Polymer brushes are representing a versatile tool to adjust and control surface phenomena like wetting, adsorption and adhesion. Moreover, certain complementary surface modification methods can support improving their intended applications. A promising strategy is the combination of the grafting-to method under initiator and solvent free conditions with an additional electron-beam irradiation step which can lead to polymer brushes with high-selective properties.The experiments were focused on the investigation of the surface reactions taking place during the dose-dependent irradiation of the thin polymer brush layers with the aim to modify their properties. A comprehensive and detailed study of the initiated functionalization reactions associated with their influence on the alteration of surface properties of different homo and binary polymer brush layers was performed using sensitive surface characterization techniques such as XPS, contact angle measurements, and ellipsometry.

The capability of mixed polymer brush systems to modify and switch physico-chemical properties of biointerfaces is demonstrated in this paper. Mixed brushes were composed of the temperature sensitive polymer poly(N-isopropyl acrylamide) (PNIPAAM) and two polyelectrolytes, poly(2-vinylpyridine) (P2VP) and poly(acrylic acid) (PAA). Using these polymers binary brushes were prepared, the first one consisting of PNIPA AM P2VP representing a system sensitive for temperature and pH changes, and the second one consisting of two polyelectrolytes (P2VP-PAA) showing sensitivity to pH changes of the surrounding fluid. Selected aspects concerning the environmental sensitivity as well as the protein adsorption affinity of these brushes are discussed. The stimuli-response towards temperature (PNIPAAm-P2VP) and pH (P2VP-PAA) of these binary polymer brushes was utilized to switch the adsorbed amount of protein and finally to regulate cell adhesion at brush coated surfaces.

Temperature-sensitive poly(N-isopropylacrylamide) (PNIPAAm) brushes with different molecular weights M(n) and grafting densities σ were prepared by the "grafting-to" method. Changes in their physicochemical properties according to temperature were investigated with the help of in situ spectroscopic ellipsometry and in situ attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy. Brush criteria indicate a transition between a brush conformation below the lower critical solution temperature (LCST) and an intermediate to mushroom conformation above the LCST. By in situ ellipsometry distinct changes in the brush layer parameters (wet thickness, refractive index, buffer content) were observed. A broadening of the temperature region with maximum deswelling occurred with decreasing grafting density. The brush layer properties were independent of the grafting density below the LCST, but showed a virtually monotonic behavior above the LCST. The midtemperature ϑ(half) of the deswelling process increased with increasing grafting density. Thus grafting density-dependent design parameters for such functional films were presented. For the first time, ATR-FTIR spectroscopy was used to monitor segment density and hydrogen bonding changes of these very thin PNIPAAm brushes as a function of temperature based on significant variations of the methyl stretching, Amide I, as well as Amide II bands with respect to intensity and wavenumber position. No dependence on M(n) and σ in the wavenumber shift of these bands above the LCST was found. The temperature profile of these band intensities and thus segment density was found to be rather step-like, exceeding temperatures around the LCST, while the respective profile of their wavenumber positions suggested continuous structural and hydration processes. Remaining buffer amounts and residual intermolecular segment/water interaction in the collapsed brushes above the LCST could be confirmed by both in situ methods.

Protein adsorption, as the primary process occurring when a foreign surface comes into contact with a biosystem, was studied on thin polymer brush films consisting of poly(N-isopropylacrylamide) (PNIPAAm) and poly(2-vinylpyridine) (P2VP). These films were prepared by the "grafting to" method. The protein resistance of stimuli responsive PNIPAAm-brushes toward serum albumin was recorded and compared with protein adsorption on P2VP brushes. To achieve a better understanding of protein resistance, PNIPAAm brushes with different molecular weights were investigated below and above the lower critical solution temperature of 32 degrees C. To use these findings for the adjustment and switching of protein adsorption, in a first attempt the adsorption on a mixed brush system consisting of PNIPAAm and P2VP chains was studied. This system showed temperature-dependent adsorption behavior due to the presence of PNIPAAm, representing a smart surface with stimuli-responsive changes in the physicochemical surface properties. With this mixed brush, the adsorbed amount of protein could be controlled, depending on composition and the temperature of the surroundings.

