Brushes shrinking

The polyelectrolyte brush shrinks strongly on addition of electrolytes. At low or moderately low salt concentrations (cs=0.01 mol L-1) the force profiles resemble those of a soft brush. At salt concentrations of cs= 0.03 mol L, however, the profile of the static force resembles more closely that of a hard surface. Interestingly, if the behavior of the PEL brush is studied close to the collapse point significantly increased compressibility can be observed. However, the compressibilty shows no bistability, which indicates that the transition between the brush and the collapsed state is not a true first-order transition, although this would be expected from mean-field theory. One possible explanation of this behavior would be that the polydisper-sity of the surface-attached chains smoothens the transition. 

The use of polymeric coatings in catalysis is mainly restricted to the physical and sometimes chemical immobilization of molecular catalysts into the bulk polymer [166, 167]. The catalytic efficiency is often impaired by the local reorganization of polymer attached catalytic sites or the swelling/shrinking of the entire polymer matrix. This results in problems of restricted mass transport and consequently low efficiency of the polymer-supported catalysts. An alternative could be a defined polymer coating on a solid substrate with equally accessible catalytic sites attached to the polymer (side chain) and uniform behavior of the polymer layer upon changes in the environment, such as polymer brushes. 

Why is it that insects like beetles can walk on water Why do the bristles of a brush immersed in water cling together as the brush is pulled out Phenomena such as these arise because of a special property of interfaces that separate two phases. Let us consider another example first. Everyone has had the experience of pouring more beverage into a cup or glass than that container could hold. In addition to the spills this causes, such an experience provides an opportunity to observe surface tension. Most liquids can be added to a vessel until the liquid surface bulges above the rim of the container. The liquid behaves as if it had a skin that prevents it —up to a point —from overflowing. Stated technically, a contractile force, which tends to shrink the surface, operates around the perimeter of the surface. This is what we mean when we talk about the surface tension of a liquid. All phase boundaries behave this way, not just liquid surfaces however, the evidence for this is more apparent for deformable liquid surfaces. 

Figure 1 Responsiveness in a polymer brush. In a good solvent (a) favourable interactions between polymer segments and solvent molecules lead to the chains being stretched - the loss of configurational entropy attributable to the chain stretching is outweighed by the lowering of energy due to the polymer solvent interaction. If this interaction becomes less favourable (b), due to a change in temperature or pH, for example, the chains reversibly shrink...

Increasing the salt concentration one expects the lateral interactions to be reduced and the polyelectrolyte to be less stretched. This is indeed observed. Also as typical for the salted brush the brush length increases upon compression [44]. For this case one theoretically expects the dependence on density as depicted by the full line describing the measurements for 0.1 mol L-1 salt. At still higher salt contents one expects counter-ion condensation [45], hence part of the counter-ions do not contribute to the osmotic pressure. In this case one expects a further shrinking of the brush with salt content. In the extreme of a collapsed polymer one expects the length to be inversely proportional to the molecular area which corresponds to the steepest slope at highest salt content. 

The behavior of neutral brushes in good solvent conditions has been generalized to any solvent quality [218,219]. In 0 solvent, the excluded volume, which is the second virial coefficient, is equal to zero. As a consequence, the thickness L results from the balance between the three-body interaction forces (positive and proportional to c3) and the chain elasticity. In a poor solvent, the chains shrink, and the elasticity term is irrelevant. The determination of L thus results from the equilibrium between the two-body and three-body interaction forces. The expression of L as a function of N, a, d, and the second and third virial coefficients is obtained by the authors, who demonstrate that the brush configuration is preserved in both the 0 and the poor solvent with a linear variation of L with N. The brush thickness continuously decreases as the solvent quality decreases. Experimentally, this can be suitably achieved by varying the temperature. The effect of changing solvent quality on the flocculation behavior of emulsions stabilized by copolymers was discussed by March and Napper [154] (Sec. III.B). Even if the random copolymers used in this study are not expected to form brushes, it was clear that flocculation was observed in 0 solvent conditions while it was suppressed in good solvent conditions. 

Fig. 7 PSS (hydrophilic) layer structure change by salt concentration (1) The btush is not influenced by salt addition in the bulk at low concentration. (2) Added salt ions enter the btush layer tind a screening effect results in shrinking of the btush chains. (3) Structural transition from the carpet + brush to carpet-only PSS layer by further addition of salt. Reprinted from [72] with permission. Copyright 2007 American Chemical Society...

Metal nanoparticles embedded inside temperature-responsive polymer brushes (Lu et al., 2006) or cross-linked pNlPAM shell (Lu dai,2009a) were tested for the oxidation of alcohols to corresponding aldehydes or ketones (Lu et al., 2009a). The nanoparticles were fully accessible to the reactants at the temperature below the LCST of pNIPAM. Swelling-shrinking could be... 

Finally, recent SANS experiments by Auroy and Auvray probed the behavior of brushes immersed in a solution of homopolymers in a good solvent. Their results confirm the theoretical expectations. In particular, they observed the shrinking of the brush due to increased screening. Also observed was the associated decrease in the correlation length or blob size. Both results are of special interest as direct confirmation of the blob picture. 

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