Membrane Gas Separation

CO
2
 Dif
fusi
vity [cm
2
/s]
1/FFV
Figure 12.6 CO 
2
diffusivity as a function of 1/FFV in Pebax
 
®
 
/PEG membranes [modifi ed 
plot from ref. 75 ]
When gas diffusivity is plotted versus 1/FFV, a linear relationship is obtained (Figure 
12.6 
). As is known, models based on free volume have been used to describe gas 
diffusion in polymers [80 – 84] , and the correlation between the logarithm of diffusion 
coeffi cient and 1/FFV often gave a straight line. Because of FFV calculation from density, 
gas diffusivity and PALS measurements are completely independent; the good correlation 
between them supports the idea of increasing the total free volume in rubbery - like mem-
branes, in our case increase of total free volume in Pebax 
® 
due to the PEG presence. 
Therefore, besides CO 
2
solubility increase in Pebax 
® 
/PEG blend membranes, the total 
free volume increase due to the plasticizer is also considered as an important factor for 
improvement of gas permeability. In searching for the design of novel membrane rubbery -
like materials it is desirable to develop strategies to enhance the chain fl exibility and the 
total free volume in order to increase the gas permeability keeping the selectivity intact.
The two most promising Polyactive copolymers, i.e. 1500PEO77PBT23 and 
4000PEO55PBT45, were also used for the preparation of blends with PEG. It was made 
in order to tailor a blend membrane material with improved CO 
2
separation performance. 
Blend membranes prepared from both copolymers showed excellent miscibility with 
PEG200 up to 50 wt.%. Table 12.3 presents the permeabilities of different gases and 
selectivities for 1500PEO77PBT23/PEG200 blend membranes. CO 
2
permeability slightly 
increases from 115 
Barrer (pristine copolymer) to 134 
Barrer (blend with 30 
wt.% of 
PEG), then it decreased to 110 Barrer. As can be seen, all gases behave in similar way, 
i.e. fi rst the permeability increases at lower content of PEG ( 
<
30 wt.%), and then at higher 
PEG content it decreases to values close to the pristine copolymer. This behaviour has 
been related to the crystallinity increase or chain ordering because of strong hydrogen 

---

238
Membrane Gas Separation
bonding, since a new melting point was observed between 50 and 150 ° C (discussed later). 
The selectivities are almost constant.
The diffusivity of gases plotted as a function of PEG content is shown in Figure 12.7 , 
and the H 
2
diffusivity decreases after the addition of 10 wt.% of PEG. The diffusivity of 
other gases has a similar trend. Therefore we concluded that the amorphous phase fraction 
Table 12.3  Permeability and perm selectivity of 1500 PEO 77 PBT 23 blend membranes 
with PEG [Reprinted with permission 67 ]
Permeability, Barrer
PEG (wt.%)
0
10
20
30
40
50
H 
2
11.3
11.8
11.5
12.1
10.8
10.1
N 
2
2.52
3.1
3.2
2.8
3.8
3
CH 
4
6.76
7.63
8.3
8.9
7.6
7.5
CO 
2
115
130
126
134
114
110
Permselectivity
CO 
2
/H 
2
10.2
11.0
11.0
11.1
10.6
10.9
CO 
2
/N 
2
45.6
42.1
40.0
48.7
30.1
36.9
CO 
2
/CH 
4
17.0
17.0
15.2
15.1
15.0
14.7
Source: 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. 
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
0
10
20
30
40
50
PEG [wt. %]
Dif
fusi
vity [cm
2
/ s]·10
6
H2
N2
CH4
CO2