1. Synthesis of cost-effective polymeric membrane by phase-inversion
method and effect of sonication mode, coagulation time and cross-
linking time on its swelling.
Abstract: The preparation of cost-effective polymeric membrane with better tensile strength and
increased permeation flux is done via phase-inversion method using ethanol as coagulant by
adding 1 gm. NaCl and the effect of cross-linking with (continuous and pulse) and without
sonication, coagulation time and cross-linking time on the membrane swelling is done at a fixed
concentration of cross-linking agent,0.5% glutaraldehyde in Cross-linking solution. The Swelling
Test is performed by dipping the membrane in milli-pore water for different tome periods. 1 hr, 6
hrs, 12 hrs, 24 hrs, 48 hrs and 72 hrs. respectively. The result shows that the membrane swelling
is least when the cross-linking is done via continuous mode and is observed highest when done
without sonication, due to better penetration of the cross-linking solution with in the membrane
by transmission of continuous sound waves through sonifier, resulting in lesser swelling due to
better interaction of the membrane with cross-linking agent in continuous mode where as in
pulse(degas) mode the membrane interacts with the cross-linking solution in discontinuity
resulting in comparatively lesser penetration of the CLA in membrane. As a result PVA
Membrane Swelling depending upon the mode of cross-linking follows the order: Continuous
mode < Degas mode < Without Sonication which shows the advantage of using sonifier for
cross-linking and its specific (Continuous) mode. The swelling further increases with increase in
coagulation time for a given cross-linking time due to increase in phase-inversion time PVA
concentration at the membrane surface increases and so does the hydrophilicity of the PVA
Membrane at the surface which leads to more water absorption and thus increased swelling.
2. Introduction: The PVA Membrane 12% by weight alone lacks mechanical strength and breaks
easily upon slight load or expansion due to its lesser tensile strength. In order to enhance its
mechanical strength and castability 1 gm NaCl is mixed with 11gm PVA which further increases
the permeation flux of the polymeric membrane without decreasing the rejection property of the
membrane. Beyond 1% permeation flux increases but rejection property of the membrane also
starts decreasing simultaneously. NaCl modified PVA Membrane has a slightly lower swelling
and a little smaller contact angle of water than the KCl modified PVA Membrane when the mass
fraction of salts is the same. Polyvinyl alcohol (PVA) polymer seems attractive for the
preparation of membranes because of its high hydrophilicity and good film forming properties. It
is little affected by greases, hydrocarbons and is chemically stable against organic solvents. But
because of its higher solubility and intensive swelling in water, the modification of PVA is
necessary for industrial application. Heat-treatment, cross-linking, acetalization, blend etc are the
usual methods of modification. Glutaraldehyde, a linear 5-Carbon dialdehyde is the most suitable
cross-linking agent because of its commercial availability and low cost in addition to its high
reactivity and is more efficient than other aldehydes in generating thermally and chemically
stable crosslinks.
Literature-Review:
AL Ahmad, NM Yusuf, BS Ooi, 2011, studied the effect of adding glutardehyde as cross-
linking agent on the pore size distribution of the membrane and concluded that the pore size of
the membrane decreases with increasing cross-linking time. The sonication has been done
immediately after the preparation of 12% PVA solution before casting for the removal of trapped
bubbles. He further concluded that cross-linking decreases the hydrophilicity of the membrane
which is confirmed by the increase in contact angle of water for the prepared membrane and pure
3. water permeation flux is believed to have direct relationship with the permeation flux. Thus
Glutaraldehyde cross-linked PVA membrane are suitable for ultrafiltration for particle size
greater than 100nm.
Zhang Qrxiu and Qiu Yunren, 2003 studied the effect of adding alkali metal chloride on the
properties of modified PVA-CA blend ultrafiltration membrane prepared by the phase- inversion
method and concluded that on increasing the mass fraction of alkali metal salt in the PVA
solution till 1% the permeation flux of the membrane increases much more than that of
unmodified PVA-CA membrane without affecting the rejection property of the membrane but
when the mass fraction of salt is greater than 1.5%, the permeation flux increases much more
than that of unmodified PVA-CA membrane but the rejection property of the membrane also
decreases which is undesirable for the membrane Also with the increase of mass fraction of salt,
the contact-angle decreases and the swelling increases.
Further the NaCl modified PVA-CA membrane has slightly lower swelling than KCl modified
membrane when the mass fraction of alkali metal salt (NaCl/KCl) is same in PVA solution.
Experimental set-up and procedure:
Reagent: PVA with a degree of polymerization >98%, sodium chloride, absolute ethanol (
minimum 99.9%), Glutaraldehyde aqueous solution (50% by weight), dry Sodium Sulfate, conc.
