## Dissertations, Theses, and Masters Projects

1994

Dissertation

#### Degree Name

Doctor of Philosophy (Ph.D.)

Applied Science

Gina L Hoatson

#### Abstract

$\sp2$H-NMR is a powerful spectroscopic technique for investigating dynamics in solids. to extend the range of motional rates that can be quantitatively investigated, a new approach for measuring the quadrupolar spin-lattice relaxation time $T\sb{1Q}$ was developed. This uses a Broadband Jeener-Broekaert (BBJB) sequence with echo-detection, which avoids the frequency-discriminated excitation profile and spectral baseline distortion intrinsic to the conventional Jeener-Broekaert (JB) experiment. By combining the BBJB experiment with an Inversion-Recovery sequence with Quadrupole-Echo detection (IRQE), two independent longitudinal relaxation times, $T\sb{1Q}$ and $T\sb{1Z}$, can be measured. Spectral densities of motion $J\sb1(\omega\sb{o}$) and $J\sb2(2\omega\sb{o}$) are extracted from these relaxation times. The new approach was demonstrated on a nematic liquid crystal binary mixture of 4-methyl-4$\sp\prime$-cyanobiphenyl-d$\sb{11}$ (1CB) and 4-n-pentyl-4$\sp\prime$-cyanobiphenyl-d$\sb6$ (5CB). Measurements on mixtures containing 10 and 25 mol% 1CB revealed that rotational motion can be described by the third rate model and the correlation times and activation energies of 1CB and 5CB are concentration independent. In hexamethylbenzene, it was demonstrated that the orientation dependence of spectral densities provides geometric and kinetic information. This technique works well for motions which are in the fast regime ($k\ge10\sp7 s\sp{-1}$) and contribute to relaxation. at ambient temperature, the experimental data were fit by a simulation which included simultaneous threefold and sixfold rotations, with geometric distortions of the electric field gradient tensors of the methyl group. The best-fit jump rates for threefold methyl rotation was $k\sb3 = 5.0\times10\sp{11} s\sp{-1}$ and sixfold aromatic rotation was $k\sb6 = 3.85\times10\sp8 s\sp{-1}$, with out-of-plane and in-plane distortions of 2.5$\sp\circ$ and 1.2$\sp\circ$ respectively. Relaxation times of the bisphenol-A polycarbonate (BPA-PC) and its monomer (BPA) were measured between 250K to 400K. The data were fit to several dynamic models. Simple threefold methyl rotation accounts for the spectral density anisotropies of BPA but not those of the polymer BPA-PC. Inclusion of a semilogarithmic distribution of jump rates, $k\sb3$, improved the agreement qualitatively but not quantitatively. Modulation of threefold methyl jumps by libration of the $C\sb{3V}$ axis was treated with the Stochastic Liouville formalism. Best agreement is found if the two motions are correlated. Activation energies for methyl group rotation are 19.2 $\pm$ 2.0 kJ/mol and 13.0 $\pm$ 0.8 kJ/mol in monomer and polymer respectively.

#### DOI

https://dx.doi.org/doi:10.21220/s2-e444-3457