image
Home Journals Progress in Chemistry
Progress in Chemistry

Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

About  /  Aim & scope  /  Editorial board  /  Indexed  /  Contact  / 
Online first  /  Current issue  /  All issues  /  Top access  /  RSS

Progress in Chemistry >

2023 , Vol. 35 >Issue 8: 1199 - 1213

DOI: https://doi.org/10.7536/PC221215

Review

Structure Design and Tailoring Strategy of Polymeric Materials for Fabrication of Nanofiltration Membranes via Phase Inversion

  • Tao Liu ,
  • Junping Miao ,
  • Longlong Wang ,
  • Yunxia Hu , *
Expand
  • State Key Laboratory of Separation Membranes and Membrane Processes, School of Materials Science and Engineering, Tiangong University,Tianjin 300387, China
*Corresponding author e-mail:

Received date: 2022-12-28

  Revised date: 2023-06-14

  Online published: 2023-07-18

Supported by

National Natural Science Foundation of China(21978215)

Fold

Abstract

The non-solvent induced phase separation (NIPS) method has significant advantages including easy processing and tailorable membrane structure in the preparation of nanofiltration membranes with high-flux and selectivity. Increasing attention has been drawn from the membrane field to further improve the precise separation and permeability of the membrane. In this review, the effects of the thermodynamics and kinetics on the membrane structure and properties during the NIPS process are systematically described, and the research progress is summarized to illustrate how the polymeric membrane materials including polysulfone and polyethersulfone affect the membrane structure and separation performance. Furthermore, the characteristics of amphiphilic block copolymer materials and their outstanding advantages in the fabrication of high-flux nanofiltration membranes are comprehensively reviewed. Finally, the potential research focus is proposed to inspire the membrane community to develop high-performance nanofiltration membranes via NIPS in the future.

Contents

1 Introduction

2 Research progress of nanofiltration membrane prepared by phase inversion

2.1 Formation mechanism of nanofiltration membrane prepared by phase inversion

2.2 Materials for preparation of nanofiltration membrane by phase inversion

2.3 Optimization of nanofiltration membrane structure and separation performance

3 Amphiphilic block copolymers and the fabricated nanofiltration membranes

3.1 Amphiphilic block copolymer membrane materials and their characteristics

3.2 Research progress of block copolymer nanofiltration membrane

4 Conclusion and outlook

Key words： nanofiltration membrane; non-solvent induced phase separation (NIPS); permeability; selectivity; block copolymer

Cite this article

Tao Liu , Junping Miao , Longlong Wang , Yunxia Hu . Structure Design and Tailoring Strategy of Polymeric Materials for Fabrication of Nanofiltration Membranes via Phase Inversion[J]. Progress in Chemistry, 2023 , 35(8) : 1199 -1213 . DOI: 10.7536/PC221215

1 Introduction

Nanofiltration (NF), which appeared in the 1980s, is a pressure-driven separation technology with separation performance between ultrafiltration and reverse osmosis. The pore size of nanofiltration membrane is generally between 0.5 and 2.0 nm, and the molecular weight cut-off (MWCO) is between 200 and 2000 Da. Compared with reverse osmosis technology, nanofiltration technology has higher permeate flux and lower operating pressure (0. 3 ~ 1.0 MPa), and has the advantages of low investment cost, low operation energy consumption and low maintenance cost in large-scale application[1]. In addition, most of the nanofiltration membranes are charged, and the separation ability of nanofiltration membranes is affected by the size exclusion and Donnan effect[2]. Based on the above characteristics, nanofiltration can be used in the fields of material separation and concentration, water purification and softening, municipal and industrial and agricultural sewage recycling, etc[3].
At present, the preparation methods of nanofiltration membrane are mainly divided into interfacial polymerization, phase inversion, layer-by-layer self-assembly and coating-crosslinking. Interfacial polymerization is the most commonly used and industrialized method for the preparation of nanofiltration membranes, which can use aqueous phase monomers (such as piperazine) and oil phase monomers (such as trimesoyl chloride) to form a dense polyamide separation layer on the surface of the base membrane, and then prepare a composite nanofiltration membrane with an ultra-thin separation layer[4]. The thin and dense polyamide separation layer (only 10 – 100 nm in thickness) allows for fast water permeation (commercial polyamide nanofiltration membranes, such as Dow's NF90, have a water permeability coefficient of up to 14 L/(m2·h·bar)) and efficient rejection of salts and small molecules (their MWCO is usually less than 500 Da)[5][6⇓~8]. However, there are still some problems in the preparation of nanofiltration membranes by interfacial polymerization. One is that the polyamide separation layer has a dense structure, and the pore size is difficult to be precisely controlled at the nanometer scale, resulting in low screening accuracy of the membrane for small organic molecules and salts. Another reason is that that polyamide separation lay formed by this method is not very resistant to active chlorine and oxidizing agent. When exposed to water containing fungicides (such as hypochlorous acid) for a long time, the crosslinked structure of the polyamide separation layer will be destroyed, thus losing the sieving performance[9]. In addition, the interfacial polymerization process often uses a large number of toxic and harmful organic solvents, which is not friendly to the environment. At the same time, the interfacial polymerization process has high requirements for processing equipment and high investment cost.
At present, another mature technology for industrial preparation of nanofiltration membrane is non-solvent induced phase separation (NIPS), in which polymer materials, porogens and solvents or diluents are fully mixed to form a thermodynamically stable homogeneous casting solution. And then respectively prepare a nascent flat plate or a nascent hollow fiber liquid membrane by a coating or extrusion mode; Finally, a non-solvent is introduced to promote the phase separation of the nascent liquid film to form a polymer-poor phase and a polymer-rich phase respectively, and the polymer-poor phase and the polymer-rich phase are solidified to form the film[10⇓~12]. Nanofiltration membrane prepared by NIPS method can control the pore size (1 ~ 3 nm) at the nanometer scale, and realize the concentration of small molecular organic matter, the separation of small molecular organic matter and salt, and the separation of monovalent salt and multivalent salt[13][14]. In addition, the NIPS membrane method has simple operation and low equipment requirements, and can be used for industrial production of flat nanofiltration membranes and hollow fiber nanofiltration membranes[15⇓~17]. At present, the common commercial phase inversion flat nanofiltration membranes include NP010 and NP030 produced by Minard Company in Germany and NTR7450 nanofiltration membrane produced by Nitto Denko. However, this kind of nanofiltration membrane is made of polyethersulfone (PES) or sulfonated polyethersulfone (SPES) by phase inversion. Due to the influence of its molecular structure and membrane preparation characteristics, the porosity of the nanofiltration membrane is low and the skin layer is thick, resulting in poor water permeability of the membrane and limited application space.
In this paper, the research progress of nanofiltration membranes prepared by phase inversion method at home and abroad is reviewed, and the theoretical basis and influencing factors of membrane pore formation by phase inversion method are summarized, focusing on the analysis of the influence mechanism of different polymer materials on membrane pore structure and separation performance, and finally the existing problems and future development direction of nanofiltration membranes prepared by phase conversion method are pointed out.

2 Research progress in preparation of nanofiltration membrane by phase inversion method

Phase inversion is a process in which a polymer solution in a homogeneous state is transformed into two liquid States by some external interference (liquid-liquid phase separation). After a certain degree of phase separation, one of the phases (the phase with high polymer concentration) solidifies into a solid mass, while the other phase (the phase with low polymer concentration) becomes a void. The membrane morphology can be regulated by controlling the phase inversion process[10]. This section will focus on the theoretical basis of the preparation of nanofiltration membranes by phase inversion method, and summarize the commonly used nanofiltration membrane materials in the literature, the structure-activity relationship between the structure and performance of nanofiltration membranes, and the common methods to regulate the structure and optimize the performance of nanofiltering membranes.

2.1 Theoretical Basis of Nanofiltration Membrane Preparation by Phase Inversion

At present, the preparation methods of polymer nanofiltration membranes are mainly NIPS. The microstructure and separation performance of the membrane are determined by the thermodynamic properties of the casting solution and the diffusion rate of solvent-nonsolvent in the phase inversion process. In order to further understand the mechanism of membrane formation in the process of phase inversion and the influence of various parameters on the membrane structure, we first analyzed and summarized the basic theories of thermodynamics and kinetics in the process of membrane formation by NIPS.

2.1.1 Thermodynamic properties of casting solution

In describing the thermodynamic properties of membrane casting solution, Gibbs free energy of mixing (ΔGm) is usually used for analysis, which is defined as:
Δ G m = Δ H m - T Δ S m #
Where ΔHm is enthalpy of mixing and ΔSm is entropy of mixing. When ΔGm≤0, it means that the mixing process can proceed spontaneously and the system presents a homogeneous phase; When the ΔGm>0, the system occurs phase separation and multiphase state. For the casting solution system, because the molecular weight of polymer can reach tens of thousands, the ΔSm is very small, so the stability of the casting solution is mainly affected by the ΔHm (that is, affected by the interaction parameters between solvent and polymer). Therefore, the thermodynamic stability of the casting solution can be effectively controlled by adjusting the properties of the polymer and the solvent[10,18].
Fig. 1 is the phase diagram of a typical polymer/solvent/nonsolvent ternary component, in which the thick solid line is the binodal line, the thin solid line is the spinodal line, and the region between the binodal line and the polymer-solvent axis is the miscibility gap[10]. In the process of NIPS membrane preparation, when the homogeneous primary liquid membrane (zone Ⅴ) is immersed into the non-solvent, the composition of the primary membrane will enter the corresponding zones Ⅰ, Ⅱ, Ⅲ, Ⅳ, Ⅵ in the phase diagram with the immersion of the non-solvent from the surface of the primary membrane into the membrane and the precipitation of the solvent, and the phase separation behavior will occur and the membrane will be solidified[19]. The film structure formed after phase separation in different regions of the phase diagram is shown in fig. 2. The results show that when the binodal line is close to the polymer-solvent axis and the miscibility gap is small (i.e., the Ⅰ and Ⅵ regions are small), instantaneous phase separation is easy to occur[19⇓⇓⇓~23]. On the contrary, when the miscibility gap is large (that is, when the Ⅰ and Ⅵ regions are large), delayed phase separation is easy to occur. In instantaneous phase separation, the composition curve of the nascent liquid membrane crosses the binodal line in a very short time (generally t < 1 s), which means that liquid-liquid phase separation occurs rapidly at and below the membrane/bath interface, resulting in a porous structure, and the membrane obtained after instantaneous phase separation is prone to exhibit a membrane structure with high porosity and macropores[24⇓~26]. However, in the delayed phase separation, the membrane/bath interface and the part below it are in the single-phase region for a long time, and the components are still miscible, so it takes a longer time to achieve the full replacement of solvent-nonsolvent, and the membrane casting solution system will be separated. This makes it too late for the nascent film to produce enough polymer-poor and -rich phases before the polymer gelation (solidification) occurs, so that the resulting film tends to have a dense structure[20,27,28].
图1 聚合物/溶剂/非溶剂三元体系相图[13]

