Membrane technology occupies an important position in chemical technology and is widely used in varied fields of industry and daily life. With the continuous development of thin film technology, the preparation of porous polymer films with controlled pore size and size distribution, excellent mechanical properties and functional versatility has become an urgent problem to be solved. Compared to non-porous dense polymer films, micro- and nanoporous polymer films generally have lower density, lighter weight, higher specific surface area, and feature acoustic and thermal insulation, good permeability and high plasticity. This review begins with a summary of the methods used to prepare micro/nano-porous polymer films, including template methods (hard and soft templates), phase separation, electrostatic spinning, etching and some other methods. The methods for characterizing the structure and related properties of membranes in porous polymer films are also introduced. The applications of porous permeable membranes are then summarized in gas separation, energy, environmental engineering and bioengineering. Finally, the drawbacks and challenges of current methods for the preparation of micro- and nanoporous polymer films are summarized and an outlook is given on their future innovative applications.
Contents
1 Introduction
2 Design and fabrication of micro/nano-porous polymer films
2.1 Template methods
2.2 Electrospinning
2.3 liquid-liquid phase separation
2.4 Etching methods
2.5 Other methods
3 Structure and properties characterization of micro/nano-porous polymer films
Thin films play an important role in modern production and life, and have a wide range of applications. In many applications, polymer films are preferred over other films because of their advantages such as simple preparation, high efficiency, easy scale-up, low space requirements, and good flexibility[1]. A polymer permeable membrane is a type of polymer film that acts as a discrete, thin interface that slows the permeation of chemicals that come into contact with it. This interface may be molecularly homogeneous, that is to say completely homogeneous in composition and structure, or it may be chemically or physically inhomogeneous, for example, a thin film containing pores of limited size or consisting of some form of layered structure[2]. Common thin films include isotropic films such as non-porous dense films, microporous films, charged films, etc. Anisotropic films such as Loeb-Souriraja anisotropic films, composite anisotropic films, ceramic films, metal films, and liquid films (see fig. 1).
In recent years, the preparation methods of porous polymer films have been continuously explored to obtain new materials with wide applications. With the increasing demand for more complex structures and functions, how to prepare porous polymer films with excellent mechanical properties, functional diversity, controllable pore size and distribution has become an urgent problem to be solved. There are many ways to make pores in porous polymer films.Template method (including hard template and soft template), electrospinning, phase separation method (including thermally induced phase separation, non-solvent-induced phase separation, vapor-induced phase separation, solvent-induced phase separation and reaction-induced phase separation), etching (solvent etching, laser etching) and other methods such as freeze-drying, supercritical fluid drilling and so on[3][4][5][6][7][8][9][10][11][12][13][14]. In recent years, researchers have used these pore-forming methods to selectively remove certain components in the membrane system, and continuously prepared various functional micro-nano porous polymer membranes, which have been applied to oil-water separation, seawater desalination, medical dressings, battery separators and so on[15][16][17][18]. Compared with non-porous dense polymer films, micro-nano porous polymer films generally have the advantages of low relative density, high specific surface area, light weight, sound insulation, heat insulation, good permeability and strong plasticity. As a potential film material, micro-nano porous polymer films play an indispensable role in the fields of health care, environment (water purification, heavy metals and organic pollutants enrichment and separation), energy (seawater desalination, batteries, power generation, etc.), Information (sensing) and so on.These functional films can not be separated from production and life practice, and the application scope of micro-nano porous polymer films is expanding[19⇓⇓⇓~23][24⇓⇓⇓⇓⇓~30][31⇓⇓⇓⇓⇓~37]. In recent years, the research on porous polymer films has focused on exploring the pore-forming methods and film functionalization, but so far there is still no universally applicable pore-forming method for all kinds of polymers. Therefore, how to obtain a universal pore-forming method has become a major challenge for research. In addition, the control of pore shape, size, size distribution and density is also a major problem in the study. This review will summarize the common methods of polymer pore formation and their advantages and disadvantages, as well as the common characterization methods of porous structure and permeability membrane performance on the basis of previous studies.Finally, the preparation methods of micro-nano porous polymer membranes and the shortcomings in the application of permeable membranes were analyzed, and the future application prospects were put forward.
2 Design and Preparation of Micro-nano Porous Polymer Film
Although porous polymer films have shown outstanding performance, they still need to be designed and constructed to improve their performance and strength in order to be used in practical production and life. How to obtain functional films, especially porous polymer functional films, has become a challenge. Researchers are exploring various methods to prepare micro-nano porous polymer films (Table 1). Template method, electrospinning, phase separation and other related pore-forming methods will be briefly described below in combination with previous work.
2.1 Template method
Template method is one of the most important methods to prepare micro-and nano-porous polymer films. Template synthesis of nanomaterials is a technology developed in the 1990s, and it is also a very effective method widely used in recent years[48⇓~50]. This method can effectively control the morphology, size and structure of materials, and is a very widely used synthesis method[38⇓⇓~41]. In the template method, the selected template should be easy to obtain and controllable in shape. The target material is mixed with the template, and then the template is removed, and finally the material with special structure or morphology is synthesized. As the template for pore-making, it is mainly divided into two categories: hard template and soft template.
2.1.1 Hard template method
The preparation of porous polymer films usually includes three steps: (1) polymer precursor liquid casting template; (2) polymerize that precursor liquid; And (3) remove that template. The hard template method is the most commonly used method to prepare polymer films with easily controlled pores (Figure 2a)[51⇓~53]. Commonly used hard templates include monodisperse inorganic oxide nanoparticles, polymer nanoparticles, anodic aluminum oxide (AAO), etc.
Among them, AAO porous templates prepared by electrochemical method and porous polycarbonate (PC) membranes prepared by "track etching" method (represented by PC-PTM) are widely used in industrial membranes. These membranes contain cylindrical through-holes across the membrane surface with pore sizes up to 10 nm and pore densities close to 109 pore /cm2[54]. Martin et al. Synthesized polypyrrole (PP), polyaniline (PANI) and polythiophene (PTH) nanostructures using AAO and PC-PTM as templates[55⇓⇓~58]. Similarly, Xiong et al. Also described the formation mechanism of ordered PANI nanotubes in AAO channels by chemical synthesis[59]. The morphology of polyaniline synthesized in AAO membrane and PC-PTM membrane was compared. The results showed that the morphology of polyaniline microtubules and the arrangement of polyaniline molecular chains were affected by the characteristics and pore size of the template as well as the synthesis method[60]. In addition, colloidal particles (Figure 2b), porous silicates, mesoporous zeolites, carbon nanotubes, and highly oriented pyrolytic graphite (HOPG) have also been used as templates to fabricate conductive polymer nanostructures[61⇓~63][64][65][66][67]. For example, macroporous conducting polymers (PANI and PHT, etc.) Can be synthesized using colloidal particle arrays as templates[60,68,69].
