Chair for Multicomponent Materials

Free volume in membranes and other non-crystalline materials studied by positron annihilation

Free volume and positron annihilation

After injection (typically from radioactive sources) and thermalization positrons in condensed matter preferentially reside in regions of reduced atomic density and undergo annihilation with electrons. Typical traps for positrons are vacancies, dislocations, grain boundaries and microscopic cavities in amorphous materials. Here the annihilation characteristics are statistically averaged quantities over the annihilation sites. In insulating materials positronium, a hydrogen-atom-like particle consisting of an electron and a positron, generally forms. The positron lifetime depends on the electron density which is reduced at most defects. Thus the lifetime, in the range of 0.1 - 5 ns depending on the material and type of defect, can often be regarded as a fingerprint of the defect. In polymers, in particular, a simple semi-empirical model, derived from quantum theory, directly relates the positron lifetime to the cavity size.

For the investigation of thin film and surface properties we used the moderated positron beams with well-defined implantation energy, available at the Research Reactor Munich and in Tsukuba, Japan.



During the last years, we have mainly used positron annihilation lifetime spectroscopy to study free volume in the following context:

Most investigations were supported by the German Research Foundation (DFG) or the Ministry of Education and Research (BMBF).

Selected Examples

For our positron annihilation studies in metallic glasses we refer to the research field “Metallic glasses and glass forming melts: diffusion and glass transition”.


Porous liquids

Applying Positron Annihilation Lifetime Spectroscopy, Tönjes Koschine and Klaus Rätzke in an international collaboration proved the existence of permanent porosity in special free-flowing liquids whose bulk properties are determined by their permanent porosity. To achieve this, the liquids are made up of designed cage molecules that provide a well-defined pore space and that are highly soluble in solvents whose molecules are too large to enter the pores. The results were published in Nature and got much international attention. 

Porous liquids

Sketch of a porous liquid containing cages that cannot be entered by the solvent molecules.



Liquids with permanent porosity, Nicola Giri, Mario G. Del Pópolo, Gavin Melaugh, Rebecca L. Greenaway, Klaus Rätzke, Tönjes Koschine, Laure Pison, Margarida F. Costa Gomes, Andrew I. Cooper & Stuart L. James; Nature 527, 216–220, (2015) doi:10.1038/nature16072


Microscopic cavities in polymers for membrane applications

Membranes widely used for gas separation. Further improvement can be achieved via a detailed knowledge on average hole size and its distribution, which can be obtained from positron annihilation lifetime spectroscopy. Within the last years, we analyzed several membrane materials for separation processes, starting with Teflon AF, a well-known gas separation copolymer where found an unusual temperature dependence of the free volume which we attributed local side group movement. Other high free volume polymers like the Polymers of Intrinsic Microporosity (PIM) showed similar behavior. We extended our methods to hexafluoropropylene polymers, pervoration membranes, and to polymer nanocomposites.

Selected publications

Emmler, T.; Heinrich, K.; Fritsch, D.; Budd, P. M.; Chaukura, N.; Ehlers, D.; Rätzke, K.; Faupel, F.: Free volume investigation of polymers of intrinsic microporosity (PIMs) PIM-1 and PIM1 copolymer with increased stiffness,Macromolecules Vol. 43 (2010) 6075-6084.

Dlubek, G.; Pionteck, J.; Rätzke, K.; Kruse, J.; Faupel, F.: The temperature dependence of the free volume in amorphous Teflon AF1600 and AF2400: A pressure-volume-temperature and positron lifetime study, Macromolecules May 41 (2008) 6125-6133.

Rudel, M.; Kruse, J.; Rätzke, K.; Faupel, F.; Yampolskii, Y.P.; Shantarovich, V.P.; Dlubek G.: Temperature dependence of positron annihilation lifetimes in high permeability polymers: amorphous Teflons AF, Macromolecules, 41(3) (2008) 788-795.

Lima de Miranda, R.; Kruse, J.; Rätzke, K.; Fritsch, D.; Abetz, V.; Budd, P.M.; Selbi, J.D.; McKeown, N.B.; Ghanem, B.S.; Faupel, F.: Unusual temperature dependence of positron lifetime in a polymer of intrinsic microporosity, Physica Status Solidi RRL 1(5) (2007) 190-192.

Structural investigation on functionalized 6FDA-copolyimide filmsR. Konietzny, C. Barth, S. Harms, K. Rätzke, P. Kölsch, C. Staudt Polymer International, in press (6/2011) doi: 10.1002/pi.3123

Gas transport and free volume in hexafluoropropylene polymers N.A. Belov, A.A.Zharov, A.V.Shashkin, M.Q.Shaikh K.Raetzke, Yu.P.Yampolskii Journal of Membrane Science 383 (2011) 70– 77.


