49

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.