Grafting of epoxy chains onto graphene oxide for epoxy composites with improved mechanical and thermal properties

Epoxy composites filled with both graphene oxide (GO) and diglycidyl ether of bisphenol-A functionalized GO (DGEBA–f–GO) sheets were prepared at different filler loading levels. The correlations between surface modification, morphology, dispersion/exfoliation and interfacial interaction of sheets and the corresponding mechanical and thermal properties of the composites were systematically investigated. The surface functionalization of DGEBA layer was found to effectively improve the compatibility and dispersion of GO sheets in epoxy matrix. The tensile test indicated that the DGEBA–f–GO/epoxy composites showed higher tensile modulus and strength than either the neat epoxy or the GO/epoxy composites. For epoxy composite with 0.25 wt% DGEBA–f–GO, the tensile modulus and strength increased from 3.15 ± 0.11 to 3.56 ± 0.08 GPa (∼13%) and 52.98 ± 5.82 to 92.94 ± 5.03 MPa (∼75%), respectively, compared to the neat epoxy resin. Furthermore, enhanced quasi-static fracture toughness (K_IC) was measured in case of the surface functionalization. The GO and DGEBA–f–GO at 0.25 wt% loading produced ∼26% and ∼41% improvements in K_IC values of epoxy composites, respectively. Fracture surface analysis revealed improved interfacial interaction between DGEBA–f–GO and matrix. Moreover, increased glass transition temperature and thermal stability of the DGEBA–f–GO/epoxy composites were also observed in the dynamic mechanical properties and thermo-gravimetric analysis compared to those of the GO/epoxy composites.     

Fabrication and characterization of 45S5 bioglass reinforced macroporous calcium silicate bioceramics

Bioactive and degradable macroporous bioceramics play an important role in clinical applications. In the present study, 45S5 bioglass reinforced macroporous calcium silicate ceramics (45BG-reinforced MCSCs) were fabricated. The effect of bioglass additives on compressive strength and open porosity of the samples was investigated, and the bioactivity and degradability of the obtained reinforced samples were also evaluated. The 45S5 bioglass additive was found to be effective to increase the strength of the MCSCs by the liquid-phase sintering mechanism. The optimum amount of bioglass additives was 5 wt.% and the compressive strength of the reinforced samples was approximately 2 times higher as compared to the pure macroporous calcium silicate ceramics (MCSCs). The compressive strength of the reinforced samples with about 50% porosity reached 112.47 MPa, which was similar to those of the cortical bones. After soaking in simulated body fluid (SBF), hydroxycarbonate apatite (HCA) layer was formed on the surface of the 45BG-reinforced MCSCs. Furthermore, the degradation rate of the reinforced samples was just about one-third of those pure MCSCs. Our results indicated that degradable 45BG-reinforced MCSCs possess excellent mechanical strength and bioactivity, and may be used as bioactive and degradable biomaterials for hard tissue prosthetics or bone tissue engineering applications.     

Synthesis and characterization of cross-linked polymeric nanoparticles and their composites for reinforcement of photocurable dental resin

Reactive polymeric nanoparticles were formed for reinforcement of photocurable dental resin. Cross-linked polymeric nanoparticles were synthesized by emulsion polymerization of mono- (methyl methacrylate; MMA) and trifunctional (trimethylol propane trimethacrylate; TMPTMA) monomers. The nanoparticles were dispersed in bisphenol A glycol dimethacrylate (Bis-GMA) based dental resin matrix in the range of 5–25 wt% to form photocurable nanocomposites.The effect of reactive polymeric particles on the mechanical properties of photocurable dental resin was investigated. Polymerization shrinkage, polymerization shrinkage stress, viscosity, diametral tensile strength, compressive, and flexural strength of the nanocomposites have been studied.It was observed that the cross-linked nanoparticles significantly influenced the mechanical properties of the reinforced dental resin nanocomposites.     