Fig.1 Schematic representation of a ternary phase diagram of the polymer/solvent/nonsolvent system[13]. Copyright 2011, American Chemical Society

图2 不同铸膜液成分发生分相后所形成的膜结构:(Ⅰ,Ⅵ)致密结构;(Ⅱ)海绵孔结构;(Ⅲ)双连续结构;(Ⅳ)粒状结构(黄色代表聚合物富相,绿色代表聚合物贫相)[27]

Fig.2 The membrane structure formed by phase inversion of the different casting solution components: (Ⅰ,Ⅵ) Dense structure; (Ⅱ) Sponge structure; (Ⅲ) Bi-continuous or lacy structure; (IV) Nodules (yellow represents polymer rich phase and green represents polymer lean phase)[27]. Copyright 1990, Elsevier

For the preparation of nanofiltration membrane, the proportion of polymer components at the membrane/bath interface can be increased by increasing the solid content of polymer in the casting solution, so that the proportion of polymer lean phase formed by phase separation is reduced, the pore size of the membrane is reduced, and the porosity is reduced[29]; In addition, when the polymer concentration in the initial casting solution is further increased, the first micronucleus formed in the polymer-poor phase is gradually away from the membrane/bath interface, the delayed phase separation is more obvious, and the dense skin thickness of the final separation membrane is thicker[30,31]. In addition to increasing the solid content, increasing the hydrophilicity of the polymer (when water is used as a non-solvent coagulation bath, improving the hydrophilicity of the material can improve the tolerance of the system to the non-solvent) and selecting a good solvent for the polymer can enhance the thermodynamic stability of the casting solution.The binodal line is far away from the polymer-solvent axis, which is beneficial to the phase separation behavior into the Ⅰ and Ⅵ regions, and the obtained membrane is more likely to show a dense skin structure[32].

2.1.2 Mass transfer kinetics

In the process of phase inversion membrane formation, the thermodynamic behavior of the casting solution can not fully explain the mechanism of membrane structure change, and the influence of mass transfer kinetics on the membrane formation process should also be considered[23,33]. After the nascent liquid membrane is immersed in the coagulation bath (Fig. 3), when the flux J1 of the non-solvent into the casting solution is greater than the flux J2 of the solvent into the coagulation bath, that is, when the J2/J1 value is small, the resulting polymer membrane usually tends to form a thinner skin structure with higher porosity, and the lower part of the skin usually presents finger-like pores[34]; When the J2/J1 is higher, the polymer film has a denser and thicker skin structure, and the lower part of the skin is mostly sponge-like pores[35,36].
图3 NIPS相分离过程中溶剂-非溶剂交换过程[13]

Fig.3 Non-solvent/solvent exchange process in the NIPS process[13]. Copyright 2011, American Chemical Society

In the process of membrane formation, the viscosity of casting solution, hydrophilicity and hydrophobicity of materials, the composition of coagulation bath, the properties of additives in casting solution and the temperature of coagulation bath can affect the J1 and J2[13]. When the viscosity of the casting solution increases, the rheological resistance will inhibit the movement of polymer chains in the casting solution, and the diffusion flux J1 of the non-solvent into the casting solution will slow down[37]. Increasing the hydrophilicity of the membrane forming material will improve the affinity between the casting solution and the non-solvent (when water is used as the non-solvent), and then increase the flux J1 of the non-solvent into the casting solution[25]. In addition, in the adjustment of the coagulation bath, increasing the content of alcohol substances (such as methanol, ethanol, etc.) In the coagulation bath can improve the affinity between the coagulation bath and the solvent, and then promote the diffusion of the solvent into the bath (increase the J2)[38⇓~40]. The increase of coagulation bath temperature can increase the kinetic energy of non-solvent molecules, so it can further increase the diffusion rate of non-solvent into the casting solution, increase the porosity of the membrane and reduce the skin thickness[41,42]. In addition, the introduction of hydrophilic polymers such as polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG) into the casting solution can increase the affinity between the non-solvent (water) and the casting solution, and then increase the invasion rate of the non-solvent into the casting solution; On the other hand, the addition of hydrophilic polymer can increase the viscosity of the casting solution and affect the invasion rate of the non-solvent into the casting solution[19,24,26]. Taking PVP as an example, when the content of PVP is low (the viscosity of casting solution is not significantly increased), the invasion of non-solvent will be promoted, and the membrane structure will be looser. When the content of PVP is further increased, the high viscosity of the casting solution will inhibit the invasion of the non-solvent, promote the occurrence of delayed phase separation, and make the membrane structure more compact[19]. In addition, surfactants, such as sodium dodecyl sulfate (SDS), can promote the invasion of non-solvent into the casting solution by reducing the surface tension of the casting solution and the coagulation bath. Therefore, the membrane pore size increases with the increase of SDS in the coagulation bath[43].

2.2 Preparation of nanofiltration membrane materials by phase inversion method

At present, the traditional polymer materials used to prepare nanofiltration membranes include polysulfone, polyethersulfone, cellulose acetate and polyacrylonitrile. The characteristics of molecular chains of different materials, such as hydrophilicity and hydrophobicity, flexibility of molecular chains, types of functional groups carried and crystallinity, determine the thermodynamic properties of the casting solution system composed of them, which makes them show different phase separation behaviors in the process of membrane formation, and then show different membrane structures and separation properties.

2.2.1 Poly (ether) sulfones

Poly (ether) sulfone (PES) materials have been widely used in the preparation of porous membranes from microfiltration to nanofiltration due to their good film-forming properties, thermal stability, chemical resistance and high mechanical strength[8,21,25,29,44,45]. Compared with cellulose acetate and other hydrophilic materials, the hydrophobicity of poly (ether) sulfone materials makes the casting solution show poor thermodynamic stability and narrow miscibility gap[46]. Therefore, the casting solution prepared with poly (ether) sulfone materials often shows instantaneous phase separation characteristics, and the separation layer of the obtained membrane is easy to show larger pore size (d > 3 nm), while the supporting layer is most common in the finger-like pore structure[47,48][49,50]. In the preparation of nanofiltration membrane, in order to increase the compactness of polysulfone (PSf) membrane, Chung et al. increased the solid content of PSf in the casting solution, and found that when the solid content of PSf reached 29%, the compactness of PSf membrane was significantly improved, and the macroporous structure in the membrane disappeared (Fig. 4)[46]. However, increasing the membrane density by increasing the solid content often leads to a sharp decrease in membrane porosity and a significant thickening of the dense skin layer, and the water permeability of the membrane is often lower than that of the 5 L/(m2·h·bar)[51,52].
图4 PSf固含量(标记于图中左上角)对膜断面结构中大孔形成的影响[46]

Fig.4 Effects of PSf concentration (shown in the upper corner) on the formation of macro-voids[46]. Copyright 2008, Elsevier

In order to further optimize the membrane structure and improve the water permeability of the membrane on the premise of ensuring the rejection performance of the membrane, researchers introduced hydrophilic functional groups into the main chain or side chain of poly (ether) sulfone, such as sulfonic acid, amino, carboxyl or other oxygen-containing functional groups[53][54,55][56]. On the one hand, the material modification method can obviously improve the accommodation capacity of the membrane casting solution to the coagulation bath (water), improve the thermodynamic stability of the membrane casting solution, enhance the delayed phase separation, and is beneficial to reducing the pore size of the membrane obtained by phase conversion and forming a dense membrane structure[57]; On the other hand, the improvement of the hydrophilicity of the film-forming material can improve the affinity of the casting solution to the coagulation bath (water), and then promote the invasion rate of the non-solvent water into the casting solution, so as to promote the formation of a thin skin layer and a high porosity structure of the obtained film[58]. Therefore, the hydrophilic modification of materials plays a role of mutual promotion and restriction in the formation of membrane structure in thermodynamics and kinetics. In addition, the introduced hydrophilic group also provides more water permeable channels and improves the affinity between the membrane and water, thereby improving the water permeability coefficient of the membrane[59]. At present, many enterprises such as Toray, Osmonix, Minard and Nitto have prepared a number of commercial polyethersulfone nanofiltration membranes, such as NTR7450 and N30F[60,61]. These nanofiltration membranes are all made of sulfonated polyethersulfone materials as the rejection layer, showing MWCO below 1000 Da, showing excellent rejection performance for divalent salts and small molecular organics, and the water permeation flux of the membrane can reach more than 5 L/(m2·h·bar). Chung et al. Blended polyethersulfone and sulfonated polysulfone, and systematically explored the effect of the proportion of sulfonated polysulfone in the casting solution on the membrane structure and membrane performance. It was found that with the increase of the proportion of sulfonated polysulfone in the casting solution, the cross-sectional structure of the membrane changed from the original finger-like pore structure to the sponge-like pore structure (Fig. 5), and the membrane pore gradually decreased. Compared with the pure PES membrane (3. 1 nm), the pore size of PES/sulfonated PSF (mass ratio 9/1) membrane decreased to 1. 9 nm, and the pure water permeation flux of the membrane reached 15 L/(m2·h·bar)[47,62].
图5 不同PES/SPSf共混比的中空纤维膜的断面形态[47]

