Surface-functionalized Multi-walled Carbon Nanotube And Graphene,and Their Epoxy-based Composites

It has been of vital significance to toughen the epoxy effectively without decreasing strength.Since compounding with nanofillers is a practical approach to improve the mechanical properties of polymer materials,the carbon nanomaterials,such as carbon nanotube and graphene,have been regarded as favorable nanofillers to reinforce and toughen the epoxy simultaneously due to their unique structures.How to tailor the interface precisely is the key and challenging problem for high-performance composites.In this dissertation,the controllable synthesis of poly(glycidyl methacrylate)-poly(hexyl methacrylate)block copolymer(PGMA-b-PHMA)covalently grafted carbon nanomaterials,and their effects on the interface structures and mechanical properties of epoxy-based composites were systematically studied.The grafted PGMA-b-PHMA effectively improved the dispersion of carbon nanomaterials in the epoxy matrix,and constructed a double-layered interface in the epoxy-based composites.The relationship between their micro-structures and mechanical properties,and the toughening mechanism were discussed.On that basis,the reduced graphene oxide(RGO)was bifunctionalized with polydopamine(PDA)with high adhesion and flexible branched polyethyleneimine(PEI)by hydrothermal reaction and ultrasonic electrostatic adsorption,which paved an efficient and green way for simultaneously reinforcing and toughening the polymer composites.The main results were presented as following:(1)PGMA-b-PHMA block copolymers with the fixed PGMA block and various PHMA block length were synthesized by reversible addition-fragmentation chain transfer polymerization,then grafted on multi-walled carbon nanotube(MWNT)by click reaction.The influences of PHMA block length on the grafting density and dispersibility of the synthesized PGMA-b-PHMA@f MWNT in different organic solvent were discussed.PGMAb-PHMA@f MWNTs with the grafting density of 2.1~7.6 polymer chains per 104 C atoms were synthesized,and their dispersion concentration in tetrahydrofuran reached to 2.10~2.75 mg/m L.These synthesized PGMA-b-PHMA@f MWNTs provided a solid basement for studying epoxy-based composites.(2)Epoxy/PGMA-b-PHMA@f MWNT composites were fabricated by solution compounding.The effect of PGMA-b-PHMA@f MWNT on the mechanical properties of the composites,and the relationship between their micro-structures and macroscopic mechanical properties were investigated.The grafted PGMA-b-PHMA block copolymer enhanced the composite dispersion,and constructed a double-layered interface in the epoxy-based composites.As the block length of PGMA was fixed with a molecular weight of ~6500 and PHMA block was adjusted with molecular weight of 4480~8500,loading 0.05 wt% PGMAb-PHMA@f MWNT increased the fracture toughness of the composite by 45.4~46.0%,and the tensile strength and Young's modulus of the composite were reinforced with increments of 14.4~16.5% and 10.8~13.7% respectively.It is worth mentioning that the 0.05 wt% loading was only one-twentieth of the loading in other reported epoxy/MWNT composites with the same toughness increment.The toughening mechanism of epoxy/PGMA-bPHMA@f MWNT composites was proven to owe to the microcrack bridging of MWNTs and plastic deformation of the epoxy matrix.The double-layer interface in epoxy-based composites could be optimized by the block copolymers grafted on MWNTs,providing a new theory and method for developing high-performance polymer-based composites.(3)Based on the optimized structure of the above PGMA-b-PHMA@f MWNT with the best modification to epoxy,PGMA-b-PHMA block copolymer with PGMA block length of molecular weight 6290 and PHMA block length of molecular weight 4480 was grafted on the reduced graphene oxide(RGO)by click reaction.The received PGMA-b-PHMA@f RGO was compounded with epoxy to fabricate the composites.The effects of the dimension and structure of carbon nanomaterials on their grafting densities and solvent dispersibilities were discussed as well as corresponding mechanical properties of epoxy-based composites.Loading 0.2 wt% PGMA-b-PHMA@f RGO increased the fracture toughness of the composite by 41.8%,and the tensile strength and Young's modulus of the composite were reinforced with increments of 17.5% and 27.9% respectively.Compared with epoxy/PGMA-bPHMA@f MWNT composites,the grafted PGMA-b-PHMA block copolymer has higher efficiency in toughening epoxy/RGO composites.(4)The reduced graphene oxides(RGO)was bifunctionalized with polydopamine(PDA)with high adhesion and flexible branched polyethyleneimine(PEI)by hydrothermal reaction and ultrasonic electrostatic adsorption.The received PEI-f-PDA@RGO was compounded with epoxy to fabricate the composites,and the related toughening mechanism was discussed.The bifunctionalization of PEI and PDA enhanced the dispersion of RGO in the epoxy matrix,and improved the RGO-matrix interfacial interaction.Loading 0.2 wt% PEI-f-PDA@RGO increased the fracture toughness of the composite by 53.9%,and the tensile strength and Young's modulus of the composite were reinforced with increments of 1.3% and 36.3% simultaneously.The toughening mechanism of epoxy/PEI-f-PDA@RGO composites was proven to owe to the deflection of RGO sheets to microcracks in composites.This modification method combining hydrothermal reaction with ultrasonic electrostatic adsorption is simple and environmentally friendly,and showed a great potential application in the high-performance polymer-based composites.