Characterization of Local Elastic Modulus in Confined Polymer Films via AFM Indentation

Abstract

The properties of polymers near an interface are altered relative to their bulk value due both to chemical interaction and geometric confinement effects. For the past two decades, the dynamics of polymers in confined geometries (thin polymer film or nanocomposites with high-surface area particles) has been studied in detail, allowing progress to be made toward understanding the origin of the dynamic effects near interfaces. Observations of mechanical property enhancements in polymer nanocomposites have been attributed to similar origins. However, the existing measurement methods of these local mechanical properties have resulted in a variety of conflicting results on the change of mechanical properties of confined polymers. Here, an atomic force microscopy (AFM)-based method is demonstrated that directly measures the mechanical properties of polymers adjacent to a substrate with nanometer resolution. This method allows us to consistently observe the gradient in mechanical properties away from a substrate in various materials systems, and paves the way for a unified understanding of thermodynamic and mechanical response of polymers. This gradient is both longer (up to 170 nm) and of higher magnitude (50% increase) than expected from prior results. Through the use of this technique, we will be better able to understand how to design polymer nanocomposites and polymeric structures at the smallest length scale, which affects the fields of structures, electronics, and healthcare.

Measuring interphase stiffening effects in styrene-based polymeric thin films

“…Normalized results from PS show positive interphase effects on a carbon surface (1.3e1.4 fold increase in E 0 ), that are of a significantly lower magnitude. The low interphase enhancements, along with SSBR1 and SSBR2 results, indicate that interphase stiffness enhancements reduce as the bulk polymer mobility reduces and are consistent with AFM data on glassy polymers [30].…”

“…As the film thickness decreases, the effective or measured modulus is more influenced by the interphase emanating from the substrate. More detailed results for direct measurement of interphase modulus, and the gradient of modulus away from the surface, can be extracted through a modified setup [30].…”

The Elastic Mechanical Response of Nanoscale Thin Films of Miscible Polymer/Polymer Blends

“…41 Note that while PMMA/SiO x interfacial interactions are highly specific leading to the enhancements of T g in thin films, 20 the opposite is true for PMMA/Al 2 O 3 . 48 However, regardless of the differences between the polymer/substrate interfacial interactions, PMMA/SiO x and PMMA/Al 2 O 3 exhibited similar mechanical responses.…”

Fast evaluation of local elastic constants and its application to nanosized structures

“…Overall, this new approach will be a powerful tool for the modeling and simulation of other inhomogeneous materials, such as the interphase of polymer-based nano-composites [45], interface formed between different crystallographic directions [46], etc. It will enables validation of the various continuum models on the prediction of the effective modulus for heterogeneous materials; the approach can be used to estimate the mechanical performances that are difficult to obtain experimentally; and it will give us a means to explore and compare the roles of polymer-substrate interactions and geometrical constraints on elastic properties.…”