Fig.5 Cross-sectional morphology of hollow fiber membranes with different PES/SPSf blend ratios (in wt%)[47]. Copyright 2017, Elsevier

In addition, phenolphthalein polyethersulfone (CPES) is a derivative of polyethersulfone family materials, which improves the hydrophilicity of materials by introducing phenolphthalein side groups into the main chain of polyethersulphone[63]. Blanco et al. Found that the water permeability coefficient of CPES membrane was about 12 times higher than that of polysulfone membrane, reaching the 167 L/(m2·h·bar). At the same time, the sulfonation of CPES materials can improve the rejection of CaCl2 from 4% to 64%, and the water permeability coefficient is 6 L/(m2·h·bar)[25]. On the basis of CPES materials, Zhang et al. Prepared a new polyethersulfone material (PES-TA) containing tertiary amine groups, and the hydrophilicity of the material was further improved[64,65]. The MWCO of the membrane is 1875 Da, the cross section of the membrane shows abundant finger-like pore structure, the rejection rate of MgCl2 is 63%, and the water permeability coefficient is 8 L/(m2·h·bar)[65].
So far, the water permeability coefficient of the prepared nanofiltration membrane can be improved to a certain extent by introducing hydrophilic groups into the molecular chain of poly (ether) sulfone materials. However, due to the small molecular size of the hydrophilic group, on the one hand, it is difficult to form more penetrating pores in the separation layer of the membrane; On the other hand, in the nanofiltration membrane, the polymer molecular chain is packed very compactly, and it is difficult to effectively support the molecular chain to form a permeable channel only through small-sized hydrophilic groups. Therefore, the improvement of the water permeation flux of the membrane by the material is very limited, and the water permeability coefficient of the nanofiltration membrane prepared by the modified material is still difficult to exceed the 15 L/(m2·h·bar) at present. In addition, the modification methods of these materials often involve multi-step reactions, and the reaction conditions are harsh, so it is difficult to achieve large-scale preparation[66]. Therefore, how to develop a membrane-forming material with simple synthesis process and more effective improvement of membrane porosity is the key to further improve the permeability coefficient of poly (ether) sulfone nanofiltration membrane.

2.2.2 Cellulose acetate

Cellulose acetate (CA) is another common membrane material, which has great advantages in the preparation of nanofiltration membranes and reverse osmosis membranes because of its cheapness and hydrophilicity[67,68]. In the process of phase inversion, the hydrophilic nature of CA molecules, on the one hand, makes the casting solution have a high capacity to accommodate non-solvent water. Compared with hydrophobic materials such as polyvinylidene fluoride (PVDF) and PSf, it has a higher thermodynamic stability and miscibility gap (Figure 6), and the pore size of the membrane is smaller[46]. On the other hand, the hydrophilicity of its molecules also increases the diffusion rate of non-solvent water into the casting solution, which can improve the porosity of the membrane to a certain extent[69].
图6 PSf、P84及CA材料所成铸膜液的三元相图,CA材料所成铸膜液的混溶间隙最大、热力学稳定性最强,P84材料次之,PSf最弱[46]

Fig.6 The phase diagrams of the PSf, P84, and CA polymer systems. The miscibility gap and thermodynamic stability of casting solution made of CA are the largest, followed by P84 and PSf the weakest[46]. Copyright 2008, Elsevier

According to the degree of substitution of hydroxyl groups by acetyl groups in CA, it can be divided into cellulose diacetate (CDA, acetyl content 37% ~ 40%) and cellulose triacetate (CTA, acetyl content more than 60%)[69]. Although the increase of acetylation degree can improve the heat resistance and acid and alkali resistance of CA, the hydrophilicity of the material decreases, resulting in the decrease of water permeability coefficient of CA membrane with the increase of acetylation degree, and the pore size of the membrane increases[70,71].
In 1960, Loeb and Sourirajan of the University of California used CDA as a material to prepare an asymmetric separation membrane with a NaCl rejection rate as high as 94.4% by NIPS method, but the water permeability coefficient of the membrane was only 0.7 L/(m2·h·bar)[73]. Subsequently, in order to adjust the structure of CDA membrane and improve the membrane flux, Haddada et al. Explored the effects of polymer solid content and annealing time on the structure and performance of the membrane. It was found that when the solid content of CDA was 20% and the thermal annealing temperature was 70 ℃, the cross-sectional structure of the membrane showed a dense separation layer with a thickness of 1 ~ 2 μm, and the lower part of the separation layer was composed of sponge-like pores with a pore size of about 1 μm. The pure water permeability coefficient of the membrane is 3 L/(m2·h·bar), and the rejection of NaCl can reach 50%[74]. Chung et al. Prepared CA hollow fiber nanofiltration membrane by dry-wet spinning method, and the membrane section showed a gradually loose structure from top to bottom (Fig. 7). The pore radius of the membrane can be adjusted from 0. 6 nm to 0. 3 nm by increasing the annealing temperature to promote the movement of CA molecular chains around the pore wall and realize the rearrangement of molecular chains[72]. Although CA materials can be used to prepare nanofiltration membranes, their use is still limited due to their poor thermal stability (used below 30 ℃), low mechanical strength and weak acid and alkali resistance (only suitable for pH 4 ~ 6 of feed solution)[75][70,76⇓⇓⇓⇓ ~81].
图7 (a)不同热处理温度下所成CA膜的外皮层形貌(CA-1# 未处理,CA-2# 60℃处理,CA-3# 90℃处理),(b)CA-3#膜断面不同区域的形貌[72]

Fig.7 (a) Outer skin layer morphology of the CA membranes at different heat-treatment temperatures: CA-#1(untreated), CA-#2 (60 ℃) and CA-#3 (90 ℃), (b) cross-section, inner surface and outer surface morphology of the CA-#3 membrane[72]. Copyright 2010, Elsevier

2.2.3 Other materials

In addition to poly (ether) sulfone and cellulose acetate materials, linear polyamide and polyacrylonitrile materials are also used to prepare nanofiltration membranes by phase inversion method[82⇓~84][14,85]. Among them, linear polyamide materials not only have excellent acid and alkali resistance, but also have good heat resistance and easy processability. Because its molecular chain contains a large number of amide groups and there is hydrogen bonding force between molecular chains, it can inhibit the formation of macropores in the process of phase inversion and is easy to prepare nanofiltration membranes. The material is represented by poly (m-phenylene isophthalamide) (PMIA), polyimide (PI) and nylon 6 (PA6), and can realize the preparation of a nanofiltration membrane with MWCO between 1000 and 1800 Da, and the water permeability coefficient of the membrane can reach 5~20 L/(m2·h·bar)[82⇓~84][39,86][87]. In addition, the membrane formed by the material, such as PI membrane, has imide active crosslinking sites, and the membrane can be post-crosslinked by chemical crosslinking or thermal crosslinking to further adjust the membrane structure and pore size[88,89].
Table 1 summarizes the polymer materials used to prepare nanofiltration membranes and their membrane properties reported in the current literature. It can be seen from the table that the molecular structure of these polymer materials is mostly modified by hydrophilic modification such as sulfonation, hydrophilic material grafting and hydrophilic segment copolymerization. The nanofiltration membrane prepared by the method has good rejection capacity for divalent salts and small molecular organics, but the water permeability coefficient of the membrane is still generally low, and the 20 L/(m2·h·bar) is difficult to break through. The reason for the low water permeability of the membrane is that when the NIPS method is used to prepare the nanofiltration membrane, it is still necessary to increase the solid content and viscosity of the casting solution to prepare the membrane with a thick and dense skin structure, thereby reducing the effective pore size of the membrane, and the porosity of the membrane is relatively low[90⇓~92]. Therefore, the key to improve the water permeability of the nanofiltration membrane prepared by NIPS method is to optimize the structure of the nanofiltration membrane prepared by phase inversion, improve the porosity of the membrane and reduce the skin thickness.
表1 相转化法所制纳滤膜的材质及分离性能参数指标

Table 1 Main information (material, MWCO/pore size, water permeance and rejection) of NF membranes prepared through a phase inversion process

Casting material MWCO/
pore size		 Permeance
(L/(m2·h·bar)		 Solute rejection ref
Polyamic acid 800 Da 4 R(Na2SO4) = 94% 93
Sulfonated poly(phthalazinone biphenyl ether sulfone) - 7 R(Na2SO4) = 84% 94
Polyetherimide - 7 R(Na2SO4) = 76% 95
Sulfonated nitro-polyphenylsulfone - 8 R(Na2SO4) = 65% 57
Polyacrylonitrile-co-methylacrylate - 15 R(Na2SO4) = 30% 96
Polyacrylonitrile-graft-poly(ethylene oxide) 1 nm 9 R(Congo Red) = 100%
R(Ethyl Orange) = 81%		 97
Poly(trifluoroethyl methacrylate)-r-
sulfobetaine methacrylate		 1 nm 8 R(Brilliant Blue R) = 100% 98
Carboxylated cardo poly(arylene ether ketone)s 4 nm 30 R(Congo red) = 100% 59
Poly(m-phenylene isophthalamide) - 20 R(Alizarine red) = 98% 84

2.3 Optimization of Nanofiltration Membrane Structure and Separation Performance

As mentioned above, the main reason for the low water permeability of phase inversion nanofiltration membrane is the high concentration and viscosity of the casting solution in the membrane preparation process, and the thick skin and low porosity of the membrane prepared by delayed phase separation. Inorganic salts, hydrophilic nanoparticles or hydrophilic polymers are blended with common membrane forming materials such as polysulfone, polyethersulfone and the like to effectively improve the porosity of the membrane formed by phase conversion, thereby improving the water permeability coefficient of the formed membrane; The surface of the ultrafiltration membrane is crosslinked by using the chemical characteristics of the membrane forming material, which can effectively improve the compactness of the separation layer and avoid the separation layer being too thick, thereby improving the water permeability coefficient of the nanofiltration membrane[99,100].

