New liquid-crystalline polyesters 1 with 4-(pheny1azo)azobenzene moities as mesogenic side groups and a polyester backbone structure characterized by a systematic variation of the length of the flexible non-mesogenic main-chain segments were synthesized. The polyesters were investigated by differential scanning calorimetry, polarizing microscopy, miscibility studies and X-ray diffraction. With increasing number of methylene groups in the backbone spacer, the thermal stability of the nematic and/or smectic phases first decreases and then increases.
Within the class of dimesogenic liquid-crystalline polymers, series of new LC-polyesters with laterally and terminally linked mesogenic units were synthesized. The mesogenic moieties perpendicularly arranged to the main chain are endowed either with identical or with unequal wing groups. Depending on the wing group and the spacer length, the polyesters can form monotropic or enantiotropic nematic and smectic phases. These polymers were subjected to characterization studies using optical microscopy. differential scanning calorimetry and X-ray diffraction techniques. The phase behavior is compared with that of combined and cross-shaped polymeric mesogens.
A newly synthesized series of liquid-crystalline main chain polymers with crossing mesogens is investigated using dielectric spectroscopy. The crossing mesogenic units are endowed with hexyl as well as with CN-wing groups. We examined the change of mobility of these crossing segments with increasing spacer length in the main chain, the relaxation behaviour at the phase transition nematic-isotropic and the temperature dependence of the absorption intensity. The main results are: there is an alternation of the dielectric relaxation times with respect to the length of the spacer in such a way that the molecular mobility is raised in case of an odd number of methylene groups in the spacer. The steps in the relaxation frequencies at the clearing temperatures are obviously due to a change in the short-range order caused by the formation of the nematic structure. Owing to a strong dipol-dipol interaction between neighbouring segments, a decrease of the absorption intensity with decreasing temperature was observed. Furthermore, apart from the low-frequency absorption a high-frequency relaxation process could be monitored.
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