Highly aligned,ultralarge-size reduced graphene oxide/polyurethane nanocomposites: Mechanical properties and moisture permeability

Polyurethane (PU) nanocomposite films containing highly-aligned graphene sheets are produced. Aqueous dispersion of ultralarge-size graphene oxide (GO) is in situ reduced in waterborne polyurethane, resulting in fine dispersion and high degree of orientation of graphene sheets, especially at high graphene contents. The PU/reduced GO nanocomposites present remarkable 21- and 9-fold increases in tensile modulus and strength, respectively, with 3 wt.% graphene content. The agreement between the experiments and theoretical predictions for tensile modulus proves that the graphene sheets are indeed dispersed individually on the molecular scale and oriented in the polymer matrix to form a layered structure. The moisture permeability of the nanocomposites presents a systematic decrease with increasing graphene content, clearly indicating the impermeable graphene sheets acting as moisture barrier. The synergy arising from the very high aspect ratio and horizontal alignment of the graphene sheets is responsible for this finding.     

功能化石墨烯/埃洛石纳米管对聚丙烯的协同强韧化改性研究

以功能化氧化石墨烯(GO)-埃洛石纳米管(HNTs)杂化材料(GO@HNTs)为纳米填料,以聚丙烯(PP)为基体,通过熔融共混法制备了不同GO@HNTs 含量的GO@HNTs/PP纳米复合材材料,并对所得杂化填料和PP纳米复合材料的结构与性能进行系统研究。研究结果表明,功能化GO与HNTs之间存在化学相互作用,二者之间形成的“屏障效应”抑制了彼此在PP基体中的团聚。仅添加0.5%GO@HNTs杂化纳米填料后,PP复合材料的拉伸强度和冲击强度分别较纯PP提高了17.5%和80.4%,与单独添加相同含量的GO或HNTs所得复合材料的力学性能相比,GO@HNTs杂化纳米填料对PP基体具有明显的协同增强增韧改性作用。与纯PP相比,GO@HNTs/PP试样表现出更高的储能模量、损耗模量和玻璃化转变峰值。由于GO@HNTs的“异相成核效应”和“物理热阻效应”,有效提高了PP纳米复合材料的结晶温度、熔融温度、结晶度和耐热分解温度。     

Click coupled stitched graphene sheets and their polymer nanocomposites with enhanced photothermal and mechanical properties

The chemically stitched graphene oxide (GO) sheets were obtained using a click chemistry reaction between azide-functionalized GO and alkyne-functionalized GO. The click coupled GO (GO-click-GO) sheets showed the largely increased electrical conductivity and near infrared laser-induced photothermal properties compared to the GO sheets, which result from formation of triazole ring as a bridging linker between the GO sheets. The polyurethane (PU) nanocomposites incorporating the GO-click-GO sheets exhibited enhanced mechanical properties of breaking stress and modulus than the GO/PU nanocomposites. The modulus of GO-click-GO/PU nanocomposites was higher than that of the GO/PU nanocomposites at the same filler loading of 0.1 and 0.5 wt%. The GO-click-GO/PU nanocomposites also showed a significantly improved photothermal properties compared to the GO/PU nanocomposites at the same filler loading. The click coupled stitched GO sheets in this study can be used as the superior reinforcing fillers for mechanically and photothermally high performance polymer nanocomposites.     

Excellent photocatalytic performance under visible-light irradiation of ZnS/rGO nanocomposites synthesized by a green method

ZnS/graphene nanocomposites with different graphene concentrations (5, 10 and 15 wt.%) were synthesized using L-cysteine as surfactant and graphene oxide (GO) powders as graphene source. Excellent performance for nanocomposites to remove methylene blue (MB) dye and hexavalent chromium (Cr(VI)) under visible-light illumination was revealed. TEM images showed that ZnS NPs were decorated on GO sheets and the GO caused a significant decrease in the ZnS diameter size. XRD patterns, XPS and FTIR spectroscopy results indicated that GO sheets changed into reduced graphene oxide (rGO) during the synthesis process. Photocurrent measurements under a visiblelight source indicated a good chemical reaction between ZnS NPs and rGO sheets.     

