A novel analysis method based on Hertz theory was used to determine the mechanical properties from force-distance curves obtained over a wide range of temperatures and frequencies on poly(n-butyl methacrylate) (PnBMA) and two polystyrene (PS) samples, having different molecular weight and hence different glass transition temperature Tg. The analysis technique extends the elastic continuum contact theories to the plastic deformations and permitted to calculate the stiffness in the plastic regime of deformation, the yielding force, the parameters of the WLF and Arrhenius equations, and the Young's modulus. The Young's modulus and the shift coefficients of the polymers determined through AFM measurements were in excellent agreement with the values from DMA measurements and/or the literature values.
Force-distance curves were also acquired on a model polymer blend of PS/PnBMA at different temperatures. The analysis method was used to determine the Young's modulus of PS and PnBMA away from the interface and close to the interface with a resolution of 800 nm. The differences in Tg of the two polymers resulted in different viscoelastic behavior. The modulus of PnBMA and PS was in excellent agreement with the DMA and AFM data from the measurements on individual films. The morphology of the PS/PnBMA blend was characterized using the Young's modulus of the constituting polymers. A several µm long transition region was observed in the vicinity of the interface, where the modulus of PnBMA decreased from the value on PS to the value on PnBMA away from the interface. This experiment shows the capability of AFM of surveying local mechanical properties and studying heterogeneous samples. Such spatially resolved measurements cannot be achieved with any other technique.