1 The membrane was cast at a high speed and exposed to the atmosphere for 15 seconds. Then, the membrane was immersed in a coagulation bath with a concentration of 30% DMAc at 60°C and a relative humidity of 60%. The prepared membrane was washed several times with deionized water to remove residual solvent and then stored in deionized water. The PVDF thin film was polarized using thermal polarization with silicone oil as the polarization medium to improve its residual polarization intensity. The polarization conditions were: polarization electric field strength of 50 MV / m, polarization temperature of 100℃, and polarization time of 30 min. Finally, the polarization field strength was maintained and the film was cooled to room temperature.

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