Nanoscale Mechanical Characterization of Polymers by AFM Nanoindentations:  Critical Approach to the Elastic Characterization

Abstract

AFM nanoindentations show a dependence of penetration, i.e., the relative motion between the sample and the tip (indenter), on material elastic properties when using the same load. This relationship becomes visible by using of samples being homogeneous down to the scale of nanoindentation. They were prepared from materials covering a broad range of mechanical behavior: from rubbery networks to glassy and semicrystalline polymers. The elastic modulus can be obtained applying Sneddon's elastic contact mechanics approach. To do this, some calibrations and instrumental features have to be measured accurately. All the polymers tested show that the contact between the tip and the sample is dominated by elastic behavior with negligible plastic deformation. In contrast to a standard metallic material like lead, the penetration dependence on load follows an exponent of 1.5, consistent with elastic contact mechanics. This can be justified on the basis of the large elastic range polymers exhibit, on the constraints due to the geometry of the deformation during indentation and to the critical yielding volume needed in order to induce plasticity. For the polymers studied, this volume is chosen in such a way that a significant material volume is irreversibly deformed. Elastic moduli taken from AFM force curves show a very good agreement with bulk values obtained by macroscopic tensile testing, on all the polymers tested. This result confirms that AFM nanoindentations in polymers take place mostly in the elastic range and opens the possibility to characterize the mechanical behavior of polymers on an unparalleled small scale compared to commercial DSI (depth sensing instruments), which use a much blunter indenter.

Existence of a tribo-modified surface layer of BR/S-SBR elastomers reinforced with silica or carbon black

“…The load-penetration depth curve can be used together with a contact mechanics model to calculate the stiffness of the sample. This technique has been successfully applied to both polymers and rubbers [26][27][28]. The viscoelastic properties of the samples can be obtained by performing measurements at various temperatures and/or indentation ratios [29,30].…”

Quantitative mapping of surface elastic moduli in silica-reinforced rubbers and rubber blends across the length scales by AFM

“…The AFM-based HarmoniX Imaging has been introduced as a mechanical property mapping technique providing high resolution ([10 nm laterally) quantitative mechanical characterization of stiffness, elastic modulus, adhesion, and energy dissipation of a polymer surface [16][17][18][19].…”

Photochemical surface modification of PP for abrasion resistance

“…The AFM-based nanoindentation has become an increasingly popular tool for characterizing polymer surfaces and thin films with widely different properties [16][17][18][19][20]. During operation of the AFM nanoindentation in force mode, the probe tip is first lowered into contact with the sample, then indented into the surface, and finally lifted off the sample surface.…”