NAMS 2002 Workshop - ICOM 2008

Generally these results were in good agreement with the literature data
determined mostly by sorption measurements [2-4].
A log-log plot of the diffusion coefficient vs. critical volume of the penetrant
showed the commonly observed linear relationship for permanent gases and
linear hydrocarbons. However, large differences were found between the trends
of dissimilar species in molecular shape (e.g. linear vs. branched or ring
structures) or in molecular interactions (e.g. inert molecules vs. polar or hydrogen
bonding species which may exhibit clustering [4]). For instance, n-alcohols show
a similar trend as n- alkanes, but their diffusion is nearly half an order of
magnitude slower than that of alkanes with a comparable critical volume, or it is
similar to alkanes which have nearly twice their crucial volume. Similarly,
cyclohexane diffusion is over two orders of magnitude slower than hexane
diffusion and MTBE is 200 times slower than diethylether. The anomalous
behaviour of the lower alcohols is further illustrated by the highly unusual
transient in the methanol and ethanol permeation curves, suggesting the
presence of multiple diffusion coefficients.
The experimental results were discussed in terms of the penetrant’s molecular
properties and the free volume of the polymers. For this purpose the free volume
of the two Hyflon samples was studied by molecular dynamics (MD) simulations.
Several independent atomistic bulk models were constructed for each sample
and the cavity size distribution was investigated by the particle insertion grid
method [5]. It will be shown that the molecular modelling approach offers
important insight in the possible behaviour of the penetrants in the free volume
elements of the polymer.
References
1. M. Macchione, J.C. Jansen, G. De Luca, E. Tocci, M. Longeri and E. Drioli, Polymer 48 (2007)
2619-2635.
2. R.S. Prabhakar, B.D. Freeman, I. Roman, Macromolecules 2004, 37, 7688-7697.
3. A.Yu. Alentiev, Yu. P. Yampolskii, V.P. Shantarovich, S.N. Nemser, J. Membr. Sci 126 (1997)
123-132.
4. A. Tokarev, K. Friess, J. Machková, M. `ípek, Yu. Yampolskii, J. Polym. Sci. Part B, 44 (2006)
832-844.
5. D. Hoffmann, M. Heuchel, Yu. Yampolskii, V. Khotimskii, V. Shantarovich, Macromolecules, 35
(2002) 2129.