Membrane Gas Separation

13.2.6
AFM Surface Imaging 
AFM imaging in the contact mode and in the friction mode was done with a ‘ Nanoscope 
III A 
’ 
from Digital Instruments (Santa Barbara, USA) using a 100 
μ
m piezoelectric 
scanner. The cantilevers used were characterized by a spring constant of 0.16 N m 
−
1
. A 
standard pyramidal tip of silicon nitride was used. The measurements were carried out in 
air and at a constant force in the 10 
−
9
to 10 
−
8
N range.
13.2.7
Surface Modifi cation of Pebax ® Membrane by Cold Plasma 
The surface modifi cation of the polymer fi lm was carried out in a 13 L glass reactor 
equipped with a microwave generator working at 433 MHz. Nitrogen and hydrogen were 
fed at controlled fl ow rates on top of the reactor after a vacuum purge under 10 
−
9
bar. The 
plasma gases were generated in the discharge cavity with the energy transferred from the 
generator via a surfatron waveguide. The polymer fi lm reacted with the plasma gases that 
fl owed on its surface under a pressure of 10 
−
4
bar towards the reactor bottom. The exact 
conditions of the surface modifi cation will be given in the results section.
13.3
Results and Discussions 
13.3.1
Sorption and Permeation of CO
2
and N
2
in Extruded Pebax ® Films 
The CO 
2
extraction from fl ue gases would be economically feasible if the membranes can 
be produced at a low price. Pebax 
® 
polymers can give an opportunity to produce 
low cost membranes, since they are industrially produced and are easily processable by 
extrusion into thin fi lms of good quality according to the website of Arkema ( www.pebax.
com ). Such fi lms could be laminated onto a microporous support for a production of 

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262
Membrane Gas Separation
membranes in extrusion - lamination processes similar to those used for packaging fi lms. 
Such an opportunity motivated us to study extruded Pebax 
® 
fi lms in sorption and 
permeation. 
The permeation, diffusion and sorption coeffi cients obtained from the ‘ time - lag ’ experi-
ments are summarized in Table 13.2 for seven extruded - Pebax ® grades. The values of 
these coeffi cients vary in a large range according to the chemical composition of the 
Pebax ® fi lms. The most signifi cant features of the data are (1) the high sorption selectivity 
in favour of CO 
2
gas (CO 
2
sorption is up to 50 times higher than that of nitrogen); 
(2) the high permeability of polyether - rich Pebax ® grades, and (3) the low diffusion 
selectivity, which can be in favour of CO 
2
or of N 
2
. These features are characteristic of 
a rubbery material. The sorption experiments carried out on the microbalance showed a 
Henry - type isotherm and confi rmed the high CO 
2
sorption calculated from the ‘ time - lag ’
data for all grades (Figure 13.3 ). The 1657, 6100 and 1205 grades showed the best CO 
2
permeability (ca. 100 Barrer), and the 1657 grade the best ideal selectivity (  
α
  = 48).
As Pebax ® is a very complex material, more detailed studies on the physical structure 
and characteristics are needed to understand better the transport properties of Pebax ® of 
different grades. The transport properties through Pebax ® fi lm would depend on the 
volume fraction of the PA and polyether blocks, the PA crystallinity, the density and 
availability for interactions of polar groups, the block nature, length and their organization 
in the fi lms. Indeed, the two - step gas permeation mechanism requires fi rst a sorption of 
gas molecules and then gas diffusion in the free volume of the entangled polymer chains. 
It should be noted that the close molecular sizes of the studied gases (kinetic diameter 
values of 0.33 and 0.36 nm for CO 
2
and nitrogen, respectively) makes the diffusion selec-
tivity close to unity for all membranes. The best fi lms for the acid gas abatement would 
be those with high sorption coeffi cients for CO 
2
. 
As the CO 
2
molecule (polarizability value of 26.5 
×
10 
−
25
cm 
3
, compared with 
17.6
×
10 
−
25
cm 
3
for nitrogen) is highly polarizable, it can interact with the polar groups 
in the membrane. It seems normal that Pebax ® 1657 is the best membrane, due to its 
highest content in polar ether (EO) and amide groups. However, the situation was not 
quite clear for the other membranes, and a more detailed study on the fi ne membrane 
structure was required. 
The studies of the fi lms by DSC provide us with information about the phases and their 
physical properties. The DSC thermograms for the different Pebax ® grades are shown in 
Figure 13.4 . From the values of glass transition temperatures and melting points of the 
homopolymers, we were able to assign the measured values of the thermal characteristics 
to different phases. The glass transition temperature values reported in the literature are 
−
55 ° C for homopolymer PEO with a molecular mass of approximately 300 g mol 
−
1
, 51 
and 36 ° C for high molecular weight PA6 and PA12 [31] (no value for PTMO is available 
since it has variable T
g
values depending on its sequence length). The melting points for 
homopolymer standards ( M
n
≈
1000 g mol 
−
1
) are 35, 15, 225 and 178 ° C for PEO, PTMO, 
PA6 and PA12, respectively [31] .
The detected temperature values for the fi rst - order (endothermic peak) and the second -
order transitions (heat capacity jump) (Table 13.3 ) were close to the values of the glass 
transition temperature and melting points of the pure homopolymers. The slightly smaller 
values of the PA and polyether melting points probably refl ect the low molecular weight 
of the blocks and the less ordered crystallite structure. They can be therefore attributed 

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