Low Graphene Content Research Articles

Mechanical properties of graphene films enhanced by homo-telechelic functionalized polymer fillers via π–π stacking interactions

Most researches on graphene/polymer composites are focusing on improving the mechanical and electrical properties of polymers at low graphene content instead of paying attention to constructing graphene’s macroscopic structures. In current study the homo-telechelic functionalized polyethylene glycols (FPEGs) were tailored with π-orbital-rich groups (namely phenyl, pyrene and di-pyrene) via esterification reactions, which enhanced the interaction between polyethylene glycol (PEG) molecules and chemical reduced graphene oxide (RGO) sheets. The π–π stacking interactions between graphene sheets and π-orbital-rich groups endowed the composite films with enhanced tensile strength and tunable electrical conductivity. The formation of graphene network structure mediated by the FPEGs fillers via π–π stacking non-covalent interactions should account for the experimental results. The experimental investigations were also complemented with theoretical calculation using a density functional theory. Atomic force microscope (AFM), scanning electron microscope (SEM), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), thermal gravimetric analysis (TGA), UV–vis and fluorescence spectroscopy were used to monitor the step-wise preparation of graphene composite films.

Graphene nanoplatelets-reinforced polyetherimide foams prepared by water vapor-induced phase separation

The present work considers the preparation of medium-density polyetherimide foams reinforced with variable amounts of graphene nanoplatelets (1–10 wt%) by means of water vapor-induced phase separation (WVIPS) and their characterization. A homogeneous closed-cell structure with cell sizes around 10 µm was obtained, with foams exhibiting zero crystallinity according to X-ray diffraction (XRD). Thermogravimetric analysis under nitrogen showed a two-step thermal decomposition behaviour for both unfilled and graphene-reinforced foams, with foams containing graphene presenting thermal stability improvements, related to a physical barrier effect promoted by the nanoplatelets. Thermo-mechanical analysis indicated that the specific storage modulus of the nanocomposite foams significantly increased owing to the high stiffness of graphene and finer cellular morphology of the foams. Although foamed nanocomposites displayed no further sign of graphene nanoplatelets exfoliation, the electrical conductivity of these foams was significant even for low graphene contents, with a tunnel-like model fitting well to the evolution of the electrical conductivity with the amount of graphene.

Enhanced mechanical properties of nanocomposites at low graphene content based on in situ ball milling

In this study, epoxy‐based nanocomposites with low content mechanically exfoliated graphene were successfully prepared via one‐step in situ ball milling method. The effect of graphene on mechanical properties of the nanocomposites was investigated. The results showed that samples with loadings less than 0.1% weight of mechanically exfoliated graphene increased by 160% in tensile strength and 65% in Young's modulus. The experimental value of Young's modulus was also compared with the predictions of the well‐established Halpin‐Tsai model. In addition, the adding of graphene did not decrease the impact strength of epoxy. The microstructural results showed that the as‐prepared graphenes were single‐ and few‐layer graphene sheets and preserved perfect structure. Thus enhancements of mechanical properties in the nanocomposites could be ascribed to the strong interfacial interaction between the stiff graphene nanosheets and the epoxy matrix. POLYM. COMPOS. 37:1190–1197, 2016. © 2014 Society of Plastics Engineers

Effects of the graphene doping level on the optical and electrical properties of ITO/P3HT:Graphene/Au organic solar cells

The effects of the graphene doping level on the optical and electrical properties of ITO/P3HT:Graphene/Au solar cells are investigated. The spectroscopic ellipsometry (SE) analysis proves that the insertion of low graphene content in the P3HT matrix reduces the optical bandgap of the P3HT:Graphene nanocomposite and the thickness of the film. As a result the optical absorption properties of the device are enhanced in the visible range. The device elaborated with the lower graphene content displays a Jsc=9.93×10−4A/cm2, a Voc=0.55V and FF=41%, under illumination. These results are in good agreement with the SE analyzes since the device elaborated with the lower graphene content exhibits the higher optical parameters. The origin of the improved energy conversion efficiency for low graphene doping level is discussed in this work.

