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.     

Investigation of the nanomechanical properties of nylon 6 and nylon 6/clay nanocomposites at sub-ambient temperatures

The nanomechanical properties of nylon 6, nylon 6/exfoliated clay and nylon 6/non-exfoliated clay nanocomposites have been investigated from room temperature to ?10 °C in a controlled environment with humidity less than 1% RH. The hardness, elastic modulus and creep resistance of nylon 6 were improved in the nanocomposites across the temperature range. However, the effective reinforcement of the clay depended on the temperature due to the change between the glassy and transition states in the nylon. The exfoliated clay nanocomposite showed the greater improvements than in the non-exfoliated clay nanocomposite at all testing temperatures due to the improved constraint of the polymer chains by the clay platelets in the exfoliated structure. The surface mechanical properties of nylon 6 and the nanocomposites were also found to be highly sensitive to the moisture level during the tests; increasing the humidity in the room temperature tests resulted in a dramatic decrease in hardness and stiffness due to plasticisation by water molecules. The kinetics of the re-humidification process on nylon 6 were studied by monitoring the change in nanoindentation response. Analysis of the indentation creep revealed a significant change in the strain rate sensitivity when the humidity of the near-surface region probed by nanoindentation was in the vicinity of the glass transition.     

Clay exfoliation and organic modification on wear of nylon 6 nanocomposites processed by different routes

The role of nanoclay on the wear characteristics of nylon 6 nanocomposites processed via different routes is examined in this paper. Pristine clay and organoclay were used in melt-extrusion with the matrix resulting in a largely aggregated and a highly exfoliated morphology, respectively. High exfoliation of pristine clay was also achieved by a water-assist process in melt compounding. Nylon 6/pristine clay composite had the worst wear resistance due to the large aggregated clay particles. For the two nylon 6/exfoliated clay nanocomposites, the one with the organically modified clay outperformed that with the pristine clay exfoliated by water. Detailed study on the wear track and subsurface below of the nylon 6/clay composites using both transmission and scanning electron microscopy provided new insight into the differences in their deformation and damage mechanisms. It was revealed that the interfacial adhesion of clay to matrix, and not the exfoliated morphology of clay, played a critical role in wear. However, exfoliated clay morphology is preferred to aggregate morphology. Hence, the superior wear-performance of nylon 6/organoclay nanocomposite is brought about by a combined effect of fine dispersion of clay platelets in nylon 6, high interfacial interaction between nylon 6 and clay layers, and effective constraint on surrounding nylon 6 material exerted by the clay platelets.     

Surface modification of hydroxyapatite to introduce interfacial bonding with polyactiveTM 70/30 in a biodegradable composite

A method was developed to improve the interfacial bonding between hydroxyapatite and a biodegradable copolymer Polyactive^TM 70/30. Hydroxyapatite was first surface modified by the polyelectrolytes polyacrylic acid or poly(ethylene-co-maleic acid) in aqueous solutions. Subsequently the surface-modified hydroxyapatite was used as filler in composites with Polyactive^TM 70/30. The strength, elongation at break and elastic modulus of the composite in aqueous environment were significantly improved by this method. Based on these experimental results, it is believed that the interface improvement is due to hydrogen bonding and/or dipole interactions formed between polyelectrolyte molecules and polyethylene glycol segments in the polymer matrix. Due to the introduction of interfacial bonding by using such method, a new biodegradable bone-bonding composite can be made.     

