Membrane Gas Separation

Figure 12.7 Diffusivity of different gases (H 
2
 , N 
2
 , CH 
4
and CO 
2
 ) in copolymer 
1500PEO77PBT23 . Reprinted with permission from Advanced Functional Materials, Tailor -
 made polymeric membranes based on segmented block copolymers for CO 
2 
 separation, by 
A. Car, C. Stropnik, W. Yave, K. - V. Peinemann, 23, 2815 – 2823. Copyright (2008) Wiley - VCH

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Tailoring Polymeric Membrane Based on Segmented Block Copolymers
239
0
40
80
120
160
200
PEO [wt.%]
P
er
mea
bility [bar
rer]
H
2
N
2
CH
4
CO
2
56
60
64
68
72
76
0
1
2
3
4
5
Dif
fusi
vity [cm
2
/s]·10
6
Figure 12.8 Permeability and diffusivity of 4000PEO55PBT45 blends with PEG as function 
of total PEO. Reprinted with permission from Advanced Functional Materials, Tailor - made 
polymeric membranes based on segmented block copolymers for CO 
2 
 separation, by A. 
Car, C. Stropnik, W. Yave, K. - V. Peinemann, 23, 2815 – 2823. Copyright (2008) Wiley - VCH
decreased in the blends with higher PEG content. Blend membranes were optically homo-
geneous, since all samples were completely transparent. Thus, no phase separation is 
expected. Transport in blends depends upon composition, miscibility and phase morphol-
ogy. In homogeneous blends, the diffusion process is infl uenced by the interaction between 
the components [85,86] , while in heterogeneous blends the interfacial phenomena and the 
rubbery or glassy nature of phases are important [87] . Consequently, the permeability 
depends on heterogeneity of the system and the method of blend preparation [88] . A 
selection of polymer and additive with good compatibility are important in order to have 
a homogeneous system. However, strong interactions which can produce a perfectly 
homogeneous blend cannot be required, because hydrogen bonding can decrease the 
permeability due to the free volume decreases.
Copolymer 4000PEO55PBT45 has longer PEO segment than 1500PEO77PBT23, 
hence the addition of PEG with 200 g/mol effectively hindered the hydrogen bonding. 
It can be assumed that somehow the PEG chains are accommodated between polymer 
segments and create higher fractional free volume (more amorphous phase). The CO 
2
permeability in pristine copolymer is 96 Barrer and it is increased two fold ( 
∼
190 Barrer) 
by addition of 50 wt.% of PEG (as 77.5 wt.% calculated by Equation 12.8 ). Thus, the 
incorporation of EO units as PEG into the 4000PEO55PBT45 polymer matrix clearly led 
to a material with improved gas transport properties (Figure 12.8 ). This fact has been 
attributed to the structural (increase in the content of EO units) and morphological 

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240
Membrane Gas Separation
(decrease in crystallinity) changes in the membrane As can be observed in Figure 12.8 , 
the diffusivity of all gases is increased, as is the case with Pebax, and this is in agreement 
with the increases in the free volume.
It is worth to mention that permeabilities for 4000PEO55PBT45/PEG blend mem-
branes shown in Figure 12.8 are according to recalculated values of total PEO content, 
which was done by using the following equation:
wt
m
m
⋅
=
⋅
+
%
.
PEOtotal
copolymer
PEG
0 55
200
(12.8)
where 0.55 represents the mass fraction of PEO in the copolymer, m
copolymer
and m
PEG200
are the mass of copolymer and PEG in 100 g of blend, respectively. 
Figure 12.9 describes the strategy of preparation of a tailor - made blend membrane. By 
adding 50 wt.% of PEG as additive into the copolymers, membranes with improved prop-
erties can be produced. However, a limit may exist because of hydrogen bonding, which 
results in decreased chain mobility and CO 
2
solubility. In order to develop novel mem-
brane materials with better performance as compared to those developed here, other 
strategies must be studied i.e. strategies for a simultaneous increase in solubility and dif-
fusivity (hindering the hydrogen bonding).
The enhancement of permselectivity, solubility selectivity and diffusivity selectivity of 
4000PEO55PBT45/PEG blend membranes are presented in Table 12.4 . The successful 
0
50
100
150
200
250
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000