  The viscoelastic properties of thin polymeric films represent a central
issue, especially for nanotechnological applications. In particular, it is
highly relevant the dependence of viscoelasticity on the temperature. For
polystyrene it is known that the glass transition temperature is dependent on
the film thickness. At present, there is wide agreement on the importance of
the two interfaces that the films form with the air and with the substrate. The
relevance of molecular entanglement has been also stressed for the case of
suspended films. However, the role of molecular entanglement on the glass
transition temperature of supported films still remains elusive. In order to
investigate the viscoelastic properties of thin films on the nanoscale, we have
employed a scanning probe microscope suitably modified in order to monitor the
indentation of a tip into a polymeric film during a given lapse of time with
the application of a constant load. Thin polystyrene films have been prepared
on a range of different substrates: native silicon oxide, hydrogen-terminated
silicon and polystyrene brushes. In particular, we have considered polystyrene
molecules with molecular weight values below and above the critical value for
the occurrence of molecular entanglement. We find that, for samples where
molecular entanglement can occur accompanied by a strong interaction with the
substrate either by means of chemical bonds or physisorption, the glass
transition temperature of thin films increases back to values comparable with
those of thick films. This phenomenon is envisioned to be of great relevance in
those cases where one needs to improve the adhesion and/or to control the
viscoelastic properties of thin films.
