阅读说明：本技术 一种双层聚合物分离膜及其制备方法 (Double-layer polymer separation membrane and preparation method thereof ) 是由 孙世鹏 王振远 付正军 汤梦梦 邵丹丹 邢卫红 于 2020-04-07 设计创作，主要内容包括：本发明提出一种通过具有特殊官能团的单体改性聚合物制备层间黏结性优良的双层膜的方法。本发明通过在双层膜制备过程中,向铸膜液或纺丝液中添加具有活性基团和特征基团的改性单体,利用单体的活性基团与铸膜液或纺丝液中的聚合物发生接枝反应、利用单体的特征基团增强与其他聚合物的分子间相互作用,以提高聚合物之间的相容性,制备界面黏结性优良的双层膜。该方法同时适用于双层平板膜和双层中空纤维膜的制备,并且可以实现在温和的制备条件下制备界面处具有互相贯穿结构的双层膜。界面处的互相贯穿的结构使得制备的双层膜具有优良的层间黏结性,同时具有更低的传质阻力。(The invention provides a method for preparing a double-layer film with excellent interlayer adhesiveness by using a monomer modified polymer with a special functional group. In the preparation process of the double-layer membrane, the modified monomer with the active group and the characteristic group is added into the membrane casting solution or the spinning solution, the active group of the monomer and the polymer in the membrane casting solution or the spinning solution are subjected to grafting reaction, and the characteristic group of the monomer is utilized to enhance the intermolecular interaction with other polymers, so that the compatibility among the polymers is improved, and the double-layer membrane with excellent interface cohesiveness is prepared. The method is simultaneously suitable for preparing the double-layer flat membrane and the double-layer hollow fiber membrane, and can realize the preparation of the double-layer membrane with an interpenetrating structure at the interface under mild preparation conditions. The interpenetration structure at the interface ensures that the prepared double-layer film has excellent interlayer cohesiveness and lower mass transfer resistance.)

1. A two-layer polymeric separation membrane comprising a first polymeric layer and a second polymeric layer; a modifier is also included in the first polymer layer.

2. The bilayer polymer separation membrane of claim 1, wherein in one embodiment, the first polymer is a high molecular polymer comprising an imine structure; the second polymer is a high molecular polymer containing carbonyl, carboxyl or ether bond;

in one embodiment, the first polymer is a polyimide, polyetherimide, polyamideimide, or the like; the second polymer is polyetherimide, polyethersulfone, polysulfone and the like;

in one embodiment, the modifier is a fluoro aromatic amine monomer containing fluorine or fluoromethyl and an amino group;

in one embodiment, the modifying agent is 4-fluoro-2- (trifluoromethyl) benzylamine.

3. The bilayer polymer separation membrane of claim 1, wherein in one embodiment the bilayer membrane is a flat sheet or hollow fiber;

in one embodiment, the bilayer membrane is a microfiltration membrane, ultrafiltration membrane, nanofiltration membrane, reverse osmosis membrane, pervaporation membrane, vapour permeable membrane, gas separation membrane or forward osmosis membrane.

4. The method for preparing a bilayer polymer separation membrane according to claim 1, comprising the steps of:

step 1, preparing a first polymer, a modifier, a solvent and an additive into a first solution;

step 2, preparing a second polymer, a solvent and an additive into a second solution;

and step 3, coating the first solution and the second solution in a laminating way, and obtaining the separation layer after phase inversion.

5. The method for preparing a double-layered polymer separation membrane according to claim 4, wherein the step 3 is a step of preparing a flat-plate type separation membrane, comprising the steps of: coating the first solution and the second solution on a support carrier by a co-knife coating technology, and after a certain period of phase separation treatment, allowing the solvent to enter a gel bath to be cured into a double-layer flat membrane;

in one embodiment, said step 3 is a process for preparing hollow fibers, comprising the steps of: and (3) extruding the first solution, the second solution and the core liquid from a spinning nozzle by adopting a co-extrusion technology, entering a coagulating bath through an air gap at a certain distance, and solidifying into the double-layer hollow fiber membrane.

6. The method of preparing a bilayer polymer separation membrane according to claim 4 wherein, in one embodiment, the first polymer, modifier, additive are present in the first solution in the range of: 10-35%, 0.1-20% and 0-40%; in one embodiment, the second polymer, additive, and the second solution are present in the range of: 10-35% and 0-20%.

7. The method of preparing a bilayer polymer separation membrane of claim 4 wherein, in one embodiment, the coagulation bath is water; in one embodiment, the solvent is one or a mixture of NMP and THF, and the additive is LiCl.

8. Use of the bilayer polymeric separation membrane of claim 1 for liquid separation or gas separation.

9. Use of the modifier of claim 1 in an agent for preparing a bilayer polymer membrane.

10. The use according to claim 9, wherein the modifier is used to increase the interpenetration between the macromolecules of the first and second solutions and to increase the bonding strength between the two layers of polymer.

Technical Field

The invention belongs to the technical field of membrane separation, and particularly relates to a method for preparing a double-layer membrane with excellent interlayer cohesiveness by using a monomer modified polymer with a special functional group.

Background

The process industry serves as a supporting industry for national economic development and provides basic material guarantee for social development. The separation and purification of materials in the process industry consumes a great deal of energy and is accompanied by the discharge of a great deal of solid, liquid and gas wastes. The membrane separation technology has the advantages of low energy consumption, high separation precision, environmental friendliness and the like as a novel separation technology, and can be coupled with the traditional separation and purification process to realize technical innovation and industrial upgrading so as to eliminate out-of-date capacity and reduce environmental pollution. The membrane separation technology has shown huge application value and social and economic benefits in the fields of petrifaction, food, medicine, water treatment and the like.

