Hyperbranched Polyethylene Copolymers Synthesis, Characterizations And Their Noncovalent Interactions With Graphene

As a type of unique two-dimensional carbon nanomaterials, graphene possesses wide application prospects in various fileds, such as energy, environment, biomedical, sensor and polymer modification, and so on. It is of significant importance to obtain stable, high-concentration solutions of graphene free of structural defects for its researches and various emerging applications. Two hyperbranched polyethylene (HBPE) copolymers are herein synthesized, including HBPE-b-PMMA block copolymer and HBPE@POSS copolymer covalently tethered multiple POSS nanoparticles. Their structures were characterized and noncovalent interactions with graphene in organic solvents were systematically investigated. The details are summarized as followings:(1) Chain walking ethylene polymerization was carried out with a functionalized Pd-diimine catalyst (Pd-Br) to give a HBPE macroinitiator (HBPE-Br), which was further used to initiate ATRP of MMA, giving a HBPE-b-PMMA block copolymer. Characterizations on the resulting copolymer were performed via various methods including proton nuclear magnetic resonance (1H NMR), gel permeation chromatography (GPC), dynamic light scattering (DLS), thermogravimetry analysis (TGA), differential scanning calorimetry (DSC), fourier transformed infrare (FTIR) spectroscopy and melt rheology measurement. It is confirmed that the copolymer consists of HBPE and linear PMMA block, with a MMA unit proportion of30.6mol%relative to total units.(2) A series of stable graphene solutions were obtained in various organic solvents via noncovalent liquid exfoliation of natural graphite with the resulting HBPE-b-PMMA block copolymer, under the assistance of ultrasonication. The structure of the resulting graphene flakes was characterized with UV-Vis spectroscopy, High-resolution transmittance electron microscopy (HRTEM), atom force microscopy (AFM), Raman spectroscopy and wide-angle X-ray diffraction (WAXRD) etc. The copolymer was found to effectively exfoliate graphite under the assistance of ultrasonication in tetrahydrofuran (THF), chloroform and toluene, leading stable, high-concentration solutions of graphene free of structural defects with a thickness of2～4layers. Results from FTIR and TGA confirm the presence of strong noncovalent interactions between the copolymer and graphene flakes.(3) HBPE@POSS copolymer was synthesized via chain walking copolymerization of ethylene and POSS monomer with an acetonitrile Pd-diimine catalyst under ethylene pressure of1atm and35℃. The resulting copolymer was characterized with1HNMR, GPC, FTIR, TGA, DLS and melt rheology test, and it is confirmed that the copolymer has a hyperbranched chain topology, with a POSS proportion of2.43mol%relative to total ethylene units. The copolymer was further found to effectively exfoliate graphite in THF or chloroform under the assistance of ultrasonication, leading stable, high-concentration graphene solutions. The as-prepared graphene was found to has a thickness of2～4layers with low structural defects. By means of the noncovalent interactions between HBPE and graphene, POSS nanoparticles could be introduced onto the surface of graphene flakes.(4) Polymer-based graphene nanocomposites with changed graphene loadings were obtained via noncovalent exfoliation of graphite with the HBPE-b-PMMA copolymer in toluene and subsequent precipation in methanol. The structures and performances of the as-prepared composites were characterized and it is shown that introducing graphene into the copolymer as a matrix leads to simultaneous enhance of thermal stability, mechanical and electrical properties.