Phase behavior and rheology of single-walled carbon nanotubes (SWNTs) in superacids with application to fiber spinning
Davis, Virginia Angelica
Pasquali, Matteo; Smalley, Richard E.
Doctor of Philosophy
This dissertation describes the first discovery and characterization of a SWNT liquid crystalline phase. The protonation of SWNTs in superacids enables their dispersion at concentrations up to 12% vol. (10% wt.); this concentration is an order of magnitude higher than has been achieved for pristine SWNTs in any other solvent. The dispersed SWNTs behave as rigid rods; with increasing concentration the dispersions transition from a dilute phase of Brownian non-interacting rods, to a semidilute phase in which rod rotation is inhibited, to a biphasic regime where an isotropic phase coexists with a liquid crystalline phase, and finally to a single phase liquid crystal. This phase behavior was determined using a combination of rheology, optical microscopy, and differential scanning calorimetry. For monodisperse rods interacting only through hard rod repulsion, the phase boundaries are only a function of the rods' aspect ratio. For SWNTs in superacids, the phase transitions are also affected by the extent of SWNT protonation as well as aspect ratio polydispersity. SWNT protonation has significant effects on the concentration at which the system becomes biphasic, the liquid crystalline morphology, and the response of the system to non-solvents. In contrast, SWNT protonation has little effect on the concentration at which the system becomes a single liquid crystalline phase; this transition appears to be primarily controlled by SWNT polydispersity. The ability to form a lyotropic nematic SWNT liquid crystal directly impacts the development of applications requiring macroscopic assemblies of highly aligned SWNTs. Highly aligned macroscopic SWNT fibers ranging from 30 to 100 mum in diameter and up to 100 m in length have been produced by solution spinning liquid crystalline SWNT-102% H2SO4 dispersions. The direct impact of liquid crystalline phase behavior and liquid crystal morphology on fiber microstructure was demonstrated by producing fibers from SWNTs dispersed in 102% H2SO4 and ClSO3H. The results of this research provide a key example of how liquid crystalline phase behavior can be used to facilitate the development of macroscopic structures comprised of anisotropic nanomaterials.