RESUMO
Molecular relaxations determine the viscoelastic properties of polymers, which, in turn, control macroscopic processes like dewetting. Here, we demonstrate experimentally that the onset of rim instabilities is correlated with the longest relaxation ("reptation") time of the dewetting polymer. Conversely, such experiments allow us to determine the reptation time of polystyrene in thin films as a function of molecular weight. Our approach opens up new possibilities for testing rheological properties of polymers confined in thin films.
RESUMO
In attempting to reduce the size of functional devices, the thickness of polymer films has reached values even smaller than the diameter of the unperturbed molecule. However, despite enormous efforts for more than a decade, our understanding of the origin of some puzzling properties of such thin films is still not satisfactory and several peculiar observations remain mysterious. For example, under certain conditions, such films show negative expansion coefficients or show undesirable rupture although energetically they are expected to be stable. Here, we demonstrate that many of these extraordinary effects can be related to residual stresses within the film, resulting from the preparation of these films from solution by fast evaporation of the solvent. Consequently, depending on thermal history and ageing time, such films show significant changes even in the glassy state, which we quantify by dewetting experiments and corresponding theoretical studies. Identifying the relevance of frozen-in polymer conformations gives us a handle for manipulating and controlling properties of nanometric thin polymer films.