CN109621747B - Preparation method of hollow fiber micro/ultrafiltration membrane - Google Patents

Preparation method of hollow fiber micro/ultrafiltration membrane Download PDF

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CN109621747B
CN109621747B CN201910037619.2A CN201910037619A CN109621747B CN 109621747 B CN109621747 B CN 109621747B CN 201910037619 A CN201910037619 A CN 201910037619A CN 109621747 B CN109621747 B CN 109621747B
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lcst
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CN109621747A (en
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肖通虎
陈伟
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Ningbo University
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/06—Organic material
    • B01D71/30—Polyalkenyl halides
    • B01D71/32—Polyalkenyl halides containing fluorine atoms
    • B01D71/34—Polyvinylidene fluoride
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0002—Organic membrane manufacture
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/08—Hollow fibre membranes

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Abstract

The invention discloses a preparation method of a hollow fiber micro/ultrafiltration membrane, which is mainly characterized in that a hollow fiber micro/ultrafiltration membrane with an easily-regulated structure is obtained by preparing a high-molecular membrane casting solution system with a low critical solution temperature and preparing a membrane at a proper spinning nozzle and gel bath temperature. The polymer casting solution is homogeneous and stable at low temperature, but phase separation occurs at high temperature, and the corresponding film-making mechanism is different from that of the traditional thermally induced phase separation method. The hollow fiber membrane with the novel structure has the characteristics of large flux and high strength.

Description

Preparation method of hollow fiber micro/ultrafiltration membrane
Technical Field
The invention belongs to the technical field of polymer membrane separation, and particularly relates to a preparation method of a hollow fiber micro/ultrafiltration membrane.
Background
Microfiltration and ultrafiltration (abbreviated as micro/ultrafiltration) membrane separation technologies are widely applied to the fields of material filtration separation, concentration purification, purification and the like. Common forms of micro/ultrafiltration membranes are flat plate and hollow fiber forms. The hollow fiber membrane module has the advantages of high packing density, high production capacity and the like, and is widely applied. The micro/ultra-filtration hollow fiber membrane prepared from polyvinylidene fluoride (PVDF) has outstanding performances in the aspects of solvent resistance, acid and alkali resistance, ultraviolet ray resistance, weather resistance, cleaning resistance and the like. The preparation method of the PVDF hollow fiber membrane is mainly a phase separation method, and is generally classified into a non-solvent induced phase separation method (NIPS) and a thermal induced phase separation method (TIPS). The NIPS method takes the phase separation and film formation of a high molecular solution caused by the exchange of a solvent and a non-solvent formed by concentration difference as a mechanism, and the TIPS method takes the phase separation and film formation of the high molecular solution caused by the thermal induction of temperature difference as a mechanism. In the current TIPS method for preparing PVDF membrane, the polymer solution is generally an Upper Critical Solution Temperature (UCST) system, i.e. when the temperature of the polymer solution is decreased from high temperature to its critical solution temperature, phase separation occurs to form a micro/ultrafiltration membrane. The single NIPS method for preparing the PVDF hollow fiber micro/ultrafiltration membrane has numerous process parameters needing to be regulated and controlled, and the membrane has low porosity, large resistance and low permeation flux; the preparation of PVDF hollow fiber membrane by TIPS method requires high polymer dissolution temperature, which is usually higher than melting point temperature of PVDF, and is up to 180 ℃ or more, and has high energy consumption, and most of the solvents used for dissolving polymers need to be extracted by organic solvents, and the process flow is complex and the environmental pollution is large. In view of the above, the present invention provides a preparation method of a hollow fiber micro/ultrafiltration membrane based on a Lower Critical Solution Temperature (LCST) system. The Lower Critical Solution Temperature (LCST) system refers to a system in which a polymer homogeneous solution is subjected to phase separation when the temperature is increased from low temperature to exceed the lower critical solution temperature.
Disclosure of Invention
The invention aims to provide a preparation method of a hollow fiber micro/ultrafiltration membrane based on a low critical solution temperature system aiming at the defects of the existing PVDF membrane and the preparation technology thereof.
