Nematic-Like Regimes of the NMR spin-lattice Relaxation Rate in the Smectic Phase of Thermotropic Liquid Crystals.

R. H. ACOSTA; D. J. PUSIOL

Angra do Reis, Brasil

Congreso; VII Encontro de Usuarios de RMN; 1999

In nematic mesophases of thermotropic liquid crystals, molecules maintain its long axis locally oriented, resembling a tridimensional oriented liquid. Smectic structures however includes a higher ordering degree; {em i.e}, molecules are arranged in layers. Within each mesophase the local director suffers thermally stimulated orientational order fluctuation called Order Fluctuations of the Director (OFD). This OFD are represented by elastic overdamped hydrodynamical modes, expanding in the three dimensions in the case of nematic mesostructures giving arrise to a square root law ($T_1 u_{L}^{1/2}$) or in bidimensional layers -undulations modes- in smectic mesophases with a linear law, $T_1 u_{L}^{1}$.NMR spin lattice relaxation rate $T_1$ accounts to the fluctuations producing modulation in the orientation of the internuclear vector of two protons with respect to the external magnetic field. It is well known that in NMR studies of thermotropic liquid crystalline mesophases the proton spin-lattice relaxation Larmor frequency dispersion $T_1( u_L)$ is essentially determined by the superposition of several competing relaxation mechanisms: {em i)} individual molecular motions, like self-diffusion $(T_1)_{SD}$ and molecular rotations $ (T_1)_{Rot}$, and {em ii)} collective molecular motions or order fluctuations of the local director $(T_1)_{OFD}$. OFDs has been proved to be the relaxation mechanism dominating $T_{1}( u _{L})$ in the frequency range below the MHz´s.The NMR spin-lattice proton relaxation dispersion in the smectic mesophase of two liquid crystals: 4cyano-4´-8-alkybiphenyl (8-CB) and 4,4´-bis-heptyloxy-azoxybenzene (HpAB) are studied over several decades of Larmor frequencies. The results shows that the order fluctuation of the local smectic director contribution to $T_1( u _L)$ suffers a transition in between two power regimes: from $T_1( u _L) propto u_{L} ^1$ to $T_1 u_{L}^{1/2}$ on going from low to high Larmor frequencies. We explain this behaviour by assuming, in the smectic mesophases, short coherence lenght nematic-like cooperative molecular reorientations.