Study On The Viscoelasticity Of Polymer Thin Films By Monitoring The Development Of Wetting Ridge At Their Surfaces

Viscoelasticity of polymer thin films reveals the inherent chain dynamics in thin films which is critical to the application and processing of materials. The viscoelasticity of polymer thin film deviates from that of the bulk due to the nano-confinement effect. Measuring the viscoelasticity of polymer thin film remains an unsolved problem. Although considerable approaches have been developed for the viscoelasticity measurement of thin film, much controversy remains due to apparently con?icting results. Developing new experimental approaches are of signi?cance, both in terms of unveiling the mechanisms controlling the chain dynamics confined in thin film and for providing effective ways for resolution of the controversies in understanding the chain dynamics of polymer in constrained environment. In this thesis, the relationship between the viscoelasticity of the thin polymer films and the development of the wetting ridge at their surfaces was studied in details. The main conclusions are shown as follows:(1)A novel method was developed to detect the viscosity of thin polystyrene(PS) films by correlating the relationship between the viscoelasticity of the PS films and the development of wetting ridge at their surfaces. The development of the wetting ridge caused by the surface tension of ionic liquids at different temperatures was studied. It was found that when the time was long enough, the height of the wetting ridge increases linearly with time. The slope(k) represented the deformation rate of the PS films under the surface tension of the ionic liquds and was found to be closely linked with the viscosity of the PS films. Then the relationship between the slope(k) derived from different temperatures,PS molecular weights and surface tensions of ionic liquids and the viscosity of PS(η) was studied. It was found that k and η obeys the following formulation: η=0.37γsinθ/k. This result suggests that the viscosity of polymer films can be evaluated by monitoring the linear increase of the height of the wetting ridge with time.(2) The viscosity of thin polystyrene(PS) films on different substrates was studied using the current method. The viscosity increased with the decrease of film thickness for the PS thin films supported on Si-H, while the viscosity decreased with the derease of film thickness for the PS thin films supported on Si O2. The difference between the two kinds of thin films was attributed to the interface effect between the PS and the two different substrates. The critical thickness below which the viscosity of thin films showed thickness dependence was found to be around 10 Rg, for the PS thin films both on Si O2 and Si-H, which is in consistence with literature.(3) The plateau modulus and viscos flow temperature(Tf) of polystyrene films were studied by monitoring the height of wetting ridge at different temperatures. The height of the wetting ridge at different temperatures was studied. It was found that there exists a plateau area where the height of the wetting ridge remains unchanged with temperature and molecular weight. This area was found to be corrected with the plateau modulus of the PS films. The modulus of PS films estimated from this plateau height was ~105.85 which is nearly the same with the plateau modulus of PS reported in literature. This result suggests that the plateau modulus of Polymer films can be extracted from the plateau height of the wetting ridge. The critical temperature(T*) above which the plateau area ends was studied. It was found that T*and Tf shares the same molecular weight dependence and obey the following relationship: Tf=10℃+ T*. This result shows that the viscous flow temperature of polymer films can be extracted from this critical temperature.