Sulfuric Acid 98% are all of analytically pure grade.
Procedure: PVA Membrane Synthesis: 11 gm PVA and 1 gm NaCl is mixed with 88 ml milli-
pore water using a magnetic stirrer at 90 degree Celsius at 500 RPM for 2.5 hrs. It is then casted
on a glass plate using a casting knife to maintain a uniform thickness. The glass plate is then
immediately put under the tray filled with ethanol for the desired coagulation time which is
4. varied as 0.5 hr, 1 hr, 1.5 hrs, 2 hrs. The membrane is then put out of the coagulant and is left for
drying at room temperature for 12 hrs. The membrane is then cut into the shape of petri-dish and
is then put into the cross linking solution of the following composition, glutaraldehyde 0.5%,dry
sodium sulfate- 45 gms (to prevent the membrane getting soluble in water), conc. Sulfuric Acid
98% strength(0.5% in cross linking solution), milli-pore water 1000 ml. It is then cross-linked
via sonifier using both the modes continuous, pulse(degas) and without sonication by dipping
simply in a tray filled with cross-linking solution for different coagulation time 0.5 hr, 1 hr, 1.5
hr. & 2 hrs. It is then put out of cross-linking solution and put out on the glass plate till it gets
dried.
Swelling Test : 6 circular membranes of radius 1cm is cut from the dried cross-linked PVA
membrane and is dipped into milli-pore water for the following time-period (in hrs) respectively
1,6,12,24,48 & 72. The membrane is then weighed on a high precision electronic weighing
balance. The weight of the swollen Membrane (W s) is noted down. Swelling Degree is
calculated by dividing the weight of water absorbed by the original weight of the membrane (W
o) multiplied by 100.
% S.D. = [(W s- W o) / W o] *100
Results and Discussions: The graph between % swelling degree and swelling time(hrs) is
plotted for different coagulation and cross-linking time with various modes of cross-linking:
Degas (pulse) mode
Continuous mode
No Sonication ( in bath) cross-linking
17. As is clear from the graph, the continuous mode gives the least swelling after 72 hours of
swelling test while the highest swelling is observed when the cross linking is done without
sonication due to better interaction of the cross linking agent (0.5% glutaraldehyde) with the
membrane via continuous transmission of ultra-sonic vibrations in continuous mode and better
removal of air bubbles in cross-linking solution whereas in pulse(degas) mode the extent of
penetration and interaction of the cross-linking agent and the membrane comparatively reduces
due to irregularities in transmission of ultrasonic vibrations in the form of pulse. Moreover for a
given cross-linking time, the swelling degree increases with increase of coagulation time, as the
time required for phase inversion in coagulation bath increases resulting in enhanced PVA
concentration and reduced water concentration at the PVA- Ethanol interfacial surface which
results in the increased hydrophilicity of the PVA membrane at the surface facing coagulant
(ethanol). The other part of the PVA membrane with dense water concentration after
coagulation supported on glass plate is rendered less hydrophilic after getting cross-linked, thus
only the part of the membrane facing ethanol decides the trend of variation of % swelling degree
which increases with increase of coagulation time giving rise to more absorption of water and
hence greater swelling for a given cross-linking time.
CONCLUSION:
For the same coagulation & cross-linking time, cross-linking by contiuous sonicaton
mode gives lesser swelling than the degas mode followed by No sonication(in bath cross-
linking) for same concentration of cross-linking agent; due to better penetration of the
cross-linking solution with in the membrane by transmission of continuous ultrasonic
waves through sonifier, resulting in lesser swelling due to better interaction of the
membrane with cross-linking agent, where as in pulse(degas) mode the membrane
18. interacts with the cross-linking solution in discontinuity resulting in comparatively lesser
penetration of the Cross-linking agent in membrane. As a result PVA Membrane
Swelling follows the order depending upon the mode of cross-linking : Continuous
mode < Degas mode < Without Sonication
.% Swelling Degree. increases with increase of coagulation time irrespective of the
mode of crosslinking (degas, continuous or No sonication) mode for a given cross
linking time as follows: 0.5 hr. coagulation< 1 hr. coagulation< 1.5 hr.
coagulation< 2 hr. coagulation. as the time required for phase inversion in
coagulation bath increases resulting in enhanced PVA concentration and reduced
water concentration at the surface which results in the increased hydrophilicity of the
PVA membrane on its surface because of the hydrophilic nature of PVA giving rise
to more absorption of water for a given time period and thus greater swelling.
For a given coagulation time, 1 hr. cross-linking gives the least swelling. The
swelling degree follows the order: 1 hr. crosslink < 1.5 hr. crosslink< 0.5 hr.
crosslink irrespective of the mode of sonication for a given coagulation time.
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