Membrane Gas Separation

Membranes of Hyfl on AD 
 ® 
 : 
Free Volume Distribution by 
Photochromic Probing and Vapour 
Transport Properties
Johannes Carolus Jansen 
a
, Karel Friess 
b
, Elena Tocci 
a
, Marialuigia Macchione 
c
,
Luana De Lorenzo 
a
, Matthias Heuchel 
d
, Yuri P. Yampolskii 
e
and Enrico Drioli 
a


a
 Institute of Membrane Technology, ITM - CNR, c/o University of Calabria, Rende, Italy
 
b
 Department of Physical Chemistry, Institute of Chemical Technology, Prague, Czech Republic
 
c
 University of Calabria, Rende, Italy 

d
 GKSS Research Center, Institute of Chemistry, Teltow, Germany
 
e
 A. V. Topchiev Institute of Petrochemical Synthesis, Moscow, Russia
4.1
Introduction and Scope 
Fluorinated polymers have quite different properties compared to their hydrocarbon 
counterparts, due to the nature of the C – F bond. They often have a particularly high 
thermal and chemical stability, insolubility in common organic solvents and resistance to 
swelling by condensable gases or vapours. Such properties are an advantage for those 
membrane applications where the separation takes place under harsh conditions. 
Crystalline perfl uoropolymers such as Tefl on 
® 
are extremely stable but their crystalline 
nature makes them mostly suitable for porous membranes, either for fi ltration of liquid 
media, or for instance for membrane distillation processes which take advantage of the 

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60
Membrane Gas Separation
extremely high hydrophobicity of perfl uorinated polymers. This prevents wetting of the 
pores by water and thus enables the transport through the vapour phase alone. 
The increasing pressure on industry to develop more sustainable technologies is an 
additional driving force for radical changes in the materials choice. In this respect, in 
recent years, amorphous glassy perfl uoropolymers (PFPs) such as Cytop 
® 
, Tefon AF 
® 
and 
Hyfl on AD 
® 
are under the attention of the membrane community for their application 
in gas and/or vapour separation processes. The latter is possible due to the presence of 
bulky groups in the polymer chain which adds a relatively high permeability to their 
already outstanding set of properties. In theory this allows their application in fi elds with 
particularly harsh conditions where other polymers may fail due to chemical attack, 
thermal instability and plasticization by condensable species. 
An important aspect for the understanding of the transport properties of such polymers 
is the study of their structure, down to the molecular level. Especially in the absence of 
swelling phenomena, when the penetrant solubility in the perfl uoropolymer matrix is rela-
tively low, as is usually the case for non - fl uorinated hydrocarbon vapours, the main factor 
which determines the transport properties is the Free Volume (FV). Knowledge of the 
free volume is therefore often of great importance for the understanding of the transport 
properties of dense membrane materials. 
The scope of this chapter is to determine the average free volume size and size 
distribution for Hyfl on AD 
® 
(Figure
4.1 
) by the photochromic probe method and to 
study the transport of organic vapours in membranes of this polymer. The fi nal aim is 
to correlate the transport data in the polymer with the free volume element (FVE) size 
distribution and to gain a better understanding of structure 
– 
property relationships in 
perfl uoropolymers.