Cryogenic electron microscopy (Cryo-EM) techniques are among the most powerful to characterize self-assembling soft materials (colloids, polymers, and microemulsions, etc.) at the nanometer scale, without any need for implicit models or assumptions about the structure. We can even visualize structure under dynamic conditions, capturing each stage of development. In this thesis, cryo-EM has been used to investigate the formation and structure of a variety of self-assembling soft materials. Visualization is complemented by small angle X-ray scattering (SAXS), dynamic light scattering, and conductivity measurements. In each case, cryo-EM provides new insights, not otherwise available, into the nanostructure development. Self-assembly phenomena at the molecular level are critical to the performance of tremendous number of applied systems ranging from personal care products to industrial products. To evaluate these self-assembled materials, multiple characterization techniques are required. We investigated aggregation behavior of cesium dodecyl sulfate (CsDS) ionic surfactant in aqueous solution. Coupled with the real space data from cryogenic transmission electron microscopy (Cryo-TEM) and the inverse space data from SAXS, the experimental result of CsDS in aqueous solution gave a new insight in CsDS micellar structures and their development as a function of concentration. Cryo-TEM showed the presence of the liquid-like hydrocarbon core in the CsDS micelles and relatively thick shell structures at a low CsDS concentration. The core-shell sphere structure micelle shifted to core-shell cylindrical micelle structure at high concentration. The morphology and structure of paclitaxel silicate (PTX) prodrug, encapsulated with amphiphilic poly(ethylene glycol)-b-poly(lactic-co-glycolic acid) diblock copolymers were studied. The six different silicate PTX prodrug candidates were characterized with cryo-TEM. Direct imaging with cryo-TEM illustrated structure of prodrug morphology prepared with flash nanoprecipitation method. Cryo-TEM also visualized drug release kinetics of the silicate PTX prodrugs, suggesting diffusion or degradation release mechanisms. Lastly, formulation of nanoemulsion through phase change emulsification of water/alkane/alkylphenolethoxylate non-ionic surfactant microemulsions is investigated. The amount of cosurfactant determined phase behavior characteristics of microemulsions. The initial structure of the microemulsions determined resulting nanoemulsions. The oil-in-water microemulsions became an exfoliating lamellar intermediate structure during quenching and dilution. The process produced small, simple, and uniform nanoemulsions. Complimentary electron microscopy techniques such as cryo-TEM, cryogenic scanning electron microscopy, and freeze-fracture electron microscopy elucidated microstructural development at each stage. This result enables to control both intra- and inter-molecular forces that govern structure and properties relationship, therefore, to correlate the optimization and the performance of engineered liquid systems within the specified constraints.
University of Minnesota Ph.D. dissertation. May 2015. Major: Material Science and Engineering. Advisor: Alon McCormick. 1 computer file (PDF); xxi, 249 pages.
Lee, Han Seung.
Cryogenic Electron Microscopy Studies: Structure and Formation of Self-assembled Nanostructures in Solution.
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