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.

Figure 1

Surface hole relaxation at 287 K of 60 nm thick $i$-PMMA ($\mathrm{Mw}=212.4\mathrm{k}$) film on Si; the solid curve is the single exponential fit. The inset shows some surface holes in the AFM image (left) and the line scan (right) for one surface hole (marked as a cross).

Figure 2

Temperature dependence of the average lifetime for the first few nm of the $M_w$ 889k $i$-PMMA surface for films with $h\gtrsim 180\mathrm{nm}$. The solid curve is the relaxation curve for bulk $i$-PMMA [19] and the dashed curve is the same curve with $T_{\infty }$ shifted by 41 K from the bulk curve.

Figure 3

Normalized surface hole lifetime vs thickness of $i$-PMMA films of different molecular weights on two kinds of substrates: 212.4k $i$-PMMA on Si (◯), 889k $i$-PMMA on Si (□), 212.4k $i$-PMMA on Al (△), 889k $i$-PMMA on Al (▿); the solid curves serve as guides for the eye. The inset shows the surface hole lifetimes, and ellipsometrically measured $T_g$ values of thick $i$-PMMA films ($h\gtrsim 180\mathrm{nm}$) on Si for three molecular weights: 212.4k (◯), 436k (◊), and 889k (□).