Molecular coherence above the nematic-isotropic phase transition

F. VACA CHAVEZ; F. BONETTO; D. PUSIOL

Sendai, Japón

Conferencia; 18th International Liquid Crystal Conference (ILCC); 2000

The Japanese Liquid Crystal Society

NMR spin relaxation measurements in liquid crystals provide detailed information about molecular dynamic. Individual molecular motions like translational diffusion and anisotropic rotational processes of single molecules can be distinguished from the peculiar feature of liquid crystalline mesophases; namely collective molecular reorientations or fluctuations of the  director (OFD). The T1 measurement can be used to study the short range orientational order fluctuations above the nematic-isotropic (N-I) phase transition. In thermotropic liquid crystals, the N-I phase transition is a weak first order transition. Above the clearing point, short range nematic order is encountered and the orientation of the molecules are correlated spatially over a distance known as coherence length, x. When the Larmor frequency used to study the short range orientational order fluctuations in the isotropic phase is above a certain value, T1 is temperature and frequency dependent. But below this value, T1 is only temperature dependent and may be described in terms of the Landau-De Gennes theory (L-DG). The temperature dependence of the proton spin-lattice relaxation time, T1(T), was measured in the isotropic phase of  4-PCH liquid crystal at 28 MHz, 1.235 MHz and 48 KHz Larmor frequencies. From the experiments we observe that: a) T1(T) at high Larmor frequencies (MHz range) is determined by non-collective motions such as molecular rotations. b) The measurement of T1(T) at low Larmor frequency show the existence of  strong orientational correlation between neighbouring molecules. We conclude that the critical order fluctuations in the isotropic phase of 4-PCH liquid crystal can be studied by means of proton spin relaxation measurements, and the L-DG theory is appropriate to describe it, if the frequency used is in the range  where the OFD are the more effective relaxation mechanism, it means, the KHz range.