NAMS 2002 Workshop - ICOM 2008

will be shown that homogeneous films could be obtained successfully by the
clever choice of mutually miscible fluorinated and non-fluorinated solvents.
Dense membranes, doped with a series of stilbene and azobenzene
photochromic probe molecules with different sizes, were thus prepared by
solution casting. The cis-trans isomerisation reaction was found to be completely
reversible and repeatable over numerous cycles when irradiating at 440 nm and
350 nm, respectively. The ratio of the degree of probe isomerisation at the
photostationary state in the solid polymer film, compared to that in a solution, is a
quantitative measure of the availability of free volume elements of the
corresponding size. A plot of this ratio as a function of the total isomerisation
volume of the probe molecules represents the FVD in the polymer. For two
different grades of Hyflon AD the experimentally determined FVD curve shows a
typical sigmoidal shape. The free volume size ranges from about 250 to 520 Å 3 in
Hyflon AD60X and from about 380 to 600 Å 3 in Hyflon AD80X. This is in
agreement with the higher gas permeability of Hyflon AD80X and with data
obtained by 129-Xe NMR spectroscopy and PALS. For the molecular dynamics
simulations, several independent atomistic bulk models were constructed. The
cavity size distribution was investigated by the particle insertion grid method [8].
It will be shown that the molecular modelling approach offers additional insight
into the free volume distribution compared to the experimental method, for
instance on the pore-interconnectivity.
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