Tuning the dynamics of polymer films by adjusting the polymer-substrate interaction and substrate compliance
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Studies have shown that the dynamics of polymer supported films are strongly influenced by the specificities of the supporting surface. One key factor to the influence is the polymer-substrate interaction: the stronger this interaction is, the slower the dynamics become as the film thins. In the first part of this thesis, it is demonstrated that the dynamics of polymer films (measured by effective viscosity, ηeff) can be controlled by adjusting the polymer-substrate interactions. Polystyrene (PS) films supported by oxide-covered silicon was used as the model system. A combination of ultraviolet ozone (UVO) treatment of the polymer, and variable treatments of the substrate to change the concentrations of surface Si-OH groups, was used to adjust the polymer-substrate interaction. The ηeff of films was found to increase with the attractive interactions between the UVO-induced oxygenated groups in the polymer and the Si-OH on the substrate. Interesting implications about the dynamic properties of polymer surfaces were also drawn from the ηeff results. In the second part of this thesis, the supporting substrate was replaced by silicon covered by a soft, compliant layer of polymethylsiloxane (PDMS). Previous results showed that PS slips strongly on this surface. The ηeff of entangled PS on PDMS were measured and the films with Mw > ~393 kg/mol were found to slip in the rubbery elastic state, which is not discussed by prevailing theory. Based on the results, this study establishes for the first time that strongly slipping solid films obey the same linear relation as strongly slipping liquid films and a single friction coefficient is able to describe all the data. A microscopic model is proposed to explain the observations.