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CHAPTER 3
MEMBRANE FABRICATION AND CHARACTERIZATION
3.1 INTRODUCTION
Loeb and Sourirajan [36] invented the asymmetric polymer membrane and
this invention is treated as one of the significant findings in the field of membrane
science. Researchers have concentrated on developing techniques to prepare
asymmetric polymer membranes and some of the common methods are wet phase
inversion (immersion-precipitation), vapor-induced phase separation, thermally
induced phase separation and dry casting [11, 208]. Among the methods for
preparation of asymmetric polymer membranes, the most widely used one is the wet
phase inversion method [11, 166, 209].
The structure of the membrane plays a major role in the performance of the
separation and permeation. The synthetic membranes can be classified mainly into
two types, symmetric and asymmetric membranes. In symmetric membranes the
pore size or the properties are uniform throughout the membrane whereas in case of
asymmetric membranes, the pore size is not uniform from the top to the bottom layer
of the membrane, therefore the particles gets rejected mostly at the surface without
entering the inner layers and hence the pores does not get plugged in. After the
invention of Loeb and Sourirajan [36], asymmetric membranes had a major
breakthrough towards industrial applications.
In membrane technology, mostly polymer based membranes like cellulose
acetate [210, 211], polysulfone [212, 213], polyethersulfone and polyacrylonitrile
etc., [214-216] have been fabricated towards the development of highly efficient
membranes. Among these, polysulfone based membranes are widely used for water
treatment due to its interesting properties like flame retardant, possesses high
mechanical, thermal and oxidative stability and moreover it is soluble in most of the
50
organic solvents with wide pH tolerance, wide temperature limit, creep resistance,
dimensional stability, increased flow rate and better trapping ability [52, 213]. For
our investigation 9%, 12%, 18% polysulfone (PSf) and silver immobilized
polysulfone (Ag-PSf) membranes were taken and their fabrication and
characterization were discussed in this chapter.
3.2 POLYSULFONE MEMBRANES
3.2.1 Fabrication of polysulfone membranes
A homogeneous casting solution was prepared by dissolving PSf and pore
former polyvinyl pyrrolidone (PVP) in the solvent N-methyl pyrrolidone (NMP).
The solution was cast on a clean glass plate using a casting knife (Elcometer Model
3570) with a thickness of 150µm. The thin layer of the cast solution was immersed
in the deionized water (DI), in which the polymer solution was phase inversed and
the membrane film was formed due to the coagulation with water. The as prepared
membrane was kept for 24 hrs. in a water bath conditioned at 25ºC. The membranes
were kept in fresh DI water for atleast one week before testing [217-221].
Membranes of three different concentrations (9%, 12% & 18%) were fabricated by
changing the weight % of the polysulfone in the casting solution as given in table
3.1.
Table 3.1 Composition of the polymer used to fabricate membranes
Polymer(wt. %)
Poreformer(wt. %)
Solvent(wt. %)
Coagulation bath
9% 7.5% NMP DI water
12% 10% NMP DI water
18% 15% NMP DI water
51
3.2.2 Characterization of PSf membranes
3.2.2.1 Surface morphology using Field Emission Scanning Electron Microscopy
(FESEM)
Scanning electron microscopy (SEM) and energy dispersive spectroscopy
(EDS) observations of the Ag-PSf membranes were carried out using a FEI’s, Quanta
200 FE-SEM and an EDAX’s Genesis EDS respectively. FESEM images were taken
on the membrane samples to examine the morphology of the membranes formed,
thereby to investigate any adverse change according to the change in concentration
of the membranes.
Fig.3.1. SEM micrographs showing the surface morphology of (a) 9% (b) 12%
(c) 18% membranes
(a) (b)
(c)
52
Figures 3.1 a, b and c show the morphology of the PSf membranes. The size
of the pores in 9% membranes were in the range of 300-500nm, in 12% membranes
it is around 200nm and the pore size observed in 18% membranes were around 100-
200nm. The pores were almost uniformly spaced but not spherical in shape instead,
elliptical shaped pores are formed. The uniformity is more pronounced for higher
concentration membranes as seen from the images.
