Transition of Linear to Exponential Hole Growth Modes in Thin Free-Standing Polymer Films

We have studied the formation and growth of holes in free-standing polystyrene films. For thick (∼μm) films and sufficiently high temperatures, exponential hole growth vs time was observed. Using the theory of deBregeas et al. [Phys. Rev. Lett. 1995, 75, 21], relating growth rate to viscosity (η), we confirmed bulk scaling η ∝ MW^3.2±0.2, where MW is the polymer molecular weight, and obtained good quantitative agreement with bulk viscosities. As temperature is lowered and/or films are made thinner, a transition to a linear growth mode of hole radius vs time is observed. The molecular weight dependence of growth velocity (V) for temperatures near the glass transition temperature (T_g) is V ∝ MW^0.5±0.2. Our results also show an intermediate regime where growth is exponential, but bulk viscosities are not observed. Ellipsometry measurements, atomic force microscopy cross-section analysis, and optical images show no rim formation and uniform film thickening as holes grow.