Rheology and structure of aqueous bentonite-polyvinyl alcohol dispersions

The influence of polymer on flow behaviour of Balikesir, Turkey bentonite dispersions (2%, w/w) was studied for non-ionic polymer, polyvinyl alcohol (PVA). In a range of 3.3 × 10⁶ −3.3 × 10⁵ mol/l, PVA was added to the bentonite dispersions in different concentrations and its behaviour was observed on rheology parameters. Thixotropy was detected by a hysteresis loop of the flow curves. The data were interpreted taking into account the interactions of colloidal clay particles, bentonitic clay concentrations, structure, and concentrations of added PVA. The particle size analysis was explained by surface orientation of PVA to the clay particles dispersed in aqueous solution. Zeta potential determination also emphasized that PVA molecules got attached on the face and edge surface of clay particles. The morphology of bentonite dispersions was analysed by scanning electron micrograph (SEM). FTIR studies carried out in parallel to rheology studies showed that hydrogen bonds were formed between surface of the clay, and absorbed PVA molecules and adsorbed water. The presence of PVA did not prevent extensive swelling of bentonite.     

Natural silica fiber as reinforcing filler of nylon 6

Short silica fiber (SF) content on the mechanical and morphological properties of composites based on nylon 6 and rubber-toughened nylon 6 matrices was examined. Binary nylon 6/SF and ternary [nylon 6/EPDM-g-MA (ethylene–propylene–diene grafted with maleic anhydride)/SF] composites containing 0–20 wt% SF were formulated. The flexural modulus increased with the SF content at all fiber compositions investigated; however, the value of this property gradually diminished when 20 wt% rubber was added to the polymer. Notched Izod impact strength of 80/20 nylon 6/EPDM-g-MA blend was reduced up to 50% with the addition of 5 wt% SF. However, these composites still retained good stiffness and toughness and presented a good interfacial adhesion between the phases. The results suggest that silica fibers can be employed as an alternative reinforcement of nylon 6 matrices, resulting in materials with useful properties.     

X-ray photoelectron spectroscopic study of non-stoichiometric nickel and nickel–copper spinel manganites

The surface of non-stoichiometric nickel and nickel–copper spinel manganites has been investigated by X-ray Photoelectron Spectroscopy (XPS). The oxidation states of the nickel, copper and manganese cations present on the surface of the samples were determined from the analysis of the M 2p_3/2 core levels (M=Ni, Cu, Mn). In particular, both Cu²⁺ and Cu⁺ were evidenced in the structure whereas only bivalent nickel was observed. The partial substitution of manganese by copper led to a chemical shift towards lower binding energy in the Ni 2p_3/2 region, which was explained by the displacement of some Ni²⁺ cations from tetrahedral to octahedral sites of the spinel structure. Finally, the surface atomic ratios Ni/Mn for nickel manganites, Ni/(Mn+Cu) and Cu/(Mn+Ni) for nickel–copper manganites, determined from XPS data, were compared to the ratios corresponding to the bulk composition. This study shows in all cases a nickel enrichment at the surface which is not affected by the copper content of the oxide. On the contrary, the ratio Cu/(Mn+Ni) was found to be lower than the corresponding bulk value.     

碳纤维／PF尼龙复合材料性能及界面研究

制备了碳纤维／ＰＦ尼龙复合材料，采用Ｘ光电子能谱仪及化学滴定法定量分析了碳纤维表面的含氧状况，利用扫描电镜研究了碳纤维的表面形态及碳纤维／ＰＦ尼龙复合材料的界面形态，探讨了碳纤维表面形态和含氧量对复合材料力学性能和界面粘结状况的影响。结果表明，规整ＣＦ表面的羧基与ＰＦ尼龙分子中的胺基发生化学键合是复合材料具有良好力学性能及界面具有良好粘结的主要因素。     

A Surface‐Functionalized Ionovoltaic Device for Probing Ion‐Specific Adsorption at the Solid–Liquid Interface

Aqueous ion–solid interfacial interactions at an electric double layer (EDL) are studied in various research fields. However, details of the interactions at the EDL are still not fully understood due to complexity induced from the specific conditions of the solid and liquid parts. Several technical tools for ion–solid interfacial probing are experimentally and practically proposed, but they still show limitations in applicability due to the complicated measurements. Recently, an energy conversion device based on ion dynamics (called ionovoltaic device) was also introduced as another monitoring tool for the EDL, showing applicability as a novel probing method for interfacial interactions. Herein, a monitoring technique for specific ion adsorption (Cu²⁺ and Pb²⁺ in the range of 5 × 10^?6–1000 × 10^?6m ) in the solid–liquid interface based on the ionovoltaic device is newly demonstrated. The specific ion adsorption and the corresponding interfacial potentials profiles are also investigated to elucidate a working mechanism of the device. The results give the insight of molecular‐level ion adsorption through macroscopic water‐motion‐induced electricity generation. The simple and cost‐effective detection of the device provides an innovative route for monitoring specific adsorption and expandability as a monitoring tool for various solid–liquid interfacial phenomena that are unrevealed.     