The technical scheme adopted by the invention for solving the technical problems is as follows: a preparation method of a hollow fiber micro/ultrafiltration membrane is characterized by comprising the following steps:
1) preparing a polyvinylidene fluoride (PVDF) casting solution with a low critical solution temperature: preparing a PVDF membrane casting solution with proper concentration according to the mass percentage of 5-40% of polymer PVDF, 1-20% of second polymer, 50-85% of solvent and 1-20% of small-molecule pore-forming agent, firstly completely dissolving the small-molecule pore-forming agent in the solvent, then adding PVDF and the second polymer, stirring, heating and dissolving to form a completely dissolved polymer PVDF membrane casting solution system with Low Critical Solution Temperature (LCST), standing and defoaming for later use;
2) extruding the prepared PVDF membrane casting solution in a proper spinning nozzle at a certain spinning temperature to spin hollow fiber nascent state membrane filaments, controlling the flow rates and spinning speeds of the membrane casting solution and core solution, standing in an air section for a certain time, immersing in an external gel bath at a certain temperature, allowing the nascent state membrane to undergo phase separation, solidifying from a liquid phase to form a membrane, and winding the membrane filaments on a membrane winding machine;
3) and (3) placing the membrane in water to further completely dissolve out the solvent, magnesium chloride and the like in the membrane to obtain the polyvinylidene fluoride micro/ultrafiltration membrane.
The polyvinylidene fluoride (PVDF) casting solution in the step 1) has a Lower Critical Solution Temperature (LCST), the casting solution is a uniform high molecular solution at a temperature lower than the LCST, the original uniform PVDF solution is subjected to phase separation and becomes turbid at a temperature higher than the LCST, and the LCST is between 1 and 125 ℃, preferably between 35 and 80 ℃.
The PVDF membrane casting solution in the step 1) is a system composed of four or more components, and comprises a polymer PVDF, a second polymer, a solvent and a micromolecular pore-forming agent, wherein the micromolecular pore-forming agent must contain magnesium chloride, but other micromolecular compounds can be added at the same time, and the micromolecular pore-forming agent magnesium chloride can be anhydrous magnesium chloride or magnesium chloride containing crystal water.
The second polymer in step 1) is a polymer material having a good compatibility with PVDF, and may be a non-water-soluble polymer, or a water-soluble polymer, and preferably a polymer having a carbonyl group in a structural unit, such as polyvinylpyrrolidone (PVP), but is not limited thereto.
The solvent in the step 1) is a solvent capable of dissolving PVDF, such as Dimethylformamide (DMF), dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), or a mixed solvent containing the solvent, and is not limited to the above, and the solvent preferably has a carbonyl group in a structural unit and a solvated magnesium complex ionic structure capable of performing electron donor-donor interaction with a small molecular pore-forming agent magnesium chloride to form a multi-solvent molecule, such as [ Mg (DMAc) ]6]2+And a difference in solubility parameter Δ δ from PVDFs-pLess than 3MPa0.5Such as dimethylacetamide (DMAc).
The micromolecular pore-foaming agent magnesium chloride in the step 1) can generate an electron donor-donor interaction effect with the macromolecule PVDF and the second macromolecule in the step 1).
The heating dissolution temperature in the step 1) is lower than 130 ℃, and simultaneously can be higher than the LCST temperature, but preferably is lower than the LCST temperature for dissolution, and the temperature of the polymer casting solution after complete dissolution is kept to be lower than the LCST temperature for standby.
The spinning temperature in step 2) is usually lower than the LCST temperature to keep the casting solution in a thermodynamically stable state, but the initial polymer casting solution may be metastable across the binodal line by raising the temperature, preferably lower than the LCST temperature.
The spinneret proper in the step 2) can be of a double-channel structure, the inner channel is a core liquid channel, the outer channel is a membrane casting liquid channel, or can be of a three-channel structure, the inner channel is a core liquid channel, the middle channel is a membrane casting liquid channel, the outermost channel can be used for carrying gel medium or membrane casting liquid, when the gel medium flows through the outermost channel, the composition and temperature of the gel medium can be different from those of the outer gel bath, and when the membrane casting liquid flows through the outermost channel, the composition, temperature and thickness of the membrane casting liquid can be different from those of the middle channel.
The certain time of the stay in the air section in the step 2) is 0 to 15 seconds.
The outer gel bath in step 2) is a non-solvent for PVDF, such as water, ethanol, and an aqueous solution containing a solvent, but not limited thereto.
The bore fluid in step 2) is a non-solvent of PVDF, such as water, ethanol, polyethylene glycol, glycerol, and aqueous solution containing solvent, but not limited thereto.
