Permeability thickness dependence of polydimethylsiloxane (PDMS) Membranes

The paper describes a reliable technique to prepare PDMS membrane used to determine the permeability to CO2 and He. SEM controls morphology of surface membrane and SEM/FIB measures the membrane thickness. The data show that permeability becomes thickness dependent below some tens of micrometers. The Islam׳s model, based on the non-equilibrium sorption–desorption process at the interface, fits quite well with the experimental data. The comparison shows that flux models based on interface reaction could apply to describe quantitatively the thickness-dependence of the permeability. The model allows to determine the surface reactions rate constants for CO2 and He on PDMS. The introduction of a specific characteristic thickness Lc provides a functional form, which describes very accurately the results of the present paper and other experiments on polymers. By reducing the membrane thickness below 200 nm, both permeability and selectivity change, indicating Knudsen diffusion as flux mechanisms responsible for permeation. In this range of thickness, SEM images reveal membrane defects.