Nuclear magnetic resonance and the theory of liquid crystals

Two strands of thought run through this thesis. The first is that a theoretical description of a particular state of matter, the liquid crystalline state, can be well represented by a single particle distribution function. The second is that the best probe of the single particle distribution is Nuclear Magnetic Resonance spectroscopy. In two of the chapters a theory for the orientational ordering and thermodynamic properties of flexible liquid crystal molecules is laid out. The theory of the orientational ordering is compared with results obtained from deuterium NM on the alkyl-chain deuterated n-alkylcyanobiphenyls and n-alkoxycyanobiphenyls, which can provide order parameters for different parts of the chain. Carbon-13 NPLR has also been used to obtain order parameters for liquid crystals. This is demonstrated in chapter 4 where the differences in chemical shift between NUR signals in the isotropic phase spectra and the nematic phase spectra are used to calculate the principal order parameter for several of the alkyl and alkoxy derivatives of cyanobinhenvl. NNR spectroscopy can provide information on the dynamic properties of liquid crystalline systems through the measurement of relaxation rates. In chanter 5 spectral densities are measured for several sites along the alkvl chain of one liquid crystal at two diffent resonance frequencies. Finally a return is made to the calculations of thermodynamic properties, but of liquid crystalline mixtures rather than of pure systems. Two theories are presented and compared. One is a hard particle theory, the other derived from the Maier-Saupe theory.