Re-entrant mesophase

Note 1 The recommended subscript for the designation of re-entrant mesophases is re. Note 2 Re-entrant mesophases are most commonly observed when the molecules have strong longitudinal dipole moments (see example). 

Note 3 Sequences of re-entrant mesophases have also been found in binary mixtures of non-polar liquid-crystalline compounds. 

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. 

Cr Cub, Cubv d E G HT Iso Isore l LamN LaniSm/col Lamsm/dis LC LT M N/N Rp Rh Rsi SmA Crystalline solid Spheroidic (micellar) cubic phase Bicontinuous cubic phase Layer periodicity Crystalline E phase Glassy state High temperature phase Isotropic liquid Re-entrant isotropic phase Molecular length Laminated nematic phase Correlated laminated smectic phase Non-correlated laminated smectic phase Liquid crystal/Liquid crystalline Low temperature phase Unknown mesophase Nematic phase/Chiral nematic Phase Perfluoroalkyl chain Alkyl chain Carbosilane chain Smectic A phase (nontilted smectic phase)... 

Fig. 45 Fluorous metallomesogens at the lamellar-columnar cross-over (a) induction of a nematic phase by mixing metallomesogens with smectic and columnar phases, (b) metallomesogens showing re-entrant isotropic mesophases (Isore) (77° C) [40, 41, 293]...

The nature and number of any mesophases that are formed by a given material, and the temperatures at which they exist must be determined experimentally. This requires recourse to a selection of physical techniques, some of which are described below. However, it is helpful to know that, in most cases, the thermodynamic ordering of the various (fluid) mesophases maintains a fairly constant order that is given in Scheme 1 (although note that out-of-sequence or re-entrant phases are known).It is rather unlikely that a single material will show all of these phases and it is found empirically that certain combinations are more likely than others. ... 

When fluorine groups were introduced in the 2- and 3-positions of the terminal aromatic ring of the biphenyl unit ((48) M = Cu R = OC8Hn w=l) no columnar phase was observed, but a monotropic nematic phase instead, probably due to the reduction of the molecular symmetry When the lateral chain was elongated, the columnar phase disappeared at the expense of an enantiotropic nematic phase ((48) M = Cu R = OC H2 +i, n = 6, 8, 10-12, 14 and m = 2 n=l2, m = 3 and n = 4, 8, m = 4), which became monotropic ((48) M = Cu, R = OC H2 +i, n=l2,m = 4 n = 8, m = 8 and n=12, w = 12,). Furthermore, re-entrant (Section 7.9.3.2.5) nematic phases, between two crystalline phases, were also observed for the compounds in the series m = 2. Both the temperatures of the crystal-to-mesophase (163-219 °C) and mesophase-to-isotropic liquid (175-224 °C) decreased very rapidly with increasing m and n. The stability of the nematic mesophase was also found to depend on the bulkiness of the lateral chain. For instance, using a branched side group such as HMeEt did not suppress the monotropic phase but reduced the transition temperatures considerably. The cross-over between the Coh and the nematic phase would then correspond to a decrease of the lateral interactions between molecules due to the steric hindrance of the chains. 

Of course not aU of these mesophase have to appear in a single liquid crystalline system. For very few liquid crystals, exceptions from this sequence rule are known to exist. In these liquid crystals a mesophase with a higher symmetry reappears on cooling, even though a less symmetric mesophase has already formed at higher temperatures. Such phases are called re-entrant and are indicated with a subscript RE . Re-entrant behavior was first observed for a N-SmA-NRE-Cr phase sequence [39], but it was also found for other types of mesophases [40, 41]. It is not completely clarified when and why re-entrant phases appear. Different approaches to explain the re-entrant behavior were made, e.g. on the basis of frustration, geometric complexity or competing fluctuations [42, 43]. 

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

The orthogonal arrangement of the disc-like molecules in the columns of and D id phases makes these phases uniaxial, while the tilted phases (Drd and Doh.d and Dt) are optically biaxial. There are two additional columnar phases labeled as and that have not yet been classified. The columnar phases were discovered before the observation of a nematic phase for disc-like molecules. Both chiral nematic phases and the re-entrant behavior have now been observed in discotics. The phase diagram and molecular structure of a typical discotic liquid crystal are shown in Fig. 1.11. Finally, it is noted that another classification scheme for the discotic mesophases has been used [1.26], which is based on the notation used for the conventional smectics. 

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]. 

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

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