Deconvolution of Nanomechanical Atomic Force Microscopy across Interfaces in Polymer Systems in Experiment and Simulation

Collinson, David William

doi:https://doi.org/10.21985/n2-4d5a-pv31

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Deconvolution of Nanomechanical Atomic Force Microscopy across Interfaces in Polymer Systems in Experiment and Simulation

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High performance polymers and their composites have wide ranging application in advanced and emerging material systems. The macroscale performance of these advanced materials is often defined by interfaces that induce nanoscale changes in the microstructure or molecular conformations (termed the ‘interphase’) of the polymer. Atomic force microscopy (AFM) is an experimental method that allows for probing surface mechanical properties at nanoscale resolution and extremely low forces, providing an avenue for direct characterization of complex polymer systems. However, it remains challenging to quantitatively characterize nanomechanical properties with AFM, especially on polymers and soft materials, and requires a detailed understanding of AFM operation and material behavior. The first chapter of this thesis is a series of literature reviews that examines AFM methods and their capabilities, the nature of the interphase between polymers and an attractive surface, and the role of interfaces in the mechanical performance of additively manufactured materials. The second chapter of this thesis is first focused on combined finite element analysis (FEA) and quasi-static AFM to deconvolute the interphase from multi-body effects in rubber and carbon black nanocomposites. Next, a combination of FEA and molecular dynamics simulations was used to investigate the viscoelastic response of polymers under confinement. Later in the second chapter, high resolution dynamic AFM in concert with numerical simulations is used to image and analyze a series of model glassy polymers below the glass transition temperature. For the polymers studied, it was found that a higher dispersion of surface relaxation times combined with the characteristic heterogeneity length scale below Tg is associated with an increased fragility of glass formation for the polymer. Finally, bimodal, nanomechanical AFM is used to identify the bound layer and an additional extrinsic layer in the interphase of a silica-poly(ethyl-methacrylate) nanocomposite which was shown to be associated with the polymer β transition. In the third chapter of this thesis, AFM nanomechanical mapping is used to investigate the processing-structure-property relationship for the fused filament fabrication of ABS (acrylonitrile-butadiene-styrene), PLA (poly(lactic acid)) and PEEK (poly(ether-ether-ketone)) which all have degraded mechanical properties when additively manufactured. Previous work has identified that the welds between deposited layers is a major source of weakness in additively manufactured materials, but a connection to the underlying polymer microstructure has been lacking. AFM on additively manufactured ABS shows that the reduced fracture strength of inter-fiber welds is due to an altered distribution of butadiene rubber particles within the weld zone, preventing effective toughening of the brittle styrene-acrylonitrile matrix. For PLA and PEEK, both semicrystalline polymers, preferential crystallization in the weld region was found to limit polymer mobility and prevent effective weld formation. In response, a two-step annealing procedure was designed based on the Avrami crystallization kinetics to improve weld strength up to 6-8× for fully crystallized PEEK. In summary, this thesis brings AFM together with simulation and complementary techniques to interrogate not only the nanomechanical properties of complex polymer systems, but the processes and practices required for quantitative accuracy.

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