Dewetting Rheology for Determining Viscoelastic Properties of Non-equilibrated Thin Polymer Films

We performed systematic dewetting experiments on isotactic poly(para-methylstyrene) (iPpMS) films in order to explore the temperature dependence of the viscoelastic behavior of these films. We quantified the amount of residual stresses σres induced through film preparation by spin-coating. As anticipated, σres was found to be independent of the temperature Tdew at which dewetting was done. A particular focus was on the temperature dependence of the relaxation time τ of σres, which was measured with the help of three independent dewetting parameters. Within error, all three values of τ were identical and followed an Arrhenius behavior yielding an activation energy of 60 ± 10 kJ/mol.  The initial dewetting velocity, being proportional to the ratio of surface tension of iPpMS and the corresponding viscosity, increased significantly with Tdew. Assuming a linear stress-strain response, we deduced that the elastic deformation responsible for the maximum height of the dewetting rim increased with temperature although σres did not vary with temperature. Correspondingly, the shear modulus of iPpMS films was found to decrease monotonically with increasing temperature. Using a Maxwell-type model, the corresponding viscosity of the film showed the expected decrease with increasing temperature. Our experiments demonstrated that dewetting provides, on the one hand, a suitable approach for determining viscoelastic properties of thin polymer films and, on the other hand, a means for quantifying preparation-induced residual stresses and their relaxation at temperatures above the glass transition.