NAMS 2002 Workshop - ICOM 2008

functional groups becomes available. The third approach is positron annihilation
lifetime spectroscopy (PALS) technique, which can measure the mean size and
size distribution of subnanometer-size vacancies in polymers. The published
mean hole volume detected by PALS is employed to predict solvent diffusion
coefficients with the help of the EOS. The predictions are generally consistent
with published diffusion data. In addition, the analysis of hole size distribution can
prove the reliability of the EOS-approach and group contribution-approach. The
fourth approach is molecular simulation, which can investigate free- volume holes
from microscopic point of view. The simulation is performed on PVAc and PMA,
which are structural isomers of each other. The quantitative relation between the
simulation and the free volume defined by the Simha-Somcynsky EOS, PALS
measurements and free-volume theory is given. The infinite dilution diffusion
coefficients in PVAc are predicted using simulation method, and the predictions
are in good agreement with experimental data.
This study provides a consistent feature to describe solvent transport in polymer
matrix with both macro- and microscopic structure. The prediction ability of the
original free-volume theory is improved by introducing the EOS and group
contribution method. Therefore, the modified model is useful to understand mass
transport in polymeric dense membranes and to develop a novel approach for
membrane materials design.
Acknowledgement The authors gratefully acknowledge the financial assistance
from the Major State Basic Research Development Program of China (973
Program) (No. 2003CB615701) and the National Natural Science Foundation of
China (No. 20676068).
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