Membranes - Introduction to Carbon Capture and Sequestration - page 325

T g

Glassy Rubbery

The excess free volume;
adsorption obeyed
Langmuir in microvoids

T

V g (T)

V g

Overall free volume
in polymer

V l (T)

V l

V 0 (T)

V 0

Occupied Volume

Temperature

Figure 7.5.4 Phase diagram of a typical polymer
Figure adapted from [7.10]. This animation can be viewed at: http://www.worldscientifi c.
com/worldscibooks/10.1142/p911#t=suppl

we drop the temperature below the glass transition temperature, the
individual polymer chains are now locked in place, but there is an excess
volume present. This excess volume represents micropores inside the
material that exist because the previously moving chains are no longer
taking up that extra room. Adsorption of CO 2 in that medium looks very
much like adsorption in a porous medium. Glassy polymers “push” the
limits of the Robeson plot because they appear to the adsorbing mole-
cule as a porous material. A theoretical basis for this upper bound is
given by Freeman [7.11].
To further break the boundaries of the Robeson limit researchers are
looking at materials that have the potential for molecular control of
adsorbate solubility by way of “ facilitated transport .”

Facilitated transport

In materials in which transport is governed by the diffusion solubility, we
have limited opportunities to change the solubility of adsorbates. When