Ranxing Li

May 2013
The Influence of Interface and Solvent Quality on the Dynamics of Polymer Films
Committee Members: Advisor: Ophelia Tsui, MSE/Physics; Rama Bansil, MSE/Physics; Xi Lin, MSE/ME; Soumendra N. Basu, MSE/ME; Srikanth Gopalan, MSE/ME

Abstract: As polymer films become thinner, approaching the coil size of the polymer chains, the properties of the films are seen to deviate from the bulk. For the purpose of product design and quality control, understanding the thickness dependence of the properties of polymer thin films has become more and more important. This is especially true in microelectronics applications where the thin films are reaching this limit. In this thesis, the influences of (1) the interactions at the polymer-substrate interface and (2) the conformation of the polymer chains caused by the solvent quality on the dynamics associated with the viscosity of polymer films supported by a substrate are studied using a recently developed method that is uniquely suitable for nanometer films.

The interfacial interaction between the polymer and substrate could influence the movement of polymer chains and thereby impact the dynamics and equilibration process of polymer thin films. For example, poly(methyl methacrylate) (PMMA) is known to have strong interactions with silicon. The glass transition temperature (Tg) of PMMA films supported by silicon also displayed a corroborative increase above the bulk Tg with decreasing film thickness. To investigate the effect of this interfacial interaction on the viscosity of the films, the thickness dependence of the viscosity of PMMA thin films supported by silicon was studied. The results show that the viscosity of PMMA thin films increases as the film thickness decreases. A two-layer model was developed, with a bulk-like layer stacked on top of a slow layer at the substrate, consistent with the well-known fact that PMMA interacts strongly with silicon.

In addition to the interfacial interaction, the conformation of polymer chains in thin films, in particular the entanglement density, can affect the equilibration process and viscoelastic properties. The entanglement density in a film has been shown to depend on the quality of the solvent used in the spin-coating process to make the film. The effect of adjusting the solvent quality, leading to the change of the structural properties of polystyrene films, on the way they approach equilibrium upon annealing above the Tg, is examined.