Solid-state, wide-line, $\sp2$H Nuclear Magnetic Resonance spectroscopy was used to study the structure and dynamics of exchange of deuterated hydroxyl groups on the surface of silica gel. Spectra were obtained from three different kinds of silica gel, and detailed studies were carried out using Aldrich 70-230 mesh, high-purity silica. In ambient temperature relaxation experiments, the longitudinal relaxation time, T$\sb1$ was determined to be 70 $\pm$ 30 msec. The transverse relaxation time, T$\sb2$ was found to be inhomogeneous, with minimum and maximum values of 50 $\mu$sec and 325 $\mu$sec, respectively. By means of a quadrupolar echo modification of a Carr-Purcell-Meibloom-Gill (CPMG) spin echo experiment, it was determined that deuterium nuclei migrate slowly across the silica surface and that the inherent spectral line width of the spin packets was less than 1 KHz. At 72 K, T$\sb1$ was estimated to be 0.1-0.5 seconds, and CPMG experiments indicated that some motion still existed on the surface. In experiments at 72 K and 92 K, NMR spectra were collected from samples of silica degassed under vacuum at various temperatures. Using the technique of spectral subtraction to isolate individual spin species, we compared the experimental data with simulated spectra. We found that at least four deuterium spin species exist on the silica surface; one of these is almost certainly bound D$\sb2$O, and the others are deuterated hydroxyl groups, which were discriminated by their quadrupolar coupling constants, asymmetry parameters, and exchange energy.

We have developed a modular rheo-optical apparatus to study the flow properties of liquid crystals. Its main components are shearing device, strong magnetic field, and optical microscope. We performed experiments on well defined initial morphologies with uniform molecular alignment. The monodomains were achieved with strong magnetic fields (4.7T). Time resolved conoscopy is the primary optical technique in our investigation. We propose a simple relation between the distribution of alignment angles over the sample thickness and the conoscopically measured angle, to quantitatively measure the alignment angle in shear flow. We followed the relaxation of a shear induced splay deformation in small molecule model systems (N-(p-methoxybenzylidene)-p-butyl aniline (MBBA), pentyl-cyano-biphenyl (5CB) and a commercially available mixture OMI4244, and devised a model, based on the diffusion equation, to determine the rotational diffusivity from the relaxation process. The director alignment behavior of the SMLC's in shear flow is well described by the two dimensional Leslie-Ericksen model. The effect of director elasticity can clearly be seen in our experiments, resulting in a decrease of the steady state alignment angle at smaller Ericksen numbers. We found that there is no strain rate dependence of the director vorticity from 0.002/s to 2/s for poly-($\gamma$-benzyl-D/L-glutamate) (PBG). We determined ${\alpha\sb2/\alpha\sb3}$ = 44 for a 20% solution of 280.000 molecular weight PBG in m-cresol at 20$\sp\circ$C. The conoscopic interference pattern vanished after 8 strain units from an initially planar alignment and shearing could be reversed up to 10 strain units to completely recover the initial monodomain. Liquid crystalline polymers (LCP) are known to arrange into periodic director patterns during flow. We studied this for shear flow of lyotropic poly $\gamma$-(benzyl-glutamate) as a model system, which is a well characterized synthetic poly ($\alpha$ amino acid) with rigid chain architecture and well defined conformations. The molecules were are aligned uniformly as the starting condition. This so called monodomain morphology was obtained by use of strong magnetic fields. The shear apparatus is placed in an optical microscope, which is set up for conoscopy to allow direct observation of the shear induced rotation of the molecules. After a small strain during which the molecules rotate around the vorticity axis, they 'break out' sideways and form a three dimensional spatially periodic pattern. The shear induced instabilities have been observed under crossed polars as spatially periodic patterns (bands), some researchers observing them during flow and and others after cessation of shearing$\sp{(27, 42, 58, 59)}$. Bands, which develop during shear flow of poly (benzyl glutamate) (PBG) have been reported in 1980$\sp{(36)}$. However, later workers have failed to reproduce their findings and periodic pattern are believed to occur only after cessation of flow. Our findings solve a long standing controversy in the literature about the formation of periodic director pattern during flow. By varying the initial molecular orientation with respect to the flow direction we could show that the periodic pattern does not depend on the shear direction; it is governed by the director of the initial monodomain.

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No Abstract provided / Thesis / Master of Science (MSc)

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This thesis investigates the physics of various polymeric systems via the tool of computer simulation. The problems addressed include the tunable conformations of dendritic polyelectrolytes, the complexation of oppositely charged linear polyelectrolytes to charged dendrimers, the molecular mechanisms of polymer crystallization, and the retractive response of polymers perturbed from their equilibrium conformations. The algorithms employed include both equilibrium Metropolis Monte Carlo and Brownian dynamics methods. The computational investigations are augmented by scaling and variational theory treatments. The results of these studies complement experimental ventures in the respective fields and provide new insight into the limitations of existing theoretical descriptions.* *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation).

<P> Polymers are one of the most prevalent types of molecules in modern life. These long macromolecules make up everything from DNA to plastics to Jell-0™. An interesting class of polymers are block copolymers, which are composed of two (or more) chains, or blocks, of chemically distinct monomers covalently bonded end-to-end to form a single polymer. Different types of polymers tend to avoid each other, but since block copolymers are joined together the polymer species can not macroscopically phase separate. Instead, they separate on the scale of the size of the polymers, forming nanostructures. For a diblock copolymer melt, which is made from two types of polymers, these nanostructures can be, depending on the ratio of the length of one block to the other, spheres, cylinders, lamellae, or the more bizarre gyroid phase. </p> <P> Self-consistent field theory (SCFT) as formulated by Helfand in 1975 has in recent years been successfully applied to the study of the phase behaviour of diblock copolymers. However, most of the studies assume that the polymers are monodisperse, while almost all polymer melts are polydisperse. This work examines the effect of polydispersity in the block lengths on phase behaviour of diblock copolymer melts, by developing the SCFT for polydisperse block copolymers. The theory is examined using a perturbation method, as well as the random-phase approximation (RPA). The perturbation parameter is the ratio K of the weight-averaged molecular weight and the number-averaged molecular weight, which is a common measure of polydispersity. </p> <P> The results show polydispersity shifts the transition from a disordered phase to an ordered phase to a higher temperature, and increases the period of the nanostructures. It is also observed that polydispersity leads to larger non-lamellar phase regions in the phase diagrams. Results from the RPA also suggest that macrophase separation occurs for large polydispersities. </p> / Thesis / Master of Science (MSc)

The theory of Atoms in Molecules is applied to a series of borane molecules. A study of the topological features of the charge density, p(r), yields a quantum mechanical definition of atoms, bonds, and the average properties of atoms within a molecule. Other topological features of the boranes studied include rings and cages, formed by bond paths in p which link the atoms. These bond paths which form rings are bent inward in order to maximize the binding in these electron-deficient molecules. An important result of this analysis is the unambiguous assignment of the connectivity of the boranes. The theory of atoms and molecules allows one to quantum mechanically partition molecules into atomic basins, and calculations of average values for electron count and energies of individual atoms are performed by integration over these basins. Comparisons of atomic properties are performed to determine the transferability of atomic properties across the series. The Laplacian of p, V2p, yields information about sites of electrophilic and nucleophilic attack in molecules. These sites are found to correspond to the positions of critical points in V2p, which are localized to specific atoms in the molecule. The boranes and carboranes are ranked according to their susceptibility to electrophilic and nucleophilic attack, and such reactions with boranes are predicted to be regiospecific. / Thesis / Master of Science (MSc)

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No description available.