Synthesis and characterization of well-defined poly(l-lactide) functionalized graphene oxide sheets with high grafting ratio prepared through click chemistry and supramolecular interactions

Two simple and effective methods, “click” chemistry and supramolecular interactions, are demonstrated here to synthesize well-defined poly(l-lactide) (PLLA) functionalized graphene oxide (GO) sheets. We provide a simple method to introduce azide groups on GO sheets by the ring opening reaction of sodium azide with the epoxide groups of GO. The GO-N₃ sheets can easily undergo “click” reaction with alkyne-terminated PLLA by “grafting onto” method to produce GO/PLLA composites with high grafting ratio and exfoliated structure. Interestingly, GO-N₃ can be grafted with oxygen-containing polymers such as PLLA, polymethyl methacrylate (PMMA) or polyethylene oxide (PEO) via supramolecular interactions between the azide groups and these oxygen atoms on polymers, producing GO/polymer composites with low grafting ratio and intercalated structure. These “grafting onto” methods are useful to produce a variety of GO/polymer composites with different structure via “click” reaction or supramolecular interactions, which have potential applications in material science.     

Chemical Functionalization of Graphene Oxide by Poly(styrene sulfonate) Using Atom Transfer Radical and Free Radical Polymerization: A Comparative Study

Poly(sodium styrenesulfonate)-functionalized graphene was prepared from graphene oxide, using atom transfer radical polymerization and free radical polymerization. In atom transfer radical polymerization route, the amine-functionalized GO was synthesized through hydroxyl group reaction of GO with 3-amino propyltriethoxysilane. Atom transfer radical polymerization initiator was grafted onto modified GO (GO-NH₂) by reaction of 2-bromo-2-methylpropionyl bromide with amine groups, then styrene sulfonate monomers were polymerized on the surface of GO sheets by in situ atom transfer radical polymerization. In free radical polymerization route, the poly(sodium 4-styrenesulfonate) chains were grafted on GO sheets in presence of Azobis-Isobutyronitrile as an initiator and styrene sulfonate monomer in water medium. The resulting modified GO was characterized using range of techniques. Thermal gravimetric analysis, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy results indicated the successful graft of polymer chains on GO sheets. Thermogravimetric analysis showed that the amount of grafted polymer was 22.5 and 31?wt% in the free radical polymerization and atom transfer radical polymerization methods, respectively. The thickness of polymer grafted on GO sheets was 2.1?nm (free radical polymerization method) and 6?nm (atom transfer radical polymerization method) that was measured by atomic force microscopy analysis. X-ray diffractometer and transmission electron microscopy indicated that after grafting of poly(sodium 4-styrenesulfonate), the modified GO sheets still retained isolated and exfoliated, and also the dispersibility was enhanced.     

Highly Exfoliated Poly(Ethylene Terephthalate)/Clay Nanocomposites via Melt Compounding: Effects of Silane Grafting

The ammonium intercalated montmorillonite (A-MMT), especially acid treated montmorillonite (H-S-A-MMT), which experienced silane grafting treatment, exhibited highly exfoliation state in poly(ethylene terephthalate) (PET) matrix. It can be explained that the shear stress was transferred effectively from molten polymer to clay layers during melting processing due to the enhanced polymer-clay interaction by the grafted silane, leading to the slippage and peeling of clay sheets. Furthermore, the equilibrium melting point reduction of highly exfoliated nanocomposite calculated by the Hoffman-Weeks formula exhibited great miscibility between clay and polymer, which was proved by the Nishi-Wang equation.     

