NAMS 2002 Workshop - ICOM 2008

Gas Separation II – 6
Tuesday July 15, 5:00 PM-5:30 PM, Kaua’i
Analysis of the Size Distribution of Local Free Volume in Hyflon® AD
Perfluoropolymer Gas Separation Membranes by Photochromic Probes
J. Jansen (Speaker), ITM-CNR, Rende (CS), Italy - jc.jansen@itm.cnr.it
E. Tocci, ITM-CNR, Rende (CS), Italy - e.tocci@itm.cnr.it
M. Macchione, Università della Calabria, Rende (CS), Italy - marmach@unical.it
L. De Lorenzo, ITM-CNR, Rende (CS), Italy - Ldelore@unical.it
M. Heuchel, GKSS Research Center, Teltow, Germany - matthias.heuchel@gkss.de
E. Drioli,ITM-CNR, Rende (CS), Italy - e.drioli@itm.cnr.it
This paper reports on the first successful application of the photochromic probe
technique for the evaluation of the free volume distribution (FVD) in the
amorphous glassy perfluorpolymer Hyflon® AD, a copolymer of 2,2,4- trifluoro-5trifluorometoxy-1,3-dioxole
and tetrafluoroethylene. Hyflon AD is highly
permeable to permanent gases, offering interesting perspectives for use in gas
separation membranes [1,2], especially because of its high thermal, chemical,
ageing and weather resistance and excellent inertness to most organic solvents.
As in other amorphous perfluoropolymers [3], the high gas permeability of Hyflon
is related to the high Fractional Free Volume (FFV), usually estimated by Bondi's
group contribution method [1,4-6]. Besides the total FFV, knowledge of its
distribution is important for the understanding of the transport properties of the
Hyflon membranes.
The aim of the present work is to use the photochromic probe method and
molecular dynamics (MD) simulations to determine the size distribution of local
free volume elements in Hyflon AD membranes and to correlate this to their
transport properties. Experimentally, photochromic probing is relatively simple
compared to other probing methods, like 129-Xe NMR spectroscopy and
Positron Annihilation Lifetime Spectroscopy (PALS). It is based on the principle
that photo-isomerizable molecules require a certain free volume to undergo
isomerisation when dispersed in the polymer matrix [7]. Using a series of probe
molecules with different size, spectrophotometric analysis of the degree of
isomerisation of each probe will yield the FVD.
In the case of perfluoropolymers a major technical difficulty is the sample
preparation, because the hydrocarbon probe molecules are usually insoluble in
the fluorinated solvent for the polymer and the polymer is insoluble in the solvent
for the probes. The main challenge of the present work is therefore to find a
suitable method to obtain a homogeneous dispersion of the dye molecules in
Hyflon AD films, to be subjected to subsequent spectrophotometric analysis. It