CN115253711B - Method for modifying hollow fiber polymer membrane by stretching-heat treatment - Google Patents

Abstract

The invention provides a method for modifying a hollow fiber polymer membrane by stretching-heat treatment. The method comprises the following steps: a. preparing a hollow fiber polymer membrane; b. stretching the hollow fiber polymer film along the length direction of the film wire and fixing the hollow fiber polymer film on a stainless steel bracket; c. immersing the stretched hollow fiber polymer membrane together with the stainless steel bracket in a filling medium; d. heating the soaked hollow fiber polymer film; e. and drying the hollow fiber polymer membrane after the heating treatment at room temperature to obtain the modified hollow fiber polymer membrane. The invention prepares the high-performance hollow fiber polymer membrane through three post-treatment processes of stretching, membrane hole filling and heat treatment. The method has the advantages of simple process, no pollution to the environment, easy operation, no need of harsh treatment conditions and low cost, and is an effective method for preparing the high-performance hollow fiber polymer membrane.

Description

Method for modifying hollow fiber polymer membrane by stretching-heat treatment

Technical Field

The invention relates to the technical field of hollow fiber membrane preparation, in particular to a method for modifying a hollow fiber polymer membrane by stretching-heat treatment.

Background

The membrane separation technology is considered as the most effective technical means for realizing sewage reclamation and guaranteeing the safety of drinking water. The hollow fiber polymer membrane component has the advantages of high membrane packing density per unit volume, large filtering area, small occupied area, relatively low cost and the like, and becomes the membrane product with the fastest development, maximum regulation and maximum output value in the field of separation membranes.

Currently, the main preparation methods of hollow fiber polymer membranes include a non-solvent induced phase separation (NIPS) method, a Vapor Induced Phase Separation (VIPS) method and a Thermally Induced Phase Separation (TIPS) method, or a combination of the above methods. The process for preparing the hollow fiber polymer membrane mainly by using the NIPS method mainly comprises the following steps: (1) Preparing a film liquid, namely dissolving a high molecular polymer and an additive into a solvent at a set temperature to form a homogeneous film liquid; (2) Film forming, extruding film forming liquid from a spinneret to form a hollow fiber shape, then entering a gel bath, and realizing phase separation by bidirectional diffusion of a solvent and a non-solvent or reduction of the temperature of the film forming liquid, wherein the formed film structure is influenced by dynamic and thermodynamic factors in the phase separation process; (3) And (3) post-treatment, namely repeatedly soaking and cleaning the hollow fiber membrane just formed to remove the residual additives and solvents in the membrane, then soaking the membrane filaments in glycerol or a mixed solution of glycerol and water for 8-24 hours, and finally taking out and airing. Development of high performance hollow fiber membranes is a continuing goal pursued by membrane technology workers. The scientific researchers mainly improve the film performance by optimizing the formula of film forming liquid and the film forming process conditions. The importance of post-processing does not place sufficient importance. However, improper post-treatment severely affects the performance of the hollow fiber polymeric membrane. For example, hollow fiber membranes can undergo severe structural deformation during drying. This is due to the fact that the polymer molecules are frozen without sufficient relaxation during film formation and shrink during later drying or use. Studies have shown that the shrinkage of the membrane pore size of self-supporting polymer membranes after drying is as high as 46%. This is also an important reason for the membrane filaments to be soaked with glycerol solution before drying. Therefore, it is necessary to develop a suitable post-treatment method for hollow fiber polymer membranes.

In addition, film structural stability is the basis for film performance stability. The initial flux of a general hollow fiber polymer membrane decays rapidly, and the water flux decays with the storage time of membrane wires, which is caused by the unstable membrane structure. Hollow fiber membranes, particularly for Membrane Distillation (MD), where membrane structural distortion reduces the LEP of the membrane _w And (3) the value of the membrane is that the gas-liquid interface at the membrane hole is unstable in the MD process, membrane wetting occurs, salt easily invades the membrane hole, and the interception capability of the membrane is reduced. This is because the hydraulic disturbance caused by the flow of the feed liquid continuously impacts the surface of the membrane, and the size or shape of the membrane pores is deformed. Therefore, the improvement of the stability of the polymer membrane structure and the guarantee of the stable permeation selection performance of the membrane are the problems which are needed to be solved at present.

