Liquid crystals solids/melts

These RM(CO)5 compounds are white to pale yellow liquids or solids melting below 100° C. They are very volatile. Crystals of CH3Mn(CO)5, for example, evaporate when placed in an open container (93). Most of these compounds, especially the acyl and perfluoroalkyl derivatives, which have electronegative substituents on the carbon atom bonded to the manganese atom, are indefinitely stable in air. 

Liquid crystal materials can be grouped into two main classifications, thermotropics and lyotropics. A thermotropic phase is one that can form by heating or cooling a material. Just as we see a phase transition between solid and liquid as we heat and melt ice, in thermotropic liquid crystals, additional melting points can be observed in between the solid and the liquid phases. These are the thermotropic liquid crystalline phases. A lyotropic liquid crystal phase is formed by molecules dissolved in a solvent, and phases form at certain concentrations in that solvent. In this chapter, we focus on descriptions of the thermotropic liquid crystal phases. Lyotropics, although also liquid crystals, are described in detail in Chapter 3 on surfactants. 

A characteristic feature of LCP is the molecular stmcture. These polymers consist of rigid rod-like macromolecules, which align in the melt to produce liquid crystal structures. If a liquid crystal polymer melt is subjected to shear or stretching flow, as in the case of all thermoplastic processing operations, then the rigid macromolecules order themselves into fibres and fibrils, which are frozen when the melt cools. This is how the specific morphology of LCP is formed in the solid state. 

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). 

An increase in rod-like arrangement of the macromolecules can also arise by stretching a polymer either in its solid state, either in the melt or even in solution (for polymers leading to lyotropic liquid crystals such as aromatic polyamides). This is the basis of the development of synthetic fibres including high modulus polyethylene Dyneema , polyamide Nylons and Kevlar , polyester Tergal or Dacron fibres. 

Fig. 17 B/E-p dependence of the critical temperatures of liquid-liquid demixing (dashed line) and the equilibrium melting temperatures of polymer crystals (solid line) for 512-mers at the critical concentrations, predicted by the mean-field lattice theory of polymer solutions. The triangles denote Tcol and the circles denote T cry both are obtained from the onset of phase transitions in the simulations of the dynamic cooling processes of a single 512-mer. The segments are drawn as a guide for the eye (Hu and Frenkel, unpublished results)...

Here /g,hq and y ,ss are the activity coefficients of component B in the liquid and solid solutions at infinite dilution with pure solid and liquid taken as reference states. A fus A" is the standard molar entropy of fusion of component A at its fusion temperature Tfus A and AfusGg is the standard molar Gibbs energy of fusion of component B with the same crystal structure as component A at the melting temperature of component A. 

To p. 11. According to N. H. Fletcher, J. Crystal Growth 28, 375 (1975), the free energy of solid - melt interfaces in many systems (e.g., water - ice) is determined above all by the low entropy of the liquid layers adjacent to the solid surface. This loss of entropy occurs because the above layers are more ordered than the melt far from the solid. 

An example of a binary eutectic system AB is shown in Figure 15.3a where the eutectic is the mixture of components that has the lowest crystallisation temperature in the system. When a melt at X is cooled along XZ, crystals, theoretically of pure B, will start to be deposited at point Y. On further cooling, more crystals of pure component B will be deposited until, at the eutectic point E, the system solidifies completely. At Z, the crystals C are of pure B and the liquid L is a mixture of A and B where the mass proportion of solid phase (crystal) to liquid phase (residual melt) is given by ratio of the lengths LZ to CZ a relationship known as the lever arm rule. Mixtures represented by points above AE perform in a similar way, although here the crystals are of pure A. A liquid of the eutectic composition, cooled to the eutectic temperature, crystallises with unchanged composition and continues to deposit crystals until the whole system solidifies. Whilst a eutectic has a fixed composition, it is not a chemical compound, but is simply a physical mixture of the individual components, as may often be visible under a low-power microscope. 

In the case of interface equilibrium (open system conditions), the partition coefficient is valid only at the interface between solid and liquid (or at zero distance from the interface) and at time of crystallization (or melting) t ... 

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