Membrane Gas Separation

2.2
The PIM Concept 
Most polymer chains are fl exible, in the sense that they can change conformation by 
rotation about backbone bonds. If suffi cient free volume is available, it is possible for 
large - scale movements of fl exible polymer chains to occur, giving a rubbery or fl uid 
material. In order to trap additional free volume in the glassy state, chain mobility must 
be constrained. This can be done by creating a polymer backbone that cannot easily 
change conformation. The extreme example is a ladder polymer, for which rotation about 
the backbone axis requires a bond to be broken. However, rigid polymer chains, such as 
ladder polymers, tend to pack together effectively and interact strongly, giving dense 
materials that are diffi cult to process. Packing must be disrupted for a high free volume 
polymer to be achieved. One way to do this is to introduce ‘ sites of contortion ’ , which 
force the backbone to twist and turn erratically. Thus, the two key features of a ‘ polymer 
of intrinsic microporosity ’ (PIM) are that the backbone incorporates (1) sequences that 
are ‘ rigid ’ in the sense that rotational movements are highly restricted (e.g. comprising 
fused rings giving a ladder structure) and (2) sites that introduce kinks into the backbone. 
A suitable ‘ site of contortion ’ is a spiro - centre where a single tetrahedral carbon atom is 
part of two fi ve - membered rings. These features are clearly seen in the polymer referred 
to as PIM - 1, which is most commonly prepared from 5,5 
′
,6,6 
′
- tetrahydroxy - 3,3,3 
′
,3 
′
-
tetramethyl - 1,1 
′
- spirobisindane and 1,4 - dicyanotetrafl uorobenzene (Figure 2.1 ) [6 – 10] . 
The highly effi cient double aromatic nucleophilic substitution reaction gives a planar 
dibenzodioxane linkage, generating a fused ring (ladder) sequence. The spiro 
- 
centre 
provides a site of contortion that puts a bend in the polymer backbone. This design concept 
developed out of research originally aimed at producing catalytically active polymer 
networks with high surface area [11 – 14] .
PIM - 1 was fi rst prepared by a lengthy polymerization (72 h) at modest temperature 
(65 
° 
C) in dimethylformamide (DMF)
[6,7] 
. Subsequently, a rapid, high temperature 
(155 ° C) procedure has been developed [15] . The product, which precipitates during the 
polymerization, is a yellow, fl uorescent polymer that is soluble in tetrahydrofuran, 
chloroform, dichloromethane, o - dichlorobenzene and acetophenone. It can be cast from 
solution to form free - standing membranes with a fi lm density of about 1.1 g cm 
−
3
. It is a 
glassy polymer with a high degradation temperature (above 350 ° C) by thermogravimetric 
analysis. There is no evidence of a glass transition below the degradation temperature. 
As a membrane, in pervaporation it is selective for organics over water [7] , and in gas 
separation it exhibits an outstanding combination of permeability and selectivity [5,16] , 

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Gas Permeation Parameters and Other Physicochemical Properties
31
as is discussed further below. The rationale for describing it as ‘ microporous ’ comes from 
gas adsorption studies and this is discussed fi rst.