Slow molecular processes in a nematic liquid crystal confined to random porous network formed by aerosil particles

Frequency dispersion measurements of proton spin-lattice relaxation rates (R-1) of liquid crystal 4-propyl-4'-pentylazoxybenzene in bulk and confined samples (in random porous network of aerosil nano-particles) are reported in isotropic and nematic phases. Significant low-frequency increase in R-1 in confined samples indicates slow molecular reorientations mediated by translational displacements near the adsorbing porous surface. The resulting dispersion behavior of R-1 (similar to omega (p)) reflects the nature of the random surface (p = 0.5 for equi-partition of the diffusive modes). The observed temperature-independent exponent in the isotropic phase (p = 0.34) indicates the abundance of low-wavelength surface modes. Its temperature-dependent higher values in the nematic phase (from 0.59 to 0.65 on cooling), and increased spin-lattice coupling via this mechanism, show progressive onset of longer wavelength modes. A detailed analysis shows the effect of confinement on the order director fluctuations, molecular reorientations, and translational diffusion of the molecules.