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Preparation of Synthetic Membranes
Chapter III (1) Introduction, III-1-2 (2) Polymeric membranes, III.3-5
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Outline Introduction Phase inversion membranes
Preparation of synthetic membranes Sintering, Stretching, Track-etching, Template leaching, Phase inversion, etc. Phase inversion membranes Preparation techniques for immersion precipitation Phase separation in polymer systems Influence of various parameters on membrane morphology Preparation techniques for composite membranes
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III. 1-2 Introduction Porous membrane Dense membrane Carrier membranes
(microfiltration/ultrafiltration) (gas separation/pervaporation) Fig. III-1 Three basic types of membranes P71
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Three basic types of membranes
Porous membranes Separation depends on pore size and pore size distribution Nonporous membranes Separation depends on intrinsic properties of membranes Thickness of the membrane matters!!! Carrier mediated transport membranes Separation depends on affinity and reactivity of membranes Extremely high selectivity possible Two types: mobile carrier & fixed site carrier …………….. …………….. ……………..
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Membrane Material & Preparation
Polymers: most common Inorganic: more stable hybrid In principle all types of materials can be used as membranes. However, the selection of a type of material is dependent on the cost, on the separation task, the desired structure of the membrane and the operating conditions under which it has to perform. The most commonly used membrane materials are organic polymers. There are a large number of polymer materials available. Some of the advantages of polymers are flexibility, permeability and ability to be formed into a variety structures. On the other hand, polymers are generally not thermally stable, which can be a problem for many separation tasks. Symemetric / Asymmetric / composite Sintering Sintering Stretching Stretching Track-etching Track-etching Template leaching Template leaching Sol-gel process Sol-gel process Solution coating Solution coating Phase inversion Phase inversion Ref: Mulder, Basic principles of separation technology
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List of most important membrane preparation techniques
Sintering Stretching Track-etching Template leaching Sol-gel process Phase inversion technique Coating - Membrane modification
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Sintering Compressing a powder consisting of particles of given size and sintering at high temperatures. For both polymeric and inorganic membranes with outstanding chemical, thermal and mechanical stability Sintering temperature depends on the material (polymers, metals, ceramics, carbon, glass) Pore size & distribution depends on the particle size & distribution (0.1-10µm) Porosity 10-20% HEAT pore
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Stretching Stretch extruded film perpendicular to the extrusion & crystallite orientation Only semicrystalline polymers (PTFE, PE, PP) used Rapture to make reproducible microchannels Pore size 0.1-3µm Porosity is very high (up to 90%) Stretched PTFE membrane
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Extrusion of thermoplastic polymers
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Track-etching Thin membranes (up to 20µm) perpendicularly exposed to a high energy bean of radiation to break chemical bonds in the polymer The membrane is then etched in a bath which selectively attacks the damaged polymer. Features uniform cylindrical pores Pore size µm Surface porosity <10% Narrow pore size distribution Track-etched 0.4 µm PCTE membrane
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Track-etching process
radiation source polymer film etching bath Membrane with capillary pores t t t t3 energetic heavy ions Acid or basic solution
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Phase inversion Phase inversion covers different techniques
A polymer transformed in a controlled manner from a liquid to a solid state Phase inversion covers different techniques For example: preparing asymmetric membranes: A dense(r) skin layer integrally bonded in series with a thick porous substructure Same material in each layer 4. Phase inversion Chemical PI involves preparing a concentrated solution of a polymer in a solvent. The solution is spread into a thin film, then precipitated through the slow addition of a nonsolvent, usually water, a concentrated solution of a polymer in a solvent. precipitated through addition of a nonsolvent (immersion in) sometimes from the vapour phase. Thermal PI a solution of polymer in poor solvent is prepared at high temperatures. After being transformed into its final shape, a sudden drop in solution temperature causes the polymer to precipitate. The solvent is then washed out. a solution of polymer prepared at high temperatures. a sudden drop in solution temperature causes the polymer to precipitate. The solidification initiated by the transition from one liquid state into two liquids (liquid-liquid demixing) By controlling the initial stage of the phase transition the membrane morphology can be controlled.