Jatropha curcas oil based alkyd/epoxy/graphene oxide (GO) bionanocomposites: Effect of GO on curing,mechanical and thermal properties

Jatropha curcas oil based alkyd/epoxy/GO bionanocomposites were prepared by direct solution blending of alkyd/epoxy blend matrix with GO nano filler. Structures and properties of the bionanocomposites were characterized with Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, differential scanning calorimetry, thermogravimetric analysis, and tensile testing. X-ray diffraction and transmission electron microscopy study demonstrates the formation of highly exfoliated GO layers and its homogeneous dispersion throughout the polymer matrix with 1 and 3 wt% GO. However, the intercalated structure is predominant with 5 wt% GO. The homogeneous dispersion and the strong interaction of the GO layers and the polymer matrix induced the significant improvement in thermal and mechanical properties of the bionanocomposites. The tensile strength and elastic modulus of the bionanocomposite increased by 133% and 68% respectively with 3 wt% GO loading. The thermal stability of the bionanocomposite improved by 39 °C and T_g is shifted toward higher temperature by 20 °C as compared to the pristine polymer. Incorporation of GO significantly decreases the curing time of the alkyd/epoxy resin blend.     

Reinforcement with reduced graphene oxide of bioactive glass scaffolds fabricated by robocasting

45S5 bioactive glass composite scaffolds reinforced with reduced graphene oxide (rGO) were fabricated for the first time by robocasting (direct-writing) technique. Composite scaffolds with 0–3 vol.% content of rGO platelets were printed, and then consolidated by pressureless sintering at 550 or 1000 °C in Ar atmosphere. It was found that the addition of rGO platelets up to 1.5 vol.% content enhanced the mechanical performance of the 45S5 bioactive glass scaffolds in terms of strength and toughness. Best performance was obtained for 1 vol.% rGO, which yielded an enhancement of the fracture toughness of ∼850 and 380% for sintering temperatures of 550 and 1000 °C, respectively, while the compressive strength increased by ∼290 and 75%. rGO addition thus emerges as a promising approach for the fabrication of novel bioglass scaffolds with improved mechanical performance without deterioration of their bioactivity, which may then find use in load-bearing bone tissue engineering applications.     

In situ chemical reduction and functionalization of graphene oxide for electrically conductive phenol formaldehyde composites

We report an efficient one-step approach to reduce and functionalize graphene oxide (GO) during the in situ polymerization of phenol and formaldehyde. The hydrophilic and electrically insulating GO is converted to hydrophobic and electrically conductive graphene with phenol as the main reducing agent. Simultaneously, functionalization of GO is realized by the nucleophilic substitution reaction of the epoxide groups of GO with the hydroxyl groups of phenol in an alkali condition. Different from the insulating GO and phenol formaldehyde resin (PF) components, PF composites are electrically conductive due to the incidental reduction of GO during the in situ polymerization. The electrical conductivity of PF composite with 0.85 vol.% of GO is 0.20 S/m, nearly nine orders of magnitude higher than that of neat PF. Moreover, the efficient reduction and functionalization of GO endows the PF composites with high thermal stability and flexural properties. A striking increase in decomposition temperature is achieved with 2.3 vol.% of GO. The flexural strength and modulus of the PF composite with 1.7 vol.% GO are increased by 316.8% and 56.7%, respectively.     

Mesua ferrea L. seed oil based acrylate-modified thermostable and biodegradable highly branched polyester resin/clay nanocomposites

Mesua ferrea L. seed oil based highly branched polyester resin was modified by methyl methacrylate through grafting polymerization technique. The nanocomposites of this acrylate-modified polyester and 1–5 wt% loadings of organically modified montmorrilonite (OMMT) nanoclay were prepared by an ex situ technique using strong mechanical mixing and ultrasonication. Formation of nanocomposites was confirmed by X-ray diffractometeric (XRD), scanning electron microscopic (SEM) and transmission electron microscopic (TEM) analyses. The absence of d_0 0 1 reflections of OMMT in XRD and TEM study revealed the partial exfoliation of OMMT by the polymer chain. The homogeneous surface morphology was also ascertained from SEM. Mechanical and thermal studies of the nanocomposites showed an appreciable improvement in tensile strength and thermal stability by OMMT loading. The enhancement of tensile strength by 2.5 times and thermal stability by 32 °C for 5 wt% OMMT filled nanocomposite was observed compared to that of pristine system. The rheological behavior of the nanocomposites was also investigated and shear thinning was observed. Biodegradation of the nanocomposite films was assayed using two strains of Pseudomonas aeruginosa, SD2 and SD3 and one strain of Bacillus subtilis, MTCC736. The nanocomposites exhibited enhanced biodegradability as compared to pristine acrylate modified polyester. All the results showed the potentiality of the nanocomposites as advanced thin film materials for suitable applications.     

