Best practices and recommendations for accurate nanomechanical characterization of heterogeneous polymer systems with atomic force microscopy
Graphical abstract
Introduction
Since atomic force microscopy (AFM) was developed in the 1980s, [1] AFM has been used for the characterization of surfaces and their properties by scanning a sharp tip attached to the end of a cantilever across a sample surface, where deflections of the cantilever can be related to nanoscale changes in surface topography and properties.
Since then, the past two decades have seen the rise of an expanding range of AFM modes and techniques [2], [3], [4], [5], [6] that map nanoscale mechanical properties with ever growing sophistication, transforming AFM into an indispensable tool for the characterization of the nanoscale properties of polymers and other soft materials. [7], [8], [9] Depending on the choice of the excitation signal and feedback mechanism(s) a range of quasi-static and dynamic AFM operational modes can be accessed. Quasi-static AFM modes, also commonly referred to as static modes, conduct force spectroscopy measurements by driving the tip of an AFM cantilever into contact with a sample surface while tracking the cantilever deflection as a function of the probe movement towards and into the sample. Dynamic AFM modes excite the cantilever at a specified frequency or range of frequencies close to the cantilever resonance while the tip is near (commonly referred to as ‘non-contact mode’), or in intermittent contact (commonly referred to as ‘tapping mode’) with, a sample surface where perturbations to the cantilever oscillation behavior due to tip-sample forces allows the user to infer information about the surface properties. Table 1 provides a brief summary of current quasi-static and dynamic AFM modes and their capabilities for measuring mechanical properties. Beyond the “simple” acquisition of sample topography conducted by traditional AFM, the advanced modes have been developed to measure electrical, thermal, surface chemistry, [10] as well as mechanical properties – the focus of this review – which provide sources of contrast between sample components that is not available from other high resolution imaging techniques (SEM, TEM, etc.).
Despite the numerous advancements in AFM methods, the viscoelastic (VE), adhesive nature of polymers and soft materials presents a significant challenge in quantitative AFM measurement due to complex contact mechanics and non-linear sample deformation. Multi-phase materials provide additional complications to AFM measurement as the response of a material to AFM indentation is a convolution of contributions from a volume underneath the AFM tip and may include several phases. At the same time, local property measurements are the most insightful on multi-phase systems to shed light on complex material interactions. Perhaps the most well-known consequence of the finite probed volume is the ‘substrate effect’ (also known as the ‘thin film effect’ or ‘stress interaction effect’) where the measured force from an indentation into a sufficiently thin film will have a contribution from the supporting substrate, artificially increasing or decreasing the measured modulus of the film depending on the relative stiffness of the substrate.[39] While most well-known for its relevance in the study of supported thin films, the substrate effect is also an important consideration in the study of composite and blend systems generally, as the various bodies in the sample can influence indentations in the neighboring phase.
One of the most promising areas for local property measurement of polymers by AFM is in elucidating the nature of the polymer interphase. The interphase is a nanoscale (1-100 nm) region of polymer with altered properties relative to bulk regions resulting from chemical and/or physical interactions between local polymer chains and the surface of a neighboring domain. The changes in polymer conformations and dynamics within the interphase is thought to be responsible for many of the enhanced mechanical, dielectric, transport, and thermal properties observed in thin films and polymer nanocomposites (PNCs). [40,41] Polymer thin films, supported thin films in particular, [42] are in some ways analogous to PNCs due to similarities between the interactions of local polymer chains with an embedded nanoparticle and with a stiff chemically active surface. Advancements in understanding of the interphase impacts a variety of application domains including microelectronics and energy storage devices, drug delivery systems, structural composites and nanocomposites, polymer blends, polymer adhesion and tribology.
The interphase layer and its formation also tell a fundamental story about polymers and the underlying material physics. Aspects of glass formation remain an unsolved problem in solid state physics, [43,44] with multiple theories still competing to explain the mechanisms of how a glass-former (such as a polymer) undergoes such drastic change in physical properties at the glass transition temperature. The ability to understand the interphase morphology, its local properties, and how it arises in real systems can provide useful information about entanglement, chemistry, and glass formation in polymers.