Significant improvement in static and dynamic mechanical properties of graphene oxide–carbon nanotube acrylonitrile butadiene styrene hybrid composites

Herein, hybridization of graphene nanosheets and carbon nanotubes (CNTs) has been made to solve the problem of restacking of graphene nanosheets and agglomeration of CNTs. The multiwalled carbon nanotubes (MWCNTs), reduced graphene oxide (RGO) and graphene oxide–carbon nanotubes (GCNTs) reinforced acrylonitrile butadiene styrene (ABS) composites have been prepared using micro-twin-screw extruder. The effect of these reinforcements on static and dynamic mechanical properties of composites is studied. The ultimate tensile strength and elastic modulus for 7 wt.% GCNT–ABS composites show enhancement of 26.1 and 71.3% over pure ABS matrix, respectively. Various parameters such as coefficient “C” factor (the ratio of storage modulus of the composite to polymer in glassy and rubbery regions), degree of entanglement, crosslink density and adhesion factor have been calculated to analyze the interaction between fillers and polymer matrix. The 3-D hybrid structure of GCNTs overcomes the associated problem of CNTs agglomeration and graphene restacking. GCNT hybrid composites show higher dispersion as well as effectiveness for increased filler amount as compared to RGO and MWCNTs based composites. GCNTs prove its superiority over MWCNTs and RGO by showing a synergistic effect in the glass transition temperature and storage modulus. Raman spectroscopy and scanning electron microscopy are used to confirm the interaction and distribution of the filler and matrix, respectively.     

Morphology, mechanical and thermal properties of graphene-reinforced poly(butylene succinate) nanocomposites

Graphene nanosheets (GNSs) reinforced poly(butylene succinate) (PBS) nanocomposites are facilely obtained by a solution-based processing method. Graphene nanosheets, which are derived from chemically reduced graphite oxide (GO), are characterized by AFM, TEM, XRD and Raman spectra. The state of dispersion of the GNSs in the PBS matrix is examined by SEM observations that reveals homogeneous distribution of GNSs in PBS matrix. A 21% increase in tensile strength and a 24% improvement of storage modulus are achieved by addition of 2.0 wt% of GNS. The electrical conductivity and thermal stability of the graphene-based nanocomposite are also improved. DSC measurement indicates that the presence of graphene sheets does not have a remarkable impact on the crystallinity of the nanocomposites. Therefore, the high performances of the nanocomposites are mainly attributed to the uniform dispersion of GNSs in the polymer matrix and strong interfacial interactions between both components.     

Crystallization,mechanical performance and hydrolytic degradation of poly(butylene succinate)/graphene oxide nanocomposites obtained via in situ polymerization

Poly(butylene succinate) (PBS)/graphene oxide (GO) nanocomposites were fabricated via in situ polymerization with very low GO content (from 0.03 to 0.5 wt%). The microstructures of the nanocomposites were characterized with Raman spectroscopy, fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), sedimentation experiments and atomic force microscopy (AFM). The results showed that PBS chains have been successfully grafted onto GO sheets during in-situ polymerization, accompanied by the thermo-reduction from GO to graphene. The grafted GO displayed a great nucleating effect on PBS crystallization, resulting in largely improved crystallization temperature and decreased spherules size. A simultaneous enhancement in tensile strength and elongation was achieved for PBS/GO nanocomposites fiber. Meanwhile, increase in hydrolytic degradation rate was also observed for these nanohybrids. Our result indicates that using very low content GO is a simple way to achieve good dispersion yet with remarkable property enhancement for polymer/GO nanocomposites.     

Hierarchical architecture of ultrashort carbon nanotubes/polyaniline nanocables coated on graphene sheets for advanced supercapacitors

Multilayer super-short carbon nanotubes (SSCNTs) could be prepared by tailoring raw multiwalled carbon nanotubes (MWCNTs) with mechanical-stirring and ultrasonic oxidation-cut method. The SSCNTs/polyaniline/reduced graphene oxide (SSCNTs/PANI/RGO) ternary hybrid composite was fabricated by reducing SSCNTs/PANI/GO precursor prepared by self-assembly from the complex dispersion of graphene oxide (GO) and the as-prepared SSCNTs/PANI nanocables, followed by redoping and reoxidation of the reduced PANI to restore the conducting structure of PANI in the ternary composite. The microscope images indicated that SSCNTs/PANI nanocables could uniformly distribute in the conductive network of graphene sheets and prevent the agglomeration of graphene. Such the hierarchical structure perfectly facilitates the contact between PANI for faradaic energy storage and electrolyte ions, and efficiently utilizes the double-layer capacitance of SSCNTs and graphene sheets at the electrode–electrolyte interfaces. The maximum specific capacitance of the SSCNTs/PANI/RGO composite achieved 845 F g^?1, which was much higher than that of pure PANI and SSCNTs/PANI nanocables. Moreover, the ternary composite also showed the good cycling stability, retaining about 96% of its initial capacitance after 1000 cycles because of the synergistic effect and conductive network of SSCNTs/PANI nanocables and graphene sheets. Therefore, the combined effects between SSCNTs/PANI nanocables and graphene sheets taking advantage of both charging and faradaic processes could readily explain the excellent electrochemical performance for supercapacitors.     