Disclosure of Invention

The invention aims to provide a method for modifying a hollow fiber polymer membrane by stretching-heat treatment, which can improve the structural stability of the hollow fiber polymer membrane, improve the porosity of the membrane surface, increase the mechanical strength of the membrane and ensure the synchronous improvement of the permeability and the stability of the membrane.

The invention is realized in the following way: a method of stretch-heat treating a modified hollow fiber polymer membrane comprising the steps of:

a. preparing a hollow fiber polymer membrane;

b. stretching the hollow fiber polymer membrane prepared in the step a along the length direction of the membrane wire, and fixing the hollow fiber polymer membrane on a stainless steel bracket;

c. soaking the hollow fiber polymer membrane in the step b together with the stainless steel bracket in a filling medium for 4-24 hours;

d. c, heating the hollow fiber polymer membrane soaked in the step c at 90-160 ℃ for 1-3 hours; preferably, the heating temperature is 120-130 ℃ and the heating time is 1 hour;

e. and d, drying the hollow fiber polymer membrane subjected to the heating treatment in the step at room temperature to obtain the modified hollow fiber polymer membrane.

Preferably, in step b, the draw ratio after the draw treatment is generally selected to be in the range of 0.96 to 1. In the step b, the hollow fiber membrane (not dried) prepared mainly by a non-solvent induced phase separation method is stretched along the length direction of the membrane filaments, so that the orientation of the membrane filaments in the length direction is increased, and the shrinkage of the membrane filaments in the length direction is avoided.

Preferably, the filling medium in step c is water, glycerol, polyethylene glycol 200, polyethylene glycol 400 or a solvent such as ethylene glycol. In the step c, the membrane filaments are soaked in a filling medium, so that capillary shrinkage stress generated by the volatilization of moisture in membrane holes in the subsequent heat treatment is eliminated, the high porosity of the membrane surface is maintained, and the shrinkage in the radial direction is reduced.

Preferably, step a) prepares a hollow fiber polymer membrane specifically by:

a-1, dissolving a polymer and an additive into a solvent to form a homogeneous film-forming liquid;

a-2, extruding the homogeneous film-making solution through spinning equipment and completing phase separation;

and a-3, finally soaking in water for 8-24 hours to clean the residual solvent and additive in the film.

Preferably, the polymer in the step a-1 is a common membrane preparation material such as polysulfone, polyvinylidene fluoride or polyethersulfone.

Preferably, in the step d, the hollow fiber polymer membrane soaked in the filling medium in the step c is placed in a closed pressure-resistant metal container (a cooking kettle) together with a stainless steel bracket for heating treatment; and a heating device is arranged outside the metal tank or the cooking kettle so as to heat the hollow fiber polymer film in the metal tank or the cooking kettle. The heat treatment temperature is selected in relation to the melting temperature of the film-forming polymeric material, and is typically 30-40 ℃ lower than the melting temperature of the polymeric material.

If wet hollow fiber polymer membranes are used as the filling medium, they are generally heat treated in a pressure-resistant metal vessel containing water. In the case of using a high boiling point solvent such as glycerin, polyethylene glycol 200, polyethylene glycol 400, etc. as the filling medium, the heating treatment may be performed in a container containing the corresponding filling medium, or the heating treatment may be performed in hot air after immersing the filling medium.

In step d, by heat treatment, not only is the relaxation stress inside the polymer released, but also the tensile strength of the film is improved.

Finally preparing the hollow fiber ultrafiltration membrane or the hollow fiber microfiltration membrane. For hollow fiber ultrafiltration membranes, the permeability of the membrane can be improved. The polyvinylidene fluoride hollow fiber microfiltration membrane applied to membrane distillation can improve the permeability and the anti-wettability of the membrane.

Aiming at the problem that the polymer film is easy to shrink or deform in the process of drying or using, the invention starts from the fundamental purpose of stabilizing the film structure to improve the permeability and long-term stability of the film. The invention relates to three post-treatment processes, comprising stretching, filling film holes and heating, and the hollow fiber polymer film has the following advantages that:

(1) The stretching treatment ensures that the polymer molecular chain has better orientation in the length direction, and simultaneously avoids the shrinkage of the membrane yarn in the length direction in the heat treatment process, thereby ensuring that the defect of the membrane structure can not be generated.

(2) After the membrane pores are filled with filling medium, capillary shrinkage stress generated by volatilization of water in the membrane pore channels of the hollow fiber polymer membrane is eliminated when the hollow fiber polymer membrane is heated, and the high porosity of the membrane surface is ensured.

