Structure and Stability of Ultrathin Polymer and Nanocomposite Films at the Air-Water Interface

The main part of this work focuses on the structural investigations at the air-water interface using two different approaches. The first one is an indirect approach that aims for a structural characterization at the interface using thermodynamic quantities such as surface coverage, surface pressure and isothermal compressibility using the Langmuir-Blodgett technique. In the second approach the structural characterization is performed using local visualizing techniques like Brewster-Angle-Microscopy or X-ray surface scattering techniques. The systems studied for this purpose are all confined into a quasi-2D conformation including ultrathin polymer films, single nanoparticle films and polymer nanocomposites at the air-water interface. The indirect approach mainly focuses on the investigation of quasi-2D polymer films at the air-water interface. From the in water insoluble polymer’s point of view, the air-water interface represents a solvent that can provide either good or θ solvent conditions. The interface can be characterized using 2D scaling laws from polymer physics. In one of the systems a remarkable effect is found. The solvent properties at the air-water interface can be tuned from θ to good conditions in amphiphilic block copolymers by increasing the volume fraction of the hydrophilic block. This result was quite astonishing since the volume fractions of the hydrophilic block were less than 5 %. The same system exhibits a unique phase transition that can be linked to the hydrophobic part in the block copolymers. X-ray surface scattering reveals that upon compression to a critical surface pressure, the hydrophobic part in the blocks is able to dewet from the air-water interface. Lateral structures of several micrometer are formed within the film which can be observed by optical and scattering techniques. However, the existence of the hydrophilic block is crucial for their observation because their lateral size seems to depend on the mobility of the chains at the interface. The in-situ structural development of single nanoparticle films was observed for different particle sizes. It was possible to directly observe a structural transition that is also apparent in the indirect measurement of the thermodynamic quantities of the particle film. Finally, nanoparticles were successfully introduced in a polymer matrix at the air-water interface. Depending on the nanoparticle size, they seem to be evenly distributed or forced out of the polymer film upon increasing the area fraction of the particles.