Localized in situ polymerization on graphene surfaces for stabilized graphene dispersions

We demonstrate a novel in situ polymerization technique to develop localized polymer coatings on the surface of dispersed pristine graphene sheets. Graphene sheets show great promise as strong, conductive fillers in polymer nanocomposites; however, difficulties in dispersion quality and interfacial strength between filler and matrix have been a persistent problem for graphene-based nanocomposites, particularly for pristine graphene. With this in mind, a physisorbed polymer layer is used to stabilize graphene sheets in solution. To create this protective layer, we formed an organic microenvironment around dispersed graphene sheets in surfactant solutions, and created a nylon 6, 10 or nylon 6, 6 coating via interfacial polymerization. Technique lies at the intersection of emulsion and admicellar polymerization; a similar technique was originally developed to protect luminescent properties of carbon nanotubes in solution. These coated graphene dispersions are aggregation-resistant and may be reversibly redispersed in water even after freeze-drying. The coated graphene holds promise for a number of applications, including multifunctional graphene-polymer nanocomposites.     

Graphene oxide and hydroxyapatite as fillers of polylactic acid nanocomposites: preparation and characterization

Graphene and its derivatives have attracted great research interest for their potential applications in electronics, energy, materials and biomedical areas. When incorporated appropriately, these atomically thin carbon sheets are expected to improve physical properties of host polymers at extremely small loading. Herein, we report a novel two-step method for the preparation of PLLA/Hap/graphene oxide nanocomposites with augmented mechanical properties when compared to PLLA/Hap and neat PLLA. The presence of graphene oxide (GO) had a positive effect on the dispersion of hydroxyapatite particles on the polymeric matrix contributing for a good homogeneity of the final nanocomposite. PLLA nanocomposites prepared with 30% (w/w) of Hap and 1% (w/w) of GO showed the highest hardness and storage modulus values indicating an efficient load transfer between the fillers and the PLLA matrix. These materials may find interesting biomedical applications as for example bone screws. The following step on the study of these materials will be in vitro tests to access the biocompatibility of these new nanocomposites.     

Fabrication and mechanical properties of well-dispersed multiwalled carbon nanotubes/epoxy composites

Multiwalled carbon nanotubes (MWCNTs)/epoxy nanocomposites were fabricated by using ultrasonication and the cast molding method. In this process, MWCNTs modified by mixed acids were well dispersed and highly loaded in an epoxy matrix. The effects of MWCNTs addition and surface modification on the mechanical performances and fracture morphologies of composites were investigated. It was found that the tensile strength improved with the increase of MWCNTs addition, and when the content of MWCNTs loading reached 8 wt.%, the tensile strength reached the highest value of 69.7 MPa. In addition, the fracture strain also enhanced distinctly, implying that MWCNTs loading not only elevated the tensile strength of the epoxy matrix, but also increased the fracture toughness. Nevertheless, the elastic modulus reduced with the increase of MWCNTs loading. The reasons for the mechanical property changes are discussed.     

Carbon Nanotube Pullout,Interfacial Properties,and Toughening in Ceramic Nanocomposites: Mechanistic Insights from Single Fiber Pullout Analysis

While debonding and subsequent pullout at fiber‐matrix interfaces can improve fracture toughness in ceramic nanocomposites, the magnitudes of these contributions are currently the subject of ongoing debate. To provide quantitative insight into these mechanisms, ceramic matrix nanocomposites were fabricated with a polymer‐derived ceramic matrix, using multiwalled carbon nanotubes (MWCNTs) that exhibit relatively long pullout lengths. In situ micromechanical pullout tests on individual MWCNTs were used to directly measure the strength of the fiber‐matrix interface. Similar pullout lengths were also observed in bulk and thin film composites, where the fracture toughness of the composite films was measured and found to be higher than that of the matrix material. The interfacial properties from the micromechanical test and the pullout lengths from the composite films were then used to estimate the energy release rates for fiber debonding and pullout. Based on the observed MWCNT and composite failure mechanisms, these results are discussed in terms of their relation to previous estimates of toughening in MWCNT‐ceramic nanocomposites, and in terms of design possibilities for further fracture toughness improvements.     

