Surface energy of silk fibroin and mechanical properties of silk cocoon composites

Silkworm cocoons are biological composite structures protecting the silkworms against environmental damage and physical attack by natural predators. In particular, some outdoor reared silk cocoons exhibit outstanding mechanical properties that are relevant to the higher level protection required to enhance the survival chance of silkworms while supporting their metabolic activity. The performance of composite materials strongly depends on the adhesion between the fiber reinforcement and matrix, with the surface properties of the fibers playing a key role in determining the level of adhesion achieved. For this reason it is important to study the surface properties of silk fibroin to further understand the composite properties of the cocoons. In this work, both the mechanical properties of the silk cocoons and silk fibroin were studied. The surface topography was examined using scanning probe microscopy (SPM), which revealed distinct longitudinal ridges and striations along the fiber axis of the four silk fiber types. The fibers were found to exhibit heterogeneity in surface energy as evidenced from inverse gas chromatography (IGC) measurements. The combination of excellent mechanical properties and the more energetically heterogeneous surface nature of the wild A. pernyi silk fibroin fibers correlates well with the excellent mechanical properties of the A. pernyi cocoons. This journal is

Effect of surface functionality of PAN-based carbon fibres on the mechanical performance of carbon/epoxy composites

The performance of composite laminates depends on the adhesion between the fibre reinforcement and matrix, with the surface properties of the fibres playing a key role in determining the level of adhesion achieved. For this reason it is important to develop an in-depth understanding of the surface functionalities on the reinforcement fibres. In this work, multi-scale surface analysis of carbon fibre during the three stages of manufacture; carbonisation, electrolytic oxidation, and epoxy sizing was carried out. The surface topography was examined using scanning electron microscopy (SEM), which revealed longitudinal ridges and striations along the fibre-axis for all fibre types. A small difference in surface roughness was observed by scanning probe microscopy (SPM), while the coefficient of friction measured by an automated single fibre tester showed 51% and 98% increase for the oxidised and sized fibres, respectively. The fibres were found to exhibit heterogeneity in surface energy as evidenced from SPM force measurements. The unsized fibres were much more energetically heterogeneous than the sized fibre. A good correlation was found between fibre properties (both physical and chemical) and interlaminar shear strength (ILSS) of composites made from all three fibre types. © 2014 Elsevier Ltd.

Electric contributions to magnetic force microscopy response from graphene and MoS2 nanosheets

Magnetic force microscopy (MFM) signals have recently been detected from whole pieces of mechanically exfoliated graphene and molybdenum disulfide (MoS2) nanosheets, and magnetism of the two nanomaterials was claimed based on these observations. However, non-magnetic interactions or artefacts are commonly associated with MFM signals, which make the interpretation of MFM signals not straightforward. A systematic investigation has been done to examine possible sources of the MFM signals from graphene and MoS2 nanosheets and whether the MFM signals can be correlated with magnetism. It is found that the MFM signals have significant non-magnetic contributions due to capacitive and electrostatic interactions between the nanosheets and conductive cantilever tip, as demonstrated by electric force microscopy and scanning Kevin probe microscopy analyses. In addition, the MFM signals of graphene and MoS2 nanosheets are not responsive to reversed magnetic field of the magnetic cantilever tip. Therefore, the observed MFM response is mainly from electric artefacts and not compelling enough to correlate with magnetism of graphene and MoS2 nanosheets.