In addition, Guo et al. Prepared polyethersulfone (PES)/polyvinylpyrrolidone (PVP) submicron porous proton exchange membranes by polymer blending method using monodisperse silica solid spheres as hard templates[71]. After doping with phosphoric acid (PA), a PA-rich network is formed due to the submicron pores in the mixture, which is beneficial to PA loading and proton transfer, thus improving the proton conductivity and achieving a balance between proton conductivity and mechanical strength. Some researchers also use common particles (sugar and salt) in daily life to create the required holes. For example, Shin et al. used brown sugar powder as a hard template to cure polydimethylsiloxane (PDMS), and then selectively dissolved the brown sugar particles (i.e. sacrificial particles) in the cured PDMS in hot water to prepare a porous polymer membrane (Fig. 3A)[72]. In addition, Xue et al. Prepared porous calcium alginate/silver nanoparticle hydrogel films with superhydrophilicity and underwater superoleophobicity by soluble salt template method[73]. These templates do not use toxic organic solvents, are inexpensive and easy to handle, and are suitable for many polymer materials (monomers, oligomers, resins)[74]. In addition, the work of Magerl et al. Shows that porous polymer films can also be obtained by using the principle of offset printing to prepare templates, which form hydrophobic and hydrophilic areas with the help of photolithography and are selectively wetted with a calcium chloride solution in the hydrophilic areas[75]. A polymer solution is then applied to this surface to form a hydrophobic layer of water droplets. Unlike standard offset printing, this layer does not transfer to another surface in the liquid state, but is separated from the plate after curing (Figure 3B).
The pore morphology, pore size and pore density of the micro-nano porous polymer film prepared by using the hard template can be accurately adjusted by the hard template, but the template needs to be removed by post-treatment, which makes the preparation process longer. On the other hand, the size of the porous polymer film is often limited by the size of the template, thus limiting its scope of application.
2.1.2 Soft Template Method
The morphology of soft templates is diverse, but the structural stability is usually poor. Soft templates are usually aggregates with certain structural characteristics formed by intermolecular or intramolecular interaction forces (hydrogen bonding, chemical binding and electrostatic interaction) in the synthesis of nanomaterials. The aggregate has an obvious structural interface, and inorganic substances can present a specific distribution through the unique structural interface, thereby obtaining the nano-material with the specific structure of the template. Common soft templates mainly include various block polymers (such as triblock polymers P123, F127), liquid crystals, vesicles, micelles, microemulsions and so on[76][77][78][79].
Hou et al. reported a new type of soft template ionic liquid with good fluidity and molecular size for the preparation of porous nanofibers composed of pores of about 4 nm, and the topology of these nanopores can be designed according to the type and loading of ionic liquid[4]. After adding the polymer precursor solution, a unique porous nanofiber composite membrane was prepared. This porous polymer composite membrane can release conductive groups while absorbing a large amount of water, and finally achieve excellent proton conduction. Tanaka et al. prepared the ordered mesoporous carbon membrane by using the triblock copolymer Pluronic F127 as a template. The mesoporous carbon membrane carbonized at 600 ℃ has good permeability, high hydrothermal stability and good alkali resistance. In addition, the soft template provides the possibility for the morphology control of the ordered mesoporous carbon[80]. Zhang et al. Proposed a way to synthesize porous polydimethylsiloxane sponge on a large scale by paraffin oil emulsification technology. This synthesis method is economical and simple, and the shape and thickness can be adjusted to meet different needs in the synthesis process[81].
Due to the application of the soft template, the traditional sugar template forming and centrifugal operation are avoided, and the prepared PDMS sponge has a porous structure and a swellable skeleton, thereby greatly improving the absorption capacity.Organic matter and oil in the water can be efficiently separated, and the functionality is not lost after multiple cycles, showing excellent oil removal capacity and good recoverability. In addition, Kovalenko et al showed that the initial state of the emulsion had a great influence on the porous structure and properties of the final material when preparing porous materials by emulsion polymerization[82]. The use of surfactants with different solubilities in the emulsion continuous phase can tailor the interaction of the emulsion droplets to produce a material with interconnected or isolated pores. The two systems exhibit completely different behavior on drying, which results in a large-pore aerated material in the interconnected case and a small-pore collapsed material in the second case (fig. 4).
2.2 Electrospinning
Nanofibers have attracted more and more attention in recent years. Electrospinning, as a flexible method to fabricate nanofibers, has been widely used due to its good adaptability and easy production[5,42⇓⇓ ~45]. Porous polymer films prepared by electrospinning generally have high specific surface area and porosity, and the pore size can be adjusted artificially according to the process parameters[83]. A typical electrospinning setup consists of three main components: a high-voltage power supply for charging the polymer solution; A syringe with a pump, the polymer solution being extruded through a capillary connected to the syringe filled with the polymer solution; And a grounded collector for storing the nanofibers (figs. 5a-c). The viscous electrospinning solution in the injector is extruded by the feed pump at a constant rate, and the charged jet of the solution is ejected from the tip of the Taylor cone and split into many tiny jets between the tip and the collector. As the solvent evaporates, solidified continuous fibers form on the collector. Electrospinning methods include hybrid, coaxial, needleless and emulsion techniques (Fig. 5d ~ G)[84].
For the polymers to be electrospun, as long as they can be dissolved in an appropriate solvent to obtain a solution or melted without degradation, most of the polymers can be directly used for the preparation of porous polymer membranes by electrospinning. Ma et al. prepared micro-nano porous polystyrene (PS) by controlling the concentration of polystyrene/dimethylformamide (PS/DMF) solution using electrospinning technology[85]. When the molecular weight of PS is 250 000 and the concentration of PS/DMF solution is 5% (mass fraction), the surface shows superhydrophobicity. Tajitsu et al. developed a new wearable piezoelectret sensor using electrospun poly (tetrafluoroethylene) (PTFE) as a force-sensitive material to detect pressure signals from the heartbeat[86].
Although electrospinning has certain requirements for the polymer used, it has been favored by researchers because it can directly form a porous structure containing a large number of micro/nano pores in the polymer membrane. In the process of electrostatic spinning, the polymer solution sprayed from the needle tip is split and solidified into nanofibers in the air, and the nanofibers are continuously gathered and stacked on the collector to form a porous film. So far, more than one hundred natural and synthetic polymers, such as collagen, elastin, cellulose, polyurethane and their mixtures, have been successfully made into nanofibers by electrospinning. These poly blend nanofiber films, which are electrospun from a mixture of synthetic and natural polymers, exhibit robust biochemical, structural, and mechanical properties that cannot be achieved with any single polymer. This technique not only produces nanostructures that inherit the properties of each constituent polymer, but also allows the use of natural polymers, including proteins and polysaccharides, which are otherwise difficult to prepare in the form of fibers[87]. In addition, through the operation of process and formulation variables (including concentration, viscosity, molecular weight) and environmental parameters (including temperature and humidity), films with desired pore size can be prepared to optimize the performance of nanofibers.
