Membrane Gas Separation

13.3.3
Pebax ® 1657 Membrane Modifi ed by Cold Plasma 
Although Pebax ® – PEG blends can exhibit high performances in the CO 
2
extraction from 
fl ue gases, a limitation in industrial operations can be foreseen: the long - term stability of 
the blend membranes, especially at high temperatures. In fact, a low molecular weight 
PEG is more a plasticizer than a true polymer in the blends; its chains are too short to be 
permanently blocked in a polymer network by simple entanglements. This was evidenced 
by the PEG migration towards the surface observed in the blends of high PEG300 con-
tents. In order to obtain a more permanent modifi cation of Pebax ® , we propose the use 
of cold plasma to modify the Pebax ® 1657 fi lm surface with chemical groups capable of 
strong CO 
2
absorption. 
When a cold plasma in N 
2
, NH 
3
or H 
2
/N 
2
gas is put in contact with a membrane, the 
chemical modifi cation by reactions of the plasma active species with the membrane 
surface results in alterations of transport properties associated with changes in the per-
meant sorption into that surface. Under the conditions where the modifi cation occurs on 
10
0.1
1
0
10
20
30
Ideal selectivity CO
2
/N
2
40
50
60
70
80
90
100
CO
2
permeability/Barrer
1000
Pebax®
Pebax® 1657 /
PEGEPI
Pebax® 1657 /
PEG300
Upper bound limit
10000
Figure 13.9 Robeson ’ s plot of ( 
×
 ) the different Pebax ® , ( 
䊊
 ) the Pebax ® 1657/PEGEPI 
blends and ( 
䊏
 ) the Pebax ® 1657/PEG300 blends. ( - - - ) is the polymer upper bound

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274
Membrane Gas Separation
the external surface, the underneath of the membrane would entirely control the diffusion 
of the penetrants towards the downstream side. Our aim in this work was to graft amine 
groups [45] onto the surface of a Pebax ® 1657 fi lm to enhance CO 
2
sorption into the 
membrane while keeping unchanged the transport by diffusion through the Pebax ® 1657, 
which is already quite good for mixtures of CO 
2
with permanent gases. Several modifi ca-
tion tests in N 
2
/H 
2
cold plasma were performed on Pebax ® 1657 fi lms to determine the 
operating conditions for the largest change in fi lm surface properties. The conditions for 
the best surface property modifi cation (Table 13.7 ) were obtained by assessment tests 
with measurements of the contact angle of the modifi ed surface with pure water. In fact, 
we considered that the contact angle with water is the lowest for a modifi cation where 
hydrophilic amine groups are grafted to the fi lm surface.
The friction mode AFM images (Figures 13.5 and 13.10 ) of the surface before and 
after plasma treatment under the conditions given in Table 13.7 show the difference in 
the surface properties. The uniform colour of the image of the surface - treated fi lm is in 
big contrast with that of the pristine fi lm, where crystalline zones, amorphous PA zones 
Table 13.7  Best conditions used for the surface modifi cation by H
2
 / N
2
plasma 
Parameter
Value
Gas fl ow
10 STP cm 
3
min 
−
1
Power
45 W
Treatment time
3 min
Distance between the bottom of the excitation 
source and the sample
2 cm
Vol.% H 
2
15
(a)
0
2.50
5.00
0
2.50
5.00
0
2.50
5.00
0
2.50
5.00
400.0 nm
200.0 nm
0.0 nm
μ
m
μ
m
(b)
Figure 13.10 Surface morphology of plasma treated Pebax ® 1657 membrane obtained by 
AFM in contact (a) and in friction (b) modes. The contact mode image gives the surface 
topology

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CO
2
 Permeation with Pebax®-based Membranes for Global Warming Reduction
275
and molten polyether zones can be clearly distinguished. We infer from the uniform fric-
tion characteristics of the plasma - modifi ed surface that the grafted amine groups were 
fairly uniformly distributed on the surface.
However, the grafted layer is apparently extremely thin, since no changes were detected 
in DSC thermograms. The CO 
2
permeation test indicated that the plasma - modifi ed mem-
brane exhibited a greatly enhanced permeability (144 Barrer at 25 ° C). 
We speculated that, in line with the mechanism proposed for facilitated transport of 
CO 
2
with amine molecules, the CO 
2
sorption was enhanced due to the following reversible 
complexation reaction [46,47] , in addition to the normal gas sorption according to Henry ’ s 
law into a rubbery/molten polymer phase:
RNH
CO
RNHCOO
H
2
2
+
⇔
+
−
+
RNH
H
RNH
2
3
+
⇔
+
+
In the case of our plasma - modifi ed membrane where amine groups were only grafted on 
the surface, CO 
2
facilitated transport cannot occur through the membranes. Instead, the 
high interfacial concentration in CO 
2
served as entrance concentration for the classical 
diffusion through the unmodifi ed Pebax ® 1657 polymer, leading to an overall improve-
ment of the CO 
2
permeability coeffi cient.