Emulsion Polymer Conformations

Some aspects of the core-shell model of Grancio and Williams (1970) have already been examined in relation to graft-type ABS plastics (Section 3.1.2.2). Here, too, the unfavorable heat of mixing between polymer loops and water seems to be controlling. A more detailed view of these experiments will now be presented. 

While a quantitative molecular explanation of overcoating in latex polymerization is lacking, a qualitative picture can be synthesized with the aid of the polymer solution statistical mechanics developed by Flory (1953) and the phase separation statistics of Meier (1969, 1970). Let us examine 

In this case the molecules on both sides of the phase boundary are both polymeric. The heat of mixing will still be positive, but much less so. In fact, in many such cases extensive (but incomplete) solution does take place. 

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To apply the above method one should use a dispersion with monodisperse particles with a radius that is not much larger than 8. This limits the method for direct use with emulsions, which are polydisperse and with relatively large radii. However, small model particles of polystyrene may be used, and the assumption is made that the polymer conformation at the interface is similar to that at the o/w interface. 

Usually pH has to be adjusted with a suitable base, since the acrylate is a polyacid, to get an optimal extended polymer conformation [16] which leads to thickening of emulsions. Final emulsion viscosity depends on acrylate concentration and is independent of temperature control during preparation in contrast to common emulsions. In emulsions acrylate adsorbs at the oil/water interface at low concentrations of acrylate stabilization results from electro-steric effects and in the upper concentration range an associative thickening mechanism is postulated [17]. 

When polymers are used as constituents of coatings, paints, and lacquers, they require solvents as dispersing agents. Whether true solutions are formed or emulsions, the solvents used have to conform to environmental specifications, but should be sufficiently volatile so as to permit rapid drying of the applied polymer and pigment, if present. One should examine chapter 6 and the appendix for green examples of coatings solvents. 

In addition to giving information about the shape and internal structure of colloidal aggregates, SANS studies can also be used profitably to determine the thickness and conformation of polymer layers adsorbed onto the surface of colloidal particles such as latex nanoparticles, and in some special cases, the surface of emulsion droplets. ° In such studies, the particles on which the polymer is adsorbed must generally be very accurately contrast matched to the solvent so as to allow information to be obtained only about the adsorbed layer. SANS studies have also been recently used in combination with differential scanning calorimetry and visual inspection of the solutions, to draw up a (simplified) partial phase diagram of the aggregation behavior of a polymeric surfactant in water.t ... 

The mechanical stability of polymers was related to the polymer s conformation in some of the earlier drag-reduction studies. Above a critical stress, degradation was faster the more contracted and entangled the polymer s conformation (5-7). In petroleum applications the mechanical instability of synthetic relative to carbohydrate polymers was well-recognized. The relative stability problems (possibly related to DUEVs (8)) encountered in the use of high molecular weight hydrolyzed poly(acrylamide) (HPAM) led to the development of an inverse-emulsion polymerization technique (9). (Current research directions using this technique are discussed in Chapter 9.)... 

Thus, K-casein in its native stabilizing role exists, probably as small disulphide linked polymers, bound to the micellar surface (the ill-defined boundary between the hydrophobic interior of the micelle and the aqueous phase). C-terminal polypeptides (61 residues) of the protein project from the surface into the solution. In this position, the macropeptide moiety of the protein is conformationally free (H), constrained only by its interactions with its neighbours (J ), and the bond 105-106 of the protein is held in a particularly advantageous position for attack by enzymes such as chymosin ( ). The importance of this will be apparent when emulsions stabilized by K-casein are being discussed. The enzymic action has a relatively small but detectable effect on the hydrodynamic diameters of the particles, and a large effect on their electrophoretic mobilities, which decrease by between one-third and one-half, depending on the solution conditions ( ). 

The foregoing discussion has clearly revealed that the structure of latex particles deriving from emulsion polymerization depends strongly on the mode in which the second monomer is introduced. A question intimately connected to seeded emulsion polymerization is related to possible inhomogeneities in swollen latex particles due to the wall-repulsion effect [99,100]. Emulsion polymerization can be used to obtain high molecular weight polymers whose radii of gyration may be of the order of the radius of the latex particles. If such a particle is swollen by a solvent, the polymer chains tend to avoid the surface of the particle because of their restricted conformations near a wall. Therefore the polymer concentration near an impenetrable surface is diminished. 

There have been many studies devoted to characterization of these practically important systems. Reverse emulsion droplets have been used as chemical micro-reactors to produce nanosize inorganic and polymer particles with special properties that are not found in the bulk form (38-42). These microemulsion systems have also been a topic of research for biological systems and the AOT head groups have been found to influence the conformation of proteins and increase enzyme activity (43-6). The unique environment created in the small water pools of swollen reverse micelles allows for increased chemical reactivity. The increase in surface area with decreased in size of the droplets also can significantly increase reactivity by allowing greater contact of immiscible reactants. 

Understanding the adsorption and conformation of polymeric surfactants at interfaces is key to understanding how these molecules act as stabilizers for suspensions and emulsions. Most basic theories on polymer adsorption and conformation have been developed for the solid/liquid interface (9). The same concepts may be applied for the liquid/liquid interface, with some modifications whereby some part of the molecule may reside within the oil phase, rather than simply staying at the interface. Such modifications do not alter the basic concepts, particularly when one deals with the stabilization by these molecules. 

The conductivity values of the PAM salts measured as pellets are given in Table 4.12. The conductivity of the PAM salts are in the range of 0.3-0.9 S/cm. The higher conductivity of PAM obtained by the inverted emulsion method could be due to a more homogenous protonation of the imine nitrogen and a more ordered chain conformation of the polymer. The conductivity of the PAM-camphor sulfonic acid was found to be higher than the other salts (see Table 4.12). 

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