Switching Layer Stability in a Polymer Bilayer by Thickness Variation

We use optical and scanning force microscopy to explore the possibility of switching the stability of a bilayer of poly(methyl methacrylate) (PMMA) on polystyrene by simply changing the film thickness. We show that for thin PMMA layers on thicker polystyrene films the PMMA layer is unstable to thickness fluctuations. However, polystyrene layers are unstable when they are substantially thinner than the now stable PMMA film. Dewetting morphologies are cataloged as a function of the thickness of individual polymer layers by identifying which layer is unstable by which mechanism, be it spinodal dewetting or heterogeneous thermal nucleation. Our results are in good agreement with a linear stability analysis of the influence of long-range dispersion forces, but also indicate the influence of film preparation and small variations of material properties.

Figure 1

SFM topography data and schematic diagrams showing (top) the dewetted morphology of the capping PMMA layer in a bilayer of a 22 nm thick PMMA layer ($M_w=19.3\mathrm{kDa}$) upon an 87 nm thick PS layer ($M_w=273\mathrm{kDa}$) after annealing at $160°\mathrm{C}$ for 5 min, and (bottom) the dewetted morphology of a buried PS layer in a bilayer of a 144 nm thick PMMA layer ($M_w=19.3\mathrm{kDa}$) film on 16 nm thick PS ($M_w=22.2\mathrm{kDa}$) after annealing at $160°\mathrm{C}$ for 4 min after the PMMA was selectively dissolved in an acetic acid rinse.

Figure 2

Plot of dominant rupture length scale derived from the SFM topography for (top) a PMMA layer of thickness $h$ dewetting from PS layers of 87 and 9 nm, and (bottom) a buried PS layer of thickness $L$ dewetting a capping PMMA layer of 144 nm. Fits are to Eq. (6) using the appropriate free energy. The dashed lines delimit a region that we consider provides satisfactory fits to $\lambda$ and are used to calculate the uncertainty in the limits of stability shown in Fig. 3. Fits are made using an effective interfacial energy [13] $\gamma =1.45\mathrm{mJ}{\mathrm{m}}^{-2}$ for the PMMA layer [13] and $\gamma =1.52\mathrm{mJ}{\mathrm{m}}^{-2}$ for buried PS layer at $160°\mathrm{C}$. (This is simply the PS/PMMA interfacial tension.)

Figure 3

Phase diagram for the Si/PS/PMMA experimental bilayer: (⋄) PMMA unstable and PS stable; (○) PS unstable and PMMA stable; (□) bilayer destroyed by thermal nucleation; (▵) unidentified. The fits represent the stability transition boundaries predicted by Eqs. (5, 6). The gray shaded region indicates unstable PMMA and stable PS, the white region indicates the region of unstable PS and stable PMMA, and the area of overlap is indicated by a mix of gray and white. The thinner broken lines represent the error on the PMMA fits; for clarity the corresponding PS errors are not shown. Within error the stability limits coincide, and therefore there is an immediate transition from an unstable PMMA layer to an unstable PS layer such that no region of stability in both layers is observed.