2.3 Liquid-liquid phase separation
liquid-liquid phase separation (LLPS) is an effective method to prepare porous polymer structures. During phase separation, the thermodynamically unstable polymer solution phase separates into two coexisting liquid phases in thermodynamic equilibrium. One of the phases, the polymer-rich phase, solidifies to form a solid matrix, while the other phase, the non-solvent-rich polymer-poor phase, eventually forms pores in the matrix[46]. LLPS can be achieved through eight pathways: thermally induced phase separation (thermally induced phase separation, TIPS), non-solvent-induced phase separation (non-solvent induced phase separation, NIPS), vapor-induced phase separation (vapor induced phase separation, VIPS), vaporization-induced phase segregation (evaporation induced phase separation, EIPS), dry-cast phase separation [dry cast phase separation, DCPS], and wet-cast phase separation [thermally induced phase separation.
2.3.1 Thermally induced phase separation
TIPS process is often used to prepare micro-nano porous polymer films. In the process of TIPS membrane preparation, the polymer is dissolved in the diluent at high temperature, and then the homogeneous polymer solution is separated by cooling. After the polymer is cured, the diluent is removed to finally obtain the desired porous polymer film[6].
In the TIPS process, diluent-rich droplets begin to form and grow after the homogeneous polymer diluent solution cools and undergoes liquid-liquid thermally induced phase separation. This growth process is called coarsening. When the system is further cooled to solidify, the resulting film consists of a polymer matrix and diluent-filled pores. Thus, the size of the droplet is directly related to the resulting pore size, i.e., the larger the droplet size, the larger the pore. TIPS is therefore suitable for a variety of polymers, including those for which the immersion precipitation method cannot be used to prepare microporous membranes due to solubility problems. The typical polymer is polyolefin, which has good heat resistance and solvent resistance, and low cost, and is a suitable material for microporous membranes[88].
Matsuyama et al. Prepared microporous hollow fiber membranes from two high density polyethylenes (HDPE) with molecular weights of 250,000 (HDPE25) and 130,000 (HDPE13) by the TIPS method using a batch extrusion apparatus as shown in fig. 6a[89]. Due to the faster cooling rate and the higher polymer concentration near the outer surface, an asymmetric structure with smaller pores on the outer surface is obtained in all cases. In addition, Zhang et al. prepared highly hydrophilic and stain-resistant HDPE/PE-b-PEG microporous membranes by blending HDPE with amphiphilic copolymer polyethylene-polyethylene glycol (PE-b-PEG) using a liquid-liquid thermally induced phase separation (L-L TIPS) process (Fig. 6B)[90]. The cross section and surface morphology of HDPE/PE-b-PEG blend membranes were observed by SEM, which showed that the pore size of HDPE/PE-b-PEG blend membranes was larger than that of pure HDPE membranes.
2.3.2 Nonsolvent-induced phase separation
The non-solvent-induced phase separation (NIPS) technique is also widely used for the preparation of porous membranes (fig. 7A). In the process of NIPS, the non-solvent of the polymer is directly introduced into the homogeneous polymer solution, which divides the solution into two phases: one is the polymer-rich phase, which becomes the matrix of the membrane after the precipitation of the polymer solution; The other is a polymer-poor phase, which forms membrane pores after removal from the precipitation solution[7].
Kim et al. continuously prepared sponge-like polyimide membranes with large pore size (0.1 ~ 10 μm) and high permeability by NIPS method (Fig. 7B)[91]. The existing studies generally use vacuum to remove the solvent and form pores in the drying process, but in this study, the vacuum atmosphere is no longer needed, so that the whole preparation process can be carried out continuously. The prepared polyimide membrane has uniform micropores, and the pore size of the polyimide membrane can be controlled by changing the type and the height of a non-solvent during soaking and curing. It is also found that the pore size decreases with the increase of the height of the non-solvent, and the average porosity of all samples is 80%. In addition, Zhao et al also prepared polyethersulfone (PES) ultrafiltration membrane by non-solvent-induced phase separation method, and synthesized a new block copolymer for in-situ modification of porous membrane by radical polymerization[92]. X-ray photoelectron spectroscopy (XPS) confirmed that the spontaneous or forced segregation on the surface of the copolymer resulted in the enrichment of hydrophilic and fluorine-containing segments during the non-solvent-induced phase separation. The modified membrane has obvious pH responsiveness, permeability (fig. 7c, d) and good antifouling performance.
2.3.3 Vapor-induced phase separation
In 1918, Zsigmondy and Bachmann proposed a new method to induce the solidification of homogeneous polymer solutions in a gas phase environment, which is called vapor-induced phase separation (VIPS). VIP S involves a wet and dry casting process[93]. In this process, the polymer solution is exposed to non-solvent vapor (usually humid air), and water vapor is slowly transferred from the gas phase to the polymer solution. In contrast to NIPS, VIPS is characterized by the fact that the nonsolvent phase is a gas, which means that this phase separation is a process of gas inflow and solvent outflow. By controlling the exposure time in humid air, the relative humidity, the air temperature, etc., films with unique morphologies can be obtained. Therefore, the membranes prepared by VIPS have a wide range of applications[8].
Ke et al. Used the VIP method to expose the block copolymer micelle solution to an organic non-solvent vapor atmosphere to evaporate the solvent, and prepared a superhydrophobic film with high moisture permeability[94]. After complete evaporation of the solvent, the films exhibit a three-dimensional network of nanoscale to microscale bicontinuous matrices and channels. In addition, the matrix of the film is composed of interconnected submicron particles, and this special layered structure makes the film superhydrophobic and highly adhesive. It was found that the micellar state of the block copolymer contributed to the phase separation during VIPS, resulting in the formation of a layered structure in the final membrane. Due to this layered structure, the film has CA > 150 ° and can strongly adsorb water droplets. Thin film coatings with a layered structure can also be prepared on non-planar substrates by this method. Due to the high porosity, water vapor can freely pass through the film. The pure water flux measured by the membrane is 3320 L/(m2·h·MPa), and the membrane can be used for microfiltration separation of yeast. At that same time, the film is easy to clean and convenient to reuse.
In addition, Hao et al. Prepared a hollow fiber membrane with controllable surface geometry and uniform internal structure by thermally induced phase separation, dip-coating and gas phase separation[95]. In the preparation process, a polyvinylidene fluoride (PVDF) hollow fiber membrane was first prepared by a TIPS method, then the outer surface of the membrane was impregnated with another PVDF solution, and finally the outer surface of the membrane was reconstituted by VIPS (Fig. 8), and finally a porous network structure was successfully prepared on the surface of the original PVDF membrane. The pore size of the membrane was affected to some extent by the dip-coating process and the VIPS treatment conditions. The recombined surface acts as a new separation layer. The hollow fiber membrane with porous network structure prepared by TIPS, dip coating and VIPS has high permeability, controllable pore size and high mechanical strength. In addition, the surface-treated membranes exhibited low fouling performance compared to the untreated PVDF membranes.
