Probing Properties of Polymers in Thin Films Via Dewetting

In our quest to make functional devices smaller, the thickness of polymer films has reached values even smaller than the diameter of the unperturbed molecule. Many experimental studies have been devoted to the determination of the behavior of such thin films as a function of film thickness and temperature. However, despite enormous efforts over the last few decades, our understanding of the origin of some puzzling properties of such thin films is still not satisfactory and several peculiar observations remain rather mysterious. In this context, we explore the consequences of film preparation, i.e., the transition from a dilute polymer solution to the glassy state, with respect to the properties of polymers in thin films. This transition is likely to result in residual stresses arising from out-of-equilibrium chain conformations due to rapid solvent loss. Consequently, depending on thermal history and ageing time, such films exhibit significant changes even in the glassy state, which we can quantify by performing detailed studies of visco-elastic dewetting of thin polystyrene films on solid substrates. We explore relaxation times, residual stresses, and temporal changes to the stability of non-equilibrated thin films as they progress toward stable equilibrium behaviors. We present some tentative ideas on the relation between the observed atypical mechanical and relaxational behavior and the metastable states introduced by sample preparation.

3D view (measured by atomic force microscopy) of a typical hole obtained by dewetting a polystyrene film on a PDMS-coated substrate at temperatures close to the glass transition of polystyrene.

h(x,t) is the profile of the film, h ₀ is the initial height of the film, H(t) is the height of the front, L(t) is the dewetted distance, W(t) is the width of the rim, and v(x,t) is the velocity of the film. Typical 3D cross-sections show the asymmetric shape of the rim at early stages and comparatively low temperatures (left) and the more symmetric shape at late stages or at high temperatures (right).