2.3.1 Blending method

Blending is a common method used to enhance the porosity of the membrane and to adjust the membrane structure[101]. In recent years,Common additives include hydrophilic polymers (such as polyethylene glycol, polyvinylpyrrolidone, and polyethyleneimine), inorganic particles (such as titanium dioxide, silicon dioxide, zinc chloride, and lithium chloride),Nanomaterials (such as carbon dots, carbon nanotubes, graphene oxide and halloysite nanotubes) and porous nanoparticles (such as covalent organic framework materials, metal-organic framework materials)[21][102][26][50][103,104][21][84][105][106][107⇓~109][110,111][112][113]. Different additives have slightly different mechanisms for increasing porosity. For hydrophilic polymer materials, in the process of film formation, a part of the lost hydrophilic polymer is lost with the precipitation of the solvent, and the space originally occupied by the lost hydrophilic polymer forms film pores after phase inversion, thereby improving the porosity of the film; On the other hand, the hydrophilic polymer that is not lost in the membrane matrix can increase the hydrophilicity of the membrane, which in turn increases the water permeability of the membrane. For inorganic particles, nanomaterials and porous nanoparticles, when they are added to the casting solution, on the one hand, the invasion rate of non-solvent into the membrane can be increased, and then the porosity can be increased; On the other hand, due to the incompatibility of the additive and the polymer material, the nanoscale gap between the additive and the polymer material further increases the porosity of the formed film. In addition, the porosity of the porous nanoparticles themselves also helps to improve the porosity of the membrane. At present, the water permeability coefficient of the nanofiltration membrane formed by phase inversion can be improved to the 40L/(m2·h·bar) by blending modification[50,52,107]. However, on the one hand, the compatibility between additives and polymers is poor, which makes it difficult to further improve the porosity due to the limited amount of additives. On the other hand, the self-agglomeration problem causes defects in the membrane, which increases the pore size of the membrane, and the pore size of the membrane is often greater than 3 nm[48,107,110].

2.3.2 Post-processing method

Although the blending modification can effectively improve the separation performance of the membrane, especially in the preparation of decolorized nanofiltration membrane with membrane pore size greater than 2 nm, the effect of membrane water flux improvement is significant. However, the water flux enhancement effect of blending modification for salt rejection nanofiltration membrane with membrane pore size less than 2 nm is not ideal (water permeability coefficient is difficult to break through the 10 L/(m2·h·bar)). This is attributed to the fact that the blending modification not only improves the density of the skin layer of the phase inversion nanofiltration membrane, but also increases the thickness of the skin layer (> 300 nm), resulting in low water flux and other problems[114]. Therefore, on the basis of the ultrafiltration membrane or the loose nanofiltration membrane formed by the phase inversion method, by utilizing the chemical characteristics of the membrane-forming material (for example, the functional group carried by the membrane-forming material can participate in chemical crosslinking), and by further shrinking the cavity after crosslinking on the membrane surface, the retention capacity of the membrane can be improved, and the increase of the thickness of the membrane separation layer can be greatly reduced[115] [116].
Akbari et al. Used PSf ultrafiltration membrane as the base membrane, N, N-dimethylacrylamide as the crosslinking agent, acrylic acid as the monomer, and used the photosensitivity of PSf material (sulfone group is broken under ultraviolet light and free radicals are generated) to generate polyacrylic acid on the surface of PSf membrane under ultraviolet light induction, thus effectively reducing the pore size of the membrane[117][118]. Compared with the untreated membrane, the Na2SO4 rejection of the modified membrane was increased from 16% to 85%, and the water permeability of the membrane could be maintained at a 10 L/(m2·h·bar). Compared with the nanofiltration membrane obtained by PSf phase conversion, the water flux of the nanofiltration membrane is improved by about 3.5 times[21]. In addition, Zhang et al. Carboxylated polyaryletherketone to prepare membrane materials with abundant electronegative carboxyl functional groups (PAEK-COOH) in the side chain[119]. The material was then used to prepare a membrane, and positively charged polyethyleneimine (PEI) was introduced into the coagulation bath. During the phase inversion process, the positively charged PEI and negatively charged PAEK-COOH attracted each other due to electrostatic interaction, forming a thin and dense polyelectrolyte layer on the membrane surface (the thickness of the separation layer could be reduced to 151 nm) (Fig. 8). The rejection rate of the membrane for MgCl2 is improved from 40% to 90%, and the water permeability coefficient can be kept at a 7 L/(m2·h·bar). Lin et al. Used the blend of PMIA and PEI as the casting solution and prepared the membrane by NIPS, and then used p-dichlorobenzyl (XDC) for surface crosslinking treatment. The rejection rate of the prepared membrane for MgCl2 was 86%, and the pure water permeability coefficient could reach 14 L/(m2·h·bar)[120].
图8 通过静电吸引诱导相转化过程形成PAEK-COH-PEI荷正电纳滤膜[119]

Fig.8 Formation of the PAEK-COOH-PEI nanofiltration membrane with positive charge via the electrostatic interaction during the phase inversion process[119]. Copyright 2017, Elsevier

Although the post-treatment method can inhibit the increase of skin thickness as much as possible when improving the density of the separation layer of the salt-intercepting nanofiltration membrane, so as to obtain a higher water permeability coefficient. However, the water permeability coefficient of the membrane prepared by this method is still difficult to break through the 15 L/(m2·h·bar). We speculate that this may be due to the dense and low porosity of the basement membrane cortex.

3 Amphiphilic block copolymer and nanofiltration membrane thereof

It can be seen from the previous discussion that the water permeability coefficient of nanofiltration membranes made by NIPS is generally low when the polymer materials with a single chain segment are used as the membrane forming materials, such as poly (ether) sulfone or cellulose acetate, regardless of the hydrophilic modification of the membrane forming materials or the addition of porogens in the membrane forming process. Therefore, in order to further improve the water permeability of phase conversion nanofiltration membrane, it is necessary to finely regulate the microscopic pore structure of the membrane formed by phase conversion, such as increasing porosity, optimizing pore size distribution, and finely regulating pore size and chemical properties. In recent years, researchers have started from designing and optimizing the physical structure and chemical properties of membrane materials, using amphiphilic block copolymer materials as membrane materials, finely regulating the microstructure and chemical properties of membrane pores, and preparing high-flux nanofiltration membranes for precise separation[121,122].
The amphiphilic block copolymer is a kind of polymer composed of thermodynamically incompatible hydrophilic segment A and hydrophobic segment B and linked by covalent bonds, and its molecular structure is shown in Figure 9[123]. Due to the differences in physicochemical properties of different segments in the molecular chain, amphiphilic block copolymers can undergo phase separation under suitable thermodynamic conditions, but the segments are linked by covalent bonds, and the size of phase separation is usually at the molecular scale, which is called microphase separation process[124]. In addition to microphase separation, the pore size and porosity of the membrane, especially the hydrophilicity, charge, reaction function activity, etc., can also be flexibly regulated by regulating the molecular structure and chemical properties of the amphiphilic block copolymer[125].
图9 A-B型两亲性嵌段共聚物的结构示意图[123]

Fig.9 Schematic of A-B amphiphilic block copolymers[123]. Copyright 2012, The American Association for the Advancement of Science

3.1 Amphiphilic block copolymer membrane material and membrane preparation characteristic

A and B segments with different chemical structures and properties in amphiphilic block copolymers can be enthalpically driven to undergo phase separation, and the magnitude of this driving force is affected by the Flory-Huggins interaction parameter χAB between A and B segments, which is defined as:
χ A B = ε A B - 1 2 ( ε A A + ε B B ) k T
The total driving force between A and B segments is χABN, where N is the degree of polymerization of the block copolymer, εAB, εAA, and εBB are the interaction forces between each structural unit A and B, A and A, and B and B, respectively, and kT is the thermal energy. When χABN is greater than the critical interaction force (χABN)C between the two segments, microphase separation occurs between the two segments; However, when χABN is less than the critical interaction force (χABN)C between the two segments, the two segments are miscible and form a homogeneous system with two segments A and B[126].
Compared with traditional film-forming polymer materials, the microphase separation of amphiphilic block copolymer has the advantage that the distribution morphology of different components in the polymer can be finely adjusted at the nanometer scale[126]. Taking diblock polymers as an example, many studies have shown that the final block component distribution of block copolymers is affected by three parameters[126⇓~128]. One is the volume fraction f of the two segments, which determines the morphology of the microphase separation. The second is the interaction parameter χABN, which determines the strength of microphase separation. The third is the total degree of polymerization N of the block polymer. As shown in Fig. 10, with the change of the volume fraction f of block A and the interaction parameter χABN between the two segments, the microphase separation process can realize the construction of ordered structures such as spherical phase, columnar phase, cubic bicontinuous phase and lamellar phase, respectively[129,130]. According to the phase diagram obtained from the theoretical simulation and the experimental process, when the different segments in the diblock copolymer have the same volume fraction and the interaction force between the two segments is χABN<10.5[126].
图10 (a)AB二嵌段共聚物微相分离所成形态,(b)由自洽场理论预测的AB嵌段共聚物的理论相图,(c)聚异戊二烯-聚苯乙烯嵌段共聚物的实验相图[130]

Fig.10 (a) Equilibrium morphologies of AB diblock copolymers in bulk, (b) theoretical phase diagram of AB diblocks predicted by the self-consistent mean-field theory; (c) experimental phase diagram of polyisoprene-block-polystyrene copolymers[130]. Copyright 2012, Royal Society of Chemistry

