Unlike most other porous materials, which have broad pore size distributions and poorly defined pores, microporous and mesoporous molecular sieve materials are frameworks with pores of well-defined size and geometry. The pore diameters are tunable and are of molecular dimensions allowing the materials to exclude molecules based on size and shape. As a result, powders of both microporous and mesoporous molecular sieves have found applications in catalysis and selective adsorption. Many new and potentially beneficial applications would be possible if thin films of these materials could be synthesized with a desired orientation of the pore structure. Previously, mesoporous silica films had been grown only with the pores parallel to the substrate. In order to achieve directional control of the pores, we synthesized mesoporous silica films in externally applied fields. Growth in shear flow fields gave the best results. However, this technique is limited to imparting directional orientation to channels, which remain in the plane of the substrate. We have utilized this method of channel orientation in an attempt to form mesoporous membranes by depositing mesostructured precursors in the pores of a macroporous substrate. Also, we have researched the structure of a new microporous titanosilicate molecular sieve, designated ETS-4. Thermally treated samples of this material have been shown to be nitrogen selective for methane/nitrogen mixtures. By using the methods of Fourier synthesis recycling and Rietveld refinement of x-ray and neutron diffraction data, we have revealed the shifting of cation positions and a lattice contraction of the structure during heating. Microporous and mesoporous materials are formed from the bottom-up and have pores that are of dimensions unattainable with topdown lithographic techniques. As a result, the pores of these materials give us the potential to template the growth of materials on the nanometer length scale. In order to evaluate their potential application in thermoelectric devices, we have modeled the thermoelectric transport properties of composite films using the semi-classical theory of electron transport. Our results suggest that oriented microporous and mesoporous films would make excellent hosts for nanowires and may yield thermoelectric devices with efficiencies a factor of 4 higher than current materials.

The fracture behavior of 2-D SiC/SiC woven composite was investigated at both ambient and elevated temperatures in this research. At ambient temperature, the fracture initiation toughness and R-curve behavior for composite were characterized and related to in-situ microscopic obseniations of damage accumulation and crack advance. Matrix cracking, crack deflection and branching were observed and dominated fracture behavior in the early loading stage. The key to toughening appeared to be associated with the mechanics of crack arrest at fiber bundles in the woven architecture. After the primary crack extension, the composite behaved non-linearly and a J-integral technique was applied to investigate the R-curve behavior. Substantial fibrous pull-out was observed in this regime of crack advance. An insight into the origin of the $J\sb{R}$-curve of a SiC/SiC woven composite was obtained by experimental characterization of the closure stress-crack opening displacement, $\sigma(u)$, relationship in the process zone of the crack. Application of a previously derived theoretical function, $\sigma\sb{b}(u)$, solely based on fiber bridging, showed consistent results with experimental data. The slow crack growth (SCG) behaviors of three different 2-D SiC/SiC composites were investigated at elevated temperatures from 700$\sp\circ$C to 1200$\sp\circ$C. In Dupont composite, crack length was measured in-situ at temperatures by an optical telescope allowing crack growth rate, V, as a function of applied stress intensity, K, to be obtained directly. Catastrophic failure followed after a limited crack extension suggesting limited SCG behavior at the intermediate temperatures. In contrast, an extensive SCG behavior was observed at 1200$\sp\circ$C. Microstructure observation indicated that crack bridging was absent at the intermediate temperatures but was present at 1200$\sp\circ$C. These results were consistent with the role of temperature on the oxidation and mechanical properties of the fiber bundles. In Goodrich composite, an introduction of boron as well as an increase of carbon interface thickness were found to enhance the time dependent deformation at the crack frontal region. Consequently, instead of the stress intensity factor, K, the energy rate C parameter exhibited a better correlation with the crack velocity.

The effects of shear flow on the phase behavior of a miscible blend of polystyrene with poly(vinyl methyl ether) have been investigated by a combination of fluorescence and rheometry. For this purpose, a Rheometrics RMS-800 rheometer was modified for stress and in-situ fluorescence measurements under well-controlled temperature and deformation conditions. Following equilibration at approximately 25 K below the coexistence temperature, slow heating to temperatures in the two-phase region shows that shearing at a constant rate increases the apparent coexistence temperature. Within the experimental error, the elevation above the quiescent spinodal temperature is related to the shear rate as $\Delta T/T\sb{s}$ = (0.015 $\pm$ 0.002)$\sbsp{\gamma}{\cdot 0.59 \pm 0.04}$; this correlation is independent of the composition in the range of 20-60% (w/w) polystyrene. A shear-induced demixing also occurs in the same blend at temperatures as much as 40 K below the LCST. There appears to be no threshold value of the shear rate or the shear stress for the onset of demixing, but a certain value of rate of work done on the sample roughly describes the onset of demixing at a fixed temperature. When shear does induce demixing, the fluorescence intensity remains much higher than that of the quiescent blend for as long as 24 hours, showing that the segregated phase structures can be stable under shear. However, in other cases, a demixing appears for a shorter time, along with relatively high stresses, shortly after the inception of shear but disappears as the stresses drop at longer times. Transmission electron microscopy on sheared samples confirms the presence of a shear-induced structure. The influence of phase separation on the linear viscoelastic behavior was also studied by measuring the complex moduli $G\sp\prime$ and $G\sp{\prime\prime}$ of the blend at temperatures between 25 and 155$\sp\circ$C. For the 20/80 and 40/60 PS/PVME blends, the terminal zone was in the accessible frequency window and phase separation was accompanied with a large increase in G$\sp\prime$ and $G\sp{\prime\prime}$. In contrast, the complex modulus of the 60/40 PS/PVME blend could not be measured near the terminal zone and the G$\sp\prime$ and G$\sp{\prime\prime}$ did not exhibit any significant changes near the phase transition temperature.

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