The EIPS technique, also known as the dry casting process, was first proposed by Kesting in 1973. For EIPS, various polymers are typically dissolved directly into a mixture of solvents containing high concentrations of solvent and other non-solvents.
As the volatile solvent evaporates, the less volatile non-solvent also begins to coalesce into droplets and remains there long enough for the polymer to solidify. Finally, a porous structure is formed by removing these non-solvent rich droplets[96]. Zhao et al. Prepared silicone rubber membranes with ordered micropores on the surface by solvent volatilization induced phase separation method[9]. The ternary solution of liquid silicone rubber (LSR) precursor, liquid paraffin (LP) and n-hexane was cast into two-phase film. By removing the liquid paraffin phase from the silicone rubber film, micropores are formed (fig. 9 a). At the same time, the effects of liquid paraffin concentration, casting temperature, initial casting solution thickness, air circulation and the addition of surfactant Span-80 on the structure of membrane surface holes were also studied. The average pore diameter increases with the increase of the concentration of liquid paraffin or the initial thickness of the casting solution. The results show that the formation of the pore structure on the membrane surface is related to the phase separation, and the pore formation mechanism is the nucleation, growth and coalescence process of the liquid paraffin phase in the membrane.
In addition, Pervin et al. Used a mixture of poly (methyl methacrylate) -tetrahydrofuran-water (PMMA-THF-H2O), according to the distribution of pores in the polymer film formed by EIPS, by adjusting the composition of the polymer solution and the ratio of H2O/PMMA during phase separation.The morphology of the finally formed membrane was found to be: (a) non-porous, i.e., dense membrane, (B) membrane with pores only on the bottom surface, (C) asymmetric membrane, i.e., membrane with bottom porous structure and top dense layer, (d) porous membrane with uniform pores distributed throughout the membrane, and (e) membrane with pores only on the top surface (fig. 9b)[46]. It was also found that the morphology of PMMA films could be adjusted by changing the ratio of low molecular weight PMMA to high molecular weight PMMA in the casting solution. These porous PMMA structures have good biocompatibility and can be used in the fields of cell culture, drug delivery and wound dressing. Mandal et al. prepared a porous polyvinylidene fluoride-co-hexafluoropropylene porous film by solvent volatilization induced phase separation (Fig. 9c), and the prepared film has high solar reflectivity, high emissivity and excellent cooling performance[97].
2.3.5 Reaction-induced phase separation
Reaction induced phase separation (RIPS) is to mix the solvent with the prepolymer to make the system in a homogeneous state before the polymer solidifies. With the increase of polymer molecular weight and the change of interaction and conversion rate caused by chemical modification, phase separation eventually occurred, and porous structure was obtained after solvent removal[98]. Klein et al. Demonstrated a simple and widely applicable method to precisely control polymer porosity over two orders of magnitude (from micrometer to nanometer scale) by controlling the crosslinking reaction rate in a phase separation system[10]. Once the epoxy components are reacted to form a solid porous matrix, the sacrificial phase octadecanol is removed by vacuum assisted evaporation. The pore size can be controlled by changing the curing temperature and adding the catalyst methylimidazole.
2.3.6 Other phase separation method
In addition to the above methods, researchers are still exploring more phase separation methods to prepare porous polymer films for various applications. For example, Zhou et al. reported a new method for preparing a novel asymmetric porous sulfurized poly (ether thione) (SPES) membrane (Fig. 10), which can be used as a proton exchange membrane for an advanced redox battery (VRFB)[99]. A unique porous structure is formed through the phase separation induced by the ionic liquid during the preparation. The thickness of the dense layer can be controlled by adjusting the content of the ionic liquid in the casting solution, thereby adjusting the proton conductivity and vanadium ion permeability of the porous membrane. Due to the high ion selectivity of asymmetric porous SPES (APSPES) membranes, ultra-low vanadium ion permeability and high proton conductivity can be achieved simultaneously.
In addition, freeze-drying can also be used to prepare porous polymer films[13]. Freeze-drying, also known as sublimation drying, consists of a polymer phase and a solvent phase. For the polymer solution prepared at room temperature or high temperature, it can be frozen into a solid by contacting with a cold liquid (such as liquid nitrogen), and then directly freeze-dried to remove the frozen solvent. The frozen solvent is equivalent to a porogen. After removing the solvent by sublimation under vacuum, a porous structure (as shown in fig. 11) is produced, thereby preparing a porous polymer film[100]. Some researchers use ice as a template to make holes, freeze phase sublimation and then sinter, and finally prepare a porous structure with one-way channels, in which the pores are the result of ice crystal phase sublimation[101]. The key to successful freeze-drying is that the sample must remain frozen throughout the process to ensure that the pore structure is not altered. Unfortunately, although freeze-drying has been applied to a variety of polymer materials, reasonable control of the microstructural morphology has not yet been achieved.
There are other ways to induce phase separation in polymer solutions or gels. supercritical fluids (SCFs) are solvents used at the critical point temperature and pressure. SCFs exhibit liquid-like density, gas-like diffusion, and very low surface tension. Their solvent properties can be adjusted simply by changing the temperature and pressure. In recent years, SCFs are regarded as "green" solvents, which are often used in chemical reactions and extraction, and also used as foaming agents to produce porous polymer membranes. Supercritical carbon dioxide (SC CO2) is one of them. Since the CO2 has mild critical points (TC=31.1℃ and PC=73 atm), the supercritical state can be reached relatively easily. The process operation cost is low compared to other SCFs. At the same time, SC CO2 can replace some organic solvents as porogens and non-solvents in the phase inversion process, which can be separated simply by releasing carbon dioxide under reduced pressure, without organic solvent residues in the formed structure, and directly obtain dry porous membranes. SC CO2 has been widely concerned because of its non-toxic, non-flammable and easy to separate from the system[14].
2.4 Etching method
Etching methods include chemical etching, laser etching, and trace etching. Jung et al. prepared a porous Nafion membrane with controllable pore size by using sol-gel process and hydrofluoric acid etching process, and developed an enhanced ionic polymer-metal composite brake by using the porous Nafion membrane through electroless plating and ion exchange process, and found that the porosity, water absorption and ion exchange capacity of the porous Nafion membrane increased with the decrease of pore size[11]. Tiwari et al. reported a polyethylene terephthalate (PET) temperature-responsive porous membrane prepared using a 248 nm KrF excimer laser[12]. PET films can be selectively laser-etched to obtain porous films with highly ordered, regular, and well-defined pore sizes. Polyvinylidene fluoride (β-PVDF) nanoporous membranes were prepared by Cuscito et al. Using heavy ion irradiation and trace etching techniques. The pore diameters were measured to be in the range of 20-50 nm by scanning electron microscopy and small-angle neutron scattering[102]. Mao et al. prepared a polymethyl methacrylate (PMMA)/chlorinated polyethylene (CPE) blend surface porous film by cyclohexane/toluene/acetone (90/10/10) mixed solvent impregnation method, selectively etched the outer polymer component, and studied its cooling and anti-icing performance (Fig. 12)[103]. After etching, it was found that the thickness and pore size of the porous layer increased with the increase of CPE content.