The drive in computational materials science towards big data and machine learning approaches to the development of novel materials requires high fidelity data sets with accurate representation of the sample microstructure for quantification and prediction. [45] Beyond the enormous range of material constituents available for fabricating PNCs, the interactions between filler particles and polymers add an additional wrinkle to attempts to predict or tailor the macroscale properties. Nanomechanical AFM has the potential for direct measurement of the impact of constituent materials and their interactions on the interphase and microstructure in heterogenous systems such as PNCs. However, conducting nanomechanical AFM on PNCs remains a significant challenge as the multiple phases present result in changes to deformation behavior and local sample topography, among other issues, that require careful treatment for quantitative measurements.
AFM has seen significant use in characterization of biological materials and indentation measurements of the mechanical properties of cells, bacteria, proteins, and other soft biological systems. While early experiments were limited to static indentations and topography, advancement in high-sensitivity low force AFM modes, [46,47] experimental protocols, [11,14,48,49] and the combination of AFM measurements with complementary techniques [9] have made AFM a sophisticated analysis technique for biological systems. AFM studies on biological systems is a crucial and growing field, and there are several recent reviews on the subject [8,9,50]. Our review is focused on polymers and their composites and blends, but the outcomes and content of our review are broadly applicable to AFM on all soft materials, including biological materials.
In this review, we examine recent advances in the elastic and viscoelastic characterization of polymers and the impact of instrument calibration and other experimental considerations. Section 2 is an overview of the AFM modes that are most suitable for nanoscale characterization of elastic and viscoelastic mechanical properties of polymers. We also provide a summary of commonly used contact mechanics models for elastic properties as well as contact models that include viscoelastic material behavior for quasi-static and dynamic AFM modes. Section 3 summarizes sources of error within AFM mechanical property measurements from the calibration procedure as well as the measurement artifacts that result from non-linear deformation behavior and structural effects due to the presence of multiple phases in an indented system. In light of these considerations, we provide some recommendations and best practice for acquiring and interpreting AFM data on polymer systems. Finally in Section 4 we examine recent work on measuring local properties near surfaces in nanostructured polymers in the context of the lessons learned from Sections 2 and 3 and suggest future directions to refine AFM measurements for the detailed measurement of local mechanical properties in complex, heterogeneous polymers.
Section snippets
Elastic and Viscoelastic Property Measurement from Quasi-static and Dynamic AFM
The viscoelastic, adhesive nature of polymers make them difficult systems to study with AFM, and so it is often the case that there is not just one AFM technique that works across every system of interest. In this section, we provide an overview of the AFM modes which are most suitable for extraction of elastic and viscoelastic mechanical property data from polymer blends and composites. We first introduce quasi-static modes, then dynamic modes, in each section summarizing methods for elastic
Nanomechanical AFM experiments
The contact mechanics models discussed in Section 2.1.1 are predicated on the assumption of ideal contact conditions, where the volume beneath the tip is comprised of a single material and the contact area is symmetric. Indentation of polymers of scientific and engineering interest, including PNCs, polymer blends, biomaterials, films and other systems are rarely as experimentally simple as the picture described by contact mechanics models due to complexity from the presence of multiple
Application of AFM Modes to Nanoscale Property Mapping on Polymers
In this section, we summarize the application of AFM modes on soft, heterogenous materials, focusing on the best approaches to extract small-scale elastic and viscoelastic mechanical property gradients. These measurements are especially insightful for understanding the fundamental physics of “interphase” polymer as mentioned earlier (Section 1.2.1), as well as in characterizing important regions in applications and systems of immense technical interest. While both AFM indentation measurements
Summary
In summary, we have provided the dedicated AFM user and curious experimentalists a guidebook for assessing the capabilities of quasi-static and dynamic AFM modes for elastic and viscoelastic characterization of polymers, as well as current best practices for achieving accurate and reliable results. In Section 2, current AFM modes (Table 1) are reviewed and their current capabilities for elastic and viscoelastic property measurement are discussed. Section 3 details the efforts over the past
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge support from NSF BCS-1734981, AFOSR FA9550-18-1-0381 and NSF-CMMI-DMREF grant 1818574. The authors would also like to thank Heer Majitha for her valuable discussion and comments as well as Christopher M. Stafford for his constructive feedback.
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Currently at Department of Materials Science, Stanford University, Stanford, California, 94305, USA.