Figure 2: (a) Schematic representation of the solution-diffusion process of a molecule (A) through a polymer matrix. The diffusion takes place in the free volume (). To avoid reverse diffusion, the free position has to be occupied by another molecule or a polymer segment (B). (b) The transport through a solution-diffusion membrane consists of three relevant steps: sorption, diffusion, and desorption ( designates the chemical potential, is the mole fraction, is the concentration, is the partial pressure, is the flux, is the layer thickness, the subscript indicates a material component, and the superscripts , , , and indicate the material component in the feed (), the material component in the permeate (), the physical state (gaseous: ), and the membrane matrix ()).