Properties of carbon fiber and composites modified with different‐sized graphene oxide sheets

The graphene oxide (GO) sheets with different size distributions were effectively separated by a centrifugation method. The exfoliated single‐layer structure and the size of GO sheets were verified by scanning electron microscopy (SEM), atomic force microscope (AFM), and metallurgical microscope, respectively. Two different‐sized GO sheets water suspensions were obtained, which were then directly dispersed in carbon fiber (CF) sizing agent, respectively. The influences of the different‐sized GO sheets on CF and CF composites were explored. The workability in later process of CF and CF surface morphology were characterized by abrasion resistance, fluffs and breakage, stiffness, and SEM. SEM micrographs demonstrated that a nonuniform distribution of the large‐sized GO was lapped on CF whereas the small‐sized GO was uniformly leaned on CF. The interlaminar shear strength of the small‐sized GO/CF reinforced composite could reaches the maximum value. It indicated that the interfacial region between CF and polymer matrix could be enhanced by adjusting the size of GO sheets. POLYM. COMPOS., 37:2719–2726, 2016. © 2015 Society of Plastics Engineers     

In situ exfoliation of graphite oxide nanosheets in polymer nanocomposites using miniemulsion polymerization

Poly(styrene-co-butyl acrylate) (poly(St-co-BA)) nanocomposite latices based on graphene oxide (GO) were synthesized by miniemulsion polymerization. The polymerization procedure involved dispersing an aqueous solution of graphite oxide in a monomer phase, followed by emulsification in the presence of a hydrophobe and a surfactant into miniemulsions. The focus was to investigate the suitability of miniemulsion for the synthesis of polymer nanocomposites based on a graphene derivative (i.e., GO) with exfoliated structure in a one-step nano-incorporation technique. Poly(St-co-BA) nanocomposites containing the exfoliated GO nanoplatelets, which have improved mechanical and thermal properties were successfully synthesized by the miniemulsion process. The nanostructure of the nanocomposites was investigated by transmission electron microscopy (TEM) and X-ray diffraction (XRD). TEM and XRD indicated that the nanocomposites mainly showed exfoliated morphologies, except at relatively high GO content. TEM also revealed that the nanocomposite latices had the so-called ‘‘armored’’ structure, where the nanosized GO sheets are distributed around the edges of the copolymer particles.     

Preparation and properties of exfoliated nanocomposites through intercalated a photoinitiator into LDH interlayer used for UV curing coatings

The exfoliated polymer/layered double hydroxide (LDH) nanocomposites based on MgAl were prepared through intercalating a photoinitiator into LDH interlayer, following by UV irradiation induced polymerization. The fragmental photoinitiator, 2-hydroxy-2-methyl-1-phenylpropane-1-one (1173) firstly reacted with isophorone diisocyanate (IPDI) to obtain the semiadduct, 1173-IPDI, and then reacted with the LDH modified by aminoundecanoic acid, obtaining LDH-1173 with an intercalated microstructure, which was characterized by FTIR, XRD, and TGA measurements. The obtained LDH-1173 was mixed with the multifunctional acrylate oligomer and monomer, and then exposed to a UV lamp to prepare a polymer/LDH nanocomposite. From the XRD, TEM and HR-TEM analysis, as well the photopolymerization kinetics investigation, it was found that the LDH-1173 effectively initiated the photopolymerization of acrylates, and formed exfoliated polymer/LDH nanocomposites. However, the mostly intercalated polymer/LDH nanocomposites were obtained for the systems with additional 1173 except for LDH-1173 addition. Compared with the pure polymer material, both the exfoliated and intercalated polymer/LDH nanocomposites exhibited the enhancements in mechanical and thermal properties, as well as hardness.     

