Date of Award

12-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Materials Science and Engineering

Advisor

Kornev, Konstantin G

Committee Member

Rao , Apparao

Committee Member

Luzinov , Igor

Committee Member

Foulger , Stephen H

Abstract

We developed a wet spinning process for the formation of polymeric fibers with high loading of single walled carbon nanotubes. The dissertation consists of five chapters. In the first chapter, the research goals were formulated and the art and technologies of fiber spinning from carbon nanotubes were critically analyzed. The next three chapters report the original results. Last chapter summarizes all the findings.
In the second chapter, we describe a surfactant based method of stabilization of carbon nanotube dispersions. The conditions of stability of nanotube dispersions in aqueous solutions of sodium dodecyl sulfate were analyzed. Using surface tension isotherms, the phase diagram was experimentally constructed. The diagram covers a broad range of nanotube concentrations. The proposed method allowed us to analyze highly concentrated opaque dispersions, which are hard to study using traditional optical techniques.
In the third chapter, we explain the process of electrostatic assembling of polyelectrolytes and nanotubes coated with sodium dodecyl sulfate. Taking sodium alginate as an example of a suitable polymer, we successfully wet spun fibers with various carbon nanotube loadings. The maximum concentration of nanotubes in the spun polymer fibers was 23 wt %, which is significantly greater than the percolation limit. It was shown that the Young's modulus of these fibers non-monotonically depends on nanotube concentration. The dependence was explained using Halpin-Tsai and Voigt models. Scanning electron microscope micrographs and resistivity analysis of the fibers suggest that the nanotube-alginate system undergoes a morphological transition from a composite structure of discrete nanotube bundles embedded in an alginate matrix to a complex continuous structure consisting of a nanotube network interwoven into a macro-molecular network of alginate. These nanotube - alginate fibers have unprecedented high flexibility and very high electrical conductivity - similar to semimetals (between germanium and carbon).
In the fourth chapter, we report on a method to stabilize single walled carbon nanotube-alginate fibers in aqueous solutions enriched with Na+ and K+ ions through covalent crosslinking of alginate. The unmodified wet spun nanotube-alginate fibers are unstable in electrolyte solutions such as phosphate buffered saline. This instability makes them unsuitable for biomedical applications as biosensor platforms or actuators. Therefore, these fibers were chemically modified through incorporation of covalent crosslinking to provide stability in solutions enriched with Na+ and K+ ions. Nano-pores were also introduced in the chemically modified fibers. We demonstrated that the modified alginate-nanotube fibers are stable in electrolyte solutions and achieve volumetric swelling up to 16 times their original volume in buffer solutions in 10 minutes. Loading the fibers with nanotubes, we achieved much better tensile and compression properties compared to the covalently crosslinked alginate fibers without nanotubes. The chemically modified nanotube-alginate fibers also show instantaneous pH-dependent swelling, promising interesting sensory applications.

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