Several substances of different chemical structures exhibiting the smectic B polymorphism in a wide temperature range [accompanied by the nematic (N) phase in few cases] were studied with the aid of the dielectric relaxation and X- ray methods. The dielectric relaxation time sk connected with the end-over-end rotations of molecules was determined. It elongates considerably at the N–B tran- sition whereas the activation barrier hindering the flip–flop motion decreases. The parameters of the hexagonal unit cell were determined as functions of temperature. The packing parameter, p 1⁄4 ZVmol=Vu.c., was calculated which exceeds value of c. 0.6, thus is typical for the plastic crystals. The dynamical properties of molecules are discussed in relation to their packing in the smectic layers. The applicability of the Arrhenius equation to calculation of the activation barrier from the tempera- ture dependence of sk in the B phase seems to be doubtful in view of the experi- mental data collected. It was concluded that all examined substances have rather the hexatic than the crystalline B phase.
|Autori:||Urban S; Czub J; Przedmojski J; Dabrowski R; Geppi M|
|Titolo:||Dielectric and X-ray studies of substances with the smectic B phase|
|Anno del prodotto:||2007|
|Digital Object Identifier (DOI):||10.1080/154214007.01732340|
|Appare nelle tipologie:||1.1 Articolo in rivista|