Conformational and Mechanical Characterization of Organic Thin Films on Surfaces by Neutron Reflection and Atomic Force Microscopy
Engineering thin, organic materials with tailored properties requires both the understanding of the conformation of thin organic films and their conformational response to changes in the environment, and the accurate characterization the mechanical properties of the materials as a thin layer on surfaces. These issues have not yet been sufficiently addressed due to the paucity of appropriate tools and data interpretation approaches to reveal the nanometer scale conformation and mechanics of surface-grafted, thin, organic films. In this dissertation, I report on the characterization of conformational and mechanical properties of thin organic films, and the development of techniques that allow more detailed and reliable measurement of these material properties. First, I co-developed a novel approach to evaluate neutron reflectivity data and to simulate the conformational structure for thin stimulus-responsive polymer brushes. In this approach, we used a molecular-based lattice mean-field theory, augmented with experimentally obtained parameters to describe the polymer chains. The approach and fitting results required fewer fitting parameters, and captured the thermal response of the sample self-consistently.
Second, I demonstrated the capability of force-modulation microscopy in imaging surface-grafted, organic thin films in aqueous environments, with high spatial resolution and sensitivity to conformational details that affect the contact mechanics. To this end, I developed a new parameter-selection approach. This approach allowed both highly sensitive mapping of subtle differences in the molecular packing of thiol molecules on the substrate surface, and generation of high-contrast contact-stiffness images of end-grafted protein patterns on a surface. Finally, I discussed model selection and error estimation in calculating the reduced Young's modulus of soft materials on surfaces. I found that the detailed characterization of probe apex profiles, using probe-reconstruction techniques, provide only marginal improvements in calculating the reduced Young's modulus of thin films, compared with analytical models of equivalent probe radii; however, I found that a hybrid worn-cone model is appropriate for large indentations on soft materials, and benefits from the characterization of the probe apex profile. Additionally, we rendered error maps of several common scenarios, referenced to indentation and probe radius values, in the determination of the reduced Young's modulus.
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