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Polymer Science and Engineering: The Shifting Research Frontiers
blending existing polymers did not begin until the late 1970s and 1980s. Now, the area of polymer blends is one of the routes to new materials that is most actively pursued by the polymer industry.
There are several driving forces for blending two or more existing polymers. Quite often, the goal is to achieve a material having a combination of the properties unique to each of the components, such as chemical resistance and toughness. Another issue is cost reduction; a high-performance material can be blended with a lower-cost polymer to expand market opportunities. A third driving force for blending polymers of different types is addition of elastomeric materials to rigid and brittle polymers for the purpose of toughening. Such blends were the first commercial example of polymer blend technology and, even today, probably account for the largest volume of manufacturing of multicomponent polymer systems. The main problem is that frequently when polymers are blended, many critical properties are severely depressed because of incompatibility. On the other hand, some blends yield more or less additive property responses, and others display certain levels of synergism. The problem is knowing how to predict in advance which will occur and how to remedy deficiencies.
From a fundamental point of view, one of the most interesting questions to ask about a blend of two polymers is whether they form a miscible mixture or solution. The thermodynamics of polymer blends is quite different from that of mixtures of low-molecular-weight materials, owing to their molecular size and the greater importance of compressibility effects. Because of these, miscibility of two polymers generally is driven by energetic rather than the usual entropy considerations that cause most low-molecular-weight materials to be soluble in one another. The simple theories predict that miscibility of blends is unlikely; however, recent research has shown that by carefully selecting or designing the component polymers there are many exceptions to this forecast. The phase diagram for polymer blends is often opposite of what is found for solutions of low-molecular-weight compounds. Polymers often phase separate on heating rather than on cooling as expected for compounds of low molecular weight. Theories to explain the behavior of miscible polymer blends have emerged, but theoretical guidance for predicting the responsible interactions is primitive. With the advent of modern computing power and software development, molecular mechanics calculations of this type are being attempted. Neutron scattering has provided considerable insight about the thermodynamic behavior of blends and the processes of phase separation.
One of the earliest blend products was a miscible mixture of poly(phenylene oxide) and polystyrene. The former is relatively expensive and rather difficult to process. The addition of polystyrene lowers the cost and makes processing easier. Numerous other commercial products are now based on miscible or partially miscible polymer pairs, including polycarbonate-polyester blends and high-performance ABS materials.
For mixtures that are not miscible, the most fundamental issues relate to