(3) The heat treatment not only releases the relaxation stress in the polymer, but also the polymer is easy to crystallize secondarily, so that the tensile strength of the polymer film is improved, and the stability of the film structure is improved.

Compared with the prior art, the technical scheme of the invention has the following beneficial effects:

(1) The invention realizes the improvement of the appearance of the hollow fiber polymer membrane by a stretching-heat treatment method, and enhances the stability of the membrane structure. The prior post-treatment method of the hollow fiber polymer membrane is to directly dry in room temperature air, or to impregnate glycerol and then dry in room temperature air, or to directly heat and dry. These post-treatment measures all cause membrane shrinkage, the pore size of the membrane is reduced, and the permeability is greatly reduced. The invention effectively prevents the shrinkage of the hollow fiber polymer membrane, stabilizes the membrane structure and improves the permeability and operation stability of the membrane, in particular to a hollow fiber micro/ultrafiltration membrane by three post-treatments of stretching, membrane hole filling and heat setting. This benefits from the fact that this post-treatment method enables to obtain hollow fiber polymer films with high surface porosity, high overall porosity and high tensile strength.

(2) The post-treatment method of the hollow fiber polymer membrane provided by the invention has the advantages of simple process, no pollution to the environment, easiness in operation, no need of harsh treatment conditions and low cost, and is an effective method for preparing the high-performance hollow fiber polymer membrane.

Drawings

FIG. 1 is a process flow diagram of a stretch-heat treatment modified hollow fiber polymer membrane of the present invention.

FIG. 2 is a scanning electron microscope image (a, cross section of membrane; b, inner surface of membrane) of the polyvinylidene fluoride hollow fiber micro-filtration membrane prepared in example 1, example 2, comparative example 1, comparative example 2 of the present invention.

FIG. 3 is a graph showing changes over time of membrane flux and condensate conductivity of the polyvinylidene fluoride hollow fiber microfiltration membranes prepared in example 1, example 2, comparative example 1, and comparative example 2 of the present invention, in which a 3.5% NaCl aqueous solution was concentrated in membrane distillation.

FIG. 4 is a scanning electron microscope image (inner surface, cross section and outer surface of membrane) of polysulfone hollow fiber ultrafiltration membrane prepared in example 3, example 4, comparative example 3 of the present invention.

FIG. 5 shows XRD diffraction peak patterns of polysulfone hollow fiber ultrafiltration membranes prepared in example 3, example 4, and comparative example 3 of the present invention.

FIG. 6 shows pure water flux of polysulfone hollow fiber ultrafiltration membranes prepared in example 3, example 4, and comparative example 3 of the present invention and membrane flux and BSA rejection rate when Bovine Serum Albumin (BSA) is filtered.

In FIG. 1, a wet hollow fiber polymer membrane; 2. a stainless steel bracket; 3. filling a medium; 4. a metal tank or a cooking kettle; 5. a heating device; 6. high performance hollow fiber membranes after stretch-heat treatment.

Detailed Description

The invention is described in detail below with reference to the drawings and the specific embodiments. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.

As shown in fig. 1, the present invention prepares a high-performance hollow fiber polymer film by a post-treatment method of a stretch-heat treatment, which is sequentially operated as follows:

(1) Hollow fiber polymer membrane preparation

Selecting a film-forming polymer and a solvent, firstly dissolving the film-forming polymer (such as polysulfone, polyvinylidene fluoride, polyether sulfone and the like), an additive and the like into the solvent to form a homogeneous film-forming liquid, then spinning and completing phase separation, and finally soaking in a gel bath for 8-24 hours to clean the solvent and the additive remained in the film, so as to prepare the wet hollow fiber polymer film 1. Preferred are the preparation of wet polyvinylidene fluoride (PVDF) micro/ultrafiltration membranes and polysulfone (PSf) micro/ultrafiltration membranes.

(2) Stretching treatment

The wet hollow fiber polymer membrane 1 prepared in the step (1) is stretched along the length direction of membrane wires and fixed on a stainless steel bracket 2. The stretching ratio is selected from 0.96 to 1, and the stretching ratio is preferably 0.98 to 1. The draw ratio is the ratio of the original length of the film filaments to the length after drawing. For example, the initial length of the wet film yarn is 50cm, the length after stretching is 51cm, and the stretching ratio is 50/51. A draw ratio of 1 means that the wet film yarn was not drawn but was fixed to a stainless steel bracket to prevent shrinkage, so that the drawing treatment was also counted as being performed when the draw ratio was 1 in the present application.