Improved rheological and electrical properties of graphene/polystyrene nanocomposites modified with styrene maleic anhydride copolymer

Styrene–maleic anhydride copolymer (SMA) was used as a compatibilizer to improve the interfacial interaction between graphene sheets and polystyrene matrix. SMA mediated graphene sheets are closely encapsulated by a polymer layer and uniformly dispersed compared to the graphene sheets without SMA. The presence of SMA not only improves the melt modulus and its frequency-independence, but also results in an earlier transition from liquid-like to solid-like, indicating the formation of a percolated conducting network at a low content of graphene with the assistance of SMA. Accordingly, the different frequency dependences of electrical conductivity also lead to a much lower threshold of 0.17 vol.% for SMA modified nanocomposites compared to 0.52 vol.% for the unmodified counterparts, further confirming the earlier formation of a graphene network in the matrix. The improved rheological and electrical properties of the polystyrene nanocomposites are attributed to the homogeneous dispersion of the graphene sheets and the enhanced interfacial interaction between the graphene sheets and the polystyrene matrix.

Characterization and simulation of tensile behavior of graphene/polypropylene nanocomposites using a novel strain-rate-dependent micromechanics model

In this article, a newly developed model is utilized to simulate the strain-rate-dependent constitutive equation of graphene/polypropylene nanocomposites, using the mechanical properties of the constituent materials of nanocomposites. The model is a combination of the micromechanics and Goldberg models called strain-rate-dependent micromechanics (SRDM) model. The Johnson-Cook material model is used by the explicit finite element code LS-DYNA to simulate the strain-rate-dependent mechanical behavior of the standard tensile test specimen made of graphene/polypropylene nanocomposites under dynamic loading and its constants are calculated by the SRDM model. Polypropylene reinforced with 0.5, 1.0, and 2.0 wt% graphene sheets were prepared via coating polypropylene with graphene particles. Then, by melt blending in a twin-screw extruder followed by an injection molding process, the nanocomposite samples are manufactured. Good enhancements of Young’s modulus and yield stress at very low graphene contents are achieved. To evaluate the current model, the results are compared with the experimental result of the standard tensile test specimen. A good agreement between the experimental data and the SRDM model is achieved.

Cytotoxicity assessment of MDA-MB-231 breast cancer cells on screen-printed graphene-carbon paste substrate

Graphene is a novel carbon-based material widely studied in bio-electrochemical fields because of its high electrical conductivity and excellent electrocatalytic activity. However, its biological applications have been limited due to the lack of understanding of its compatibility with numerous biological entities. In this paper, cytoxicities of MDA-MB-231 breast cancer cells (MDA cells) on carbon paste (CP) and graphene-carbon paste (GCP) substrates are assessed. GCP was prepared by mixing graphene powder into carbon paste with different graphene contents. Cytotoxic effect was evaluated from cell viability, cell adhesion, ROS production and fluorescence staining studies. Cell viability on GCP substrate was found to initially increase as graphene content increases from 0 to 2.5wt% but then decrease as the content increases further. In addition, the viability decreases with time for all substrates. Similarly, graphene concentration affected the number of adherent cells in the same manner as the cell viability. Likewise, reactive oxygen species (ROS) induced by carbon substrate increased with time and decreased with small graphene inclusion, confirming that low graphene content led to lower cytotoxicity. Moreover, confluence of MDA cells on substrate evaluated using Hoechst 33342 fluorescence staining was also found to be enhanced at low graphene concentration. Therefore, low-content graphene incorporation can effectively improve biocompatibility of carbon-based materials with MDA-MB-231 breast cancer cells, enabling potential applications such as electrochemical electrode for cell study.

Three-dimensional graphene/LiFePO4 nanostructures as cathode materials for flexible lithium-ion batteries

Three-dimensional graphene/LiFePO4 nanostructures for flexible lithium-ion batteries were successfully prepared by solvent evaporation method. Structural characteristics of flexible electrodes were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). Electrochemical performance of graphene/LiFePO4 was examined by a variety of electrochemical testing techniques. The graphene/LiFePO4 nanostructures showed high electrochemical properties and significant flexibility. The composites with low graphene content exhibited a high capacity of 163.7mAhg−1 at 0.1C and 114mAhg−1 at 5C without further incorporation of conductive agents.

