Ultrasound effects on the Spin-lattice Relaxation Time in Nematic Liquid Crystals

F. BONETTO; E. ANOARDO; R. KIMMICH

Campinas, Sao Paulo, Brasil

Workshop; V Seminario de Mostradores de Informaçao, IV Escola Iberoamericana de Mostradores de Informaçao; 2002

Rede Iberoemericana de Mostradores de Informaçao

In this work we discuss the influence of ultrasound waves in the molecular dynamic of different nematic liquid crystals as revealed by the Larmor frequency dispersion of the spin lattice relaxation time (T1). Experimental and theoretical studies of nuclear magnetic relaxation in nematic liquid crystals indicate that T1 essentially reflects two kind of molecular mechanisms: collective motions called Director Order Fluctuation (ODF) and individual molecular motions, such as self-diffusion and rotations [1]. Collective re-orientations modes are mostly contributing to spin-lattice relaxation at low-frequencies. Experimental evidences exist that contributions from ODF to the total T1 relaxation parameter are dominant in the kHz range [2,3]. Because the aim of the work was to study the influence of a sonic perturbation in a dynamical region where ODF is a dominant mechanism, experiments were carried out using the fast field cycling technique [4,5]. Experimental T1 profiles with and without audio were recorded by a commercial fast field cycling NMR relaxometer. The ultrasound was applied through a 3 mm glass sonotrode coupled to a generator working at 30 kHz.  We present here T1 dispersion curves in a Larmor frequency window that ranges from 20 kHz to 4MHz at various temperatures in the nematic range of three different compounds: PAA (p-azoxyanisole), 5CB (p-pentyl-p´-cyanobiphenyl) and 8CB (p-octyl-p´-cyanobiphenyl). A new model was developed to explain the data in presence of ultrasound waves [6]. From the parameters obtained in the curve fittings it is possible to quantify how the ultrasound waves affect the relaxation profile. Besides, they could give some information about the changes in the macroscopic properties (like viscosity or diffusion coefficient) of these materials under sonication. 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] R. Koellner, K. H. Schweikert and F. Noack, Liq. Cryst. 13, 483 (1993). [4] Kimmich R, Bull. Magnetic Resonance, 1, 195 (1980). [5] Noack F, Prog. in NMR Spectroscopy, 18, 171 (1986). [6] F. Bonetto, E. Anoardo and R. Kimmich, Chem. Phys. Let., 361, 237 (2002).