Membrane Gas Separation

4.2.3
Membrane Properties 
The membranes were prepared as a dense fi lm of about 60
μ
m by solution casting and 
solvent evaporation, as described in the previous section. No photochromic dyes were 
used in the membranes for the transport measurements. For the FV analysis, six different 
Casting solution with
polymer, solvent, dye 
Solvent evaporation in
controlled atmosphere:
1) Ambient conditions
2) In vacuum oven 
Dense Hyflon AD membrane
( < 0.1% dye ) 
Polymer solution in
fluorinated solvent
(Rapid stirring!)
Concentrated
dye in CH
2
Cl
2
Release 
with water
Fluorinated solvent
(Stirring)
Casting
Hyflon AD
Figure 4.4 Membrane preparation procedure. All steps but the preparation of the 
polymer solution are carried out in the dark

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68
Membrane Gas Separation
probes were used (Table 4.1 ). Their fi nal concentration in the dried membrane is approxi-
mately 1.7
×
10 
−
6
mol cm 
−
3
. This is less than 0.1% of the fi nal weight and corresponds to 
a total number of molecules equal to ca. 10 
18
cm 
−
3
. In terms of weight (or volume) this is 
practically negligible compared to the overall free volume in Hyfl on AD of ca. 23 vol. - % 
[27] . Also in quantitative manner this is much lower than the concentration of FV ele-
ments, usually between 2
×
10 
20
cm 
−
3
and 8
×
10 
20
cm 
−
3
[1] . It may therefore indeed be 
assumed that the probes themselves do not have a notable infl uence on the free volume. 
The more than 200 - fold excess of available FV holes also reduces the probability that 
two molecules might occupy the same FVE to practically zero, justifying the assumptions 
made in the introduction above.