Effect of confinement on molecular processes in the liquid crystal 8OCB: application of NMR relaxometry

Abstract

Dispersions of proton spin–lattice relaxation rates (R₁) in the nematic and smectic phases of the liquid crystal 4′-octyloxy-4-cyanobiphenyl (8OCB), embedded in a nano-porous medium formed by an aerosil matrix, are investigated over a wide frequency (ν) range (10⁴ Hz to 5 × 10⁷ Hz) at two average pore sizes. In the low frequency (sub-MHz) region we observe a significant increase of R₁ in the confined samples, mediated by slow translational displacements (RMTD), arising from the formation of adsorption layers near the porous surface. Dispersion in this region is well accounted for by a power law behaviour (R₁ ∼ w^-p), bracketed by low and high cutoff frequencies reflecting the limiting length scales of such displacements in these layers. The exponent p increases from 0.45 to 0.6 with a decrease of the temperature within the nematic phase, due to the progressive onset of longer wavelength diffusive modes within these layers. This signifies the onset of order director fluctuation (ODF) modes within the adsorption layers. Confinement is also seen to place restrictions on the long wavelength ODF modes of the bulk-like region of the liquid crystal, thus weakening considerably their dispersive effects in the low frequency region, relative to the bulk samples. In the smectic phase, p decreases appreciably signalling a depletion of such long range cooperative modes on layer formation. Analysis of the R₁ data also shows that individual dynamic processes like translational diffusion and reorientations of the molecules within the voids are largely unaffected due to confinement.