Chain overlap, polymer adsorption

Steric stabilization by adsorbed homopolymers suffers from the conflicting requirements that the liquid be a poor solvent to ensure strong adsorption but a good solvent to impart a strong repulsion when the adsorbed polymer chains overlap. At low polymer concentrations, an individual polymer chain can become simultaneously adsorbed on two (or more) surfaces, resulting in an attractive interaction known as bridging flocculation (see Sect. 4.6.4). 

Usually, in the polymer adsorption theory, there are three stages adsorption from very dissolved solutions, when the substitution degree of surface is low, and macromolecules are situated at a large distance from each other The transitional stage, in which the distance between macromolecules is not large and their chains partially overlaps the plateau regime, when the macromolecule chains are overlapped, stretched into the solution, and only a small part is bound with the surface. This notion of polymer adsorption makes the theoretical calculations of macromolecule state in the adsorption layer more complicated. 

Repulsive forces between Fe oxide particles can be established by adsorption of suitable polymers such as proteins (Johnson and Matijevic, 1992), starches, non-ionic detergents and polyelectrolytes. Adsorption of such polymers stabilizes the particles at electrolyte concentrations otherwise high enough for coagulation to occur. Such stabilization is termed protective action or steric stabilization. It arises when particles approach each other closely enough for repulsive forces to develop. This repulsion has two sources. 1) The volume restriction effect where the ends of the polymer chains interpenetrate as the particles approach and lose some of their available conformations. This leads to a decrease in the free energy of the system which may be sufficient to produce a large repulsive force between particles. 2) The osmotic effect where the polymer chains from two particles overlap and produce a repulsion which prevents closer approach of the particles. 

Fig. 8. Schematic representation of the possible conformations of adsorbed (co)polymers prepared using macromonomer technique a brush adsorption of graft copolymer b terminally-attached adsorption c the mushroom-brush transition for strongly overlapping chains proposed by de Gennes [130] and Alexander [155]...

The polymer concentration in the adsorption layer is greater than in solution. The macromolecules in the adsorption layer are situated close to each other even if adsorption occurs from dilute solutions. The chains of macromolecules overlap, forming the network that is continually fluctuating. Bonds between two chains form and are destroyed continually. The number of chains between two bonds and the distance between them, e, termed the screening of correlation length, depend on the polymer concentration and chemical potential of chain [49]. The dependence between e and the polymer concentration in solution can be described by... 

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