methods of polymeric membrane preparation by: ehsan saljoughi

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Characterization of

membranes

Dr. Ehsan Saljoughi Department of Chemical Engineering, Faculty of

Engineering, Ferdowsi University of Mashhad

Email: [email protected]

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Methods of polymeric membranes characterization Scanning electron microscopy (SEM)

Atomic force microscopy (AFM)

Bubble point method

Mercury intrusion method

Solute rejection measurements

Soaking method

Thermo gravimetric analysis (TGA)

Differential scanning calorimetry (DSC)

Methods of mechanical properties measurement

Surface analysis methods X-ray photoelectron spectroscopy

Fourier transform infrared (FTIR) spectroscopy

By: Dr. Ehsan Saljoughi

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Scanning electron microscopy (SEM)

is a very convenient and simple method for

characterizing and investigating the porous

structure and substructure of microfiltration and

other asymmetrical membranes, respectively.

A clear and concise picture of the membrane can

be obtained in terms of top layer, cross-section

and bottom layer. In addition, the porosity, pore

size distribution and geometry of the pores can be

estimated from the photographs.


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A narrow beam of electrons with kinetic energies

in the order of 1-25 kV hits the membrane sample.

The incident electrons are called primary (high-

energy) electrons, and those reflected are called

secondary (low-energy) electrons.

secondary (low-energy) electrons mainly

determine the imaging.

Burning of membrane can be avoid by coating the

sample with a conducing layer, often a thin gold

layer, to prevent charging up the surface.


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Thickness= 164 µm


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FESEM and TEM


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Atomic force microscopy (AFM) Is a rather new method to characterize the surface

of a membrane.

Pore size, mean pore size, surface porosity and

roughness parameters can be obtained from the

AFM images.

A sharp tip with a diameter smaller than 100 °A is

scanning across a surface with a constant force.

London-vander waals interactions will occur

between the atoms in the tip and the surface of the

sample and these forces are detected. This will

result in line scan or profile of the surface.


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0

0.005

0.01

0.015

0.02

0.025

0 50 100 150 200

Pore size, dp, (nm)

Pro

bab

ilit

y d

en

sit

y f

un

cti

on

Tw een-20 (0 w t.%)

Tw een-20 (2 w t.%)


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The advantage of this technique is that no

pretreatment is required and the measurement

can be carried out under atmospheric conditions.

A disadvantage is that high surface roughness

may result in the images which are difficult to be

interpreted. Moreover, high forces may damage

the polymeric structure.


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Contact angle measurement


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Bubble-point method Provides a simple means of characterizing the maximum pore size in a

given membrane.

The top of the filter is placed in contact with a liquid (e.g. water) which fills all the pores when the membrane is wetted. The bottom of the filter in contact with air and as the air pressure is gradually increased bubbles of air penetrate through the membrane at a certain pressure.


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Bubble-point method The relationship between pressure and pore radius is

given by the Laplace equation:

If small pores are present, it is necessary to apply high

pressures or to replace the water with another liquid

e.g., by an alcohol. Because the surface tension at the t-

butanol/air interface is 20.7 mN/m whereas at the

water/air interface is 72.3.

Prp

2


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is a variation of the bubble-point method.

Pore size and pore size distribution can be

determined.

Mercury is forced into a dry membrane with the

volume of mercury being determined at each

pressure.

Again, the relationship pressure and pore size is

given by the Laplace equation. Because mercury

does not wet the membrane Laplace equation is

modified to:

cos2

Prp


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θ is the contact angle of mercury with polymeric material (often 141.3 °).

Surface tension at the mercury/air interface is 0.48 N/m. Also as mentioned θ is 141.3°. Hence Laplace equation reduces to:

Where rp is expressed in nm and P in bar.

Since the volume of mercury can be determined very accurately, pore size distribution can be determined quite precisely.

