High pressure study on induced S_Aphases in binary liquid crystal mixtures

Abstract

Binary mixtures of terminal polar and non-polar liquid crystals exhibiting induced smectic phases are studied under high pressure. For terminal polar compounds with smectic phases, there are two types of T, x phase diagrams known up to now. Diagrams with a nematic gap between the induced phase and the smectic phase of the terminal polar compound and diagrams with an uninterrupted miscibility of the smectic phases. We find a continuous transformation between these phase diagrams with pressure. At a certain pressure, the phase transition lines form a cross separating two nematic and two smectic phases. IntroductionMixtures of terminal polar and non-polar liquid crystals usually show phase diagrams with a very stable smectic phase (induced smectic phase) of the orthogonal type in the range of medium concentrations El, 21. If the pure terminal polar component also exhibits a smectic phase, two different types of phase diagram can be observed which are shown schematically in figures 1 and 2. An uninterrupted miscibility of the induced S, phase and the SA phase of the polar component (see figure I ) is usually found in mixtures of 4-n-octyloxy-4'-cyanobiphenyl(80CB) with terminal non-polar esters having long alkyl chains. Esters with short alkyl chains usually exhibit phase diagrams with a nematic gap [3] between the two smectic phases (see figure 2). On both sides of the nematic gap, the reentrant phenomenon, i.e. the phase sequence Nr,-SA-N, is observed. The question arises as to whether these two types of phase diagram are principally different. Such a principal difference could be caused by slight differences in the layer structure which is, in fact, different: a monolayer (S,,) for the induced phase [3, 41 and a bilayer (SAd) structure for the pure polar component [5].A hint that both types of phase diagram can be transformed into one another is given by the phase diagram for 4-n-butyloxyphenyl-4'-n-pentyloxybenzoate and 80CB [6]. This phase diagram shows besides the maximum of the induced S, phase a minimum in the phase boundary line N-S, on the 80CB side.It is known that the application of pressure reduces the stability of the S,, phase [7], whereas nearly all other liquid crystal phases are stabilized. Therefore, it is possible to change significantly the form of a phase diagram with an induced smectic phase by application of pressure. We have now tried to find the missing link between the different types of phase diagram with induced smectic phases by studying suitable binary systems under high pressure.

The Induced SA Phase in New Menthyl-Containing Copolymers

“…The presence of hydrogen bonds, donor and acceptor interactions, − and strong dipole−dipole interactions often leads to the development of additional induced mesophases in such systems. The most common case concerns the development of the S A phase in the blends of nematogens with strong polar, donor−acceptor interactions between the components. − …”

“…The most common case concerns the development of the S A phase in the blends of nematogens with strong polar, donor-acceptor interactions between the components. [6][7][8][9] For example, the development of the induced S A phase proceeds in polymer systems where one comonomer is a donor and another comonomer is an acceptor. 10 The development of the induced S A phase is also observed for the copolymers and blends of homopolymers in which the initial components are nonmesomorphic.…”