Abstract
Many interesting organic materials consist of multiple components that form distinct domains. These materials range from biological deposits such as wood and bone to manufactured materials such as plastics and pharmaceuticals. Various important macroscopic properties of these materials, such as strength, elasticity and solubility, are often determined by the microscopic properties of the domains formed by their different components, such as their composition, size and morphology. The lack of long-range order due to the variety of domains generally precludes detailed analysis by X-ray and other diffraction techniques. While various electron microscopies can be used to study these domain structures, they only work to a certain extent. The intrinsic, light-elemental nature of organic material usually results in poor contrastamong different components, and this severely limits the resolution of these micrographs. In comparison, ssNMR offers a unique opportunity in probing the spatial relations among the domains. With its high sensitivity to different chemical environments, ssNMR allows spectrally a much sharper distinction of different chemical components, thus enabling the nuclear spins that represent these different components to act as local structural probes within their domains.
My dissertation research is focused specifically on harnessing the power of advanced solid-state NMR to isolate and even quantitatively measure the size of domains in complex organic materials formed by different chemical or dynamical profiles. We have studied various interesting materials such as wood, degradable polymers, and polymer-MOF (metal-organic framework) composites. In addition, we have developed new NMR methods and improved existing ones to measure domain sizes through 1H spin diffusion analysis.
Specific results include the following:
1. Improved data analysis method that allows measurements of long periods in rigid
polymer systems even with similar components using 1H spin diffusion in 2D HetCor
NMR.
2. Determination of secondary cell wall ultrastructure of young oakwood through combined
13C and 1H spin diffusion NMR and mathematical modeling.
3. Analysis of biodegradable high-density polyethylene-like polyesters formed by
condensation of diol linkers with long chain diesters of various lengths. ssNMR can
identify sizes of crystalline and amorphous regions, as well as the conformation and
dynamics of the chain segments, which helps explain the mechanical properties of these
materials.
4. Analysis of several different solid-phase organic photoactive molecule systems.
Properties such as domain formation, packing, conformation, and the process of thermal
reversion were determined.
5. Theoretically rigorous modification of the Avrami equation for partially cooperative
conversions, which introduces prenucleation time as a physical quantity in order to
bypass unphysical noninteger Avrami exponents.