Demixing types

The best physical explanation is given by a calculation of the concentration profiles. To calculate the concentration profiles in the polymer film during the (delayed demixed type of) phase inversion process, some assumptions and considerations must be made[35] ... 

Tablet Press. The main components of a tablet compression machine (press) are the dies, which hold a measured volume of material to be compressed (granulation), the upper punches which exert pressure on the down stroke, and the lower punches which move upward after compaction to eject the tablets from the dies. Mechanical components deflver the necessary pressure. The granulation is fed from a hopper with a feed-frame on rotary-type presses and a feeding shoe on single-punch presses. A smooth and even flow ensures good weight and compression uniformity. Using the proper formulation, demixing in the hopper is minimized.

The rheo cells can easily be replaced by various types of mixers, propellers or paddles (Figure 2.1.10). It is then possible to analyze the temporal evolution of chemical/physical reactions of mixing, demixing and sedimentation of materials in process engineering, e.g., during the mash process or fermentation [28, 29]. The stirring mechanics and speed can be optimized for various materials of different particle sizes and viscosity. 

Some remarks can be made about the evolutions in the presence of Ca2+, although it is difficult to think that some brine could contain only this type of cations without monovalent cations. Before any aging, a copolymer of 17% of acrylate content begins to demix for CaCl2 > 2.5 10 2 M/l and this limit is lower for higher... 

Fig. 14 Type I demixing of PVCL in water for three samples of different chain length (type I). Mv = 9000gmol"1 Tcp (+), rdem, ( ) Mv = 20000gmoL1 Tcp ( ), Tdem ( ) Mv = 275 000 gmol-1 Tcp ( ), Tdem (A). (Reprinted with permission from Ref. [157] copyright 2000 Elsevier)...

Lachwa, J. et al.. Changing from an unusual high-temperature demixing to a UCST-type in mixtures of l-alkyl-3-methylimidazolium bis (trifluoromethyl) sulfonyljamide and arenes. Green Chem., 8, 262, 2006. 

The phase-behaviour of such a system thus may change completely with variations in pressure, temperature, molar mass, and chemical composition of the polymer(s). Thus, polymer systems may show upper critical, lower critical, or hourglass-type demixing behaviour. 

Until recently, very little quantitative information was available on blends of block copolymers. The literature is summarized in Table 6.3. Hoffman et al. (1970) reported microscopic demixing of blends of PS-PB diblocks, with two maxima in the domain size distribution, but with no evidence tor macrophase separation. These findings must be treated with caution in the light of more recent results. Hadziioannou and Skoulios (1982) used SAXS and SANS to investigate the morphology of binary blends of PS-PI diblocks, and binary PS-PI/PS-PI-PS or PS-PI/PI-PS-PI blends or blends of the two types of triblock. They found that the blends were microphase separated, and that the sharpness of the interface was not reduced in blends compared to neat copolymers. The transition between a lamellar and a cylindrical structure was shown to depend primarily on blend composition. In contrast, the transition from a lamellar to a disordered phase at... 

In the standard classification of Scott and Konynenburg [145,146] the simplest case is type I behavior, where liquid-liquid demixing is absent. 

While the early work on molten NH4CI gave only some qualitative hints that the effective critical behavior of ionic fluids may be different from that of nonionic fluids, the possibility of apparent mean-field behavior has been substantiated in precise studies of two- and multicomponent ionic fluids. Crossover to mean-field criticality far away from Tc seems now well-established for several systems. Examples are liquid-liquid demixings in binary systems such as Bu4NPic + alcohols and Na + NH3, liquid-liquid demixings in ternary systems of the type salt + water + organic solvent, and liquid-vapor transitions in aqueous solutions of NaCl. On the other hand, Pitzer s conjecture that the asymptotic behavior itself might be mean-field-like has not been confirmed. 

Prior to this discovery, in 1954 Silberberg and Kuhn (62) were first to study the polymer-in-polymer emulsion containing ethylcellulose and polystyrene in a nonaqueous solvent, benzene. The mechanisms of polymer emulsification, demixing, and phase reversal were studied. Wetzel and Hocks discovery would then equate the pressure-sensitive adhesive to a polymer-polymer emulsion instead of a polymer-polymer suspension. Since the interface is liquid-liquid, the adhesion then becomes one type of R-R adhesion (35, 36). According to our previous discussion, diffusion is not operative unless both resin and rubber have an identical solubility parameter. The major interfacial interaction is physical adsorption, which, in turn, determines adhesion. Our previous work on the wettability of elastomers (37, 38) can help predict adhesion results. Detailed studies on the function of tackifiers have been made by Wetzel and Alexander (69), and by Hock (20, 21), and therefore the subject requires no further elaboration. 

Although aniline does possess a dipole moment the cohesion energy is predominantly of the London type. The critical solubility point—technically actually the i i demixing point—with hydrocarbon mixtures (aniline point) is an important characteristic quantity. The aniline point rises with the molecular weight in a homologous series. For aromatic hydrocarbons the critical solubility point is the lowest, for paraffins the highest, olefins and cyclic hydrocarbons are in between, in agreement with the variation of the specific cohesion (Table 33). 

The formation of uniform network polymers from lignin requires that phase separation prior to gel formation remain limited. The process of component demixing is controlled by both an enthalpic and an entropic contribution to Gibbs free energy. Whereas, the entropic factor requires the use of low molecular weight fractions for uniform gel formation, the enthalpic parameter necessitates chemical modification. The type of modification needed depends on the medium in which gel formation is performed. Examples of improving the solubility and compatibility with various reaction media are cited. 

On mixing solutions of a protein and a polysaccharide, four kinds of mixed solutions can be obtained. Figure 3.1 shows that two single-phase systems (1 and 3) and two-types of biphase systems (2 and 4) can be produced. The two-phase liquid systems 2 and 4 differ in the distribution of biopolymers between the co-existing phases. The biopolymers are concentrated either in the concentrated phase of system 2 because of interbiopolymer complexing, or within separated phases of system 4 because of incompatibility of the biopolymers. The term biopolymer compatibility implies miscibility of different biopolymers on a molecular level. The terms incompatibility or limited thermodynamic compatibility cover both limited miscibility or limited cosolubility of biopolymers (i.e., system 2) and demixing or phase separation... 

Food systems are often not composed of one type of constituent but of a mixture thereof Usually one does not operate at conditions where all constituents mix homogeneously, and even if that were the case, the formulation window turns out to be rather narrow (e.g., the case of Fastis, an anise-holding drink (Grillo 2003)). In most cases one finds that demixing occurs, leading to a situation of each constituent forming its own phase separated from the other phases by a so-called interface. 

At the present time, mechanisms for segregation even in the simple tumbling drum remain obscure, and work on more complex and industrially common blender geometries is extremely limited. Three distinct types of demixing are moderately well characterized in tumblers. They are radial demixing, axial demixing, and competitive patterned demixing. We describe each of these in turn. 

The autocorrelation A(Af) is equal to zero for a random distribution of the two elements. It is positive if species separated by Af tend to be of equal type and negative in case of a preference for different atoms separated by Af. If Af is the vector between nearest neighbours (NN), positive A(Af) means demixing tendency, negative A(Af) a tendency towards ordering. The maximum positive value that A(Af) can approach is +1 (in case of complete phase separation where one never finds an unlike atom in a distance Af), whereas the minimum value is given by... 

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