Chapter 4 - Membrane Characterization (1)

7/23/2019 Chapter 4 - Membrane Characterization (1)

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MEMBRANE CHARACTERIZATION

A small change in one of the membrane

formation parameters can change the (toplayer) structure and consequently have adrastic eect on membrane performance.

REPRO!"#$#%#&' is also problem

hy need embrane "haracteri*ation +&o relate structural membrane propertiessuch as pore si*e, pore si*e distribution,free volume and crystallinity to membraneseparation properties

&o obtain impression about si*e of particlesand molecules and ions to encountered, a-ide range of particles and molecules -ithvarious dimensions are available

E$RAE "/ARA"&ER#0AO $E"OE1

PRO2RE11#3E%' ORE #44#"!%&1 A1 &/EPORE 1#0E E"REA1E1.

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embranes -ill be classi5ed in t-o maingroupsi. porous

ii. nonporous

46!4 7 58ed pores are present, can becharacteri*ed by several techniques

e5nation of porous (#!PA")i. acropores 9 :; nmii.esopores < nm =pore si*e = :; nmiii. icropores = < nm

Pore diameter or more arbitrarily pore width

4 7 porous media containing macropores,

!4 also porous -ith mesopores in top layer.

&herefore, it is not the membrane material-hich is characteri*ed, but the pores in themembrane

Pore and pore si*e distribution (P1) mainlydetermines -hich particles or moleculesare retained and -hich -ill pass throughthe membrane

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&he material is of little importance in

determining the separation performance.

ense pervaporation6gas separation

membranes 7 no 58ed pores are presentand the material itself mainly determinesthe performance.

&he morphology of the polymer materialused for membrane preparation directlyaects its permeability (crystalline,amorphous, glassy, rubbery)

4actors such as temperature and the

interaction of the solvent and solute -iththe polymeric materials, have a largein>uence on the segmental motions(molecular orientation).

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CHARACTERIZATION OF POROUS

MEMBRANE

1hape of the pore or its geometry 7 is not

-ell de5ned in membrane characteri*ationtechniques.

#n order to relate pore radii to physicalequations, several assumptions have to bemade about the geometry of the pore.

Poiseuille equation 7 pores are considered

to be parallel cylinders,

?o*eny@"arman eqn 7 pores are the voidsbet-een the close@paced spheres of equaldiameter

Actuality, such pores do not e8ist. /o-ever,

in order to interpret the characteri*ation

results, it is often essential to maeassumptions about the pore geometry

#n addition, it is not the pore si*e -hich is

the rate@determining factor, but the

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smallest restriction. 7 #ndeed, somecharacteri*ation techniques determine thedimensions of pore entrance rather than

the pore si*e. better info aboutBpermeation relatedC characteristics

P1 in a porous !4 and 4 membrane 7

means pores do not have same si*e bute8ist as a distribution of si*e.

1urface porosity 7 is also a very important

variable in determining the >u8 through themembrane. 7 1P is a factor of acombination -ith the thicness of the toplayer or the length of the pore.

&-o dierent types of characteri*ationmethod for porous membranes D@

i. 1tructure@related parameter 7determination of pore si*e, P1, toplayer thicness and surface porosity.

ii.Permeation@related parameters 7determination of the actual separationparameters using solutes that are more

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or less retained by the membrane (cut@o measurements)

#t is often very dicult to relate the (i) and(ii) because the pore si*e and shape is notvery -ell de5ned.

&he con5guration of the pores (cylindrical,paced@spheres) used in simple model

descriptions deviate sometimes dramaticallyfrom the actual morphology, in

MICROFILTRATION

Pores D ;.F 7 F; m range

&echniques used such as Electron

microscopy, $ubble point method, ercuryintrusion porometry, permeationmeasurements

E @ 1E, &E (&ransmission Electronmicroscopy).

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1E 7 provides a very convenient and simplemethod for characteri*ing and investigatingthe porous structure of 4.

&he resolution limit lies in the ;.;F m (F;

nm).

1E principle

A narro- beam of electrons -ith ineticenergies in the order of F@ected are called secondaryelectrons.

1ec electrons (lo- energy) are not re>ectedbut liberated from atoms in the surface 7mainly determine the imaging 7 -hat isseen on the screen or micrograph.

&o avoid sample damaged or burned due tothe electron beam, sample is coated (thingold layer coating)

1E 7 allo-s a clear vie- of the overallstructure of a 4 membrane (top surface,

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8@section and bottom surface) and anyasymmetry structures. Also the geometryof the pores can be clearly visuali*ed.

Atomic 4orce icroscopy (A4)

%atest development

1uitable for 4 (even though only on thesurface morphology)

A sharp tip -ith a diameter smaller than F;;Angstrom is scanning across a surface -ith aconstant force. %ondon@van der aals

interactions -ill occur bet-een the atoms inthe tip and surface of the sample and theseforces are detected.

