Notizen: Specific Volume Studies on Some Nematic Liquid Crystals

Abstract

The temperature variation of the specific volume of two nematic liquid crystals (HBT and OBT) has been observed in both the nematic and isotropic regions. A sudden jump is observed in the vicinity of the nematic-isotropic transition indicating a first order phase transition. Pretransitional effects are found to occur only on the nematic side of the transition. This accords with the Maier-Saupe theory. Some parameters such as 5k , A, the adiabatic compressibility, the Rao number, and the van der Waals constant are also determined. Now a days the liquid crystalline state is recognized as a true state intermediate between the crystalline solid and the isotropic liquid state [1][2][3][4][5][6] . Generally, the mesophase-isotropic phase transitions are regarded as of first order though some recent measurements on this transition show some second order admixture. Indeed, the cholestric and nematic liquid crystals exhibit the smallest first order transitions to the isotropic liquid known among pure compounds. In first order phase transitions a discontinuity or a steep change in specific volume is observed, while in second order transitions a point of inversion occurs (i. e. no discontinuity occurs in the first derivatives of Gibbs' free energy ' at the transition) .This paper, dealing with specific volume measurements of two nematic liquid crystals, (p-nhexyloxybenzylidene)-p-toluidine (HBT) and (p-noctyloxybenzylidene)-p-toluidine (OBT), is part of our systematic study on liquid crystals [8][9][10][11][12][13][14] .Both HBT and OBT were procured from M/S E. Merck (Germany) in pure form and were used as such. The transition temperatures were determined by means of a polarizing microscope and are listed below68.6°C 69.0°C 75.0°C Solid ^ Smectic ^ Nematic ^ Isotropic escThe specific volume was measured by means of a sensitive pycnometer u , which was placed in a regulated hot air oven (regulation better than + 0.1 ~C). The temperature variation of the volume of the pycnometer was also taken into account.The temperature dependences of the specific volume, the adiabatic compressibility (ß.id) and the Rao number (Rn) are shown in Figs. 1, 2 The specific volume increases linearly with temperature up to ~1 C below the nematic-isotropic transition, and again so in the isotropic region. A small but sudden discontinuity is observed which clearly indicates that the transition is of first order. The sudden increase of the specific volume with temperature near the nematic-isotropic transition is attributed to a sudden change from the ordered nematic to the disordered and less densely packed liquid state. From the Table 1 it can be observed that the thermal expansion coefficient [(l/u)" (di>/df)] is larger for the nematic than for the isotropic phase. This indicates that the increase of order with decreasing temperature is larger in the nematic than in isotropic phase.The breadths of the nematic-isotropic transitions are ~1.6°C and ~1.0°C in HBT and OBT respectively. The nematic-isotropic transition in our case is less sharp than i...

Synthesis and Characterization of Nonsymmetric Liquid Crystal Dimer Containing Biphenyl and Azobenzene Moiety

“…is includes the dramatic dependence of their transitional properties on the length and parity of the �exible spacers linking the mesogenic units [12], the observation of a nematic-nematic transition [13], and the occurrence of alternating smectic phases [14]. Particularly, molecules containing azo groups are well known to show reversible isomerization transformations upon irradiation with ultraviolet and visible light [15,16]. e azobenzene class of chromophores is characterized by the azo linkage (-N=N-) that bridges two phenyl rings.…”