Membrane Gas Separation

4.7
Conclusions 
The photochromic probe method was successfully used to analyze the FVE size distribu-
tion in membranes of the glassy amorphous perfl uoropolymer Hyfl on AD. Two grades 
of Hyfon with a slightly different content of the rigid cyclic co - monomer showed a rela-
tively narrow FV size distribution, and the entire distribution of Hyfl on AD80X, with the 
highest cyclic co - monomer content is shifted to somewhat higher volumes compared to 
the AD60X grade. The results of the photochromic probe method, indicating a FVE size 
distribution ranging from ca. 250 – 560 Å
3
in Hyfl on 
® 
AD60X and from ca. 280 – 640 Å
3
in 
Hyfl on 
® 
AD80X, are in fair agreement with literature data from other experimental tech-
niques like inverse gas chromatography, positron annihilation lifetime spectroscopy, 
129
Xe - NMR spectroscopy. Molecular dynamics simulations show a combination of single 
small FVEs and large interconnected FVEs, extending in three dimensions. 
Organic vapour transport reveals fundamental differences between a polar substance 
like methanol and the less polar dichloromethane. An extremely wide transient region for 
methanol vapour permeation may be related to clustering of the alcohol molecules through 
the formation of hydrogen bridges between the hydroxyl groups. This is supported 
by careful analysis of the time lag behaviour, which indicates a spectrum of diffusion 
constants, rather than a single one. 
Based on their van der Waals volume, even large clusters of at least three methanol 
molecules are still considerably smaller than the FVE size observed experimentally. Their 
presence in the fi lm should therefore not be hindered for sterical reasons. The total con-

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Amorphous Glassy Perfl uoropolymer Membranes of Hyfl on AD®
81
centration of the vapours at the feed pressure used, calculated by the tangent method, is 
10.0 cm 
3
STP
cm 
−
3
(38 mg cm 
−
3
) for DCM and 3.18 cm 
3
STP
cm 
– 3
(4.5 mg cm 
- 3
) for methanol. 
In the condensed (liquid) state this corresponds to a volume far below the available free 
volume of ca. 23%.