Re-entrant Phase Transitions in Liquid Crystals

Cladis, P.E. Re-entrant phase transitions in liquid crystals. In Demus, D., Goodby, J., Gray, G.W., Spiess, H.-W., Vill, V. (eds.) Physical Properties of Liquid Crystals, pp. 289-303. Wiley-VCH, Weinheim (1999)... 

The re-entrant or Tammann loop-shape phase diagram as observed in proteins is also found in other systems and has been connected to exothermic disordering [88]. In this particular case, nematic - smectic A transitions in liquid crystals and the phase behaviour of a crystalline polymer, poly(4-methyl-pentene-l), the phase behaviour can be understood by... 

While the phase was also found in ni-tro compounds with two benzene rings [28], re-entrant phenomena in liquid crystals made a giant leapforward in 1979, when the stable Nj-e-SmA transition [29] and multiple re-entrance [30] was discovered at 1 atm... 

As the temperature is decreased the chiral nematic structure transforms to a higher order phase. The phase may go through a first order phase transition and crystallize in which case the optical properties are of little interest herein. It may transform to a glass, in which case the optical properties, such as birefringence, pitch, etc., are frozen and may be used in static, or time and environment-independent devices or applications (as discussed in Sec. 2.5 of this Chapter), or it may go through a second order or second order plus a weak first order phase transition to a higher order liquid crystalline phase. Here, for simplicity, we are not considering the so-called re-entrant phases [ 14]... 

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. 

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. 

A point where three fluctuation dominated phase transition lines meet in a 2-dimen-sional parameter space is also expected to exhibit universal features. An extensively studied liquid crystal candidate was the N-SmA-SmC point in mixtures [83], in a pure compound under pressure [84] and at the re-entrant N-SmA-SmC multicritical point [85]. The situation may be summarized as follows. The systems studied showed qualitative and quantitative similarities. However, the exponents exhibited were not in the expected universality class for three second order phase transition lines meeting at a point. This is likely because, in the N-SmA-SmC case, the N-SmC transition line is first order [86] as is the N-SmA transition line, [26] leaving only the SmA-SmC second-order phase transition line. 

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