SYNTHESIS AND CHARACTERIZATION OF NEW LIQUID-CRYSTALLINE MONOMERS FORNONLINEAR OPTICS - X-RAY STUDY OF REENTRANT NEMATIC BEHAVIOR WITH SMECTIC-LIKE FLUCTUATIONS OF C-TYPE

The syntheses and structural properties of four liquid crystalline mon omers are presented. They were prepared by two different reaction path ways. Transition temperatures and phase characterization were done by DSC, polarization microscopy and X-ray diffractometry. The dependence of phase transition temperatures on the alkyl chain lengths and the la teral substitution of the aromatic core are discussed. Two of the comp ounds investigated show highly ordered smectic phases. The improvement of the conjugation by means of a lateral hydroxy group disturbs the o ccurrence of the E and B phases. The presence of a nematic re-entrant phase with smectic-like fluctuations of the C-type was observed in a c ompound with a lateral hydroxy group. The existence of this re-entrant nematic phase was confirmed by X-ray measurements both on cooling and heating of the sample.