PMRD investigations of molecular dynamics and phase transitions in the liquid crystal 6CHBT

Abstract

Proton Magnetic Relaxation Dispersion (PMRD) measurements were carried out over a wide Larmor frequency range near the isotropic-nematic transition temperature (T NI) and in the mid nematic phase of a low viscous liquid crystal 4-(trans-4′-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT), with a view to examining the pre-transitional effects just above isotropic-nematic transition (T > T NI) and the role of director fluctuations (DF) on the nematic phase stability of this system. The results near the transition indicate critical slowing down of the short-range nematic order modes just above T NI with a mean field critical exponent characterizing the coherence length (ξ). The observed temperature independence of the correlation time associated with the reorientations (τR) and the extension of DF mode spectrum, in the mid-nematic phase, to relatively higher resonance frequencies seem to suggest fairly low hindering barriers for the tumbling of the molecules about their short axes.