Recently Chan et al. published a paper describing a method to use the wrinkling of a thin elastic film on which is adhered a thin polymer film to extract the viscoelastic properties of the polymer. Results were presented for the rubbery plateau modulus and viscosity of a polystyrene (PS) material of approximately 600 kg/mol molecular mass. The wrinkling technique itself, for the elastic case, is fully described in a number of previous publications. Our comments are concerned with the viscoelastic analysis and the data presented for the polystyrene relative to that in the macroscopic state. In particular, we show that there is an error of several orders of magnitude in the reported macroscopic viscosity for the polystyrene and these were, seemingly, consistent with the thin film results. This suggests that the method is not perfected to provide viscoelastic data in the long time regime of polymers. We also show that the values of the rubbery modulus determined for the thin polymer film are highly sensitive to the assumptions made for the Poisson s ratio of the polymer (nearly incompressible) rubbery state. The viscosity estimates are also sensitive to the assumed value of Poisson s ratio, but not sufficiently to explain the orders of magnitude differences with literature reports. Because of these problems with the constrained thin film analysis, we examine here an analysis of the data using a conventional analytical solution for a thick compliant layer, i.e. where the material is not confined, and again find that the results for the polystyrene film are not consistent with measured bulk properties. This suggests that these thin film wrinkling experiments may be in a region where the confinement is not well defined and/or that further refinement to the viscoelastic model reported by Chan et al. is required to fully capture the viscoelastic properties of confined polymer films.
Citation: Soft Matter
Pub Type: Journals
wrinkling, viscoelastic, polymer, film, viscosity, modulus