Synthesis Of Hyperbranched Polyethylene-g-Poly(Methyl Methacrylate) Copolymer And Its Application In The Production Of Graphene

Liquid-phase exfoliation of graphite in specific solvents under the assistance of ultrasonication or shear force is an extremely promising route for graphene production,as it can conveniently render dispersions of high-quality graphene through simple process using abundant natural graphite as starting materials,which is highly desirable for the liquid-phase processing and applications of resultant graphene.There are two major issues in this research area,one is how to efficiently exfoliate graphite into graphene in common solvents,and the other is how to functionalize the exfoliated graphene nanosheets.On the basis of our earlier researches,herein,we have synthesized a series of binary copolymers,HBPE-g-PMMA consisting of a hyperbranched polyethylene(HBPE)backbone and multiple poly(methyl methacrylate)(PMMA)side chains.The effectiveness of the as-synthesized copolymers as stabilizer for liquid-phase exfoliation of graphite into graphene in a series of common organic solvents has been systematically investigated.The application performance of the resultant graphene in flexible polymer composite films and graphene-based micro supercapacitors is also examined.All the results are summarized as followings:(1)A hyperbranched polyethylene covalently bearing multiple isobutyryl bromide terminal groups,HBPE@Br,was first synthesized via chain walking copolymerization of ethylene and BIEA,with Pd-diimine catalyst at an ethylene pressure of 0.1 MPa and25~oC.Then,the HBPE@Br was used as macroinitiator to initiate ATRP of methyl methacrylate(MMA)at 90~oC to render a series of HBPE-g-PMMA copolymers under different polymerization times.The structure,compositions,chain topology of resultant copolymers were characterized by a series of different techniques,including proton nuclear magnetic resonance(~1H NMR),fourier transformed infrared spectroscopy(FTIR),melt rheological measurement,gel permeation chromatography(GPC)and dynamic laser light scattering(DLS).It is confirmed that the resultant copolymers consist of an approximately global HBPE backbone(degree of branching:83branches/1000 C)and multiple covalently tethered PMMA side chains(graft ratio:0.61~3.07 g/g PE).The copolymers are highly soluble at ambient temperature in a series of conventional organic solvents,such as chloroform,dichloromethane,tetrahyfrofuran and toluene etc.Meanwhile,flexible polymer films could be conveniently obtained from the resultant copolymers with adequate PMMA ratio,such as 2.57 g/g PE,via simple solution-casting process.(2)A series of graphene dispersions were fabricated via liquid-phase exfoliation of graphite under sonication in various different common organic solvents,by using the resultant HBPE-g-PMMA as stabilizer.The graphene concentrations and corresponding graphite exfoliation efficiencies were then quantified for various resultant dispersions,by UV-vis absorbance spectroscopy according to Lambert-Beer law,and the effects of various parameters,including solvent type,polymer composition,graphite feed concentration,and polymer/graphite mass ratio were systematically examined.The morphology,strural defects,lateral size and thickness of resultant graphene flakes were characterized by high-resolution transmittance electron microscopy(HRTEM),atomic force microscopy(AFM),wide-angle X-ray diffraction(WAXRD),Raman and X-ray photoelectron spectroscopy(XPS).Also,the noncovalent interactions between the graphene and copolymer are investigated by TGA and FTIR analyses.On this basis,a mechanism for the exfoliation of graphene has been proposed.It is found that the resultant HBPE-g-PMMA copolymer can promote grahite exfoliation in chloroform at efficiencies higher than those achieved with the pure HBPE.A graphene concentration,0.13 mg/mL,can be achieved with the HBPE-g-PMMA-0.5 h in chloroform.The as-obtained graphene flakes appear as approximately rectangular shape with a length of100 to 400 nm,a width of 50 to 200 nm,and a thickness of 2 to 4 layers.The efficient exfoliation of graphite under the assistance of the HBPE-g-PMMA is attributed to the noncovalent CH-πinteractions between the copolymer and graphene,which makes the former reversibly adsorbed on the exfoliated graphene surface to effectively prevent their aggregation.(3)Further,a set of graphene/HBPE-g-PMMA composite films were prepared from the dispersions via simple solution-casting process.The dispersion status of graphene nanosheets in the resultant films are observed by scanning electron microscopy(SEM)and the electrical performance of the films were measured.It is shown that the resultant graphene nanosheets can be dispersed homogeneously into the HBPE-g-PMMA as a matrix,with a considerable drop in electrical resistance by 4orders,as compared to the pure polymer film,at a graphene loading of 4.8 vol%.Moreover,the resultant film remains flexible.However,the films become brittle with further increasing the graphene loading.(4)Further,a series of graphene films were made by vacuum filtering the resultant graphene dispersion using a PVDF membrane and their thicknesses were adjusted by changing the volume of dispersions and graphene concentrations.Subsequently,various graphene-based micro-supercapacitors(G-MSCs)of different thicknesses were obtained by“scratch method”,with their electrochemical performance being measured by cyclic voltammetry(CV)and constant current charge and discharge(GCD).The results show that the surface capacitance of G-MSCs rises with the increase in the thickness of resultant graphene film.When the thickness of G-MSCs is 19μm,the surface capacitance of the film can reach 5.08 mF/cm~2 at the current density of 0.011mA/cm~2.