Pushing AFM to the boundaries — interphase mechanical property measurements near a rigid body

Understanding the mechanical properties of polymer nanocomposite materials is essential for industrial use. Particularly, the determination of the polymer modulus at the nanofiller-polymer interphase is important for optimizing the interfacial mechanical properties. Nanoindentation via Atomic Force Microscopy (AFM) is well established for measuring the modulus of the interphase region with nanoscale spatial resolution. However, indentation into heterogeneous materials presents a confounding issue often referred to as the "substrate effect", i.e., the structural stress field caused by the rigid body is convoluted with the actual modulus of the interphase region. While finite element analysis (FEA)-based methods can be used to deconvolute the interphase modulus from measured apparent modulus-distance profiles, the experimental validation of this method is still needed. Here, we provide this validation using AFM nanoindentation on a layered model composite which consists of three layers with different moduli to recapitulate the properties of the matrix, the filler, and the interphase of real polymer nanocomposites. By systematically varying the thickness of the “artificial” interphase layer and the AFM probe radius, we obtain modulus - distance profiles over a wide range of indentation conditions. We validate a method to deconvolute the substrate effect using an empirically derived master curve obtained from FEA analysis. Furthermore, we showed that the effect of the artificial interphase on modulus - distance profiles can be distinguished only if the interphase layer is thick enough compared to the contact radius of the probe. Finally, we established an innovative and quantitative framework to predict the interphase thickness from mechanical nanoindentation measurements, and we discussed the lower, practical limit for interphase thickness determination. In summary, we provide a broadly applicable method to extract interphase mechanical properties of multiphase soft materials, and practical guidelines for choosing optimal characterization conditions.