242
Membrane Gas Separation
The Maxwell model was calculated only for Pebax/PEG system in order to compare the
data with an additive model.
Pebax
®
/PEG and 40000PEO55PBT/PEG exhibit similar trend (experimental data): the
higher the
PEG content, the higher the permeability. 1500PEO77PBT/PEG blends,
however, are different: permeability increases and then is reduced below theoretical
values calculated by the additive model. It can be observed that every copolymer shows
different behaviour; hence, the selection of an appropriate additive for a specifi c copoly-
mer is very important.
DSC curves obtained for the 1500PEO77PBT23 copolymer and its blends with PEG
are presented in Figure 12.11 . In the fi rst heating (solid lines) and second heating (dashed
lines) the melting temperatures of PEO phase are between 10 and 35 ° C; for the second
heating the melting temperatures are shifted to higher values. The
pristine copolymer does
not show the melting temperature of PBT, and the blends present peak around 140 ° C in
the fi rst heating, which disappear in the second heating. These results show that an ordered
structure is present in the as - cast membranes and therefore indicate that the microphase
separation between PEO and PBT was induced, as revealed in endothermic peaks.
On the other hand, pristine 4000PEO55PBT45 copolymer showed two characteristic
T
m
values (40 ° C for PEO and 213 for PBT, see Table 12.2 ). It is consistent with the
microphase separated structure in block copolymers [91] .
For the blend samples T
m
for
the PEO crystalline phase was shifted to higher values when PEG is added into the
polymer matrix (Figure 12.12 a). These results are in contradiction to those observed in
Pebax/PEG system reported earlier [24] . High PEG content slightly increases the crystal-
linity: from 19% (pristine polymer) to 24% (blend with 50% of PEG).
T
m
values of PBT
50
70
90
110
130
150
170
190
210
0,00
0,10
0,20
0,30
0,40
0,50
0,60
0,70
0,80
0,90
1,00
Φ
[volume fraction of PEG]
P
er
mea
bility
Teor(Pebax)
Exp(Pebax)
Teor (1500PEO77PBT)
Exp (1500PEO77PBT)
Teor (4000PEO55PBT)
Exp (4000PEO55PBT)
Maxwell (Pebax)
151
143
188
110
Figure 12.10 Experimental and estimated values of permeability in Pebax
®
/PEG,
1500PEO77PBT /PEG and 40000PEO55PBT/PEG blend membranes [modifi ed plot from
ref. 24 ]
Tailoring Polymeric Membrane Based on Segmented Block Copolymers
243
in blend samples are not well defi ned (
Figure 12.12 b), they are shifted to lower values
with PEG content, and blends with high PEG content presented only an amorphous phase.
Thus, the decrease of crystallinity in the PBT phase led to greatly improved diffusivity
for all gases (Figure 12.8 ), especially for H
2
and CO
2
. Thermal properties and the crystal-
linity values are in a good agreement with permeability values: the higher the content of
the amorphous phase, the higher is the rate of gas transport. The observed peak (
∼
140 ° C)
in a previous blend based on 1500PEO77PBT23 copolymer was not detected in the
present blend (4000PEO55PBT45/PEG). Therefore, the lower permeability obtained in
1500PEO77PBT23/PEG blends has been mainly attributed to the ordered structure found
by DSC analysis. Although this ordered
phase is not thermally stable, its presence in fresh
fi lms could have affected the gas permeability.
Some results of atomic force microscopy (AFM) of pristine copolymers and their
blends are presented in Figure 12.13 . The blends contain 40 wt.% of PEG and they are
representative samples for discussion of morphology. Microphase separation can be
observed in Polyactive
®
/PEG blend (Figure
12.13
b) and not in pristine copolymer
(Figure 12.13 a). As observed in DSC analysis, the blends presented an ordered structure
(
Figure 12.11 , peak around 140 ° C), in good agreement with AFM images. The
PEG in
Polyactive
®
can induce microphase separation because of strong hydrogen bonding, and
the PBT hard phase is surrounded by PEO and PEG phase, thus the CO
2
permeability is
decreased.
-150 -100 -50
0
50
100
150
200
250
300
0
2
4
6
8
10
40% PEG
50% PEG
0% PEG
30% PEG
20% PEG
Hea
t flow
T[°C]
1.heating
2.heating
10% PEG
Figure 12.11 DSC thermogram of copolymer 1500PEO77PBT23 and its blends with
PEG200. The termograms have been displaced vertically for easier viewing . 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
244
Membrane Gas Separation
The morphology of Pebax
®
samples (Figures 12.13 c and 12.13 d) are different in com-
parison with Polyactive
®
samples. The pristine Pebax
®
presents ordered structures as a
result of microphase separation, and its blend with PEG does not exhibit any organized
structure, hence a higher gas permeability is observed in these blend membranes. In
Polyactive/PEG membranes the hydrogen bonding can
be stronger than in Pebax
®
/PEG
blend, this can induce an organization of phases and consequently the CO
2
permeability
will be reduced.
30
40
50
60
0,0
0,2
0,4
0,6
0,8
1,0
1,2
40%PEG
30%PEG
20%PEG
10%PEG
0%PEG
Hea
t flow [mW]
T [°C]
50%PEG
Exo
(a)
160
170
180
190
200
210
220
0,1
0,2
0,3
0,4
0,5
50%PEG
30%PEG
20%PEG
40%PEG
10%PEG
Hea
t flow [mW]
T [°C]
0%PEG
Exo
(b)
Figure 12.12 DSC thermograms for (a) PEO and (b) PBT blocks in copolymer
4000PEO55PBT45 . 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
