Polymers electrosteric stabilization

Salt effects in polyelectrolyte block copolymer micelles are particularly pronounced because the polyelectrolyte chains are closely assembled in the micellar shell [217]. The situation is quite reminiscent of tethered polymer brushes, to which polyelectrolyte block copolymer micelles have been compared, as summarized in the review of Forster [15]. The analogy to polyelectrolyte brushes was investigated by Guenoun in the study of the behavior of a free-standing film drawn from a PtBS-PSSNa-solution [218] and by Hari-haran et al., who studied the absorbed layer thickness of PtBS-PSSNa block copolymers onto latex particles [219,220]. When the salt concentration exceeded a certain limit, a weak decrease in the layer thickness with increasing salt concentration was observed. Similar results have been obtained by Tauer et al. on electrosterically stabilized latex particles [221]. 

The role of polymers on colloid stability is considerably more complicated than electrostatic stability due to low molecular weight electrolytes considered in Chapter 11. First, if the added polymer moieties are polyelectrolytes, then we clearly have a combination of electrostatic effects as well as effects that arise solely from the polymeric nature of the additive this combined effect is referred to as electrosteric stabilization. Even in the case of nonionic... 

Ottewill, R.H. and Satgurunathan, R. (1995) Nonionic lattices in aqueous media. 4. Preparation and characterisation of electrosterically stabilized particles. Colloid Polym. Sci., 273, 379-86. 

FIGURE 10.16 Diagram of electrosteric stabilization (a) negatively charged particles surrounded by cationic counterions with nonionic polymers adsorbed, (b) positively charged polymers called polydectrolytes intermingled with anionic counterions attached to uncharged particles. 

In addition, if the polymer is charged as with polyelectrolytes, the charge of the polymer gives rise to electrosteric stabilization-a combi-... 

With polymers that have ionizable groups, adsorption of a polymer will alter the charge of the surface altering the electrostatic interaction energy and also provide steric protection for the colloid, because the ionized groups will give better than theta conditions for the poisoner in an aqueous solution. This type of polymer stabilization is called electrosteric stabilization because both the electrostatic and the steric play a role in stabilization. The equations for this total interaction are simply the sum of electrostatic and steric terms as well as the van der Waals attraction. 

On the other hand, several reports have been published that point out that when a polymeric surfactant acting as an electrosteric stabilizer is used, the rate of radical entry into a polymer particle should decrease due to a diffusion barrier of the hairy layer built up by the polymeric surfactant adsorbed on the surface of the polymer particles [34-36]. Coen et al. [34] found that in the seeded emulsion polymerization of St using a PSt seed latex stabilized elec-trosterically by a copolymer of acrylic acid (AA) and St, the electrosteric stabilizer greatly reduced the radical entry rate p compared to the same seed latex... 

Polymer 42 (R=Me) is a powerful stabilizer in emulsion polymerization of ST. The particle size increases with increasing length of the hydrophobic block. Due to very efficient electrosteric stabilization the resulting dispersions have outstanding colloid stability even at high salt concentration [64, 75],... 

The hairy particles stabilized by non-ionic emulsifier (electrosteric or steric stabilization) enhance the barrier for entering radicals and differ from the polymer particles stabilized by ionic emulsifier [35]. For example, the polymer lattices with the hairy interface have much smaller values of both the radical entry (p) and exit (kdes) rate coefficients as compared to the thin particle surface layer of the same size [128,129]. The decrease of p in the electrosterically stabilized lattices is ascribed to a hairy layer which reduces the diffusion of oligomeric radicals, so that these radicals may be terminated prior to actual entry. For the electrostatically stabilized lattices with the thin interfacial layer, exit of radicals occurs by the chain transfer reaction [35]. This chain transfer reaction results in a monomeric radical which then exits out of the particle by diffusing through the aqueous phase and this event is competing with the propagation reaction in the particle [130]. The decrease of kdes in the electrosterically stabilized latex... 

The ultrasonification process is connected with the rapidly increased oil-water interfacial area as well as the significant re-organization of the droplet clusters or droplet surface layer. This may lead to the formation of additional water-oil interface (inverse micelles) and, thereby, decrease the amount of free emulsifier in the reaction medium. This is supposed to be more pronounced in the systems with non-ionic emulsifier. Furthermore, the high-oil solubility of non-ionic emulsifier and the continuous release of non-micellar emulsifier during polymerization influence the particle nucleation and polymerization kinetics by a complex way. For example, the hairy particles stabilized by non-ionic emulsifier (electrosteric or steric stabilization) enhance the barrier for entering radicals and differ from the polymer particles stabilized by ionic emulsifier. The hydro-phobic non-ionic emulsifier (at high temperature) can act as hydrophobe. 

The major route to colloidal (effectively water soluble) PAn has been through the chemical oxidation (S2082-) of the monomer in the presence of polymeric steric stabilizers and electrosteric stabilizers (polyelectrolytes), such as poly(vinyl alcohol), polyGV-vinyl pyrrolidone), polyethylene oxide), polystyrene sulfonate), dodecylben-zene sulfonate, and dextran sulfonate. It has been found that the stabilizer can act simultaneously as a dopant, imparting new functionality to the polymer or additional compatibility for the final application. 

