Influence of film-substrate adhesion on the mechanical properties of thin polymer films

The mechanical properties of mechanical double layers (thin amorphous films of poly(n-butyl methacrylate) on considerably stiffer silicon substrates) were investigated by force-displacement curves acquired with an atomic force microscope. The substrate surface was modified to provide a hydrophilic and a hydrophobic moiety. Such a modification affects the adhesion between film and substrate, since the polymer film adheres strongly on the hydrophilic side and weakly on the hydrophobic one. In this way, we were able to investigate the influence of the adhesion between the polymer film and substrate on the mechanical properties of the mechanical double layer. We were able to show that force-displacement curves on the two moieties of the sample have a different shape, and we propose an interpretation of the mechanisms determining the force-deformation curves on films with small adhesion to the substrate. This experiment can also provide an approach to the question of how the mechanical properties of a bulk material change with the volume to surface ratio, since a film with vanishing adhesion to the substrate is the only experimental accessible setup approximating a free-standing thin film.