Preparation And Functionalization Of Graphene And Its Application In Composites

Graphene is a single layer graphite sheet that consisting of sp2carbonatoms covalently bonded in honeycomb crystal lattice. Due to itsfascinating properties such as giant electron mobility, high thermalconductivity and high thermal stability, graphene has aroused considerableresearch interest and exhibited broad application prospect in physics,chemistry, materials and biology, etc. Recently, owing to its intrinsicexcellent mechanical strength, large surface area, electric and thermalconductivities, graphene nanosheets are widely used as ideal nanofillers forenhancing and extending the properties in polymer composites. However,for graphene-based composite materials, there are some main problemsstill waiting for resolve in its application of academia and industry, e.g.,preparation and functionalization of graphene, improving the dispersibilityof graphene and its derivative in polymer matrices, and enhancing interfaceinteraction between graphene nanosheets and polymer matrix. Therefore,we carried the research works around these above problems. In thisdissertation, we innovative utilized various implementation methods forpreparation and functionalization of graphene and further realized andexpanded the composite applications of graphene in composite materials.The details of research works are as follows:(1) Using a modified Hummers method, graphite oxide was obtained byoxidation of graphite. It can be exfoliated into graphene oxide (GO)nanosheets and formed uniform and stable GO aqueous dispersion with sonication. Through free-radical polymerization of acrylamide (AAm) andSA in the presence of GO in aqueous system followed with ionicallycrosslinking of calcium ions, a novel graphene oxide (GO)/sodium alginate(SA)/polyacrylamide (PAM)(GSP) ternary nanocomposite hydrogel withexcellent mechanical performance have been fabricated. As-preparedGO/SA/PAM (weight ratio SA/AAm=1/2) ternary nanocompositehydrogel with5wt%of GO displays a compressive stress as high as1.543MPa at the compressive deformation of70%. The tensile strength andmodulus of the hydrogel achieved~201.7and~30.8kPa, respectively.Meantime, the GSP nanocomposite hydrogels can recover a largeproportion of elongation at break and exhibit the good elasticity.Additionally, the GSP ternary nanocomposite hydrogel exhibitedgood adsorption properties for water-soluble dyes.(2) Multiwalled carbon nanotubes (MWNTs) have been widely used asnanofillers for polymer reinforcement. However, it has been restricted bythe limited available interface area of MWNTs in the polymer matrices.Oxidation unzipping of MWNTs is an effective way to solve this problem.Unzipped multiwalled carbon nanotube oxides (UMCNOs) obtained byoxidation unzipping MWNTs were used as novel nanofillers formechanical reinforcement of chitosan (CS) matrix. The UMCNOs/CSnanocomposite films with different amounts of UMCNOs were fabricatedby solution-casting the mixtures of UMCNOs and CS acetic acid aqueousdispersions. The structures and mechanical properties of thenanocomposite films were characterized by XRD, FT-IR, SEM, and tensiletests. The results demonstrated that Compared to neat chitosan, theUMCNOs/CS nanocomposite films showed~105.9%increase in tensilestrength from69.3to142.7MPa, and~165.3%increase in Young’s modulus from2.6to6.9GPa with incorporation of only0.2wt%ofUMCNOs into the chitosan matrix.(3) The unzipped multiwalled carbon nanotube oxides (UMCNOs)exhibit excellent enhancement effect with low weight fractions, butagglomeration of UMCNOs at a relatively higher loading still hamperedthe mechanical reinforcement of polymer composites. We interestinglyfound that the dispersion of UMCNOs in polymer matrices can besignificantly improved with the combination of pristine MWNTs. Thehybrids of MWNTs and UMCNOs (U/Ms) can be easily obtained byadding the pristine MWNTs into the UMCNOs aqueous dispersionfollowed with sonication. With π-stacking interaction, the UMCNOs wereattached onto outwalls of MWNTs. The mechanical testing of the resultantpoly (vinyl alcohol)-based composites demonstrated that the U/Ms can beused as ideal reinforcing fillers. Compared to poly (vinyl alcohol)(PVA),the yield strength and Young’s modulus of U/Ms-PVA composites with aloading of0.7wt%of the U/Ms approached~145.8MPa and6.9GPa,which are increases of~107.4%and~122.5%, respectively. Thereinforcement effect of U/Ms is superior to the individual UMCNOs andMWNTs due to the synergistic interaction of UMCNOs and MWNTs.(4) The bulk aramid macroscale fibers can be effectively split intoaramid nanofibers (ANFs) in dissolution of dimethylsulfoxide (DMSO)with the presence of potassium hydroxide (KOH). We first introduced theANFs into the structure of graphene nanosheets through noncovalentfunctionalization with π-π stacking interaction. Aramid nanofibersfunctionalized graphene sheets (ANFGS) were successfully obtained byadding the graphene oxide (GO)/DMSO dispersion into the ANFs/DMSOsolution followed with the reduction of hydrazine hydrate. Combining the two ultra-strong materials, ANFs and graphene nanosheets (GS), theANFGS can be acted as novel nanofillers for polymer reinforcement.ANFGS can be used as ideal nanofillers for reinforcing the mechanicalproperties of poly (methyl methacrylate)(PMMA). With loading of0.7wt%of ANFGS, the tensile strength and Young’s modulus of ANFGS/PMMAcomposite film approached63.2MPa and3.42GPa, increased by~84.5%and~70.6%respectively. The thermal stabilities of ANFGS/PMMAcomposite films were improved with addition of ANFGS. Additionally, thetransparencies of ANFGS/PMMA composite films have a certain degreeof UV-shielding effect due to the ultraviolet light absorption of ANFs inANFGS.(5) Based on the method of direct liquid-phase graphene exfoliation，we present a green and facile way for preparing graphene–Ag nanohybridsassisted with gum arabic (GA). GA functionalized graphene sheets (GA-G) were prepared by directly exfoliating graphite flakes in the gum arabic(GA) aqueous solution with sonication. After sonication for8h, the CGScan run up to0.69mg mL-1with the initial graphite concentration of140mg/mL and GA concentration of100mg mL-1. The distributions of thelateral dimensions and thickness for graphene flakes were mainlyconcentrated at the range of0.5-2μm2and2-6nm, respectively. The silverions can be directly reduced and immobilized on the surface of GA-Gnanosheets by GA. The Ag/GA-G hybrid materials can be used as suitablesubstrates of surface-enhanced Raman spectroscopy (SERS) for detectionof4-aminothiophenol (4-ATP) at detectable level of concentration of10-6M in aqueous environment.(6) Glycidyl methacrylate-modified gum arabic (GMA-GA) wasobtained by chemically modification of gum arabic (GA) with glycidyl methacrylate (GMA). Through two different pathway reactions, epoxyring-opening and transesterification, the vinyl groups of C=C coming fromGMA were coupled onto the polysaccharide structure of GA. Aftermodification, the GMA-GA can still be used for liquid-phase directexfoliation of graphite. With assist of GMA-GA, the maximumconcentration of graphene flakes can reach~1.12mg/mL. Using thistechnique, GMGS can be easily obtained by centrifugal separation aftersonication. The functionalized graphene flakes, coupled with vinyl groupscoming from the GMA-GA were introduced into poly (acrylic acid)(PAA)hydrogel through in situ polymerization for enhancing the mechanicalproperty of hydrogels. Compared to PAA hydrogel, the compressivestrength and elastic modulus of GMGS/PAA composite hydrogel with5wt%of functionalized graphene flakes reach~49.2and~66.9kPa, increasedby~846.1%and~243.7%, respectively.