Membrane Gas Separation

Molecular weight of PEO [g/mol]
CO
2
 per
mea
bility [Bar
rer]
designed polymer
4000PEO77PBT23
4000PEO55PBT45
blend with PEG
4000PEO84PBT16
Figure 12.2 Experimental and predicted data of CO 
2
permeability (for different block 
copolymer) as a function of molecular weight of PEO block [modifi ed plot from ref. 67 ]

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Tailoring Polymeric Membrane Based on Segmented Block Copolymers
233
Table 12.2  Thermal properties of PEO - PBT multi - block copolymers [Reprinted with 
permission 67 ]
Polymer
PEO
PBT
T
g
( ° C)
T
m
( ° C)
X
c
(%)
T
m
( ° C)
X
c
(%)
600PEO77PBT23
−
42
–
–
110
3
1000PEO80PBT20
−
45
19.6
16
–
–
1500PEO77PBT23
−
49
27.0
24
–
–
300PEO55PBT45
−
20
–
–
152
6
4000PEO55PBT45
−
49
40.5
19
213
9
4000PEO77PBT23 *
−
53
45.0
42
∼
180
–
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. 
P
M
M
CO
w
w
2
5
2
2 2746
0 1081
2 10
(
)
= −
+
⋅
(
)
− ⋅
(
)
−
.
.
(12.3)
From those points (theoretical data) one can conclude that the CO 
2
permeability would 
have a maximum value for the copolymer containing PEO with a molecular weight 
between 2500 and 3000 g/mol. However, the commercially available polymer is that with 
1500 g/mol (1500PEO77PBT). In order to validate the model and to have real information 
about the behaviour of these copolymers, the polymer 4000PEO77PBT23 was designed 
by us, and synthesized by PolyVation (Netherlands) [67] . The permeability of the syn-
thesized polymer was lower than expected, as observed in Figure 12.2 . 
The low permeability in 4000PEO77PBT23 copolymer has been attributed to high 
crystallinity of PEO phase ( X
c
= 0.42), moreover the T
m
was high (45 ° C), which shows 
that at room temperature the crystallities are big. The data for commercial polymers are 
given for comparison in Table 12.2 [67] . Therefore, besides the PEO content in the 
polymer system, the amorphous/crystalline phase ratio of PEO is also very important for 
improving the CO 
2
permeability, because an increase in the content of the crystalline 
phase leads to a strong decrease of solubility and diffusivity, and consequently the perme-
ability drops. As mentioned above, one way to enhance the gas permeability is destroying 
the crystalline phase, which will improve the solubility as well as diffusivity. Addition 
of 50 wt.% of PEG (200 g/mol) reduced the crystallinity of the PEO phase ( T
m
= 38 ° C 
and X
c
= 0.23) in the polymer designed (4000PEO77PBT23), thus a membrane with 
higher permeability has been obtained. The high permeability observed in that membrane 
is also due to the increased chain motion ( T
g
decrease from
− 
53 to
− 
79 ° C).
Table 12.2 shows thermal properties (obtained from second heating) of different PEO -
PBT copolymers; since the PEO content and molecular weight vary (300 – 4000 g/mol) 
different glass transition temperatures ( T
g
) are observed for each sample. It was not possible 
to detect by DSC method, the T
g
of PBT blocks and according to literature it should be 
around 60 ° C [62] . T
g
in block copolymers with 77 – 80 wt.% of PEO content slightly 
decreases according to the PEO molecular weight increase, whereas T
m
and degree of crys-
tallinity ( X
c
) increase. Interpretation of T
g
and T
m
is consistent for copolymers containing 
55 wt.% of PEO as well. For the PBT phase, an increase in PBT fraction seems to lead to 
higher values of T
m
and X
c
. T
m
for crystalline PBT is between 110 and 213 ° C [67,74] . The 
length of PBT chains ( M
w
) is unknown; hence their infl uence on the thermal properties can 
* designed polymer .

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234
Membrane Gas Separation
not be discussed. Taking into account these results, block copolymers with high M
w
, high 
fraction of PEO and low crystallinity could exhibit desired properties for gas separation. 
Figure 12.3 illustrates thermogravimetric results of PEO - PBT copolymers performed 
under argon. The samples start to degrade at approximately 330 ° C. There is no signifi cant 
difference in temperature of degradation between different copolymers, but it is possible 
to distinguish that samples with high PEO content start to degrade at lower temperatures. 
All samples show good thermal stability, and thus, they are promising membrane materi-
als, which can be used for gas separation at moderately high temperature ( 
∼
100 ° C). The 
temperature of exhaust gas in power plants for example is > 55 ° C; hence the membranes 
developed from these copolymers have potential application in this scenario.