Membrane Gas Separation

12.3 Block Copolymers and their Blends with Polyethylene Glycol 
It has been observed [24] that for PEG (200 g/mol) modifi ed Pebax 
® 
membrane for CO 
2
separation the CO 
2
permeability increased by a factor of about 2 (from 73 to 151 Barrer) 
and the separation factor CO 
2
/H 
2
also increased by PEG addition (50 wt.%). This enhance-
ment was attributed to the appearance of additional ethylene oxide (EO) units and free 
volume increase. Higher content of EO units results in an increase in the solubility of 
CO 
2
. Later, the total free volume increase and hence the increase of the permeability 
was demonstrated by measurements of density and by positron annihilation lifetime 
spectroscopy (PALS) analysis [75] . 
0
100
200
300
400
500
0
20
40
60
80
100
4000PEO55
300PEO55
600PEO77
1000PEO80
1500PEO77
Mass loss [%]
Temperature [°C]
Figure 12.3 Thermal gravimetric analyses of PEO - PBT multi - block copolymers . 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
235
Figure 12.4 presents the density decrease and the total fractional free volume (FFV) 
increase by addition of PEG into the Pebax 
® 
. The experimental values of density, which 
are lower than those calculated by the additive model, suggest that the total free volume 
is increased. The FFV was estimated from a defi nition of the volume fraction of the fi ller 
by using density values [76] , and by solving the following equations:
φ
ρ
ρ
ρ
F
N
F
F
P
P
F
F
=
(
)
+
(
)
W
W
W
(12.4)
φ
φ
ρ
ρ
ρ
ρ
ρ
ρ
F
T
F
N
P
P
F
F
P
P
F
P
V
F
F
C
=
+
⎡
⎣⎢
⎤
⎦⎥
+
+
⎡
⎣⎢
⎤
⎦⎥
=
⎡
⎣⎢
⎤
⎦⎥
⎡
⎣⎢
⎤
W
W
W
W
V
W
1
⎦⎦⎥
=
W
F
C
F
ρ
ρ
(12.5)
FFV
FFV
FV
=
+
P
V
(12.6)
where
φ
F
N
and
φ
F
T
are the nominal and the true fi ller volume fraction respectively, W
F
, W
P
, 
 
ρ
 
F
and  
ρ
 
P
, are the weight percents and densities of fi ller and pure polymer respectively 
and,  
ρ
 
C
, V
V
, FFV
P
and FV
V
are the density of composite, the void volume, the fractional 
free volume of pure polymer and the void volume fraction. Note that the total FFV has 
been calculated by adding the void volume fraction created by PEG addition to the free 
volume fraction of pristine copolymer. 
1,1100
1,1150
1,1200
1,1250
1,1300
1,1350
1,1400
1,1450
0,00
0,10
0,20
0,30
0,40
0,50
Φ