The temperature of the outer gel bath and the temperature of the core liquid in the step 2) can be lower than the LCST temperature and can also be higher than the LCST temperature, but at least one of the temperatures is higher than the LCST temperature and higher than the temperature of the casting solution, the temperatures of the two temperatures are preferably higher than the LCST temperature when the microfiltration membrane is prepared, the temperature of one temperature is preferably lower than the LCST temperature when the ultrafiltration membrane of the single-skin separation layer is prepared, the temperature of the other temperature is higher than the LCST temperature when the ultrafiltration membrane of the single-skin separation layer is prepared, and a three-channel structure spinning nozzle is preferably adopted when the ultrafiltration membrane of the separation layer in the middle and the two surfaces are macroporous support layers and two casting solutions with the LCST are extruded simultaneously.
When the nascent-state membrane in the step 2) is gelled, at least one of the temperature of the outer gel bath and the temperature of the core liquid is higher than the LCST temperature of the membrane casting solution and higher than the temperature of the membrane casting solution, so a Thermally Induced Phase Separation (TIPS) mechanism exists in the phase separation process of the nascent-state hollow fiber membrane under the action of temperature difference.
The PVDF hollow fiber micro/ultrafiltration membrane obtained in the step 3) has the outer diameter of 0.4mm-4mm, the inner diameter of 0.2mm-3mm and the average pore diameter of 0.01 μm-1.0 μm.
Compared with the prior art, the invention has the characteristics and advantages that:
1. the invention adopts a PVDF casting solution system based on a Low Critical Solution Temperature (LCST). PVDF is a semi-crystalline polymer, the structure and the characteristics of the PVDF are completely different from materials such as polysulfones, and no report of successfully forming a PVDF system with Low Critical Solution Temperature (LCST) is found at present. The invention successfully forms a PVDF membrane casting solution system with Low Critical Solution Temperature (LCST) through a quaternary system which simultaneously comprises a macromolecule PVDF, a second macromolecule, a specific micromolecule pore-foaming agent magnesium chloride and a solvent matched with the macromolecule PVDF, and opens up a new way for preparing the PVDF hollow fiber membrane based on the PVDF membrane casting solution system.
2. The mechanism of forming the low critical solution temperature of the PVDF casting solution system based on the Low Critical Solution Temperature (LCST) is not phase separation caused by the fact that the hydrogen bond action of the system is damaged in the temperature rise process, nor is the PVDF casting solution system based on the Low Critical Solution Temperature (LCST) because magnesium chloride can generate electron donor-donor interaction with a specific solvent and a second polymer of the system, wherein solvated magnesium complex ions of formed specific multi-solvent molecules can generate structural change and cause the interaction change of different components in a quaternary system when the temperature rises, so that the dissolving capacity of the polymers in the system is reduced, and the phase separation is promoted. If lithium chloride or zinc chloride is used to replace magnesium chloride and other components are not changed in the casting solution system with Low Critical Solution Temperature (LCST), the new system has no Low Critical Solution Temperature (LCST) in the temperature range; further, when the second polymer or magnesium chloride is removed from the system and the system is changed from the quaternary system to the ternary system, the ternary system does not have a low critical solution temperature in the above temperature range.
3. The preparation method of the hollow fiber micro/ultrafiltration membrane based on the low critical solution temperature system is different from the traditional NIPS method and the TIPS method for preparing the membrane. The phase separation in the film preparation process at least comprises a TIPS phase separation process based on a Lower Critical Solution Temperature (LCST), and the TIPS phase separation process is different from the traditional TIPS phase separation process. In the traditional TIPS phase separation process, a PVDF solution is stable in a high-temperature state, and phase separation occurs at a low temperature, and corresponds to a UCST system; in the TIPS phase separation process, the PVDF solution is stable in a low-temperature state and is subjected to phase separation at a high temperature, and the preparation process of the PVDF hollow fiber membrane is different from the preparation process of the PVDF hollow fiber membrane by a traditional method corresponding to an LCST system.
