Polymers layer overlap

FIGURE 14.6 Schematic representation of polymer layer overlap. 

Altematively, tire polymer layers may overlap, which increases tire local polymer segment density, also resulting in a repulsive interaction. Particularly on close approach, r < d + L, a steep repulsion is predicted to occur. Wlren a relatively low molecular weight polymer is used, tire repulsive interactions are ratlier short-ranged (compared to tire particle size) and the particles display near hard-sphere behaviour (e.g., [11]). 

Two mechanisms of steric stabilization can be distinguished entropic stabilization and osmotic repulsion. Entropic stabilization arises when two opposing adsorbed polymer layers of adjacent particles overlap, resulting in compression and interpenetration of their... 

In many practical instances (see Vignette 1.5), electrostatic repulsion is not a convenient option. In such cases, a suitable polymer that adsorbs on the particle surfaces may be added to the dispersion. The resulting polymer layer masks the attraction and may also provide a repulsive force, partly due to pure steric effect, when the polymer layers on two interacting particles attempt to overlap with each other. This is what is known as polymer-induced stability. Polymer-induced stability is often referred to as steric stability for the above... 

FIG. 13.13 Interaction between polymer-coated particles. Overlap of adsorbed polymer layers on close approach of dispersed solid particles (parts a and b). The figure also illustrates the repulsive interaction energy due to the overlap of the polymer layers (dark line in part c). Depending on the nature of the particles, a strong van der Waals attraction and perhaps electrostatic repulsion may exist between the particles in the absence of polymer layers (dashed line in part c), and the steric repulsion stabilizes the dispersion against coagulation in the primary minimum in the interaction potential. 

Ideally, it would be desirable to be able to develop quantitative expressions for the interaction energies so that we can deal with coagulation or flocculation, at least in the case of fairly dilute dispersions, the way we did in Sections 13.3-13.4 for electrostatic stabilization. It is possible to develop approximate expressions for interaction energy due to various individual effects such as osmotic repulsion, attraction or repulsion due to the overlap of the tails of the adsorbed (or grafted) polymer layers, interaction of the loops in the layers, and so on (see Fig. 13.15). However, the complicated nature of polymer-induced interactions makes these tasks very difficult. In this section, we merely illustrate some of the issues that need to be considered in developing a fundamental quantitative understanding of polymer-induced forces. In Section... 

When two particles, each with a radius R and containing an adsorbed surfactant or polymer layer with a hydrodynamic thickness 5, approach each other to a surface-surface separation distance h that is smaller than 25, the surfactant or polymer layers interact with each other, with two main outcomes [1] (i) the surfactant or polymer chains may overlap with each other or (ii) the surfactant or polymer layer may undergo some compression. In both cases, there will be an increase in the local segment density of the surfactant or polymer chains in the interaction region this is shown schematically in Figure 8.1. The real-hfe situation perhaps lies between the above two cases, however, with the surfactant or polymer chains undergoing some interpenetration and some compression. 

Bondy ° observed coagulation of rubber latex in presence of polymer molecules in the disperse medinm. Asaknra and Oosawa published a theory that attributed the observed interparticle attraction to the overlap of the depletion layers at the surfaces of two approaching colloidal particles (Figure 5.28). The centers of the smaller particles, of diameter, d, cannot approach the surface of a bigger particle (of diameter D) at a distance shorter than d 2, which is the thickness of the depletion layer. When the two depletion layers overlap (Fignre 5.28), some volume between the large particles becomes inaccessible for the smaller particles. This gives rise to an osmotic pressnre. 

Steric forces arise from the overlap of the adsorbed layers and can be repulsive or attractive in nature depending on whether or not the outermost layers on the particles prefer to be in contact with the solvent. If the solvent power of the medium for the exposed portions of the exterior layer, for example, those of an adsorbed polymer layer, is satisfactory, they will be compatible with the medium and the suspension will remain stable. On the other hand, if the solvent power for the adsorbed layer is minimal, there will be a tendency for the exposed portions of the adsorbed layer on one particle to interpenetrate into those of a layer on another particle and thereby promote aggregation. 

Figure 3.10 Origin of the steric repulsion between particles (a) particles far apart, H 26, (b) successive stages of overlap of the polymer layers as H goes from 26 to 6. 6 a(rlf -, where r is the number of segments in the polymer, l is the length of each segment, and a is an expansion coefficient (a = 1 for a random coil).

Figure 13 Steric stabilization caused by the overlap of adsorbed polymer layers upon close approach of dispersed solid particles.

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