Grafted Molecular Layers for Control of Surface Properties

Sammanfattning: The goal of this thesis work was to develop responsive surface grafted brushlayers for control of surface properties and to gain insights in the molecular mechanisms that control these properties. Three types of grafted layers were investigated, as outlined below. In the first system studied, poly(AAc) was synthesized by a grafting from approach, utilizing a photopolymerization reaction from a macroinitiator cast onto QCM substrates. The responsiveness in terms of frequency change, Δf, of the resulting brushes to changes in bulk pH was studied with QCM. Further, the friction properties of poly(AAc) was elucidated with colloidal probe AFM as a function of pH and counterion valency. High friction (μ=0.27) was found in presence of CaCl2 at high pH (7.5), but not under any other condition explored. It was concluded that the high friction was due to intralayer COO--Ca2+--OOC bridges. QCM-D was utilized for studying viscoelastic properties of growing poly(AAc) films during in situ photopolymerization. By Voigt modeling, the thickness, shear elasticity and shear viscosity were extracted. These parameters were observed to undergo sudden transitions at a critical thickness, and from this thickness the grafting density of the growing poly(AAc) layers was determined. In addition, the sensitivity to changes in Δf and ΔD with respect to the thickness of the poly(AAc) films was investigated, and the results showed that high sensitivity in ΔD is retained at higher film thicknesses than for Δf, and that the sensitivity with respect to noise can significantly alter the thickness that is best suited forthe study of viscoelastic changes in sensor applications. The work with QCM-D also involved the detection of structural variations within a thick brush layer of poly(AAc). Since lower overtones have higher penetration depth, these frequencies sense polymer segments further out in the brush. It was found that the apparent pKa of the poly(AAc) was higher for lower overtones, indicating therelative ease of acid dissociation in segments further out in the polyelectrolytebrush. In the second system studied, phenylethylamine (PEA) was electrografted to flatsurfaces of glassy carbon (GC). The nanomechanical properties, such as topography, deformation, adhesion and dissipation, were investigated using PeakForce quantitative nanomechanical mapping (QNM). One main finding is that globular domains of 40 to 50 nm indiameter appear in the electrografted PEA layer. They are assigned to clusters of PEA formed due to less rapid reactions between radicals and the GC surface compared to reactions with already grafted PEA. The interactions between the PEA layer and a silica sphere were further investigated by surface force measurements. A main finding is that the PEA surfaces were heavily charge regulated due to the limited net charge of the PEA layer compared to that of silica. In the third system studied, the mechanical response of polyelectrolytes as a function of applied load, probing angle and pH was investigated with PeakForce QNM. The used polyelectrolyte was poly(2-dimethylaminoethyl methacrylate)(PDMAEMA) with pKa of 6.5 to 7.5 and grafted to silica nanoparticles. While most research on polyelectrolyte brushes is conducted by employing flat and smooth surfaces, with a roughness on the nanometer scale, real surfaces are rarely ever ideally flat but rather they possess topographic irregularities on nano- and micro-scales, which locally imparts high curvatures. The spherical geometry of the core-shell nanoparticles serves as a model for real surfaces with respect to topographical irregularities of real surfaces. The nanoscale brushes were probed with an ultrasharp AFM tip, providing nanoscale resolution of topography, deformation, adhesion and dissipation. It was found that the mechanical response of the polyelectrolyte corona is dependent on the applied load and the polar angle of the tip-brush interaction. All nanomechanical data show a non-monotonic variation with horizontal position, and the peak values are shifted in magnitude and position as a function of peak force. The effect of pH on deformation was further investigated. The results showed that the brush is more resistant against compression over the centre than it is to deflection at larger horizontal positions, and this effect is amplified by charging the brush. This work provided understanding of the direction dependence of the mechanical properties and is relevant for the design of brush boundary lubricating agents for rough surfaces, where the polymer chains are both being bent and compressed under the influence of load and shear.