Surfactants in Aqueous Solution A very important component that is usually present in the lyophobic colloids is the surfactant. These molecules are amphiphilic, that is, a part of the molecule is much more polar than the other part. On the basis of the nature of the polar groups in the surfactant molecule, they are classified as ionic (anionic or cationic) and nonionic. When ionic-type surfactants are adsorbed onto polymer particles, they provide stabilization by electrostatic repulsion between them and when the nonionic type are adsorbed instead the mode of stabilization is by steric repulsion. Electrosteric stabilization is provided by polyelectrolyte chains that give place to both modes of repulsion electrostatic and steric. 

It is possible to have combinations of electrostatic and steric stabilization, which has been termed electrosteric stabilization. The electrostatic component may originate from a net charge on the particle surface (see Fig. 1.7a) and/or charges associated with the polymer attached to the surface (i.e. through an attached polyelectrolyte) (see Fig. 1.7b). Electrosteric stabilization is common in biological systems. 

Fig. 1.7. Diagrammatic representation of electrosteric stabilization (a) charged particles with nonionic polymers (b) polyelectrolytes attached to uncharged particles (not to scale).

In addition to electrosteric stabilization, it is possible to have combinations of depletion stabilization with both steric and/or electrostatic stabilization. The combination of depletion and steric stabilization is quite common at high concentrations of free polymer in the dispersion medium. 

Since the beginning of colloids science, however it is also known that the agglomeration of colloids and dispersed particles can be prevented or controlled by stabilization [8]. The attractive interactions between the colloidal particles, caused by van-der-Waals forces, need to be compensated by repulsive interactions. The latter can be based either on electrostatic repulsion due to same-sign surface charges (electrostatic stabilization), or on repulsion via a polymer shell formed through adsorption of polymers to the particle surface (steric stabilization, in presence of polyelectrolytes termed electrosteric stabilization due to additional charged-induced repulsion) [9, 10]. The stabilization by control of the interaction forces between colloidal particles has been in the focus of extensive research efforts. Already... 

In steric stabilization, adsorbed polymer molecules must extend outward from the droplet surface, yet be strongly enough attached that they remain adsorbed in the presence of applied shear. It is also possible to have particles stabilized by both electrostatic and steric stabilization these are said to be electrosterically stabilized. 

These precursor polymers lead to hydrophilic thermo-sensitive block copolymers, which form at room temperature (RT) transparent solutions which convert during heating into electrosterically stabilized PSS-PNIPAm, PAA-PNIPAm, PDADMAC-PNIPAm, and PDEAMEMA-PNIPAm block copolymer particles. 

Lochhead RY. Electrosteric stabilization of oil-in-water emulsions by hydrophobically modified poly(acrylic acid) thickeners. In Shulz DN, Glass JE. Eds. Polymers as Rheology Modifiers. ACS Symposium Series No. 462. Washington, DC American Chemical Society, 1991 101-120. 

Electrosteric stabilization, consisting of a combination of electrostatic and steric repulsion, achieved by the adsorption of charged polymers (polyelectrolytes) onto the particle surfaces. 

The main processes by which particles dispersed in a liquid can acquire a surface charge are (1) preferential adsorption of ions, (2) dissociation of surface groups, (3) isomorphic substitution, and (4) adsorption of charged polymers (polyelectrolytes). Preferential adsorption of ions from solution is the most common process for oxide particles in water, whereas isomorphic substitution is commonly found in clays. The adsorption of polyelectrolytes is the main charging mechanism in electrosteric stabilization and will be discussed later. The dissociation of ionizable... 

As outlined earlier, suspensions can also be stabilized by electrosteric repulsion, involving a combination of electrostatic repulsion and steric repulsion (Fig. 4.4c). Electrosteric stabilization requires the presence of adsorbed polymers and a significant double layer repulsion. It is commonly associated with suspensions in aqueous liquids, but several studies have indicated that electrostatic effects can be important in some nonaqueous systems (46). A common way of achieving electrosteric stabilization in aqueous liquids is through the use of polyelectrolytes, i.e., polymers that have at least one type of ionizable group (e.g., carboxylic or... 

Ramakrishnan S., McDonald C.J., Carbeck J.D., Prudhomme R.K. Latex composite membranes Structure and properties of the discriminating layer. J. Mem. Sci. 2004 231 57-70 Romero-Cano M.S., Martin-Rodriguez A., Nieves F.J.D.L. Electrosteric stabilization of polymer colloids with different functionality. Langmuir 2001 17(11) 3505-3511 Rau D.C., Parsegian V.A. Direct Measurement of the intermolecular forces between counterion-condensed DNA double helices— Evidence for long-range attractive hydration forces. Biophys. J. 1992 61(1) 246-259... 

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