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Abstract : Two series of liquid-crystalline compounds having three phenyl rings separated by flexible spacer —CHCH3CH2—COO— and by rigid azo and azoxy group, were studied by DSC, optical and X-ray methods. For esters of dl-3-4′-nitro-phenylbutyric acid with 4′-alkoxy-phenylazo-phenol-4 having dodecyloxy or longer terminal chains, as well as for related azoxy compounds, a narrow even below 5 K reentrant or inverted nematic phase appearing between partly bilayer and monolayer smectics A was observed. For higher homologues of the azoxy series additional smectic phases appear, leading to the occurrence of new multicritical points, e.g. the critical end point Ad – Cd – Nre. On each of the lines, which separate nematic from smectic A phases, transitions are of weakly first or second-order and more than one tricritical point can occur. On the A1 – N-A1 – Nre line, a simple N – A1 tricritical point is observed at TNI-TAN = 0.834. The presence of further critical points depends on the components of the binary system involved. Four of the azoxy compounds studied undergo a second order phase transition between partly bilayer smectics, Ad and Cd. Such a transition is accompanied by a jumb in the specific heat, varying linearly with the length of the molecular tails. Various temperature dependences of the layer spacing in the Ad phase are observed for subsequent homologues from the azoxy series.





Autor: W. Pyżuk E. Górecka J. Mieczkowski J. Przedmojski

Fuente: https://hal.archives-ouvertes.fr/



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