4. Compared with the single NIPS method for preparing the hollow fiber micro/ultrafiltration membrane, the preparation method of the hollow fiber micro/ultrafiltration membrane based on the low critical solution temperature system does not need to regulate and control a plurality of process parameters, and the aperture of the epidermal layer is easy to regulate and control; compared with the single traditional TIPS method for preparing the hollow fiber micro/ultrafiltration membrane, the method does not need to operate the membrane casting solution with the temperature higher than the melting point of PVDF (more than 180 ℃), can form a membrane by phase separation even under the condition of the gel temperature lower than 90 ℃, and has low energy consumption and simple process flow. The PVDF hollow fiber membrane is prepared by gel at the temperature higher than LCST, and an ultrafiltration membrane with a skin separation layer, an ultrafiltration membrane with a separation layer in the middle and two surfaces as macroporous support layers, a microfiltration membrane without a skin layer and an asymmetric microfiltration membrane with a gradient pore structure can be very easily prepared by changing the structure of an outer gel bath, a core liquid and a spinneret in a spinning process, which is difficult to realize for the preparation of a flat membrane. When the ultrafiltration membrane is prepared, the separation layer is ultrathin, the main body layer is a sponge pore structure with high porosity and large pore diameter, and when the microfiltration membrane is formed, the microfiltration membrane is an asymmetric microfiltration membrane which has no epidermal layer, has high porosity, adjustable pore diameter and narrow pore diameter distribution or has a high porosity and gradient sponge pore structure, and is different from a conventional membrane structure, so that the hollow fiber membrane with a novel structure has the characteristics of large flux, low resistance, high strength and suitability for multiple fields.
Drawings
FIG. 1 is a SEM image of the cross-sectional structure of a PVDF hollow fiber microfiltration membrane prepared in example 1 of the invention.
Detailed Description
The present invention will be described in further detail with reference to examples.
Example 1
Preparing a PVDF membrane casting solution according to the mass percent of a second polymer, namely 12 percent of polyvinylpyrrolidone (PVP), 68 percent of solvent DMAc and 8 percent of small-molecular pore-forming agent magnesium chloride, firstly completely dissolving magnesium chloride in DMAc, then adding PVDF and PVP, stirring and heating at 40 ℃ for constant-temperature dissolution to form a completely uniform PVDF membrane casting solution, and standing and defoaming. And then, extruding the prepared PVDF membrane casting solution into a spinning head with a double-channel structure at 40 ℃ to spin hollow fiber nascent state membrane filaments, wherein an inner layer channel is a core solution channel, an outer layer channel is a membrane casting solution channel, and the core solution is water with the temperature of 90 ℃, controlling the flow rate and the spinning speed of the membrane casting solution and the core solution, staying in an air section for 1 second, immersing the membrane filaments into outer gel bath water with the temperature of 90 ℃, causing the nascent state membrane to generate phase separation, solidifying from the liquid phase to form a membrane, winding the membrane filaments on a membrane winding machine, placing the membrane in water, and further completely dissolving out a solvent, magnesium chloride and the like in the membrane to obtain the polyvinylidene fluoride hollow fiber membrane.
The PVDF casting solution system has a Lower Critical Solution Temperature (LCST) of 50 ℃. The gel temperature in the membrane preparation process of the hollow fiber membrane based on the low critical solution temperature system is 40 ℃ higher than the LCST temperature. The prepared microfiltration membrane with a sponge-like pore structure is shown in figure 1. The hollow fiber membrane had an outer diameter of 1.2mm, an inner diameter of 0.7mm, an average pore diameter of 0.2 μm, an elongation at break of 250%, and a breaking strength of 3.5 MPa.
Example 2
Preparing a PVDF membrane casting solution according to the mass percent of 16 percent of macromolecular PVDF, 4 percent of second macromolecular polyvinylpyrrolidone (PVP), 72 percent of solvent DMAc and 8 percent of small molecular pore-forming agent magnesium chloride, firstly completely dissolving magnesium chloride in DMAc, then adding PVDF and PVP, stirring and heating at 45 ℃ for constant temperature dissolution to form a completely uniform PVDF membrane casting solution, and standing and defoaming. And then, extruding the prepared PVDF casting solution in a spinning head with a double-channel structure at 45 ℃ to spin hollow fiber nascent state membrane filaments, wherein an inner layer channel is a core solution channel, an outer layer channel is a casting solution channel, and the core solution is water with the temperature of 45 ℃, controlling the flow rate and the spinning speed of the casting solution and the core solution, directly immersing the hollow fiber filaments in external gel bath water with the temperature of 95 ℃, enabling the nascent state membrane to generate phase separation, solidifying from a liquid phase to form a membrane, winding the membrane filaments on a membrane winding machine, and placing the membrane in water to further completely dissolve out a solvent, magnesium chloride and the like in the membrane, thereby obtaining the PVDF hollow fiber membrane.
