Rheological and Structural Properties of Graphene Oxide Liquid Crystal in Polymer Solution

Abstract

Graphene oxide (GO), an oxygenated form of the graphene sheets, is evolving from laboratory-scale research to the level of real-world industrial applications. GO has been actively researched in the various fields, combining the merits of graphene such as high electrical/thermal conductivities and extraordinary mechanical properties and its own advantages such as liquid crystallinity, low cost, manageability, and process scalability. Thanks to these unique properties, GO has been employed as a key material in high-quality application such as transparent electrodes, wearable devices, flexible sensors, supercapacitors, and catalysts. While the excellent performance of GO products has been attracted attention and actively researched, the fundamental research for GO as a colloid is still insufficient. In practice, the performance of the GO-based product comes from the colloidal behavior such as colloidal stability and dispersions; however, most previous studies have focused on the characterization of physical properties at a macroscopic level without detailed concerns in terms of colloidal behavior in microscopic level. Especially, GO dispersion in the presence of polymers remains even more elusive though polymers are frequently added to control the stability of colloids and indeed added to GO suspension aiming to improve the GO-based product. To realize the potential applications of GO, the know-why research is required rather than the know-how research. In this regard, this dissertation aims to present a systemic research for colloidal properties of GO and its rheological properties and ultimately suggest guidelines for GO processing. The colloidal stability needs to be firstly ensured for GO processing. Thus, the colloidal stability and rheological properties of GO in the polymer solution is demonstrated in Chapter 2 and Chapter 3. The model system employed in this dissertation is composed of aqueous GO suspensions and poly(ethylene glycol) (PEG), which is a well-known water-soluble polymer and can adsorb onto the GO surface by hydrogen bonding. The liquid crystallinity and the rheological properties of GO suspension are investigated with varying GO concentration, polymer concentration, and polymer molecular weight. However, the rheological properties and structure of GO are often varied in the dynamic GO processing accompanying the shear. Thus, Chapter 4 and Chapter 5 are dedicated to investigating the microstructure of GO with varying shear stress or strain directly correlating it to its rheological properties. The detailed microstructure and rheological properties of GO suspension are studied employing small-angle X-ray scattering, oscillatory rheometer, and polarized optical microscopy.