Decoupling between Diffusivity and Effective Viscosity in Poly(isobutyl methacrylate) Films with a Thickness-Independent Glass Transition

We report measurements of self-diffusion ($D)$ and effective viscosity ($\eta _{\mathrm{eff}})$ on silica-supported poly(isobutyl methacrylate) (PiBMA) thin films. These films had been found to exhibit thickness ($h_{\mathrm{0}})$ independence in the glass transition temperature, $T_{\mathrm{g}}$ ($=$ 58 $^{\mathrm{o}}$C). At $T \quad =$ 106 $^{\mathrm{o}}$C, $D$ was independent of $h_{\mathrm{0}}$, consistent with $T_{\mathrm{g}}$. On the other hand, $\eta_{\mathrm{eff}}$ decreased with decreasing $h_{\mathrm{0}}$, indicating decoupling between $D$ and $\eta _{\mathrm{eff}}$. We contemplate that the decoupling is caused by dynamic heterogeneity in the film and that $D$ and $\eta_{\mathrm{eff}}$ are different dynamic averages. Specifically, by using a layer model, where the film is divided into sub-layers with thickness $h_{\mathrm{i}}$ and local viscosity $\eta_{\mathrm{i}}$, and assuming that $D$ \textasciitilde $k_{\mathrm{B}}T$/\textless $\eta_{\mathrm{i}}$\textgreater and $\eta _{\mathrm{eff}}$ \textasciitilde \textless $h_{\mathrm{i}}^{\mathrm{3}}$/(3$\eta_{\mathrm{i}})$\textgreater , we are able to account for all the measurements.