1H and 2H NMR studies of dynamic orientational, translational, and dipolar orders in the doubly reentrant liquid crystal 4-cyanobenzoyloxy-

Abstract

We report NMR studies with $^{1}\mathrm{H}$ and $^{2}\mathrm{H}$ on the doubly reentrant polar liquid crystal 4-cyanobenzoyloxy-[4-octylbenzoyloxy]-p-phenylene (CBOBP) and its chain-deuterated homologue CBOBP-d17. The phase transition sequence is crystal-${\mathit{S}}_{\mathit{A}1}$ (smectic A of monolayer type)-RN (reentrant nematic)-${\mathit{S}}_{\mathit{A}\mathit{d}}$ (smectic A of partially bilayered type)-N (nematic)-I (isotropic). It has been found by a technique of director realignment in the magnetic field that rotational viscosity changes drastically at the phase transitions reflecting the creation/annihilation of one-dimensional translational order. On the other hand, the orientational order parameter of the molecular core changes smoothly through the phase changes RN-${\mathit{S}}_{\mathit{A}\mathit{d}}$-N, while a first-order discontinuity has been detected for the ${\mathit{S}}_{\mathit{A}1}$-RN transition. The proton NMR line shape for CBOBP-d17 has revealed a decrease (or loss) of antiferroelectric dipolar order in the ${\mathit{S}}_{\mathit{A}1}$ phase compared to the ${\mathit{S}}_{\mathit{A}\mathit{d}}$ phase. Segmental order parameters of the deuterated methylene groups exhibited anomalous upward deviation in the RN phase, showing that the alkyl chains are in a more restricted environment than in the ${\mathit{S}}_{\mathit{A}\mathit{d}}$ phase despite the more fluid macroscopic properties. The ${\mathit{S}}_{\mathit{A}1}$ phase is characterized by distinctly high segmental orders. These results are discussed in line with a view that the two ${\mathit{S}}_{\mathit{A}}$ phases are stabilized by different mechanisms involving competition between dipolar and conformational orders, and that the intervening RN phase appears as a frustrated disordered phase.