Crystalline defects, equilibrium phase diagrams

Ideally, the solidus should be established for all cases. Except in a few rare situations a complete phase diagram, where both the liquidus and solidus are presented, is not available. Determining the solidus for polymers, even on a compositional basis, is a formidable matter. As theoretical considerations have indicated, the sequence distributions in both phases are actually required for polymers rather than the composition. This makes the task of determining the solidus a very difficult one. Moreover, if a mixed crystalline phase is observed a decision has to be made as to whether it represents an equilibrium or defected state. 

It is evident that the study of crystalline copolymers presents some very difficult as well as intriguing problems. The theoretical development of equilibrium relations and phase diagrams can be accomplished in principle when it is recognized that one must be concerned with the sequence propagation probability and not the composition. One major difficulty is that although the liquidus of the phase diagram can be determined quite easily, the solidus is extremely difficult to establish in most cases. In addition, it is virtually impossible to establish equilibrium on an experimental basis. As we have enumerated above there is a large number of important contributions to the deviations from equilibrium. The major ones are the finite crystallite size (relative to the equilibrium requirement), possible defected structures within the lattice and most importantly the structure of the interphase. 

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