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III.3 Phase inversion techniques
Precipitation by solvent evaporation Simple evaporation, coating Precipitation from the vapour phase Vapour phase: nonsolvent + saturatedsolvent Prepare porous without top layer Precipitation by controlled evaporation Polymer dissolved in mixture of solvent and nonsolvent Prepare membranes with skinned layer Thermal precipitation Polymer solution is cooled to enable phase separation Prepare membranes with skinned layer Immersion precipitation
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III.4 Immersion precipitation
Polymer solution cast on a support (/or not) Immersed in a coagulation bath containing a nonsolvent Precipitation (solidification) occurs because of the exchange of solvent and nonsolvent Membrane structure results from a combination of mass transfer and phase separation Asymmetric membranes obtained- most commercial membranes prepared by this technique
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1. Flat membranes Preparation parameters:
GKSS equipment Preparation parameters: Polymer concentration (viscosity) Casting thickness Evaporation time Humidity Temperature Additives (composition of the casting solution) Solvent/solvents & non-solvent Fig. III-5 Preparation of flat sheet membranes
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Factors affect membrane structure
Choice of polymer, choice of solvent/nonsolvent Composition of casting solution Composition of coagulation bath Gelation and crystallization behavior of the polymer Location of the liquid-liquid demixing gap Temperature of the casting solution and coagulation bath Evaporation time
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2. Tubular form membranes
Hollow fiber (d<0.5mm), self-support Capillary (d: 0.5-5mm), self-support Tubular (d>5mm), on support Techniques for preparation of HF and capillary membranes Dry-wet spinning (wet spinning) Melt spinning Dry spinning
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Dry-wet hollow fiber spinning process
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Spinning of hollow fiber membranes
Preparation parameters (dry-wet process) Extrusion rate of the polymer solution Flow rate of the bore fluid Tearing rate Residence time in air gap Dimensions and types of the spinneret Composition of polymer solution Composition and temperature of coagulation bath
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Membrane SEM images For the free fall (i.e., no elongational draw) hollow fibers, a clear double-layer structure of macrovoids can be observed (Fig. 31.8a). One layer near the outer skin edge is finger-like, whereas the other near the inner skin edge is teardrop-like. When doubling the takeup speed, the number of finger-like macrovoids decreases. When tripling the takeup speed, only single layers of finger-like macrovoids can be observed. However, the size and dimension of macrovoids become larger and longer and their total number is clearly reduced. Further increasing the takeup speed, Figure 31.8e shows that the finger-like macrovoids disappear at 53.5 m/min, which is about 6 times of the free fall speed. Here the elongational draw ratio, f, Compared to other methods to eliminate macrovoids, using high elongational draw ratios appears to have many advantages. Not only does it reduce fiber dimension but it also increases fiber production rate.is defined as the ratio of the cross-section area of the spinneret for dope flow to the solid cross-section area of the precipitated hollow-fiber membrane as follows: Fig 31.8 Hollow fiber membrane by phase inversion process, using high elongational draw ratios to elimiate macrovoids, reduce fiber dimension and increase fiber production Ref. TAI-SHUNG NEAL CHUNG, Chapter 31, FABRICATION OF HOLLOW-FIBER MEMBRANES BY PHASE INVERSION, in Advanced membrane technology and application.