The separation membrane is used as the core of a membrane separation technology and is mainly divided into an integral asymmetric structure membrane and a composite membrane according to the membrane configuration. The integral asymmetric structure membrane is usually prepared by a phase inversion method, and the preparation process is simple and is beneficial to process amplification. But is limited by the preparation process in which the selective separation and support layers of the membrane are integrally phase-inverted from the same polymeric material, the separation performance of an integrally asymmetric membrane is generally low. Compared with an integral asymmetric membrane, the composite membrane can obtain better separation performance due to the fact that the structure and the property of each layer can be independently designed. However, the traditional composite membrane usually requires a multi-step preparation process, which causes problems of long production period, high reagent consumption, large production equipment investment and the like.

The double-layer film prepared by the co-blade coating or co-extrusion technology combines the advantages of the integral asymmetric film and the composite film, and has a simple one-step preparation process and a composite film structure form with each layer independently designed. For example, in a one-step co-extrusion process, a bi-layer membrane can be achieved with the lowest material cost and optimized performance by using a membrane material with excellent separation performance as the separation layer and an inexpensive membrane material with excellent mechanical properties as the support layer. The double-layer membrane has shown wide application prospect in the membrane separation fields of ultrafiltration, nanofiltration, pressure delay permeation, pervaporation, gas separation and the like.

The interlayer binding force of the double-layer film is weak due to various factors such as the compatibility difference and the mechanical property difference of two layers of polymers, the matching problem of film forming conditions of two layers of casting solution (spinning solution) and the like or the coupling effect of the two layers of casting solution (spinning solution), so that the double-layer film is layered in preparation or application. The problems of reduced separation performance and shortened service life caused by the delamination phenomenon of the double-layer film are the technical bottlenecks which hinder the development of the double-layer film. How to optimize the interlayer structure and expand the polymer selection range, and the preparation of a bilayer film with excellent interlayer adhesion by using incompatible polymers becomes a technical problem to be solved urgently in the development of bilayer films.

Disclosure of Invention

The invention provides a double-layer film with excellent cohesiveness prepared by monomer graft modification polymer with special functional group, and provides a preparation method of the film and application of a modifier.

In a first aspect of the present invention, there is provided:

a bi-layer polymeric separation membrane comprising a first polymeric layer and a second polymeric layer; a modifier is also included in the first polymer layer.

In one embodiment, the first polymer is a high molecular polymer containing an imine structure; the second polymer is a high molecular polymer containing carbonyl, carboxyl or ether bond.

In one embodiment, the first polymer is a polyimide, polyetherimide, polyamideimide, or the like; the second polymer is polyetherimide, polyethersulfone, polysulfone, or the like.

In one embodiment, the modifier is a fluoro aromatic amine monomer containing fluorine or a fluoromethyl group and an amino group.

In one embodiment, the modifying agent is 4-fluoro-2- (trifluoromethyl) benzylamine.

In one embodiment, the bilayer membrane is a flat sheet or hollow fiber.

In one embodiment, the bilayer membrane is a microfiltration membrane, ultrafiltration membrane, nanofiltration membrane, reverse osmosis membrane, pervaporation membrane, vapour permeable membrane, gas separation membrane or forward osmosis membrane.

In a second aspect of the present invention, there is provided:

the preparation method of the double-layer film comprises the following steps:

step 1, preparing a first polymer, a modifier, a solvent and an additive into a first solution;

step 2, preparing a second polymer, a solvent and an additive into a second solution;

and step 3, coating the first solution and the second solution in a laminating way, and obtaining the separation layer after phase inversion.

In one embodiment, step 3 is a step of preparing a flat-plate type separation membrane, comprising the following steps: and (3) coating the first solution and the second solution on a support carrier by a doctor blade coating technology, and after a certain period of phase separation treatment, allowing the solvent to enter a gel bath to be solidified into a double-layer flat membrane.

In one embodiment, said step 3 is a process for preparing hollow fibers, comprising the steps of: and (3) extruding the first solution, the second solution and the core liquid from a spinning nozzle by adopting a co-extrusion technology, entering a coagulating bath through an air gap at a certain distance, and solidifying into the double-layer hollow fiber membrane.

In one embodiment, the first polymer, modifier, and additive are present in the first solution in the range of: 15-25%, 1-10% and 0-5%.

In one embodiment, the second polymer, additive, and the second solution are present in the range of: 15-25% and 0-5%.

In one embodiment, the coagulation bath is water.

In one embodiment, the solvent is NMP and the additive is LiCl.

In a third aspect of the present invention, there is provided:

the application of the double-layer polymer membrane in liquid separation or gas separation.

In a fourth aspect of the present invention, there is provided:

use of the modifier described above in an agent for the preparation of a bilayer polymer membrane.

In one embodiment, the agent is used to increase the interpenetration between the macromolecules of the first solution and the second solution, and to increase the binding force between the two layers of polymers.

Drawings

FIG. 1 is a schematic representation of a bilayer membrane prepared according to the present invention.

FIG. 2 is a prepared FTB conjugateATR-FTIR spectra of the films.

Fig. 3 shows a two-layer film. The top layer is respectively: (a) the region is pure(b) Region is 2 wt% addition of FTB-245(c) Zone is 4 wt% addition(d) The region being 6% by weight of the addition

The bottom layer is Ultem.

FIG. 4 is a cross-sectional profile of a double-layer hollow fiber membrane of 4-fluoro-2- (trifluoromethyl) benzylamine graft modified P84 polyimide/Ultem polyetherimide.

FIG. 5 is a dotted line showing the upper and lower layer casting solutions.

Fig. 6 is an optical microscope observation of the phase transition process. (a) The region is pure

Coating solution; (b) the domains are 3 wt%Coating solution; (c) the domains are 6 wt%And (4) coating the film solution.

