Molecular dynamics in a mixture of 8OCB-d(17) and 6OCB showing nematic andreentrant nematic phases: A deuterium nuclear magnetic resonance study

A deuteron NMR study of the molecular dynamics of 4-n-octyloxy-4'-cyanobiph enyl (8OCB) in the mixture of 72 wt % of 8OCB and 28 wt % of 4-n-hexyloxy-4 '-cyanobiphenyl is presented. The mixture has the same composition as the o ne used before (Shen and Dong, 1998) except in the deuteration of the compo nent molecule. The deuteron Zeeman and quadrupolar spin-lattice times and q uadrupolar splittings were measured in the nematic, smectic A, and reentran t nematic phases at 15.1 and 46 MHz. The additive potential method was empl oyed to construct the potential of mean torque based on the observed splitt ings. The spectral density data from the relaxation times were interpreted in terms of the internal conformational motions of the chain decoupled from the molecular small-step rotational diffusion and the order director fluct uations. The latter motion was found to be essential to the fit of experime ntal results in the nematic phase. The fitting parameters obtained using a global target fitting method are acceptable when compared with those obtain ed from other deuteron and proton NMR studies of the same mesophases. (C) 2 000 American Institute of Physics. [S0021-9606(00)50132-9].