Effect of graphene addition on the mechanical and electrical properties of Al2O3-SiCw ceramics

This paper presents a study on graphene-reinforced Al₂O₃-SiCw ceramic composites and the relationship between graphene oxide (GO) loading and the resulting mechanical and electrical properties. Well-dispersed ceramic-GO powders were fabricated using a colloidal processing route. Dense composites were obtained via spark plasma sintering, a technique that has the ability to reduce GO to graphene in situ during the sintering process. The mechanical properties of the sintered composites were investigated. The composite with only a small amount of graphene (0.5 vol.%) showed the highest flexural strength (904 ± 56 MPa), fracture toughness (10.6 ± 0.3 MPa·m^1/2) and hardness (22 ± 0.8 GPa) with an extremely good dispersion of graphene within the ceramic matrix. In addition to these exceptional mechanical properties, the sintered composites also showed high electrical conductivity, which allows the compacts to be machined using electrical discharge machining and thus facilitates the fabrication of ceramic components with sophisticated shapes while reducing machining costs.     

Microtopography and mechanical properties of vacuum hot pressing Al/B4C composites

Al/B₄C composites with various volume contents of B₄C (5%, 10%, 15%, 20%, and 25%) reinforcing the Al matrix, have been fabricated by vacuum hot press sintering at 680 °C, with a soaking time of 90 min and external pressure of 30 MPa. Mechanical properties, phase composition, and microstructure of the Al/B₄C composites are discussed to reveal the physical properties of the composites. Field emission transmission electron microscopy and selected area electron diffraction have been employed to verify the interior structure and crystal growth direction, respectively. The Vickers hardness, fracture strength, tensile strength, and maximum force attained the optimal values of 108.45 ± 4.02 HV, 585.70 ± 23.26 MPa, 196.18 ± 2.48 MPa, and 4.44 ± 0.17 kN, respectively, for 25 vol% B₄C/Al composites. The static compression strength increased before the 15 vol% B₄C addition and then decreased, acquiring the highest value of 292.15 ± 2.09 MPa for 15 vol% B₄C/Al composites. In general, the relative density and ductility of these composites consistently increased, with an increase in the volume content of Al, achieving a maximum of 99.22% and 54.63 ± 7.34%, respectively, for 5 vol% B₄C/Al composites.     

Controlled direction of electrical and mechanical properties in nickel tethered graphene polyimide nanocomposites using magnetic field

Oriented hybrid nickel tethered graphene polyimide resin nanocomposites with different degrees of orientation were prepared by in-situ magnetic field solvent casting method. Magnetization of the hybrid Ni-graphene polyimide nanocomposites exhibited a maximum in the magnetic field direction and a minimum perpendicular to the magnetic field direction indicating the orientation of the superparamagnetic nickel nanoparticles. In-plane dc electrical conductivity of the 1.3 vol.% Ni-graphene was 2.5 times higher when cast in a high magnetic field compared to films cast without an applied magnetic field. The through-plane dc conductivity of the 1.3 vol.% oriented Ni-graphene polyimide nanocomposites decreased with increasing magnetic field strength and reached insulation (10⁻¹² S/cm) for the films cast in high magnetic field. The in-plane tensile modulus of the polyimide exhibited a 35% increase when 0.16 vol.% Ni-graphene was added to the polyimide and cast in a low-strength magnetic field. Further addition of Ni-graphene, up to 1.3 vol.%, to the polyimide resulted in nearly constant tensile moduli. Tensile strength of nickel graphene nanocomposites showed up to 2-fold increase compared to the neat polyimide. Scanning electron microscopy (SEM) revealed that the Ni-graphene nanosheets were oriented in the magnetic field direction.     