Fig. 7 is a result of calculation of compatibility between polymers. Wherein region (a) shows FTB-graftedAndthe difference in solubility parameters between; (b) region display

Surface energy and polarizability of the film surface.

FIG. 8 is a block diagram consisting of (1)

Andblend, (2)And

infrared spectrum of the constituent polymer blends.

FIG. 9 shows three liquidsContact angle on the film surface.

FIG. 10 is a view for explaining intermolecular interaction between polymers of upper and lower layers, in which the region (a) is

and(b) The domains being FTB-graftedAnd

FIG. 11 is the phase behavior of a polymer mixture, wherein region (a) is pureAnd

mixing the solution; (b) the area isAnd

the solution was mixed.

FIG. 12 is a diagram for explaining the phase behavior of a polymer solution. Wherein (a) is pureandMixing the solution; (b) the region is pureAnd

the solution was mixed.

Figure 13 is the morphology of the interfacial region of the bilayer membrane.

Fig. 14 is a graph of water separation performance of two-layer hollow fiber NF membranes at different FTB incorporation: (a) the zone is pure water permeable; (b) the regions are the retention of different single salts; solvent separation performance of 4 wt% FTB doped double layer hollow fiber membrane: (c) the region is the ultraviolet-visible absorption spectrum of the tetracycline solution; (d) the region is the uv-vis absorption spectrum of the vitamin B12 solution.

FIG. 15 is a cross-linked pristineSEM photograph of the double layer hollow fiber membrane soaked in DMF.

Detailed Description

The invention provides a method for preparing a double-layer film with excellent interlayer adhesiveness by using a monomer modified polymer with a special functional group. In the preparation process of the double-layer membrane, the modified monomer with the active group and the characteristic group is added into the membrane casting solution or the spinning solution, the active group of the monomer and the polymer in the membrane casting solution or the spinning solution are subjected to grafting reaction, and the characteristic group of the monomer is utilized to enhance the intermolecular interaction with other polymers, so that the compatibility among the polymers is improved, and the double-layer membrane with excellent interface cohesiveness is prepared. The method is simultaneously suitable for preparing the double-layer flat membrane and the double-layer hollow fiber membrane, and can realize the preparation of the double-layer membrane with the interpenetrating structure at the interface under the mild preparation condition. The interpenetration structure at the interface ensures that the prepared double-layer film has excellent interlayer cohesiveness and lower mass transfer resistance. The preparation method has certain universality, can expand the selection range of the polymer for preparing the double-layer film and expand the application field of the double-layer film.

Schematic diagram 1 illustrates:

in the traditional method for preparing the double-layer film, due to the compatibility difference between polymers or the problem of parameter matching in the film forming process, the bonding force at the interface of two layers of the prepared double-layer film is weak (macromolecules at the interface are not easy to permeate into each other), and the double-layer film is easy to delaminate.

The method grafts the modified monomer on the macromolecules (such as the upper layer) in one polymer layer, improves the compatibility of two layers of polymers, enhances the interaction between molecules, leads the macromolecules at the interface of the two layers to permeate mutually, and forms a double-layer film with excellent adhesiveness.

In the present invention, the upper and lower polymer layers (which do not refer to a spatial relationship in the upper and lower layers, and may be defined as the first polymer layer and the second polymer layer, and only defines a spatial positional relationship in which the polymer layers are stacked on each other).

For the first polymer, based on the technical idea of the present invention, it should be a high molecular polymer containing an imide structure, such as polyimide, polyetherimide, polyamideimide, etc.; the second polymer should be a high molecular polymer containing carbonyl, carboxyl or ether bonds, for example: polyetherimides, polyethersulfones, polysulfones, and the like. On this basis, the modifier used should be a fluoroaromatic amine monomer containing fluorine or a fluoromethyl group and an amino group. On the other hand, the addition of the modifier can improve the compatibility between the first polymer and the second polymer, enhance the interaction between the first polymer and the molecules containing carboxyl and ether in the second polymer, and improve the compatibility and permeability between the two polymers, and the mutual diffusion of high molecules at the interface is enhanced in the film forming process of the bilayer film due to the two reasons. The macromolecules at the interface are mutually diffused and run through, so that the interlayer adhesiveness of the double-layer film is excellent.

In the following examples, for example, a polyimide/polyetherimide two-layer film excellent in interlayer adhesiveness or a polyimide/polyethersulfone two-layer film excellent in interlayer adhesiveness can be produced by graft-modifying a polyimide with 4-fluoro-2- (trifluoromethyl) benzylamine.

For example, the following raw materials may be used:

polymer A:(polyimide)

Polymer B:

(polyetherimide)

Modifying agent: 4-fluoro-2- (trifluoromethyl) benzylamine (FTB)

Polyimide and1000 Polyetheramides are candidate polymers for the selection and support layers, respectively, since (1)The nano-filtration material is a high-performance nano-filtration material, has a rigid structure, and has relatively high material cost. However,with moderate material costs, with a rather flexible and porous structure. (2)And

has unique performance and poor compatibility, which is realizedAnd

adhesive tape with double-layer filmDifficulties and challenges arise. With the fluoro-substituted aromatic amine monomer 4-fluoro-2 (trifluoromethyl) benzylamine, the presence of the amine in FTB can be combined with

Imide reaction in the molecule. Addition of fluorine atoms with high electronegativity can be varied

Polarity of the polymer and promotion of the reaction withIntermolecular interaction of carboxyl groups contained in the molecule and ether. The method can possibly expand the selection range of non-miscible polymers for preparing non-layered double-layer films. The penetrating structure of the interface region obtained by this method can further broaden the application fields of the bilayer film.

