Synthesis, Characterization, and Gas Permeation Properties

General Introduction
prepared by melt extrusion or solution casting and exhibit high permselectivity and
rather low gas permeability. An asymmetric membrane is made up of two distinct
layers, a thin non-porous one responsible for the separation performance and a porous
one to provide the physical support.
In accordance with the type of the membrane, the membrane separation
mechanisms for a binary gas mixture have been primarily classified into five
categories, porous membranes (I–IV) and non-porous membranes (V), as illustrated in
Figure 2. 22c,33 With porous membranes, in the absence of any interaction between the
gas molecules and the membrane „Knudsen flow‟ or Knudsen diffusion mechanism (I)
is known to operate whereas „surface diffusion‟ (II) takes place if the gas molecules
interact with the surface inside the pores and „capillary condensation‟ (III) acts for the
transport of gases or vapors which tend to condense inside the pores. The molecular
sieve mechanism (IV) prevails for the porous membranes, with average pore-size
greater than the molecular size of one of the gases, thus exhibiting extremely high
separation performance; however, it is quite difficult to prepare well-ordered
angstrom-size (i.e., gas size) pores in the polymer membranes.
Figure 2. Basic Mechanisms of Gas Permeation through Membranes
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