Many ultrahigh-performance micro-and nano-porous polymer films have been fabricated by etching techniques, and the number and pore size of the pores obtained by etching are two almost independent parameters that can vary in a wide range: pore size from 10 nm to tens of microns, and pore density from 1~1010 to /cm2[47]. Although the advantages of etching technology are obvious, the difficulty of etching process, the poor reproducibility of thin film material properties and the high cost of the final product are still the challenges of this technology.
2.5 Other methods
In addition to the methods described above, new hole making methods are constantly being invented and applied, such as 3D printing, melt extrusion stretching, biodegradation and so on. Duan et al. prepared 3D porous polydimethylsiloxane (O-PDMS) by using 3D printing technology, and then integrated it with carbon nanotube and graphene conductive network to form high ductility conductor (OPCG), proving that OPCG with stacking fault structure has high conductivity and can withstand large strain and repeated deformation, showing great potential in the application of next generation scalable electronic products[104]. Sadeghi et al. Selected five different polypropylene resins to prepare microporous membranes by melt extrusion and stretching. The preparation process of microporous membranes includes the preparation of precursor membranes, annealing, cold and hot stretching and other stages[105]. Li et al. Prepared two-dimensional ordered porous polymer films from polymers in vacuum by using the breath-figure method[106]. This technology can change the pore size by controlling the air humidity, vacuum degree and polymer concentration. Zhang et al. Prepared porous polymer membranes by microwave-assisted reaction of citric acid and sodium bicarbonate, and studied the effects of reaction conditions such as temperature and time on the pore structure of porous polymer membranes, and found that the shorter the reaction time at higher temperature, the higher the porosity of the membranes[107]. Guo et al. Successfully synthesized microporous poly (vinyl alcohol)/starch composite polymer membranes by biodegradation method, and the micropore diameter was 1 ~ 10 μm[108].
In addition, membranes with uniform and straight nanopores have important applications in different fields, but their applications are limited due to the lack of highly controllable preparation methods[109]. Most of the methods for generating uniform pores must selectively remove the sacrificial components in the precursor material to generate pores, and the incorporation and subsequent removal of the sacrificial components often make the pore-forming process complicated and inefficient, and bring greater uncertainty to the control of pore uniformity. Wang et al. Used the selective swelling of block copolymers to generate ordered nanopores non-destructively, which provided an idea for the preparation of uniform porous membranes[110]. Block copolymer systems with ordered morphologies can be precisely converted into voids without loss of order, homogeneity, and overall stability. In addition, the liquid crystal polymer system is also of great significance for accurately controlling the pore diameter and producing a uniform pore film. Gin et al. Suggested that Room-temperature ionic liquids (RTILs) with different morphologies could be used to design separation membranes[111]. Majewski et al. Suggested that the self-assembly of diblock copolymers can form mechanically robust films with precise and adjustable periodic alignment by two physically separated but chemically linked polymer materials[112]. Henmi et al. Constructed a membrane with ordered three-dimensional ion nanochannels by in-situ photopolymerization of thermotropic liquid crystalline monomers, which has high filtration performance and ion selectivity[113]. Self-organized nanostructured membranes have great potential for supplying high-quality water.
In a word, in the process of designing porous membranes, it is necessary to design porous membranes with different pore size, pore density and pore morphology according to different purposes. Moreover, each method has its advantages and limitations, and different methods need to be selected for different purposes[114]. Electrospinning, for example, can be prepared in large areas to produce membranes with high mechanical strength. However, it is often difficult to obtain ordered and uniform pores by this technique. Self-assembly of block copolymers can provide films with well-controlled pore size, but it is still a challenge to produce large pore size and large area porous films in microscale. Track etching, on the other hand, allows finer control of pore shape, pore size, and porosity, but requires high equipment. 3D printing is also a promising technology because it can be used to produce complex geometries with high reproducibility, but its accuracy still needs to be improved to achieve nanoscale structural features, and its manufacturing cost is relatively high. Therefore, it is essential to understand the advantages and disadvantages of each technology and choose the right one for different purposes. For example, for tissue engineering, mechanical strength is one of the key factors, therefore, electrospinning is the most commonly used method. Membrane separation needs to control the pore size and pore density, so it needs a method that can control the pore size and pore density of the membrane, such as template method. In order to improve the performance of porous polymer membranes in the future, for example, some porous polymer membranes have the problems of poor stability at high temperature or in organic solvents, poor conductivity and low electrocatalytic activity, it is necessary to integrate other technologies, dope other materials or explore new polymer materials with specific properties[3,5,14].
3 Characterization of Structure and Properties of Micro-nano Porous Polymer Film
3.1 Structure Characterization
For the micro-nano porous polymer films, the degree of polymerization and the presence of functional groups can be characterized by FTIR and UV-vis spectroscopy. In addition, gas adsorption-desorption and thermogravimetric analysis can quantify the pore size distribution and total porosity of the polymer. Solid-state NMR and Raman spectroscopy can be used to analyze the structure of polymers. In addition to traditional combustion analysis, other element-sensitive analytical techniques, such as ICP-OES and ICP-MAS, can be used when the material contains elements other than C, H, N, O.
As one of the important properties of porous materials, there are many methods to determine the relevant data. Liu et al. Introduced the common methods to determine the pore size and its distribution of porous materials.Including direct section observation method (microscopic method), bubble method, penetration method, mercury intrusion method, gas adsorption method, liquid-liquid method, suspension filtration method, X-ray small angle scattering method and so on, which can be selected according to the specific situation. Several commonly used pore characterization methods are introduced as follows:[115]
Microscopic method: In order to observe the general distribution, shape and size of the pores in the polymer film, and obtain more comprehensive information about the pore structure,The micro-morphology of micro-and nano-porous polymer films can be analyzed by environmental scanning electron microscopy (ESEM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning transmission X-ray microscopy (STXM), and scanning probe microscopy (SPM)[116]. Although the actual analysis is non-destructive, sample preparation often requires operations such as cutting of the material, which can cause damage to the film. In addition, the resolution of the scanning electron microscope is usually 5 to 10 nm, so porous membranes with pore sizes less than 5 nm need to be observed by a transmission electron microscope. If the material is difficult to process, an environmental scanning electron microscope is required. Since ESEM allows the introduction of a gaseous environment in the specimen chamber, it is possible to examine the surface of almost any specimen, wet or dry, insulating or conducting[117]. In addition, the limited magnification, the difficulty of observing the nanopore, the focal length and the depth of field affect the actual size of the observed object, which will also affect the actual observation results.
