The experimental viscosity dependences of reorientation times of small molecules in liquids are compared to those predicted by the hydrodynamic slip model. It is found that the slip model successfully predicts the viscosity dependence for most small molecules in solution in the absence of strong solute-solvent interactions.It has long been known that equations derived from hydrodynamics are successful in predicting rotational diffusion coefficients of large molecules.1 In

Measurements of reorientational relaxation times of simple aromatic compounds have been made by both depolarized Rayleigh light scattering and 13C NMR spin-lattice relaxation. Combination of the reorientation times determined by these techniques makes it possible to extract the reorientation times about the different molecular axes. The viscosity dependence of reorientation times about the individual molecular axes has been measured for benzene, mesitylene, toluene, and nitrobenzene. The viscosity dependence, which is highly anisotropic, is discussed in terms of a ``slip'' model of reorientational motion.

Measurements of orientational relaxation times of chloroform and nitrobenzene in a variety of solvents have been made by depolarized light scattering. At constant concentration a plot of reorientational relaxation time of nitrobenzene versus solution viscosity was found to fit a straight line with nonzero intercept. The reorientational relaxation time of both solutes increased with increasing solute concentration (at constant viscosity). For nitrobenzene and chloroform, plots of the reorientational relaxation time versus solute concentration were linear. This concentration dependence is attributed predominantly to pair correlations. The ``static'' correlation parameter f and the ``dynamic'' correlation parameter g are determined for both chloroform and nitrobenzene. The carbon-13 spin-lattice relaxation time (T1) for both solutes in solution has also been determined. The reorientational relaxation time of chloroform determined by NMR agrees well with the reorientational relaxation time determined by depolarized light scattering extrapolated to zero solute concentration.

The driving force behind the research on advanced materials has largely been aerospace and defense applications. In such applications the customer-induced limitations and infrastructural economic factors are secondary to the application-oriented parameters. Automobiles, on the other hand, represent the primary consumers of advanced materials in civilian applications. Automobiles are high-technology, low-cost machines which need to be robust for various climatic conditions and driver behavior. Advanced materials play an important role in the design and fabrication of various components of automobiles, and the use of new materials continues to increase. Recent interest in developing highly fuel efficient vehicles with low emissions has focused efforts toward materials, designs, and devices and is spurring research into advanced materials for weight reduction. The goal is to achieve fuel efficiency by weight reduction and a more efficient powertrain. In this review article, we summarize the efforts of the 1970s and the 1980s on the development of ceramic gas turbine engines with emphasis on materials processing and properties. This is followed by a discussion of metal-matrix composites and reinforced plastics, the structural materials of current interest. The materials issues related to automotive exhaust reduction catalysts are discussed since materials continue to play an important role in designing catalysts to meet the new EPA regulations. The understanding of materials chemistry is expected to play an important role in designing new materials and developing new processes that can be used economically to mass produce vehicle components. We also summarize reports based on ceramic precursor technology and sol-gel processes that show promise in the fabrication of automotive components.