The molecular formula is shown as follows:

Polyimide

polyetherimide

4-Fluoro-2-(trifluoromethyl)benzylamine

the cross-linking grafting process of FTB and P84 is as follows:

based on the above process, the bilayer film in the present invention means that two layers of polymer raw materials are sequentially applied in a synchronous manner, and finally a bilayer film formed by laminating a first polymer and a second polymer is formed. The structure of the double-layer membrane herein may be flat or hollow fiber. When the plate is flat, coating is carried out on a supporting material in a mode of scraping and coating solutions of two polymers in an overlapped mode, and a double-layer film is obtained after phase inversion; if the hollow fiber is adopted, the hollow fiber can be extruded through a spinning head in a co-extrusion mode, the core liquid is extruded out of the spinning head, and the hollow fiber membrane with double layers is obtained after solidification through a coagulating bath.

More specifically, the preparation steps are as follows:

the preparation process of the flat membrane comprises the following steps:

step one, preparing a casting solution (spinning solution):

casting solution (spinning solution) a: mixing the first high molecular polymer, modifier, solvent and additive in certain proportion to form casting solution A.

Casting solution (spinning solution) B: and uniformly mixing the second high molecular polymer with a solvent and an additive according to a certain proportion to prepare a casting solution (spinning solution) B.

Step two, blade coating (spinning) of a double-layer film:

and (3) coating the defoamed casting solution A, B on a support carrier by adopting a co-knife coating technology, and after a certain period of phase separation treatment, allowing a solvent to enter a gel bath to be cured into a double-layer flat membrane.

The preparation process of the hollow fiber membrane comprises the following steps:

and (3) extruding the defoamed spinning solution A, B and the core solution from a spinning nozzle by adopting a co-extrusion technology, entering a gel bath through an air gap with a certain distance, and curing to form the double-layer hollow fiber membrane.

Materials used in the following experiments

By using

Polyimide (HP Polymer GmbH) as a bilayer filmThe layer material is selected. Provided by SABIC

1000 polyetheramide as the polymeric material of the support layer. N-methyl-2-pyrrolidone (NMP, 99%, Macklin) as a solvent and tetrahydrofuran (THF, 99%, national chemical reagents, Inc.) as a volatile cosolvent in a solution of the selective layer. 4-fluoro-2- (trifluoromethyl) benzylamine (FTB,>99%, Leyan) pair

Polyimide is modified. The lithium chloride (LiCl,>99%, alatin) and silicon dioxide (SiO)₂，>99.8%, Maclin) as an additive in the support layer casting film solution. The bilayer membrane was crosslinked using 1, 6-hexanediamine (HDA, 99%, national chemical Co., Ltd.). Sodium sulfate (Na) available from national chemical agents Co., Ltd₂SO₄99%), sodium chloride (NaCl, 99%), magnesium sulfate (MgSO)₄99%), magnesium chloride (MgCl)₂99%) was dissolved in an aqueous solution, and the nanofiltration performance was tested. Tetracycline (Mw:444.435Da) and vitamin B12(Mw:1355.37Da) extracted from alatin were dissolved in methanol and tested for solvent-resistant nanofiltration performance.

Preparing casting solution, measuring its cloud point, viscosity and phase separation behavior

Polyimide andthe polyetherimide polymer was dried in a vacuum oven at 90 deg.C overnight to remove moisture. Firstly, the first step is to

The polyimide was dissolved in a mixture of NMP and THF and stirred for 1 day to form a homogeneous solution. Then is atA certain amount of FTB monomer was added to the polymer solution and stirred for 1 day.

The polyetherimide was dissolved in NMP at 70 deg.C, and a certain amount of lithium chloride and silica were added and stirred for 2 days. After a homogeneous solution was formed, the solution was allowed to stand for 2 days to eliminate air bubbles.

With addition of FTB determined by titration at room temperatureSolutions andcloud point of the solution. A homogeneous casting solution was prepared in a glass bottle and kept under stirring during the titration experiment. Small drops of water were added to the homogeneous solution in batches until the solution became cloudy and was kept under stirring for more than 24 hours. The total amount of added water was calculated. The viscosities of the various casting film solutions were measured with a Brookfield viscometer (DV 2T). The viscosity is obtained at a shear rate of 10 s-1.

The incorporation of a certain amount of FTB was observed with a polarizing microscope (PLM, Olympus BX53, Japan)

Casting solution and process for producing the same

The phase transition of the casting solution was recorded with a CDD camera. Detailed procedures can be found in the literature (T.Z.Jia, J.P.Lu, X.Y.Cheng, Q.C.Xia, X.L.Cao, Y.Wang, W.Xing, S.P.Sun, surface engineered sulfonated polyarylenes ether copolymers (SPEB) which is derived from fatty Polyarylenes (PAN) which is derived from film composite nanofilations membranes, J.Membr.Sci.623580 (2019) 214-.

Preparation of double-layer flat membrane and double-layer hollow fiber membrane

With addition of a quantity of FTB monomer

Casting solution andthe casting solution was knife coated on a glass plate using a coupled doctor blade and then immersed in a bath of a water coagulant to phase-convert.

The specific operation steps are as follows:

the high molecular polymer P84 polyimide and Ultem polyetherimide were placed in a vacuum oven at 90 deg.C for 12 hours to remove moisture.

The P84 polyimide polymer was dissolved in NMP solvent and magnetically stirred for 24 hours to form a homogeneous solution. Adding a 4-fluoro-2- (trifluoromethyl) benzylamine modifier into the macromolecule solution, and magnetically stirring for 24 hours at room temperature to form uniform membrane casting solution A.

Dissolving Ultem polyetherimide polymer and LiCl additive in NMP solvent, heating in water bath at 70 deg.C, and magnetically stirring for 24 hr to obtain uniform casting solution B.