Envisioning the composition effect on structural,magnetic, thermal and optical properties of mesoporous MgFe2O4-GO nanocomposites

The effect of variation in composition on the structural, magnetic, optical and photo catalytic activity of magnesium ferrite (MgFe₂O₄) -graphene oxide (GO) nanocomposites was studied. Magnetic nanocomposites of GO and MgFe₂O₄ nanoparticles (NPs) with varying w/w ratio were synthesized by facile sonication method. X-Ray diffraction patterns confirmed the presence of spinel ferrite phase in the nanocomposites with the crystalline size 8–32 nm. Fourier transformation infrared (FT-IR) spectra of the nanocomposites displayed absorption bands corresponding to GO and MgFe₂O₄ NPs along with red shift of bands corresponding to C=O, C=C and O-H stretching. Thermo gravimetric analysis confirmed higher stability of nanocomposites over pristine GO. Saturation magnetization increased from 3.63 to 11.10 emu/g with the increase in content of MgFe₂O₄ NPs in the nanocomposites. Scanning electron microscopy analysis along with energy dispersive spectroscopy (SEM-EDX) confirmed the presence of MgFe₂O₄ NPs along with GO sheets. Immobilization of clusters of MgFe₂O₄ NPs onto GO sheets was evident from transmission electron micrographs (TEM) of all the nanocomposites. BET surface area of the nanocomposites ranged from 63.04 to 165.29 m²/g and was maximum when GO:MgFe₂O₄ w/w ratio was 1:0.5. It was markedly higher than pristine GO and MgFe₂O₄ NPs. Optical studies revealed lowering of the band gap in the nanocomposites upto 2.21 eV as compared to pristine MgFe₂O₄ NPs. Photoluminescence (PL) spectra of nanocomposites displayed quenching of PL intensity with increase of GO content. Band gap also displayed similar trend. The synthesized nanocomposites were used as photocatalysts for methylene blue dye degradation under visible light irradiation. The nanocomposite with GO to MgFe₂O₄ ratio 1:0.5 displayed best activity with complete degradation of dye in 30 min. The results confirmed that the composition of GO based magnetic nanocomposites can be tailored for efficient removal of contaminants.     

High electrical anisotropy in hydrochloric acid doped polyaniline/phyllosilicate nanocomposites: Effect of phyllosilicate matrix,synthesis pathway and pressure

Pressed tablets from polyaniline/phyllosilicate nanocomposites have been prepared under various conditions in order to optimize anisotropic conductivity of composite by ordering of flat phyllosilicate particles intercalated with polyaniline (PANI). Powder samples of PANI/phyllosilicate nanocomposites have been prepared using two phyllosilicates, montmorillonite and vermiculite, with a different layer charge. Two precursors were used, anilinium hydrochloride and anilinium sulfate. Prepared PANI/phyllosilicate composites were subsequently doped by hydrochloric acid via rinsing after polymerization process and for the DC conductivity measurements pressed into tablets. Applied pressure was 28 MPa and 128 MPa. Highly anisotropic conductivity has been achieved in pressed tablets. The in-plane conductivity for PANI/montmorillonite was 0.084 S/cm, i.e., 1000 × higher than in the direction perpendicular to the tablet plane. Increase of pressure up to 128 MPa led to dramatic decrease of conductivity.     

Covalently-grafted polyaniline on graphene oxide sheets for high performance electrochemical supercapacitors

A simple route to achieve covalently-grafted polyaniline (PANI)/graphene oxide (GO) nanocomposites has been developed. The synthesized composites showed a uniform hierarchical morphology of the PANI thin film and short rod-like nanostructures that had densely grown on the GO sheets, in contrast to the nonuniform morphology of noncovalently-grafted PANI/GO. Compared to pure PANI and noncovalently-grafted PANI/GO composites, the covalently-grafted PANI/GO composites possessed a much larger specific surface area and pore volume, which increased the accessible surface area for the redox reaction and allowed faster ion diffusion. This unique hierarchical morphology maximized the synergistic effect between PANI and GO, resulting in excellent electrochemical performance (capacitance 442 F/g of PANI/GO (6:1) vs. 226 F/g of pure PANI) and improved cycling stability (83% @ 2000 cycles of PANI/GO (6:1) vs. 54.3% @ 1000 cycles of pure PANI). The enhanced electrochemical performance demonstrates the advantage of the PANI/GO composites prepared via this covalent grafting method.     

