Liquid Solution -Crystal Phase Separation

The thermodynamic stimulus for crystallization (liquid-crystal phase separation) is the equality of the chemical potentials of each component in the crystal phase G o,.cr and in the solution 6 , at a certain temperature Y l and concentration x,f. 

Cite the following fact to illustrate the difficulties in the phase analysis of polymer systems. For the poly(vinyl alcohol)- -water system, some researchers propose a state diagram of amorphous phase separation with an upper critical solution temperature, others — ainor]>hons separation with a lower critical solution temperature about 100 C there are some who think that there is no region of amorphous separation below 150 -instead, they observe liquid-crystal phase separation. Such are the discrepancies on the basic question of thermodynamics ... 

When cholesteric liquid crystals are encapsulated in droplet form, the bistability can be preserved when droplet size is much larger than the pitch [64]. There arc two methods which are used to encapsulate Ch liquid crystals phase separation and emulsification. In phase separation [69], the Ch liquid crystal is mixed with monomers or oligomers to make a homogeneous mixture. The mixture is coated on plastic substrates and then another substrate is laminated on. The monomers or oligomers are then polymerized to induce phase separation. The liquid crystal phase separates from the polymer to form droplets. In the emulsification method [70-73], the Ch liquid crystal, water, and a water dissolvable polymer are placed in a container. Water dissolves the polymer to form a viscous solution, which does not dissolve the liquid crystal. When this system is stirred by a propeller blade at a sufficiently high speed, micron-size liquid crystal droplets are formed. The emulsion is then coated on a substrate and the water is allowed to evaporate. After the water evaporates, a second substrate is laminated to form the Ch display. 

Changes in the temperature or composition of the solution may result in a conformational transition of one of the components. For example, intramolecular forces may stabiUze a helical conformation. Such an ordered structure may also be favoured because of the associated decrease in the volume occupied. A helical conformation can affect the phase-separation behaviour in several ways, for example by presenting attractive or repulsive groups on its surface, promoting either polymer-polymer or polymer-solvent interactions, or, if the helical molecule is sufficiently stiff and rod-like, by an isotropic-liquid crystal phase separation. Flory has predicted the general features of the corresponding phase diagram (Fig. 7). 

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. 

Purification of a chemical species by solidification from a liquid mixture can be termed either solution crystallization or ciystallization from the melt. The distinction between these two operations is somewhat subtle. The term melt crystallization has been defined as the separation of components of a binaiy mixture without addition of solvent, but this definition is somewhat restrictive. In solution crystallization a diluent solvent is added to the mixture the solution is then directly or indirec tly cooled, and/or solvent is evaporated to effect ciystallization. The solid phase is formed and maintained somewhat below its pure-component freezing-point temperature. In melt ciystallization no diluent solvent is added to the reaction mixture, and the solid phase is formed by cooling of the melt. Product is frequently maintained near or above its pure-component freezing point in the refining sec tion of the apparatus. 

To produce novel LC phase behavior and properties, a variety of polymer/LC composites have been developed. These include systems which employ liquid crystal polymers (5), phase separation of LC droplets in polymer dispersed liquid crystals (PDLCs) (4), incorporating both nematic (5,6) and ferroelectric liquid crystals (6-10). Polymer/LC gels have also been studied which are formed by the polymerization of small amounts of monomer solutes in a liquid crystalline solvent (11). The polymer/LC gel systems are of particular interest, rendering bistable chiral nematic devices (12) and polymer stabilized ferroelectric liquid crystals (PSFLCs) (1,13), which combine fast electro-optic response (14) with the increased mechanical stabilization imparted by the polymer (75). 

In the two systems just considered, gelation is taken to occur from a one-phase solution. However, under some circumstances, there is the possibility that liquid-liquid phase separation precedes crystallization, and that a gel forms as a result of this process. A crystallization process may or may not be superimposed on this phase separation at a later time, and the crystals may or may not further stabilize the gel. One may... 

Evaporators employ heat to concentrate solutions or to recover dissolved solids by precipitating them from saturated solutions. They are reboilers with special provisions for separating liquid and vapor phases and for removal of solids when they are precipitated or crystallized out. Simple kettle-type reboilers [Fig. 8.4(d)] may be adequate in some applications, especially if enough freeboard is provided. Some of the many specialized types of evaporators that are in use are represented on Figure 8.16. The tubes may be horizontal or vertical, long or short the liquid may be outside or inside the tubes, circulation may be natural or forced with pumps or propellers. 

Although polymorphism in plastic crystals is less frequent than in liquid crystals, it does exist. Tetrakis(methylmercapto)methane, C(SCH3)4, for example, has four crystal modifications of which the three high temperature forms have a high degree of plasticity 100). Also, it has been observed that plastic crystals are frequently mutually soluble 16b), a consequence of the less restrictive crystal structures. Phase separation of these solutions occurs often on transition to the fully ordered crystal, giving rise to quite complicated phase diagrams102). 

Crystallization. Low temperature fractional crystallization was the first and for many years the only commercial technique for separating PX from mixed xylenes. As shown in Table 2, PX has a much higher freezing point than the other xylene isomers. Thus, upon cooling, a pure solid phase of PX crystallizes first. Eventually, upon further cooling, a temperature is reached where solid crystals of another isomer also form. This is called the eutectic point. PX crystals usually form at about —4° C and the PX-MX eutectic is reached at about —68° C. In commercial practice, PX crystallization is carried out at a temperature just above the eutectic point. At all temperatures above the eutectic point, PX is still soluble in the remaining Cg aromatics liquid solution,... 

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