Confocal laser scanning microscopy elucidation of the micromorphology of the leaf cuticle and analysis of its chemical composition

Electron microscopy techniques such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have been invaluable tools for the study of the micromorphology of plant cuticles. However, for electron microscopy, the preparation techniques required may invariably introduce artefacts in cuticle preservation. Further, there are a limited number of methods available for quantifying the image data obtained through electron microscopy. Therefore, in this study, optical microscopy techniques were coupled with staining procedures and, along with SEM were used to qualitatively and quantitatively assess the ultrastructure of plant leaf cuticles. Leaf cryosections of Triticum aestivum (wheat), Zea mays (maize), and Lupinus angustifolius (lupin) were stained with either fat-soluble azo stain Sudan IV or fluorescent, diarylmethane Auramine O and were observed under confocal laser scanning microscope (CLSM). For all the plant species tested, the cuticle on the leaf surfaces could be clearly resolved in many cases into cuticular proper (CP), external cuticular layer (ECL), and internal cuticular layer (ICL). Novel image data analysis procedures for quantifying the epicuticular wax micromorphology were developed, and epicuticular waxes of L. angustifolius were described here for the first time. Together, application of a multifaceted approach involving the use of a range of techniques to study the plant cuticle has led to a better understanding of cuticular structure and provides new insights into leaf surface architecture.

Evaluating the corrosion behaviour of magnesium alloy in simulated biological fluid by using SECM to detect hydrogen evolution

Scanning electrochemical microscopy (SECM) in surface generation/tip collection mode is investigated as an assessment tool for studying the corrosion behaviour of magnesium in simulated biological fluid. The technique provides a local map of hydrogen (H2) evolution which alone can be used as a direct measure of corrosion. The H2 generated during corrosion of magnesium is oxidized at the probe(i.e. a Pt ultra micro-electrode);with the magnitude of the current generated due to oxidation being indicative of the intensity of H2 evolution at a local scale on the magnesium surface. This method was calibrated using a cathodically polarized Pt disk to simulate H2 evolution in a controlled condition on a homogeneous surface. Potential interference from dissolving Mg or high local pH was also investigated. The technique was implemented for studying H2 evolution at the surface of AZ31 as a model Mg alloy.SECM results combined with SEM-EDX and profilometry data revealed that local domains of higher H2 evolution on the surface of AZ31 are in close proximityof the observed pitting sites.

Studying the cathodic disbondment of coatings using electrochemical impedance spectroscopy

The disbondment of protective organic coatings is a widely reported pipeline coating failure mode in the oil and gas industry. Traditional methods of evaluating cathodic disbondment of pipeline coatings are based on visual inspection of pipeline conditions, and laboratory testing of cathodic disbondment resistance (CDR) using standard methods such as ASTM G8. Although some other laboratory-based techniques, such as scanning kelvin probe and scanning acoustic microscopy have been used to study the cathodic disbondment (CD) of coatings, these are often difficult to apply in practical testing. Over the past decade, electrochemical impedance spectroscopy (EIS) has been employed as a potential method for measuring CD. This paper reports preliminary results from an EIS study designed to characterise CD behaviour of epoxy coatings under excessive cathodic protection. EIS data correlated well with the area of disbonded coating. Analysis of EIS data can provide valuable information on the initiation and rates of CD.

The influence of graphene on the electrical communication through organic layers on graphite and gold electrodes

The influence of graphene on the electrical communication through organic layers fabricated on graphite and gold electrodes is investigated. These layers were prepared by in situ reductive adsorption of 4-aminobenzoic acid in the presence of NaNO2 and HCl to have surface bound carboxylic acid functionalities, followed by covalent attachment of 1-aminopyrene via an amide coupling reaction to have surface bound pyrene groups for graphene immobilization via noncovalent π-π stacking interaction. The coverage of the layers created via reductive adsorption on graphite electrodes was found to be much higher than that on gold electrodes. It was revealed that graphene significantly enhances the electrical communication through the layers on graphite electrodes but on gold electrodes the enhancement effect through the layers was only minor. However, when gold electrodes were modified with a self-assembled monolayer (SAM) of propanethiol the subsequent immobilization of graphene resulted in a significant enhancement of the electrical communication. It is also found that immobilization of graphene could affect the electron transfer between the redox probe, pyrene and the underlying electrodes. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to characterize the graphene sheets. Cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS) were also used to characterize the stepwise modified electrodes. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.