The PVDF casting solution system has a Lower Critical Solution Temperature (LCST) of 50 ℃. The outer gel temperature in the hollow fiber membrane preparation process based on the low critical solution temperature system is 45 ℃ higher than the LCST temperature, and the core liquid temperature is 5 ℃ lower than the LCST temperature. The obtained ultrafiltration membrane has an ultra-thin inner skin layer and a spongy pore structure as a main body. The hollow fiber membrane had an outer diameter of 1.4mm, an inner diameter of 0.8mm, an average pore diameter of 0.03 μm, an elongation at break of 270%, and a breaking strength of 3.8 MPa.
Example 3
Preparing a PVDF membrane casting solution according to the mass percent of a second polymer, namely 12 percent of PVDF, 4 percent of polyvinylpyrrolidone (PVP), 73 percent of DMAc and 11 percent of magnesium chloride serving as a small-molecular pore-forming agent, firstly completely dissolving magnesium chloride in DMAc, then adding PVDF and PVP, stirring and heating to 35 ℃ for constant-temperature dissolution to form a completely uniform PVDF membrane casting solution, and standing and defoaming. And then, extruding the prepared PVDF casting solution in a spinning head with a three-channel structure at 35 ℃ to spin hollow fiber nascent state membrane filaments, wherein an inner channel is a core solution channel, a core solution is 93 ℃ water, an intermediate channel is a casting solution channel, an outermost channel is a gel medium channel, a gel medium is 93 ℃ water, the flow rates and spinning speeds of the casting solution, the outermost gel medium and the core solution are controlled, the PVDF casting solution, the core solution and the gel medium are immersed in 35 ℃ outer gel bath water after staying for 5 seconds in an air section, the nascent state membrane is solidified from a liquid phase to form a membrane, the membrane filaments are wound on a membrane rolling machine, and the membrane is placed in water to further completely dissolve out a solvent, magnesium chloride and the like in the membrane, so that the polyvinylidene fluoride hollow fiber membrane is obtained.
The PVDF casting solution system has a Lower Critical Solution Temperature (LCST) of 43 ℃. The temperatures of a gel medium and a core liquid of an outermost layer channel in the hollow fiber membrane preparation process based on the low critical solution temperature system are both 50 ℃ higher than the LCST temperature, thermally-induced phase separation has occurred in an air section, and the nascent state hollow fiber membrane is further solidified and formed through solvent and non-solvent exchange after being immersed into external gel bath water with the temperature lower than the LCST. The prepared hollow fiber membrane structure is a microfiltration membrane with a spongy pore structure, the outer diameter of the membrane is 1.0mm, the inner diameter of the membrane is 0.5mm, the average pore diameter is 0.15 mu m, the elongation at break is 260%, and the breaking strength is 3.3 MPa.
Example 4
Spinning the hollow fiber membrane by a three-channel spinneret. Preparing a PVDF membrane casting solution according to the mass percent of 12 percent of high-molecular PVDF, 4 percent of second high-molecular polyvinylpyrrolidone (PVP), 73 percent of solvent DMAc and 11 percent of small-molecular pore-forming agent magnesium chloride, firstly completely dissolving magnesium chloride in DMAc, then adding PVDF and PVP, stirring and heating at 35 ℃ for constant-temperature dissolution to form a completely uniform PVDF membrane casting solution, standing, preserving heat and defoaming, and allowing the membrane casting solution to pass through a middle layer channel of a three-channel spinning head. In addition, preparing another PVDF membrane casting solution according to the mass percent of 13 percent of the high-molecular PVDF, 3 percent of the second high-molecular polyvinylpyrrolidone (PVP), 74 percent of the solvent NMP and 10 percent of the small-molecular pore-forming agent magnesium chloride, firstly completely dissolving the magnesium chloride in the NMP, then adding the PVDF and the PVP, stirring and heating the mixture to be at 70 ℃ for constant-temperature dissolution to form a completely uniform PVDF membrane casting solution, standing, preserving heat and defoaming, and leading the membrane casting solution to pass through an outermost layer channel of a three-channel structure spinning head.