Free-volume and storage stability of epoxy nanocomposites (BMBF project)

Epoxides are high-end materials, not only as adhesives, but also for encapsulation and insulation. One of the drawbacks is the problem of unwanted curing reactions prior to application due to the mixing the constituents. The general aim of this project was to develop multifunctional nanomodules for thermally controlled release of accelerator for crosslinking one-component epoxies. This allows usage of end-user ready one-component precursors of high end epoxides without mixing. The project was carried out with partners from the universites of Saarbrücken, Kaiserslautern and Luxenburg, the IFAM in Bremen and Industry partners (Kömmerling, Degussa, Sika) under the management of Siemens.

Our focus was on developing a suitable model for prediction of storage stability of the one-component precursors. Using positron annihilation lifetime spectroscopy at elevated temperatures and measuring the reduction in free volume during isothermal curing allowed us to observe in situ the onset of curing of the respective epoxide-nanocomposites. Applying a well established kinetic model (Johnson-Mehl-Avrami-Kolmogerov) we were able to predict the storage stabilities at room temperature on a reliable basis. Our results were corroborated by spectroscopic investigations of cooperation partners.


Ortho-positronium lifetime as a measure of degree of curing for a model epoxy. Note plateau at the beginning, indicating an incubation time for onset of curing and the delay in curing with decreasing temperature.

Selected publications

Shaikh, M.Q.; Rätzke, K.; Gaukler J.C.; Possart, W.; Faupel, F.: Reactive epoxies with functional zeolite fillers: IR spectroscopy and PALS studies, Journal of Materials Research Vol. 26 Number 22 (2011) 2877-2886.

Rätzke, K.; Shaikh, M.-Q.; Faupel, F.; Noeske, P.-L.M.: Shelf stability of reactive adhesive formulations: A case study for dicyandiamide-cured epoxy systems, International Journal of Adhesion and Adhesives 30 (2010) 105-110.


Free volume changes of polymer-azobenzene blends (SFB 677)

Using Positron annihilation lifetime spectroscopy, we could measure free volume changes of azobenzene molecules in PMMA during light-induced switching. Those photoswitchable molecules play a central role in the Collaborative Research Center SFB677 for light-induced conductance switching and other phenomena.

Selected publication

Harms, S.; Rätzke, K.; Pakula, C.; Zaporojtchenko, V.; Strunskus, T.; Egger, W.; Sperr, P.; Faupel, F.: Free volume changes on optical switching in azobenzene-PMMA blends studied by a pulsed low-energy positron beam, Journal Polymer Science B: Polymer Physics 49  (2011) 404.


Free volume in molecular liquids

Positron annihilation lifetime spectroscopy (PALS) was employed to characterize the temperature dependence of the free volume in several molecular liquids. From the PALS spectra the size (volume) distribution of local free volumes (subnanometer size holes), its mean, and mean dispersion were calculated. A comparison with the macroscopic volume from pressure-volume-temperature (PVT) experiments yielded the hole density and the specific hole free volume and thus a complete characterization of the free volume properties.  These data were used together with structural relaxation data from broad-band dielectric spectroscopy (BDS) to review various theoretical free volume models also in terms of Tg and glass fragility.

Selected publications

Dlubek, G.; Shaikh, M.Q.; Rätzke, K.; Pionteck, J.; Paluch, M.; Faupel, F.: Subnanometre size free volumes in amorphous Verapamil hydrochloride: A positron lifetime and PVT study through Tg in comparison with dielectric relaxation spectroscopy, European Journal of Pharmaceutical Science 41 (2010) 388-398.

Dlubek, G.; Shaikh, M.-Q.; Rätzke, K.; Paluch, M.; Faupel, F.: Free volume from positron lifetime and pressure-volume-temperature experiments in relation to structural relaxation of van der Waals molecular glass-forming liquids, Journal of Physics: Condens. Matter 22 (2010) 235104(10pp).

Dlubek, G.; Shaikh, M.; Rätzke, K.; Pionteck, J.; Paluch, M.; Faupel, F.: The Temperature dependence of free volume in phenyl salicylate and its relation to structural dynamics: A positron annihilation lifetime and pressure-volume-temperature study, Journal of Chemical Physics 130 (2009) 144906.

Dlubek, G.; Qasim Shaikh, M.; Rätzke, K.; Faupel, F.; Paluch, M.: The temperature dependency of the free volume from positron lifetime experiments and its relation to structural dynamics: Phenylphthalein-dimethylether, Physical Review E78 (2008) 51505.


Free volume and porosity in barnacle adhesives

Positron annihilation lifetime spectroscopy with a moderated beam was also applied to the adhesive layer of barnacles. The latter stick on every material, even on Teflon-coated ship hulls. Understanding the mechanisms of this extraordinary adhesion might help to develop new glues. Our investigations showed that the structure of the barnacle adhesive is highly porous up to the sub-nm scale. We suppose that the flexibility and the fracture behavior of the adhesive originate from this pronounced porosity.

Selected publication

Open volume in bioadhesive detected by positron annihilation lifetime spectroscopy K. Rätzke, M. Wiegemann, M. Q. Shaikh, S. Harms, R. Adelung, W. Egger, P. Sperr Acta Biomaterialia, 6 (2010) 2690-2694