And standing the casting solution A and the casting solution B for 12 hours for defoaming.

The method comprises the steps of adjusting the height of a scraper of a polymer membrane casting solution B to be 150 mu m and the height of a scraper of a polymer membrane casting solution A to be 200 mu m by adopting a phase inversion method and a co-scraping coating (co-casting) technology, scraping the polymer membrane casting solutions A and B on a carrier (a glass plate or non-woven fabric), immersing the carrier in a gel bath (pure water), and solidifying the membrane casting solution into a double-layer flat membrane.

And respectively extruding the polymer membrane casting solution A, the polymer membrane casting solution B and the core solution from a three-hole spinning nozzle by using a dry-wet spinning and co-extrusion (co-extrusion) technology by using an injection pump. The nascent fiber is immersed in a gel bath (water) after passing through a section of air gap to solidify the double-layer hollow fiber membrane. And (3) after the solidified hollow fiber membrane filaments are collected by a filament collecting device, immersing the hollow fiber membrane filaments in pure water for 2 days, and replacing the solvent. Then the membrane filaments were soaked in a 30% glycerol aqueous solution for 2 days and then air-dried.

The detailed preparation process of the bilayer flat plate membrane can also be seen in (Q.C.Xia, J.Wang, X.Wang, B.Z.Chen, J.L.Guo, T.Z.Jea, S.P.Sun, A hydrolicity 598 gradient control membranes, J.Membr.599Sci.539(2017) 392. 402).

Table 1 summarizes the detailed coating ingredients and their viscosities.

TABLE 1

The double-layer hollow membrane is prepared by using a three-hole spinning head and adopting dry-jet wet spinning and coextrusion technologies. Two ISCO injection pumps simultaneously spray liquid to the inside and the outside of the spinneret plate, and a liquid chromatography pump simultaneously sprays liquid to the inside of the spinneret plate. The co-extruded coating material experiences an air gap before entering the water coagulant. The precipitated hollow fibers were collected by a take-up drum. The nascent hollow fibers were rinsed with tap water for 2 days to remove residual solvent. Part of the hollow fibers used for module manufacture was immersed in a 40 wt% glycerol aqueous solution for 2 days and then dried in ambient air. Another hollow fiber uses a solvent exchange process involving three times n-hexane and three times methanol to eliminate surface tension.

Membrane characterization

Adopts Fourier transform infrared spectroscopy (FTIR) to analyzeChemical change of the top layer upon combination with a quantity of FTB monomer. Attenuated Total Reflectance (ATR) mode was applied using an FTIR spectrometer (Thermo Scientific, Nicolet iS50) over the range of 400-4000 cm-1 for a total of 64 scans per sample. To further quantify

The FTB content on the polyimide was analyzed for elemental analysis using X-ray photoelectron spectroscopy (XPS, Escalab250Xi, Thermo Scientific).

The appearance of the double-layer flat membrane is observed by a Field Emission Scanning Electron Microscope (FESEM) and Hitachi S4800. The cross-sectional morphology of the double-layer hollow fiber membrane was observed using a Scanning Electron Microscope (SEM) and Hitachi TM 3000. Prior to FESEM or SEM analysis, the samples were broken up in liquid nitrogen and sputtered with gold.

FTB monomers and XPS were studied using FTIR and XPS

And (3) carrying out grafting reaction on the polyimide in the top layer casting film solution. 1780. 1718 and 1360cm^-1The signal intensity of the imide band at (A) gradually decayed, and 1534cm^-1With amide band at

The gradual increase in the FTB monomer content in the solution increased (FIG. 2), indicating thatThe imide group in (b) reacts with the amine in the FTB and forms an amide group. 1317cm detected by XPS as the amount of FTB monomer added increases^-1The resulting increase in the trifluromethyl base band (FIG. 2) and F/O atomic ratio (Table 2) further confirmed the successful incorporation of FTB into the P84 polymer chain. FTB monomer andthe reaction mechanism between polyimides is as shown above. FTB graft modification

Denoted as FTB-P84. Containing one FTB monomer per repeat unit

Denoted S-FTB-P84, each repeating unit comprising two FTB monomersDenoted as D-FTB-P84. Addition of FTB monomer in

Polymer and method of making sameAmide and trifluoromethyl units are introduced into the chain, which can be used as proton donor and acceptor and form intermolecular interaction with the carbonyl-containing polymer. It is shown that the characteristic peak of the imine bond of the P84 polyimide is weakened, the characteristic peak of the amide bond is increased and appeared, and the characteristic peak of the fluoromethyl group is appeared at the same time by the graft modification of 4-fluoro-2- (trifluoromethyl) benzylamine. The successful grafting of the 4-fluoro-2- (trifluoromethyl) benzylamine modifier to the P84 polyimide polymer chain is proved, and the fluoromethyl and amide groups are introduced.

Table 2 XPS elemental analysis results of the surface layer of the P84 film prepared in table 2

Containing a certain amount of FTBSolutions andthe solution was co-drawn on a glass plate using a coupled casting knife. The membrane was then immersed in a coagulation bath at room temperature for phase inversion. Fig. 3 illustrates the morphological evolution of two-layer films prepared by adding different FTBs to the top-layer casting solution. When using pure FTB monomer

When the casting film solution is used as the top layer casting film solution, the top layer is solidified in water after co-casting

Directly from the bottom layerSeparation (region (a) of fig. 3). The addition of 2 wt% FTB monomer to the top layer solution allowed the bilayer membrane to achieve macroscopic adhesion during coagulation. However, the bilayer membrane slightly delaminates when ruptured in liquid nitrogen (region (b) of fig. 3). Further increasing the FTB monomer content in the top solution allows for a delamination-free interfacial region(region (c) and region (d) of FIG. 3). Illustrating the bilayer membrane with no added 4-fluoro-2- (trifluoromethyl) benzylamine modifier, the upper P84 polyimide layer and the lower Ultem polyetherimide layer were peeled directly. With the addition of the 4-fluoro-2- (trifluoromethyl) benzylamine modifier, the adhesiveness at the interface of the double-layer film is gradually enhanced, and after a certain amount of 4-fluoro-2- (trifluoromethyl) benzylamine graft modified P84 polyimide is added, the excellent adhesiveness of the upper layer and the lower layer of the double-layer film is realized.

