Local Mechanical Properties Characterization of Soft Polymeric Material via Atomic Force Microscopy Nanoindentation and Finite Element Simulations

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In polymer nanocomposites (PNCs), the physical and chemical interactions at the polymer matrix-filler interface lead to local variations in polymer properties, creating a substantial interphaseregion in the vicinity of the interface. Quantifying the significance of the interphase effect in the presence of substrates or nanoparticles is of essential importance in the design and performance of polymer nanocomposites and nanostructured polymers where a significant portion of the polymer matrix lies within the interphase region. Going well beyond the capability of conventional experiments, Atomic Force Microscopy and its nanometer sized probes provide a tool for the direct determination of modulus for the interphase region with a nanoscale resolution.', 'In this thesis, a quantitative and systematic study is performed to quantify the altered mechanical properties and the associated length scales in polymeric materials extending from the substrate in multiple model nanocompositesystems with well-defined polymer-substrate interfaces is provided. The effects of both chemical and physical interactions on the mechanical response of the interfacial polymer region are investigated, including how the modulus of one interphase region can be perturbed by another interphase in the close proximity. The results from AFM are compared with fluorescence spectroscopy characterization, resulting in the first report in the literature of two independent, yet complementary techniques utilized on the exact same samples to yield qualitative and quantitatively agreement on stiffness confinement effects of polymer interphases in multiple polymer-substrate systems. ', 'In order to obtain a more accurate interpretation of the mechanical properties of the interphase from experimental results, and decipher questions that cannot be answered experimentally, three dimensional (3D) and two dimensional (2D) finite element analysis (FEA) models are developed to simulate the indentation experiments on model nanocomposites samples. The results shed light on issues such as the direct influence of the adjacent substrate on stress fields during indentation into nearby polymer, which can then be quantified and excluded from experimental data analysis. These simulations also provide insight into experimental artifacts arising from tip deformation and tip radius. In addition, the 2D FEA simulation is developed to enable a comparative study benchmarking results from different modeling approaches to address the issue of result interpretation for further understanding of the underlying physics of the interphase effect.

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  • 11/20/2019
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