Polymers steric stabilisation

Stability of Disperse Systems Containing Adsorbed Nonionic Surfactants or Polymers Steric Stabilisation... 

If the polymer layers increases the stability of the dispersion, it is denoted steric stabilisation. The polymer must fulfil two key criteria (i) the polymer needs to be of sufficient coverage to coat all the particle surfaces with a dense polymer layer, and (ii) the polymer layer is firmly attached to the surface. How this is engineered is beyond the scope of this article, but the consequences of not satisfying these criteria are informative in understanding the effect that polymers have on the overall interparticle interaction. Since complete or incomplete coverage of the particles results in very different properties (i.e stability or instability), this is clearly one way in which minimal change in initial conditions can lead to major differences in product. 

In studying the stability of colloidal dispersions it is of considerable advantage if the particles concerned are monodisperse and spherical. For aqueous, charge-stabilised systems polymer latices have proved invaluable in this regard. With non-aqueous systems, steric stabilisation is usually required. In this case it... 

Already the ancient Egyptians knew that one can keep soot particles dispersed in water when they were incubated with gum arabicum, an exudate from the stems of acaia trees, or egg white. In this way ink was made. The reason for the stabilizing effect is the steric repulsive force cause by adsorbed polymers. In the first case these are a mixture of polysaccharide and plycoprotein, in the second case it is mainly the protein albumin. Steric stabilisation of dispersions is very important in many industrial applications. Direct quantitative measurements were... 

Steric stabilisers are usually block copolymer molecules (e.g. poly (ethylene oxide) surfactants), with a lyophobic part (the anchor group) which attaches strongly to the particle surface, and a lyophilic chain which trails freely in the dispersion medium. The conditions for stabilisation are similar to those for polymer solubility outlined in the previous section. If the dispersion medium is a good solvent for the lyophilic moieties of the adsorbed polymer, interpenetration is not favoured and interparticle repulsion results but if, on the other hand, the dispersion medium is a poor solvent, interpenetration of the polymer chains is favoured and attraction results. In the latter case, the polymer chains will interpenetrate to the point where further interpenetration is prevented by elastic repulsion. 

Surfactants or polymers adsorbed on the particle surface are able to keep particles that far ap-part that the Van der Waals attraction cannot become effective. This phenomenon is called steric stabilisation. 

The sterically stabilised dispersions produced can be weeikly flocculated by the addition of free (non-adsorbing) polymer such as poly(ethylene oxide). 

Non-Aqueous Processes. Dispersions of composite particles in non-aqueous media (12) have been prepared. The particles were sterically stabilised to prevent flocculation and aggregation. This was achieved by physical absorption of amphipathic graft or block copolymer (13,14) or by covalent attachment of diluent-soluble oligomer or polymer chains (15) at the particle surface so that by definition different polymers were situated at the surface and in the bulk of the particles, even for single-polymer particles. Composite particles were prepared by slow addition of the second monomer which was fully miscible with the diluent phase, obviating a monomer droplet phase further monomer-soluble initiation and amphipathic graft stabiliser was included as appropriate so that the process comprised continued dispersion... 

The distribution of segments in loops and tails, p(z), which extend in several layers from the surface. p(z) is usually diflBcult to obtain experimentally, although recently the application of small-angle neutron scattering has been used to obtain such information. An alternative and useful parameter for assessing steric stabilisation is the hydrodynamic thickness, Sf, (the thickness of the adsorbed or grafted polymer layer plus any contribution from the hydration layer). Several methods can be applied to measure 5, as will be discussed below. 

Polymers are also essential for the stabilisation of nonaqueous dispersions, since in this case electrostatic stabilisation is not possible (due to the low dielectric constant of the medium). In order to understand the role of nonionic surfactants and polymers in dispersion stability, it is essential to consider the adsorption and conformation of the surfactant and macromolecule at the solid/liquid interface (this point was discussed in detail in Chapters 5 and 6). With nonionic surfactants of the alcohol ethoxylate-type (which may be represented as A-B stmctures), the hydrophobic chain B (the alkyl group) becomes adsorbed onto the hydrophobic particle or droplet surface so as to leave the strongly hydrated poly(ethylene oxide) (PEO) chain A dangling in solution The latter provides not only the steric repulsion but also a hydrodynamic thickness 5 that is determined by the number of ethylene oxide (EO) units present. The polymeric surfactants used for steric stabilisation are mostly of the A-B-A type, with the hydrophobic B chain [e.g., poly (propylene oxide)] forming the anchor as a result of its being strongly adsorbed onto the hydrophobic particle or oil droplet The A chains consist of hydrophilic components (e.g., EO groups), and these provide the effective steric repulsion. 

When >0.5, becomes negative (attractive) this, combined with the van der Waals attraction at this separation distance, produces a deep minimum causing flocculation. In most cases, there is a correlation between the critical flocculation point and the 0-condition of the medium. A good correlation is found in many cases between the critical flocculation temperature (CFT) and the 0-temperature of the polymer in solution (with both block and graft copolymers the 0-temperature of the stabilising chains A should be considered) [2]. A good correlation was also found between the critical volume fraction (CFV) of a nonsolvent for the polymer chains and their 0-point under these conditions. In some cases, however, such correlation may break down, and this is particularly the case for polymers that adsorb by multipoint attachment. This situation has been described by Napper [2], who referred to it as enhanced steric stabilisation. 

In recent years systematic studies using a wide variety of synthetic polymers have led to a considerable elucidation of the detailed mechanism of stabilisation of this kind, which has been given the name steric stabilisation. 

Figure 3.12 Typical forms of the total-interaction free energy for (a) electrostatically stabilised systems [curves (i), (ii), and (iii) refer to increasing electrolyte concentration], (b) sterically stabilised systems [curves (i), (ii), and (iii) refer to constant density of polymer chains, but decreasing <5, arising from decreasing values of a].

At this point we see qualitatively that the mechanisms of steric stabilisation and depletion flocculation are closely related. In the former instance the concentration of polymer segments in the space between the particles increases as the particles come together, leading to a repulsion caused by the osmotic flow of solvent into this space in the latter case the concentration between the particles is lower than that in the bulk, and diffusion of solvent out of the interparticle space results in an attraction. 

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