It can be seen that with the FTB monomer in

The doping amount in the polyimide solution is gradually increased, so that continuous form conversion from delamination to partial attachment can be realized, and the adhesion of a double-layer film can be further realized. A mild and effective method to solve the bilayer membrane delamination problem shows good application prospects.

The preparation method of the double-layer flat membrane is successfully transplanted to the preparation of the double-layer hollow fiber nanofiltration membrane by adopting dry-jet wet spinning and co-extrusion technologies. The detailed spinning conditions are shown in table 3.

TABLE 3

Similar morphological changes from delamination to slight adhesion and further adhesion were observed with increasing amounts of FTB added (fig. 4). The outer layer is about 5 μm, the majority has a sponge structure, and the interior has

The layers are about 200 μm, with the majority of the large pores being long fingers. Illustrating the bilayer membrane with no added 4-fluoro-2- (trifluoromethyl) benzylamine modifier the outer P84 polyimide layer peeled directly from the inner Ultem polyetherimide layer as shown in region (a) of figure 4. With the addition of the 4-fluoro-2- (trifluoromethyl) benzylamine modifier, the adhesion at the bilayer membrane interface gradually increases, as shown in region (b) of fig. 4. When a certain amount of 4-fluoro-2- (trifluoromethyl) benzylamine graft modified P84 polyimide is added, the inside and outside of the double-layer filmThe layer achieves excellent adhesion as in the region (c) (d) of fig. 2.

Influence of FTB grafting modification on thermodynamic property and film forming kinetics of casting solution

The interpenetration of the polymer molecules can form an interpenetrating network structure in the interface area of the double-layer film, so that the double-layer film has strong integrity and low transfer resistance. Cast film

Measure and makeAnd

the cloud point curve of the casting solution is shown in FIG. 5.The cloud point curve of the bottom layer solution is very close to the polymer solvent axis, indicating very limited resistance to water. And

compared with the solution, pureThe cloud point curve of the solution is much further from the axis of the polymer solvent. Thus, is obtained byCompared with the casting film solution, pure

The phase inversion rate of the casting solution is much slower. With following

The increase in FTB monomer content in solution shifts the cloud point curve in the opposite direction to the polymer-solvent axis. Therefore, the distance between the cloud point curves of the upper and lower layer solutions increases, indicating that the difference between the precipitation rates of the upper and lower layers is large. The gradual delayed precipitation rate of the top layer casting film solution may be the topThe interpenetration of the layer polymer with the underlying polymer provides more residence time, a factor that promotes bilayer adhesion.

The phase inversion process was further observed by microscopy (fig. 6). A small drop of solution was dropped onto a glass slide and covered with a glass cover. After the casting solution is contacted with a non-solvent (water), exchange of the solvent and the non-solvent occurs at the water-casting solution interface. When the water intrusion exceeds the critical concentration for liquid-liquid separation, polymer dilute phase nuclei form and coalesce.An increase in the FTB monomer content of the coating solution does not significantly alter the growth rate of the finger-like macropores. Therefore, as the amount of FTB monomer added to the top casting film solution increases, the rate of water penetration from the top casting film solution into the interface region does not slow. As is apparent from the above-mentioned characterization,the thermodynamics of the cast film is

The adhesion of the interface region of the bilayer membrane plays a more important role in providing prerequisites.

Influence of FTB grafting modification on polymer interpenetration at double-layer membrane interface

Essentially, bilayer membrane adhesion results from interpenetration of polymer molecules, i.e., spontaneous mixing of polymer molecules or segments, which interpenetration is due to mixed Gibbs free energy (Δ G) when the cast membrane casting solutions meet at the interface when co-cast or co-extruded^M/RT) is thermodynamically determined and can be described by the following equation:

in the formula (16), Φ A, Φ (B) and N_A(N_B) Denotes the volume fraction and degree of polymerization of the polymers A (B), respectively, and chiIs a polymer-polymer interaction parameter. Δ G_Hthe/RT term reflects the contribution of specific intermolecular interactions between polymers to the free energy of mixing. The above formula gives two main factors of spontaneous mixing of polymer molecules: (1) the relative affinity of the polymers to each other, which can be described by an interaction parameter or a solubility parameter; (2) specific intermolecular interactions between polymers, such as hydrogen bonding.

The solubility parameter reflects the nature and the size of the interaction force and embodies the principle of 'similar and compatible'. The solubility parameter differences between the polymer pairs are small, indicating that they have better mutual affinity and possibly better compatibility. PureAndthe solubility parameter components are calculated in groups according to the chemical structures of the components, and the specific calculation process is as follows:

the solubility parameter of a polymer can be calculated by group contribution methods, which depend on the cohesive energy of the constituent functional groups and their molar volume in the repeating units. Total solubility parameter of polymer_totalAnd its component of dispersing force_dDipole component_pAnd hydrogen bond component_hCan be obtained from the following equation:

table 4 shows the values of the parameters used in the above formula.