Concentrations of the polymers play a role in the formation of pores. As the
concentration increases, there is a formation of thin skin at the top of the surface and
round shaped pores beneath the skin. This is due to the fact that when concentration
of the polymer is increased, the casting mixture would be highly viscous due to the
presence of lesser solvent, and the solvent leaves the membrane comparatively
slower than the lower concentration membranes thereby forming smaller size and
fewer pores in the case of higher concentration. Usually asymmetric membranes
were formed by gelation followed by liquid-liquid phase inversion. The formation of
the skin is due to the gelation of polymer top layer and the porous sub layer was due
to the liquid-liquid phase inversion by coagulating using non-solvent bath. The
coagulation of the membrane film can be carried out at different conditions like,
precipitating the membrane by immediately immersing into the coagulation bath,
immersing the film after some time of evaporation and finally precipitating the film
after complete evaporation of the film without immersing in the bath. Among these
different methods, the rate of precipitation for the immediate immersion is very rapid
comparative with the other methods, as there will be rapid exchange of the NMP and
the water thereby creating uniform sized pores. When the casting solution is exposed
to the atmosphere for a prolonged period of time, the casting solution changes from
transparent to cloudy state, which indicates the occurrence of the phase separation by
absorbing the water from the atmosphere. Hence even traces of water, in the air, is
sufficient to cause the phase inversion.
The figures 3.1 (a, b, c) also show the formation of elongated and uniform
sized pores in which the pores are not spherical in shape and might be due to
formation of macrovoids. The formations of macrovoids are due to the rapid
exchange or penetration of the nonsolvent into the weak points of the membrane
53
surface and and the skin layer at the top of the membrane can hinder the nonsolvent
entering into the sublayer in order to create many nuclei thereby forming macrovoids
and more void structures. The formation of uniform pores might be due to the
formation of uniform nucleation and growth [11, 158, 159, 175].
3.2.2.2 Zeta potential measurement
The surface charge of PSf membrane were measured using zeta potential
analyzer from Microtrac-Zetatrac analyser and is based on the streaming potential,
conductivity, electrolyte composition, pH and pressure. Zeta potential is measured
by electrophoresis technique. The membranes were crushed and dispersed in the
solvent which is allowed to settle down for few hours and few volume of aliquot is
withdrawn from the supernatant. The particles will move relative to the electrolyte
in which it is suspended in and the pH under the influence of the electric field. The
velocity imparted to the charged particle is measured from which the electrophoretic
molbility is calculated [199]. Zeta potential is directly proportional to the
electrophoretic mobility which is calculated using Helmoltz- Smoluchowski equation
given in equation (3.1):
= 4 U/ (3.1)
- Zeta Potential
- Viscosity of suspending fluid
- Dielectric constant of dispersion medium
U - Electrophoretic Mobility = v/ V / L
V - Velocity of particle
V - Voltage
L - Distance
The zeta potential value of the PSf membrane in N, N-Dimethylformamide
(DMF) is given in table 3.2., measured at pH 7. The polarity of the PSf shows that it
carries positive surface charge.
54
Table 3.2 Zeta potential of PSf membrane at pH 7
Membraneconcentration
Zeta potential(mv) Polarity Conductivity
(µs/cm)
9%
12%
18%
11.92
6.90
12.07
positive
positive
positive
7
11
8
3.2.2.3 BET studies
The surface area and pore volumes of the membranes were measured using
gas adsorption-desorption (BET) method using nitrogen gas as an adsorbate. As
discussed under the section 2.5.8, the surface area and the porosity can be measured
using the BET method. Gas adsorption allows probing of entire surface including
irregularities and pore interiors and the amount adsorbed is a function of temperature,
pressure and the strength of attraction or interaction potential.
To find the surface area, a graph is plotted between 1/[Q(P0/P-1)] versus
P/Po to give a straight line which is shown in fig.3.2.. Best fit of the straight line
using least square regression is used to find the slope and the intercept and the
surface area is calculated as discussed under section 2.5.8. The surface area values of
PSf membranes for all the concentrations are shown in table.3.3. As the
concentration of the membrane has increased, the surface area of the membrane has
also increased.
55
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
1/[Q
(P0/
P-1)
]
Relative pressure (P/P0)
18%12% 9%
PSf
Fig.3.2 BET surface area plot of PSf membranes
Table.3.3 BET Surface area measurement of three different PSf membranes
Concentration BET surface area(PSf) (m2/g)
9% 10.15
12% 11.93
18% 18.85
The pore size distribution graph of the PSf membranes of 9%, 12% and 18%
are shown in figure 3.3, which is plotted between the pore volume and the pore size.