Preparation of polyacrylonitrile/graphene oxide by in situ polymerization

Highly oriented molecular structure is essential for high‐performance carbon fibers. The addition of a small amount of graphene sheets may enhance the degree of molecular orientation of precursor fibers during spinning and stabilization by limiting the disorientation of the chain segments. Graphene sheets merge into the carbon fiber structure during carbonization. The structure and properties of polyacrylonitrile containing graphene oxide (GO) prepared by in situ polymerization were investigated. With increasing GO loading, the molecular weight of the polymer decreased gradually from 69 000 g mol^?1 for the sample without GO to 60 600 g mol^?1 for the sample with 2.5 wt% loading of GO. Scanning electron microscopy and X‐ray diffraction results indicated that GO was dispersed in single layers in the polymer matrix. The degree of crystallization of the polymer with 0.5 wt% GO was increased by 8%. Moreover, differential scanning calorimetry and thermogravimetric analysis showed that an appropriate amount of GO, e.g. 0.5 wt%, made the carbon yield of the polymer increase by 5.0 wt%, because the GO in the composite improved the intermolecular crosslinking reaction. Copyright © 2012 Society of Chemical Industry     

Effects of different kinds of clay and different vinyl acetate content on the morphology and properties of EVA/clay nanocomposites

A series of EVA/clay nanocomposites and microcomposites have been prepared via melt-blending. Using four kinds of EVA with different vinyl acetate (VA) contents: 28, 40, 50 and 80 wt%, and four kinds of clay: three are organophilic clay (OMMT) and one unfunctionalized clay (Na-MMT), the effects of different VA content of EVA and the kinds of the clay on the morphology and properties of EVA/clay nanocomposites were systematically investigated. In previous studies, there are only two distinct nanostructures to distinguish polymer/clay nanocomposites: the intercalated and the exfoliated. But in this paper, we proposed a new nanostructure—‘the wedged’ to describe the dispersion degree of clay in nanocomposites, it means the sheets of clay were partly wedged by the chains of polymer. The wedged, the intercalated and the partially exfoliated structures of EVA/clay nanocomposites were characterized by X-ray diffraction (XRD) and by high-resolution transmission electron microscopy (HRTEM). The enhanced storage modulus of EVA/clay nanocomposites was characterized by dynamic mechanical thermal analysis (DMTA). The enhanced degree in the storage modulus of the OMMT on EVA/clay nanocomposites with the partially exfoliated and intercalated structure is much higher than that with wedged structure, and that with the higher VA content is higher than that with the lower. The thermal stabilities of EVA/clay nanocomposites were also studied by thermal gravimetric analysis (TGA).     

Effect of processing conditions on morphology and mechanical properties of compatibilized polypropylene nanocomposites

This work focuses on the influence of processing conditions on the nanocomposites structure, i.e. intercalated or exfoliated, and on the enhancement of mechanical properties of polypropylene (PP) nanocomposites. These nanocomposites were prepared using the melt intercalation technique in a co-rotating intermeshing twin screw extruder. In order to optimise processing conditions, both screw speed and barrel temperature profile were changed. The role of the compatibilizer (maleic anhydride grafted polypropylene) was also studied. The results obtained show that the barrel temperature is a very important parameter: using lower processing temperature, the apparent melt viscosity and, consequently, the shear stress are higher and, therefore, the exfoliation of the clay is promoted. Even using optimised processing conditions, exfoliation of clay can be achieved only when an high compatibility between polymer and clay exists: the PP nanocomposites containing maleic anhydride show an exfoliated structure and a sensible enhancement of mechanical properties while PP nanocomposites without compatibilizer show a structure mainly intercalated and a lower improvement of mechanical properties.     

Grafting of hydrophilic monomers onto aminopropyl‐functionalized sodium montmorillonite via surface‐initiated redox polymerization

