Structural characterisation of supported thin polymer films

(eng) X-ray reflectometry was used to elucidate the question of the density evolution of polymers near solid substrates, by determining density profiles on thin polymer films. Secondly, atomic force microscopy allowed us to characterise the structure of free surfaces of polymers films and finally neutron reflectometry led to complete the study of the structure of thin polymer films, for both the buried and free interfaces. The XRR and NR studies of different polymer films deposited onto silicon wafers covered with their oxide indicate perturbations of the density profile near the solid substrate. The importance of these effects would correlate with the strength of the interactions developed between the polymer and silicon oxide. The analysis of the free surface of the films by AFM showed that, when polymer films are strictly thicker than 4Rg, the power spectral densities follow satisfactorily the capillary wave fluctuation theory. This is, to our knowledge, the first time that this behaviour is put in evidence in direct space. For thinner films, despite the fact that long-range van der Waals forces should stabilise polystyrene films on silicon substrates against height fluctuations, evidences were provided that amplification of capillary waves of specific wavelengths is nevertheless possible for films as thick as 15 nm, ultimately leading to dewetting in a “spinodal-like” process. The process was shown not to be limited to polystyrene films. Such forces most probably result from the out-equilibrium structure of the film after spin-coating, especially when the thickness of the film is in the range of a few radii of gyration. This indicates that the structure of spin-coated films may not be fully relaxed, even after long annealing times. Since such out-of-equilibrium films are those used for technological applications, our study is thus fully relevant.