Self-dewetting of perfluoroalkyl methacrylate films on glass

Side chain crystallization in spin cast films of polymethacrylates with perfluoroalkyl substituents (i.e., F(CF2)(n)(CH2)(m)-, n = 8, m = 2 and n = 10, m = 6) is shown to yield double layers of the hydrocarbon backbone and fluorocarbon side chains. Disordering to a smectic mesophase has been observed by differential scanning calorimetry and X-ray scattering. Regular packing of the side chains in the top layer at the polymer-air interface resulted in a very low critical surface tension of ca. 6 mN/m. Above the bulk isotropization temperature of the material, the multilayered structure collapsed resulting in a peculiar self-dewetting which could be observed directly by scanning force microscopy. On the basis of the dewetting behavior, three states have been distinguished depending on the film thickness: (i) At distances at least more than seven bilayers away from the flat substrate, the film melted according to the bulk isotropization temperature and demonstrated macroscopic dewetting of an ordered sublayer. (ii) Also this sublayer was not stable above the isotropization transition, but collapsed in a peculiar stepwise manner. A bilayer started to dewet the next bilayer only after the top layer had collapsed into microscopic droplets. This repetitive process proceeded until (iii) a thin film with the thickness of only one and a half bilayers remained to cover the substrate. This layer was stable even when the temperature was raised 50 degrees C above the isotropization temperature. Apparently, the geometric constraint by the ''hard wall'' and the interactions with the substrate caused this stabilization. The self-dewetting is discussed in terms of a progressive disordering of the side chains with increasing distance from the wall.