Hydrophilic polymer/sodium montmorillonite (Na‐MMT) hybrid nanomaterials were prepared via surface‐initiated redox polymerization of 2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid (PAMPS‐g‐MMT), acrylamide (PAAm‐g‐MMT) and styrenesulfonic acid sodium salt (PSSA‐g‐MMT) from surface of aminopropyl‐functionalized sodium montmorillonite (AMMT) dispersed in an aqueous medium. Cerium(IV) ammonium nitrate/nitric acid and aminopropyl groups on the surface of AMMT were used as oxidant and reducing groups, respectively. AMMT was prepared by covalently attaching 3‐aminopropyltriethoxysilane onto the surface of Na‐MMT. Hydrophilic monomers (AMPS, AAm and SSA) were then grafted onto AMMT dispersed in water via redox initiation at 40 °C. Structure, morphology and thermal properties of the AMMT, PAMPS‐g‐MMT, PAAm‐g‐MMT and PSSA‐g‐MMT hybrid materials were characterized using Fourier transform infrared (FTIR), X‐ray diffraction (XRD) and thermogravimetric (TGA) analyses, respectively. FTIR results indicated that hydrophilic monomers were successfully grafted onto the surface of MMT. Grafting amounts of the hydrophilic polymers were estimated from TGA thermograms to be 28.8, 118.8 and 14.4% for PAMPS, PAAm and PSSA, respectively. XRD patterns showed an exfoliated morphology for PAMPS‐ and PAAm‐grafted MMT hybrid nanomaterials and an intercalated/exfoliated morphology for the PSSA‐grafted MMT one. The effect of the nature of hydrophilic monomer on the grafting efficiency is discussed in detail. © 2013 Society of Chemical Industry     

“Clicking” graphite oxide sheets with well-defined polystyrenes: A new Strategy to control the layer thickness

Polystyrene (PS) graft graphite oxide (GO) was synthesized by Cu (I)-catalyzed 1, 3-dipolar cycloaddition (“click” coupling) of azido modified graphite oxide with well-defined, alkyne-terminated polystyrene. This method produced PS blocks graft on GO surface, while ordered layer structure between GO sheets was observed for the first time in GO/Polymer composite. The layered structure was characterized and confirmed by 1D X-ray diffraction (XRD) and atomic force microscopy (AFM), and the layer thickness was observed to be controlled with intercalated PS length between GO layers.     

Preparation of graphene oxide/natural rubber composites by latex cocoagulation: Relationship between microstructure and reinforcement

Graphene oxide(GO) has recently attracted substantial interest as a possible reinforcing agent for next generation rubber composite materials. In this research, GO was incorporated in natural rubber(NR) composites through latex co-coagulation technique. The microstructures of GO/NR composites were characterized through a combination of transmission electron microscope, scanning electron microscope, X-ray diffraction, Fourier transform infrared spectroscopy, and Differential scanning calorimeter. The results showed that highly exfoliated GO sheets were finely dispersed into NR rubber matrix with strong interface interaction between GO and NR. The mechanical properties of the GO/NR composites were further evaluated. The results showed that the tensile strength, tear strength and modulus can be significantly improved at a content of less than 2 phr. Especially,GO exhibited specific reinforce mechanism in NR due to the stress-induced crystallization effects of NR. The stress transfer from the NR to the GO sheets and the hindrance of GO sheets to the stress-induced crystallization of NR were further displayed in stress–strain behavior of GO/NR composites. These enhanced properties were attributed to the high surface area of GO sheets and highly exfoliated microstructures of GO sheets in NR.     

A new approach to polymer/montmorillonite nanocomposites

A novel method for preparation of exfoliated/intercalated nanocomposites is reported based on two steps, i.e. preparation of treated-montmorillonite (MMT) solution and solution blending with polymers. After in situ polymerization of dimethyldichlorosilane between layers and separation of most polydimethylsiloxane (PDMS), the treated-MMT solution shows good storage stability. Although elemental analyzer shows no residue PDMS, NMR proves residue PDMS still exists in the solution. The residue PDMS is believed to graft onto the MMT layer surface via condensation of hydroxyl groups of PDMS and those that existed on MMT surface. Lower relaxation time of end-capped CH₃ of alkyl ammonium grafted onto layer surface via ion exchanging in the solution shows that the layer spacing was increased significantly or even exfoliated. When the solution was blended with some polar polymers, exfoliated nanocomposites were found. When it was blended with some nonpolar polymers, however, intercalated nanocomposites were obtained. The reason was explained in the light of compatibility between polymer matrix and MMT as well as alkyl ammonium and PDMS grafted on the layer surface. For intercalated nanocomposites, different layer spacing corresponds to different chain flexibility and the presence of multi-peaks is caused by the processing of these blends.     

