Weak polyacid brushes : synthesis, swelling behavior, complex formation and micropatterning
In this work, densely end-grafted polymer monolayers, so-called "polymer brushes", were covalently attached to solid planar surfaces and investigated with respect to their properties in contact with aqueous solutions of varying composition. The synthesis of these polymer brushes was performed in two steps. First, an end-functionalized azoinitiator was covalently anchored to silicon wafers and planar glass substrates to form a self-assembled monolayer. Then, a surface-initiated free radical polymerization of vinyl monomers was triggered by thermal cleavage of the initiator ("grafting from" polymerization). By varying the monomer concentration and the polymerization time, the molecular weight of the surface-attached polymer chains and the anchoring density, a characteristic parameter that determines the degree of chain stretching of a polymer brush, could be reproducibly adjusted. Furthermore, different photopatterning strategies were implemented into the concept of surface-initiated polymerization. Using different approaches, positive and negative images of a mask and, in particular, laterally chemically microstructured polymer brushes could be obtained. The focus of this work was on the investigation of the properties of poly(methacrylic acid) (PMAA) brushes, a weak polyelectrolyte whose charge density can be tuned by adjustment of external parameters such as the solution pH-value. The PMAA brushes were swollen in aqueous solutions of different ionic composition and their properties were studied as a function of the valency, size (polarizability), concentration and nature of the ion. In another study, the complex formation of PMAA brushes with oppositely charged surfactants of different alkyl chain lengths was investigated. Stable complexes were formed with multivalent cations and oppositely charged surfactants. The complex formation behavior of PMAA brushes in contact with various small metal ion and oppositely charged surfactant solutions was investigated in detail, both, through in situ and ex situ measurements and significant new insights into the complex formation mechanism in such systems could be obtained. In particular, it was shown that already trace concentrations of certain ions and surfactants are sufficient to strongly alter the conformation of surface-attached molecules. This results in a strong modification of the layer properties such as the accessibility of functional groups. On the other side, seemingly small variations in the chemical details, such as interchanging the two divalent cations magnesium and copper which have the same charge and similar ionic radii, can have tremendous effects on the way these ions affect the polyelectrolyte brush structure. Moreover, these systems do not always behave as one would intuitively expect. For instance, the influence of monovalent silver ions on the weak polyacid brush structure is much closer related to that of divalent calcium ions than to that of monovalent sodium ions. These findings have important implications for the fundamental understanding of biological processes such as signaling, bioadhesion or the stabilization of micelles as in many biological systems (surface) biopolymers adopt constrained conformations similar to those of polymer brushes and a mix of different ions is usually present. Moreover, an understanding of the influence of certain ions on the molecular structure of thin surface-bound polyelectrolyte brushes is crucial for technological applications such as the electrosteric stabilization of colloids, the preparation of surfaces with ultra low friction properties or for the development of novel materials for drug delivery, biosensor or nanoreactor applications which rely on the fact that many proteins bind to such systems under preservation of their natural structure.
Polymerbürste, Polyelektrolytbürste, Polymethacrylsäure, Quellverhalten, Kation, polyelectrolyte, brush, swelling, radical polymerization, cation
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