H-1 AND H-2 NMR-STUDIES OF DYNAMIC ORIENTATIONAL, TRANSLATIONAL, AND DIPOLAR ORDERS IN THE DOUBLY REENTRANT LIQUID-CRYSTAL 4-CYANOBENZOYLOXY-[4-OCTYLBENZOYLOXYL]-P-PHENYLENE AND ITS DEUTERATED ANALOG

We report NMR studies with H-1 and H-2 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-S-Al (smectic A of monolayer type)-RN (reentrant nematic)-S-Ad (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-S-Ad-N, while a first-order discontinuity has been detected for the S-Al-RN transition. The proton NMR line shape for CBOBP-d17 has revealed a decrease (or loss) of antiferroelectric dipolar order in the S-Al phase compared to the S-Ad 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 S-Ad phase despite the more fluid macroscopic properties. The S-Al phase is characterized by distinctly high segmental orders. These results are discussed in line with a view that the two S-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.