Alloy is a term commonly associated with metals and implies a composite which may be sinqle phase (solid solution) or heterophase. Whichever the case, metallic alloys generally exist because they exhibit improved properties over the base metal. There are numer­ ous types of metallic alloys, including interstitial solid solutions, substitutional solid solutions, and multiphase combinations of these with intermetallic compounds, valency compounds, electron compounds, etc. A similar situation exists with polymers. There are numerous types of composites, or "alloys" of polymers in existence today with new ones being created continuously. Polyblends are simple physical mixtures of the constituent polymers with no covalent bonds occuring between them. As with metals, these may be homogeneous (single phase) solid solytions or heterogeneous (multiple phase) mixtures. With polymers, the latter case is by far the most prevalent situation due to the thermodynamic incompatibility of most polymers. This is due to the relatively small gain in entropy upon mixing the polymers due to contiguity restrictions imposed by their large chain length.