Membrane Gas Separation

7 
Vapor Sorption and Diffusion 
in Mixed Matrices Based on Tefl on 
 ® 
 AF 2400
Maria Chiara Ferrari , Michele Galizia , Maria Grazia De Angelis
and Giulio Cesare Sarti
Dipartimento di Ingegneria Chimica, Mineraria e delle Tecnologie Ambientali (DICMA), 
Alma Mater Studiorum - Universit à di Bologna, Bologna, Italy
7.1
Introduction 
Polymers are currently the dominant materials for gas separation membranes because 
of their processability that allows the economic production of modules for large scale 
separation. Their separation ability, however, may be low in comparison with more rigid 
molecular sieving media and this may limit their performance. A signifi cant improvement 
can be achieved by dispersing an inorganic more selective phase in a polymeric process-
able phase, with the possibility of taking advantage of the peculiar characteristics of both 
materials. This class of composites is usually referred to as mixed matrix membranes 
(MMM) where the highly selective rigid phase is represented by zeolites, carbon molecu-
lar sieves or silica particles. The polymeric membranes employed are glassy polymers 
that already show good selective and/or gas permeation properties on their own, such as 
polyimides, polysulfone, amorphous Tefl on 
® 
, PTMSP and PMP [1] . 
Measurements of gas transport in AF 2400 fi lled by non - porous hydrophobic fumed 
silica (FS) nanoparticles showed that the inorganic fi ller enhances gas permeability [2] ; 
such an increase is more pronounced for larger penetrants and leads to a lower size selec-
tivity of the mixed matrix membrane with respect to the unloaded polymer. The sorption 

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Membrane Gas Separation
isotherm of these materials, reported per unit mass of solid membrane, is not greatly 
affected by the inorganic fi ller: the latter adsorbs only marginal amounts of penetrants. 
Surprisingly, on the contrary, the diffusion coeffi cients are increased signifi cantly by the 
addition of the nanoparticles, which are in fact obstacles to diffusion through the mem-
brane, thus determining an important increase in permeability. Fumed silica is non - porous 
and therefore a penetrant is adsorbed only on its surface, and diffusion around it requires 
a more tortuous path. The observed sorption and diffusion behaviour can be explained by 
considering that the nano - fi ller changes the packing structure of the polymeric matrix 
increasing the overall fractional free volume (FFV) of the polymer phase and thus increas-
ing its sorption capacity; that can compensate for the lower contribution to sorption 
offered by the particles. The additional free volume created affects diffusivity even more 
signifi cantly, in spite of the presence of obstacles in the diffusive path. That physical 
interpretation is in agreement with PALS measurements that in the super - glassy polymers 
such as amorphous Tefl ons and PTMSP found two different populations of holes whose 
distribution is modifi ed in MMM after the addition of FS, with an increase of the fraction 
of the larger FFV elements [3] . 
In a previous work [4] we already proposed a method to predict the enhancement 
in diffusivity due to the addition of fumed silica particles to high free volume matrices 
such as PTMSP and Tefl on 
® 
AF 2400. The model was tested only on the bases of avail-
able literature data while in this work we performed a detailed characterization of Tefl on 
® 
AF 2400 mixed matrices to further inspect and document the validity of the approach 
proposed.