NMR Relaxometry Study of Slow Processes in Liquid Crystals

by Ms. M. Rajeswari

Monday, February 13, 2012 from 16:00 to 17:00 (Asia/Kolkata)
at Colaba Campus ( AG-69 )

TIFR, Colaba Mumbai 400005

Description

Dispersion profiles of nuclear spin-lattice relaxation rates measured over several decades of frequency provide a unique opportunity to probe different dynamic processes, and recent application of field-cycling technique accessing sub-MHz resonance frequencies had led to the detection of ultra slow dynamics, for example in soft materials. This talk presents our recent work on certain interesting slow molecular processes in liquid crystals, based on proton and fluorine relaxometry. These systems are particularly interesting because of their rich dynamic molecular organizations spanning their correlation times from μs to few tens of ns. We have taken specially synthesized molecules where the single fluorine is present on the aromatic core and many other protons. The spin-lattice relaxation rate dispersions of ¹H and ¹⁹F in the isotropic phase of these liquid crystals point to their differing relaxation pathways and hence their sensitivity to qualitatively different time modulations. The strong coupling of fluorine nuclei to the lattice is seen to detect the slowly relaxing local structures via the spin-rotation interaction. The relaxation rates measured at very low frequencies provides a level crossing of the two nuclear species facilitating an additional mechanism of cross-relaxation. Another interesting problem we worked on relates to liquid crystal systems confined to random porous matrices formed by aerosil nano-sized particles. The confinement induces an ordering near the surface, resulting in slow reorientational dynamics in the surface-ordered layer. These induced slow processes are analyzed in terms of reorientations mediated by translational displacements (RMTD). The power law behavior of this mechanism is different in different mesophases, indicating the predominance of low or high wavelength modes in different mesophases. The relaxation rate enhancement upon confinement gives an opportunity to probe the surface structure factor and kinetics in the adsorption layer.