Interpenetrating network formation in isotactic polypropylene/graphene composites

Morphological structure development of the in-situ polymerized isotactic polypropylene/graphene nanocomposites was investigated during the annealing process. The material was prepared via an in-situ slurry polymerization method with graphene-supported Ziegler–Natta catalyst. An interpenetrating network was observed at a relatively low graphene concentration after the sample was annealed for a long period (one to several hours). Alternative Conductivity impedance spectroscopic measurements showed a remarkable electrical conductivity increase of several orders of magnitude after the network was formed. At very low graphene content, although macroscopic network cannot be formed, the electrical conductivity can be enhanced through annealing as well. It implied that the rearrangement of the clusters or aggregates was necessary and such rearrangement favored the network formation. The network forming process was investigated through optical and electron microscopy methods and discussed based on the viscoelastic phase separation model. The effect of network on the crystallization behaviors was also investigated. It showed that the overall crystallization was improved by the presence of graphene platelets, but depressed after the network was formed, which was due to the spatial confinement and the decrease of total surface area available for heterogeneous nucleation.

Highly reversible and ultra-fast lithium storage in mesoporous graphene-based TiO2/SnO2 hybrid nanosheets

A facile and scalable step-wise approach has been developed to fabricate graphene-based TiO2/SnO2 hybrid nanosheets (TiO2@SnO2@GN). With uniform staggered distribution of rutile SnO2 and TiO2 nanocrystals in a 2D mesoporous manner, TiO2@SnO2@GN with low content of graphene (∼5 wt%) manifests superior reversible capacity and excellent rate capability as an anode material in lithium-ion batteries.

Freeze-dried WS2 composites with low content of graphene as high-rate lithium storage materials

Few layered WS2–graphene nanosheet composites are prepared by a simple and scalable hydrothermal reaction and a subsequent freeze-drying method. The freeze-dried WS2–graphene composite exhibits good cycling stability and outstanding high-rate capability of lithium storage. The reversible capacity remains 647 mA h g−1 after 80 cycles at a current density of 0.35 A g−1. Comparable capacities of 541 and 296 mA h g−1 can still be maintained when cycling at even higher current densities of 7 and 14 A g−1 (7 and 14 mA cm−2) respectively.

Hierarchical titania mesoporous sphere/graphene composite, synthesis and application as photoanode in dye sensitized solar cells

Hierarchical TiO2 mesoporous sphere/graphene composites (HTMS/Gs) were prepared by incorporation proper amounts of graphene oxide (GO) into the reaction system toward hierarchical TiO2 mesoporous spheres (HTMSs). The HTMS/Gs inherit the merits of high specific surface area derived from both HTMS and graphene, as well as the well conductivity of graphene. Power conversion efficiencies (PCEs) of dye sensitized solar cells (DSSCs) using HTMS/Gs as photoanode materials were also investigated. The graphene content in HTMS/G exhibited great influence on PCE. Lower graphene content in HTMS/G showed superior dye adsorption capacity, lower charge recombination and thus higher photocurrent density over bare HTMS. However, excessive graphene promoted the recombination of photo-generated electrons, which deteriorated the PCE. Due to the high dye adsorption capacity and the prolonged electron recombination lifetime, the HTMS/G contains 5.68wt.% graphene, denoted as HTMS/G5.68, boosted up the short circuit current density (Jsc) to 16.17mA/cm2, and a PCE of 7.19% was achieved, which is 21.8% higher than that of bare HTMS.

