PEL brush

However, it should be noted that this is only a somewhat simplified summary of the theory of PEL brushes. For a more detailed discussion the reader is referred to Refs. [6-35]. 

In a first example, for a densely grafted PEL brush system positively charged quaternized poly-4-vinylpyridine brushes have been prepared by following a two step approach [2, 63, 64, 66]. In the first step a neutral poly-4-vinylpyridine monolayer is prepared and, subsequently, charges are introduced by a second, polymer-analogous quaternization step. The grafting density of the parental neutral brush is adjusted by varying the polymerization time (Fig. 12) [63, 64]. The substrates were planar silicon substrates in... 

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. 

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... 

Fig. 31 Schematic depiction of the formation of PEL multilayers through PEL brushes (a) strong/strong system (b) strong/weak system no implication is made about phase boundaries between the different layers and interface roughness...

The strategy for synthesis of mixed PEL brushes is based on a combination of the concepts used for synthesis of homopolymer PEL brushes, when two main grafting to [94-99] and grafting from [63, 64, 66, 100] approaches... 

Polyacrylic acid (PAA)—P2VP mixed brushes were prepared by a similar synthetic procedure, by grafting of carboxyl-terminated poly(ferf-butyl acrylate) (PtBuA) and P2VP. Afterwards, PtBuA was hydrolyzed in the presence of p-toluene sulfonic acid. The same strategy was employed to graft mixed PEL brushes on polymer surfaces. In this case plasma treatment was used to functionalize surface of polymer substrates. We introduced amino groups on the surface of PA-6 and PTFE by treatment of the polymer samples with NH3 plasma. Then the carboxyl terminated homopolymers were grafted step by step from the melt to the solid substrate via amide bonds. 

Large diversity of functional surfaces can be fabricated with approach of mixed PEL brushes. Here we consider two most important examples (Fig. 33) ... 

The mixed PEL brush is sensitive to pH in a wide range of acidic-basic properties of solution. That we illustrate with the dependence of the brush thickness vs. pH. This relationship is presented by a symmetric U-shaped... 

Polyelectrolyte brushes exhibit interesting characteristics with respect to both theoretical and practical aspects because their behavior is fundamentally different from that of uncharged polymer brushes [32,33]. In the case of strong PEL brushes, in which the charge density... 

Fig. 9 Transfection efficiency of PLL-bmsh/pEGFP-C3. Polymer complexes at various chargemixing ratios with 1.0 pg pEGFP-C3 on PBME cells PEL-brush complex 2, z /z = 10.6 PLL-brush complex 3, z /z = 6.3 linear PEL complex 1, z /z = 8.9 linear PEL complex 2, z /z = 6.3. Lipofectamine was used as a positive control and GFP expression was measured as % of total cell population, 48 h after transfection...

To avoid the limitations of grafting-to procedures we have generated polyelectrolyte molecules by growing the chains at the surface of the substrate as described above for neutral polymers. Either an ionic monomer such as styrene sulfonate is polymerized directly [38] or the PEL monolayer generation is performed in a two-step process [39]. Eirst, a neutral brush is grown at the surface, which is then converted into the PEL brush in a polymer-analogous reaction. In this case it is very important that the degree of functionalization is close to f = 1,... 

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