resistance are needed to survive the high temperatures required for processing these materials. In addition, these elastomers must be dispersed within the matrix to an appropriate morphology (or size scale) and adequately coupled to the matrix. These two issues are often interrelated and specific to the particular matrix material. Continued efforts will be required to produce a better understanding of the various toughening mechanisms that are applicable to engineering polymers.

Numerous opportunities exist to achieve better understanding that would shorten the time to develop new blends and alloys. There is an interesting parallel between this field and alloying in metallurgy, and the polymer community may be able to learn from the long experience of metallurgists. Both fields involve a broad spectrum of issues including synthesis, processing, physical structure, interfaces, fracture mechanics, and lifetime prediction. The United States is currently in a position of technical leadership; however, companies and universities around the world are also aggressively pursuing research and development in this field.

Structural Composites

Polymer composites can provide the greatest strength-to-weight and stiffness-to-weight ratios available in any material, even the lightest, strongest metals. Hence, high-performance and fuel-economy-driven applications are prime uses of such composites. One of the most important attributes is the opportunity to design various critical properties to suit the intended application. Indeed, performance may be controlled by altering the constituents, their geometries and arrangement, and the interfaces between them in the composite systems. This makes it possible to "create" materials tailored to applications, the single greatest advantage and future promise of these material systems. Structural composites are of interest in aerospace applications and in numerous industrial and consumer uses in which light weight, high strength, long fatigue life, and enhanced corrosion resistance are critical. Much needs to be done to advance processibility and durability, to provide a more comprehensive database, and to improve the economics of these systems. A wide range of future needs encompasses synthesis, characterization, processing, testing, and modeling of important polymer matrix composite systems.

In general, the future of polymer matrix composites is bright. The engineering community is now in the second generation of applications of composites, and primary structures are now being designed with these materials. There is a growing confidence in the reliability and durability of polymer composites and a growing realization that they hold the promise of economic as well as engineering gain. Commercial programs such as high-speed civil transport will not succeed without the use of polymer composites. Integrated synthesis, processing, characterization, and modeling will allow the use of molecular concepts for the

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