1 Introduction Fabrication of Water Treatment Membrane Using Templating Method A Critical Review Fabrication of Water Treatment Membrane Using Templating Method A Critical Review ABSTRACT The progress in the use of templating method for the controlled synthesis of nano, meso and macro porous materials opens many new application areas, one of which is the water treatment membrane. This paper presents a critical review on the fabrication of water treatment membrane using the templating method. Three templating methods are investigated including the block copolymer templating method, the polystyrene beads templating method, and the particles templating method. The future research directions are also discussed. KEYWORDS Template; Porous Membrane; Block Copolymer; Polystyrene Bead Providing sustainable supply of clean water is one of the greatest challenges in the 21 st century. Among the various water treatment technologies, the membrane technology is the most promising technology. It is well known that a membrane is a porous material which serves as a selective barrier for fluids. The obvious advantages of the membrane technology as compared to other methods include: As a result, many commercial products have been developed and it is expected that this technology will dominate the market in the coming years. Figure 1 gives the chemical and physical properties of the base materials generally used for membrane fabrication including: better performance in chemical resistance and physical strength, which are very important to the membrane stability in long-term usage of water treatment. As a 55 materials exhibit poor hydrophilicity, which is of great importance in the application. Accordingly, a hydrophilicity, generally evaluated by the contact angle between water and membrane surface. Fig. 1: Mechanical properties (tensile strength and hardness), hydrophilicity (evaluated by the water contact angle) and chemical resistance (including halogen, oxidant, ester, ketone, aliphatic solvent, aromatic solvent, base and acid) of frequently used membrane materials. The numbers represent the performance: 1 extremely poor; 2 d poor; 3 limited; 4 good; 5 extremely good. Beside the base material, the selectivity, permeability and trans-membrane pressure of the membrane are the key parameters for the performance of the membrane. These parameters are mainly determined by the pore structure membranes with desirable performance cannot be monomers is indispensable. In recent years there has been considerable progress in the use of templating method for the controlled synthesis of nano, meso and macro-porous materials.
Vol. 6 No.1/ Feb. 2012 three steps: First, a template with a specific structure Finally, the starting template is removed leaving behind a replica of the original template. The development of the fabrication of the membrane with controllable pore Templates can be inorganic, organic, or biological materials. Figure 2 gives an overview of commonly [1]. They cover a length of scales ranging from sub-nanometer to several hundred micrometers. Templates can be formed kinetics. They are widely used for producing nanotubes as well as micelles, micro emulsions and latex particles. There have been several review articles [1]-[3] on the fabrication of composite membrane. This review paper focuses on the fabrication of water treatment membrane using the templating method. Fig. 2: Templates of different scale [1]. 2 O. Though, in the nature, there exists hardly any single water molecule. The smallest body of water is about three or four water In order to separate the water from impurities, the also proper material properties. Presently, there have been a number of successful commercial water treatment membrane remains as a goal. This paper presents a critical review on various templating methods that can be used for making water conclusions and future work. 2 Block Copolymers Template for Meso- and Macroporous Membrane enerally, block copolymer template could be in the form of micelle, lyotropic phase and bulk phase depending on the concentration of block copolymer in the solution. Block copolymer micelles have a core consisting of the insoluble A-blocks and a shell or corona of the soluble B-blocks. For example, the AB-block copolymers composed of polybutadien-b- with aggregation number of 8 are shown in Figure 3. The shape of micelles can be controlled by controlling the block lengths. By increasing the lengths of the micelles, Increasing the concentrations block copolymers tends materials. For templating purposes it is possible to control the morphology and the periodic length via block lengths and polymer concentration. block copolymers shown in Figure 6. They can be in the form of spherical and cylindrical micelles as well as vesicles form in dilute solution. The morphology of these lengths and polymer concentration. Thus it is possible to prepare tailor-made nanostructured templates of certain templates for the preparation of mesoporous inorganic of the pores are predetermined by the hydrophobic Fig. 3: AB-block copolymers composed of Polybutadienb-polyethylenoxid and the self-organization into a micelle with aggregation number of 8. 56