During the microphase separation of the block copolymer, the hydrophobic segments aggregate to form the main body of the membrane, providing mechanical strength; While the hydrophilic segment constitutes the membrane pore. Compared with the blending method, the mass ratio of the hydrophilic segment can be increased to 20 wt% due to the chemical bonding between the hydrophilic segment and the hydrophobic segment[31,131]; Compared with the group modification method, the molecular size and morphology of the hydrophilic segment are easier to adjust. For this reason, the block copolymer can achieve a greater increase in membrane porosity.
Another feature of amphiphilic block copolymers is the diversity of molecular structure as well as material chemistry[132]. In addition to linear block copolymers, linear-comb and linear-hyperbranched block copolymers have also attracted more and more attention[133][134,135]. The diversity of material structure not only makes the aggregation structure of block copolymer more abundant[123,128]. At the same time, the existence of linear-comb and linear-hyperbranched structures also provides a basis for the multifunctional nature of materials, which can realize the post-functionalization of materials and the flexible regulation of chemical properties[136,137][138⇓~140].
Compared with traditional polymer materials, the microphase separation and the diversity of molecular structure and chemical properties of amphiphilic block copolymers in the process of phase inversion membrane preparation can endow the membrane with unique structural and chemical properties, such as increasing the porosity of the membrane, adjusting the size of the membrane, optimizing the pore size distribution of the membrane and improving the anti-fouling property of the membranes (Fig. 11)[141⇓⇓⇓⇓~146]. In addition, the existence of the hydrophilic segment in the amphiphilic block copolymer promotes the invasion of the non-solvent into the membrane during the phase inversion process, thereby reducing the thickness of the skin layer of the formed membrane and increasing the porosity[31]. In this paper, we focus on the effect of amphiphilic block copolymer on pore size control, porosity and post-functionalization in the preparation of nanofiltration membranes.
图11 两亲性嵌段共聚物与传统高分子材料通过相转化法成膜的结构差异[122]

Fig.11 The difference of membrane structure between block copolymers and traditional materials by phase inversion[122]. Copyright 2020, American Chemical Society

Hu et al. Synthesized a polysulfone block copolymer membrane material PSf-b-PEG by stepwise polymerization, and prepared membranes with PSf-b-PEG, PSf, and a mixture of PSf and PEG, respectively (Fig. 12). It was found that the overall porosity and surface porosity of the membrane formed by the PSf-b-PEG block copolymer could reach 88% and 15%, respectively.The porosity of PSf-b-PEG block copolymer is much higher than that of polysulfone (69%, 6%) and PSf-PEG blend membrane (82%, 9%), which shows the significant advantages of PSf-b-PEG block copolymer in the preparation of high porosity separation membrane[31]. In addition, the separation layer thickness of the PSf-b-PEG block copolymer membrane is 167 nm, which is also much lower than that of the other two membranes (430 nm for PSf membrane and 270 nm for blend membrane), mainly due to the low viscosity of the PSf-b-PEG casting solution and the fast non-solvent-solvent diffusion rate during the phase inversion process. The high porosity and thin skin of the PSf-b-PEG block copolymer film make it have great advantages in improving the water permeability of the film.
图12 PSf-b-PEG嵌段共聚物、PSf及PSf与PEG的混合物所成膜的孔隙率及分离层厚度[31]

Fig.12 Porosity and separation layer thickness of the membrane formed by PSf-b-PEG, PSf and PSf/PEG mixture[31]. Copyright 2020, Elsevier

3.2 Research progress of block copolymer nanofiltration membrane

The research on block copolymer materials and their microphase separation behavior has been relatively mature, and they can significantly improve the porosity of membranes. However, its research and application in the field of separation membrane is still in its infancy[147]. At present, the pore size of most block copolymer membranes prepared by phase inversion method is between 5 nm and 100 nm, and the preparation of nanofiltration membranes using block copolymer materials has only been widely concerned by scholars in the past two years[148][30,121,149]. Although it is difficult to prepare nanofiltration membranes with pore size less than 5 nm by one-step phase inversion method, block copolymers are easier to modify than traditional materials due to their hydrophilic segment structure and rich functional groups. Therefore, by introducing complexing agents into the casting solution or post-treatment of the membrane, the preparation of separation membranes with pore sizes below 5 nm or even smaller has been realized, and the efficient screening of small organic molecules and inorganic salts in water has been realized[150][131].
At present, the common block copolymer materials used to prepare nanofiltration membranes mainly include PSf block copolymer and polystyrene (PS) block copolymer[124,151]. This section will summarize the research progress of domestic and foreign scholars in the preparation of nanofiltration membranes using the above two types of materials, focusing on the idea of polymer structure design at the molecular scale.The influence of molecular structure on the membrane structure was explored, and the influence of post-treatment on the pore size and chemical properties of the membrane and the separation performance of the nanofiltration membrane were verified[130,152,153]. Finally, we will further analyze the shortcomings of this kind of materials and the direction of improvement.

3.2.1 PS-based block copolymer

PS-based block copolymer is one of the most commonly used materials for the preparation of separation membranes by phase inversion method. There are many kinds of materials, including polystyrene-poly (4-vinylpyridine) (PS-b-P4VP), polystyrene-poly (2-vinylpyridine) (PS-b-P2VP), polystyrene-polyisoprene (PS -b-PI) and polystyrene-polyethylene oxide (PS -b-PEO). Due to the strong repulsion between PS segment and hydrophilic segment, the microphase separation process of PS-based block copolymer is obvious. At the same time, because of the living polymerization, the molecular chain structure of PS-based block copolymer is regular and the molecular weight distribution is narrow (PDI < 1.2), so the membrane is mostly uniform and has a highly uniform pore structure. However, most of the current reports on this type of membrane belong to the category of ultrafiltration, and the membrane pores are large. In order to realize the preparation of nanofiltration membrane, the membrane pore can be reduced to the nanofiltration range through the post-treatment of swelling, complexation and grafting by means of the multi-active sites and easy modification of block copolymer.
Yu et al. Used PS-b-P4VP as the membrane preparation material, first prepared the membrane by phase inversion, then soaked the membrane in a solution containing HAuCl4·3H2O, and reduced the pore size of the formed membrane from 20 nm to 3 nm by complexation of Au3+ and P4VP chain segments on the pore wall of the membrane (Fig. 13)[152]. Due to the ultrahigh porosity of the membrane and the thin skin layer thickness (about 100 nm), the water permeability coefficient of the PS-b-P4VP nanofiltration membrane can reach 100 L/(m2·h·bar), which can be several times higher than that of the nanofiltration membrane made of traditional materials.
图13 通过Au3+和P4VP链段的络合作用,制备孔径可调的PS-b-P4VP嵌段共聚物膜:(a)Au3+和单一胶束的络合作用,(b)经Au3+络合还原后膜孔径的变化[152]

Fig.13 The preparation of a PS-b-P4VP membrane with controlled pore sizes through electroless gold deposition: (a) gold decoration on a single micelle of PS-b-P4VP and (b) pore evolution of the PS-b-P4VP membrane with gold deposition[152]. Copyright 2014, Wiley

In addition, Zhang further designed the molecular structure of PS-based materials and prepared a polystyrene (PS) -based block copolymer (PS-b-P (HTMB-r-I)) with abundant hydroxyl functional groups by living polymerization. The material was first used to prepare the membrane, and then the membrane was post-treated with 1,3-propane sultone, during which the hydroxyl functional groups on the pore wall of the PS-b-P (HTMB-r-I) block copolymer membrane were converted to sulfonic acid groups (fig. 14). The post-treatment process can not only reduce the pore size of the membrane, but also increase the electronegativity and hydrophilicity of the membrane. The membrane can achieve efficient sieving of organic dyes with molecular size of 1 ~ 2 nm while maintaining a water permeability coefficient of 60 L/(m2·h·bar)[154].
图14 PS-b-P(HTMB-r-I)嵌段共聚物原始膜I0(a)及磺化膜SM(b)的示意图、聚合物化学结构及SEM图片,(c)原始膜I0和磺化膜SM水渗透性能的比较,(d)原始膜I0和磺化膜SM对酸性橙Ⅱ染料和活性绿19染料的分离性能[154]

Fig.14 Schematic representation, chemical structure and SEM images of (a) the PS-b-P(HTMB-r-I) membrane I0, (b) the sulfonated membrane SM. (c) Comparison of water permeance of the pristine membrane I0 and the sulfonated membrane SM under trans-membrane pressure of 1 bar. (d) The separation behavior of small organic molecules (i.e. orange II and reactive green 19) using the membranes I0 and SM[154]. Copyright 2020, Royal Society of Chemistry

Although PS-based materials have significant advantages in the preparation of separation membranes with uniform pore structure, their synthesis conditions are harsh and the cost is high, which makes it difficult to achieve large-scale preparation. In addition, the strength and toughness of this kind of material are poor, and the film formed is still difficult to be applied in practice.

