MEASUREMENT OF ORIENTATIONAL ORDER AND MOBILITY OF A NEMATIC LIQUID-CRYSTAL IN RANDOM NANOMETER CONFINEMENT

Confinement of a nematogen in submicrometer cavities can produce subst antial effects on the nematic-isotropic phase transition and on order and orientation of the nematic phase. We combine the methods of C-13 a nd H-1-NMR and broadband dielectric spectroscopy to probe liquid cryst alline order and molecular mobility. We find that the dynamics of meso genic molecules (5CB) in the free volume of nanoporous sol-gel glass ( 5 nm pore diameter) is as fast as in the free bulk phase. In addition, a boundary layer of molecules with reduced mobility covers the pore w alls. Isotropic-nematic transition as well as crystallization of the c onfined nematogen are considerably suppressed. In order to extract the temperature dependence of the nematic order parameter from NMR data, a mathematical model is applied which describes the combined influence s of orientational order and diffusional averaging in randomly oriente d pores on NMR spectra of the nematogens. (C) 1997 American Institute of Physics.