Measurement and modelling of tensile moduli of polymer blend thin films with phase separated structures

The mechanical properties of polymer composite thin films are critically important to their applications, but its investigation is still far from complete. In this study, phase separated polyurethane/polystyrene blend thin films with a variety of thicknesses ranging from ~135 to ~675 nm are mainly employed, and their apparent moduli are measured by tensile testing on water surface. Unlike bulk materials, the phase height difference is a common topography in blend thin films, which causes an unavoidable deviation between the measured moduli and the predicted values calculated from any existing model. An equivalent box model with void geometry is developed to describe the polymer blend thin films with a cratered morphology, where both the composition and the phase structure of the dispersed phase are considered. The predicted values from the new model demonstrate a good agreement with the measured moduli. In addition, thin films with both components in rubbery state or both components in glassy state are tested, and the validity of the proposed model is verified as well.