NAMS 2002 Workshop - ICOM 2008

Polymeric Membranes I – 2
Monday July 14, 10:15 AM-10:45 AM, Moloka’i
Unusual Temperature Dependence of Positron Lifetime in a Polymer of
Intrinsic Microporosity
K. Rätzke (Speaker), Technische Fakultät der CAU, Lehrstuhl für Materialverbunde, Kaiserstr. 2,
Germany, kr@tf.uni-kiel.de
R. Lima De Miranda, Technische Fakultät der CAU, Lehrstuhl für Materialverbunde, Kaiserstr. 2,
Germany
J. Kruse, Technische Fakultät der CAU, Lehrstuhl für Materialverbunde, Kaiserstr. 2, Germany
F. Faupel, Technische Fakultät der CAU, Lehrstuhl für Materialverbunde, Kaiserstr. 2, Germany
D. Fritsch, Institut für Polymerforschung, GKSS-Forschungszentrum Geesthacht, Germany
V. Abetz, Institut für Polymerforschung, GKSS-Forschungszentrum Geesthacht, Germany
P. Budd, School of Chemistry, The University of Manchester, Manchester, UK
J. Selbie, School of Chemistry, The University of Manchester, Manchester, UK
N. McKeown, School of Chemistry, Cardiff University, Cardiff, UK
B. Ghanem, School of Chemistry, Cardiff University, Cardiff, UK
The performance of glassy polymeric membranes for gas separation is mainly
determined by the availiability of free volume. Polymers of intrinsic microporosity
are interesting candidates due to the high abundance of accessible free volume.
Positron annihilation lifetime spectroscopy (PALS) is a generally accepted
method for investigation of free volume in polymers due to the so-called standard
model developed by Tao and Eldrup. This simple quantum mechanical model
assumes the Ps to be confined to spherical holes with infinitely high walls and
gives a direct relationship between pick-off lifetime of orthopositronium and the
size of the free volume holes. Since hole sizes in amorphous polymers are
relatively broadly distributed, the discrete o-Ps lifetime obtained from fits to
lifetime spectra and hence the hole radius has to be regarded as an average
value.
We performed measurements of the temperature dependence of PALS in two
polymers of intrinsic microporosity (PIM-1 and PIM-7) in the range from 143 to
523 K [1] . The mean value of the free volume calculated from the orthopositronium
life time is in the range of typical values for high free volume
polymers. However, the temperature dependence of the local free volume is nonmonotonous
in contrast to the macroscopic thermal expansion. The tentative
explanation is linked to the spirocenters in the polymer. The intensity shows no
anomalous temperature behavior, but is dependent on the pretreatment of the
samples. Experiments are underway to clarify this unusual behavior and will be
presented on the conference.