Re-entrant nematic

Many technological applications of liquid crystals, as in electro-optic display devices, are based on multicomponent mixtures. Such systems offer a route to the desired material properties which cannot be achieved simultaneously for single component systems. Mixtures also tend to exhibit a richer phase behaviour than pure systems with features such as re-entrant nematic phases [3] and nematic-nematic transitions possible. In this section, we describe simulations which have been used to study mixtures of thermotropic calamitic mesogens. 

These structures were firstly observed for terminally polar mesogens [11, 12]. However, recent experiments give clear evidence of the presence of smectic A layering [37, 38], re-entrant nematic behaviour [39], two-dimensional lattices [40, 41] and smectic layering with incommensurate periodicities [42] for non-polar sterically asymmetric LCs. 

Example The following compound exhibits, as temperature decreases, an isotropic (I) phase, nematic (N), smectic A (SmA) re-entrant nematic (Nre), re-entrant smectic A (SmAre) mesophases and a crystalline (Cr) phase, with transitions at the specified temperatures. 

The structure of the smectic A phase, therefore, is governed by the relative lengths of the spacer and terminal chains. For short spacers, n = 3-7, the CBOnO.lO series exhibits the SmAa phase while for long spacers, n= 10-12, the SmAc phase is observed. The disappearance of smectic behaviour for intermediate chain lengths (see Fig. 11) implies that neither smectic modification is favourable and hence nematic behaviour results. There is a strong similarity here to re-entrant nematic behaviour which is also driven by two different length scales. 

The mean field theory predicts a bicritical point when the second order N-Ai and N-A<, phase boundaries meet a first order A -Aj boundary. However, when the effect of fluctuations is taken into account, the existence of such a point becomes doubtful. On the experimental side, phase diagrams with an Aj-Nj -A, point have been reported, (where Nr is the re-entrant nematic, see 5.6.2) the topology of these diagrams resembling that of the magnetic bicritical point. But high-resolution experiments carried out subsequently in the immediate vicinity of an... 

Prasad SK, Nair GG. 2001. Effects of photo controlled nanophase segregation in a re entrant nematic liquid crystal. Adv Mater 13(1) 40 43. 

Figure 11. The mechanism of formation of the re-entrant nematic mesophase.

In 1975 Cladis discovered the sequence of phases nematic, smectic, and again nematic at atmospheric pressure. The lower-temperature nematic phase was designated as the re-entrant nematic phase (N ). By 1977 Cladis [68] was successful in giving evidence of a pressure-induced re-entrant nematic phase (Fig. 2). The investigated compounds were cyano Schiff bases and cyanobiphenyls with terminal n-alkyl or... 

The studied compounds all have a terminal cyano group which means a very strong dipole. Cladis et al. [69] proposed a structural model of the bilayer smectic A phase for this kind of molecules (Fig. 3). The molecules are assumed to be associated in antiparallel pairs, which results in a weak interacting between the different polar parts of the pairs and a less dense packing of the molecules. Thus a transition of such bilayer SmA phase to a re-entrant nematic phase seems to be evident, because in this phase empty spaces of the structure are filled up more efficiently. Probably for similar reasons Pollmann et al. [70-72] found a pressure-induced re-entrant cholesteric phase behavior for ternary mixtures of cholesteryl n-alkanoates which, however, are terminally nonpolar (see Fig. 4). The mixtures of... 

Figure 3. Schematic arrangement of antiparallel associated pairs in the nematic, smectic A, and re-entrant nematic phase. (From [64], reproduced by permission of American Physical Society.)...

Guillon et al. [73] investigated the pressure dependence of the SmA phase layer spacing of A-(4-cyanobenzylidene)-4-n-octyloxyaniline by X-ray-diffraction measurements. While the layer spacing decreases, when the SmA phase is pressurized towards the solid phase, it remains constant when this smectic phase is pressurized towards the re-entrant nematic phase. 

Shashidhar and Rao [74] performed high pressure X-ray studies on liquid crystals with re-entrant behavior with an opposed diamond anvil cell. They found that the layer spacing of the SmA phase of 4-n-octyloxy-4 -cyanobiphenyl first decreases more or less linearly with increasing pressure up to 140 MPa, then increases at still higher pressures. Since this compound shows re-entrant nematic behavior at high pressures, this result confirms the prediction of Cladis et al. that the occurrence of a re-entrant nematic phase is associated with an expansion of the SmA phase layer spacing. 

Shashidhar et al. [75] studied the influence of pressure on the SmA- (re-entrant) nematic and N-I phase boundaries of mixtures of 4-n-hexyloxy- and 4- -octyloxy-4 -cyanobiphenyl. The maximum pressure where the SmA and re-entrant nematic phase, respectively, still exist, decreases with increasing mole fraction, x, of the hex-yloxy homolog till at x 0.30 the SmA phase disappears. Just in this mole fraction region the slope of the N-I transition... 

In the re-entrant nematic transition, a liquid phase without translational order (nematic) occurs at a lower temperature or higher pressure than one with one-dimensional translational order (SmA). With decreasing tern-... 

Goodby and coworkers [24] elegantly showed that re-entrance in cyano compounds resulted from a sensitive balance between dipolar and steric factors. They synthesized two benzene ring materials with an ester link that either reinforced (resulting in exclusively re-entrant nematic behavior) or opposed (resulting in the appearance of a SmC phase below the SmA) the cya-nophenyl mesomeric relay [24], The conclusion is that, when dipolar forces domi-... 

Some investigations have been devoted to the behaviour of elastic constants in particular regions of the mesophase diagram for example, studies near the N-SmA-SmC tricrit-ical point [86,89,93,94] and studies of elasticity in a re-entrant nematic phase [113]. 

Figure 11. Kinematic shear viscosity coefficients Vi, V2 and V3 of a re-entrant nematic mixture as a function of temperature in the nematic, smectic and re-entrant nematic phases.

While we said that lowering the temperature produces typically a more complex liquid crystalline phase, this is not always so. Re-entrant nematic (RN) phases, existing at temperatures below smectic phases, are also possible. Sackmann, Demus and collaborators studied binary MLC + MLC systems forming RN phases. While it was previously presumed that such phases can be formed in unary and binary systems of strongly polar compounds only, their compounds contained terminal non-polar groups. In some cases Sackmann et al found a smectic A phase in the middle of a phase diagram, while pure components themselves form no such phases. PLCs also exhibit RN phases, as first established by Shibaev, Plate and their collaborators and then also by Claudine Noel and her colleagues. ... 

The lower-temperature nematic phase was referred to as re-entrant nematic phase (Nr). Examples of more complex re-entrant behaviour are now known for pure compounds. " ... 

N = nematic mesophase (RN being a re-entrant nematic phase)... 

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