Fabrication of Water Treatment Membrane Using Templating Method A Critical Review Fig. 4: A and B, Electron micrographs; C, optical micrographs of the three most frequent forms of selforganization of block copolymers. A spherical micelles (PS-PI/DMF) [4], B cylindrical micelles (PB-PEO/water) [5], C vesicles (P2VP-PEO/water). C (above right) shows was encapsulated. Fig. 5: Electron micrographs of a: (A) cubic; (B) hexagonal; and (C) lamellar superlattice of block copolymers in lyotropic liquid-crystalline phases. The structure of the lyotropic phases was fixed by g-irradiation [5] Fig. 6: Self-organization structures of block copolymers and surfactants: spherical micelles, cylindrical micelles, vesicles, FCC- and BCC-packed spheres (FCC, BCC), hexagonally packed cylinders (HEX), various minimal surfaces (gyroid, F-surface, P-surface), simple lamellae (LAM), as well as modulated and perforated lamellae (MLAM, PLAM) [5] templating domains in the lyotropic phase.the first were produced by Mobil Co [6], [7]. They are amorphous According to the structure of the lyotropic phases, porous cubic, hexagonal, and lamellar structures can be prepared [8], [9]. By using the sol/gel process it is possible [22], 2, TiO [23] 2, NiO, ZrO 2, 57 Al 2 O 3, Nb 2 O, Ta 2 O,WO 3 2 2, as well as mixed, ZrTiO, Al2TiO, and ZrW 2 O [23] 8. The typical synthesis procedures for the oxides stated above include: dissolved in a solution and the respective inorganic chloride precursor was added with stirring. 2. The resulting sol solution was gelled in a controlled gelled temperature and time, during which the a metal oxide network. 3. Alternatively, the sol solution can be used to prepare porous inorganic material is obtained with the pore Besides the use of block copolymers as template for the formation of inorganic material, there are some researchs of regularly spaced nano channels in a glassy polymer. A film had been prepared by casting from a solution of a toluene, followed by slow solvent evaporation leading to a microphase separation into a bicontinuos gyroid morphology. The resulted thick films had then been the PI blocks. The channel diameters in the bicontinuous had been obtained by the same approach but using a. Fig. 7: Silicates with: (A) spherical pores of cubic order; (B) hexagonally ordered cylindrical pores; (C) lamellar pores, [5, 8] prepared from lyotropic phases of block copolymers et al [26] used a triblock copolymer polyisopren- ordered nano-channels by. The copolymer had been
mixed with the homopolymer C and membrane was casted from solutions in a common solvent. After drying crosslinking of the AB phase. Thereafter, the C had been extracted, and regular pore morphology had been confirmed the highly porous nature of the films, but the lack of water permeability suggested that the nanochannels might be discontinuous on a macroscopic et al. had found that polystyren-block-polylactide [27], [28] and ordered monoliths with connected and hydrophilic [29] with a low polydispersity. Alignement of the phase separated polymer was achieved using cooling from the melt in a channel die. Finally, the polylactide was hydrophilic polyacrylamide covering the pore surface. Another result of an organic nanoporous membrane. The phase separated gyroid morphology corresponds to middle block, which can change its conformation Another step towards a better orientation via a pore- introducing the melt of a microphase-separated polystyreneblock-polybutadiene into the pores of an anaphora membrane via capillary action [33]. The polymer, which forms cylindrical micro domains in the bulk, presents those cylindrical domains aligned parallel to the pore walls in the membrane. Mayes et al showed a promising example for such a transfer of microphase-separated morphologies of well-defined block copolymers into a real-world separation membrane. They had prepared composite electron microscopy and NF experiments suggested that hydrophilic nanochannels in a hydrophobic matrix as transmembrane barrier in the skin layer and a hydrogellike outer membrane surface had been obtained. 58 Vol. 6 No.1/ Feb. 2012 Fig. 8: Nanoporous membranes from phase separated polystyren-blockpolylactide (PSt-b-PL) copolymers with varied copolymer structure (molar mass in kg/mol, molar fraction of PL), after selective hydrolysis of the PL phase: (a) 32, 0.28 (DcylZ15 nm), (b) 58, 0.38 (DcylZ31 nm), (c) 92, 0.36 (DcylZ45 nm), (d) 40, 0.42 (DcylZ42 nm) the diameter of the cylinder, Dcyl, as determined by SAXS agrees well with the pore diameter found from SEM [28]. 3 Polystyrene Bead Templating Method Organic or inorganic membrane with macroporous structure is of great interest in separation process, [37], catlaysis and optical [38] applications. A number of methods have been developed for producing such kinds of membranes by block copolymers directing array of spherical pores [39] isolated from each other in the bulk. Methods based on replication take advantage of template structures that can be easily assembled from a variety of species, such as aggregates of surfactant molecules, air bubbles, solid particles, and bacteria. These methods are simple The key route of the replication method or templating method is the preparation of the replica which serves as template of this type since its simple and practical procedure for forming spherical particles with macroscale and meso-scale over relatively large areas. These as sacrificial templates to generate macroporous membranes of organic polymers or inorganic ceramics. dimension can be precisely controlled in the range from on both top and bottom surfaces of the membrane film
