Membrane Gas Separation

4.1.3
Relevance of Free Volume for Mass Transport Properties 
It is generally accepted that mass transport in dense polymer membranes takes place 
according to the well - known solution - diffusion mechanism [18,19] . For non - swelling and 
non - plasticizing species the amount of penetrant that can dissolve in the glassy polymer 
matrix depends on the available sorption sites and often a typical dual mode sorption 
behaviour is observed [20] . The number of Langmuir sorption sites is strongly related to 
the free volume distribution of the sample. 
Also penetrant diffusion depends strongly on the free volume distributions, because it 
takes place through a kind of ‘ hopping ’ mechanism where the penetrant molecules move 
around in a free volume element until they fi nd enough energy to exceed the energy barrier 
for a jump to the neighbouring FVE [21 – 23] . These energy barriers depend on many 
different factors, such as the rigidity of the polymer matrix, the size of the molecule and 
the degree of interconnectivity of the free volume. Therefore, knowledge of the free 
volume is of great help for a better understanding of the transport properties of dense 
polymeric membranes. The gas transport properties of solution - cast and melt - pressed 
Hyfl on AD60X membranes have been published previously and the results show that the 
solution - diffusion model can be applied to this polymer [24] .
4.2
Membrane Preparation 
4.2.1
Materials 
Two different grades of glassy amorphous perfl uorpolymers were used, Hyfl on 
® 
AD60X 
and Hyfl on 
® 
AD80X (Solvay Solexis), having glass transition temperatures of 130 ° C [24]
and 134 ° C [25] , respectively. In this study 1 - methoxy - nonafl uorobutane (3M, trade name 
HFE 7100, b.p. 60 ° C, mol. wt. 250 g mol 
−
1
), was used as the solvent to prepare solution -
cast Hyfl on AD fi lms. Photochromic molecules (Table 4.1 ) were purchased from Aldrich 
in their stable
trans 
isomeric form and they were used without further purifi cation. 
Dichloromethane (Carlo Erba Reagenti, DCM) was used as received, serving as the 
solvent to prepare the concentrated mother solution of the photochromic molecules.