Crack healing and strength recovery in SiC/spinel nanocomposite

The crack healing behavior and the strength recovery of the newly introduced SiC/spinel nanocomposite were investigated. SiC/spinel nanocomposite containing 27.26 wt% SiC was prepared by the ball milling of talc, graphite and aluminum powders with subsequent annealing at 1200 °C for 1 h in a vacuum. The SEM results showed that the surface cracks produced by Vickers indenter on the prepared SiC/spinel pellets can be completely healed after sintering at 1545 °C for 1 min in air atmosphere. Furthermore, an almost complete strength recovery of the specimens can be obtained in those samples heat treated at 1550 °C for 1 min, as evaluated by diametral tensile strength (DTS) test. The formations of mullite and aluminosilicate glassy phases are the major factors which are responsible for the crack healing and strength recovery in the structure. It was found that the healing efficiency of those specimens healed at 1550 °C for 1 min is 99%.     

Structural,mechanical and biological properties of hydroxyapatite-zirconia-lanthanum oxide composites

Novel hydroxyapatite-zirconia-lanthanum oxide composites for bioceramic applications were synthesized and their structural, mechanical and biological properties were studied. Pure HA was produced via precipitation method and the composites were obtained by several fabrication steps: powder milling, mixing, cold pressing and sintering at 1100 °C for 1 h. The experimental results indicated that the composites consisted of hydroxyapatite as the main phase with a trace amount of tricalcium phosphate. Calcium zirconate (CaZrO₃) was also formed by the reaction between zirconia and calcium oxide (CaO) which is the thermal decomposition product of hydroxyapatite. Addition of zirconia and lanthanum oxide resulted a more loose and porous structure on the surface. The diametral tensile strength of the composites was higher with respect to pure hydroxyapatite. The microhardness of the composites, except the one with the composition of 90 wt% HA and 10 wt% Zr, was relatively lower than that of pure HA but these composites had higher machinability. Cell culture studies with osteoblast-like Saos-2 cell line showed that composites and pure hydroxyapatite were biocompatible. Based on these findings, hydroxyapatite-zirconia-lanthanum oxide composites hold potential to be used in hard tissue replacement and regeneration therapies.     

On the mechanical,thermophysical and dielectric properties of Nextel™ 440 fiber reinforced nitride matrix (N440/Nitride) composites

The Nextel? 440 fiber reinforced nitride matrix (N440/Nitride) composites were fabricated by precursor infiltration and pyrolysis (PIP) route. The results demonstrated that the original N440 fiber had a phase composition of amorphous SiO₂ and γ-Al₂O₃. Its single filament tensile strength was 3.03 GPa (at room temperature), while it dropped to 72.6% and 35.1% at 1200 °C and 1400 °C, respectively. The phase content of N440/Ntride composites was mainly γ-Al₂O₃ and amorphous BN, as well as mullite phase (formed at > 1100 °C). The composites owned a flexural strength up to 76.0 MPa at room temperature. The stair-stepping decrease in the load-displacement curve and fiber pull-outs in the fracture surface indicated a good fiber/matrix interface and toughness. By heating at 1400 °C, the composites still possessed 67.4% of original bending strength. It was found that the high temperatures caused strong fiber-matrix bonding and severe fiber degradation. The specific heat, CTE and thermal conductivity of the composites were 0.325–0.586 J g^?1 K^?1, (3.2–4.0) × 10^?6 K^?1 and 0.78–3.47 W m^?1 K^?1, respectively. The composites possessed a dielectric constant of 4.25–4.35 and loss tangent of 0.004–0.01 at 8–12 GHz. The good overall performances enabled the N440/Nitride composites advanced high-temperature wave-transparent applications.     