Effect of graphene doping of holographic polymer‐dispersed liquid crystals

AbstractDoping a polymer matrix with a minute amount of graphene (0.05–0.25%) had significant effects on the grating formation kinetics and electro‐optical performance of a holographic polymer‐dispersed liquid crystal. Low graphene contents (≤0.1%) reduced the viscosity and induced rapid diffusion, curing, grating formation, and high diffraction efficiency with a diffraction overshoot of 0.05%. Conversely, high graphene contents increased the compound viscosity, and the entire process proceeded slowly. Graphene increased the polymer conductivity and local electric field, reduced the operating voltage from 65 to 25–50 V, and increased the contrast ratio from 7 to 8–23 with a concomitant decrease in rise time. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

A novel LiFePO4/graphene/carbon composite as a performance-improved cathode material for lithium-ion batteries

A novel LiFePO4/graphene/carbon composite as a performance-improved cathode material for lithium-ion batteries had been prepared by using an in situ solvothermal method to synthesize LiFePO4/graphene powders as precursors and then followed by a carbon-coating process. The prepared samples of LiFePO4/graphene/carbon was investigated comparatively with LiFePO4/carbon and LiFePO4/graphene composites by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and various electrochemical testing techniques. The results indicated that the co-modification of LiFePO4 with graphene and carbon coating could construct an effective conducting network, which significantly enhanced the electrochemical activity of LiFePO4/carbon based composite. Particularly, the LiFePO4/graphene/carbon composite with a low content of graphene exhibited a high initial discharge capacity of 163.7mAhg−1 at 0.1C and 114mAhg−1 at 5C, as well as an excellent cycling stability.

In situ production of high filler content graphene-based polymer nanocomposites by reactive processing

This work deals with the preparation of graphene dispersed in a monomer (tetraethylene glycol diacrylate) and the subsequent polymerization of the latter to the corresponding polymer nanocomposite, which is the first obtained so far by direct polymerization of the graphene-dispersing medium. The method used for its obtainment allows reaching the highest concentration of graphene reported until now in any medium (9.45 mg mL−1); besides, a certain amount of graphene nanoribbons is also well visible. Furthermore, this goal is achieved by directly sonicating graphite without any chemical manipulation, which generally results in a final material still containing a significant number of defects. Because of its obtainment in the monomer itself, no filtration of graphene is needed, thus avoiding the reaggregation process to graphite, which partially compromises any previous exfoliation process. The obtained graphene-based polymer nanocomposites, fully characterized by Raman and transmission electron microscopy, differential scanning calorimetry, thermogravimetry, and dynamic–mechanical thermal analysis, exhibit a very homogeneous distribution of the graphene sheets within the polymer matrix. In addition, the interactions between the polymer and nanofiller are very strong, as evidenced by a significant increase in the Tg values even in the presence of a very low graphene content, together with a strong increase in the mechanical features (flexural and storage moduli). Finally, the thermo-oxidative stability of the polymer matrix is not affected by the presence of graphene nanosheets.

Enhanced Mechanical Properties of Nanocomposites at Low Graphene Content

In this study, the mechanical properties of epoxy nanocomposites with graphene platelets, single-walled carbon nanotubes, and multi-walled carbon nanotube additives were compared at a nanofiller weight fraction of 0.1 +/- 0.002%. The mechanical properties measured were the Young's modulus, ultimate tensile strength, fracture toughness, fracture energy, and the material's resistance to fatigue crack propagation. The results indicate that graphene platelets significantly out-perform carbon nanotube additives. The Young's modulus of the graphene nanocomposite was approximately 31% greater than the pristine epoxy as compared to approximately 3% increase for single-walled carbon nanotubes. The tensile strength of the baseline epoxy was enhanced by approximately 40% with graphene platelets compared to approximately 14% improvement for multi-walled carbon nanotubes. The mode I fracture toughness of the nanocomposite with graphene platelets showed approximately 53% increase over the epoxy compared to approximately 20% improvement for multi-walled carbon nanotubes. The fatigue resistance results also showed significantly different trends. While the fatigue suppression response of nanotube/epoxy composites degrades dramatically as the stress intensity factor amplitude is increased, the reverse effect is seen for graphene-based nanocomposites. The superiority of graphene platelets over carbon nanotubes in terms of mechanical properties enhancement may be related to their high specific surface area, enhanced nanofiller-matrix adhesion/interlocking arising from their wrinkled (rough) surface, as well as the two-dimensional (planar) geometry of graphene platelets.