Thin films and coatings

In spite of a wealth of experimental and theoretical studies on this topic, there is still an intense controversy over the role played by interfaces on the occurrence of Tg. We will show that by performing Atomic Force Microscopy measurements of pull-off force measurement as a function of the temperature, we were able to probe the dynamic of supported thin polystyrene (PS) thin films. We demonstrated the existence of two transition temperatures that can be associated to the relaxation of polymer chains located at different depth regions within the polymer film. Independently of the film thickness, we have confirmed the presence of a region of high mobility for the polymer chains at the free interface. We claim here that our results demonstrate, in agreement with other techniques, the stratification of thin polymer film depth profile in term of relaxation behavior.

Stability of thin polymer films is of paramount importance for a variety of applications. In a second part of our presentation, by adopting a simple top-down method based on good solvent rinsing, we were able to prepare flat polystyrene films with a controlled thickness ranging from 1.3 nm to 7.0 nm. Their stability was scrutinized after a classical annealing procedure above the glass transition temperature. The Lifshitz van der Waals intermolecular theory that predicts the domains of stability as a function of the film thickness is in accordance with our experimental observations. We surmise that it is due to the solvent rinsing procedure that removes the residual stress and/or the density variation of the polystyrene films inhibiting thermodynamically the dewetting.