(3) Membrane pore filling

And (3) immersing the wet hollow fiber polymer film stretched and fixed in the step (2) together with the stainless steel bracket into a filling medium 3, wherein the filling medium 3 is placed into a metal tank or a cooking kettle 4, and the immersion time is 4-24 hours. The filling medium 3 may be water, glycerin, polyethylene glycol 200, polyethylene glycol 400, or a solvent such as ethylene glycol. Preferred pore filling media are water and glycerol, which are less costly.

(4) Heat treatment of

And (3) heating the wet hollow fiber polymer film soaked in the filling medium in the step (3) together with the stainless steel bracket for fixing in a closed metal tank or a cooking kettle 4 at the temperature of 90-160 ℃ for 1-3 hours. The preferred heat treatment temperature range is 100 to 150 ℃. The wet hollow fiber polymer membrane in the metal tank or the retort 4 can be heated by the heating device 5.

Finally, the membrane after heat treatment is dried at room temperature to obtain the high-performance hollow fiber membrane 6 after stretching-heat treatment. The steps (2) to (4) can influence the membrane structure and the performance, and the hollow fiber polymer membrane with high surface porosity and stable pore structure can be obtained by the synergistic effect of the steps. The steps (2) to (4) are not necessary, and only the order of the steps (2) and (3) can be changed, but the method is not limited to the four steps, and corresponding links can be added according to actual needs.

In order to more clearly and intuitively illustrate the technical advantages of the present invention, the polyvinylidene fluoride (PVDF) hollow fiber microfiltration membrane and the polysulfone (PSf) hollow fiber ultrafiltration membrane prepared in the examples and the prior art will be described briefly, and tensile strength and performance are compared, so as to reflect that the present invention has universality in the field of hollow fiber membrane preparation. It will be apparent that the described embodiments are only some, but not all, embodiments of the invention.

Example 1

This example is a PVDF hollow fiber microfiltration membrane for membrane distillation. (1) Preparing a wet hollow fiber PVDF membrane, namely dissolving PVDF and an additive into a solvent triethyl phosphate to form a homogeneous membrane preparation solution, extruding through spinning equipment, completing phase separation, and finally soaking in water for 12 hours to clean the solvent and the additive remained in the membrane, thus preparing the wet PVDF hollow fiber microfiltration membrane. (2) Stretching, namely stretching and fixing the prepared wet PVDF hollow fiber microfiltration membrane on a stainless steel bracket, wherein the stretching ratio is 43/44. (3) And filling membrane holes, and soaking the stretched and fixed wet PVDF hollow fiber microfiltration membrane into water for 4 hours. (4) And (3) heat treatment, namely heating the stretched wet PVDF hollow fiber microfiltration membrane in a sealed pressure-resistant metal container cooking kettle filled with water, wherein the heating temperature is 120 ℃, and the heat treatment time is 1 hour. And then naturally cooling the membrane filaments.

Example 2, comparative example 1 and comparative example 2 were substantially identical in operation to example 1, except for the difference in the filling medium, whether the stretching treatment and whether the heating treatment were performed. The specific operating conditions are shown in Table 1, and the overall porosity, surface porosity and tensile strength of the films are shown in Table 2.

The wet PVDF hollow fiber microfiltration membrane after the stretching treatment in example 2 was immersed in glycerin for 6 hours, and then the wet PVDF hollow fiber microfiltration membrane after the stretching treatment and the immersion in glycerin was taken out in an oven (in an air medium) to be subjected to heat treatment. While comparative example 1 was not stretched, soaked in water for 6 hours, and then directly dried in air. The wet PVDF hollow fiber microfiltration membrane of comparative example 2 was directly heat treated in an oven (in air medium) at 100 c without stretching and without membrane pore filling. The post-treatment process conditions for the wet PVDF hollow fiber microfiltration membranes of example 1, example 2, comparative example 1 and comparative example 2 are given in table 1.

Table 1 post-treatment process conditions for the wet PVDF hollow fiber membranes of example 1, example 2, comparative example 1 and comparative example 2

Sample of	Whether or not to stretch treatment	Whether or not there is a filling medium	Whether or not to heat treat
				Example 1	Stretching at a stretching ratio of 43/44	Is water	The heating temperature is 120 ℃ for 1h
Example 2	Stretching at a stretching ratio of 43/44	Is glycerol	The heating temperature is 120 ℃ for 1h
				Comparative example 1	Non-stretching (non-fixed)	Is water	If not, drying at room temperature
Comparative example 2	Non-stretching (non-fixed)	Whether or not	The heating temperature is 100 ℃ for 1h

The PVDF hollow fiber microfiltration membranes prepared in example 1, example 2, comparative example 1 and comparative example 2 were subjected to the porosity and tensile strength test, and the results obtained are shown in table 2 below.

