Phase re-entrant

The re-entrant phase diagram experimentally observed for proteins has also been observed for a number of water-soluble polymers. As expected, the sidechains of the polymers are able to form hydrogen bond interactions with water. Poly(ethylene oxide) [92] and poly(vinyl methyl ether) [93] form weak hydrogen bonds via their ether groups. Poly(N-vinylpyrrolidone), poly(N-isopropylacrylamide) [94], poly(N-vinylisobutyramide) [94], poly(N-vinylcaprolactam) and poly(2-acrylamido-2-methyl-l-propanamide) [95] contain amide groups in their side chains. It is well known that hydrogen bonds to ethers are much weaker than those formed with amides. It is clear that these polymers also show hydrophobic interactions. 

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. ... 

How are all these phases positioned in the phase sequence lowering the temperature The sequence described below is different only in some rare cases of re-entrant phases. However, there is no material known at present where all the phases are present in the same sample. The most general phase sequence has been found in the prototype material 4-(l-methylheptyloxycarbonyl)phenyl 4 -octyloxybiphenyl-4-carboxylate (MHPOBC). It has all phases mentioned but the recently discovered SmC/j phase. However, in the phase sequence of the material exhibiting the six-layer SmC phase other phases are missing. In different materials various phases are missing, for example, in racemic MHPOBC... 

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)... 

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]. 

The results of the X-ray diffraction measurements can be seen in Fig. 5.22. In case of the N phase, the measurements clearly show a rather diffuse azimuthal orientation (Fig. 5.22b) and a low scattering intensity (Fig. 5.22a), which reflects the low order of the cholesteric structure. The same can be found for the phase (Fig. 5.22d), which proves the existence of the re-entrant phase. Additional to this scattering maximum, a second incommensurate scattering maximum can be found... 

Wette, R, Klassen, I., Holland-Moritz, D., Her-lach, D. M., Schoppe, Lorenz, N., Reiber, H., Palberg, T., and Vogt, S. V. (2010). Complete description of re-entrant phase behavior in a charge variable colloidal model system. J. Chem. Phys. 132, 131102-131106. 

The re-entrant phase exists only in the supercooled region of the liquid. 

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. 

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]... 

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