56
0 20 40 60 80 100 120 140 160 180 200 220 240
0.0020.0040.0060.0080.0100.0120.0140.0160.0180.0200.0220.0240.0260.0280.0300.0320.0340.0360.0380.040
18%
12%
9%
PSf
Pore
vol
ume
(cm
3/g)
Pore radius (Angstrom)
Fig.3.3 Pore size distribution graph of PSf membranes
The pore distribution graph of the membrane shows that majority of the pores
are in the range of 2-8nm which shows that the obtained membrane belongs to UF
category of membranes. The permeability of the membrane depends upon the larger
pores compared to the small size pores although they are huge in number. This is
because the permeability is directly proportional to the fourth power of pore radius
(Ln rp4) according to Hagen-Poiseuille equation. The bigger pores even though
small in number determines the overall permeability as observed.[222] There is no
significant increase in the pore size as there is increase in concentration of the
membrane, but the pore volume or the porosity has increased as the concentration
increases. The materials which have pores less than 2nm are called microporous
materials and having pores between 2nm and 50nm are called mesoporous materials
and above 50nm are called macro porous materials. Hence membrane belongs to
mesoporous type due to its pore size range. Gas adsorption methods are used to
measure only open pores in the range of 0.4nm to 50nm [204-206].
57
3.2.2.4 Fourier Transform-Infra Red (FTIR) spectroscopy measurement
FTIR spectrum was recorded for 9%, 12%, 18% PSf membranes and it is
shown in fig.3.4. The peaks or adsorption bands at 1244cm-1 and 1585cm-1 as
shown in the figure 3.4 are characteristics of sulfone group in the polysulfone
membrane. SO2 symmetric stretching was also observed at 1150cm-1. The peaks
confirm the stretching of O=S=O of the sulfone group, asymmetric stretching of
sulfone and aryl ether group C-O-C in the polysulfone membrane.
2000 1500 1000 500-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Tran
smitt
ance
%)
Wavenumber (cm-1)
9%
2000 1800 1600 1400 1200 1000 800 600 400-1
0
1
2
3
4
5
6
7
8
1585
cm-1
1244
cm-112%
D
Wavenumber (cm-1)
1150
cm-1
2000 1500 1000 500
0.0
0.5
1.0
1.5
2.0
2.5
3.0 18%
Tran
smitt
ance
%
Wavenumber (cm-1)
Fig.3.4. FTIR spectra of PSf membranes (9%, 12%, 18%)
3.3 SILVER IMMOBILIZED POLYSULFONE (Ag-PSF) MEMBRANES
3.3.1 Preparation of silver immobilized polysulfone (Ag-PSf) membrane
The polymer casting solution is prepared by dissolving PSf and pore former
polyvinyl pyrrolidone (PVP) in the solvent N-methyl pyrrolidone (NMP) and the
same procedure followed as given in section 3.2.1. To this homogeneous mixture
0.1% of silver nitrate was added which gave a brownish yellow solution. This
mixture is kept undisturbed to eliminate air bubbles which may cause defects during
58
membrane casting process. The casting solution was cast on a glass plate and was
then precipitated in water containing 0.2% sodium borohydride which acts as
reducing agent for the formation of silver nanoparticles. The casted membrane was
finally immersed in deionized water at room temperature for 24 hrs to completely
remove the pore-former and excess solvent. The membranes were air dried before
testing for its structure and performance [52]. The concentration of the silver nitrate
is maintained constant for all the concentrations of the membrane fabricated.
3.3.2 Characterization of Ag-PSf membranes
3.3.2.1 Surface morphology using FESEM
The FESEM analysis was carried out to analyze the surface of the membrane.
The images of the silver immobilized polysulfone membranes for 9%, 12% and 18%
were shown in figures 3.5. (a, b and c) respectively.
Fig.3.5. FESEM images of the Ag-PSf membranes (a) 9% (b) 12% (c) 18%
(a) (b
(c)
59
The images show the formation of asymmetric structure with a porous top
layer and interconnected porous sub layers with fully developed pores all over the
surface of the membrane. The macrovoid surface formed in PSf membranes were
suppressed by the formation of spherical shaped pores and hence slight decrease in
the pore size due to the change in shape. The size of the pores in 9% membranes are
roughly around 100-200nm, for 12% membranes it is 100nm and mostly uniformly
sized and spaced. In 18% membranes the range is 10-100nm as it has wide range of
pores on the surface and not uniform. Other than this, there was no observed change
silver immobilized membranes. EDS measurement was also taken on the Ag-PSf
membranes to identify and confirm the presence and composition of silver
nanoparticles on the surface of the membrane. The EDS of three different
membranes were shown in figures 3.6. a & b (9%), 3.6. c & d (12%) and e and f
(18%). EDS mapping indicates the uniform distribution of the Ag nanoparticles
across the surface of the membrane.
