ISOTROPIC - NEMATIC PRETRANSITIONAL SHORT RANGE ORIENTATION CORRELATION IN PURE AND MAGNETIC PARTICLES DOPED THERMOTROPIC LIQUID CRYSTALS.

F. VACA CHÁVEZ; R. H. ACOSTA; D. J. PUSIOL

Caxambú, Brasil

Congreso; XXV Encontro Nacional de Fisica da Materia Condensada; 2002

In the nematic phase of a thermotropic liquid crystals, the molecules have random spatial placement but their molecularaxes tend to line up along one direction, called the director; and in the smetic A mesophase the molecules organizethemselves into layers. On the other hand, in the isotropic phase, there is no long range positional or orientational order,but a short range order in the molecular orientation persists, corresponding to a few molecular distances. The eect ofthis short range ordering in the Nuclear Magnetic Resonance (NMR) spin-lattice relaxation time, T1, was first suggestedby Blinc et al. and later, by Dong et al. Their results were explained in terms of the Landau-De Gennes (L-DG) theory forphase transitions. Recently, we studied the validity of L-DG at various Larmor frequencies scales and the low frequencyapproximation near the nematic - isotropic phase transition in butylcyano-phenylcyclohexane (4-PCH) liquid crystal. Inorder to compare the eect of ferrofluids in the short range, we measured the temperature dependence of T1 at dierenttime scales, specially in the kHz range, in the isotropic phase of 4’-n-pentyl-4-cyanobiphenyl (5CB) liquid crystal dopedwith magnetic particles. On the other hand, experimental and theoretical studies of isotropic - smectic A (I-A) phasetransition are scarce. In order to compare the short range in others compounds with our previous results, we measuredT1(T) in the isotropic phase near the isotropic - smectic A phase transition in 4’-n-onedecyl-4-cyanobiphenyl (11CB.)The measurements were performed by means of a home-built fast field-cycling NMR spectrometer.