Effects of sonication on spin-lattice relaxation dispersions in liquid crystals

F. BONETTO; E. ANOARDO

Florianopolis, Santa Catarina, Brasil

Workshop; VII Seminario de Mostradores de Informaçao, VI Escola Iberoamericana de Mostradores de Informaçao; 2004

Rede Iberoamericana de Mostradores de Informaçao

From spin-lattice relaxation time (T1) dispersion measurements it is possible to obtain information about different molecular dynamic processes. Experimental and theoretical studies of nuclear magnetic relaxation in nematic liquid crystals indicate that T1 is essentially determined by two kind of molecular mechanisms: collective motions called Director Order Fluctuation (ODF) and individual molecular motions, such as self-diffusion and rotations [1]. Experimental evidences exist that contributions from ODF to the total T1 relaxation dispersion is dominant in the kHz range [2,3]. On the other side, theoretical studies predict a similar T1 frequency dependence for ODF and individual motions in smectic phases, being difficult to differentiate between them [4]. When an acoustic wave propagates in a nematic liquid crystal, the fluctuations of the density couple with the director and, in consequence, a new molecular orienting term should be added to the nematic free energy expression [5,6]. Taking into account this term, a new model was developed in order to explain dispersion curves in nematics as well as in smectic A phases. Experimental T1 profiles recorded by a field cycling NMR relaxometer in the absence and the presence of an ultrasonic field are also shown. In nematics, the ultrasound induced changes can be characterized by a) a decrement in the typical slope of the dispersion (this effect is stronger at lower frequencies) and b) a nearly uniform relaxation enhancement within the whole observed Larmor frequency window [6,7]. In smectic A phases effects are similar but a new important result is obtained: ultrasonic waves could be useful to discriminate ODF from individual motions in T1 relaxation dispersions [8]. REFERENCES [1] R. Y. Dong, Nuclear Magnetic Resonance of Liquid Crystals, Springer - New York (1997). [2] F. Noack, M. Notter and W. Weiss, Liq. Cryst., 3, 907 (1988). [3] Noack F, Prog. in NMR Spectroscopy, 18, 171 (1986). [4] M. Vilfan, M. Kogoj and R. Blinc, J. Chem. Phys. 86, 1055 (1987). [5] J. Selinger, M. Spector, V. Greanya, B. Weslowsky,  D. Shenoy and R. Shashidhar, Phys. Rev. E 66, 051708 (2002). [6] F. Bonetto, E. Anoardo and R. Kimmich, Chem. Phys. Let., 361, 237 (2002). [7] F. Bonetto, E. Anoardo and R. Kimmich, J. Chem. Phys. 118, 9037 (2003). [8] F. Bonetto and E. Anoardo, J. Chem. Phys. 121, 554 (2004).