Fabrication of Water Treatment Membrane Using Templating Method A Critical Review and they are interconnected into a three-dimensional framework in the bulk. Park and Xia presented a typical procedure for the templae production and its application in porous beads, polycarbonate beads, or silica colloids. An and the beads were assembled into a ccp lattice under procedure for generaing membranes with highly ordered the precursor material, the beads were dissolved by immersing the sample in an appropriate etching solution. pores interconnected by circular "windows". The membrane fabricated using this procedure has a very diameter of the beads. resulted membrane film. The pores are fully open on top layer, the bottom layer, and the cross section of the the cross section at a higher magnification. As can be seen, the membrane is a truly three-dimensional one: the spherical pores in the bulk of the membrane are connected model of cubic close packing, each spherical pore should be connected to six other pores in the same plane and three other pores above and below the plane. The porosity the dimensional structure. Fig. 9: (A) Schematic procedure used to form crystalline assemblies from mesoscale spherical particles; (B) Schematic illustration of the procedure that uses crystalline assemblies of polystyrene beads as templates to fabricate highly ordered 3D membrane films (crosssectional view). Fig. 11: (A), (B) Top-view SEM images of a polyurethane membrane that was fabricated with a ccp lattice of 0.48 mm polystyrene beads as the template. (C), (D) Crosssectional SEM images of the same membrane [48]. polyurethane membrane that was fabricated from a nm, the procedure can be extended to the fabrication of Fig. 10: SEM images of a composite film consisting of cured polyurethane and a crystalline assembly of 0.48 with a thin layer of Au before taking SEM images. Each bright spot corresponds to the bump formed by an underlying polystyrene bead [48]. images of a polyurethane membrane film before the polystyrene beads are dissolved. The crystalline assembly 59 Fig.12. SEM images ((A) top view; (B) cross-sectional view) of a polyurethane membrane that was fabricated with a CCP lattice of 0.23 mm polystyrene beads as the template. As indicated by an arrow, each spherical cage is connected to adjacent neighbors through circular windows [48].
Vol. 6 No.1/ Feb. 2012 mesoporous membranes in which the pores are smaller procedure for the membrane fabrication may lack the 4 Silicon Particles Templating Method ilicon particles are widely available and can be used as template for the production of membrane. Feng and Werner presented a scheme as shown in Figure a volatile solvent are mixed and then applied to water surface. The thickness of particle layers is determined by the amount of colloids. The organic monomer in the voids of the silicon template is cross-linked by the irradiating with light. The silica template is then removed and the polymer membrane is formed. membranes with uniform holes. The experimental results indicated that the two-dimensional ordered arrays of pores as well as the small membrane thickness and Fig. 14: SEM of membranes prepared with 334 nm silica particles as templates: (A) before the removal of the silica particles; (B) after the removal of the silica particles; (C) SEM image (A); and (D) SEM image of (B) [49]. Fig. 13: Scheme of the preparation of porous membranes at an air/water surface [49] templating method microcopy of a layer composed of cross linked polymer the silica colloids produced a polymer membrane with bottom and the top surface of the membrane show the pore with hexagonally ordered structure and the pores are connected to each other through small circular hole. According to reference hybrid monolayers composed of hydrophobic colloids and surface active polymers can be generated using the silica colloid as sacrificed agent, and then removing the colloids to generate thin 60 Fig. 15: Scanning electron micrographs (SEM) of (a) a silicacolloidal monolayer deposited on a mica surface; (c) Free-standing porous membranes obtained by removing silica colloids by exposure to HF vapor [50]. 5 Conclusions and Future Work Using templating method for producing porous membrane can effectively control the membrane which are the crucial factors for the water treatment membrane. Based on the literatures, both inorganic and organic materials can be used to fabricate and modify water treatment membrane. Among the three templating methods reviewed, the polystyrene bead templating method seems to be the most promising. scale applications, many practical problems remain unsolved. First, the production processes via templating In fact, few large scale of templating method has been practice in industry scale. One of our research goals is to develop an industrial scale templating method in the future.
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