Doctor of Philosophy (Ph.D.)
The molecular motions are studies in two disordered materials that undergo glass transitions. Glycerol is a conventional glass former and cyclohexanol is an orientational glass former.;The technique used in the glycerol experiments was spectral hole burning. Chemical shift anisotropy produces inhomogeneous broadening of NMR lines in orientationally disordered and polycrystalline solids. By saturating or inverting a portion of the anisotropic line, 'burning a hole', molecules of certain orientations are tagged. Subsequent molecular reorientations result in spectral diffusion which is not related to spin-spin interactions. By measuring the broadening and recovery of the hole as a function of time, detailed knowledge of the reorientation is obtained. For example, the mean jump rate and the lower limit of angular reorientations are determined. The reorientation rate in supercooled glycerol is followed from 10('-2)s('-1) to 10('2) s('-1). Our measurements agreed with previous results and extended them to lower frequencies. The mean jump size was determined to be greater than 45 degrees. The hole recovery curves were not exponential, but were fitted with the Williams-Watts function, exp = ((tau)/(tau)(,0))('(beta)) with (beta) = 0.5.;The motions in the rotor phase of solid cyclohexanol are studied with proton NMR from the melt down to 5 K. Particular attention is paid to the variation of the linewidth with temperature and to the temperature and frequency dependences of T(,1). We find there are two distinct motions that cause minima in T(,1) as a function of temperature. These two motions were observed in dielectric experiments. From the proton line narrowing and C('13) high-resolution solid state spectra, the high temperature '(alpha)' motion is identified as overall molecular rotation. The low temperature '(beta)' motion is identified as a uniaxial internal rotation of the cyclohexyl ring about the CO bond, with the COH group remaining stationary. This explains both the strong spin relaxation and the weak dielectric relaxation peak associated with the (beta) motion. Both motions have distributions of correlation times, as seen from the shallow T(,1) minima and the weak temperature and frequency dependences of T(,1). From 100 K to 5 K, the temperature dependence continues to be weak. The frequency dependence remains less than (omega)(,0)('2). These results indicate that some components of the motion remain faster than the NMR frequency (omega)(,0) even at 5K. The behavior of cyclohexanol is compared to that of other disordered solids.
© The Author
Kuhns, Philip L., "NMR study of molecular motions in two disordered organic solids" (1983). Dissertations, Theses, and Masters Projects. Paper 1539623743.