Thermodynamically incompatible mixtures

Table 7.1 shows that rather similar results were also found by Makri et al. (2005) for samples of coarse emulsions containing thermodynamically incompatible mixtures of legume seed protein + xanthan gum. The protein surface load was found to be enhanced in the presence of xanthan gum, especially at elevated ionic strengths. That is, there was observed to be an increase in the adsorption of legume seed proteins at the surface of the emulsion droplets which could be attributed to an increase in the thermodynamic activity of the proteins in the system in the presence of the incompatible polysaccharide (see Table 7.1). Associated with the greater extent of protein adsorption, the authors reported an enhancement in the emulsion stability.

But there is another method — the use of heterogeneous blends of polymers [45, 46], To this end, electrical properties and distribution of the filler (carbon black) in the mixtures of polyethylene and thermodynamically incompatible polymers were investigated. 

The study of filler distribution by the methods of optical and electronic microscopy has shown that in all compositions obtained by method 4 the filler is distributed rather uniformly as in an individual polymer. In the mixtures of incompatible polymers, obtained by methods 1 and 2, the filler is distributed nonuniformly and there are zones of high concentration of the filler and almost empty ones. The size of such zones is close to the size of polymer regions known for mixtures of thermodynamically incompatible polymers — 1 to 10 p. 

The presence of a thermodynamically incompatible polysaccharide in the aqueous phase can enhance the effective protein emulsifying capacity. The greater surface activity of the protein in the mixed biopolymer system facilitates the creation of smaller emulsion droplets, i.e., an increase in total surface area of the freshly prepared emulsion stabilized by the mixture of thermodynamically incompatible biopolymers (see Figure 3.4) (Dickinson and Semenova, 1992 Semenova el al., 1999a Tsapkina et al., 1992 Makri et al., 2005). It should be noted, however, that some hydrocolloids do cause a reduction in the protein emulsifying capacity by reducing the protein adsorption efficiency as a result of viscosity effects. 

For instance, denaturation and partial hydrolysis of proteins oppositely influence their incompatibility with other biopolymers (Tolstoguzov 1991). Most biopolymers are polyelectrolytes. Factors such as pH and salt concentration affect their interactions with one another, with the solvent and their compatibility. For instance, when the pH is shifted to their isoelectric point (lEP), the thermodynamic incompatibility of proteins is usually enhanced by self-association of the protein molecules. Generally, protein-neutral polysaccharide mixtures separate into two phases when the salt concentration exceeds 0.15 M. 

Multiple emulsions can also be formed by mixing an oil-in-water emulsion with a thermodynamically incompatible biopolymer mixture. Depending on the formulation and conditions of preparation, oil-in-water-in-water or mixed oil in water/water-in-water multiple emulsions may be formed. These have potential for the controlled delivery of a range of bioactives (Kim et al. 2006). 

We may find confirmation for our statement about the thermodynamic incompatibility of linear polystyrene with the styrene-DVB copolymer in experiments by Wong et al. [269]. These authors reported similarities in the phase separation power of linear polystyrene and that of linear styrene-methylmethacrylate copolymer, the latter being a priori incompatible with the styrene-DVB network. Complete incompatibility of polystyrene with linear polydimethylsiloxane facihtates phase separation and results in the formation of a porous styrene-DVB network on adding as little as 0.5-1% of the above porogen to the initial comonomer mixture (Fig. 3.2, curves 4). It is also not surprising that the porosity of copolymers induced by linear polystyrene and linear polydimethylsiloxane is almost the same when the DVB content exceeds 10%. At a DVB content that hi, the network formed differs fundamentally from linear polystyrene, as from any alien polymer. 

As can be seen from the data presented, the displacement of values of glass transition temperature of PHB occurs about 6-7°C. Most often in the literature as reasons for this phenomenon is called a limited solubility of the mixture components in each other (from a fraction of a few per cent) [1-3]. However, for the polymers probability such variant is extremely small because of the significant thermodynamic incompatibility. More probable reasons of displacement of the glass transition temperature may be changes of the supramolecular structure of polymers when mixing as well as differences in thermal expansion coefficients of polymers in the region above and below the glass transition temperature. 

The properties of polymer mixtures depend on the method by which they are obtained and are determined by many factors by sizes of particles of the dispersed phase, by their shape and number in bulk, and by the thermodynamic affinity of the components for one another [19]. Linear polymers blend either in the course of their mutual dissolution or, in two-phase systems, under conditions of thermodynamic incompatibility of the components, when the dispersion is forced. The mixtures formed can be compatible (forming true solutions of one polymer in another), incompatible (representing a typical colloid system), quasicompatible (characterized by microscopic homogeneity at a level above heterogeneity on the molecular level), or pseudocompatible (with a strong adhesion interaction on the boundary) [106]. 

An interpenetrating network (IPN) can be defined as a mixture of two crosslinked polymers when at least one of them is synthesized and (or) crosslinked with another [114]. The components that make up an IPN are thermodynamically incompatible and a transition region of two phases is formed in such a system. The whole complex of IPN properties is determined by the availability and features of this region. 

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