Enhanced steric stabilization

Parkinson, E.L., Ettelaie, R., Dickinson, E. (2005). Using self-consistent-field theory to understand enhanced steric stabilization by casein-like copolymers at low surface coverage in mixed protein layers. Biomacromolecules, 6, 3018-3029. 

At higher surface coverages, the mean-field treatment noted in Section IV indicates that worse than theta conditions are required before an attractive minimum appears in the potential. This phenomena has been observed and termed enhanced steric stabilization by Napper (1983). Recent experiments by Edwards et al. (1984) clearly show that grafting PDMS chains onto silica particles at nl2a 12 — 18 extends the stable range substantially beyond theta conditions. 

Filler particles can be modified to decrease flocculation. Kaolin particles modified by a graft of poly(ethylene oxide) showed an increase in the upper critical flocculation temperature. Stabilization of particle dispersion was due to an enhanced steric stabilization." ... 

Random copolymers are usually less effective as steric stabilizers than block or graft copolymers, although it should not be overlooked that under certain conditions they are able to confer enhanced steric stabilization (see Chapter 9). 

Deviant Flocculation Behaviour Enhanced Steric Stabilization... 

This unprecedented observation of stability in worse than 0-solvents, when the segments of the stabilizing moieties would be expected to be mutually attractive, is the enhanced steric stabilization referred to previously. Dispersions that exhibited enhanced steric stabilization appeared to flocculate irreversibly. 

Lambe et al. (1978) studied the enhanced steric stabilization of polystyrene latices by poly(vinyl alcohol). This is included in this sub-section on copolymers because the samples studied were not fully hydrolysed. This means that the parent poly(vinyl acetate) from which they were derived was only partially (88%) hydrolysed (this is often accomplished by alcoholysis). The resultant polymer is not, however, a completely random copolymer because adjacent group effects influence the hydrolysis kinetics in such a way that some degree of blockiness is introduced. On average, these blocks consist of 2 ester groups to every 18 alcohol groups but blocks of average size 5-6 acetate groups are common. The chemical structure of the polymers should therefore formally represented by a structure intermediate between poly(vinyl acetate-6-vinyl dcohol) and poly(vinyl acetate-co-vinyl alcohol) rather than poly(vinyl alcohol) as such. The random (or statistical) copolymer can be prepared by partial reacetylation of fully hydrolysed poly(vinyl alcohol). 

As stated previously, the origins of enhanced steric stabilization are still not fully understood, although several explanations have been forthcoming. These include ... 

We note in passing that it is by no means certain that enhanced steric stabilization has actually been observed with partially hydrolysed poly(vinyl alcohol) as the stabilizer. First, it is questionable whether the concept of a 0-temperature is applicable to copolymers in general, except in rather special circumstances e.g. it is certainly applicable to alternating copolymers and perhaps to some random copolymers but it is not applicable to block or graft copolymers, except perhaps to those composed of blocks with identical 0-temperatures in the same solvent. Second, the putative 0L-temperature determined for partially hydrolysed poly(vinyl alcohol) by the phase separation method of Comet and van Ballegooijen (1966) applies to the whole... 

Stabilizer, not just to the poly(vinyl alcohol) moieties. Yet it is these latter moieties which presumably impart stability. The presence of the nominally insoluble poly(vinyl acetate) segments would be expected to depress the observed value of the -temperature. The conclusion emerges that just because the observed UCFT is greater than the putative 0 -temperature of the copolymer, this does not necessarily imply that the true. -temperature of the homopolymeric poly(vinyl alcohol) moieties is exceeded. It follows that the existence of enhanced steric stabilization with partially hydrolysed poly(vinyl alcohol) has not yet been established unequivocally, although its occurrence does seem highly likely. Unfortunately, it is not possible to use the method of Comet and van Ballegooijen (1966) to determine the actual fl -temperature of fully hydrolysed poly(vinyl alcohol) because its crystallinity and insolubility renders the method both inoperative and invalid. 

There seems little doubt that it is possible to prepare sterically stabilized dispersions in worse than 0-solvents. The origins of this enhanced steric stabilization have yet to be firmly established. Very low segment densities in... 

Destabilization of foams is a dynamic process that includes disproportionation, coalescence in addition to drainage of the thin film between bubbles. In addition to the bulk phase viscosity, all of these processes involve interfacial film properties [2, 4, 7, 10, 11]. The greater stabilizing effect may be attributed to a greater enhancement of the local viscosity in a foam lamellae which tends to inhibit film drainage, as well as to increased thickness of the mixed adsorbed layer which tends to enhance steric stabilization and inhibit bubble coalescence [3],... 

The multipoint anchoring of the polymer chains also ensures strong elastic (en-tropic) repulsion. This provides enhanced steric stabilization. 

Macromolecules adsorb onto interfaces and facilitate better coverage than monomeric emulsifiers. The adsorbed polymers are known to enhance steric stabilization mechanisms and were proved to be efficient emulsifiers in oil-in-water emulsions. 

Enhanced steric stabilization this is the case with multipoint attachment, which produces strong elastic interaction. 

Features Compat. with erde oil designed to enhance steric stabilization and thus hinder flocculation/aggregation of asphaltene molecules Properties Brn. liq. HLB 5.0 Hypermer 2434 [Croda Inc Croda Chem. Europe Ltd]... 

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