(d)
(b)
(c)
(a)
60
Fig.3.6. (a) and (b) EDS mapping and graph showing the uniform presence of
Ag nps in 9% PSf-Ag membrane respectively (c) and (d) EDS mapping and
graph showing the uniform presence of Ag nps in 12% PSf-Ag membranes (e)
and (f) EDS mapping and graph showing the uniform presence of Ag nps in
18% PSf-Ag membranes
3.3.2.2 BET studies
The surface area and pore volumes of the membranes were measured using
gas adsorption-desorption (BET) method using nitrogen gas as an adsorbate. BET
surface area graph which is plotted between 1/[Q(P0/P-1)] versus P/Po to give a
straight line as shown in fig. 3.7. Best fit of the straight line using least square
regression is used to find the slope and the intercept and the surface area is calculated
as discussed under section 2.5.8. The surface area values of Ag-PSf membranes for
all the concentrations are shown in table.3.4.
(e)
(f)
61
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
0.00
0.02
0.04
0.06
0.08
0.101/
[Q(P
0/P-
1)]
Relative pressure (P/P0)
9% 12% 20%
Ag-PSf
Figure 3.7 BET surface area plot of Ag-PSf membranes
Table.3.4 BET Surface area measurement of three different Ag-PSf membranes
Concentration BET surface area ( Ag-PSf) (m2/g)
9% 12.31
12% 13.75
18% 20.62
The pore size distribution graphs of the Ag-PSf membranes of 9%, 12% and
18% were shown in figure 3.8. The distributions of the pores are as same as
discussed under the section 3.2.2.3. The porosity or pore volume is greater for 18%
membranes than for 12% and 9% (for both PSf & Ag-PSf) membranes.
62
0 50 100 150 200 2500.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
0.045 18%
12%
9%
Ag - PSfPo
re v
olum
e (c
m3/
g)
Pore radius (Angstrom)
Fig.3.8. Pore size distribution of Ag- PSf membranes
Compared to the surface area of polysulfone membranes, there is an increase
in the surface of the silver immobilized membrane which can be seen in table 3.3,
which could be due to the presence of silver nanoparticles in the membrane.
3.3.2.3 Zeta potential measurement
The surface charge of Ag-PSf membrane was also measured to find any
change in the surface charge of the membrane after treating it with silver
nanoparticles. The principle and procedure were followed as given in section
3.2.2.2. The zeta potential value of Ag-PSf membrane is given in Table 3.5.,
measured at pH 7. The polarity of the 12% Ag-PSf shows that it carries negative
surface charge and the immobilization of silver nanoparticles have imparted negative
charge to the membrane surface which makes them to behave like a nanofiltration
membrane.
63
Table 3.5 Zeta potential of Ag-PSf membranes at pH 7
Membrane Zeta potential(mv)
Polarity Conductivity(µs/cm)
9%
12%
18%
0.95
11.05
0.94
negative
negative
negative
6
15
7
3.3.2.4 Fourier Transform-Infra Red (FTIR) spectroscopy measurement
In order to verify the bond formation between the immobilized silver
nanoparticles and the polysulfone moiety, FTIR spectra were recorded for 9%, 12%
and 18% Ag-PSf membranes and the graph is given in fig.3.7. The nature of
adhesion between the silver nanoparticles and the polysulfone membrane was studied
using these spectra. The peaks or adsorption bands at 1244cm-1 and 1585cm-1 as
shown in fig3.8 are characteristics of sulfone group in the polysulfone membrane.
The peak at 2870 cm-1 is attributed to the methyl (CH3) group present in the
polysulfone and the adsorption bands above 3000cm-1 are characteristics of C-H
stretching of aromatic rings in the polysulfone membrane. The spectra show that
there is no bond formation between the silver nanoparticles and the polysulfone
membrane which is confirmed by the absence of peak due to bonding of Ag-PSf.
