Functional Nanofibers and Colloidal Gels: Key Elements to Enhance Functionality.

摘要

Nanomaterials bridge the gap between bulk materials and molecular structures and are known for their unique material properties and highly functional nature which make them attractive for a variety of potential applications, from energy storage and pollution sensors to agricultural and biomedical products. These potential applications, coupled with advances in nanotechnology, have generated considerable interest in nanostructure research. The work presented in this dissertation focuses on two such nanostructures, electrospun nanofibers and nanodiamond particles, with an overarching goal of tailoring the material behavior for a desired outcome.;Our first research theme focuses on realizing the full potential of chitosan electrospinning by understanding the mechanism that enables fiber formation through cyclodextrin complexation as a function of solution properties, solvent types, and cyclodextrin content. We demonstrate that cyclodextrin addition not only enables chitosan fiber formation, but also extends the composition and solvent window for nanofiber synthesis while introducing a variety of mat topologies, including three-dimensional, self-supporting mats. These fiber formation improvements cannot be fully explained by conventional electrospinning parameters, but instead seem to be related to the molecular interactions between chitosan and cyclodextrin.;Our second research theme entails the modification of highly water soluble, poly(vinyl alcohol) (PVA) nanofibers dissolution properties via atomic layer deposition (ALD) post treatments. In this work, we demonstrate that applying different thicknesses of aluminum oxide nano-coatings can improve the stability of PVA nanofibers in high humidity conditions and significantly decrease the solubility of electrospun PVA mats in water, from seconds to multiple weeks. Controlling mat dissolution allows for the unique opportunity to modulate small molecule, such as drug, release from nanofibers without altering the core material so that prolonged release can be readily achieved from highly water soluble nanofibers.;The final research theme focuses on gaining a fundamental understanding of a new class of materials, nanodiamond, so that a desired microstructure can be achieved via functionalization or manipulating processing parameters. In particular, we utilize both steady and dynamic rheology techniques to systematically investigate systems of nanodiamonds dispersed in model nonpolar (mineral oil) and polar (glycerol) media. In both cases, selfsupporting colloidal gels form at relatively low nanodiamond content; however, the gel behavior is highly dependent on the type of media used. Nanodiamonds dispersed in mineral oil exhibit characteristic colloidal gel behavior, with a rheological response that is independent of both frequency and time. However, nanodiamonds dispersed in glycerol exhibit a time dependent response, with the strength of the colloidal gels increasing several orders of magnitude. We attribute these rheological differences to changes in solvent complexity, where new particle-solvent and particle-particle interactions have the potential to delay optimal gel formation. In addition to colloidal gel formation, we use large oscillatory strains to probe the effect of processing parameters on microstructure disruption and recovery. The results indicate that the formation and rearrangement of the nanodiamond microstructures are concentration dependent for both media types; however, the recovery after breakdown is different for each system. Recovery of the nanodiamond/mineral oil gels is incomplete, with the strength of the recovered gel being significantly reduced. In contrast, the original strength of the nanodiamond/glycerol gels is recoverable as the system restructures with time. The practical implications of these results are significant as it suggest that shear history and solvent polarity play a dominant role in nanodiamond processing.