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Lab-scale spinning rigs in Memfo
Ref.: Xuezhong He Blog
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Pilot scale spinning rig in Memfo
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Tubular membrane preparation
Fig.III-9 Tubular membrane preparation
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III.6 Phase separation in polymer system
General thermodynamic description of the phase separation Polymer-solvent-nonsolvent ternary system From stable homogeneous polymer solution to demixing Solvent and nonsolvent miscible If the solvent is removed from the mixture at the same rate as the nonsolvent enters, the composition of the mixture will change following the line A–B. If the solvent is removed from the mixture at about the same rate as the nonsolvent enters, the composition of the mixture will change following the line A–B. A quantitative treatment considering all thermodynamic and kinetic parameters involved in the phase-inversion process is difficult. However, a phenomenological description of the process with the aid of the phase diagram of a mixture consisting of a quantitative description of the kinetics is difficult. However, just a description of the phase separation based on the phase diagram of the polymer/solvent/nonsolvent system provides valuable information concerning the membrane structures obtained by the phase-inversion process. Binodal: coexistence curve Spinodal: limit of stability of a solution, small fluctuations in composition and density will lead to phase separation. Qualitatively, not quantitatively described! A, casting solution; B, membrane porosity; B’, polymer-lean phase; B’’, polymer-rich phase Ref. H Strathmann, L Giorno and E Drioli,1.05 Basic Aspects in Polymeric Membrane Preparation in book Comprehensive membrane science and technology
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Relationships among dope composition, precipitation kinetics, & membrane morphology
Delayed demixing - dense toplayer Instantaneous demixing – microporous toplayer In addition, the ratio (k) of solvent outflow to coagulant influx determines the macroscopic membrane porosity (Yilmaz and McHugh, 1986, 1988; Matsuura, 1994), as illustrated in Figure Depending on the initial dope composition and k value, the precipitation path may occur via nuclei growth or spinodal decomposition. Because k is not a constant across the membrane thickness and k is also a function of temperature and dope viscosity, it becomes difficult to microscopically predict local membrane porosity. Ref. TAI-SHUNG NEAL CHUNG, Chapter 31, FABRICATION OF HOLLOW-FIBER MEMBRANES BY PHASE INVERSION, in Advanced membrane technology and application.
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III.7 Influence of parameters on membrane morphology
Choice of solvent/nonsolvent system The polymer concentration The composition of the coagulation bath The composition of the polymer solution The use of additives The temperature of the polymer solution and of the coagulation bath
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Choice of solvent/nonsolvent system
Fig.III-44. Delay time of demixing for 15% cellulose acetate/sovent solution in water, P127 Delayed demixing - dense toplayer Instantaneous demixing – microporous toplayer Figure. Asymmetric membrane with a dense top layer (a) and a porous top layer (b), Ref. Braz. J. Chem. Eng. vol.28 no.3 São Paulo July/Sept. 2011
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Classification of solvent/nonsolvent
P129 In general High mutual affinity pairs – Instant demixing porous Low mutual affinity pairs – Delayed demixing nonporous
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Polymer concentration
Higher concentration results in lower top layer porosity, thicker top layer P130 P131 Fig.III-46 Calculated composition paths for the system CA/dioxan/water for varying CA concentrations in the casting solution
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Factors promotes the formation of porous membrane
Low polymer concentration High mutual affinity between solvent and nonsolvent Addition of nonsolvent to the polymer solution Vapour phase instead of coagulation bath Addition of a sencond polymer
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Formation of integrally skinned membranes
P135 Toplayer: thin & defect-free By delayed demixing Sublayer: open with negligible resistance By instantaneous demixing Generate a polymer concentration profile (as Fig III-51): By introducing an evaporation step before immersion Immersion in a nonsolvent with a low mutual affinity Fig.III-51 Volume fraction of polymer in the casting solution after a short period of time
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Formation of macrovoids
Porous sublayer - part of an asymmetric membrane Factors that favours the formation of porous membranes also favours the formation of macrovoids Instantaneous demixing A high affinity between the solvent-nonsolvent Polymer poor phase - macrovoids Weak spot for membranes for high pressures Ref.