Graphene oxide/hydroxyapatite composite coatings fabricated by electrophoretic nanotechnology for biological applications

Graphene oxide (GO) was firstly employed as nanoscale reinforcement fillers in hydroxyapatite (HA) coatings by a cathodic electrophoretic deposition process, and GO/HA coatings were fabricated on pure Ti substrate. The transmission electron microscopy observation and particle size analysis of the suspensions indicated that HA nanoparticles were uniformly decorated on GO sheets, forming a large GO/HA particle group. The addition of GO into HA coatings could reduce the surface cracks and increase the coating adhesion strength from 1.55 ± 0.39 MPa (pure HA) to 2.75 ± 0.38 MPa (2 wt.% GO/HA) and 3.3 ± 0.25 MPa (5 wt.% GO/HA), respectively. Potentiodynamic polarization and electrochemical impedance spectroscopy studies indicated that the GO/HA composite coatings exhibited higher corrosion resistance in comparison with pure HA coatings in simulated body fluid. In addition, superior (around 95% cell viability for 2 wt.% GO/HA) or comparable (80–90% cell viability for 5 wt.% GO/HA) in vitro biocompatibility were observed in comparison with HA coated and uncoated Ti substrate.     

Observation of mechanical percolation in functionalized graphene oxide/elastomer composites

We have covalently functionalized graphene oxide (GO) with octadecylamine (ODA) to form GO–ODA. This material can be dispersed in tetrahydrofuran (THF) and subsequently formed into composites with polymers such as thermoplastic polyurethane (TPU). We have characterized the mechanical properties of composites of GO–ODA in TPU. No increase in stiffness was observed at loading levels below 2.5 vol%. Reinforcement appeared to start sharply at this volume fraction and subsequently increased as a power law with increasing volume fraction. This behavior is typical of percolation and shows that the low-strain stress is not increased until the functionalized graphene flakes form a percolating network. Slightly different behavior is observed for properties related to material failure. The ultimate tensile strength increased linearly with graphene content up to the percolation threshold before subsequently falling off. Similarly the strain at break was constant below the percolation threshold but fell off dramatically above it. This work shows the importance of network formation in the reinforcement of elastomeric materials.     

The effective interfacial shear strength of carbon nanotube fibers in an epoxy matrix characterized by a microdroplet test

The tensile properties of continuous carbon nanotube (CNT) fibers spun from a CNT carpet consisting of mainly double- and triple-walled tubes, and their interfacial properties in an epoxy matrix, are investigated by single fiber tensile tests and microdroplet tests, respectively. The average CNT fiber strength, modulus and strain to failure are 1.2 ± 0.3 GPa, 43.3 ± 7.4 GPa and 2.7 ± 0.5%, respectively. A detailed study of strength distribution of CNT fiber has been carried out. Statistical analysis shows that the CNT fiber strength is less scattered than those of MWCNTs as well as commercial carbon and glass fibers without surface treatment. The effective CNT fiber/epoxy interfacial shear strength is 14.4 MPa. Unlike traditional fiber-reinforced composites, the interfacial shear sliding occurs along the interface between regions with and without resin infiltration in the CNT fiber. Guidelines for microdroplet experiments are established through probability analysis of variables basic to specimen design.     

Colloidal processing,hot pressing and characterisation of electroconductive MWCNT-alumina composites with compositions near the percolation threshold

Multiwall Carbon Nanotubes (MWCNT)-alumina nanocomposites have been fabricated by colloidal processing and uniaxial hot pressing.In the nanotube sols, the electrical conductivity is particularly high, even for low concentrations (≤0.7 vol.%). Classical conductivity models fail to explain this particular behaviour, which is likely to be related to the high aspect ratio of the nanotubes (>70).Aqueous colloidal processing was performed optimising electrostatic repulsion and conserving the homogeneity by freeze-drying. Inhomogeneities of about 50 μm appeared in the composites and a thermodynamic explanation is suggested based on the free volume of elongated and spherical particles, respectively and considering the persistence length of the nanotubes.The densification after hot pressing is incomplete (92–93%) even for the low nanotube concentrations considered (<1.4 vol.%).The composites show electrical conductivity (2.5 S/m) and the percolation threshold is ≤0.6 vol.%.The conductivity is maintained up to 500 °C in air, degradation of the nanotubes due to oxidation at higher temperatures is likely to occur, decreasing the conductivity.