COUPLING OF HYDROGEN-BONDING TO ORIENTATIONAL FLUCTUATION MODES IN LIQUID-CRYSTAL PHBA

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 showing that two processes, the formation and destruction of the hydrogen bonds and the orientational motions of the molecules contribute to 1/T//1. It is proposed 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.