Alloying of metals has been practiced for centuries, though the development of analogous intimately blended polymer systems as engineering materials is much more recent. However, such materials already form a substantial fraction of the polymer market and are growing at twice the rate of plastics as a whole. Based on the experience of the last 1-2 decades in industry, it is likely that new organic materials designed to meet future U.S. needs will arise from "molecularly engineered" blends of polymers, rather than from single-phase polymers of totally new composition. Unfortunately, existing theories of polymer mixing, developed over 50 years ago by Flory and Huggins (FH), are mean field approximations which neglect several important factors (chain self-avoidance, self contact, intra- and intersegment correlations, etc.). Thus, the fundamental rules governing the relationship between synthesis, structure and properties are not fully understood and much of current technology is based on empirical correlations. For example, there is disagreement in the literature concerning the state of mixing of many important blend systems (e.g., polyolefin melts).