Fracture mechanisms of epoxy filled with ozone functionalized multi-wall carbon nanotubes

This work focused on the fracture mechanisms and reinforcing effects of ozone-treated multi-walled carbon nanotubes (MWCNTs) in epoxy matrix. Ozone functionalization of MWCNTs was found to be of help for a better dispersion and stronger interfacial bonding with epoxy matrix, which in turn improve the strength and fracture toughness of the resin. The MWCNT/epoxy composites showed complicated failure modes than the conventional fibrous composites, which have been quantitatively investigated and correlated with the fracture toughness of the nanocomposites studied.     

石墨烯与多壁碳纳米管增强环氧树脂复合材料的制备及性能

对多壁碳管(MWCNTs)进行改性处理得到酸化碳管(MWCNTs-COOH)和环氧化碳管(MWCNTs-Epon828), 将石墨烯(Graphene)与不同的碳管分别混合, 制备出三种Graphene-MWCNTs/环氧树脂(EP)复合材料。通过拉伸和热重实验研究了石墨烯与MWCNTs的协同作用、 两者的含量以及MWCNTs功能化方法对复合材料力学和热学性能的影响。结果表明: 石墨烯与MWCNTs的协同增强明显优于MWCNTs单独增强。当石墨烯和MWCNTs质量分数仅为0.1%时, Graphene-MWCNTs-Epon828/EP的拉伸强度达最大值, 其拉伸强度、 弹性模量和断裂伸长率分别较纯EP增加了35%、 65%和34%。石墨烯和MWCNTs的加入使Graphene-MWCNTs/EP复合材料的热稳定性均有所提高。     

Effects of functional groups on the graphene sheet for improving the thermomechanical properties of polyurethane nanocomposites

Dispersibility of graphene sheets in polymer matrices and interfacial interaction are challenging for producing graphene-based high performance polymer nanocomposites. In this study, three kinds nanofillers; pristine graphene nanoplatelets (GNPs), graphene oxide (GO), and functionalized graphene sheet (FGS) were used to prepare polyurethane (PU) composite by in-situ polymerization. To evaluate the efficacy of functional groups on the graphene sheets, PU reinforced with GNPs, GO, and FGS were compared through tensile testing and dynamic mechanical thermal analysis. The Young's moduli of 2 wt% GO and FGS based PU nanocomposites were found significantly higher than that of same amount of GNPs loading as an evidence of the effect of functional groups on graphene sheets for the mechanical reinforcement. The strong interaction of FGS with PU was responsible to exhibit notably high modulus (25.8 MPa) of 2 wt% FGS/PU composite than the same amount of GNPs and GO loading even at elevated temperature (100 °C).     

Preparation and properties of reduced graphene oxide/polyacrylonitrile nanocomposites using polyvinyl phenol

Nanocomposites of polyacrylonitrile (PAN) with reduced graphene oxide (rGO) were prepared using a solution mixing technique employing polyvinyl phenol (PVP) as a compatibilizer. The PVP can facilitate composite formation by interacting with both rGO and PAN via π-π and H-bonding respectively. Various amounts of rGO were used to prepare PAN nanocomposites. The cross-sectional morphology of the composite films shows a uniform dispersion of rGO sheets in the PAN matrix. The Fourier transform infrared (FT-IR) studies revealed that good interaction of the rGO/PVP hybrid with PAN. The wide angle x-ray diffraction (WAXS) study confirms that the rGO sheets were uniformely dispersed as individual sheets in the PAN matrix. Thermogravimetric analysis shows enhanced thermal stability of the composite compared to pure PAN. The tensile strength and elastic modulus of the nanocomposites increased with increasing rGO content. A 102% enhancement in tensile strength and a 62.9% enhancement in elastic modulus were observed in the nanocomposite with 5% rGO.     

Graphene/thermoplastic polyurethane nanocomposites: Surface modification of graphene through oxidation,polyvinyl pyrrolidone coating and reduction

Herein, oxidation, polyvinyl pyrrolidone (PVP) coating and reduction are used to modify the surface of graphene in thermoplastic polyurethane (TPU)/graphene nanocomposites. It is demonstrated that graphene could be easily dispersed in TPU with PVP absorbed on reduced graphene oxide (RGO) as stabilizer during reduction. In the stress–strain curves for these composites containing GO, PVP coated GO (GO/PVP) and reduced GO/PVP (RGO/PVP) as filler, PVP coating and reduction can largely enhance the stress in low modulus region. It is thought to largely related with enhanced interfacial interaction between filler and matrix and healing of graphene structure during reduction. Consequently, the modulus of TPU/GO/PVP and TPU/RGO/PVP is significantly increased. Meanwhile, an electrical percolation threshold of 0.35 wt.% is obtained for TPU/RGO/PVP. Comparing with the results in literature, the filler surface modification used in this study has created nanocomposites with a good balance between electrical conductivity and mechanical properties.     

