Membrane Gas Separation

4.2.2
Procedures 
The polymer solution was prepared as a 5 wt. - % solution in HFE 7100 (0.25 g in 5.0 g 
of total solution). A 5.9
×
10 
−
3
mol L 
−
1
mother solution of the trans isomer of the dye 

---

Amorphous Glassy Perfl
 uoropolymer Membranes of Hyfl
 on 
AD®
65
Table 4.1  Overview of all the probe molecule structures, their van der Waals structures in the cis and trans isomeric form and their 
isomerization volume 
Photochromic Probes 
Molecular structure
van der Waals structure
Total volume 
a
( Å
3
)
Ref.
Cis isomer
Trans isomer
Azobenzene 
271
9
Stilbene 
373
9
Disperse orange 3 
Azobenzene, R = NO 
2
, 
R ’ = NH 
2
421
27
(continued overleaf)

---

66
Membrane Gas Separation
Photochromic Probes 
Molecular structure
van der Waals structure
Total volume 
a
( Å
3
)
Ref.
Cis isomer
Trans isomer
4,4 ’ - Dinitrostilbene 
449
9
Disperse red 1 
Azobenzene, R = NO 
2
, 
R ’ = N(C 
2
H 
5
)C 
2
H 
4
OH
517
27
Disperse orange 25 
Azobenzene, R = NO 
2
, 
R ’ = N(C 
2
H 
5
)C 
2
H 
4
CN
574
27
a
Total volume needed for isomerization reaction.
Table 4.1 (continued)

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Amorphous Glassy Perfl uoropolymer Membranes of Hyfl on AD®
67
molecules was prepared separately in DCM. The polymer solution containing the photo-
chromic molecules was then prepared by addition of 0.05 g of the concentrated dye solu-
tion to the Hyfl on solution under vigorous stirring. All photochromic solutions were 
prepared and stored in fl asks wrapped with aluminium foil to avoid uncontrolled light 
exposure and premature photoisomerization. Finally, the photochromic dense membranes 
were prepared by the solvent evaporation method in a Petri dish. Evaporation of the 
solvent yielded slightly coloured or colourless completely transparent haze - free fi lms. The 
procedure is schematically displayed in Figure 4.4 .
Films were fi rst dried for at least 48 h at room temperature and then in a vacuum oven 
at 50 ° C for 6 h. In all operations the exposure of the dyes to ambient light was minimized 
in order to prevent uncontrolled photoisomerization. The resulting dye concentration in 
the membrane is approx. 0.06 wt. - %, based on the polymer dry weight. It is assumed that 
during evaporation of the solvent the rapidly increasing viscosity of the solution could 
immobilize the photochromic molecules, and that the dye concentration is suffi ciently 
low to prevent noticeable aggregation or crystal nucleation of the dye. Since DCM is not 
a solvent for Hyfl on, too high concentrations in the casting solution could cause precipita-
tion of the polymer. Therefore the demixing behaviour of the ternary system Hyfl on 
® 
AD60X / HFE 7100 / DCM was determined by drop - wise addition of DCM to a Hyfl on 
solution in HFE, analogous to the procedure described previously for the determination 
of phase diagrams [26] . At all polymer concentrations from 1 – 10 wt. - % the maximum 
allowable amount of DCM in the solution without inducing phase separation was at least 
18 wt. - %, well above the DCM concentration reached in the casting solution upon addition 
of the concentrated dye solution (ca. 1 wt. - %).