Malayaite ceramic pigments: A combined optical spectroscopy and neutron/X-ray diffraction study

Ceramic pigments based on the Cr-doped malayaite structure were synthesized by solid state reaction and characterized by optical spectroscopy and combined X-ray and neutron powder diffraction in order to elucidate the still unclear chromium substitution mechanisms. The results show that coloration is actually due to simultaneous occurrence of Cr⁴⁺ and Cr³⁺ ions in the crystal lattice. Spectroscopy data confirm that Cr⁴⁺ is replacing Sn⁴⁺ in the octahedral site and, in minor amount, Si⁴⁺ in the tetrahedral site. In addition, neutron powder diffraction data suggest that Cr³⁺ substitution for octahedral Sn⁴⁺ is charge balanced by the formation of oxygen vacancies with no preference over the different oxygen sites. Upon incorporation of Cr ion, the SnO₆ octahedra exhibit an off-centre displacement of central cation which in turn induces a rearrangement of both the octahedral and tetrahedral coordination shells.     

Fracture behavior of nylon hybrid composites

Hybrid composite systems consisting of liquid crystalline polymer (LCP), short glass fibers and toughened nylon in varied ratios were studied. Dynamic mechanical results indicated that, elastomeric phase in toughened nylon 6,6 promoted a better compatibilization between nylon 6,6 and LCP in a hybrid system containing short glass fibers in comparison with one without glass fibers. Improved compatibility facilitated fibrillation of LCP phase in the skin region of the hybrid composite, thereby providing superior tensile strength. Without the presence of LCP, glass fiber reinforced toughened nylon 6,6 exhibited the least tensile strength. J-integral analysis and essential work of fracture (EWF) method were used to compare the fracture behavior of composites. Results showed that specific essential work of fracture were consistent with the critical J-integral. Matrices reinforced by LCP alone showed the best crack initiation and propagation toughnesses, followed by glass fiber reinforced and hybrid composites. The better compatibility between nylon 6,6 and LCP appeared to inhibit the interfacial debonding process, resulting in brittle fracture.     

Adhesive B-doped DLC films on biomedical alloys used for bone fixation

The long-term failure of the total hip and knee prostheses is attributed to the production of wear particles at the articulating interface between the metals, ceramics and polymers used for surgical implants and bone-fixtures. Therefore, finding an adhesive and inert coating material that has low frictional coefficient should dramatically reduce the production of wear particles and hence, prolong the life time of the surgical implants. The novel properties of the non-toxic diamond-like carbon (DLC) coatings have proven to be excellent candidates for biomedical applications. However, they have poor adhesion strength to the alloys and biomaterials. The addition of a thin interfacial layer such as Si, Ti, TiN, Mo and Cu/Cr and/or adding additives such as Si, F, N, O, W, V, Co, Mo, Ti or their combinations to the DLC films has been found to increase the adhesion strength substantially. In our study, grade 316L stainless steel and grade 5 titanium alloy (Ti-6Al-4V) were used as biomaterial substrates. They were coated with DLC films containing boron additives at various levels using various Si interfacial layer thicknesses. The best film adhesion was achieved at 8% and 20% on DLC coated Ti-6Al-4V and grade 316L substrates, respectively. It has been demonstrated that doping the DLC with boron increases their adhesion strength to both substrates even without silicon interfacial layer and increases it substantially with optimum silicon layer thickness. The adhesion strength is also correlated with the hydrogen contents in the B-DLC films. It is found to reach its maximum value of 700 kg/cm² and 390 kg/cm² at 2/7 and 3/6 for CH₄/Ar partial pressures (in mTorr ratio) for Ti-6Al-4V and 316L substrates, respectively.     

