Date of Award

Spring 2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Yuri Lvov

Abstract

Tubular nanomaterials and their composites have been extensively studied in recent years in the fields of physics, chemistry, biology, and biomedicine. Carbon nanotube is the most commonly studied tubular nanomaterial; however, toxicity and high cost make it less attractive in industry and thus restricts its applications. Halloysite nanotubes, which are available in abundance in the United States as well as in other countries around the world, is a low-cost, unique and versatile aluminosilicate mineral with a chemical formula of Al4Si4O10(OH)8·nH2O. Basically, the halloysite tube diameter is around 50 nm and the length varies with different locations ranging from 0.4-1.5 μm. The hollow nanotubular structure in the submicrometer range and large specific surface area provide opportunities for advanced applications in the fields in electronics, catalysis, biological systems, drug delivery, absorbents and functional polymeric composites. This dissertation contains the applications of halloysite in functional nanocomposites: adsorbent for dyes removal, additive for phase change heat insulating composites related to the nanotube orientation phenomena were studied. Halloysite nanotubes aligned along a particular direction were formed both by shear force and casting methods in the droplet meniscus in the microchannels.

Halloysite clay has a chemical structure similar to kaolinite, but it is rolled into tubes with external diameters of 50-60 nm, lumens of 12-15 nm and lengths of ca. 1000 nm. Halloysite exhibits higher adsorption capacity for both cationic and anionic dyes because it has negative SiO2 outermost and positive Al2O3 inner lumen surface; therefore, these clay nanotubes have a unique property of efficient bivalent absorbency. An adsorption study using cationic Rhodamine 6G and anionic Chrome azurol S has shown two times better dye removal for halloysite as compared to kaolin clay. Halloysite filters have been effectively regenerated up to 50 times by burning the adsorbed dyes at 350° C. Overall removal efficiency of anionic Chrome azurol S exceeded 99.9% for 5th regeneration cycle of halloysite. Chrome azurol S adsorption capacity decreases with the increase of ionic strength, temperature and pH. For cationic Rhodamine 6G, higher ionic strength, temperature and initial solution concentration were favorable to enhanced adsorption with optimal pH 8. The equilibrium adsorption data were described by Langmuir and Freundlich isotherms, showing that the Langmuir model describes the process better than the Freundlich model. The maximum adsorption capacity calculated from the Langmuir model is 43.6 mg/g for Rhodamine 6G and 38.7 mg/g for Chrome azurol S onto halloysite, and 21.4 mg/g for Rhodamine 6G and 36.7 mg/g for Chrome azurol S onto kaolinite. The halloysite surface modification with surfactants (dioctadecyldimethylammonium bromide) and polyelectrolytes (polyethyleneimine) allowed for varying the organic-inorganic nanocomposite adsorption properties. Electrically, bivalent halloysite tubule clay has a potential as a low-cost efficient adsorbent both for positively and negatively charged contaminant removal. Base or acid treatment of halloysite essentially increased its surface area from 40-50 to ca. 300 m2/g, which further improves the efficiency of the filtration.

In order to further develop clay nanotube composites with organic materials, we developed phase change materials (PCMs), which have gained extensive attention in thermal energy storage. Wax can be used as a phase change material in solar energy storage but has low thermal conductivity and cannot sustain its shape at higher temperatures (above phase transition from solid to liquid at 55° C). Introducing 40-50% halloysite clay nanotubes into wax yields a stable and homogenous phase change composite (wax/halloysite) with thermal conductivity of 0.36 W/m-K and no leaking until 70° C (preserving layer-shape above the original wax melting point). To increase the base thermal conductivity, nanographite and carbon nanotubes were added to the phase change material composite. Thermal conductivity of wax/halloysite/graphite (45/45/10%) composite showed a six-fold conductivity increase to 1.4 W/m·K compared to pure wax and had no liquid wax leakage until 81° C. Wax/halloysite/graphite/carbon nanotubes (45/45/5/5%) composite showed thermal conductivity of 0.85 W/m·K while maintaining the original shape until 91 ° C. Vectorial thermal energy transfer for double layers of different composition was demonstrated: heat flux difference in the opposite directions differed by 25%. This variance in layer conductivity allows for smart building roof insulators with increased absorption during hot weather, but limited thermal losses during periods of cooler temperatures. The new wax-nanoclay composite is a promising heat storage material due to good heat capacity, high thermal conductivity, and the ability to preserve its shape during wax melting. We discovered clay nanotube orientation under shear force in clay nanotube-wax layered composites and analyzed this phenomenon in the following chapter.

During drying, an aqueous suspension of charged halloysite clay nanotubes concentrates at the edge of the droplet ("coffee-ring" effect) which provides alignment of the tubes along the liquid-substrate contact line. First, the surface charge of the nanotubes was enhanced by polyanion adsorption inside of the lumen to compensate for the internal positive charges. This increased the magnitude of the ξ-potential of the tubes from -36 to -81 mV and stabilized the colloids. Then, colloidal halloysite was dropped onto the substrate, dried at 65° C, and after a concentration of ~0.05 mg/mL was reached, the alignment of the nanotubes occurred starting from the droplet edges. We described the process with Onsager's theory, in which longer nanorods, which have a higher surface charge, give better ordering after a critical concentration is reached. This study indicates a new application of halloysite clay nanotubes in polymeric composites with anisotropic properties, microchannel orientation, and production of coatings with aligned nanotubes.

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