Dynamic studies of nano-confined polymer thin films
Polymer thin films with the film thickness (h0) below ~ 100 nm often exhibit physical properties different from the bulk counterparts. In order to make the best use of polymer thin films in applications, it is important to understand the physical origins of these deviations. In this dissertation, I will investigate how different factors influence dynamic properties of polymer thin films upon nano-confinement, including glass transition temperature (Tg), effective viscosity (ηeff) and self-diffusion coefficient (D). The first part of this dissertation concerns the impacts of the molecular weight (MW) and tacticity on the Tg’s of nano-confined polymer films. Previous experiments showed that the Tg of polymer films could be depressed or increased as h0 decreases. While these observations are usually attributed to the effects of the interfaces, some experiments suggested that MW’s and tacticities might also play a role. To understand the effects of these factors, the Tg’s of silica-based poly(α-methyl styrene) (PαMS/SiOx) and poly(methyl methacrylate) (PMMA/SiOx) thin films were studied, and the results suggested that MW’s and tacticities influence Tg in nontrivial ways. The second part concerns an effort to resolve the long-standing controversy about the correlation between different dynamics of polymer thin films upon nano-confinement. Firstly, I discuss the experimental results of Tg, D and ηeff of poly(isobutyl methacrylate) films supported by silica (PiBMA/SiOx). Both Tg and D were found to be independent of h0, but ηeff decreased with decreasing h0. Since both D and ηeff describe transport phenomena known to depend on the local friction coefficient or equivalently the local viscosity, it is questionable why D and ηeff displayed seemingly inconsistent h0 dependencies. We envisage the different h0 dependencies to be caused by Tg, D and ηeff being different functions of the local Tg’s (Tg,i) or viscosities (ηi). By assuming a three-layer model, we were able to account for the experimental data and resolve the inconsistency. By extending the same ideas to the analogous data of silica-based polystyrene films (PS/SiOx), we found a resolution to the inconsistency regarding the effects of nano-confinement on the dynamics of polymer thin films.