P. S. HOPE and M. J. FOLKES Mixing two or more polymers together to produce blends or alloys is a well-established strategy for achieving a specified portfolio of physical properties, without the need to synthesise specialised polymer systems. The subject is vast and has been the focus of much work, both theoretical and experimental. Much of the earlier work in this field was necessarily empirical and many of the blends produced were of academic rather than commercial interest. The manner in which two (or more) polymers are compounded together is of vital importance in controlling the properties of blends. Moreover, particular­ ly through detailed rheological studies, it is becoming apparent that processing can provide a wide range of blend microstructures. In an extreme, this is exemplified by the in situ formation of fibres resulting from the imposition of predetermined flow fields on blends, when in the solution or melt state. The microstructures produced in this case transform the blend into a true fibre composite; this parallels earlier work on the deformation of metal alloys. This type of processing-structure-property correlation opens up many new possi­ bilities for innovative applications; for example, the production of stiff fibre composites and blends having anisotropic transport properties, such as novel membranes. This book serves a dual purpose.