Fiber-reinforced composites based on epoxy resins modified with elastomers and surface-modified silica nanoparticles

The properties of fiber-reinforced composites made using epoxy resin formulations can be improved using modified epoxy resins. As epoxies are inherently brittle, they are toughened with reactive liquid rubbers or core–shell elastomers. Surface-modified silica nanoparticles, 20 nm in diameter and with a very narrow particle size distribution, are available as concentrates in epoxy resins in industrial quantities for the past 10 years. Some of the drawbacks of toughening like lower modulus or a loss in strength can be compensated when using nanosilica together with these tougheners. Apparently, there exists a synergy as toughness and fatigue performance are increased significantly. Some of these improvements in bulk resin properties can be found for fiber-reinforced composites as well. In this article, the literature published in the last decade is studied with a focus on mechanical properties. Results are compared, and the mechanisms responsible for the property improvements are discussed. A relationship between the improvements of the fracture energy of the cured bulk epoxy resins and the fracture energy of the fiber-reinforced composites could be established.     

Palladium Nanoparticles Supported on Magnetic Carbon‐Coated Cobalt Nanobeads: Highly Active and Recyclable Catalysts for Alkene Hydrogenation

Palladium nanoparticles are deposited on the surface of highly magnetic carbon‐coated cobalt nanoparticles. In contrast to the established synthesis of Pd nanoparticles via reduction of Pd(II) precursors, the microwave decomposition of a Pd(0) source leads to a more efficient Pd deposition, resulting in a material with considerably higher activity in the hydrogenation of alkenes. Systematic variation of the Pd loading on the carbon‐coated cobalt nanoparticle surface reveals a distinct trend to higher activities with decreased loading of Pd. The activity of the catalyst is further improved by the addition of 10 vol% Et₂O to iso‐propanol that is found to be the solvent of choice. With respect to activity (turnover frequencies up to 11 095 h^?1), handling, recyclability through magnetic decantation, and leaching of Pd (≤6 ppm/cycle), this novel magnetic hybrid material compares favorably to conventional Pd/C or Pd@CNT catalysts.     

Spinning Angora Rabbit Wool‐Like Porous Fibers from a Non‐Equilibrated Gelatin/Water/2‐Propanol Mixture

Due to their porous structure, angora rabbit fibers make for some of the highest quality wool. The application of these fibers on a technical scale is not feasible due to their limited availability and high price. Here, a robust fiber preparation method is reported based on an unusual spinning process, where a non‐equilibrated, ternary system of protein, solvent, and non‐solvent is continuously processed into strong fibers with minimal energy input and harmless solvents. Gelatin—the degradation product of collagen—is chosen as the protein component because of its immense availability from slaughterhouse waste. Due to the sponge‐like structure of the ternary gelatin/water/2‐propanol spinning mixture, fibers with internal cavities are produced. The porous nature of these fibers resembles the morphology of angora rabbit fibers. Despite their high porosity, the here‐obtained gelatin fibers show clear re‐orientation of the fibrous protein and attain a mechanical performance similar to other bio‐ (e.g., wool, tendon collagen) and synthetic polymers (e.g., polytetrafluoroethylene, polyamide 6). These promising results motivate for broader investigations on the spinning of non‐equilibrium protein mixtures and suggest the use of porous gelatin fibers in textiles.     

The mechanisms and mechanics of the toughening of epoxy polymers modified with silica nanoparticles

The present paper considers the mechanical and fracture properties of four different epoxy polymers containing 0, 10 and 20 wt.% of well-dispersed silica nanoparticles. Firstly, it was found that, for any given epoxy polymer, their Young’s modulus steadily increased as the volume fraction, v_f, of the silica nanoparticles was increased. Modelling studies showed that the measured moduli of the different silica-nanoparticle filled epoxy polymers lay between upper-bound values set by the Halpin-Tsai and the Nielsen ‘no-slip’ models, and lower-bound values set by the Nielsen ‘slip’ model; with the last model being the more accurate at relatively high values of v_f. Secondly, the presence of silica nanoparticles always led to an increase in the toughness of the epoxy polymer. However, to what extent a given epoxy polymer could be so toughened was related to structure/property relationships which were governed by (a) the values of glass transition temperature, T_g, and molecular weight, M_c, between cross-links of the epoxy polymer, and (b) the adhesion acting at the silica nanoparticle/epoxy-polymer interface. Thirdly, the two toughening mechanisms which were operative in all the epoxy polymers containing silica nanoparticles were identified to be (a) localised shear bands initiated by the stress concentrations around the periphery of the silica nanoparticles, and (b) debonding of the silica nanoparticles followed by subsequent plastic void growth of the epoxy polymer. Finally, the toughening mechanisms have been quantitatively modelled and there was good agreement between the experimentally-measured values and the predicted values of the fracture energy, G_c, for all the epoxy polymers modified by the presence of silica nanoparticles. The modelling studies have emphasised the important roles of the stress versus strain behaviour of the epoxy polymer and the silica nanoparticle/epoxy-polymer interfacial adhesion in influencing the extent of the two toughening mechanisms, and hence the overall fracture energy, G_c, of the nanoparticle-filled polymers.     

