Attached polymer chains

The spatial extension of polymer molecules at an interface depends critically upon the mode of attachment. If the chains are attached terminally at one end then they are referred to as tails (or sometimes as cilia). Terminal attachment at both ends results in what are termed loops. Loopy-type adsorption, as expected for homopolymers, leads to some segments abutting the interface (these are termed trains), together with both tails and loops (see Fig. 4.7). 

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Colloidal dispersions can be stabilized by attaching polymer chains to their surface [1-9]. When neutral polymer chains grafted on two parallel plates interpenetrate, a steric repulsion is generated. If the polymer chains grafted to the plates are charged, the double layer interaction between the two plates is also affected by the presence of the chains. 

Steric Interaction. The steric interaction force /steric between two plates containing attached polymer chains can be evaluated on the basis of the scaling theory19... 

Neutral [2, 58-60] and charged polymer brushes [2, 63, 64] with high molecular masses and high grafting densities of the surface-bound macromolecules have been established. While molecular weights of the surface-attached polymer chains of more than 106 g mol-1 could be realized, the distances of the surface-attached chains were in some cases less than 3 nm. 

Once r has been calculated, the moments of the distribution function can be found. For chains in free solution, Flory has developed an elegant generator matrix technique that by-passes r and calculates the second moment directly. Unfortunately, for attached polymer chains, such as terminally... 

Scaling law theory applied to attached polymer chains 4.6.4.1. Segment density profiles... 

Everett and Stageman (1978a) have also claimed that at the CFPT, surface attached polymer chains phase separate from the dispersion medium and collapse on to the surface of the colloidal particle, this process occurring in dispersion media that are better solvents than 0-solvents. To-date, there is no experimental evidence to support their speculations for those systems that display a strong correlation between the CFPT and the 0-point. 

It is convenient from a conceptual viewpoint to delineate three domains of close approach for sterically stabilized colloidal particles (Evans and Napper, 1973a). These zones delimit domains in which different phenomenology is clearly recognizable. They are determined by the relative sizes of the spans of the attached polymer chains (assumed for simplicity to be monodisperse) and the separation between the particles. The span can be defined in this context as specifying the conformational average distance normal to the surface that the furthermost segment meanders from the surface. The span thus sets the absolute limit to the average conformational thickness of the steric layers. 

The equation derived by Fischer can be recast into a more convenient form as follows. Let Ls be the span of the attached polymer chains, which to a modest approximation can be equated to the layer thickness 5. Then if co is the weight of stabilizing moieties attached to unit area of the surface, it follows that o)=LsC2 Furthermore, the density of the attached polymer chains P2 can be set equal to the reciprocal of their partial specific volume 2- These relationships on substitution into equation (12.10) yield... 

In the foregoing chapters the effects of attached polymer chains on colloid stability have been set forth. We now turn to consider the effects of macromolecules that are not attached to the particles but rather are free in solution. Surprisingly, such free polymer is still able to affect colloid stability, being capable of generating both stability and flocculation. Stability that is imparted by free polymer is termed depletion stabilization. Aggregation that is induced by free polymer is called depletion flocculation. The latter will be discussed in this Chapter, consideration of depletion stabilization being postponed until the next Chapter. 

The onset of the steric exclusion force depends on the means of attaching the polymer chains to the substrates. For physically adsorbed polymer chains covering bodh surfaces, the steric exclusion force becomes detectable around 6Rg, where Rg is the unperturbed radius of gyration of the random polymer coil in solution (12), For terminally attached polymer chains, the repulsion commences around 12Rg (14), These values are approximate and depend on a number of factors including solvent quality, temperature, surface concentration and type of polymer chains attached to the surfaces. 

Responsive polymer bmshes are not only investigated on planar substrates, but can also be used to decorate nanoparticles (Figure 5.13). Here, the key biomedical application is the accommodation (immobilize) of biofunctional moieties such as proteins or enzymes (Wittemann Ballauff, 2006 Wittemann, Haupt, BaUauff, 2003). If there is sufficient attraction between a protein and a polymer brush to overcome excluded volume effects (the antifouling properties of the brush), much larger amounts of protein can accumulate inside a brnsh layer than can be adsorbed onto just the particles surface. The role of the attached polymer chains is thus to strongly increase the available surface area. In Fignre 5.13, we schematically show a spherical polymer brush filled with adsorbed nanoparticles. Because, protein molecules inunobilized by polymers are found to be relatively weakly bound, they keep their conformation and (enzymatic)... 

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