Surface-attached brush

Polymer Brush Surface-Attached Hydrogel Polymer Monolayer... 

Fig. 5 Surface-attached polymer brush (left) surface-attached hydrogel (center) and surface-attached polymer monolayer right)...

The polyelectrolyte macromonomers 17, 20, 22, and 23 [142,143,147,148] were prepared and applied to dispersion copolymerizations to produce polymeric particles covered with polyelectrolyte chains. Evidently, the dependence of the conformational properties of polyelectrolyte brush chains attached to the latex surface on the pH, the degree of neutralization, and the salt concentration have been the subject of growing experimental and theoretical effort. 

If neutral and charged polymer brushes are exposed to solvents, a very interesting and rich phase behavior can be observed. As a consequence, such surface-attached brushes have become the focus of considerable theoretical efforts in studies on the structure and phase behavior of such polymer chains in contact with a solvent. The thickness of a neutral brush scales linearly with the degree of polymerization, N, which is in stark contrast to the well-known characteristics of free polymer chains in a good solvent, where the radius of the coils scales as R oc AT0 58. For neutral brushes the simple scaling... 

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. 

Fig. 14 Schematic representation of a spherical polyelectrolyte brush prepared by the photoemulsion grafting from technique (details see text). The brush consists of a solid polystyrene core and surface-attached strong (PSS) or weak (PAA) polyelectrolyte brush shell (Reprinted from Ref. [71] with permission from the American Physical Society)...

Fig. 15 Waveguide spectra (p-poL) of a 740 nm thick (dry thickness) MePVP brush covalently attached to the surface of a LaSFN9/Au/Si02 substrate. The spectra were measured in different (constant) rel. humidities a = 0%, b = 35%, c = 70%, d = 87% and e = 100% rel. humidity at 25°C. The solid lines are the calculated reflection curves obtained from Fresnel modeling...

Much more interesting, however, than the swelling in humid air is the behavior of polyelectrolyte brushes in contact with water or salt solutions. The first experimental study of the swelling properties of a surface-attached polyelectrolyte monolayer obtained by a grafting to process has been reported by Auroy and coworkers [51]. Polystyrene sulfonate layers chemically... 

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. 

Figure 20 shows the dependence of the brush thickness L on the pH (A) and salt concentration (B) of the liquid phase. As expected from mean-field theory, the height of the strong polyelectrolyte brush at various grafting densities and molecular masses of the surface-attached macromolecules is independent of pH at a given constant electrolyte concentration of the medium so that the PSS chains are fully dissociated under all circumstances [71]. 

Fig. 21 Profiles of a surface-attached weak polyacid brush as obtained from FT analysis of ellipsometric data at different pH. The pH is given in the figure. Analysis of the same ellipsometric data by Fresnel modeling. Best fits to complementary error functions at different pH given in the figure. For more details see text...

Apart from the formation of ultrathin surface-attached PEL-PEL complexes it is very interesting whether the PEL brushes can be also used for the formation of PEL multilayer assemblies. The so-called layer-by-layer (LBL) technique is a simple and powerful method to form well-defined multilayered structures [80]. For the formation of such multilayer assemblies the brushes are dipped alternately into polyelectrolyte solutions, one consisting of a positively charged polyelectrolyte, the other of a negatively charged polyelectrolyte. It is usually assumed that in this LBL deposition process the driving force for each monolayer formation is charge overcompensation [81, 82]. The stability of the multilayered system formed by LBL process in different environments is one of the limitations of this process. Since the attachment of the first layer depends solely on the interaction of... 

For any practical application of polyelectrolyte brushes the influence of multivalent ions, hydrophobic ions, and other polyelectrolyte molecules present in a contacting solution on to the structure of the surface-attached layers or the cylindrical polyelectrolyte brushes are of utmost importance. In particular, study of the interaction of brushes with other polyelectrolyte molecules in solution might open an avenue for the understanding of interaction of proteins or other charged biomolecules such as DNA, as a special form of charged macromolecules, with charged surfaces. It has become clear that not only the influence of the surface on to the conformation of the protein, but also the influence of the protein on the structure of the polymer layer is important. 

Fig. 58 Schematic of inferred structure for CdSe nanocrystal infiltrated polymer brush photovoltaic device. From bottom to top ITO-coated glass slide modified by surface attachment of a bromine end-capped trichlorosilane self-assembled monolayer (SAM) (squares) polymer brushes grown from the SAM (lines) CdSe nanocrystals infiltrated into the brush network exhibiting some degree of phase separation in the plane of the film (small circles) and an aluminum cathode cap. (Reprinted with permission from [256], 2005, American Chemical Society)...

---