Table 2 data on porosity and tensile strength of PVDF hollow fiber microfiltration membranes prepared in example 1, example 2, comparative example 1 and comparative example 2

Sample of	Integral porosity (%)	Surface porosity (%)	Tensile Strength (MPa)
				Example 1	71.2	37.1	2.25
Example 2	75.7	38.8	1.88
				Comparative example 1	73.7	30.1	1.78
Comparative example 2	70.7	29.8	2.14

As shown in Table 2, the PVDF hollow fiber microfiltration membrane obtained by modification through the stretching-heat treatment method has higher surface porosity which respectively reaches 37.1% and 38.8%. The tensile strength of the film is also significantly improved. Although the PVDF hollow fiber microfiltration membrane of comparative example 2 also had improved tensile strength after heat treatment, the membrane surface porosity was the lowest.

The PVDF hollow fiber microfiltration membranes obtained in example 1, example 2, comparative example 1 and comparative example 2 were subjected to scanning electron microscopy, and the results are shown in fig. 2, in which (a) corresponds to a cross section of the membrane and (b) corresponds to an inner surface of the membrane. The scanning electron microscope image of each sample can clearly observe that the surface aperture ratio of the film after the stretching-heat treatment is higher, the film has no macropore defect, and the film structure is more complete.

Meanwhile, the PVDF hollow fiber microfiltration membranes prepared in example 1, example 2, comparative example 1 and comparative example 2 were used in membrane distillation (concentration of 3.5% NaCl aqueous solution) and the performance of simulated seawater was concentrated for a long period of time, and the results are shown in FIG. 3. As can be seen from fig. 3, the membrane fluxes of example 1 and example 2 are much higher than those of comparative example 1 and comparative example 2. In addition, the permeate side conductivities of the membranes modified in example 1 and example 2 were always at a very low level, indicating that PVDF hollow fiber microfiltration membranes modified by the invention have excellent desalination stability and anti-wetting properties.

Example 3

This example is a preparation of polysulfone (PSf) hollow fiber ultrafiltration membrane with excellent properties. (1) The preparation of the wet hollow fiber PSf membrane comprises the steps of firstly dissolving PSf and additives into a solvent N, N-dimethylacetamide (DMAc) to form a homogeneous membrane preparation liquid, then spinning and completing phase separation, and finally soaking in water for 12 hours to clean the solvent and the additives remained in the membrane, thus obtaining the wet hollow fiber PSf membrane. (2) And (3) stretching, namely fixing the prepared wet PSf hollow fiber ultrafiltration membrane on a stainless steel metal bracket, and stretching, wherein the stretching ratio is 1. (3) And filling the membrane holes, and soaking the stretched and fixed wet PSf hollow fiber ultrafiltration membrane into water for 4 hours. (4) And (3) heat treatment, wherein the wet PSf hollow fiber membrane subjected to the fixing treatment is heated in a closed pressure-resistant metal container (a cooking kettle) filled with water, and the heating temperature is 120 ℃ for 1 hour. And finally, naturally cooling the PSf hollow fiber membrane.

Example 4 and comparative example 3 were conducted in substantially the same manner as in example 3. The wet PSf hollow fiber ultrafiltration membrane after the stretching treatment (stretching ratio of 1) in example 4 was immersed in water for 4 hours, and then the wet PSf hollow fiber ultrafiltration membrane was heated in a pot filled with water at a temperature of 100℃for 1 hour. And comparative example 3 was not stretched, was not subjected to membrane pore filling, and was directly air-dried. The specific operation is shown in Table 3.

TABLE 3 post-treatment process conditions for wet PSf hollow fiber membranes in example 3, example 4, comparative example 3

Sample of	Whether or not to stretch treatment	Whether or not there is a filling medium	Whether or not to heat treat
				Example 3	Stretching with a stretching ratio of 1	Is water	The heating temperature is 120 ℃ for 1h
Example 4	Stretching with a stretching ratio of 1	Is water	The heating temperature is 100 ℃ for 1h
				Comparative example 3	Non-stretching (non-fixed)	Whether or not	If not, drying at room temperature

The modified membranes of example 3, example 4 and comparative example 3 were tested and the average pore size, porosity and tensile strength data obtained are shown in table 4.