Fire retardancy of clay/carbon nanofiber hybrid sheet in fiber reinforced polymer composites

In this paper, two kinds of clay/carbon nanofiber hybrid sheets containing 0.05 wt% and 0.20 wt% of Cloisite Na⁺ clay, were fabricated through a high-pressure filtration system. These sheets were integrated onto the surface of laminated composites like traditional continuous fiber mats through vacuum-assisted resin transfer molding process. The fire performance of the laminated composites was evaluated with cone calorimeter tests under an external radiant heat flux of 50 kW/m². Their residues were analyzed with scanning electron microscopy and thermal gravimetric analyses. It was found that the clay/nanofiber hybrid sheets survived on the combustion surface of composites and significantly reduced the heat release rate by 60.5%. The protective clay layer reduces the heat release rates and the nanofiber network reinforces the clay layer against the air bubbling and melt flow of the products degraded from the polymer resin. The clay/carbon nanofiber hybrid sheet combines the barrier and insulator effects of the clays with the re-emitting heat effect of carbon nanofibers on the combustion surface of composites.     

Stochastic analysis of interphase effects on elastic modulus and yield strength of nylon 6/clay nanocomposites

The multifunctional properties of polymer clay nanocomposites (PCNs) can be related to the interaction of clays, polymer and interphase region. Several experimental, analytical and numerical studies have been conducted to characterize the mechanical behavior of PCNs. The elastic behavior of PCNs is well documented in the literature but their other material properties like yield strength are rather vague. On the other hand, the variation of material parameters and the stochastic nature of interphase region hinder the use of deterministic methods. In this study, a stochastic analysis along with a hierarchical multiscale method is used to analyze the effect of interphase properties on the macroscopic properties of PCNs. Since the interphase layer is expected to be weaker than the polymer matrix, a weakening coefficient is defined to describe the interphase properties based on the matrix properties. This weakening coefficient and the interphase thickness are considered as the stochastic inputs. The elastic modulus and yield strength of nylon 6/clay nanocomposites are calculated using the stochastic multiscale framework. The uncertainty propagation and sobol sensitivity analysis are performed to study the effect of random inputs on the elastic modulus and yield strength of PCNs. Despite the wide range of input variations, the accuracy of the proposed stochastic multiscale framework for the prediction of the PCNs properties is estimated by validating our results against the available experimental data in the literature.

     

A biomimetic approach to enhancing interfacial interactions: polydopamine-coated clay as reinforcement for epoxy resin

A facile biomimetic method was developed to enhance the interfacial interaction in polymer-layered silicate nanocomposites. By mimicking mussel adhesive proteins, a monolayer of polydopamine was constructed on clay surface by a controllable coating method. The modified clay (D-clay) was incorporated into an epoxy resin, it is found that the strong interfacial interactions brought by the polydopamine benefits not only the dispersion of the D-clay in the epoxy but also the effective interfacial stress transfer, leading to greatly improved thermomechanical properties at very low inorganic loadings. Rheological and infrared spectroscopic studies show that the interfacial interactions between the D-clay and epoxy are dominated by the hydrogen bonds between the catechol-enriched polydopamine and the epoxy.     

