Graphene Liquid Crystals And Macroscopically Assembled Fibers

Graphene, a two-dimensional (2D) mesh of carbon atoms, has received widespread attention due to its exceptional mechanical, electrical, and thermal properties. But harnessing these attributes requires finding a way to turn the single atomic layer carbon flakes into macroscopically ordered materials. Because of the impossibility of melting processing for carbon materials, fluid assembly is the only viable approach to meet such a big challenge. Therefore, two fundamental problems should be resolved at first: orientational ordering in fluids and ordering phase-transformation from ordered fluids into ordered solid materials. Accordingly, this dissertation presented systemic studies on the finding of graphene oxide liquid crystals (LCs) and the wet-spinning assembly methodlogy for high-performance macroscopic graphene fibers.By chemical exfoliation of graphite, highly soluble, single-layer graphene oxide (GO) sheets were prepared in large scale, and their spontaneous formation of nematic LCs in aqueous dispersions was found. Detailed POM and SEM characterizations revealed the ionic concentration-dependant lyotropic liquid crystalline diagram and confirmed the orientational alignment in GO nematic LCs. Further, the thesis obtained narrowly distributed GO sheets by isopycnic differential centrifugation and found a new chiral liquid crystal of two-dimensional colloids. Synchrotron SAXS and CD examinations revealed two important characteristical attributes of this new chiral mesophase, including quasi-long-range ordered lamellar structure and strong optical activity. Freeze-fracture SEM and POM experimentally confirmed the inner twist dislocation as important chiral structural element and accordingly the ’helical-lamellar-phase’ model was proposed as the interpretative structural model of this new2D colloidal chiral mesophase. The finding of the two GO mesophases promoted deeper understanding of colloidal fluid assembly, indicating that graphene dispersive system can be served as an ideal model to further inverstigations in2D colloidal LCs. Even more important, the orientational ordering in GO mesophases sets the basis for the preparation of graphene macrocopic ordered materials. Based on GO LCs with orientational ordering in fluid state, the thesis proposed a new wet-spinning assembly stretagy to realize the fabrication of high-performance graphene fibers originated from mineral graphite. The directional flow and wet-drawing in wet-spinning process promoted the effective phase-transformation from ordered fluids of GO LCs into ordered solid materials of graphene fibers, which is verified by the uniform alignment of graphene sheets along the fiber axis and the high-performance of fibers. By this effective assembly method, the thesis designed three serials of graphene fibers with high mechanical strength, high electrical conductivity, and high porosity, respectively.(i) By tuning the interlayer interaction and optimizing the wet-spinning process, graphene fibers with high mechanical tensile strength and high toughness were obtained. The wet-drawing process and the Ca2+crosslinking promoted the tensile strength of graphene fibers to500MPa. Further, with the introduction of covlent polysulfide crosslinking, the graphene fiber performed the record tensile strength up to1.1GPa and a increasing in breakage elongation (9%), with a comparable toughness (49MJ/m3) with that of conventional stong carbon fiber T800(51MJ/m3).(ii) Wet-spinning of the complex GO LCs and Ag nanowires resulted in Ag-doped graphene fibers with high electrical conductivity (9.3X104S/m). Together with the remaining strength and flexibility, the high electrical conductive performance made Ag-doped graphene fiber promising applications as stretchable circuits.(iii) Lightweight graphene aerogel fibers with aligned pores were achieved via "freeze-dry spinning" method from GO LCs. Due to the "porous core-dense shell" structure, the aerogel graphene fibers possessed high specific surface area (884m2/g), high specific tensile strength (188kN-m/kg) and fine conductivity (4.9×103S/m).The combination of high mechanical strength, good flxiblity, high toughness and fine conductivity renders the new graphene fibers possible applications in high performance fibers, functional textiles, flexible fiberous devices and high performance composites.