Boris Mahltig

Diblock polyampholytes at the silicon–water interface: Adsorption as a function of block ratio and molecular weight

Polyampholytes are highly charged macromolecules carrying oppositely charged functional groups. This article reports on the adsorption of a weak diblock polyampholyte, poly(methacrylic acid)-block-poly[(dimethylamino)ethyl methacrylate], as a function of the copolymer composition and molecular weight. The adsorption experiments were performed on silicon substrates from aqueous polymer solutions at different pHs. The amount of adsorbed polyampholyte chains to the surface was determined by ellipsometry, whereas lateral structures were investigated by scanning force microscopy. A strong influence of pH on the adsorbed amount and the lateral structure formation at the surface was observed. Especially at the isoelectric point, drastic changes in adsorption behavior were detected. At low molecular weights, an increased adsorbed amount was detected, a behavior in contrast to common theoretical predictions. This phenomenon is explained by the high stability of absorbed micelles, which cover the silicon surface as a dense layer. We conclude that micelle formation is an important process for polyampholyte adsorption, which needs to be taken into account more explicitly.

Metal clusters in plasma polymer matrices. Part III. Optical properties and redox behaviour of Cu clusters

Using n an inert gas evaporation technique in combination with a simultaneous plasma polymerisation, copper n clusters with mean diameters between 2.9 to 5.4 nm have been prepared within plasma polymer matrices n from hexamethyldisiloxane (HMDSO) monomer. UV-visible spectroscopy of Cu clusters embedded in a plasma polymer n film revealed a red shift and a damping of the surface plasmon frequency and resonance, respectively, with n increasing thickness and changing nature of copper oxide layers surrounding the clusters. Experimental transmission n UV-visible spectra were compared with simulations using the Maxwell–Garnett theory, which is extended to n coated spheres. Owing to larger differences in the dielectric constant CuO layers lead to a stronger n red shift than Cu2O coatings. The formation and the reduction of the two copper oxide species during n successive treatments of Cu clusters with oxygen, hydrogen and carbon monoxide was observed by in situ n diffuse reflectance measurements at a wavelength n of λ=600 nm and n a temperature of 250°C. n The redox reactions with O2, CO, H2 are reversible.

Protein adsorption on preadsorbed polyampholytic monolayers

The adsorption behaviour of five different globular proteins on pure silicon substrates and on preadsorbed polyampholytic monolayers has been investigated as a function of protein concentration. The prelayers were prepared by adsorption of the ampholytic diblock copolymer poly(methacrylic acid)-block-poly((dimethylamino)ethyl methacrylate)(PMAA-b-PDMAEMA). This polyampholyte adsorbs in densely packed micelles directly from aqueous solution. Ellipsometry was used to determine the amount of adsorbed polyampholyte and protein. While ATR-IR spectroscopy gives information about the adsorption and desorption behaviour of the preadsorbed polyampholytic layer, the lateral structures of the dried films were investigated by scanning force microscopy (SFM). The amount of protein adsorbed was found to be strongly influenced by the preadsorbed polyampholyte compared to the adsorption on the pure silicon substrates. No displacement of the polyampholyte by the proteins was detected. In most cases the protein adsorption was reduced by the preadsorbed polyampholytic layer. The observed trends are explained by the change in electrostatic and hydrophilic characteristics of the substrates. Furthermore, the entropy of adsorption has to be taken into account.

Crack propagation and defect formation at polymer interfaces investigated by ultra-small angle X-ray scattering

The double cantilever beam test (DCB) was used to estimate the fracture toughness of poly(methyl-methacrylate) n(PMMA). With the help of given neutron reflectometry (NR) results for PMMA a correlation between fracture toughness and interface width was established. It displays a three regime behaviour similar to polystyrene (PS). For the first time, deformation and defect structures in the vicinity of the crack tip were investigated by scanning ultra-small angle X-ray scattering (S-USAX) where a beam from a synchrotron source is scanned over the sample. It is concluded that crack propagation during the DCB measurement is connected with large deformations and defect formation, where the plastic deformation zone extends over a large size (>400 μm). Following a model for scattering from diffuse boundaries it was concluded that the diffusiveness increases from the outer edges to the centre of the plastic zone. S-USAX thus provides details of sample failure at a microscopic level as well as information about the structure of the defect boundaries and their distribution across the plastic zone.

Desorption behaviour of regular adsorbed polyampholytic layers

The desorption behaviour of the diblock polyampholyte PMAA-b-PDMAEMA, poly(methacrylic acid)-block-poly((dimethylamino)ethyl methacrylate), preadsorbed on silicon substrates was investigated under the influence of several desorption agents. The investigated polyampholyte is known to adsorb in regular structures directly from aqueous solutions onto silicon substrates. While the adsorption process is mainly determined by electrostatic interactions, two kinds of desorption mechanism should be assumed. The first mechanism is based on changed electrostatic conditions caused for instance by a strong change in pH of the aqueous solution. The other mechanism is observed after treatment with hydrophobic organic solvent, which leads to the desorption of hydrophobic adsorbed polyampholyte chains, while the electrostatically attached ones will not be influenced. To complete the desorption experiments with organic solvents also adsorption experiments from analogous polyampholytic solutions in the same organic solvents were performed. The amount of polymer at the substrate surface after adsorption or desorption experiments was determined using ellipsometry. Atomic force microscopy (AFM) was used to investigate the surface topography of dried samples after the desorption process.