3.2.2 PS f based block copolymer

PSf is a common film-forming material, which has excellent acid and alkali resistance, chlorine resistance, thermal stability and high mechanical strength, and has significant film-forming advantages compared with PS[155]. The hydrophilic polymer and the PSf are linked by covalent bonds to prepare the PSf based block copolymer, which can endow the block material with more excellent performance. At present, the research of PSf-based block copolymers is mainly focused on PSf-b-PEG materials[156⇓~158]. In the process of membrane formation, the introduction of hydrophilic segments can not only induce microphase separation and increase the porosity of the membrane, but also promote the exchange rate of solvent-nonsolvent, thus reducing the thickness of the separation layer[30,159][31].
In order to explore the permeability and selectivity of the nanofiltration membrane made of this kind of block copolymer, Hu et al. Used PSf-b-PEG block copolymer as the membrane-forming material, and took the lead in preparing the nanofiltration membrane for dye/inorganic salt screening by using polysulfone block copolymer material by controlling the solvent ratio in the membrane casting solution and the air bath time in the membrane preparation process (Fig. 15)[30]. The results showed that the surface of PSf-b-PEG nanofiltration membrane was compact and smooth, while the cross section of the membrane showed a spongy pore structure which was gradually loose from top to bottom. The effective pore size of the membrane was 3. 8 nm, with a very narrow pore size distribution, and the water permeability coefficient was as high as 49L/(m2·h·bar). However, the PSf membrane obtained under the same preparation conditions did not show water permeability. This result confirms that PSf-b-PEG can obtain higher porosity and water permeability in the preparation of nanofiltration membrane compared with PSf material. In terms of separation performance, the membrane can achieve efficient screening for dyes and inorganic salts (the dye rejection rate is as high as 98%, and the permeability of inorganic salts is close to 100%), and the PSf-b-PEG membrane always maintains stable rejection capacity and excellent acid and alkali resistance in the pH range of 3 to 11.
图15 (a) PSf-b-PEG膜对不同分子量的PEG截留率曲线,(b) PSf-b-PEG膜的孔径分布曲线,(c) PSf-b-PEG膜的渗透性和CR/盐截留率[30]

Fig.15 (a) The different molecular weight PEG rejection profile, (b) the pore size distribution and (c) the water permeance and the CR / salts rejections of the PSf-b-PEG membrane[30]. Copyright 2021, Elsevier

In addition to PSf-b-PEG materials, the development of PSf-based block materials with active sites, complexation or crosslinking after the end of phase inversion, can further adjust the membrane pore and maintain a high flux[150]. For example, Xie et al. Prepared polysulfone-poly (tert-butyl acrylate) (PSf-b-PtBA) block copolymer by reversible addition-fragmentation chain transfer polymerization (RAFT), which contains abundant tert-butyl functional groups. The pore size of the membrane can be reduced from 50 nm to 2 nm by using the material to prepare the membrane and modifying the membrane by hydrochloric acid solution, Cu2+ and Fe3+ ions, and the pure water permeability coefficient of the membrane can be maintained above 100 L/(m2·h·bar)[160].
As mentioned above, through the optimization of the molecular structure of the polysulfone block copolymer material, the permeability coefficient of the nanofiltration membrane has been improved by several times or even ten times. However, because the synthesis of this kind of material involves stepwise polymerization, the molecular weight distribution of the obtained material is wide (PDI ≈ 2), so the monodispersity of the domain size of each chain segment after phase separation is poor, and it is difficult to prepare a homogeneous membrane[125]. In addition, the structure of the hydrophilic segment in the PSf-based block copolymer is still relatively single (mainly the linear PEG segment), and it is still difficult to further optimize the structure of the obtained membrane, especially to adjust the chemical properties.

4 Conclusion and prospect

As we mentioned above, because the water permeability coefficient of nanofiltration membranes made of poly (ether) sulfone and cellulose acetate materials is low (generally <5 L/(m2·h·bar)), the water permeability coefficient of the membrane can be improved to a certain extent by hydrophilic modification (such as sulfonation, carboxylation or amination), blending and post-treatment of the materials, but the degree of improvement is still unsatisfactory. In addition, in terms of membrane separation selectivity, because the application of nanofiltration membrane often involves the separation of monovalent/multivalent salts and inorganic salts/small organic molecules, it is very important for nanofiltration membrane to accurately control the membrane pore size and obtain a highly uniform pore size distribution in the membrane formation process. However, in the process of membrane formation by phase inversion method, the pore size of the membrane obtained by liquid-liquid phase separation is not easy to control, and the distribution is wide. The low water permeability of the membrane and the difficulty in adjusting the pore size and chemical properties of the membrane are all due to the single structure of the current membrane-forming materials, which makes it difficult to achieve fine control of the phase transformation process and membrane structure. Therefore, in order to fundamentally improve the performance of nanofiltration membranes produced by phase inversion, it is particularly critical to develop new high-performance membrane materials.
Block copolymer is a new type of membrane forming material, which has significant advantages in the preparation of nanofiltration membranes with adjustable pore size, high porosity and uniform pore size distribution. However, at present, the block copolymer materials used in the preparation of separation membranes are mainly divided into polysulfone-based and polystyrene-based block copolymers. There are many kinds of polystyrene-based block copolymers (including PS-b-P4VP, PS-b-P2VP, PS-b-PI and various derivatives), but all of them are synthesized by living polymerization, the synthesis process is complex, the synthesis conditions are harsh, and the material price is expensive. At the same time, the toughness of polystyrene-based materials is poor, and the tensile and compressive capacity of the film is insufficient, which greatly limits the large-scale preparation and application of polystyrene-based block copolymer films. In contrast, polysulfone block copolymer materials have excellent rigidity, toughness, thermal stability, chemical corrosion resistance, low toxicity, easy processability and dimensional stability due to the non-crystalline macromolecules containing sulfone groups and arylene groups in the molecular chain. Meanwhile, the synthesis of the polysulfone block copolymer material is mainly based on stepwise polymerization, the required conditions of the reaction are relatively mild, the reaction process is easy to control, the large-scale preparation is easy to realize, and the material is cheap and easy to obtain. However, at present, the type of polysulfone block copolymer is single, and the most common is PSf-b-PEG block copolymer. Although the membrane structure and pore size can be controlled by adjusting the length of PSf and PEG segments, these materials are still limited to linear structure, and their ability to adjust the packing density of polysulfone molecular chains and improve the porosity of membranes is limited. At the same time, the existing materials lack modifiable active sites (such as amino, carboxyl, hydroxyl, etc.), which makes it difficult for the film formed by this kind of materials to further carry out fine regulation of membrane surface chemical properties (such as membrane surface hydrophilic and hydrophobic regulation, charge regulation, etc.). Therefore, the author believes that in the future material synthesis, it is easier to promote the commercialization process of block copolymer nanofiltration membrane by simplifying the synthesis process, giving priority to the stepwise polymerization process, and developing low-cost polysulfone block copolymer. In addition, in the design of materials, materials with three-dimensional structure as hydrophilic segments (such as polyglycidyl, polyethyleneimine, cyclodextrin or glucoside) are more likely to promote the occurrence of microphase separation process.In the field of nanofiltration membrane, it is easier to adjust the packing density and molecular chain gap of hydrophobic chain segments (such as polysulfone chains), to adjust the nano-pore size, to increase the water permeability coefficient of the formed membrane, and to improve the separation performance of the membrane.

References

Publishing order | Descend order by publishing year | Descend order by cited within
[1]
Ahmad N N R, Ang W L, Teow Y H, Mohammad A W, Hilal N. J. Water Process. Eng., 2022, 45: 102478.

[2]
Zhang W, Zhang X. J. Membr. Sci., 2021, 632: 119358.

[3]
Oatley-Radcliffe D L, Walters M, Ainscough T J, Williams P M, Mohammad A W, Hilal N. J. Water Process. Eng., 2017, 19: 164.

[4]
Yao Z K, Guo H, Yang Z, Qing W H, Tang C Y. Desalination, 2018, 445: 115.

[5]
Nghiem L D, Hawkes S. Desalination, 2009, 236(1/3): 273.

[6]
Gong G H, Wang P, Zhou Z Y, Hu Y X. ACS Appl. Mater. Interfaces, 2019, 11(7): 7349.

[7]
Li S L, Shan X Y, Zhao Y F, Hu Y X. ACS Appl. Mater. Interfaces, 2019, 11(45): 42846.

[8]
He M B, Li T, Hu M L, Chen C, Liu B C, Crittenden J, Chu L Y, Ng H Y. Sep. Purif. Technol., 2020, 230: 115855.

[9]
Wang J, Zhang S, Wu P F, Shi W X, Wang Z, Hu Y X. ACS Appl. Mater. Interfaces, 2019, 11(12): 12043.

[10]
Strathmann H, Kock K. Desalination, 1977, 21(3): 241.

[11]
Li P, Xie H X, Bi Y F, Miao C C, Chen K, Xie T T, Zhao S C, Sun H X, Yang X J, Hou Y F, Jason Niu Q. Appl. Surf. Sci., 2023, 618: 156650.

[12]
Wang Z Y, Feng R, Wang W J, Sun Y X, Tao S N, Wang Y M, Chen Y F, Fu Z J, Cao X L, Sun S P, Xing W H. Adv. Membr., 2021, 1: 100007.

[13]
Guillen G R, Pan Y J, Li M H, Hoek E M V. Ind. Eng. Chem. Res., 2011, 50(7): 3798.

[14]
Tan L, Gong L, Wang S Y, Zhu Y Z, Zhang F, Zhang Y T, Jin J. ACS Appl. Mater. Interfaces, 2020, 12(34): 38778.

[15]
Han G, Feng Y N, Chung T S, Weber M, Maletzko C. Environ. Sci. Technol., 2017, 51(24): 14254.

[16]
Peng N, Widjojo N, Sukitpaneenit P, Teoh M M, Lipscomb G G, Chung T S, Lai J Y. Prog. Polym. Sci., 2012, 37(10): 1401.

[17]
Gao J, Wang K Y, Chung T S. J. Membr. Sci., 2020, 603: 118022.

[18]
Cervellere M R, Qian X H, Ford D M, Carbrello C, Giglia S, Millett P C. J. Membr. Sci., 2021, 619: 118779.

[19]
Sadrzadeh M, Bhattacharjee S. J. Membr. Sci., 2013, 441: 31.

[20]
Li H B, Shi W Y, Zhang Y F, Zhou R, Zhang H X. Appl. Surf. Sci., 2015, 346: 134.

[21]
Panda S R, De S. Desalination, 2014, 347: 52.

[22]
Strathmann H, Kock K, Amar P, Baker R W. Desalination, 1975, 16(2): 179.

[23]
Albrecht W, Weigel T, Schossig-Tiedemann M, Kneifel K, Peinemann K V, Paul D. J. Membr. Sci., 2001, 192(1/2): 217.

[24]
Matsuyama H, Maki T, Teramoto M, Kobayashi K. Sep. Sci. Technol., 2003, 38(14): 3449.

[25]
Blanco J F, Sublet J, Nguyen Q T, Schaetzel P. J. Membr. Sci., 2006, 283(1/2): 27.

[26]
Luo L, Chung T S, Weber M, Staudt C, Maletzko C. J. Membr. Sci., 2017, 524: 33.

[27]
Kimmerle K, Strathmann H. Desalination, 1990, 79(2/3): 283.