$ubble@point method

Provides a simple means of characteri*ingthe ma8imum pore si*es in a givenmembrane.

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&he method essentially measures thepressure needed to blo- air through a liquid@5lled membrane.

&he top of the 5lter is placed in contact -itha liquid (eg. ater) -hich 5lls all the pores-hen the membrane is -etted.

&he bottom of the 5lters in contact -ith airand as the air pressure is gradually increasedbubbles of air penetrate through themembrane at a certain pressure.

&he relationship bt- pressure and poreradius is given by %aplace equationD@

&his method can only be used to measurethe largest active pores in a givenmembrane.

BUBBLE POINT WITH GAS PERMEATION

(WET AND DRY FLOW METHOD)

$P method gives limited information, and

disadv.

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Eg. epends on the liquid use (dierent

-etting mechanisms, but not re>ected inequations), rate at -hich the pressure is

increased, anity bt- -etting liquid andmembr material etc.

$P -ith gas permeation combines the

bubble@point concept -ith themeasurement of the gas lo- through theemptied pores.

< stepsD@

gas >o- is measured through a dry

membrane as a function of the pressure isgenerally a straight line.

&hen, the membrane is -etted and again

the gas >o- is determined as a function ofthe applied pressure

At very lo- pressures the pores are still5lled -ith the liquid and the gas >o-(-hich is determined by diusion throughthe liquid) is very lo-

At a certain minimal pressure (bubble

point), the largest pores -ill be empty and

the gas >o- -ill increase by convective>o- through these pores.

At the highest press the gas >o- of the dry

membrane must be equal to the -etmembrane.

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1uitable for characteri*ation of macropores,

eg 4 -ith si*es up to :; nm.

Permeability ethod

#f capillary pores are assumed to be present,the pore si*e can be obtained by measuringthe >u8 through a membrane at a constant

pressure using the /agen@Poiseuille eqn.

G H -ater (>u8) through the membrane at adriving force of P6I,

r@pore radius (m), H liquid viscosity (Pa.s)m,H surface porosity of the membrane, Htortuosity factor.

hen the pressure is increased further the >u8increases linearly -ith pressure.

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/P assumes the pores in the membrane arecylindrical but generally this is not the case.

?o*eny@"arman eqn 7 pores are intersticesbt- close@paced spheres. 1uch pores arenot commonly found in syntheticmembrane.

ULTRAFILTRATION

"onsidered as porous, but moreasymmetric compared to 4.

"haracteri*ation 7 involves the

characteri*ation of the top layer (thicness,P1 and surface porosity).

&ypical pore diameter of top layer D


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Resolution of 1E is too lo- to determine

the pore si*es in top layer accurately.

ercury intrusion and bubble@point

methods cannot be used because the poresi*es are too small, so that very highpressures -ould be needed 7 -hich -oulddestroy the polymeric structure.

Permeation e8p can still be used. Other

techniques are D@

2as adsoption@desorption&hermoporometryPermporometry

%iquid displacement4raction or reJection measurement

&E

1O%!&E REGE"O EA1!REE&

!se concept of cut@oC to characteri*e !4membranes

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"ut o is de5ned as that molecular -eight-hich is K;L reJected by the membrane

Eg. embrane has a cut@o value of M;,;;;7 implying that all solutes -ith a molecular-eight 9 M;? are more than K; L reJected.

/o-ever, it is not possible to de5ne theseparation characteristics of a membrane asingle parameter, i.e "O. Otherparameters such as shape, >e8ibility of themacromolecular solute, its interaction -iththe membrane material and membranefouling 7 have drastic6signi5cance eect onseparation characteristics.

&o characteri*e the membrane(determination of membrane structure suchas pore si*e and etc) 1olute reJection data 7is combined -ith the models (developed forthe particular membrane system) 7 such asN

ernst Planc@equation

#rreversible &hermodynamic model (1Pieglerand ?edem odel)

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onnan 1teric Pore odel

Electrostatic model

CHARACTERIZATION OF NONPOROUS

MEMBRANE

#n nonporous, the chemical nature andmorphology of the polymeric membraneand the e8tent of interaction bet-een thepolymer and the permeates are theimportant factors to consider.

&he determination of the physicalproperties related to the chemical structureis no- more important and, the methodssuch as D@Permeability

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Other physical propertiesPlasma etching1urface analysis

PEREA$#%#&'

!sing a simple e8perimental setup

&he cell containing a homogenous

membrane of no-n thicness is pressuri*ed-ith a chosen gas. &he e8tent of gaspermeation through the membrane ismeasured by means of a mass >o- meter orby a soap bubble meter.

1ophisticated method 7 use a calibrated

volume connected to the permeate side -iththe small pressure increase in the calibratedvolume being measured -ith a pressuretransducer.

&he gas permeability or permeabilitycoecient , P can be determined

G H Pl

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Chapter 4 - Membrane Characterization (1)