Membrane Gas Separation

13.3.2
Pebax ® 1657 - based Blend Membrane: Structure and Properties 
As an acid – base reaction between the acidic CO 
2
and the electron - rich ether oxygen 
atoms of the PEG molecules can impart high CO 
2
permeability and selectivity [6,9] , we 
looked at the use of membranes made by blending the best Pebax ® copolymer (1657 
grade) with polymers containing ethylene oxide units to improve further the membrane 
performance. To reduce the overall crystallinity of the membranes by an intimate mixing 
of the second polymer in the blends, PEG of low molecular weight and poly(ethylene 
oxide - co - epichlorhydrine) (PEGEPI) were chosen as the additive polymers for the blend 
membranes. 
PEGEPI is a random copolymer which showed attractive performances in CO 
2
removal 
[41] . As all the blend membranes were prepared by solvent casting from NMP solutions, 
we measured fi rst the structure, morphology and gas permeation characteristics of the 
pure Pebax ® 1657 membrane prepared by solvent casting in the same conditions as those 
for the blend membranes; they were found to be very similar to those of the extruded 
fi lm. Melting points of PA and PE blocks and T
g
of PE block were the same; crystallinity 
of the PA block in the solvent casting Pebax ® 1657 membrane was slightly lower than 
that of 1657 extruded fi lm. The permeation results were the same for both. 
PEGEPI appeared to be miscible with the PEG phase of Pebax ® 1657: in DSC, the 
PEGEPI - Pebax ® 1657 blends showed a unique glass transition temperature (around 

---

270
Membrane Gas Separation
−
50 ° C) whose value changed with the membrane composition according to the Fox equa-
tion. All the membranes of these series exhibited lower permeability than the pure Pebax ®
1657 membrane with an improvement in selectivity (Table 13.5 ). The permeability reduc-
tion can be explained by a lower contribution to the permeability of PEGEPI than that of 
pure PEG in the membrane polyether phase. In fact, the high performances of PEGEPI 
copolymers compared with those of Pebax 
® 
1657 were only found for cross 
- 
linked 
PEGEPI of very high PEG contents: P CO 
2
= 96 Barrer and  
α
 
CO2/N2
= 63 for the copolymer 
with 90 wt.% PEG [41] . However, PEGEPI of high PEG content (much higher than that 
of the commercially available PEGEPI we used) could be attractive in blend membranes 
with Pebax 
® 
1657, due to expected selectivity improvements. Pebax 
® 
1657 would 
then impart its good mechanical properties to PEGEPI, without additional cross - linking 
processes.
The PEG - Pebax ® blend membranes were prepared with different amounts of PEG of 
300 g mol 
−
1
molecular mass. A preliminary study of blends with PEG of higher molecular 
mass showed that the miscibility of the two polymers in solid blends decreased with 
increasing PEG molecular mass. For instance, the blend with only 1 wt.% of PEG of 
3400 g mol 
−
1
molecular mass exhibited a melting peak of PEG3400 crystallites in DSC 
thermograms, while a single melting peak in DSC was found for blends with PEG300 in 
blends up to 50 wt.%. The better miscibility of low molecular mass PEG is understand-
able, if one considers its favourable mixing entropy, in addition to the favourable mixing 
enthalpy caused by hydrogen bonds between PEG hydroxyl end - groups with ether groups 
in Pebax ® . As non - miscible blends are known to be mechanically weak [42] , we focused 
our further study on Pebax ® 1657 blends with PEG300. 
The permeation data (Figure 13.7 ) indicate fi rst a reduction in CO 
2
permeability from 
that of a pure Pebax ® 1657 membrane, then a steady increase in CO 
2
permeability. The 
highest performances were obtained with a blend containing 20 wt.% PEG300: the CO 
2
permeability reached 128 Barrer and the CO 
2
/N 
2
ideal selectivity was 80.
Car et al. [9,10] reported a two - fold increase in CO 
2
permeability (up to 150 Barrer) 
without changes in CO 
2
/N 
2
selectivity when the membrane material changes from pure