Bubble method: The bubble method is mainly based on the capillary principle to determine the maximum pore size of porous materials[115]. The measurement method is to soak the sample with a liquid with good wettability to saturate the pores, and then blow the liquid immersed in the pores out of the pores of the sample with another liquid (usually compressed gas). When the gas pressure gradually increases from small to a certain value, the gas can push away the bubble from the pore (regarded as a capillary), and the pore diameter of the porous material can be obtained by measuring the pressure difference of the first bubble using the Laplace equation (where represents the pressure difference inside and outside the liquid surface, represents the surface tension coefficient, and R1 and R2 represent the principal curvature radius of the liquid surface):
Mercury porosimetry: also known as mercury porosity method[118]. It can be used to determine the pore size and volume of various porous polymer films. However, this method is more difficult to detect for pores larger than a few hundred microns. At the same time, the metallic mercury does not have good wettability for the solid surface. Only when the mercury is under a certain pressure, they will be squeezed into the porous material. The smaller the pore, the greater the pressure it needs to withstand. Therefore, mercury porosimetry is widely used to calculate the pore size distribution of mesoporous or macroporous materials. Assuming that a porous material is mainly composed of simple capillaries of different sizes and sizes, according to the principle of liquid lifting in capillaries, the relationship between the pressure of mercury and the radius of capillaries is as follows:
Where, R is the radius of the capillary (nm); σ is the surface tension of mercury; Theta represents the contact angle of the measured porous material with mercury, which varies from 135 deg to 142 deg, and P represents the pressure (N/m2) of the mercury.
BET specific surface area test method: BET test method (Brunauer-Emmet-Teller method) is widely used in the analysis and research of specific surface area and adsorption performance of different substances. It can be said that it is the most widely used test method with the most reliable test measurement results. Almost all relevant standards at home and abroad are established based on the BET equation[119]. This technique can accurately determine the total internal surface area of the material, and even very small pores at the atomic level can be detected. However, the sample must be heated and vacuumed to remove the water and gas pollutants adsorbed in the sample before adsorption determination. Generally, the minimum drying temperature of the sample is 105 ℃. In addition, the sample quantity and pretreatment, the airtightness of the system, the flow rate of the carrier gas and the equilibrium time all have a certain influence on the results.
3.2 Performance characterization
3.2.1 Water vapor permeability test
In the GB/T 12704 standard proposed by China Textile Industry Association, there are three parameters that can be used to describe the moisture permeability of materials: moisture permeability (Water vapor transmission, WVT), moisture permeability (Water vapor permeance, WVP) and moisture permeability coefficient (Water vapor permeability, PV).
At the same time, according to the definition of moisture resistance, the moisture resistance value related to the tested material can be calculated by dividing the humidity difference between the water surface in the moisture permeable cup and the surrounding environment by the actual measured moisture permeability, taking into account the area factor:[120]
Where Re is the moisture resistance of the material, the unit Pa·m2/W;Psf is the saturated water vapor pressure of the water surface in the cup, Paf is the saturated water vapor pressure of the environment, and the unit Pa;RHs is the relative humidity of the water surface in the cup (RHs=100%),RHa is the relative humidity of the environment; The Wf is the moisture permeability when the material is placed, and the unit g/(m2·h);W0 is the moisture permeability when the material is not placed and the water level is at the position of the test material; E is the heat of vaporization of water.
The test methods of moisture permeability of materials are divided into desiccant method and evaporation method. A moisture-permeable cup filled with a desiccant and sealed with a fabric sample is placed in a sealed environment at a specified temperature and humidity, and the moisture permeability coefficient of the sample can be calculated according to the mass change of the moisture-permeable cup in a certain period of time. The evaporation method is divided into a positive cup method and a negative cup method. During the measurement, the moisture permeable cup filled with a certain amount of distilled water and sealed with the sample is placed in a sealed environment with specified temperature and humidity (the positive cup method is placed upright, and the inverted cup method is placed upside down), and the moisture permeability rate, moisture permeability and moisture permeability coefficient of the sample can be calculated according to the mass change of the moisture permeable cup within a certain period of time.
3.2.2 Ion Permeability Test
The pure water flux and the salt rejection of the micro-nano porous polymer membrane can be tested by a laboratory-scale Cross-flow membrane filtration device at a constant pressure of about 25 deg C[121]. The flux of pure water is determined by measuring the volume of permeate water collected over a period of time, as shown in Equation (1):
Where J(Lm-2·h-1) is the water flux, ΔV(L) is the volume of permeate water collected at Δt(h), and α(m2) is the effective membrane area. With deionized water as the feed liquid, the pure water permeability coefficient (A) can be calculated from equation (2):
Where A (m/s Pa) is the permeability coefficient, JV(Lm-2·h-1) is the water flux, and ΔP(Pa) is the water pressure difference across the membrane. 5 mmol/L Na2SO4 or 7. 5 mmol/L NaCl solution was used as the sample solution to determine the salt rejection rate of the porous polymer membrane. The membrane rejection R is given by equation (3):
Where Cp(mol/L) and Cf(mol/L) are the permeate concentration and the injection concentration, respectively, determined by the conductivity meter. The salt permeability coefficient (B) is an intrinsic measure of the ability of a membrane to retain salt, which, according to the classical solution-diffusion theory, can be derived from equation (4):[122]
Where, B (m/s) is the salt permeability coefficient, and Δπ(Pa) is the osmotic pressure difference across the membrane.
4 Application of Micro-nano Porous Polymer Film
Porous polymer films have a wide range of applications in food processing, energy, medical and environmental fields[123⇓⇓⇓⇓~128][34,37,129⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓ ~143][144,145][16]. Osmotic membranes can control the rate of permeation of chemicals through the membrane, and in controlled drug delivery, osmotic membranes can be used for drug release, regulating the rate of drug from the reservoir to the body. In separation applications, a permeable membrane may allow one component of a mixture to freely permeate through the membrane while preventing permeation of other components. Because of these properties, polymer permeable membranes are widely used in gas separation (e.g., air dehydration, O2/N2 separation), water purification (e.g., desalination, ultrapure water production, drinking water treatment, urban and industrial wastewater treatment and reuse)[2,146,147]; Biological treatment (e.g., aseptic filtration, protein concentration, and buffer exchange)[148,149]; Medical applications (e.g., dialysis, blood oxygenation, and drug delivery)[150]; Food processing (such as clarification of beer and wine, softening of fruit juices and sugar)[151,152]; Production of chemicals (e.g. Chlorine and sodium hydroxide from chlor-alkali processes)[152]; Batteries and fuel cells[153]. Potential applications include energy generation[154,155]; Energy storage; Environmental applications such as carbon capture and selective removal of ions that contribute to eutrophication of surface waters (e.g. Nitrates and phosphates)[156][157]; Recovering the organic solvent[158]; Drug purification[159]; Catalyst recovery[159]; Membrane crystallization, distillation, emulsification, etc[160][160][161]. The following is a summary and prospect of several application fields of micro-nano porous polymer permeable membranes.