And then, extruding the prepared two PVDF membrane casting solutions in a spinning head with a three-channel structure to spin hollow fiber nascent state membrane filaments, wherein an inner channel is a core solution channel, the core solution is water with the temperature of 93 ℃, controlling the flow rate and the spinning speed of the two PVDF membrane casting solutions and the core solution, directly immersing the two PVDF membrane casting solutions and the core solution in external gel bath water with the temperature of 93 ℃, solidifying the nascent state membrane from a liquid phase to form a membrane, winding the membrane filaments on a membrane winding machine, and placing the membrane in water to further completely dissolve out a solvent, magnesium chloride and the like in the membrane, thus obtaining the polyvinylidene fluoride hollow fiber membrane.
The PVDF film casting solution system of the middle layer channel has a Low Critical Solution Temperature (LCST) of 43 ℃, 70 ℃ film casting solution of the outermost layer channel is arranged on one side in contact with the PVDF film casting solution system, 93 ℃ core solution is arranged on the other side, the temperature difference between the two sides is different, and a temperature gradient exists in the film. The PVDF casting solution system of the outermost layer channel has a Lower Critical Solution Temperature (LCST) of 78 ℃, and the outer gel bath of 93 ℃ is arranged on one side in contact with the PVDF casting solution system, and the PVDF casting solution system of 35 ℃ is arranged on the other side of the middle layer channel. The hollow fiber membrane prepared based on the low critical solution temperature system of the double-layer membrane casting solution is an asymmetric ultrafiltration membrane with a separation layer in the middle and macroporous support layers on two surfaces, the outer diameter of the membrane is 1.5mm, the inner diameter of the membrane is 0.8mm, the average pore diameter of the membrane is 40nm, the elongation at break of the membrane is 300%, and the breaking strength of the membrane is 3.6 MPa.
Comparative example 1
The outer gel bath and the core solution were both 20 ℃ water, and the PVDF casting solution system had a Lower Critical Solution Temperature (LCST) of 50 ℃ as in example 1. Because the gel temperature in the membrane preparation process is 30 ℃ lower than the LCST temperature, thermal phase separation does not occur, and the membrane is a pure non-solvent induced phase separation mechanism, the ultrafiltration membrane with a thicker skin layer and a finger-shaped pore structure is prepared. The hollow fiber membrane had an outer diameter of 1.4mm, an inner diameter of 0.8mm, an average pore diameter of 0.02 μm, an elongation at break of 200%, and a breaking strength of 2.5 MPa.
Comparative example 2
Preparing a PVDF membrane casting solution by using 12 mass percent of high-molecular PVDF, 0 mass percent of second high-molecular polyvinylpyrrolidone (PVP), 80 mass percent of solvent DMAc and 8 mass percent of small-molecular pore-foaming agent magnesium chloride, and dissolving the PVDF membrane casting solution to form a completely uniform PVDF membrane casting solution by the same steps as the step of the example 1.
The PVDF casting film liquid system does not have Low Critical Solution Temperature (LCST) between 1 ℃ and 125 ℃, and the ternary system cannot form the LCST system of the invention.
Comparative example 3
Preparing a PVDF membrane casting solution by using 12 mass percent of high-molecular PVDF, 12 mass percent of second high-molecular polyvinylpyrrolidone (PVP), 76 mass percent of solvent DMAc and 0 mass percent of small-molecular pore-foaming agent magnesium chloride, and dissolving the PVDF membrane casting solution to form a completely uniform PVDF membrane casting solution by the same steps as the step of the example 1.
The PVDF casting film liquid system does not have Low Critical Solution Temperature (LCST) between 1 ℃ and 125 ℃, and the ternary system cannot form the LCST system of the invention.
Comparative example 4
Preparing a PVDF membrane casting solution according to the mass percentage of 12 percent of high-molecular PVDF, 11.5 percent of second high-molecular polyvinylpyrrolidone (PVP), 68 percent of solvent DMAc and 8.5 percent of small-molecular pore-forming agent instead of lithium chloride, and dissolving the PVDF membrane casting solution to form the completely uniform PVDF membrane casting solution by the same steps as the step 1.
The PVDF casting film liquid system does not have a Lower Critical Solution Temperature (LCST) between 1 ℃ and 125 ℃, and the quaternary system cannot form the LCST system of the invention.