[28]
Wang D L, Teo W K, Li K. J. Membr. Sci., 2002, 204(1/2): 247.

[29]
Ma F, Ye H, Zhang Y Z, Ding X L, Lin L G, Zhao L Z, Li H. Desalination Water Treat., 2014, 52(4/6): 618.

[30]
Liu Y P, Wang J, Wang Y, Zhu H C, Xu X M, Liu T, Hu Y X. Chem. Eng. J., 2021, 405: 127051.

[31]
Wang J, Liu Y P, Liu T, Xu X M, Hu Y X. J. Membr. Sci., 2020, 599: 117851.

[32]
Mousavi S M, Zadhoush A. J. Membr. Sci., 2017, 532: 47.

[33]
Tan X M, Rodrigue D. Polymers, 2019, 11(7): 1160.

[34]
Mazinani S, Darvishmanesh S, Ehsanzadeh A, Van der Bruggen B. J. Membr. Sci., 2017, 526: 301.

[35]
Smolders C A, Reuvers A J, Boom R M, Wienk I M. J. Membr. Sci., 1992, 73(2/3): 259.

[36]
Kim I. J. Membr. Sci., 2004, 230(1/2): 183.

[37]
Kim S R, Lee K H, Jhon M S. J. Membr. Sci., 1996, 119(1): 59.

[38]
Zuo D Y, Zhu B K, Cao J H, Xu Y Y. Chin. J. Polym. Sci., 2006, 24(3): 281.

[39]
Wei Y Y, Wang Y, Wang L, Yang H, Jin H, Lu P, Li Y S. J. Membr. Sci., 2021, 620: 118829.

[40]
Deshmukh S P, Li K. J. Membr. Sci., 1998, 150(1): 75.

[41]
Thuyavan Y L, Anantharaman N, Arthanareeswaran G, Ismail A F. J. Chem. Technol. Biotechnol., 2016, 91(10): 2568.

[42]
Wang X Y, Zhang L, Sun D H, An Q F, Chen H L. J. Appl. Polym. Sci., 2008, 110(3): 1656.

[43]
Alsari A M, Khulbe K C, Matsuura T. J. Membr. Sci., 2001, 188(2): 279.

[44]
Zhu L F, Wu M Y, Van der Bruggen B, Lei L C, Zhu L Z. Sep. Purif. Technol., 2020, 242: 116770.

[45]
Hegde C, Isloor A M, Padaki M, Wanichapichart P, Yu L D. Desalination, 2011, 265(1/3): 153.

[46]
Peng N, Chung T S, Wang K Y. J. Membr. Sci., 2008, 318(1/2): 363.

[47]
Gao J, Thong Z, Wang K Y, Chung T S. J. Membr. Sci., 2017, 541: 413.

[48]
Saniei N, Ghasemi N, Zinatizadeh A A, Zinadini S, Ramezani M, Derakhshan A A. Sep. Purif. Technol., 2020, 241: 116646.

[49]
Wang L, Song X J, Wang T, Wang S Z, Wang Z N, Gao C J. Appl. Surf. Sci., 2015, 330: 118.

[50]
Sotto A, Boromand A, Balta S, Kim J, Van der Bruggen B. J. Mater. Chem., 2011, 21(28): 10311.

[51]
Zhu J Y, Zhang Y T, Tian M M, Liu J D. ACS Sustainable Chem. Eng., 2015, 3(4): 690.

[52]
Zhu J Y, Tian M M, Zhang Y T, Zhang H Q, Liu J D. Chem. Eng. J., 2015, 265: 184.

[53]
Park H, Freeman B, Zhang Z B, Sankir M, McGrath J. Angewandte Chemie Int. Ed., 2008, 47(32): 6019.

[54]
Liu C L, Sun Y X, Chen Z B, Zhang S B. Sep. Purif. Technol., 2019, 212: 465.

[55]
Zhu J H, Zhang Q F, Zheng J F, Hou S H, Mao H C, Zhang S B. J. Membr. Sci., 2016, 517: 39.

[56]
Liu Z X, Wang L, Mi Z M, Jin S Z, Wang D M, Zhao X G, Zhou H W, Chen C H. Appl. Surf. Sci., 2019, 490: 7.

[57]
Brami M V, Oren Y, Linder C, Bernstein R. Polymer, 2017, 111: 137.

[58]
Wang F, Hickner M, Kim Y S, Zawodzinski T A, McGrath J E. J. Membr. Sci., 2002, 197(1/2): 231.

[59]
Liu C L, Mao H C, Zheng J F, Zhang S B. J. Membr. Sci., 2017, 530: 1.

[60]
Boussu K, Van der Bruggen B, Volodin A, Van Haesendonck C, Delcour J A, Van der Meeren P, Vandecasteele C. Desalination, 2006, 191(1/3): 245.

[61]
Boussu K, Van der Bruggen B, Vandecasteele C. Desalination, 2006, 200(1/3): 416.

[62]
Thong Z, Gao J, Lim J X Z, Wang K Y, Chung T S. Sep. Purif. Technol., 2018, 192: 483.

[63]
Fan G Y, Jin X M, Zhou E L, Liu K J. Eur. Polym. J., 1998, 34(2): 277.

[64]
Zhang Q F, Zhang S B, Dai L, Chen X S. J. Membr. Sci., 2010, 349(1/2): 217.

[65]
Zhang Q F, Wang H F, Zhang S B, Dai L. J. Membr. Sci., 2011, 375(1/2): 191.

[66]
Mehta R, Manna P, Bhattacharya A. J. Macromol. Sci. A, 2016, 53(10): 644.

[67]
Duarte A P, Cidade M T, Bordado J C. J. Appl. Polym. Sci., 2006, 100(5): 4052.

[68]
Fei P F, Liao L, Meng J Q, Cheng B W, Hu X Y, Song J. Cellulose, 2018, 25(10): 5967.

[69]
Vatanpour V, Pasaoglu M E, Barzegar H, Teber O O, Kaya R, Bastug M, Khataee A, Koyuncu I. Chemosphere, 2022, 295: 133914.

[70]
Yasmeen E, Sayed M E, Amel E, Shaker E. Polym. Eng. Sci., 2020, 60(11): 2852.

[71]
Nguyen T P N, Yun E T, Kim I C, Kwon Y N. J. Membr. Sci., 2013, 433: 49.

[72]
Su J C, Yang Q, Teo J F, Chung T S. J. Membr. Sci., 2010, 355(1/2): 36.

[73]
Loeb S, Sourirajan S. Saline Water Conversion-Ⅱ, ACS Publications, 1962. 158. DOI:10.1021/ba-1963-0038.choll

[74]
Haddada R, Ferjani E, Roudesli M S, Deratani A. Desalination, 2004, 167: 403.

[75]
Shenvi S S, Isloor A M, Ismail A F. Desalination, 2015, 368: 10.

[76]
Sprick C, Chede S, Oyanedel-Craver V, Escobar I C. J. Environ. Chem. Eng., 2018, 6(2): 2480.

[77]
Shafiq M, Sabir A, Islam A, Khan S M, Gull N, Hussain S N, Butt M T Z. Carbohydr. Polym., 2018, 186: 367.

[78]
Ahmad A, Waheed S, Khan S M, E-Gul S, Shafiq M, Farooq M, Sanaullah K, Jamil T. Desalination, 2015, 355: 1.

[79]
Choi H G, Yoon S H, Son M, Celik E, Park H, Choi H. Desalination Water Treat., 2016, 57(16): 7545.

[80]
Ebrahim S, Mosry A, Kanawy E, Abdel-Fattah T, Kandil S. Desalination Water Treat., 2015, 57(44): 20738.

[81]
Worthley C H, Constantopoulos K T, Ginic-Markovic M, Pillar R J, Matisons J G, Clarke S. J. Membr. Sci., 2011, 385-386: 30.

[82]
Ruan H M, Guo C M, Yu H W, Shen J N, Gao C J, Sotto A, Van der Bruggen B. Ind. Eng. Chem. Res., 2016, 55(46): 12099.

[83]
Wang T, Zhao C, Li P, Li Y, Wang J. J. Membr. Sci., 2015, 477: 74.

[84]
Huang J, Zhang K S. Desalination, 2011, 282: 19.

[85]
Jiang G S, Zhang S X, Zhu Y Z, Gao S J, Jin H L, Luo L Q, Zhang F, Jin J. J. Mater. Chem. A, 2018, 6(7): 2927.

[86]
Vandezande P, Gevers L, Paul J, Vankelecom I, Jacobs P. J. Membr. Sci., 2005, 250(1/2): 305.

[87]
Wang Y C, Wang W J, Wang Q, Wang Z Y, Yan X Y, Zhao L L, Cao X L, Sun S P. Sep. Purif. Technol., 2022, 285: 120322.

[88]
Cheng X Q, Zhang Y L, Wang Z X, Guo Z H, Bai Y P, Shao L. Adv. Polym. Technol., 2014, 33(S1): 21455.

[89]
See Toh Y H, Lim F W, Livingston A G. J. Membr. Sci., 2007, 301(1/2): 3.

[90]
Hołda A K, Aernouts B, Saeys W, Vankelecom I F J. J. Membr. Sci., 2013, 442: 196.

[91]
Hołda A K, Vankelecom I F J. J. Membr. Sci., 2014, 450: 512.

[92]
Hołda A K, Vankelecom I F J. J. Membr. Sci., 2014, 450: 499.

[93]
Wei C, He Z, Lin L, Cheng Q, Huang K, Ma S, Chen L. J. Membr. Sci., 2018, 563: 752.

[94]
Guan S S, Zhang S H, Han R L, Zhang B G, Jian X G. Desalination, 2013, 318: 56.

[95]
Bandehali S, Moghadassi A, Parvizian F, Zhang Y T, Hosseini S M, Shen J N. Sep. Purif. Technol., 2020, 242: 116745.

[96]
Xu Y C, Tognia M, Guo D X, Shen L G, Li R J, Lin H J. J. Colloid Interface Sci., 2019, 546: 251.

