Graphene Oxide Toughened Epoxy Resin And Its Application In Carbon Fiber Composites

Graphene oxide?GO?,as an oxidized graphene derivative,is a kind of excellent polymer nano-reinforcement.In this paper,GO with different oxidation levels were prepared by controlling the oxidation conditions,and the structure evolution of GO with different oxidation levels were explored.The influences of GO on epoxy resin properties,including mechanical properties,thermal properties,rheological properties and curing behavior,were investigated.Emphasis was placed on studying the relationship between GO structure and its reinforcing effects on epoxy matrix.In addtion,GO with optimized structure was used for toughening carbon fiber/epoxy composite,and the influences of addition of GO in the epoxy matrix on the in-plane and interlaminar mechanical properties of carbon fiber/epoxy composites were investigated.The main research contents and results in this thesis are shown as follows:?1?Five graphene oxide with different oxidation levels were prepared by the modified Hummers'method in which the oxidant/graphite ratio and oxidation time were varied,named GO-1,GO-2,GO-3,GO-4 and GO-5.The composition and structure change of GO were investigated by Fourier transform infrared spectroscopy?FTIR?,and X-ray photoelectron spectroscopy?XPS?measurements.These test results reveal the oxidation levels from GO-1 to GO-5 show a continuous increase,which indicates increasing the ratio of the oxidant/graphite or extending the oxidation time can improve the oxidation levels of the GO.GO with lower oxidation mainly include aromatic carbon C=C,carbonyl and phenolic hydroxyl groups on the surface.As the oxidation degree increase,the characteristic band intensities of aromatic carbon C=C,carbonyl and phenolic hydroxyl groups get weakened and the bands from epoxy,ether and aliphatic hydroxyl groups gradually become dominant.This indicates carbonyl,carboxyl and phenolic groups were generated at the edges of graphene layers at the initial stage of the oxidation.With the oxidation proceeds,aliphatic hydroxyl groups were formed on the basal planes of graphene layers,and partial aliphatic hydroxyl groups further converted to epoxy and ether groups by dehydration.In addition,the reusluts of Raman spectrum and dynamic light scattering?DLS?test show that the exfoliation degree and size of GO increases with the oxidation levels.?2?The disperse states of different GO in the hardener isophoronediamine?IPDA?were observed by optical microscope.The results reveal the disperse state of GO in IPDA is closely associated with the oxidation levels of GO.Both GO with the lowest?GO-1?and highest?GO-5?oxidation levels tend to aggregate,while GO-4 has the best dispersion state in IPDA.In addition,FTIR and XPS measurements prove the GO surfaces can be chemically functionalized by IPDA during the dispersing process,which will improve the interfacial interaction between GO and the epoxy resin matrix.Then,the influences of different GO on the performances of GO/epoxy composites,including curing behaviors,rheological,thermal and mechanical properties,were respectively investigated by differential scanning calorimeter?DSC?,viscosity rheometer and dynamic mechanical analysis?DMA?.These results indicate that the addition of GO can accelerate the curing reactions of epoxy/IPDA blends and reduce the initial curing temperature because of active hydrogen on the surface of GO.Meanwhile,the incorporation of GO can increase the viscosity and decreases the pot life of the epoxy matrix.For cured epoxy resin,the storage modulus increases but the glass transition temperature decreases slightly with the incorporation of GO.Finally,the results of mechanical tests illustrate that the tensile strength,modulus and fracture toughness of GO/epoxy resin composite materials with introduction of the GO?0.2 wt%?are improved.GO-4 shows the best toughening effect,especially for fracture toughness.The incorporation of merely 0.2 wt%GO-4 would yield 56 and 128%enhancement in the critical stress intensity factor(K_IC)and the critical strain energy release rate(G_IC),respectively.?3?Carbon fiber/epoxy composites were fabricated using GO-4 modified epoxy as matrix,and their flexural property,short beam shear?SBS?strength and mode II interlaminar fracture toughness(G_IIC)were tested according to ASTM standard.The results of mechanical tests show that,compared with the unmodified carbon fiber/epoxy composite,the GO-4 modified carbon fiber/epoxy composites give about 14%,17%and 12%enhancements in flexural strength,SBS strength and G_IIC,respectively.The fracture surfaces of carbon fiber/epoxy composites were observed by SEM,and the observations show that interfacial interactions between carbon fiber and epoxy matrix are improved with the addition of GO.