Nanoscale characterization of forces, structure, and mechanical properties of functional materials
Creators: name: House, Krystal Lynn
Publisher: name: No Publisher Supplied
Year: 2022

description: The properties of many molecular materials are dictated by the structure of the molecules present and the intermolecular forces acting between the molecules from which they are composed. A better understanding of the forces driving structure is essential for the development of many technologies. This thesis aims to investigate the micro and nanoscale forces, structure, and mechanical properties of a range of functional materials and understand the effects of external factors, including confinement, temperature, solvent processing, pH, and charge. The materials studied include molecular liquids, ionic liquids, conjugated polymer films, and magnesium silicate. Starting with simple, well-studied molecules, Chapter 2 uses atomic force microscopy (AFM) measurements to investigate the interfacial nanostructure and force profiles of octamethylcyclotetrasiloxane and squalene under confinement. Liquids are known to order into solvation layers at solid interfaces resulting in oscillating force-distance profiles, but many studies to date present only periodic force-distance profiles suggesting uniformity in molecular layering and the absence of lateral structure. We demonstrate previously unmeasured lateral interfacial structure in these molecular liquids resulting in periodic and aperiodic force-distance profiles, as well as an undulating liquid structure imaged within 1 nm of the surface.Moving to more complicated liquids, Chapter 3 examines the interfacial nanostructure and force-distance profiles of aprotic, 1-alkyl-3-methylimidazolium NTf₂ ionic liquids (ILs) with varied alkyl chain length (n = 2, 3, 4, 6, 10). ILs are promising replacement solvents for various technologies, including catalysis, electrodeposition, electrochemical cells, and lubrication. To design and optimize ILs for these diverse applications, a better understanding of solid-ionic liquid interfacial structure and its temperature dependence is essential, especially when considering technology that requires or generates heat. Using AFM, we demonstrate a shift in the dominant interactions controlling structure as a function of temperature. For shorter alkyl chain length ILs (n = 2, 3, 4), solvophobic interactions create variations in force-distance profiles and increase correlation lengths in topographic images at lower temperature - these solvophobic interactions become weaker at elevated temperature. This allows Coulombic interactions to drive ion organization at higher temperature creating the appearance of increased order in the data. The interfacial nanostructure of longer alkyl chain length ILs (n = 6, 10) is insensitive to changes in temperature, as the solvophobic interactions are stronger for these liquids at both temperatures due to the longer apolar alkyl chains.Chapter 4 studies the local physical and nanomechanical properties of several conjugated polymers used for thin-film photovoltaics. Conjugated polymers have been extensively studied for semiconductor optoelectronic applications due to their interesting optical and electronic properties. However, due to relatively low crystallinity and complex morphology, semiconducting polymeric materials generally result in lower device performances than inorganic semiconductor devices. Solution processing techniques to improve intra- and inter-chain ordering are often used. In this work, using Hansen Solubility Parameters, an antisolvent was selected and added during spin coating to alter the crystallinity of the conjugated polymer films. Detailed analysis of the film morphology was conducted, and we found AFM viscoelastic mapping revealed changes in the order of the films not observable through X-ray diffraction or optical spectroscopy methods.Chapter 5 investigates the structural integrity of synthetic magnesium silicate as a function of pH and the charge of additional compounds in formulations for personal/oral care products. Synthetic magnesium silicates have broad applications in industry and are used extensively in pharmaceuticals, cosmetics, biodiesel purification, and chromatography. Compared with the polycrystalline structure of natural magnesium silicate minerals, synthetic magnesium silicates commercially available are often amorphous and porous, which could result in greater dissociation in a liquid dispersion system. Using SEM/EDS microstructural studies, we demonstrated magnesium ion dissociation is more significant at low pH in the presence of negatively charged compounds. This results in the loss of structural integrity and the formation of magnesium complexes that could significantly affect the properties of personal or oral care products prepared with synthetic magnesium silicate.