Toughening and reinforcement of poly(vinylidene fluoride) nanocomposites with “bud-branched” nanotubes

Bud-branched nanotubes, fabricated by growing metal particles on the surface of multi-wall carbon nanotubes (MWCNTs), were used to prepare poly(vinylidene fluoride) (PVDF) based nanocomposites. The results of differential scanning calorimetry (DSC) showed that the introduction of the MWCNTs and bud-branched nanotubes both increased the crystallization temperature, while no significant variation of T_m (melting temperature), ΔH_c (melting enthalpy) and ΔH_m (crystallization enthalpy) occurred. The results of wide angle X-ray diffraction (WAXD) tests showed that α-phase was the dominated phase for both pure PVDF and its nanocomposites, indicating the addition of the MWCNTs and bud-branched nanotubes did not alter the crystal structures. Dynamic mechanical analysis (DMA) tests showed that bud-branched nanotubes were much more efficient in increasing storage modulus than the smooth MWCNTs. In addition, no significant variation of the T_g (glass transition temperature) was observed with the addition of MWCNTs and bud-branched nanotubes. Tensile tests showed that the introduction of MWCNTs and bud-branched nanotubes increased the modulus. However, a dramatic decrease in the fracture toughness was observed for PVDF/MWCNTs nanocomposites. For PVDF/bud-branched nanotubes nanocomposites, a significant improvement in the fracture toughness was observed compared with PVDF/MWCNTs nanocomposites.     

Multifold interface and multilevel crack propagation mechanisms of graphene oxide/polyurethane/epoxy membranes interlaminar-toughened carbon fiber-reinforced polymer composites

Graphene oxide/polyurethane/epoxy (GO/PU/EP) membranes were directly fabricated by functionalization of graphene oxide with epoxy-grafted polyurethane (GO-UE), and the interface correlation and crack propagation mechanisms in GO/PU/EP membranes interlaminar-toughened carbon fiber-reinforced polymer composites were investigated. The functionalized GO-UE with corrugation and scrolling nature of graphene sheets was evenly dispersed in GO/PU/EP membranes below 0.50 wt% loading. Mode I fracture toughness, flexural properties and interlaminar shear strength of GO/PU/EP membranes-toughened composites were enhanced in comparison with untoughened composites and PU/EP membranes-toughened composites, which was ascribed to the multifold interface bonding between the GO-UE layers, epoxy matrix and carbon fiber. Schematic models of multilevel crack propagations were proposed based on different crack extension directions to GO-UE and the morphology evolutions of GO-UE in the interlaminar region and at the carbon fiber interface in toughened composites, which highlighted the toughening mechanisms of crack pinning, crack deflection and separation between GO-UE layers.     

Effects of gamma-irradiation on UHMWPE/MWNT nanocomposites

Ultra high molecular weight polyethylene (UHMWPE) is a polymer that is widely used in industrial and orthopaedic applications. In this work, pristine multiwalled carbon nanotubes (MWCNTs) were incorporated into UHMWPE in different concentrations (1, 3 and 5 wt.%) using a ball milling process. UHMWPE/MWCNT nanocomposites were gamma irradiated at 90 kGy to improve the interaction between MWCNTs and the polymer matrix. Structural, thermal and mechanical characterizations were conducted by means of transmission electron microscopy (TEM), Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA) and uniaxial tensile techniques. Gamma irradiation produced an increase in the melting temperature, crystallinity and temperature of maximum decomposition rate. The irradiation produced a 38% decrease in the toughness of neat UHMWPE. The incorporation of MWCNTs did not significantly affect the melting point of the neat UHMWPE but decreased the degree of crystallinity of the raw UHMWPE, which was related to a reduction in the UHMWPE lamellar density. An increase in thermal stability was also observed for the nanocomposites compared to neat UHMWPE. The tensile tests showed a 38% increase in the Young’s modulus in the reinforced nanocomposites and a small decrease in toughness (5%). Gamma irradiation of the nanocomposites increased crystallinity, which was related to an increased lamellar thickness, and also improved their thermal stability. The Young’s modulus increased by up to 71% for irradiated nanocomposites and their toughness showed no significant changes in comparison with the non-irradiated nanocomposites. The incorporation of MWCNTs reduced the negative effects of irradiation and compensated for the reduction in toughness. This fact might be attributed to the radical scavenger behaviour of the MWNT as was proved by Electron Spin Resonance (ESR) detection of the radiation-induced radicals.