2-1 Preparation of membranes-polymeric membranes.ppt

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TKP8 Membrane Technology, 2016 Fall, IKP, NTNU
Preparation of Synthetic
Membranes
Chapter III
(1) Introduction, III-1-2
(2) Polymeric membranes, III.3-5

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Outline
 Introduction
 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
Fig. III-1 Three basic types of membranes P71
(microfiltration/ultrafiltration) (gas separation/pervaporation)

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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
e.g. Lungs
Cell membranes
Biological
Glassy Rubbery
Organic
(polymeric)
Ceramic Glass Metallic Zeolitic
Inorganic
Synthetic
Membrane
Materials
hybrid
Polymers: most common
Inorganic: more stable
Sintering Stretching Track-etching
Phase inversion
Sol-gel process
Solution coating
Template leaching
Sintering Stretching Track-etching
Phase inversion
Sol-gel process
Solution coating
Template leaching
Ref: Mulder, Basic principles of separation technology
Symemetric / Asymmetric / composite

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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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HEAT
pore
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%

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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-etched 0.4 µm PCTE membrane
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 0.02-10µm
 Surface porosity <10%
 Narrow pore size distribution

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radiation source
polymer film
etching bath
Membrane with
capillary pores
Track-etching process
t0 t1 t2 t3

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Phase inversion
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
 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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 Precipitation by solvent evaporation
 Simple evaporation, coating
III.3 Phase inversion techniques
 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
Fig. III-5 Preparation of flat sheet membranes
GKSS
equipment
 Preparation parameters:
 Polymer concentration (viscosity)
 Casting thickness
 Evaporation time
 Humidity
 Temperature
 Additives (composition of the
casting solution)
 Solvent/solvents & non-solvent

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

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Pilot scale spinning rig in Memfo

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Tubular membrane preparation
Fig.III-9 Tubular membrane preparation
P81

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General thermodynamic description of the phase separation
Qualitatively, not quantitatively described!
III.6 Phase separation in polymer system
- 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
 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.

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Ref. TAI-SHUNG NEAL CHUNG, Chapter 31, FABRICATION OF HOLLOW-FIBER MEMBRANES
BY PHASE INVERSION, in Advanced membrane technology and application.
Relationships among dope composition,
precipitation kinetics, & membrane morphology
Delayed demixing - dense toplayer
Instantaneous demixing – microporous toplayer

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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
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
Fig.III-44. Delay time of demixing for 15% cellulose acetate/sovent
solution in water, P127
Delayed demixing - dense toplayer
Instantaneous demixing – microporous toplayer

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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
P131
P130
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
 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
P135

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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. http://www.polyu.edu.hk/riipt/tech-platforms/biosensing.html

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Effects of tear rate
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
porous
support
Top layer
Porous
support
non-woven
support

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III.5 Preparation techniques for
composite membranes
 Composite membrane
 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)
2
3
h
g


 
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
Fig. III-10 The formation of a composite membrane via interfaciaol polymerisation P82
Thickness<50nm

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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
Inducti
vel
y coupl
ed
M atching N etw ork
R F generator and
Pul
se generator
To vacuum pum p G as inlet
Sam pl
e
Pl
asm a zone

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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 0.05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 .

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