[97]
Asatekin A, Olivetti E A, Mayes A M. J. Membr. Sci., 2009, 332(1/2): 6.

[98]
Bengani P, Kou Y M, Asatekin A. J. Membr. Sci., 2015, 493: 755.

[99]
Guo S W, Wan Y H, Chen X R, Luo J Q. Chem. Eng. J., 2021, 409: 127376.

[100]
Cao Y, Chen X R, Feng S C, Wan Y H, Luo J Q. Ind. Eng. Chem. Res., 2020, 59(45): 19858.

[101]
Zhao C S, Xue J M, Ran F, Sun S D. Prog. Mater. Sci., 2013, 58(1): 76.

[102]
Ismail A F, Hassan A R. Sep. Purif. Technol., 2007, 55(1): 98.

[103]
Xing L, Guo N, Zhang Y, Zhang H, Liu J. Sep. Purif. Technol., 2015, 146: 50.

[104]
Yu L, Zhang Y T, Wang Y M, Zhang H Q, Liu J D. J. Hazard. Mater., 2015, 287: 373.

[105]
Koulivand H, Shahbazi A, Vatanpour V, Rahmandoost M. Mater. Sci. Eng. C, 2020, 111: 110787.

[106]
Shukla A K, Alam J, Rahaman M, Alrehaili A, Alhoshan M, Aldalbahi A. Water Air Soil Pollut., 2020, 231(3): 1.

[107]
Qiu Z Y, Ji X F, He C J. J. Hazard. Mater., 2018, 360: 122.

[108]
Zhu J Y, Tian M M, Hou J W, Wang J, Lin J Y, Zhang Y T, Liu J D, Van der Bruggen B. J. Mater. Chem. A, 2016, 4(5): 1980.

[109]
Luque-Alled J M, Abdel-Karim A, Alberto M, Leaper S, Perez-Page M, Huang K, Vijayaraghavan A, El-Kalliny A S, Holmes S M, Gorgojo P. Sep. Purif. Technol., 2020, 230: 115836.

[110]
Yu L, Zhang Y T, Zhang H Q, Liu J D. Desalination, 2015, 359: 176.

[111]
Wang Y M, Zhu J Y, Dong G Y, Zhang Y T, Guo N N, Liu J D. Sep. Purif. Technol., 2015, 150: 243.

[112]
Yuan S S, Li X, Zhu J Y, Zhang G, Van Puyvelde P, Van der Bruggen B. Chem. Soc. Rev., 2019, 48(10): 2665.

[113]
Jun B M, Al-Hamadani Y A J, Son A, Park C M, Jang M, Jang A, Kim N C, Yoon Y. Sep. Purif. Technol., 2020, 247: 116947.

[114]
Tavangar T, Karimi M, Rezakazemi M, Reddy K R, Aminabhavi T M. Chem. Eng. J., 2020, 385: 123787.

[115]
Guo S W, Chen X R, Wan Y H, Feng S C, Luo J Q. ACS Appl. Mater. Interfaces, 2020, 12(11): 13327.

[116]
Zheng J F, Li Y, Xu D L, Zhao R, Liu Y Y, Li G C, Gao Q Y, Zhang X, Volodine A, Van der Bruggen B. Sep. Purif. Technol., 2022, 282: 120155.

[117]
Yamagishi H, Crivello J V, Belfort G. J. Membr. Sci., 1995, 105(3): 237.

[118]
Homayoonfal M, Akbari A, Mehrnia M R. Desalination, 2010, 263(1/3): 217.

[119]
Liu C L, Bi W H, Chen D L, Zhang S B, Mao H C. Chin. J. Chem. Eng., 2017, 25(11): 1685.

[120]
Lin C E, Fang L F, Du S Y, Yao Z K, Zhu B K. Sep. Purif. Technol., 2019, 212: 101.

[121]
Yu H Z, Qiu X Y, Moreno N, Ma Z W, Calo V M, Nunes S P, Peinemann K V. Angewandte Chemie Int. Ed., 2015, 54(47): 13824.

[122]
Hampu N, Werber J R, Chan W Y, Feinberg E C, Hillmyer M A. ACS Nano, 2020, 14(12): 16446.

[123]
Bates F S, Hillmyer M A, Lodge T P, Bates C M, Delaney K T, Fredrickson G H. Science, 2012, 336(6080): 434.

[124]
Marques D S, Vainio U, Chaparro N M, Calo V M, Bezahd A R, Pitera J W, Peinemann K V, Nunes S P. Soft Matter, 2013, 9(23): 5557.

[125]
Guo L M, Wang Y, Steinhart M. Chem. Soc. Rev., 2021, 50(11): 6333.

[126]
Nunes S P. Macromolecules, 2016, 49(8): 2905.

[127]
Vakili H, Mohseni M, Makki H, Yahyaei H, Ghanbari H, González A, Irusta L. Polymer, 2020, 195: 122424.

[128]
Bates C M, Bates F S. Macromolecules, 2017, 50(1): 3.

[129]
Sutisna B, Polymeropoulos G, Musteata V, Peinemann K V, Avgeropoulos A, Smilgies D M, Hadjichristidis N, Nunes S P. Mol. Syst. Des. Eng., 2016, 1(3): 278.

[130]
Mai Y Y, Eisenberg A. Chem. Soc. Rev., 2012, 41(18): 5969.

[131]
Zhang Z Z, Simon A, Abetz C, Held M, Höhme A L, Schneider E S, Segal-Peretz T, Abetz V. Adv. Mater., 2021, 33(48): 2105251.

[132]
Bates F S, Fredrickson G H. Phys. Today, 1999, 52(2): 32.

[133]
Brendel J C, Schacher F H. Chem. Asian J., 2018, 13(3): 230.

[134]
Trollsås M, Kelly M A, Claesson H, Siemens R, Hedrick J L. Macromolecules, 1999, 32(15): 4917.

[135]
Klein R, Schüll C, Berger-Nicoletti E, Haubs M, Kurz K, Frey H. Macromolecules, 2013, 46(22): 8845.

[136]
Bütün V, Liu S, Weaver J V M, Bories-Azeau X, Cai Y, Armes S P. React. Funct. Polym., 2006, 66(1): 157.

[137]
Lee S, Saito K, Lee H R, Lee M J, Shibasaki Y, Oishi Y, Kim B S. Biomacromolecules, 2012, 13(4): 1190.

[138]
Feng H B, Lu X Y, Wang W Y, Kang N G, Mays J. Polymers, 2017, 9(12): 494.

[139]
Zhang Y Z, Almodovar-Arbelo N E, Weidman J L, Corti D S, Boudouris B W, Phillip W A. Npj Clean Water, 2018, 1: 2.

[140]
Zhao Z W, Wang X, Jing X X, Zhao Y J, Lan K, Zhang W, Duan L L, Guo D Y, Wang C Y, Peng L, Zhang X M, An Z S, Li W, Nie Z H, Fan C H, Zhao D Y. Adv. Mater., 2021, 33(23): 2100820.

[141]
Zhu G D, Yin Y R, Li Z N, Qiu Y Y, Yi Z, Gao C J. J. Membr. Sci., 2019, 582: 391.

[142]
Zhu G D, Yang C Y, Yin Y R, Yi Z, Chen X H, Liu L F, Gao C J. J. Membr. Sci., 2019, 589: 117255.

[143]
Sargent J L, Hoss D J, Phillip W A, Boudouris B W. J. Appl. Polym. Sci., 2018, 135(24): 45531.

[144]
Phillip W A, Dorin R M, Werner J, Hoek E M V, Wiesner U, Elimelech M. Nano Lett., 2011, 11(7): 2892.

[145]
Pendergast M M, Mika Dorin R, Phillip W A, Wiesner U, Hoek E M V. J. Membr. Sci., 2013, 444: 461.

[146]
Paradiso S P, Delaney K T, García-Cervera C J, Ceniceros H D, Fredrickson G H. ACS Macro Lett., 2014, 3(1): 16.

[147]
Peinemann K V, Abetz V, Simon P F W. Nat. Mater., 2007, 6(12): 992.

[148]
Sinturel C, Bates F S, Hillmyer M A. ACS Macro Lett., 2015, 4(9): 1044.

[149]
Zhang Z Z, Rahman M M, Abetz C, Höhme A L, Sperling E, Abetz V. Adv. Mater., 2020, 32(8): 1907014.

[150]
Xie Y H, Sutisna B, Nunes S P. Chem. Commun., 2017, 53(49): 6609.

[151]
Albert J N L, Epps T H III. Mater. Today, 2010, 13(6): 24.

[152]
Yu H Z, Qiu X Y, Nunes S P, Peinemann K V. Angew. Chem. Int. Ed., 2014, 53(38): 10072.

[153]
Hoheisel T N, Hur K, Wiesner U B. Prog. Polym. Sci., 2015, 40: 3.

[154]
Zhang Z Z, Rahman M M, Abetz C, Abetz V. J. Mater. Chem. A, 2020, 8(19): 9554.

[155]
Kheirieh S, Asghari M, Afsari M. Rev. Chem. Eng., 2018, 34(5): 657.

[156]
Yang H, Shi X S, Chu S Y, Shao Z P, Wang Y. Adv. Sci., 2021, 8(7): 2003096.

[157]
Ma D W, Ye X Y, Li Z, Zhou J M, Zhong D L, Zhang C X, Xiong S, Xia J Z, Wang Y. Sep. Purif. Technol., 2021, 259: 118100.

[158]
Wang N, Wang T, Hu Y X. ACS Appl. Mater. Interfaces, 2017, 9(36): 31018.

[159]
Chen W, Wei M J, Wang Y. J. Membr. Sci., 2017, 525: 342.

[160]
Xie Y H, Moreno N, Calo V M, Cheng H, Hong P Y, Sougrat R, Behzad A R, Tayouo R, Nunes S P. Polym. Chem., 2016, 7(18): 3076.

Options
Outlines

/

〈 〉