Free volume and intrinsic microporosity in polymers

The concept of free volume is useful for explaining aspects of the chain mobility and permeability of polymers, even though its precise definition is subject to debate. Polymers that trap a large amount of interconnected free volume in the glassy state behave in many respects like microporous materials and potentially find application in membrane separations and heterogeneous catalysis. The development is outlined of a new type of polymer, for which the molecular structure contains sites of contortion (e.g. spiro-centres) within a rigid backbone (e.g. ladder polymer). These polymers of intrinsic microporosity (PIMs) include both insoluble network polymers and soluble non-network polymers that may be processed into membranes or other useful forms. Experimental methods are discussed for elucidating the free volume or micropore distribution, and the behaviour of PIMs is compared with that of the ultrapermeable polymer poly(1-trimethylsilyl-1-propyne).