Best practices and recommendations for accurate nanomechanical characterization of heterogeneous polymer systems with atomic force microscopy

The past two decades have seen atomic force microscopy (AFM) evolve from an experimental technique to probe simple surface topography to one that can spatially map nanoscale material properties with exquisite sensitivity and high resolution. An expanding array of modes and analysis methods has made AFM a widely used technique for extracting nanoscale elastic and viscoelastic data from polymers and other soft materials. However, the assumptions required for interpretation of nanoscale mechanical data on polymers and the lack of clarity on the best practices for the different modes limits the quantitative accuracy of AFM methods and the interpretation of mechanical data. The analysis of AFM data becomes even more complex when multiple phases are present in a sample which further convolute measurements and the interpretation of the output data. Here, we present a comprehensive summary of modes and contact mechanics analyses relevant for AFM on polymers, along with assessment of sources of error and potential artifacts in measurement data on these soft, adhesive, viscoelastic and often heterogenous materials. As a result of the review into AFM best practices, we provide a series of recommendations for conducting quantitative AFM measurements on polymer systems. Finally, we investigate the impact of these advancements in the context of a specific case study: measurement of mechanical property gradients in nanostructured polymers. (c) 2021 Elsevier B.V. All rights reserved.