64
3200 2800 2400 2000 1600 1200 800 4000
20
40
60
80
100
1585
cm-1
1585
cm-1
1150
cm
-1
T%
Wavenumber (cm-1)
1150
cm
-1
1585
cm-1
3200 2800 2400 2000 1600 1200 800 4000
20
40
60
80
100
1244
cm
-1
1244
cm
-1
1150
cm
-1
9% 12%
T%
Wavenumber (cm-1)
1244
cm
-1
3200 2800 2400 2000 1600 1200 800 4000
20
40
60
80
100
2870
cm-118%
T%
Wavenumber (cm-1)
Fig.3.9. FTIR spectra of Ag-PSf membranes
3.3.2.5 UV-Visible measurement
To understand the effect and efficiency of the silver nanoparticles
immobilized onto the Ag-PSf membrane, it is important to characterize the physical
properties of the silver nanoparticles. Even though the presence of silver
nanoparticles in the membrane was confirmed by the EDS measurement, the size of
the nanoparticles was not determined as this is not directly possible in the
immobilized state. Hence the following approach was adopted for the same. The
yellow colored Ag-PSf membrane was extracted in Dimethylformamide (DMF)
solution and analyzed using UV-Vis spectroscopy. For the UV-Vis studies, DMF
solution was used as the reference. The results for the same is shown in figure 3.8,
indicating that the suspension of extracted Ag nanoparticles shows strong absorption
65
band (plasmon excitation) in visible region [223], at about ~420nm. This shows the
presence of spherical silver nanoparticles as the position of the plasmon absorption
depends upon the particle size and shape and it has been reported that the absorption
spectrum of spherical silver nanoparticles lies between 420 – 490 nm depending
upon the particle size [224].
Fig.3.10. UV-Vis absorption spectrum of the Ag extracted from Ag-PSf
membrane
3.4 COMPARATIVE STUDY BETWEEN PSf AND Ag-PSf
The polysulfone and silver immobilized polysulfone membranes of all the
three concentration (9%, 12%, 18%) were characterized and the properties are
tabulated in table 3.6. in order to compare between the PSf and Ag-PSf.
66
Table.3.6 Comparison between PSf and Ag-PSf membranes
Parameters 9% 9%-Ag 12% 12%-Ag 18% 18%-Ag
Shape
(FESEM)
Elliptical
Aligned
diagonally
spherical Elliptical
Aligned
horizontally
spherical Elliptical
Aligned
vertically
spherical
BET (m2/g) 10.15 12.31 11.93 13.75 18.85 20.62
Pore sizedistributionviaadsorption
2-8nm 2-8nm 2-8nm 2-8nm 2-8nm 2-8nm
Zetapotential
(mV)
11.92 0.98 6.90 11.05 12.07 0.94
positive negative positive negative positive negative
Conductivity
(µs/cm)7 16 11 15 8 7
FTIR
peaks
observed
due to
sulfone
group,
C=O etc.
No
changes
observed
peaks
observed
due to
sulfone
group, C=O
etc.
No
changes
observed
peaks
observed
due to
sulfone
group,
C=O etc.
No
changes
observed
Qualitative information on the surface of the membrane was given by
FESEM which shows the morphology and the shape of the pores on the top layer.
The shape of the pores in PSf membranes were almost elliptical in shape and highly
uniform due to the exchange of nonsolvent and solvent in the coagulation bath in a
controlled manner. In Ag-PSf membranes, pores are spherical in shape and highly
non-uniform, might be due to the formation of silver nanoparticles in the matrix and
the presence of reducing agent in the coagulation bath. The surface area was
increased for all the Ag-PSf membranes irrespective of the concentration due to the
presence of silver nanoparticles. The surface charge of the membrane is positive for
all PSf membranes and negative for all Ag-PSf membranes measured at pH 7.
67
Conductivity values have also increased for Ag-PSf membranes than PSf
membranes. From FTIR it was concluded that there was no formation of new bond
between the silver nanoparticles and the polysulfone.
3.5 CONCLUSION
Polysulfone and silver coated membranes of three different concentrations
were fabricated and the surface studies were carried out using different
characterization techniques like SEM, UV, BET, zeta potential and FTIR. From
SEM, the surface morphology of the PSf and the Ag-PSf were well studied and the
uniform distribution of the immobilized silver nanoparticles was also confirmed
using EDS. Using UV-visible the silver nanoparticles immobilized on the membrane
was studied. Zeta potential and BET experiments were carried out to study the
surface charge and the surface area of the membranes respectively. In FTIR the
adsorption bands are almost same for both PSf and Ag-PSf membranes and also there
was no extra peak found for the silver coated polysulfone membrane, which proves
that there is no formation of new chemical bond between the polysulfone and the
silver and has confirmed the physical adhesion between the membrane matrix and
the silver nanoparticles.