Coupling of Hydrogen Bonding to Orientational Fluctuation Modes in the Liquid Crystal PHBA

Abstract

In the liquid state, the molecules of the carboxylic acids are associated in pairs by hydrogen bonds connecting their acid groups. If these dimers are long and rigid, they can form a nematic phase. The dimer molecules of the liquid crystal p-hexyloxybenzoic acid have been deuterated on the hydrogen bonds, and the nuclear quadrupolar spin-lattice relaxation rate 1/T ₁ of these deuterons has been studied. In the nematic phase, 1/T ₁ is frequency dependent between 2.7 and 13.8 MHz; its temperature behavior is identical with that of the parameter S describing the orientational order of the nematic phase. In the isotropic phase, 1/T ₁ behaves in the same way as in conventional nematics: at high nuclear frequencies, it is still frequency dependent, and rather unsensitive to temperature in the vicinity of the nematic-isotropic transition; at low frequencies, it has no frequency dependence, but diverges when the transition temperature T _c is approached from above: The “critical frequency” which separates these two regimes is on the order of 14 MHz near T _c. These results are interpreted in the following way. Two processes may contribute to 1/T ₁: the formation and destruction of the hydrogen bonds, and the orientational motions of the molecules. It is proposed here that these two processes are strongly coupled, i.e., that the lifetimes of the various hydrogen-bonding configurations are controlled by the collective fluctuations of the molecular orientations. Assuming that the local orientations and local concentrations of broken bonds are the only slowly relaxing quantities in the system, the decay in time of their fluctuations is described by a system of coupled transport equations. The resulting spectral density for the fluctuations of the concentration of broken bonds reproduces the main features of the observed behavior.