Grazing-incidence x-ray scattering studies on surface melting in ultrathin polymer films

The aim of this study is to investigate surface crystal structures formed in ultrathin (thickness below 100 nm) polymer films by using surface-sensitive x-ray scattering techniques. This study was motivated by our current experimental finding that showed a drastic suppression (the decrease of $\sim $50\r{ }C) in the surface melting temperature (T$_{m})$ of ultrathin polymer films, which was determined as the onset of surface softening by using the shear modulation force microscopy (SMFM) method[1]. In order to clarify the relationship between the melting behavior and surface crystal structures, we integrated a variety of grazing-angle x-ray scattering techniques including reflectivity, diffuse scattering, grazing-incidence diffraction, and grazing-incidence small-angle scattering. As a result, we found that diffuse scattering, which is sensitive to surface roughness, drastically changed at T$_{m}$ determined from SMFM, while the surface crystallinity decreased with increasing temperature, but remained up to the bulk melting temperature. A model to explain the mechanism of the surface melting will be discussed. [1]Wang, Y. et al. Macromolecules, \textbf{37}, 3319 (2004).