Quantifying the stress relaxation modulus of polymer thin films via thermal wrinkling

The viscoelastic properties of polymer thin films can have a significant impact on the performance in many small-scale devices. In this work, we use a phenomenon based on a thermally-induced instability, termed thermal wrinkling, to measure viscoelastic properties of a polystyrene thin film as a function of film thickness. By monitoring the time-evolved wrinkle wavelength at fixed annealing temperatures, we infer the stress-relaxation modulus and relaxation time of the polystyrene film as a function of film thickness with the aid of the appropriate buckling mechanics models. Specifically, we are able to identify the rubbery plateau and the viscous flow regions as a function of annealing time and temperature above its glass transition. For the polystyrene films investigated, we find that the stress relaxation modulus is thickness-independent for films above 100 nm, while a slight deviation was observed below 100 nm.