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Effects of tear rate For the free fall (i.e., no elongational draw) hollow fibers, a clear double-layer structure of macrovoids can be observed (Fig. 31.8a). One layer near the outer skin edge is finger-like, whereas the other near the inner skin edge is teardrop-like. When doubling the takeup speed, the number of finger-like macrovoids decreases. When tripling the takeup speed, only single layers of finger-like macrovoids can be observed. However, the size and dimension of macrovoids become larger and longer and their total number is clearly reduced. Further increasing the takeup speed, Figure 31.8e shows that the finger-like macrovoids disappear at 53.5 m/min, which is about 6 times of the free fall speed. Here the elongational draw ratio, f, Compared to other methods to eliminate macrovoids, using high elongational draw ratios appears to have many advantages. Not only does it reduce fiber dimension but it also increases fiber production rate.is defined as the ratio of the cross-section area of the spinneret for dope flow to the solid cross-section area of the precipitated hollow-fiber membrane as follows: Fig 31.8 Hollow fiber membrane by phase inversion process, using high elongational draw ratios to eliminate macrovoids, reduce fiber dimension and increase fiber production Ref. TAI-SHUNG NEAL CHUNG, Chapter 31, FABRICATION OF HOLLOW-FIBER MEMBRANES BY PHASE INVERSION, in Advanced membrane technology and application.
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Coating To prepare composite membranes Dense top layer
(defect-free, ultrathin) Porous support (low resistance –surface pores) Coating techniques: Dip-coating spray coating spin coating Plasma polymerization Interfacial polymerization In-situ polymerization Ref: Mulder, Basic principles of separation technology
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Sample dip-coating membrane
Top layer porous support Porous support non-woven support
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III.5 Preparation techniques for composite membranes
Dense layer on porous substrate of different materials Each layer can be optimized Materials for selective layer are not limited (mechanical, chemical, thermal stability, processibility, etc.) Applications: RO, GS, PV Preparation techniques Dip-coating Spray coating Spin coating Interfacial polymerization In-situ polymerization Plasma polymerization Grafting
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Dip-coating Dip & controlled evaporate Post-treatment
Cross-linking Heat treatment Main effects on coating thickness Coating velocity, viscosity, types of polymers, types of solvent and concentration of polymers Equilibrium thickness: (III-1) p85
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Dip-coating considerations
State of polymers Glassy: coating may rupture during evaporation (Tg passed) Rubbery: mostly defect-free coating Solvent Good solvent-larger coil Poor solvent-polymer aggregate Entanglement during evaporation Hydrophilic vs. hydrophobic support surface
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Dip-coating considerations
Pore penetration Capillary force may cause pore penetration of solution Resistance increases due to the blocked pores Methods to avoid pore penetration Pre-filling the pores Chose polymer of higher MW Chose support of smaller pores Narrow pore size distribution Match surface tension of the solution to the support membranes
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Spray coating An example Also for polymeric membranes in solution
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Spin coating
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Interfacial polymerization
Thickness<50nm Immiscible solvents –reactive monomer or pre-polymer (amine type) in organic solvent, and another reactive monomer in aqueous solvent, heat-treatment to complete the reaction and cross-link the polyer Self-inhibiting by limiting the passage of reactants-extremely thin film <50nm Fig. III-10 The formation of a composite membrane via interfaciaol polymerisation P82
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Plasma polymerization: Plasma (physics & chemistry)
Plasma: a state of matter similar to gas, in which a certain portion of the particles is ionized Charged particles: equal positive ions and negative ions/electrons Ionization is generally accompanied by the dissociation of molecular bonds Ionization methods: Heating Applying strong electromagnetic field with a laser or microwave generator Glow Discharge
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Plasma polymerization setup
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Plasma polymerization
Plasma polymerization refers to formation of polymeric materials under the influence of plasma (also termed as Glow Discharge Polymerization) Plasma polymer films can be easily formed with thickness of 0.05m. These films are highly coherent and adherent to variety of substrates like conventional polymers, glass, metals. Films are highly dense & pinhole free. Multilayer films or films with grading of chemical and physical characteristics can be easily prepared . One step process .
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