Viscoelastic and glass transition properties of ultrathin polystyrene films by dewetting from liquid glycerol

We have used the liquid dewetting method originally proposed by Bodiguel and Fretigny (Eur. Phys. J. E: Soft Matter Biol. Phys. 2006, 19 (2), 185-193) to study thickness effects (ranging from 4 to 154 nm) on the viscoelastic and glass transition behaviors of ultrathin polystyrene (PS) films. PS with molecular weights of 278 and 984 kg/mol and various thermal treatments were examined. Both glass transition temperature (T_g) reduction and film stiffening in the rubbery plateau regime were observed in the PS films as film thickness decreased. The value of the plateau compliance was found to vary approximately linearly on the log plot with the film thickness. No molecular weight effect was found for the dewetting behaviors of the PS films prior to the terminal flow regime, where the majority of the current work was carried out. The present results show that the film dewetting process for the polystyrene/glycerol couple has to be described as a "non-isothermal" experiment due to the changing glass transition as the film thickens. A numerical method is proposed to correct the experimental data to the "isothermal" and constant thickness condition. Interestingly, even though in the dewetting experiment the polymer thin films were exposed to the liquid glycerol, which was originally thought to be a condition similar to that of freely standing films, the film thickness dependence of the T_g is, in fact, similar to but weaker than, what is observed for PS thin films supported on SiO₂ substrates.