Influence of nematic range on birefringence, heat capacity and elastic modulus near a nematic–smectic-Aphase transition

Abstract

The birefringence ⌬n, the specific heat C p , and the layer compressional elastic modulus B are reported for two liquid crystals near the nematic (N) to smectic-A (SmA) phase transition. As predicted long ago by MacMillan and de Gennes ͓P. G. de Gennes and J. Prost, The Physics of Liquid Crystals ͑Clarendon, Oxford, 1993͔͒ the coupling of the nematic orientational order parameter to the smectic-A layering order parameter can substantially alter the critical behavior near the N-SmA transition if the nematic range is small and the nematic order parameter susceptibility is large. In this paper, we present a direct experimental comparison of two compounds: 4-octyloxy-4Ј-cyanobiphenyl ͑8OCB͒ with a short nematic range and 4-octyloxybenzoyloxy-4Ј-cyanotolane (C 8 tolane) with a very large N range. The temperature variations of the apparent birefringence ⌬n and the specific heat C p across the N-SmA phase transition show the definite influence of the proximity of the isotropic phase in the case of 8OCB while the C 8 tolane behaves as expected for the three-dimensional XY universality class. The elastic modulus B in the SmA phase, measured at several wave vectors by the secondsound resonance technique, was studied with high resolution as a function of temperature on approaching T c (N-SmA). These elastic data confirm the B leveling off in both cases with an apparent breakdown of hydrodynamics in the case of the C 8 tolane compound.

Binary mixtures of nCB and nOCB liquid crystals. Two experimental evidences for a smectic A–nematic tricritical point

“…Likewise, Oweimreen and Hwang As far as 8OCB pure component is concerned, the SmA-N transition has been studied by means of several techniques: precise calorimetric measurements, 18,[23][24][25][26] piezothermal studies, 27 X-ray measurements, 28,29 light-scattering studies, 30 volumetric measurements, 31 elastic modulus and birefringence determinations. 32,33 Once more, conflicting results on the order of the transition and the critical exponents have been reported.…”

Imperfections in focal conic domains: the role of dislocations

“…small; 2 d 0 is the volume occupied by a molecule, and r c is microscopic, practically constant (of order for a screw dislocation, d 0 for an edge dislocation). The decrease of B, as already stated, is effective on a large temperature range before the transition, probably larger than 1 C [4,6]. The core radii scale as the correlation lengths very close to the transition, but this region is of no interest to us.…”

“…N phase transition has been the object of many investigations (for a review, see De Gennes and Prost [3]). The compression modulus, B, tends towards a value equal to or slightly different from zero, according to the author [4][5][6][7]; its variation is noticeable in a large temperature range (more than half a degree in the compounds that we have investigated). Notice that in this range, K 1 (the splay modulus) stays practically constant.…”

Kinked Focal Conic Domains in a SmA

“…It turns out that the non-nematogenic and nematogenic smectics differ by their temperature dependence for the modulus B. For non-nematogenic smectics B is very weakly temperature dependent, while for the nematogenic smectics B decreases according to some critical exponent when approaching the phase transition to the nematic phase [7][8][9][10]. Therefore the replacement of the FCD texture by the distortions involving dislocations on heating approaching the phase transition to the nematic phase can be explained by the switching from the 'isometric deformation' regime, in which the layers are mostly affected by FIGURE 6 Temperature dependence of the ellipse major axis for nonnematogenic smectic 10CB.…”