Contributions of atomic force microscopy to the mechanical property mapping of polymer blends and nanocomposites
(2018)
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- Abstract
- The potentials of AFM dynamic force spectroscopy for quantitative local mechanical property mapping of polymer blends and nanocomposites were evaluated either based on case studies with industrial importance or using well-defined model samples. It was demonstrated that a blend of two widely used toughening agents for highly crosslinked epoxies, thermoplastic phenoxy and a copolymer of methyl-methacrylate and butyl acrylate (MAM), causes a synergistic toughening compared to the case when each toughener is used alone. AFM studies using the HarmoniX mode revealed the evolution of local microstructure and mechanical properties over a relatively long interdiffusion distance (800 µm) and provided the link to the global mechanical performances of the composite panels. Afterwards, it was shown that the mechanical properties of highly crosslinked epoxy resins are homogeneous from the macroscale (several millimeters) down to the scale of AFM probe tip (~10 nanometers). This conclusion was possible thanks to a comprehensive local mechanical property mapping using the HarmoniX mode together with the macroscopic mechanical characterization using dynamic mechanical analysis and compression tests. The origins of the observed spatial and property resolution as well as the reliability of the quantified modulus values were evaluated as the main performance parameters of AFM dynamic force spectroscopy for local mechanical characterization. It is demonstrated that the spatial resolution and the mechanical property contrast are mutually related parameters that both are mainly influenced by the contact radius between the AFM probe and the sample surface. A relatively large contact radius can cause more overlap and averaging of data from adjacent surface points, which in turn can reduce the resolution and the contrast. Furthermore, the probes with a better force sensitivity and bandwidth can improve these parameters. Finally, it was demonstrated that the HarmoniX mode has better quantification capabilities when the AFM-based moduli are compared to the corresponding bulk values. This could partially be due to the way that force versus distance curves (f-d) curves are obtained in this mode. The analysis of PFT-QNM raw f-d curves provided hints about the possible source of the discrepancies about the modulus values obtained in this mode. Direct acquisition of the f-d curves, as opposed to the force curve reconstruction in the HarmoniX mode, could actually be more sensitive to different physical and chemical properties of the sample surface as well as the algorithms used for the real-time modulus calculation used in the image generation. In other words, HarmoniX f-d curves are more representative of the tip-sample interactions, which resulted in better quantification of modulus. As a matter of fact, high resolution mapping of mechanical properties in complex polymer blends with very fine microstructures imposes certain conditions on the imaging parameters like the penetration depth of AFM tip in the sample surface (only few nm). These conditions however may not be ideal of the quantification aspects. Thus, quantitative imaging of mechanical properties in complex systems could sometimes require a tradeoff between the spatial resolution and reliable quantification of the properties. Overall, dynamic force spectroscopy modes of AFM have great potentials for high resolution and high sensitivity mapping of local mechanical properties of polymer blends and nanocomposites with good quantification possibilities. Further improvements are feasible both in terms of sensitivity and quantification aspects.
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Bahrami, A. (2018). Contributions of atomic force microscopy to the mechanical property mapping of polymer blends and nanocomposites. https://hdl.handle.net/2078.5/38236