Dewetting Rheology for Determining Viscoelastic Properties of Nonequilibrated Thin Polymer Films

We performed systematic dewetting experiments on isotactic poly(para-methylstyrene) (iPpMS) films to explore the temperature dependence of the viscoelastic behavior of these films. We quantified the amount of residual stresses sigma(res) induced through film preparation by spin-coating. As anticipated, sigma(res) was found to be independent of the temperature T-dew at which dewetting was done. A particular focus was on the temperature dependence of the relaxation time tau of sigma(res), which was measured with the help of three independent dewetting parameters. Within error, all three values of tau 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 T-dew. 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 sigma(res) 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 suggest that preparation-induced residual stresses affect material properties such as elastic modulus or viscosity of iPpMS as a function of temperature.