1. Preparation of Synthetic Membranes
Chapter III
(1) Introduction, III-1-2
(2) Polymeric membranes, III.3-5

2. 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

3. III. 1-2 Introduction Porous membrane Dense membrane Carrier membranes
(microfiltration/ultrafiltration)
(gas separation/pervaporation)
Fig. III-1 Three basic types of membranes P71

4. 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
……………..
……………..
……………..

5. 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

6. List of most important membrane preparation techniques
Sintering
Stretching
Track-etching
Template leaching
Sol-gel process
Phase inversion technique
Coating
- Membrane modification

7. 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

8. 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

9. Extrusion of thermoplastic polymers

10. 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

11. Track-etching process
radiation source
polymer film
etching bath
Membrane with
capillary pores
t t t t3
energetic heavy ions
Acid or basic solution

12. 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.

13. 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

14. 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

15. 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

16. 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

17. 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

18. Dry-wet hollow fiber spinning process

19. 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

20. 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.

21. Lab-scale spinning rigs in Memfo
Ref.: Xuezhong He Blog

22. Pilot scale spinning rig in Memfo

23. Tubular membrane preparation
Fig.III-9 Tubular membrane preparation

24. 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

25. 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.

26. 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

27. 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

28. Classification of solvent/nonsolvent
P129
In general
High mutual affinity pairs –
Instant demixing
porous
Low mutual affinity pairs –
Delayed demixing
nonporous

29. 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

30. 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

31. 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

32. 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.

33. 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.

34. 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

35. Sample dip-coating membrane
Top layer
porous support
Porous support
non-woven support

36. 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

37. 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

38. 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

39. 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

40. Spray coating
An example
Also for polymeric membranes in solution

41. Spin coating

42. 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

43. 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

44. Plasma polymerization setup

45. 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 0.05m.
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 .