Comparative study of polymers mechanical properties determined by atomic force microscopy

Atomic force microscopy (AFM) is a powerful tool for measuring the mechanics of soft materials. Mechanical properties are expressed as Young’s modulus, obtained by fitting the contact mechanics model to force-indentation dependencies. The indentation and point where the AFM tip is in contact with the sample are not directly measured and can be identified only for hard surfaces. For soft surfaces, like polymers and biological materials, the dependency of cantilever displacement vs. deflection is nonlinear when the tip is in contact with the sample. Because of that cantilever force and indentation become inaccurate, and models applied to determine Young’s modulus cannot extract useful information. Also, the contact area increases with indentation. Therefore, it is unclear what part of the results depends on the geometry of the tip and what part is on sample properties. Also, most models are designed with assumptions that the sample is homogeneous, isotropic, linear elastic half-space experiencing infinitesimally small strains. In contrast, biological (and soft) materials can demonstrate opposite behavior, such as inhomogeneity, anisotropy, and nonlinearity. Moreover, the indentation size is several hundreds of nanometers, while the thickness of the sample is several micrometers and cannot be determined as infinitesimal. The experimental problem is that sharp tips lead to local strains that exceed the linear material regime. The comparison of AFM measurement results with macroscopic techniques results could be a reliable way to find appropriate experimental conditions and mathematical processing of data. We measured the PETG, PVC, and PMMA polymer samples with four different tip geometries and determined the stiffness from the linear part of force vs. indentation curves. The highest difference (3 times) between calculated Young modulus was found at PMMA samples measured with 2 um and 20 nm sphere tips.