Isotropic liquid transition

Most solid materials produce isotropic liquids directly upon melting. However, in some cases one or more intermediate phases are formed (called mesophases), where the material retains some ordered structure but already shows the mobility characteristic of a liquid. These materials are liquid crystal (LCs)(or mesogens) of the thermotropic type, and can display several transitions between phases at different temperatures crystal-crystal transition (between solid phases), melting point (solid to first mesophase transition), mesophase-mesophase transition (when several mesophases exist), and clearing point (last mesophase to isotropic liquid transition) [1]. Often the transitions are observed both upon heating and on cooling (enantiotropic transitions), but sometimes they appear only upon cooling (monotropic transitions). 

Fig. 11. DSC-trace of bis-(4 -n-octyloxybenzal)-l,4-phenylenediamine at 5 K/min. The lowest temperature transition is the fully ordered crystal-to-smectic transition, followed by four transitions between the five smectic polymorphs. Next is the smectic-nematic transition, followed by the nematic-to-isotropic liquid transition. Drawn after data by Petrie581...

A thermotropic liquid crystal (mcsogen) is a compound that, on heating the crystal or on cooling the isotropic liquid, gives rise to mesomorphism. Liquid crystallinity occurs between the crystal and isotropic liquid states. The intermediate phases, or mesophases, can be either enantiotropic, i.e., thermodynamically stable, or monotropic, i.e., thermodynamically unstable. The solid to mesophase transition is referred to as the melting point, while the mesophase to isotropic liquid transition is referred to as the clearing point. 

Fig. 9-6. Phase diagram of ferrocenes 11. Melting point nematic/isotropic liquid transition smectic A/isotropic liquid transition A isotropic liquid/smectic A transition O isotropic liquid/nematic transition.

Fig, 9-20. Comparison of the nematic to isotropic liquid transition of related systems. 

For example, the 815 cm-l peak shifted to 825 cm i and the doublet at 1730/1750 cm-l, indicative of crystal-like structures has disappeared at the highest shear rate. The spectral changes near 815 cm-1 of Fig. 15a could conceivably correspond to those of Fig. 13 which contained the spectra at different temperatures around the discotic/ isotropic liquid transition. On the other hand, no disappearance of the carbonyl structures occurred with simple phase transition. TTie disappearance of the carbonyl bands at 1730/1750 cm at the highest shear rate must therefore corresponds to a molecular realignment in which the dipole moment changes causing the infixed bands have been reoriented along the direction of observation. 

Table I. Liquid Crystal/Isotropic Liquid Transition Temperatures...

One major conclusion that can be drawn from the presented results is related to the effect of the molecular structure of the rigid, anisotropic core of the polymers under investigation on their thermotropic behaviour. Thus, if one takes both the mesophasic-to-isotropic liquid transition temperature and the temperature range of persistence of such mesophase as a qualitative index of the effectiveness of the central group X in imparting liquid crystal properties to the polymer system, the following order can be established ... 

The cholesteric-to-isotropic liquid transition is of an eutectic type. The enthalpy changes,/ are related to the cholesteric phases. The composition of the system with the lowest melting point does not coincide with the composition of the homogeneous polymer ( 8 and respectively). The... 

The thermal behavior of liquid crystalline main chain polymers is more complicated. Depending on thermal history, different transitions may occur. The DSC curves of samples quenched from the melt may show the glass transition (positive ACp), cold crystallization (exotherm), melting (endotherm)and the mesophase-isotropic liquid transition (endotherm)[ Fig. I6]. ... 

The way, or sequence, in which thermotropic transitions occur is defined in the following ways. The liquid crystal to isotropic liquid transition is called the clearing or isotropization point, and this transition, like those between liquid crystal phases, is essentially reversible and occurs with little hysteresis in temperature. The melting point of a material is usually a constant, but the recrystallization process can be subject to supercooling. Mesophases formed on the first heating cycle of a material are thermodynamically stable, and are called enantiotropic phases, whereas phases that are formed below the melt point on cooling cycles, and are revealed... 

The discussion of mixtures is worthy of further comment. It has been known for some time that mixtures of linear, nonmesomorphic molecules with nematic compounds are characterized by a sharp decrease in both the crystal-nematic and nematic-isotropic liquid transitions with increasing concentration of nonmesomorphic component. However, mixtures of two or more nematic compounds which possess subtle differences in molecular structure do not exhibit sharp decreases in the nematic-isotropic liquid transition temperatures with molar composition although eutectic points for the crystal-nematic transitions may be obtained. Thus, the nematic-isotropic liquid transition temperatures form a smooth curve over the entire range of molar composition. Similar smooth curve surfaces are observed in ternary mixtures. This remarkable stability of the nematic mesophase is undoubtedly due to the fact that all the molecules are oriented in the same direction resulting in the formation of a pseudo-lattice type structure. On the other hand, the molecules of a nonmesomorphic guest are randomly oriented in a nematic host and their presence results in the disruption of nematic order. 

Fig. 4. Entropy change for the mesophase isotropic liquid transition versus number of carbon atoms in the alkyl groups of diesters.

The liquid crystal to liquid crystal phase transformations and the liquid crystal to isotropic liquid transition (clearing... 

The hydrocarbon chain melting transition is facilitated by factors that reduce the polar headgroup network cohesion. The addition of water to cetyltrimethylam-monium tosylate produces a peak at 23°C, which is related to the melting of CTAT crystals (embedded in saturated aqueous solution below 23 C) to produce a liquid crystalline phase (in highly concentrated CTAT systems) or micellar solutions (in dilute systems). The peak is broad, probably due to the existence of a biphase transition zone. No melting peak related to the polar network was detected, probably because of the relatively weak cohesive forces in this particular polar network. The second peak detected in concentrated water-surfactant samples was due to the hexagonal mesophase-isotropic liquid transition [53]. 

The nature of the headgroup plays a crucial role in determining the phase transition, for two main reasons. One is the electrostatic interaction among head-groups at the bilayer surface, which may differently affect the stability of the bilayer structure. The other is the influence of the headgroup on the alignment of the hydrocarbon chains. For example, protonated pyridinium salts show a simple single phase transition from the solid crystalline state to an isotropic liquid, while methylated pyridinium salts exhibit solid crystalline-solid crystalline, solid crystalline-liquid crystalline, and liquid crystalline-isotropic liquid transitions [77],... 

The first Broken Symmetry of Life, we identified with a liquid crystal-isotropic liquid transition. Something (i.e., the liquid crystal order parameter) emerges from nothing, the primordial soup. 

Figure 3.17. Schematic representation of crystalline phase-liquid crystalline phase-isotropic liquid transition for HBC.

Substances that show a liquid crystalline phase, or mesophase, are called mesogens. Several thousands of compounds, both with low molecular mass and polymeric, are now known to form mesophases. They are mainly highly geometrically anisotropic in shape, rodlike or disclike (hence the terms calamitic and discotic liquid crystals), or they are anisotropic in solubility properties, like amphiphilic molecules and, depending on their detailed molecular structure, they can exhibit one or more mesophases between the crystalline solid and the isotropic liquid. Transitions to these intermediate states may be induced by purely thermal processes (thermotropic liquid crystals) or by the action of solvents (lyotropic liquid crystals). Each of these two categories can be further divided according to the structure of the mesophases and/or molecules Scheme 1 shows the classification of thermotropic mesophases. 

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