Re-entrant phase transitions

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)... 

Synthetic polymer gels are known to exist in two phases, swollen and collapsed. Polyampholyte gels should show a re-entrant volume transition in response to pH [164]. Tanaka et al. [165] have reported the existence of more than two phases in randomly distributed polyampholytes of MAPTAC and AA. Let us consider the behaviour of a sample containing 65 mol% of AA and 35 mol% of MAPTAC. At neutral pH this sample has a diameter d=do that is referred to as phase 1 (d/do=l) according to the label in the original paper. At pH=8.5 the gel swells discontinuously to phase 2.7 (Fig. 38). If the pH is lowered from 8.5 to 7.0, the gel returns to phase 1 discontinuously. If instead the pH is increased further from 8.5 to 9.8 and goes back from this point the gel collapses into phase 1 at pH=6.4. The same experiments were done from the acidic side. NMR spectra... 

The pressure-induced re-entrant phase behavior is not restricted to one re-entrant N phase and not only to N phases. Raja et al. [38] studied the effect of high pressure on the phase transitions in the seventh to tenth homologs of 4-alkoxyphenyl-4 -nitroben-zoyloxybenzoates. The decyloxy member, and only this one, exhibits a conspicuous phase diagram besides two pressure-induced SmA phases and a bounded SmC phase a quadruple re-entrance is observed SmA-N-SmAd-N-SmAj-N. 

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-... 

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. 

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 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... 

In addition to the phase transitions de-seribed so far, a number of other phase transitions involving liquid crystalline phases has been reported. These include for side-chain liquid crystalline elastomers SmC-I [32], N-SmA [6, 30], N-SmF [32], SmA-SmC [32], SmA-SmB [32] and SmC-SmF [32], Furthermore, a phase transition between a SmA phase and a re-entrant... 

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