TABLE 4

Due to the lack of a parameter for the fluoro group, the dipole and hydrogen bond components combine to a, calculated from total and d as follows:

the solubility parameter for the mixture of substances 1 and 2, component i, was calculated as follows:

_i，mixture＝φ₁·_i，1+φ₂·_i，2

in the formula (I), the compound is shown in the specification,and

representing the mole fraction of species 1 and 2. Thus, the mutual Relative Affinity (RA) can be calculated by the solubility parameter difference as described below:

obtain different proportionsAndmixtures and proportions of differentAndsolubility parameter component of the mixture. The relative affinity between the top and bottom polymers was assessed by the solubility parameter (region (a) of fig. 7). Derived purityAnd

the difference in solubility parameter between them was 3.34 cal^{1/ 2}cm^-3/2This reflects the limited mutual relative affinity between the polymer pairs. By being at

Gradually adding FTB monomer to polymer chain for modification

Andthe difference in solubility parameter between them decreased from 3.34 to 0.27cal^1/2cm^-3/2This means that

Andthe affinity between them is significantly improved. Further increase the ratio of FTB monomer

ToThe solubility difference between the polymer pairs becomes large again. In other words, in

Addition of excess FTB monomer to the polymer chain increases steric hindrance caused by the large number of branching groups and F atoms and hinders modificationThe interaction between molecules.

Interaction with Ultem molecules

Due to the strong electronegativity of fluorine atoms, fluorine radicals andincorporation of polymer chains can alter the polarizability and surface properties of the polymer. The amide groups and fluorine groups introduced by the grafting reaction can also be used as effective proton donors or acceptors to form specific intermolecular interactions with polymers containing carboxyl groups and ether groups.

The surface energy and polarizability of the membrane surface were calculated by the Lifshitz-van der Waals/acid-base (LW/AB) method. According to the LW/AB method, from a nonpolar component gamma^LWAnd an acid-base component gamma^ABThe surface energy of the solid surface γ of the composition is as follows:

γ＝γ^LW+γ^AB

electron donor acid base component gamma^ABCan be based on an electron donor gamma⁺And electron acceptor gamma^-Calculation, as follows:

solid-liquid phase gamma_SLThe interfacial energy between can be calculated as:

the subscript S represents a solid and L represents a liquid. The subscript SL represents the solid-liquid interface. Combining the above equation with Young's equation, the following relationship can be obtained:

by obtaining contact angle data measured by contact angle measuring instruments (KRUSS, DSA30) for three different polar liquids (water, glycerol and diiodomethane) and their surface tension parameters (table 5), gamma can be obtained by equation (4)^LW、γ⁺And gamma^–The value of (c). The polarizability (Pol) of the membrane surface can be further calculated as follows:

Pol＝γ^AB/γ

TABLE 5

With at

With gradual addition of FTB, the surface energy of the top layer does not follow a clear trend, but with the addition of FTB

The polarizability of (a) is significantly improved (region (b) of fig. 7). The polarizability of the polymer molecules is positively correlated with molecular interactions (e.g., dipole-dipole interactions) between the polymers.And

the specific intermolecular interactions between the two are further verified by the frequency shift of the infrared transmission peak of the polymer blend. Fluoro group (1317 cm)^-1) And amide group (1534 cm)^-1) The characteristic peak of (A) appears inAndin the mixture (regions (a) and (b) in FIG. 8)Domain, table 6).

Andcarbonyl (C ═ O) and aromatic ethers (C-O-C) in mixtures with pureAnd

the frequency of the mixture was higher than that of the mixture (region (c) (d) of fig. 7, table 6). Thus, the intermolecular interactions in the blend can be attributed to hydrogen bonds and dipole-dipole interactions between the amide groups, fluorine groups, and carbonyl groups.

TABLE 6

With and without the addition of FTB

Andthe molecular chemistry and possible ubiquitous interactions between them are shown in figure 10.

Due to the fact thatAndthe absence of effective proton donor groups in the polymer and the specific intermolecular interactions include pure

And

pi-pi stacking and n-pi complexes between. In thatAfter FTB monomer is added to polyimide chain, the hydrogen of amido can be used as proton donor, and reacted withThe ether and carbonyl groups of (a) form hydrogen bonds. Furthermore, from FTB-graftingMay be mixed with a solvent containingThe carbonyl group of the molecule and the ether form a strong interaction.

And

the enhanced mutual relative affinity and the established specific intermolecular interactions between them help to promote the compatibility and permeability of the polymer molecules in the bilayer membrane interface region. In order to verify the effects of the above two factors, the effects of the affinity between polymers and the specific interactions between molecules on the interpenetration of molecules in the interface region of the bilayer membrane, the phase behavior of polymer miscible liquids, and the interface morphology of the bilayer membrane were further studied.

The phase behavior transition of the interphase reflects the compatibility between the polymers. To simulate the mixing of the casting films encountered in the interface region during co-casting, different amounts of FTB were addedCasting solution and

the cast film and the cast film solution are mixed equivalently. The mixed cast film solution was stirred for 24 hours, and thenLeft to stand for 24 hours and then observed with an optical microscope. As observed by optical microscopy, all mixed solutions appeared macroscopically homogeneous with varying degrees of phase separation on the micrometer scale (fig. 11). PureAndthe mixed solution of (a) is composed of highly dispersed phases such as minute droplets (region (a) of fig. 11). This may be due to purity

Andis thermodynamically hindered by the poor mutual affinity and the absence of specific intermolecular interactions between polymer pairs. After the stirring, the mixture is stirred,and

the mixed solution forms a dispersed phase as shown in the region (a) of fig. 12. After the introduction of water, the dispersed phase near the water interface aggregates and precipitates into a solid. Unlike the pure P84 and Ultem mixtures, most of the mixtures of the 4 wt% FTB-P84 and Ultem cast film solutions consist of a continuous phase with a partially dispersed phase, as shown in region (b) of FIG. 11. Due to the high degree of mutual affinity and specific intermolecular interactions between FTB-bound P84 and Ultem, FTB-P84 polymer can serve as a compatibility linking the P84 and Ultem phases, as shown in region (b) of fig. 12. After the introduction of water, a continuous phase precipitates and the finger-like macropores grow through the interphase. Connectivity and interpenetration between the phase inversion mixtures can be observed.

