【作者】 许震；

【导师】 高超；

【作者基本信息】 浙江大学 ， 高分子化学与物理， 2013， 博士

【摘要】 石墨烯是具有一系列优越性能的新型二维碳纳米材料,对于复合材料、能量存储、电子器件等领域有着重要的应用价值。在利用石墨烯作为构筑单元制备宏观组装材料的研究热潮中,始终存在一个挑战：如何通过有效的液相组装方法制备有序化石墨烯宏观组装材料,从而将单片石墨烯的优越性质充分转化为宏观材料的性能。针对石墨烯宏观有序组装中液相有序化和液固有序化转变这两个关键科学问题,本论文开展了相应的研究,发现了氧化石墨烯的溶致液晶性质并由此实现了高性能多功能石墨烯宏观组装纤维的连续制备。通过合成高溶解性单层氧化石墨烯,发现了其向列相液晶,确认了液晶内部的有序排列结构,绘制了液晶相图。进而通过差速离心分级制得了窄分布氧化石墨烯,发现了二维胶体液晶领域以前未见报道的手性液晶相,确认了氧化石墨烯手性液晶的准长程有序层状结构和光学活性,探索了其内部螺旋位错手性结构,提出了“螺旋层状相”结构模型,解释了新型手性液晶的结构手性来源。氧化石墨烯液晶的发现和相态确认丰富了胶体液晶的内涵,为深入研究胶体液晶物理提供了理想的二维胶体模型体系,同时液晶的液相有序化也为高性能石墨烯宏观有序材料的宏量制备奠定了基础。提出了液晶湿纺组装的有效液固有序化转变方法,实现了具有一致排列结构的高性能石墨烯纤维的连续制备,这代表了高效利用石墨制备高性能碳基纤维的一条新路线。通过调控片间作用力和纺丝工艺,分别制备出高强度、高导电率及多孔高比表面积三大系列石墨烯纤维：(1)通过大片氧化石墨烯液晶湿拉纺丝及钙离子交联,化学还原后石墨烯纤维的拉伸强度达到500MPa；通过片间硫化共价交联,石墨烯纤维拉伸强度进一步提高至1.1GPa,其断裂伸长率提高至9%,断裂能达到49MJ/m3(与商业化碳纤维T800接近)；(2)通过氧化石墨烯液晶与银纳米线复合纺丝制备了高导电银掺杂石墨烯纤维(电导率为9.3×104S/m),可作为柔性轻质导线应用于可拉伸电子电路领域；(3)采用‘冷冻干纺’法制备了轴取向多孔石墨烯气凝胶纤维,纤维兼具高表面积(884m2/g)和高比强度(188kN·m/kg)。石墨烯纤维同时具有高强度、高导电、高韧性、大伸长高断裂能等特性,是一类新型高性能碳基纤维材料,可应用于高性能纤维、柔性功能织物、柔性线型器件、高性能复合材料等领域。更多还原

【Abstract】 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.更多还原