Comparative example 5
Preparing a PVDF membrane casting solution by using 12 mass percent of macromolecular PVDF, 12 mass percent of second macromolecular polyvinylpyrrolidone (PVP), 71 mass percent of solvent DMAc and 5 mass percent of small molecular pore-forming agent instead of zinc chloride, and dissolving the PVDF membrane casting solution to form a completely uniform PVDF membrane casting solution by the same steps as the step of the example 1.
The PVDF casting film liquid system does not have a Lower Critical Solution Temperature (LCST) between 1 ℃ and 125 ℃, and the quaternary system cannot form the LCST system of the invention.

Claims (7)

1. A preparation method of a hollow fiber micro/ultrafiltration membrane is characterized by comprising the following steps:
1) preparing a polyvinylidene fluoride (PVDF) casting solution with a low critical solution temperature: preparing a PVDF membrane casting solution with proper concentration according to the mass percentage of 5-40% of polymer PVDF, 1-20% of second polymer, 50-85% of solvent and 1-20% of magnesium chloride-containing small-molecule pore-forming agent, firstly completely dissolving the small-molecule pore-forming agent in the solvent, then adding PVDF and the second polymer, stirring, heating and dissolving to form a completely dissolved polymer PVDF membrane casting solution system with Low Critical Solution Temperature (LCST), and standing and defoaming for later use;
wherein, the second polymer is a polymer material which has better compatibility with PVDF and has carbonyl in a structural unit; the solvent is a compound having a carbonyl group in a structural unit and being capable of reacting with a smallThe molecular pore-forming agent magnesium chloride generates electron donor-donor interaction to form a solvated magnesium complex ion structure of multi-solvent molecules, and the solubility parameter difference delta between the molecular pore-forming agent magnesium chloride and PVDFs-pLess than 3MPa0.5An organic solvent of (a);
2) extruding the prepared PVDF membrane casting solution in a proper spinning nozzle at a certain spinning temperature to spin hollow fiber nascent state membrane filaments, controlling the flow rates and spinning speeds of the membrane casting solution and core solution, staying in an air section for 0-15 seconds, immersing in an external gel bath at a certain temperature, allowing the nascent state membrane to undergo phase separation, solidifying from a liquid phase to form a membrane, and winding the membrane filaments on a membrane winding machine;
wherein, at least one of the temperature of the outer gel bath and the temperature of the core liquid is higher than the LCST temperature and higher than the temperature of the casting solution;
3) and (3) placing the membrane in water to further completely dissolve out the solvent and the small molecular pore-forming agent in the membrane, thereby obtaining the polyvinylidene fluoride micro/ultrafiltration membrane.
2. The method for preparing a hollow fiber micro/ultrafiltration membrane according to claim 1, wherein the polyvinylidene fluoride (PVDF) casting solution in step 1) has a Lower Critical Solution Temperature (LCST), below which the casting solution is a uniform polymer solution, and above which the original uniform PVDF solution is phase-separated and becomes turbid, and the Lower Critical Solution Temperature (LCST) is between 1 ℃ and 125 ℃.
3. The method for preparing a hollow fiber micro/ultrafiltration membrane according to claim 1, wherein the spinning temperature in step 2) is lower than the LCST temperature to keep the membrane casting solution in a thermodynamically stable state.
4. The method of claim 1, wherein the spinneret suitable for the step 2) is a two-channel structure or a three-channel structure.
5. The method of claim 1, wherein the microfiltration membrane is prepared at a temperature higher than the LCST temperature in step 2), the ultrafiltration membrane is prepared at a single skin separation layer at a temperature lower than the LCST temperature in step one and higher than the LCST temperature in step two, and the ultrafiltration membrane is prepared at a middle part and has a macroporous support layer on both surfaces using a triple channel spinneret and simultaneously extruding two casting solutions having LCST.
6. The method of claim 1, wherein the phase separation process of the nascent membrane in step 2) is performed by a Thermally Induced Phase Separation (TIPS) mechanism under the action of temperature difference when the nascent membrane is gelled.
7. The method for preparing the hollow fiber micro/ultrafiltration membrane according to claim 1, wherein the PVDF hollow fiber micro/ultrafiltration membrane obtained in the step 3) has an outer diameter of 0.4mm-4mm, an inner diameter of 0.2mm-3mm and an average pore diameter of 0.01 μm-1.0 μm.
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