THE ROLE OF MOBILE PHASES IN CRYSTALLIZATION OF POLYMERS: SIZE DEPENDANCE IN PHASE TRANSFORMATION. S. Rastogi, Centre for Polymers and Composites Eindhoven University of Technology Den Dolech 2; P.O. Box 513 5600MB Eindhoven; Nederland

Solid phases may exist in a wide variety of forms such as polymorphs, solvates, liquid crystals and ampholytes. For a given set of experimental conditions such as temperature, pressure and composition only one solid phase will be consistent with a minimum free energy of the system. This will then be the most stable solid phase available and all other phases will be metastable with respect to this phase. However, the fact that these solids are metastable does not preclude their existence for long time periods. An extreme example of this is diamond which is metastable with respect to graphite at room temperature and pressure.

The appearance of polymorphs from a supersaturated and/or supercooled mother phase is, however, not only determined by the drive to minimise the free energy but also by the need for the system to do this by the kinetic route with the lowest activation energy. It is the balance between two processes-thermodynamic and kinetic that forms the basis of this colloquium. The importance of kinetics was recognised by Ostwald and is summarised in his 'Law of Stages' which simply states that when a system moves from an unstable to a stable state it does so by transformations to intermediate metastable states. Some of these effects will be demonstrated in polymers like polyethylene, trans-1,4 polybutadiene and poly di alkyl siloxanes.