X-Ray Study and Microscopic Study of the Reentrant Nematic Phase

Abstract

We report results from x-ray experiments on mixtures of N-(p-hexyloxybenzylidene)--aminobenzonitrile (HBAB) m N-^-cyanobenzylidene-^-?2-octloxyanilitie (C BOO A) 5 very recent microscopic observations on mixtures of cyanohexyloxybiphenyl in cyanooctylojgrbiphenyl are m agreement with these results. The evidence suggests that the reentrant nematic phase is similar to the classical nematic phase but may coexist with crystalline fluctuations. It has been shown recently^ that mixtures formed with two liquid crystals could exhibit with decreasing temperature the following sequence of phases: nematic, smectic A, and then again nematic. The nematic phase which occurs at low temperature is called the reentrant nematic phase (Fig. 1). The same phenomenom has also been observed with pure compounds under pressure.^ By increasing the pressure from the nematic phase, we observe the smectic phase and then again a nematic phase at high pressure. The nematic phase which occurs at high pressure is also called the reentrant nematic phase. All these results have been obtained using the light microscope.A model has been proposed^ for the organization of the molecules in this reentrant nematic phase. In this model, the molecules are assumed to be associated in antiparallel pairs in the smectic-A phase, which then can be destabilized as shown in Fig. 2.Here we present for the first time the results of an x-ray investigation of this unusual phase, and also very recent results of a microscopic study with a new mixture of thermotropic liquid crystals (Fig.

High-Resolution X-Ray-Scattering Study of the Nematic-SmecticA-Reentrant Nematic Transitions in 8OCB/6OCB Mixtures

“…Reciprocal space scans along #,, and q ± were fitted to the expression /(q) = l + ll| 2 foll -4b) 2+ £x 2 4-L 2 + <^-L 4 tf± 4 (1) convoluted with the instrumental resolution function. Phase transition temperatures were measured by observing the temperature at which the minimum in the width of q ± scans occurred rather than treated as adjustable parameters.…”

Pressure-driven reentrant phenomena in liquid crystals: the role of inverse layer spacing

“…From the expressions of the N-S A transition temperature, Equation (13), and the 'inverse layer spacing', Equation (7), in addition to the above experimental observations [22,23,25], it is obvious that the coupling parameters e 1 , b 1 and b 2 are pressure dependent. This makes the renormalised metastable temperature T 0 AN and the effective couplings and l eff also pressure dependent.…”

“…This is what appears as the observed reentrant phenomenon. Moreover, the layer spacing has been measured experimentally using an X-ray diffraction method [22,23] during the reentrant transition. It remains constant with increase in pressure.…”

“…This makes the renormalised metastable temperature T 0 AN and the effective couplings and l eff also pressure dependent. Considering the experimental P-T phase diagram [25] and the pressure data of q 0 [22,23], we computed the pressure dependence of e 1 , b 1 and b 2 , and hence T 0 AN , and l eff , by using the Landau parameters given in Table 1 from [28]. In doing so, we have constrained that such a pressure dependence of the above parameters satisfies the cubic Equation (9) for S. The P dependence of T 0 AN , and l eff is shown in Figures 1, 2 and 3.…”

“…It has also been observed [19][20][21] that the reentrance can be driven by increasing pressure in pure compounds and their mixtures. X-ray and microscopy studies showed [22,23] that the reentrant nematic phase is similar to the classical nematic phase but may coexist with crystalline fluctuations. Both the nematic phases are uniaxial and the defect structure of the N R phase observed in cylindrical geometry is identical to that of the N phase.…”