Relaxation dispersion and zero-field spectroscopy of thermotropic and lyotropic liquid crystals by fast field-cycling N.M.R.

Abstract

Systematic field-cycling measurements of the T ₁ relaxation dispersion in numerous nematic liquid crystals (azoxybenzenes, Schiff's bases, biphenyls, phenyl-cyclohexanes, cyclo-cyclo-hexanes) confirm our previous observations obtained for PAA and MBBA that order fluctuations of the nematic director are a significant relaxation contribution only at low Lamor frequencies v, i.e. far below the usual megahertz range. Their significance is demonstrated most convincingly by the characteristic square-root dispersion law, T ₁ ∼ v ^1/2, which occurs in the kilohertz range and which completely disappears above the nematic–isotropic phase transition. The strength of the collective relaxation mechanism varies by more than two orders of magnitude in the sequence (selection) PAA-d ₈ PAA, PAA-d ₆ PAB, OCB7, MBBA, CB7, PCH7, MBBA-d ₆ MBBA-d ₁₃ and CCH7. This finding can be understood almost quantitatively by the widely differing separations and orientations of proton pairs on the molecules, together with the different viscoelastic parameters of the nematogens. In addition, the underlying slow molecular reorientations have been observed in MBBA and PAA by intensity changes of the zero-field spectra, which are absent for high-field measurements. Similarly, smectic type order fluctuations in layered liquid crystal structures prove to be an effective relaxation mechanism only at low Lamor frequencies. This has been verified by the related linear relaxation dispersion profile, T ₁ ∼ v ¹, for both thermotropic systems (TBBA, C₁₂-AA) and lamellar lyotropic mixtures (e.g. potassium laurate in water and phospholipids in water). Our results concerning the time scale of the T ₁ ∼ v ^1/2 and T ₁ ~ v ¹ regime do not agree with conclusions drawn from conventional high-field techniques.