Membrane Gas Separation

CO
2
 Permeation with Pebax®-based Membranes for Global Warming Reduction
257
(1) using low molecular weight liquid PEO or PEG
(2) designing phase - separated block copolymers with EO - rich but short segments
(3) building highly branched, cross - linked networks with high concentrations of PEG.
Block copolymers containing EO units like poly(amide - b - ether) have been shown as a 
good material for this purpose. 
In this chapter, we followed the idea of Lin and Freeman of enhancing CO 
2
separation 
properties. We investigated high selectivity membranes made of Pebax ® block copoly-
mers and their blends with free PEG (PEG is the standard abbreviation of polyethylene 
glycol, a short - chain polyethylene oxide polymer). These membranes were developed 
using a grant from the French Environment and Energy Management Agency (ADEME) 
and the Normandy Region from 2004 to 2007. The report on the results obtained with 
these membranes won an award in the innovative technologies for the environment at the 
Lyon Pollutec meeting in 2006. 
It should be noted that Car et al. [10,11] (see also Chapter 12 of this volume) worked 
independently on similar blend membranes, which were made of Pebax ® 1657, a grade 
of commercial poly(amide - b - ether) block copolymer with six polyamide blocks, and free 
PEG. Those membranes were also shown to exhibit high selectivity and permeability 
performances, which were attributed to changes in both the chemical composition (i.e. 
higher EO content) and the morphological structure (i.e. lower material crystallinity). On 
the contrary, Jaipurkar [12] observed CO 
2
permeability and selectivity improvements for 
the blend of Pebax ® 2533 with 25% of PEG 10000, but not for the blends with other 
PEG molecular weights or composition. 
Those results motivated us, in the present work, to study the structure, physical char-
acteristics and gas transport properties of membranes made of different grades of Pebax ®
block copolymers and their blends with PEG (Table 13.1 ).
Another type of membrane with high CO 
2
separation performances is the facilitated 
transport membrane. Membranes of this type are those in which an amine - based liquid 
supported by a solid membrane promotes a selective CO 
2
permeation. They exhibit gener-
ally a remarkably high selectivity and also high permeability at low CO 
2
pressure in the 
feed gas. Unfortunately, they suffer from very poor stability. For the separation of CO 
2
, 
fi xed carrier membranes were found to have high permselectivity without the drawback 
of poor stability, but with much lower permeability [13 – 15] . However, all membranes 
with amine - based carriers suffer from a strong reduction in performance with the reduc-
tion in the relative humidity in the gas mixtures, because the presence of water in the 
Table 13.1  General features of the different Pebax ®
Pebax ® series
11
13
31
33
Pebax ® grade
1657
1878
1074
3000
1041
6100
1205
PA/PE block 
nature
PA6/PEO
PA6/PTMO
PA12/PEO
PA12/PTMO
PA/PE wt.%
50/50
66,7/33,3
50/50
75/25
50/50
Density
1.14
1.09
1.07
1.04
1.01
Melting point 
( ° C)
204
195
158
170
147

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258
Membrane Gas Separation
whole membrane is required for the CO 
2
– amine group interactions [16] , thus affecting 
the rate of the CO 
2
permeation through the membrane. We think that, if amine groups 
were grafted on the membrane surface facing the feed mixture, the CO 
2
selective sorption 
into the membrane upstream face would be enhanced even at low humidity, leading to a 
general improvement of the membrane performance. 
The plasma technique [17] either through surface modifi cation or deposition of addi-
tional layers is often used to improve the transport properties of polymer fi lms. For 
example, the CF 
4
plasma treatment of the poly(trimethylsilylpropyne) membrane [18]
induces an increase of its permselectivity towards oxygen and nitrogen compared to the 
virgin membrane. A composite polybutadiene/polycarbonate (PB/PC)
[19] 
membrane 
treated in CHCl 
3
plasma shows higher oxygen permeation with transport properties 
depending on the plasma parameters like duration and power. These alterations of trans-
port properties are associated with a chemical modifi cation of the membrane surface, i.e. 
densifi cation and/or cross - linking. 
When the plasma is created in gases such as N 
2
, NH 
3
, H 
2
/N 
2
, NO, NO 
2
or their mixtures, 
the interactions between nitrogen, ammonia and mixture of hydrogen and nitrogen lead 
to attachment of different nitrogen - containing functions (amino, amide, nitrile, imide 
groups) [20,21] . An attraction of such modifi cations is to prepare surfaces with high levels 
of hydrophilicity and with attached sites having polar and basic characters that may be 
involved in interactions through Lewis acid – base interactions [22] . Good examples of 
increased interaction strength with nitrogen and ammonia from a plasma are those of 
treated polyaramid fi bres and polyaramid reinforced resin - matrix [23] . Poly(p - phenylene 
terephthalamide) (PPTA) can be functionalized with N 
2
, NH 
3
or methylamine plasma. 
Effects of nitrogen and nitrogen oxide plasmas on polyolefi ns and polyimides were also 
investigated and compared to those of oxygen and oxygen derivatives plasma treatments 
[23 – 26] . However, in NO - plasma, the main components are fragmented species from NO 
molecules and minor components are from O 
2
, N 
2
molecules, oxygen and nitrogen atoms; 
therefore the surface - grafted groups are dominated by oxidized species with low interac-
tion ability with carbonic acid gas. 
In line with these observations, in the second part of the chapter, cold plasma in N 
2
and hydrogen was used to graft amine groups onto a Pebax ® membrane surface in order 
to enhance the overall transport through the surface - modifi ed Pebax ® membrane. We 
expect that the amine groups created on the polymer fi lm in a N 
2
/H 
2
cold plasma gas [27]
promote enhanced CO 
2
sorption at the upstream membrane surface, and therefore enhanced 
CO 
2
selectivity in respect of nitrogen.