To further confirm the contribution of intermolecular mutual affinity and intermolecular interaction to interpenetration, the bilayer membrane boundary was observed with different FTB addition levels using scanning electron microscopyTopography of the area of the face (fig. 13). Large voids are visible at the bottom surface of the top layer, demonstrating that the macro-voids of the fingers extend from the body of the top layer to the bottom layer. When pureAndwhen the solution is co-cast (region (a) of fig. 13), incompatibility between the top and bottom layer polymers thermodynamically hinders the mixing of the polymer molecules in the interfacial region, resulting in limited formation of the interfacial phase. During the phase transition, the surface layer containing a certain amount of non-solvent expands macro-pores into the interface region. Bottom part

The hydrophobicity of the layer further hinders the penetration of the non-solvent from the top layer to the bottom layer. Under the induction of non-solvent accumulation, phase inversion occurs in the newly formed interfacial phase. As shown in the region (a) of FIG. 11, the polymer is made of an immiscible polymerThe interfacial phase of the mixture composition precipitates as an isolated solid with limited connectivity. As shown in region (a) of fig. 13, isolated and micro interpenetrating structures can be seen on the top surface of the bottom layer. The interface is not able to withstand the differential shrinkage of the upper and lower layers during solidification. Thus, it is original

Delamination occurred between the two films. By being atWith the addition of the FTB monomer, the mixing of the polymer molecules in the interfacial region is enhanced due to the increased relative affinity to each other and the establishment of specific intermolecular interactions between the top and bottom layers. A gradual improvement in the interpenetration of the top layer polymer with the bottom layer polymer can be observed on the upper surface of the bottom layer (regions (b), (c), (d) of fig. 13). Further, as in FIG. 11(b) Shown in region, after phase conversion byThe constituent interfacial phases exhibit interpenetrating structures, forming a connection between each other. FTB-containing interconnect structures with enhanced intermolecular interpenetration at interfacial regions and phase inversion of interfacial phasesThe bilayer film has strong adhesion.

From the above discussion, it is clear that by adding specific functional groups, specific intermolecular interactions can be formed, mutual affinities are enhanced, and differences in compatibility can be adjusted even across polymers. The mixing of the interfacial region polymer molecules can form an interpenetrating network structure between the bilayers, as shown in fig. 1. Based on the theory presented in the present invention, the range of selecting polymers from inherently incompatible polymers for producing a delamination-free bilayer membrane can be significantly extended.

Filtration and retention Performance

The double-layer hollow fiber nanofiltration membrane prepared by the one-step method is subjected to a filtration test in a water and organic solvent system.

The separation performance of the double-layer hollow fiber membrane was evaluated from the viewpoint of pure water permeability, and a laboratory-made filtration system was used for methanol permeability and solute discharge. Pure water and methanol permeate samples were collected as a function of time and solvent permeability (P, Lm) was calculated by the following formula^-2h^-1bar^-1)：

Wherein Δ v (l) is the volume of permeate collected over a time period Δ t (h) at a transmembrane pressure Δ p (bar), a (m)²) Is the effective membrane area.

Respectively using 1000.0mg L^-1Na of (2)₂SO₄、NaCl、MgSO₄And MgCl₂The aqueous solution was subjected to a solute entrapment test in an aqueous system. With vitamin B12 and tetracycline 50.0mg L^-1The methanol solution was subjected to solute rejection tests in a solvent system.

The rejection rate is calculated by the following equation:

in the formula, C_pAnd C_fRespectively, the solute concentrations in the permeate and feed solutions. The salt and drug concentrations in the feed and permeate were measured using a conductivity meter (Mettler-Toledo) and an ultraviolet-visible spectrophotometer (AOE), respectively.

The pure water permeability of the two-layer hollow fiber membrane was affected by the amount of FTB added to the top-layer casting film casting solution (area (a) of fig. 14). The interception sequence of four salts of the cast film is Na₂SO₄>NaCl>MgSO₄>MgCl₂(region (b) of FIG. 14), particularly when the amount of FTB added is controlled to be 4% -6%, 80% or more of Na can be obtained₂SO₄Retention rate, and retention rate of NaCl above 50%. The FTB composite double-layer membrane with the mass fraction of 4 wt% is further applied to the recovery of the drugs in the organic solvent. In a pure methanol solution with a methanol permeability of 3.7LMH/bar, the membrane showed 99% retention of tetracycline and vitamin B12 (FIG. 13, panel (c) (d)).

Test for solvent resistance

In order to further verify the molecular mixing property of the double-layer hollow fiber membrane in the interfacial region and examine the tolerance of the double-layer hollow fiber membrane in an organic solvent, the prepared double-layer hollow fiber membrane is crosslinked with 1, 6-hexamethylenediamine for 15 hours and soaked in DMF for two weeks. As shown in fig. 15, the crosslinked pristine

The double-layer hollow fiber membrane is layered after being soaked in DMF. 4% by weight after crosslinking

The bilayer hollow fiber membranes still did not delaminate after two weeks immersion in DMF solvent, indicating that FTB is present in the bilayer interface region

Andthe molecules reach a good mixing state and can therefore be chemically cross-linked by HDA. Figure 15 illustrates that possible reactions exist in the outer, inner and interfacial regions of a bilayer membrane. The double-layer hollow fiber membrane with low cost can keep the structural integrity in a severe solvent, and has wide application prospect in the fields of organic solvent nanofiltration and reverse osmosis.