Solid state 19F NMR study of crystal transformation in PVDF and its nanocomposites

The polymorphism of poly(vinylidene fluoride) (PVDF) and its nanocomposites was studied by means of solid state nuclear magnetic resonance spectroscopy. ¹³C cross polarization magic angle spinning (¹³C CP MAS) NMR spectra were recorded using simultaneous high‐power decoupling on both the proton and fluorine channels. Both ¹H → ¹³C and ¹⁹F → ¹³C CP experiments were conducted, giving identical results apart from intensity variations due to the CP efficiency. Two main resonances for the CF₂ and the CH₂ groups were observed for both neat PVDF (PVDF‐C0) and the nanocomposite containing 2 wt% clay (PVDF‐C2) samples. ¹⁹F CP MAS spectra were obtained from long proton spin‐lock experiments with a shorter contact time. The results showed two strong resonances at ?84 and ?98 ppm with equal intensities, representing the α‐form crystalline structure of PVDF. It was shown that the clay induces the crystallization of PVDF in β‐form. Our earlier investigations using thermal analysis and X‐ray scattering methods also showed crystal transformation of PVDF in its clay nanocomposites. POLYM. ENG. SCI. 46:1684–1690, 2006. © 2006 Society of Plastics Engineers     

Nontriggered magnetic resonance velocity measurement of the time-average of pulsatile velocity

The feasibility of the determination of the time-average of pulsatile velocity obtained via a nontriggered magnetic resonance (MR) acquisition is studied. The advantage of this method, in comparison with a triggered acquisition, is a considerable reduction (approximately 15x) in acquisition time. However, pulsatility causes image artifacts, known as ghosts, and the Fourier transform technique required for the imaging procedure accomplishes time-averaging of the complex MR signal. Both effects can result in errors in the velocity determined. Calculations show that these errors depend on the velocity time function and the acquisition parameters. In vivo comparison of triggered and nontriggered MR velocity measurements in the femoral artery of volunteers (n = 7) shows larger statistical and systematic errors in the latter, which depend on the excitation angle. Therefore, this nontriggered average velocity measurement is only useful as a fast and rough estimation of the time-averaged velocity.     

Upgrade of the TRIUMF on-line isotope separator, TISOL

The operational TISOL thick target, on-line isotope separator at the TRIUMF, 500 MeV proton cyclotron facility has been upgraded to be a production facility with an active experimental program. A new experimental area is now available and modifications are under design to handle remotely the expected radioactively “hot” targets. Two ion source systems are now available, a heated surface of normal design and a new ECR (electron cyclotron resonance) source resulting in ion beams from a wide range of elements. Details of the new upgraded facility will be presented along with its experimental program and plans for the future. The status of the previously proposed accelerated radioactive beams facility, ISAC, for which TISOL is a prototype front-end system, will also be mentioned.     

Selective Chemical Modification of Graphene Surfaces: Distinction Between Single‐ and Bilayer Graphene

Graphene modifications with oxygen or hydrogen are well known in contrast to carbon attachment to the graphene lattice. The chemical modification of graphene sheets with aromatic diazonium ions (carbon attachment) is analyzed by confocal Raman spectroscopy. The temporal and spatial evolution of surface‐adsorbed species allows accurate tracking of the chemical reaction and identification of intermediates. The controlled transformation of sp² to sp³ carbon proceeds in two separate steps. The presented derivatization is faster for single‐layer graphene and allows controlled transformation of adsorbed diazonium reagents into covalently bound surface derivatives with enhanced reactivity at the edge of single‐layer graphene. On bilayer graphene the derivatization proceeds to an adsorbed intermediate, which reacts slower to a covalently attached species on the carbon surface.