Conflicting Confinement Effects on the T_g, Diffusivity, and Effective Viscosity of Polymer Films: A Case Study with Poly(isobutyl methacrylate) on Silica and Possible Resolution

The glass transition temperature (T_g), in-plane diffusivity (D), and effective viscosity (η_eff) were measured for the same thin film system of poly(isobutyl methacrylate) supported by silica (PiBMA/SiOx). We found that both the T_g and D were independent of the film thickness (h₀), but η_eff decreased with decreasing h₀. We envisage the different h₀ dependencies to be caused by T_g, D, and η_eff being different functions of the local T_g’s (T_g,i) or viscosities (η_i), which vary with the film depth. By assuming a three-layer model and that T_g(h₀) = ⟨T_g,i⟩, D(h₀) ∼ k_BT/⟨η_i⟩, and η_eff(h₀) = h₀³/3M_tot(η_i), where ⟨...⟩ denotes spatial averaging and M_tot is the mobility of the films, we were able to account for the experimental data. By extending these ideas to the analogous data of polystyrene supported by silica (PS/SiOx), a resolution was found for the long-standing inconsistency regarding the effects of confinement on the dynamics of polymer films.