Crystal phase separation

In practice, colloidal systems do not always reach tlie predicted equilibrium state, which is observed here for tlie case of narrow attractions. On increasing tlie polymer concentration, a fluid-crystal phase separation may be induced, but at higher concentration crystallization is arrested and amorjihous gels have been found to fonn instead [101, 102]. Close to the phase boundary, transient gels were observed, in which phase separation proceeded after a lag time. 

Powder X-ray diffraction has verified that the isomerization occurs via a crys-tal-to-crystal reaction process, and that the diffraction profiles of the crystals after photoirradiation consist of overlapped patterns of diffraction due to the crystals of 10 and 11. This indicates that the crystal domains of each isomer exist simultaneously in the crystals accompanied by crystal phase separation during the photoisomerization. Single crystal structure analysis has disclosed that the crystals of 11 as the photoproduct have a symmetry different from that of the starting crystals of 10 (Fig. 14). 

In contrast to natural structures the morphological features of structures in fabricated foods are in principle within our control. The source of the many structures of foods, even those made from a single raw material (e.g., wheat flour), lies in the ingredient mix and the fact that thermodynamic equilibrium is practically never required or achieved during processing. These metastable structures can be attained because they are favored kinetically, that is, the approach to equilibrium is slow. At any point during the development of a particular structure a process of shape stabilization sets in, usually by vitrification, partial crystallization, phase separation and/or formation of a network (Figure 12.5). 

One efficient way to increase the number density of NLO chromophores in a polymeric host, without crystallization, phase separation, and any concentration gradient, is to attach them as side chains of a polymer [15,18]. Also, the temporal stability of the poled structures of side-chain NLO polymers has been proven to be much better than that of the same guest-host system due to the higher glass transition temperature of the polymer. A number of poled side-chain NLO polymers (Fig 49.4) have employed for SHG measurements. Table 49.3 exhibits values of some side-chain NLO polymers [16-28]. 

Tg increase, cold crystallization, phase separation, embrittlement ... 

PLA PEG 1000 1460 4 23.4 Tg increase, cold crystallization phase separation, crystalline PEG 99... 

Only one simple case of the binary system (A -f B) with eutectic crystallization and no region of the mixed crystal existence (Daniels and Alberty, 1975) will be, considered here as an example of liquid-crystal phase separation (see Figure 1.11). This kind of ph2isc separation is of special importance for polymer systems as most tj pical. 

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. 

Figure 1.11. State diagram of the binary mixture A + B with a liquid-crystal phase separation region of the eutectic type in the ab sence of a mixed crystal region. 1 he dashed line denotes the liquid-liquid phase separation region (a). Composition dependenc.e of the average molar Gibbs potential (6-/) Coi.cr, Go.,( denote the molar Gibbs potential of the zth component in its individual crystal and liquid states ...

Cels with crystallite crosslinks, naturally, arc, in principle, two-phase ones with respect to the liquid-crystal phase separation, though crystallites may be silent as crystal bodies (fragments) within the resolving power of a specific analysis method (see section 6.2 for details). As to phase separation within the framework of the NP+LMWL system, this case amounts to the previous one. 

See Chapter 6 for the determination of Flory-Huggins parameter x, at li( uid-crystal phase separation. 

For some systems, the liquid-liquid and liquid-crystal phase separation regions are separated on the state diagram this is the case, for instance, with the system poly(cthylcne oxide)- -water (Malcolm and Rowlinson, 1957 Saeki et al., 1976 Uskov et al., 1976 Kjcllander and Florin, 1981) (Figure 6.9). 

Overlapping regions of liquid-liquid (UCST) and liquid-crystal phase separation were discovered for isotactic polypropylene in some solvenH-precipilant mixtures (Ogawa and Iloshino, 1971). 

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 ... 

Key words membrane contactors, hydrophobic membranes, interfaces, self-assembly, colloidal crystals, phase separation, water desalination, membrane crystallisation, membrane distillation. 

A low concentration of oxidised paraffin wax improves thermal stability [40], and unoxidised paraffin wax reduces thermal stability [41]. At higher wax concentrations in PVC, crystal phase separation as well as the presence of short wax chains in the amorphous fraction cause deterioration of thermal and mechanical properties. 

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. 

T. J. White, L. V. Natarajan, T. J. Bunning, cuid C. A. Guymon, Contribution of monomer functionahty and additives to polymerization kinetics and hquid crystal phase separation in acrylate-based polymer-dispersed hquid crystcils (PDLCs). Liq. Cryst. 34, 1377-1385 (2007). 

POLYMER FILM SMECTIC LIQUID CRYSTAL PHASE SEPARATED POLYMER SUBSTRATE... 

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