Polypropylene nanocomposite film: A critical evaluation on the effect of nanoclay on the mechanical,thermal, and morphological behavior

Polypropylene (PP)/clay nanocomposites prepared by melt blending technique using different percentages of clay with and without maleic anhydride grafted PP (MA‐PP) were studied. The intercalated and exfoliated structure of nanocomposites was characterized by X‐Ray Diffraction (XRD) and transmission electron microscopy (TEM). Because of the typical intercalated and exfoliated structure, the tensile modulus of the nanocomposites were improved significantly as compared to virgin PP. The viscoelastic behavior of the nanocomposites was studied by dynamical mechanical analysis (DMA) and the results showed that with the addition of treated clay to PP there was substantial improvement in storage modulus increases. The thermal stability and crystallization of the PP nanocomposites as studied by differential scanning calorimeter (DSC) and thermo gravimetric analysis (TGA) were also improved significantly compared to PP. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Designing of fullers‐earth‐containing poly(vinyl alcohol)‐g‐poly(2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid) nanocomposites: Swelling and deswelling behaviors

In this study, polymer–clay nanocomposites (PCNs) composed of poly(vinyl alcohol)s (PVAs), poly(2‐acrylamido‐2‐methyl‐1‐propane sulfonic acid), and fullers earth were prepared by the effective dispersal of inorganic nanoclays in the organic PVA matrix via in situ free‐radical polymerization with potassium persulfate as an initiator and N,N‐methylene bisacrylamide as a crosslinker. The monomer, 2‐acrylamido‐2‐methyl‐1‐propane sulfonic acid, was grafted onto the PVA backbone, and at the same time, fullers earth layers were intercalated and exfoliated into the grafted copolymer, especially at a low or moderate loading of the fullers earth. The synthesized PCN materials were characterized by Fourier transform infrared spectroscopy and wide‐angle X‐ray diffraction techniques. The morphological features of the synthesized materials were studied by scanning electron microscopy; this revealed that the swelling ratio of this nanocomposite increased with increasing fullers earth content. The X‐ray diffraction results indicated that the fullers earth was exfoliated in the nanocomposite matrix, and its introduction into the polymer matrix enhanced the percentage crystallinity of the polymer. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Uniaxial drawing of poly(vinyl alcohol)/graphene oxide nanocomposites

The unique potential of graphene oxide (GO) was exploited in the nanocomposites by a simple uniaxial drawing (up to three times) of poly(vinyl alcohol) (PVA)/GO nanocomposites with a small amount loading of GO. From X-ray diffraction images, the PVA crystallites were found to be oriented parallel to the drawn direction. At the same time, exfoliated GO platelets were found to be aligned parallel to the film surface. Compared with the properties of the as-cast nanocomposites, those of the uniaxially drawn nanocomposites were found to be remarkably enhanced. For the mechanical properties, not only Young’s modulus and tensile strength but also the toughness of the nanocomposites increased by the uniaxial drawing. It was revealed that 260% increase in toughness was achieved for the drawn nanocomposite with 1% w/w GO loading. Significant suppression of the swelling in water resulted in the excellent barrier properties against water, which exceeded that of the conventional high-barrier polymer, such as poly(vinylidene chloride). We revealed that this simple, fast and environmentally friendly process of uniaxial drawing exploits the excellent properties and high aspect ratio of GO in the nanocomposites.     

