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Glass Transition Temperature in Polystyrene Supported Thin Films: a SPM-based Investigation of the Role of Molecular Entanglement

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.