4.1 Gas separation
Gas separation (GS) is a pressure-driven process with diverse industrial applications. Since 1980, serial production of industrial polymer membranes has been implemented, and the GS membrane has rapidly become a competitive separation technology (Figure 13A). Unlike conventional separation plant operations, the membrane GS equipment is simple and particularly suitable for use in remote areas. In general, the GS properties of a membrane depend on: the material (permeability, separation factor), the membrane structure and thickness (permeability), the membrane morphology (e.g., flat, hollow fiber) and the functional design[162].
For gas separation, the permselectivity of the membrane material determines the efficiency of the separation process[163⇓~165]. The porous membrane is a rigid, highly porous structure with randomly distributed interconnected pores, and its gas separation is mainly the result of the combined influence of the permeability of the porous polymer membrane and the characteristics of the membrane, such as the molecular size, pore size and pore distribution of the membrane polymer (Fig. 13b)[166]. Usually, only molecules with large size differences can be effectively separated by microporous membranes. Porous membranes for gas separation show high flux levels but inherently low selectivity.In gas separation, the pore size of micro-nano porous polymer membranes must be smaller than the mean free path λ of gas molecules (about 50 nm under normal conditions). The gas flux through the aperture is proportional to the velocity of the molecule, i.e., inversely proportional to the square root of the molecular mass. Most gas separation processes using polymer membranes require the selective membrane layer to be thin, with a typical membrane thickness of less than 0.5 μm, often less than 0.1 μm.
Membrane processes for gas separation have been developed rapidly in the past few years and are competing in the market with other gas separation technologies such as cryogenic distillation. Li et al. prepared high-performance polyacrylonitrile-polydimethylsiloxane (PAN-PDMS) hollow fiber composite membranes for the separation of CO2/N2 and O2/N2 gases, and found that the inherent viscosity of PDMS could be significantly improved by partially crosslinking PDMS, and the pore size and pore size distribution of PAN hollow fibers could be adjusted by adjusting the spinning conditions and post-treatment process[167]. In addition, poly (ethylene oxide) (PEO) membranes have attracted much attention due to their high solubility for CO2 and high selectivity for gases such as CH4 and N2. Bandehali et al. Discussed the mechanism of improving the gas separation performance of PEO-supported membranes and found that the synthesis of PEO-grafted copolymers, the crosslinking of polymers and the blending with long-chain polymers played an important role in the improvement of membranes.And the incorporation of ionic liquids/nanoparticles is also effective to improve the gas separation performance of membranes. In addition, the introduction of functional groups in PEO-based polymers can also improve the permeability of CO2[168].
4.2 Environmental Engineering
Today, with the intensive development of science and technology and industry, environmental pollution has become a prominent global problem. Industrial, agricultural and domestic wastewater contains organic dyes, heavy metal ions, detergents, oils, pharmaceuticals, herbicides, flame retardants and other toxic pollutants, resulting in global water shortage and serious harm to human health[169,170]. Restraining climate deterioration and restoring the ecological environment is a multifaceted challenge, and the cornerstone of achieving green and sustainable development will be systematic treatment and innovation. As a result, the need to explore practical and economical ways and means of dealing with the environment is exploding[16]. Porous polymers have attracted much attention due to their excellent pore structure and tunable surface chemistry. Porous polymer membrane is an important adsorption and separation material in environmental treatment[171,172].
Common porous polymer membranes for water treatment can be classified into Microfiltration (MF), Ultrafiltration (UF), Nanofiltration (NF) and Reverse osmosis (RO) according to the size of solute retained in the process of water treatment (Figure 14). The molecular weight cutoff (i.e., the solute size at which 90% of the species are rejected) for UF membranes is between 5 and 500 kDa. Typical molecular weight cutoffs for NF membranes are between 100 and 300 Da. The RO membrane is designed for desalination and can remove almost all ions except uncharged solutes with molecular weight greater than 100 Da.
Fan et al. prepared a composite nanofiltration membrane by interfacial polymerization of tetraethylenepentamine (TEPA) and 1,3,5-benzenetricarbonyl chloride (TMC)[174]. The performance of the composite nanofiltration membrane was improved by adding calcium chloride in the aqueous phase during the inter-facial polymerization. With the increase of CaCl2, the pure water flux of the TEPA/TMC nanofiltration membrane was greatly improved from 18.2 L/(m2·h) to 51.3 L/(m2·h) at a pressure of 0.2 MPa. Kong et al. prepared partially sulfonated bacterial cellulose (SBC) nanofibers using bacterial cellulose as raw material through the redox reaction of NaIO4 and NaHSO3[175]. A composite nanofiltration membrane with high rejection (> 96%) and high separation flux (>320 L/(m2·h·MPa) was obtained by replacing the traditional ultrafiltration membrane with the SBC nanofiber porous membrane and regulating the interfacial polymerization reaction.
In recent years, many porous polymer separation membranes have been developed for the separation of water-soluble organic pollutants. For example, cyclodextrin is a common component of a separation membrane, and Villalobos et al. formed an asymmetric membrane on a polyacrylonitrile (PAN) porous support by polymerization of hydroxyterephthaloyl chloride (TC) and cyclodextrin[176]. The presence of hydrophobic (cavity) and hydrophilic (ester-crosslinked outer portion) regions in these films imparts high permeability to both polar and non-polar solvents. Other classes of macrocyclic compounds, such as sulfonylcalix [4] arenes and columnar arenes, have also been incorporated into mesoporous polymer membranes via interfacial polymerization[177,178]. At the same time, different kinds of cationic dyes can also be effectively removed, because cyclic aromatic hydrocarbons often have selective binding ability to them, and the penetration of negatively charged molecules is not hindered. In addition, porous polymer films are used for heavy metal ion removal, radionuclide extraction, and adsorption of other contaminants[179][180,181][182,183].
On the other hand, a large amount of oily wastewater is discharged in many industrial processes and daily life, so the separation of oily wastewater, especially emulsified oil/water mixture, is a global challenge. For the treatment of oily wastewater, membrane separation technology is considered to be the most effective method because of its high separation efficiency and relatively simple operation process. The main method of its action is to design the oil-water separation process by using the super wettability of the solid surface (that is, super hydrophobic-super oleophilic or super hydrophilic-super oleophobic) according to the difference of the interfacial tension between oil and water. The wettability of a solid surface is determined by its surface free energy and surface geometry[184]. Tao et al. Prepared a superamphiphilic polyvinylidene fluoride (PVDF) membrane, which has the characteristics of superoleophobicity under water and superhydrophobicity under oil[185]. When the membrane encounters an oil-in-water emulsion, its superhydrophilicity and underwater superoleophobicity cause the water to immediately permeate through the membrane while the oil remains on the membrane. On the contrary, during the separation of water-in-oil emulsion, its superlipophilicity and superhydrophobicity under oil ensure the penetration of oil and the effective interception of water droplets. Due to the switchable transport properties of the membrane, it is suitable for the separation of various water-in-oil and oil-in-water emulsions with particle sizes larger than 20 nm.