Pentadecane functionalized graphite oxide sheets as a tool for the preparation of electrical conductive polyethylene/graphite oxide composites

Covalent functionalization of pentadecane-decorated thermally reduced graphite oxide (GO) sheets has been studied as a tool for the preparation of polyethylene/GO composites exhibiting rheological and electrical percolation thresholds. It was accomplished through pentadecane based radical addition onto unsaturated bonds located on the GO sheets' surface using dicumyl peroxide as hydrogen abstractor. This chemical functionalization influences the affinity of the formed pentadecane grafted GO sheets for various solvents. Then, the compounding of the composites pentadecane grafted GO/PE was performed at a processing temperature of 140 °C with 25, 20, 15, 10, 8 and 5 wt% loadings. Rheological and electrical percolation thresholds were found between 10 and 15 wt% for polyethylene/pentadecane functionalized graphene oxide composites while the composite graphite/PE at the same loading percentage did not reach any percolation threshold.     

Using Agrawal integral equation and thermogravimetric analysis (TGA) to study the pyrolysis kinetics of nanocomposites of polybenzoxazine and exfoliated montmorillonite from a mono-functionalized azide polyhedral oligomeric silsesquioxane and click chemistry

In this study, an exfoliated montmorillonite was prepared through click chemistry from a singly azido-functionalized polyhedral oligomeric silsesquioxane derivative and a montmorillonite intercalated with propargyldimethylstearylammonium bromide. This exfoliated montmorillonite was then introduced into a benzoxazine matrix—prepared from paraformaldehyde, aniline, and phenol—to form polymer/exfoliated clay nanocomposites. Thermogravimetric analysis revealed that the pyrolysis kinetics had a close relationship with the structure of montmorillonite, the assembly process, the anchoring effect, the compatibility of the polymer and intercalator, and the char yield. The polymer/exfoliated clay nanocomposites had a same mechanism function, and the kinetic compensation effect equations revealed the pyrolysis essences.     

Synthesis and characterization of poly(acrylonitrile)/montmorillonite nanocomposites from surface‐initiated redox polymerization

We have developed a facile method to prepare polyacrylonitrile/montmorillonite (PAN/MMT) nanocomposites using the surface‐initiated redox polymerization of acrylonitrile (AN) in the aqueous phase. The MMT silicate surfaces were first treated with diethanolamine, and the modified MMT (DEA‐MMT) was subsequently used together with the Ce(IV) salt to serve as a redox system. The PAN chains growing on a surface‐tethered DEA expand the interlayer space, and thus lead to intercalated/exfoliated nanocomposites. The nano‐morphology of the prepared nanocomposites depends on the AN/OH molar ratio in feed. An exfoliated PAN/MMT nanocomposite was obtained when the feeding AN/OH molar ratio = 300 was used. The molecular weight of PAN in the nanocomposites prepared by the present method is also dependent on the AN/OH molar ratio in feed and can be up to ca. 160,000 g/mol. The differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA) analyses show that the increasing fraction of exfoliated silicate structures should enhance the contact interface between the silicate and polymer, resulting in the higher glass transition temperature and thermal stability. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Escherichia coli bacteria reduce graphene oxide to bactericidal graphene in a self-limiting manner

Interactions of chemically exfoliated graphene oxide (GO) nanosheets and Escherichia coli bacteria living in mixed-acid fermentation with an anaerobic condition were investigated for different exposure times. X-ray photoelectron spectroscopy showed that as the exposure time increased (from 0 to 48 h), the oxygen-containing functional groups of the GO decreased by ～60%, indicating a relative chemical reduction of the sheets by interaction with the bacteria. Raman spectroscopy and current–voltage measurement confirmed the reduction of the GO exposed to the bacteria. The reduction was believed to be due to the metabolic activity of the surviving bacteria through their glycolysis process. It was found that the GO sheets could act as biocompatible sites for adsorption and proliferation of the bacteria on their surfaces, while the bacterially-reduced GO (BRGO) sheets showed an inhibition for proliferation of the bacteria on their surfaces. It was shown that the slight antibacterial property of the BRGO sheets and the detaching of the already proliferated bacteria from the surface of these sheets contributed to the growth inhibition of the bacteria on the surface of the reduced sheets.