Study of the Molecular Dynamic in the Isotropic Phase of n-PCH Series Thermotropic Liquid Crystals

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

Lisboa, Portugal

Simposio; Joint 30º AMPERE; 2000

International Society of Magntic Resonance (ISMAR)

A lot of information about the molecular dynamics in liquid crystals can be obtained from NMR spin relaxation measurements. 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 orientational fluctuations of the  director (OFD) [1]. In thermotropic liquid crystals, the N-I phase transition is a very particular first order transition, above the clearing point, short range nematic order is encountered and the orientation of the molecules are spatially correlated over a distance known as the coherence length, x, which is in fact temperature dependent [2]. When the Larmor frequency nL 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, in that special conditions, it may be described in terms of the Landau-De Gennes theory (L-DG) [3]. The temperature dependence of the proton spin-lattice relaxation time, T1(T), was measured in the isotropic phase of  n-PCH (n = 3, 4, 5 and 7, representing propyl, butyl, pentyl and heptyl chains, respectively) liquid crystals at nL: 28 MHz, 1.235 MHz and 48 KHz. From the experiments we observe that: a) T1(T) at high nL‘s (MHz range) is determined by non-collective motions such as molecular rotations. b) The measurements of T1(T) at low nL‘s show the existence of  strong orientational correlation between neighbouring molecules. We conclude that the critical order fluctuations in the isotropic phase of N-PCH liquid crystals 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. References:  [1]  F. Noack, Encyclopedia of  Nuclear Magnetic Resonance, edited by   D.M. Grant and R.K. Harris (Willey, Chichester, 1996). [2] D. J. Pusiol and F. Vaca Chávez, Chem. Phys. Lett., 312, 91-95 (1999) [3] See for instance, R. Y. Dong, Nuclear Magnetic Resonance of Liquid Crystals, Springer-Verlag (1994).