The aim of our work was to understand the impact of electron treatment on polymer thin films, particularly on their surface properties as well as the possibilities and limitations to tune these properties. Two different polymers, polystyrene (PS) and poly-2-vinlypyridine (P2VP), were chosen to form thin polymer films by grafting of end-terminated linear polymer chains to a surface with sufficient grafting density, forming so called "polymer brushes". We were able to identify the surface properties and specify ongoing physico-chemical changes after electron beam treatment by using zeta potential and contact angle measurements. By varying the absorbed dose it was possible to tune the surface properties over a wide range. The detailed knowledge about the latitude of functionalization of the tested polymers was a prerequisite for the creation of wettability gradients by electron beam treatment by adapting a special mask of known thickness and density. Hence, electron beam treatment opens an easy reproducible way to generate surface gradients in functionality.

Poly(N-vinyl pyrrolidone) (PVP) is a widely used biocompatible polymer. PVP can be crosslinked by electron beam irradiation even in dry state. In contact with water it forms hydrogels. In a previous work we analyzed the crosslinking behavior of PVP bulk gels and thin PVP films on silicon wafers under electron beam irradiation. In this work, we applied the electron beam lithography on dry PVP films. Different patterns with a width of less than 100 nm were written into the film with the e-beam of a scanning electron microscope. The topography of the patterned film is investigated by atomic force microscopy. For further applications, we tried to create a gradient of crosslinking density within the film using a combination of e-beam irradiation and e-beam lithography. (C) 2007 Wiley Periodicals, Inc.

Binary brushes constituted from two incompatible polymers can be used in the form of ultrathin polymeric layers as a versatile tool for surface engineering to tune physicochemical surface characteristics such as wettability, surface charge, chemical composition, and morphology and furthermore to create responsive surface properties. Mixed brushes of oppositely charged weak polyelectrolytes represent a special case of responding surfaces that are sensitive to changes in the pH value of the aqueous environment and therefore represent interesting tools for biosurface engineering. The polyelectrolyte brushes used for this study were composed of two oppositely charged polyelelctrolytes poly(2-vinylpyridine) (P2VP) and poly(acrylic acid) (PAA). The in-situ properties and surface characteristics such as as surface charge, surface tension, and extent of swelling of these brush layers are functions of the pH value of the surrounding aqueous solution. To test the behavior of the mixed polylelctrolyte brushes in contact with biosystems, protein adsorption experiments with globular model proteins were performed at different pH values and salt concentrations (confinement of counterions) of the buffer solutions. The influence of the pH value, buffer salt concentration, and isoelectric points (IEP) of the brush and protein on the adsorbed amount and the interfacial tension during protein adsorption as well as the protein adsorption mechanism postulated in reference to recently developed theories of protein adsorption on polyelectrolyte brushes is discussed. In the salted regime, protein adsorption was found to be similar to the often-described adsorption at hydrophobic surfaces. However, in the osmotic regime the balance of electrostatic repulsion and a strong entropic driving force, "counterion release", was found to be the main influence on protein adsorption.

Biocompatible hydrogels based on poly(N-vinyl pyrrolidone) (PVP) were synthesized by electron beam irradiation of the dry polymer under various conditions. Sol–gel analysis of the bulk gel (in mm range) gave a dose of gelation of 94kGy. As seen for various other polymers, the network density rises with the increase in dose. At around 350kGy, PVP began to decompose. Based on these observations, films in μm range on a silicon wafer were synthesized by electron beam irradiation. Due to irradiation, the films adhered irreversibly on the wafer. Their swelling behavior was analyzed by ellipsometry.