Abstract: Background: Due to the integration of the advantages of the inorganic filler phase and the polymer phase, the organic-inorganic hybrid membrane, which can overcome the trade-off effect between permeability and selectivity, is widely used in many areas. Purpose: The aim is to study the microstructure of the PDMS-MIL-101(Cr) (Poly (dimethyl siloxane)-Material Institute Lavoisier-101(Cr)), which is a polymer-metal organic framework (MOFs) materials hybrid membrane. Methods: Positron annihilation lifetime spectroscopy (PALS) and coincidence Doppler broadening energy spectroscopy (CDBES) were used. Results: PALS results showed that the size of both the space at cross linking site and the free volume of polymer matrix decreased when the MIL-101(Cr) keep increasing, which indicates that MOFs penetrated into the free volume of polymer. Moreover, according to the CDB ratio spectra of hybrid membranes to single silicon crystal, it can be found that the effect of positron preferential annihilation with oxygen atoms was weakened with much more MIL-101(Cr) in hybrid membranes, and a fraction of positrons annihilated with the electrons of metal atoms locating at the center of subnanometer particles of MIL-101(Cr). It is inferred that after entering the free volume of polymer, MIL-101(Cr) particles may interact with oxygen atoms, thus partly block the way for positron’s access to oxygen atoms. Conclusion: Positron annihilation technique is proved to be effective to characterize the microstructure of organic-inorganic hybrid membranes.

Key words: Positron annihilation, Free volume, Hybrid membrane, PDMS-MIL-101 (Poly (dimethyl siloxane)-Material Institute Lavoisier-101)