4.3 Field of energy
At present, energy security has gradually developed into a major key issue affecting human economy, society, production and life. As we all know, traditional fossil energy is declining and gradually exhausted, and the ecological environment is also damaged by harmful substances produced in the process of using fossil energy. In 1954, Pattle discovered that when sea water meets rivers, it produces tremendous energy[186]. This kind of renewable energy, which is stored in the ocean and uses the confluence of fresh and salt water to generate electricity, is called blue energy, also known as ocean energy. Recently, Jiang et al. developed an ultrathin Janus membrane (Fig. 15) for mixed power generation of natural seawater and river water by phase separation of two block copolymers inspired by electric eel, and confirmed that the heterogeneous membrane with unidirectional ion transport characteristics can be used as an effective material for power generation by salt difference[187]. In addition, porous polymer membranes are also commonly used in seawater desalination. For example, Kong et al. Proposed a new idea of synthesizing polyamide nanofiltration membranes for seawater desalination with ultrafiltration polysulfone as the carrier[188]. Polyamide membranes with controllably dense thin layers and effective "nanopores" were prepared by adding a cosolvent (acetone) to a nonpolar organic phase (n-hexane). The results showed that the addition of more cosolvent to the n-hexane solution resulted in relatively larger pore size and higher water flux.
In addition, with the growing demand for advanced energy storage devices in the fields of portable electronic products, hybrid electric vehicles and power grids, the research and development of lithium-ion batteries (LIBs) with excellent energy density and cycle life is also imminent. As the main component of the battery, the battery separator determines the electrolyte retention, internal resistance and interface processes involved in the battery, and directly affects the safety and electrochemical performance of the battery[189].
Porous polymer films are often used as battery separators because of their low cost, suitable mechanical strength and pore structure, and good chemical stability. The most important battery separators in commercial LIBs include polyolefin membranes such as porous polyethylene (PE) and polypropylene (PP). Yamaguchi et al. prepared a membrane composed of poly (vinylsulfonic acid/acrylic acid) crosslinked gel on a porous polytetrafluoroethylene (PTFE) substrate, which had high proton conductivity and good thermal stability at 130 ° C[190]. Huai et al. Prepared polymer/ceramic phase separation porous membrane (PCPSPM) with special structure by gas phase method using polyvinyl alcohol-polyacrylonitrile (PVA-PAN) emulsion as raw material[191]. In such a porous membrane, the ceramic phase (nano SiO2 particles) is separated from the polymer matrix and dispersed in the pores. Aft that porous membrane is immerse in the electrolyte, the nano SiO2 particles are mixed with the electrolyte to form colloidal electrolyte in the pores, so that the diphasic electrolyte membrane is for, and the conductivity is improved.
4.4 Bioengineering
With the development of biomedical engineering technology, nanoporous materials, especially micro-and nano-porous polymer films, are of great importance for many biomedical applications, such as immunoseparation devices, dialysis, targeted drug delivery systems, Bioanalysis devices, biosensors and human tissue engineering[192,193]. Among them, human tissue engineering is a discipline aimed at replacing, regenerating, maintaining or improving tissue function. Membrane-like functional materials are often used in various bioengineering. In order to adapt to the environment of various microorganisms and human tissues, the membrane materials used should be biodegradable, non-toxic and biocompatible.It does not cause any immune response to the human body, and it also needs to have large specific surface area, high porosity, pore connectivity and sufficient mechanical strength.
In addition, micro-nano porous polymer films are often used in medical dressings. Researchers are constantly developing a variety of drug dressings with different characteristics and properties for clinical nurses to choose, and the new technology of drug dressings for wound treatment has been revolutionized compared with the early stage. Kim et al. proposed that the poly (lactic-co-caprolactone) (PLCL) membrane prepared by electrospinning has porous structure and biodegradability, and has a wide application prospect in the medical field[194]. Shi et al. electrospun a hydrophobic polyurethane (PU) nanofiber array onto a hydrophilic microfiber network to construct an independent self-pumping dressing (Fig. 16A)[17]. This self-pumping dressing is capable of unidirectionally draining excess tissue fluid from its hydrophobic side to its hydrophilic side, thereby preventing the tissue fluid from wetting the wound. Chen et al. prepared carboxymethyl chitosan grafted polyacrylic acid (CMCTS-g-PAA) superabsorbent polymer (Fig. 16b) by ethanol precipitation method based on the characteristics that the hydrogel would form porous and corrugated structures due to shrinkage stress during the precipitation process. During the precipitation process, the hydrogel formed porous and corrugated structures under the action of shrinkage stress[195]. The results show that the super absorbent polymer, especially the modified porous polymer, has good biocompatibility. CMCTS-g-PAA has excellent hemostatic performance due to the synergistic effect of N of chitosan, high swelling ability of porous structure and rough surface.
5 Conclusion and prospect
Micro-nano porous polymer film is a new type of film which combines the dual characteristics of porous composite structure and polymer, and plays an important role in many fields. In this paper, the preparation methods of micro-nano porous polymer films are summarized, including template method (hard template and soft template), electrospinning, phase separation method, etching method and other methods. At the same time, the characterization and test methods of porous structure and the related properties of permeable membrane were described. Finally, the applications of porous membranes in separation, energy, environment and bioengineering were summarized. In addition, current technology faces many challenges:
1。 Although researchers have explored a variety of pore-forming methods, each method has its own conditions and scope of application, such as inadequate preparation methods, complex equipment, limited scale and so on, which are urgent problems to be solved, and so far no pore-forming method has been found that can be applied to all materials. For example, the phase separation method can be used to prepare porous polyvinylidene fluoride (PVDF) for lithium battery separators, but it is not effective for other materials (such as silicone rubber).
2。 How to prepare through holes in thick polymer films by chemical or physical methods is also an unsolved problem. Nowadays, most of the holes are foam holes, and the method of preparing through holes is slightly inadequate.
3。 There are many ways to characterize the structure of materials, but the way to characterize the performance is slightly inadequate. At the same time, from the perspective of mechanism, it is of great significance to study the basic and systematic effects of surface, interface and internal structural parameters of porous polymer functional films on the performance of porous polymer films, but this work needs to be carried out in depth.
4。 Micro-nano porous polymer films have shown great potential in bioengineering, environmental treatment, seawater desalination, energy utilization and other fields, but the transformation of these laboratory results into practical applications still needs further exploration and improvement.
To sum up, the unique properties of micro-nano porous polymer films have shown advantages and application potential in many industries. How to explore a universal method to prepare micro-nano porous polymer films and apply them in actual production and life still needs the efforts of scientific researchers.
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