HBPE-b-PMMA Block Copolymer: Synthesis And Application In The Production Graphene

As a type of unique two-dimensional carbon nanomaterial,graphene shows a series of extraordinary electrical,thermal,mechanical,barrier and other properties and thus has been attracting intense attentions from various fields.It has been extensively reported that modifying polymers with graphene as a nanofiller can effectively improve the proformances of the former to give various high-performance/functionalized polymers.To achieve this,however,two major issues remain to be addressed,one is the controllable,scalable production of graphene of high quality through relatively simple processes,and the other is how to ensure the homogeneous dispersion of graphene nanosheets in specific polymer matrix and strong interfacial interactions between them.Herein,ahyperbranchedpolyethylenebearingasingle2-bromoisobutyryl bromide group?HBPE-Br?was first synthesized via an unique chain walking ethylene polymerization with a functionalized Pd-diimine catalyst?Pd-Br?.Then the as-synthesized HBPE-Br was used as a macromolecular ATRP initiator for the polymerization of methyl methacrylate?MMA?to render a diblock copolymer,HBPE-b-PMMA,which consists of a hyperbranched polyethylene?HBPE?block and a poly?methyl methacrylate??PMMA?block.A series of HBPE-Br and HBPE-b-PMMA block copolymers were synthesized under serially changedpolymerizationconditionsandtheirstructureswere characterized.Their abilities were also examined and compared as stabilizers in the liquid-phase exfoliation of graphite into graphene in organic solvents under the assistance of ultrasonication.Further,a HBPE-b-PMMA block copolymer with optimized structure was obtained and used for the scalable production of graphene.A series of PMMA/graphene nanocomposites were finally prepared by solution casting process and the effect of graphene on the properties of the composites was investigated.The details are summarized as followings:?1?Chain walking ethylene polymerizations were done at an ethylene pressure of 0.1 MPa and serially changed temperatures?5-35??for different periods?2-24 h?with the Pd-Br catalyst.The structures of the as-synthesized macroinitiators?HBPE-Br?were characterized with proton nuclear magnetic resonance?¹H NMR?,gel permeation chromatography?GPC?and melting rheology measurement,respectively.A series of graphene solutions in chloroform were subsequently obtained via noncovalently exfoliating graphite in chloroform under the assistance of ultrasonication.The graphene concentrations were quantified and compared via UV-visible?UV-Vis?absorption spectra,and their morphologyandstructureswerecharacterizedbymeansof high-resolution transmittance electron micriscopy?HRTEM?,atom force microscopy?AFM?,ramanspectroscopyandX-rayphoton spectrometer?XPS?techniques,respectively.Itisfoundthat narrow-distributed,completelyfunctionalizedHBPE-Brcanbe synthesized via chain walking ethylene polymerization with the Pd-Br at an ethylene pressure of 0.1 MPa and 5,15?for 2,4 h.The as-synthesized HBPE-Br is found to effectively exfoliate graphite in chloroform under sonication to render high-concentration,stable solution of graphene of high quality,with a thickness less than 5 layers.?2?A HBPE-Br was synthesized via chain walking ethylene polymerization with the Pd-Br at an ethylene pressure of 0.1 MPa and5?for 4 h,and used to initiate the ATRP of MMA for serially changed polymerization times from 2 to 12 h,rendering a series of HBPE-b-PMMA block copolymers.The averaged molecular weight,molecular weight distribution and composition of the resulting copolymers were characterized with ¹H NMR,GPC analysis and their chain topologies were examined via melting rheology measurement.Their abilities as stabilizers in the production of graphene in various organic solvents were also compared.It is indicated that HBPE-b-PMMA diblock copolymer can be successfully obtained via ATRP of MMA with the HBPE-Br at 90?for 4 h.The resulting copolymer is confirmed to consist a hyperbranched block and a linear PMMA block and have a MMA proportion of 26 mol%.It is further found that the obtained HBPE-b-PMMA copolymer can effectively exfoliate graphite in chloroform to give stable graphene solutions with a concentration up to0.113 mg/mL.These graphene flakes are of free defects and have a thickness less than 5 layers.?3?A HBPE-b-PMMA copolymer was synthesized under optimized conditions?0.1 MPa/5?/4 h for the ethylene polymerization stage;90?/4h for the ATRP of MMA stage?,and used for the scalable production of graphene in chloroform under sonication.A series of PMMA/graphene nanocomposites with serially changed graphene loadings were prepared through solution casting process.Their structures were characterized with scanning electron microscopy?SEM?and wide-angle X-ray diffraction?WAXRD?tests,respectively,and their electrical and thermal properties were also examined.Thermal analysis?TGA?result for the graphene shows that some HBPE-b-PMMA copolymer are irreversibly adsorbed onto the graphene flakes,testifying the presence of noncovalent interactions between them.The as-obtained graphene are found to be dispersed homogeneously in the PMMA matrix with a stronger interfacial interactions between them,originating from the presence of some HBPE-b-PMMA copolymer onto the graphene flakes.Both electrical and thermal conductivities for the PMMA can be effectively improved upon the addition of the graphene.For the addition of graphene volume at 3.12 vol%,the surface resistivity of the composite is found to decline by 5 orders as compared to the pristine PMMA,with the thermal conductivity considerably rising from 0.26 to 0.401 W·m^-1·K^-1.In summary,a novel strategy has been proposed in this dissertation for the high-efficiency scalable production of graphene and the preparation of high-performance polymer/graphene nanocomposites,which should be helpful to promote the scalable application of graphene in the field of polymer modification.