Table 4 average pore size, porosity and tensile strength data of PSf hollow fiber ultrafiltration membranes prepared in example 3, example 4, comparative example 3

Sample of	Average pore diameter (nm)	Integral porosity (%)	Tensile Strength (MPa)
				Example 3	35.1	70.3	4.52
Example 4	32.6	73.4	4.11
				Comparative example 3	28.4	64.4	3.05

As shown in Table 4, the PSf hollow fiber ultrafiltration membrane obtained by modification through the stretching-heat treatment method has higher overall porosity which respectively reaches 70.3% and 73.4%. The tensile strength of the film is significantly improved compared to the prior art (comparative example 3, direct drying process). The average pore diameter of the modified membrane is larger.

Scanning electron microscopy tests were performed on the PSf hollow fiber ultrafiltration membranes prepared in example 3, example 4 and comparative example 3, and the results obtained are shown in fig. 4. From fig. 4, it is more clearly observed that the surface aperture ratio of the modified PSf hollow fiber ultrafiltration membrane of the present invention is higher, and the separation skin layer is thinner, which all contribute to the improvement of the membrane flux.

XRD measurements were performed on the PSf hollow fiber ultrafiltration membranes prepared in example 3, example 4 and comparative example 3, and the results are shown in FIG. 5. As can be seen from FIG. 5, the polysulfone crystallization peak of the modified membrane of the present invention is significantly enhanced. This means that the post-treatment method of the stretch-heat treatment helps to improve the strength and thermal stability of the PSf film.

The PSf hollow fiber ultrafiltration membranes prepared in example 3, example 4 and comparative example 3 were subjected to a pure water flux test and a flux and a retention rate of the filtered bovine serum albumin, and the results are shown in fig. 6. As can be seen from fig. 6, the membrane in examples 3 and 4 is significantly higher than comparative example 3 in both pure water flux and in filtering bovine serum albumin, and the bovine serum albumin rejection rate is almost unchanged (see small circles above the bar graph in the figure), indicating that the modified PSf hollow fiber ultrafiltration membrane of the present invention has higher permeability.

The above-mentioned examples of the partial polymers and the partial treatment conditions of the present invention are not intended to limit the present invention in any way, and any simple modification, equivalent variation and modification of the above examples according to the technical substance of the present invention still falls within the technical scope of the present invention for preparing high-performance hollow fiber polymer membranes by stretching-heat treatment.

Claims (2)

1. A method of stretch-heat treating a modified hollow fiber polymer membrane comprising the steps of:

a. preparing a hollow fiber polymer membrane;

b. stretching the hollow fiber polymer film prepared in the step a along the length direction of the film wire, and fixing the film to a stainless steel bracket to prevent shrinkage;

c. soaking the hollow fiber polymer membrane and the stainless steel bracket in the step b in a filling medium for 4-24 hours;

d. c, heating the hollow fiber polymer film soaked in the step, wherein the heating temperature is 90-160 ℃ and the heating time is 1-3 hours;

e. drying the hollow fiber polymer membrane subjected to the heating treatment in the step d at room temperature to obtain a modified hollow fiber polymer membrane;

in the step b, the stretching ratio after the stretching treatment is 0.96-1;

the filling medium in the step c is water, glycerol, polyethylene glycol 200, polyethylene glycol 400 or ethylene glycol;

the hollow fiber polymer membrane prepared in the step a is specifically:

a-1, dissolving a polymer and an additive into a solvent to form a homogeneous film-forming liquid;

a-2, extruding the homogeneous film-making solution through spinning equipment and completing phase separation;

a-3, finally soaking in water for 8-24 hours to clean the residual solvent and additive in the film;

the polymer in the step a-1 is polysulfone, polyvinylidene fluoride or polyethersulfone;

in the step d, the hollow fiber polymer film soaked in the filling medium in the step c and the stainless steel bracket are heated in a metal tank or a cooking kettle together; and a heating device is arranged outside the metal tank or the cooking kettle so as to heat the hollow fiber polymer film in the metal tank or the cooking kettle.

2. The method for stretching-heat treating a modified hollow fiber polymer film according to claim 1, wherein in the step d, the heating temperature is 120 to 130 ℃ for 1 hour.

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