Solid-liquid crystal phase transition

M. Seredyuk, A.B. Caspar, V. Ksenofontov, Y. Galyametdinov, J. Kusz, P. Giitlich, Does the solid-liquid crystal phase transition provoke the spin-state change in spin-crossover metallomesogens J. Am. Chem. Soc. 130, 1431-1439 (2008)... 

Figure 3 Spectral switching of the cast azobenzene bilayer film by thermal and isothermal phase transition based on structural polymorphism of two-dimensional molecular packing, (a) Spectral state (1) of as-cast film moves to state (2) upon heating above the solid-to-liquid crystal phase transition (115°C). Coofing to room temperature gives state (3) and state (2) is regenerated in the second heating (60°C). State (1) appears again by an isothermal moisture treatment of state (3) as well as by UV-light irradiation of state (3). (b) Schematic illustration of structural change.

The spectral change due to the UV light irradiation is ascribable to the isothermal phase transition triggered by the photochemical isomerization reaction. The trany-isomer in the metastable solid phase with the head-to-tail chromo-phore orientation is converted to the cfT-isomer by the UV light irradiation. The backward reaction from cis to trans can also partly proceed during the UV light irradiation. When the cast film is irradiated below the solid-to-liquid crystal phase transition temperature, the trans-isomer is regenerated by the backward reaction... 

For the study of the thermotropic mesomorphism of membranes the sample preparation is very important. Lipid dispersions are generally prepared by mixing the desired amounts of solid lipid and solvent (H2O, D2O or buffer solution). When using D2O as solvent a closed vessel should be used in order to minimize exchange with atmospheric water. In order to ensure proper hydration the lipid water mixture should be first heated above the temperature of the gel to liquid crystal phase transition of the corresponding lipid, then cooled before commencing the measurements. The study of natural membranes involves re-hydration only if the membranes are isolated from the other cell components and lyophilized (6-8). The experiments on bacterial membranes currently under way in our laboratory utilize live bacteria or isolated membranes which have not been lyophilized, therefore no hydration procedure is involved (9). 

Figure 6. Vapor-liquid-solid (plastic crystal) phase diagram of adamantane. The phase transition from plastic crystal to rigid crystal phase occurs at 208.6K (l/T = 0.004794K ). This diagram is based on the data of Table II.

Depending on temperature, transitions between distinct types of LC phases can occur.3 All transitions between various liquid crystal phases with 0D, ID, or 2D periodicity (nematic, smectic, and columnar phases) and between these liquid crystal phases and the isotropic liquid state are reversible with nearly no hysteresis. However, due to the kinetic nature of crystallization, strong hysteresis can occur for the transition to solid crystalline phases (overcooling), which allows liquid crystal phases to be observed below the melting point, and these phases are termed monotropic (monotropic phases are shown in parenthesis). Some overcooling could also be found for mesophases with 3D order, namely cubic phases. The order-disorder transition from the liquid crystalline phases to the isotropic liquid state (assigned as clearing temperature) is used as a measure of the stability of the LC phase considered.4... 

One compound may exist in more than one liquid crystal form. The structure of cholestryl myristate changes from solid to smectic A at 71 °C, to chiral nematic at 79 °C, and finally to an isotropic liquid at 85 °C. Figure 16.10 shows the transitions in p-methoxybenzoic acid. Note that the order increases with increased pressure and lower temperature. Liquid crystal phases are more densely packed than the... 

Figure 2.12 is the classic pressure-temperature (FT) representation of the phase changes of a pure component. There are three primary phases of pure components solid liquid, and vapor solid-solid transitions, liquid crystal phases, and so on, are also possible but will not be considered here. The solid lines represent the sublimation curve (solid —> vapor), the vapor pressure curve (liquid —> vapor) and the melting curve (solid liquid) of the pure component. The triangle represents the triple point, at which a solid, liquid and vapor coexist in equilibrium. The circle represents the pure component critical point, where the supercritical region begins. 

Cholesteric liquid crystals are compounds that go through a transition phase in which they flow like a liquid, yet retain much of the molecular order of a crystalline solid. Liquid crystals are able to reflect iridescent colors, depending on the temperature of their environment. Because of this property they may be applied to the surfaces of bonded assemblies and used to project a visual color picture of minute thermal gradients associated with bond discontinuities. Cholesteric crystals are potentially a simple, reliable, and economical method for evaluating bond defects in metallic composite structures.f Materials with poor heat-transfer properties are difficult to test by this method. The joint must also be accessible from both sides. ... 

Tendencies to instability in nature have been interpreted in various ways in continuum theory. We recall that many substances exhibit several phase transitions as, for example, their temperature is increased. A material initially described as a rigid solid may pass through smectic and nematic liquid crystal phases prior to behaving like an isotropic liquid. In a liquid crystal polymer, the concentration of solvent sometimes has the role of temperature. In the liquid crystal phase, the orientation of the molecules in terms of the optical axis may contribute to the response of this "fluid" to external fields. It is sometimes called an internal variable". The traditional field equations for an isotropic liquid are replaced by a more elaborate collection derived on the basis of continuum theory, (Ericksen [10], Leslie, [16], Hissbrun [H]). 

In general, crystallization will be enhanced by the increased concentration of the polymer-rich regions following a phase separation (subsection on Liquid-Liquid Phase Separation, pp. 243-9). Thus, in some cases, crystallization can occur subsequently to a liquid-liquid or an isotropic-liquid crystal phase separation rather than directly from solution as a liquid-solid transition. 

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