Substrate and Chain Size Dependence of Near Surface Dynamics of Glassy Polymers

We use nanohole relaxation to study the surface relaxation of films of glassy isotactic poly (methyl methacrylate) ($i$-PMMA) films. These measurements allow us to obtain the time dependent relaxation function at a number of different sample temperatures for the first 2–3 nm of the free surface in a system often used as a model system for the effect of the substrate on thin film dynamics. The surface is observed to relax at temperatures up to 42 K below the bulk $T_g$ value, even on systems where the thin film $T_g$ is known to be greater than the bulk value. We are able to determine the range over which the substrate directly affects the free surface relaxation, and determine a surprisingly large ($M_w$ independent) limiting thickness of $\sim 180\mathrm{nm}$ where the free surface relaxation is not affected by the substrate. For thick films ($h>200\mathrm{nm}$) we find an unexpected linear $M_w$ dependence of the near surface relaxation time.