High-pressure studies of the vibrational spectra of polymers and associated materials

Presented in this thesis is the effect of pressure on the vibrational spectra of a series of eleven crystalline paraffin hydrocarbons from n-C$\sb8$H$\sb{18}$ to n-C$\sb{36}$H$\sb{74}$ plus n-C$\sb8$D$\sb{18}$ and a few branched chain compounds. Also presented are data for a series of six polyethylenes of varying molecular weight and crystallinity. The focus is on the increase in intensity of the CH$\sb2$ wagging and C-C stretching vibrations with pressure. The sum of these two intensities increased in a manner which depends primarily on chain length. The distribution of intensity between the two vibrations depends on the crystal structure. In the long chain limit, i.e. the polyethylenes, it is demonstrated that the C-C stretch borrows intensity from the CH$\sb2$ wag to the extent of overwhelming it.The results imply that the primary event is an increase in the CH$\sb2$ wag intensity due to increased intermolecular coupling. This intensity is redistributed to the C-C stretching vibration via some resonant process. This resonance is most efficient for the orthorhombic structure with four molecules per unit cell and least efficient for the triclinic structure with one molecule per unit cell. For n-C$\sb{30}$H$\sb{62}$ and n-C$\sb{36}$H$\sb{74}$, which have a monoclinic structure with two molecules per unit cell, the behavior is intermediate. In the polyethylenes the resonance is much more efficient than for the linear hydrocarbons primarily due to the very small difference in energy between the CH$\sb2$ wag and the C-C stretch.U of I OnlyETDs are only available to UIUC Users without author permissio