structural applications. Finally, polymer and polymer-fiber composite materials are finding extensive biomedical applications. All of these trends are expected to continue, and all present research challenges in synthesis and processing to achieve desired structures, properties, and performance.

Advances in computing power offer opportunities for substantial and badly needed improvements in the theoretical underpinning of polymer science. These include the modeling of stable and metastable structures of macro-molecules, of liquid crystallinity, of phase relations and phase-transition kinetics (including the now poorly understood glass transition in amorphous materials), and of nonlinear viscoelastic behavior in polymer melts (which is critical in polymer processing). The recently developed reptation concept is an interesting approach to the last problem; identification of the collapse transition of polymer solutions by use of computer-intensive Monte Carlo methods, combined with renormalization and scaling theories, illustrates the benefit of increasing computing power in modern theory.

The long-chain connectivity inherent in polymers imposes unusual constraints at or near interfaces, where abrupt changes in density, composition, and orientation can occur. Understanding polymeric interfacial problems requires sophisticated scientific studies; scaling up from analyses of small molecules will not suffice, because the behavior of polymer chains can be markedly different from that of low-molecular-weight compounds. Studies should address the physical and chemical dynamics of surfaces, interfacial regions of polymer blends and alloys, polymer melts in confined geometries, and crystal-amorphous interfaces in semicrystalline polymers. These interfaces control adhesion, wetting, colloidal stabilization, and mechanical properties, and they are intimately involved in the failure and deformation of polymers. Yet experimental and theoretical investigation of polymer interfaces has begun only recently.

Chemistry is important to all the materials classes, but none benefits more from close interaction with the field of chemistry than does polymers. The synthetic capabilities of the modern chemist can be advantageously applied in the rational design of molecules to achieve desired properties. Specificity in molecular composition, architecture, and size can be tailored with amazing precision. The attachment of functional groups at selected locations is readily accomplished. Moreover, control of chain length and composition provides for extremely well-defined materials that can self-assemble into intricate patterns sometimes mimicking those of biological systems.

Interfaces are also of crucial importance to the performance of composites (see below), many of which use polymers for the matrix, the reinforcing dispersed phase, or both. Defects or weaknesses at interfaces can severely limit the performance of composites. Figure 3.4 shows a carbon fiber-reinforced composite fracture surface. Recent research on polymers, however, suggests a route to eliminating interfaces while achieving many of the su-

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