Prp

7492


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The fractional rejection Ri may be defined

according to below equation:

Roverall is given by:

)(

)(1

Feedi

permeatei

iC

CR

F

Poverall

C

CR 1


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Cut-off is defined as the molecular weight which is 90% rejected by the membrane.

Cut-off values of a membrane are often used in an absolute fashion (if membrane has a cut-off value of 40000, implying that all solutes with a molecular weight greater than 40000 are more than 90% rejected).

Common solutes are globular proteins such as albumin, branched polysaccharide such as dextran or a linear flexible molecule such as poly ethylene glycol.

Using gel permeation chromatography (GPC) or high performance liquid chromatography (HPLC), the molecular weight distribution of both the feed and permeate in a given test run can be determined.


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Solute rejection measurements Quantitative predictions of membrane

performance are difficult to be obtained by this method, because:

Cut-off values are often defined in different ways under different test conditions (pressure, cross-flow velocity, geometry of the test cell, concentration and type of solute, molecular weight distribution of solute) that makes it difficult to compare the results.

Other factors such as adsorption of solutes and concentration polarization influence the permeation rate and membrane selectivity.


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Porosity measurement by soaking

Membranes are soaked in distilled water or other

liquids and then weighed after mopping

superficial water with filter papers.

The wet membrane was placed in an air-

circulating oven before measuring the weight of

dry membrane.

From the two weights (wet and dry membranes),

the membrane porosity is calculated using the

following equation:

1000(%)

Ah

WWP

L

drywet


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Porosity measurement by soaking

Where P is the membrane porosity.

Q0 is the weight of wet membrane (g).

Q1 is the weight of dry membrane (g).

A is the membrane surface area (cm2).

h is the membrane thickness (mm).

ρL is liquid (water) density (gr/cm3).

In order to minimize the experimental errors, the

porosity of each membrane sample should be

measured three times and the results are

presented on average.


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Mechanical/Thermal Analysis

Mechanical properties measurement

Tensile test machine

Results are elongation and tensile strength

Thermal properties measurement


Differential scanning calorimetry (DSC)


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Thermogravimetric analysis (TGA)

A sample of membrane is dried to remove

moisture and then programmed from room

temperature to a certain high temperature at a

constant rate (°C/min) under the nitrogen

atmosphere.

During mentioned increase in temperature,

degradation of polymeric membrane occurs in

three steps.


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The first step represents the volatilization of the volatile matter and/or the evaporation of residual absorbed water.

The second step represents the main thermal degradation of the polymeric chains.

The third step symbolizes the carbonization of the degraded products to ash.


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0

20

40

60

80

100

0 200 400 600 800

Wei

gh

t (%

)

Temperature ( C)

Polyethersulfone

Cellulose acetate


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Effect of heating rate: 1) 5 °C/min, 2) 10°C/min, 3) 16

°C/min, 4) 22 °C/min


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Differential scanning calorimetry (DSC) A sample of membrane is dried to remove moisture and then

programmed from room temperature to a certain temperature at a

constant rate (°C/min) under the nitrogen atmosphere.

Heat flow is plotted versus temperature.


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Dynamic Mechanical Thermal Analysis (DMTA)

Determination of Glass Transition Temperature (Tg)


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Surface analysis methods

are based on the concepts outlined schematically in below:

A solid surface is excited by means of radiation or particles bombardment and the emission products are detected.

Emission products, provide information about the presence of specific groups, atoms or bonds.


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Surface analysis methods

Important methods of surface analysis are: X-ray photoelectron spectroscopy (XPS)


transmission electron microscopy (TEM)

XPS is a surface sensitive technique and

measures the elemental composition (except H)

and chemical binding information for the top 1–5

nm depth of the surface region


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X-ray photoelectron spectroscopy (XPS)


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X-ray photoelectron spectroscopy (XPS)


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FTIR is able to provide significant qualitative detail about

the types of functional groups such as CO, OH, CN, …

FTIR offers much deeper depth of penetration (from

<200 nm to >1m)


methods of polymeric membrane preparation by: ehsan saljoughi

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