Dewetting of thin polymer films: Influence of interface evolution

The dewetting dynamics of ultrathin polymer films, e.g. in the model system of polystyrene on a polydimethylsiloxane-covered substrate, exhibits interesting behavior like a fast decay of the dewetting velocity and a maximum in the width of the built-up rim in the course of time. These features have been recently ascribed to the relaxation of residual stresses in the film that stem from the nonequilibrium preparation of the samples. Recent experiments by Coppée et al. on PS with low molecular weight, where such stresses could not be evidenced, showed however similar behavior. By scaling arguments and numerical solution of a thin film viscoelastic model we show that the maximum in the width of the rim can be caused by a temporal evolution of the friction coefficient (or equivalently of the slip length), for which we discuss two possible mechanisms. In addition, the maximum in the width is affected by the sample age. As a consequence, knowing the temporal behavior of friction (or slip length) in principle allows to measure the aging dynamics of a polymer-polymer interface by simple dewetting experiments. Introduction. – Dewetting experiments [1–3] have been proven to be a simple tool to investigate thin polymer films on various substrates [4,5]. Especially the model system of the incompatible polymers polystyrene (PS) on polydimethylsiloxane (PDMS) grafted on a silicium substrate has attracted numerous experimental investigations. There is consensus that friction and slippage at the interface, as well as the viscoelasticity of the dewetting film are crucial to understand the experiments [6, 7]. To account for the occurrence of a maximum in the width of the rim in the course of time, and to interpret the effects of sample aging on this peculiar feature of the thin films’ dewetting dynamics, residual stresses in the PS film have been shown to be important [8–10]. These stresses are supposed to stem from the sample preparation process (usually spin-coating) and represent an additional degree of freedom that can (partially) relax during dewetting. For high enough molecular weights, residual stresses have been clearly evidenced in various systems [8, 11, 12]. However, their relaxation well below the glass temperature, needed to interpret the effects of aging [8], is currently debated. An additional puzzle is the fact that for films of low molecular weight no such relaxation of stress could be evidenced, while a maximum in the rim width and its dependence on aging have still been found. Very recently, in parallel to dewetting experiments the interface between the two polymer species has been studied by neutron reflectometry [13] and interdiffusion of the PS-PDMS interface has been found, for samples aged well below the PS bulk glass temperature. In this letter we show by scaling arguments and demonstrate by numerical solution of a thin film model that a slow increase in the friction coefficient or, equivalently, a decrease of the slip length can give rise to a maximum in the rim width. This evolution of friction/slip might be due to several mechanisms: first, as suggested by the neutron reflectometry measurements [13], a slow roughening of the film-substrate (PS-PDMS) interface with concomitant increase in friction may be responsible. A second possibility would be that (for the lowest PDMS grafting densities) a small number of PS chains may enter the PDMS brush and become attached to the substrate. It is known that minute amounts of such ’connectors’ [14–16] already decrease the slip length. A second finding is that the value of the maximum rim width decreases monotonously with the friction coefficient at the beginning of the dewetting process. This could explain the experimentally observed decrease of the maximum with the sample age. Thus knowing the evolution of the friction coefficient allows in principle to re-