Polymer nanocomposites processed via self-assembly through introduction of nanoparticles,nanosheets, and nanofibers

Nanocomposites offer the theoretical potential to achieve mechanical properties surpassing those of conventional (micro-scale) composites. The underlying reasons for the high potential of nanocomposites include the uniquely high mechanical attributes of nano-scale reinforcement, effective control of defect size and growth by nano-spaced interfaces, and interactions between the polymer matrix and the large surface areas of nanomaterials. Attempts to produce nanocomposites via conventional processing techniques have encountered challenges associated with thorough dispersion and effective interfacial interactions of nano-scale reinforcement with the polymer matrix. In order to address these challenges, materials were processed into polymer nanocomposites via electrostatically driven layer-by-layer self-assembly. Electrostatically dispersed nanomaterials and oppositely charged polyelectrolytes were sequentially built upon a substrate (cellular scaffold). The self-assembled nanocomposites, after complementary cross-linking, provided a unique balance of strength and ductility, which surpassed those of conventional (micro-scale) composites. Self-assembly was found to be an effective approach to producing nanocomposites embodying uniformly dispersed nanomaterials with controlled interfacial interactions. This approach is highly versatile and enables introduction of diverse nanomaterials into polymer nanocomposites. The work reported herein evaluated introduction of diverse categories of nanomaterials incorporating nanoparticles, nanosheets, nanotubes, and nanofibers. This investigation also evaluated the potential for a biomimetic approach to processing of light-weight structural systems by self-assembly of polymer nanocomposites onto cellular scaffolds.     

乙烯基酯复合材料/橡胶界面性能研究

通过对橡胶表面进行处理并涂刷柔性过渡层,研究橡胶与乙烯基树脂复合材料的界面粘接性能。结果表明,橡胶表面经过化学处理后,出现大量细小微裂纹,使得橡胶与乙烯基酯复合材料的界面粘接强度得到大幅度提高;在经过化学处理后的橡胶表面涂刷柔性能渡层,二者界面剥离强度提高到2.90 k N/m,粘合强度达到2.30 MPa。     

Fabrication and properties of SiC fibre-reinforced Li2O·Al2O3·6SiO2 glass-ceramic composites

Ta₂O₅, Nb₂O₅ and TiO₂ were used separately as additives to a Li₂O·Al₂O₃·6SiO₂ glass-ceramic composition, to act as nucleating dopants and to aid the formation of an interfacial carbide layer (TaC and NbC) between the fibre and matrix in SiC fibre uniaxially reinforced glass-ceramic composites, The composites exhibited high modulus of rupture (>800 MPa) and fracture toughness (K _IC > 15 MPam^1/2). The interfacial amorphous carbon rich layer and carbide layer were responsible for lowered interfacial shear strength but permitted high composite fracture toughness. The composite with the TiO₂ additive in the matrix showed a lower flexural strength (<500MPa) and a smaller K _IC (-11 MPam^1/2) which resulted from the high interfacial shear strength between the SiC fibre and the matrix due to the formation of the interfacial TiC layer.     

A Hydrogel‐Film Casting to Fabricate Platelet‐Reinforced Polymer Composite Films Exhibiting Superior Mechanical Properties

The fabrication of mechanically superior polymer composite films with controllable shapes on various scales is difficult. Despite recent research on polymer composites consisting of organic matrices and inorganic materials with layered structures, these films suffer from complex preparations and limited mechanical properties that do not have even integration of high strength, stiffness, and toughness. Herein, a hydrogel‐film casting approach to achieve fabrication of simultaneously strong, stiff, and tough polymer composite films with well‐defined microstructure, inspired from a layer‐by‐layer structure of nacre is reported. Ca²⁺‐crosslinked alginate hydrogels incorporated with platelet‐like alumina particles are dried to form composite films composed of horizontally aligned alumina platelets and alginate matrix with uniformly layered microstructure. Alumina platelets are evenly distributed parallel without precipitations and contribute to synergistic enhancements of strength, stiffness and toughness in the resultant film. Consequentially, Ca²⁺‐crosslinked alginate/alumina (Ca²⁺‐Alg/Alu) films show exceptional tensile strength (267 MPa), modulus (17.9 GPa), and toughness (3.60 MJ m⁻³). Furthermore, the hydrogel‐film casting allows facile preparation of polymer composite films with controllable shapes and various scales. The results suggest an alternative approach to design and prepare polymer composites with the layer‐by‐layer structure for superior mechanical properties.