NAMS 2002 Workshop - ICOM 2008

Nanofiltration and Reverse Osmosis II - Imaging and Characterization - 2
Tuesday July 15, 9:30 AM-10:00 AM, Maui
Positron Annihilation Spectroscopy (PAS): A New Powerful Technique to
Study Membrane Structure
A. Cano-Odena (Speaker), Center for Surface Chemistry and Catalysis, Katholieke Universiteit
Leuven, Leuven, Belgium
P. Vandezande, Center for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven,
Leuven, Belgium
K. Hendrix, Center for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven, Leuven,
Belgium
R. Zaman, Center for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven, Leuven,
Belgium
K. Mostafa, NUMAT (Nuclear Methods in Materials Science), Dept Subatomic and Radiation,
Gent, Belgium
J. De Baerdemaeker, NUMAT (Nuclear Methods in Materials Science), Dept Subatomic and
Radiation, Gent, Belgium
I. Vankelecom, Center for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven,
Leuven, Belgium, ivo.vankelecom@biw.kuleuven.be
Introduction Positron annihilation spectroscopy (PAS) is a nondestructive
technique used to study defects and open volumes in materials based on the
analysis of the ³-ray radiation emitted due the annihilation of positrons (antimatter
counterpart of the electron) with electrons of the material. Positrons injected in a
polymer sample can either annihilate freely or capture an electron forming a
meta-stable bound state positronium(Ps) with two possible states depending on
the relative orientations of the spins of the electron and the positron:ortho-
positronium(o-Ps) and para-positronium(p-Ps). The use of a variable low energy
positron beam enables the study of the characteristic annihilation of the
positronium from the surface down to a couple of micron. Hence with a positron
beam a depth profile of the porosity is measured. O-Ps has a longer lifetime in
vacuum and gets preferentially localized in the free volume of the polymer. In
positron annihilation lifetime spectroscopy (PALS) the decrease in o- Ps lifetime
is related to free volume cavities radius. Despite clear benefits over more indirect
characterization techniques PAS has only recently been applied in membrane
research [1,2]. Its use will be here extended to an in- depth characterization of
the morphology of the polymeric structure allowing to correlate physical defects
at atomic scale (free volume) with membrane performance (permeability,
selectivity).
Thanks to increased environmental concerns and the search for cleaner and
energy-efficient technologies, solvent resistant nanofiltration (SRNF)[3] has
received enhanced attention as a promising technique, offering a sustainable
alternative for conventional energy-intensive and waste-generating separations.
SRNF holds a vast potential in food, (fine-)chemical, pharmaceutical and