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SYNTHESIS AND PROCESSING: GENERAL METHODS 13 and temperature history. There is an enormous variety of methods to process polymers, most of which aim at influencing overall texture (extrusion, injection molding, etc.). Of recent interest are various thin-film applications (lithography, dielectrics, optical coatings, etc.) where the polymer may be cast from an initially dilute solution or produced in situ by polymerization of a thin monomer film (both by liquid-phase and vapor-phase methods). Chemical and structural modification of polymer surfaces is also possible. In addition, reactions occurring in the two-dimensional environment of a surface can yield exciting possibilities for novel materials synthesis (e.g., the formation of polyacetylene on catalytic surfaces). Another example of much current interest involves the field- induced crystallization of PVF2 to create superior piezoelectric, pyroelectric, and ferroelectric materials. The use of polymers for nonlinear optical (NLO) processes is gaining considerable attention because of the ability to synthesize and tailor molecular structures that have inherently fast response times and large second-and third-order molecular susceptibilities. Polymers provide processing options not available with other classes of NLO materials. These encompass many options on the ultrastructure and submicron levels, including both main-chain and side-chain liquid crystalline polymers, poled isotropic polymers with functionalized chromofores, guest-host structures, polymer alloys and blends, spin-coated films, Langmuir-Blodgett films, and molecular assemblies (Heeger et al., 1988; Khanarian, 1986; Ulrich, 1987). The characteristic size of a polymer molecule is proportional to the square root of its length. Typical macromolecules are 5 to 20 nm in size, so that perturbation from the two external surfaces of a thin film on a polymer molecule is already strong at thicknesses in the 10-nm range, and completely new physical behavior is expected at thicknesses at and below the characteristic size. For example, craze microstructure changes dramatically for films less than about 0.1 µm thickness. Much work in characterization of polymeric materials in the thin-film regime is needed to explore future opportunities for applications in two-dimensional and particularly three-dimensional device architectures. REDUCTIVE PYROLYSIS The synthesis of metal-ceramic composites, so-called cermets, is usually accomplished by traditional powder metallurgy methods. The prototypical example is Co-WC, which is the workhorse material of the cutting-tool industry worldwide. This material has a bicontinuous structure, with the hard WC phase being the major constituent. It is the continuity of the WC phase in three dimensions that provides the compressive strength and elastic stiffness of the material, whereas the relatively soft and ductile cobalt residing within the interstitial space of the hard phase controls the fracture toughness. Tradeoffs in strength and toughness are achieved by control of the relative volume fractions of the two phases. The higher the volume fraction of cobalt, the
