Research On Interface And Propertiesof Graphene Oxide/Epoxy Composites

As the toughest material in the world, the one-atom-thick graphene is becoming more and more popular in the research field since the beginning of21st century, because of its excellent properties. Among a large number of researches regarding the exciting material, graphene/polymer nanocomposite is considered as one of the hottest research objectives since it always shows outstanding mechanical, thermal, optical, electrical and other functional properties. However, the mass production of graphene and its poor compatibility with the polymer matrix have limited the application of graphene in such promising field. On the other hand, graphene oxide (GO), as one of the derivatives of graphene which possesses abundant functional groups and the feasibility of mass production, has attracted great attention in the research field of polymer based nanocomposites. In order to meet the growing demands for applications, it is necessary to further improve the performances of GO/polymer composites. One common way is conducting surface modification on GO sheet to achieve better interfacial interaction between GO and the matrix, and the plenty of functional groups on its surface could be used as active sites to perform the modification.In this thesis, GO was firstly prepared and then employed as the initiating material for the “wash-and-rebuild” process in which base wash GO (bwGO) and two other modified GO with different end groups were synthesized successively. These two modified GOs were recognized as the amino groups terminated APTS-GO and the epoxy groups terminated GPTS-GO, respectively. Finally, a novel ATP-GO hybrid was also obtained by attaching the needle-like attapulgite (ATP) particle onto the surface of GO platelet. The above five GOs with different surface states were introduced into epoxy resin separately to prepare GO/epoxy nanocomposites. The tensile properties, fracture toughness, dynamic mechanical properties and thermal properties of the composites were investigated in the study.The as-prepared GO (aGO) with more functional groups could act better reinforcement on the mechanical properties of the composites than the base wash GO (bwGO) with less functional groups. When the content of aGO was1wt%, the Young’s modulus and tensile strength of the aGO/epoxy composites reached their highest value of3.1GPa and79.7MPa, which were24%and14%higher than the pure epoxy. The highest critical stress intensity factor (KIC) and critical energy release rate (GIC) was1.27MPam1/2and0.45kJ/m2respectively for the0.5wt%aGO/epoxy composites, they were both25%higher than the values for the pure epoxy. The0.2wt%aGO/epoxy composites have the highest glass transition temperature (Tg) of133℃,5.1℃higher than the neat epoxy. However, the thermal stability of the GO/epoxy composites was only enhanced by introducing less functionalized bwGO. aGO may decrease the thermal stability of the epoxy composites because the active functional groups on its surface were instable to heat.The silane functionalized GOs were found to be superior to both aGO and bwGO in reinforcing mechanical properties of the GO/epoxy composites, especially for fracture toughness. Meanwhile, the reinforcing behavior of these two functionalized GOs may vary depending on the different functional groups they possessed. The addition of0.2wt%amino-functionalized GO (APTS-GO) yielded a32%increase in Young’s modulus (3.3GPa) and16%increase in tensile strength (81.2MPa). Less reinforcement was observed with the epoxy-functionalized GO (GPTS-GO) but there was a more significant increase in ductility for GPTS-GO/epoxy, with the fracture toughness (KIC) and fracture energy (GIC) increased43%(1.46MPam1/2) and72%(0.62kJ/m2) respectively at0.2wt%loading.Remarkable enhancements on mechanical properties were achieved by using ATP-GO hybrids as the reinforcing fillers, much greater than that of the composites reinforced individually by ATP or GO. The best performance was obtained only when the composition of the hybrids was optimized. The optimum composition was1wt%ATP+0.2wt%GO in the composite, which resulted in the highest Young’s modulus, tensile strength, KIC and GIC, increased by36%(3.4GPa),16%(80.8MPa),27%(1.29MPam1/2) and19%(0.43kJ/m2), correspondingly. Moreover, the thermal properties were increased in ATP-GO/epoxy composites as well, owing to the introduction of the heat-stable ATP.Furthermore, Raman spectroscopy methods were utilized to investigate the dispersion of GOs and the interfacial stress transfer between the two phases, based upon which the mechanism for the interaction between epoxy and GO with different surface states was discussed. The reduction of functional groups led to a poorer distribution of bwGO than aGO and a lower efficiency of stress transfer in the bwGO/epoxy composites. The dispersion of APTS-GO was less uniform than GPTS-GO, while the interaction between APTS-GO and the matrix was stronger. However, the stress transfer in APTS-GO/epoxy was found to be weaker than that in aGO/epoxy composites, which was because the mobility of graphene sheets was increased as a result of the formation of soft interphase around the GO sheets caused by the modification of silane coupling agent. Thus the strain energy was partly consumed by the soft interphase. In case of ATP-GO hybrid, the dispersion of the hybrid was a little poorer than aGO, whereas the interfacial interaction was much higher.It is obvious that the modification on GO has changed its reinforcing behavior in the composites, thus could be considered as an effective way to achieve highperformance polymer nanocomposites.