The wide range of polymer permeation properties were first observed and modelled over a century ago.1, 2 However, little further practical or theoretical work was undertaken until after the turn of the century when the need for improved pneumatic tyres developed. This led to the replacement of natural rubber with butyl rubber due to its lower permeability.3 Subsequently, the need for materials with ever-improved permeation properties has grown unabated. For instance, polymers which are relatively impermeable are used in food packaging and encapsulation of electronic circuits, whilst polymers which are very selectively permeable are also used in food packaging as well as in gas and liquid separation processes. Applications also exist for polymers which can release drugs or pesticides at a controlled rate.