Quantifying the local mechanical properties of twisted double bilayer graphene

Nanomechanical measurements of minimally twisted van der Waals materials remained elusive despite their fundamental importance for device realisation. Here, we use Ultrasonic Force Microscopy (UFM) to locally quantify the variation of out-of-plane Young's modulus in minimally twisted double bilayer graphene (TDBG). We reveal a softening of the Young's modulus by 7% and 17% along single and double domain walls, respectively. Our experimental results are confirmed by force-field relaxation models. This study highlights the strong tunability of nanomechanical properties in engineered twisted materials, and paves the way for future applications of designer 2D nanomechanical systems

Tethered Bilayer Lipid Membranes on Mixed Self-Assembled Monolayers of a Novel Anchoring Thiol: Impact of the Anchoring Thiol Density on Bilayer Formation

Tethered bilayer lipid membranes (tBLMs) are designed on mixed self-assembled monolayers (SAMs) of a novel synthetic anchoring thiol, 2,3-di-o-palmitoylglycerol-1-tetraethylene glycol mercaptopropanoic acid ester (TEG-DP), and a new short dilution thiol molecule, tetraethylene glycol mercaptopropanoic acid ester (TEG). tBLM formation was accomplished by self-directed fusion of small unilamellar vesicles of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine. The influence of the dilution of the anchoring thiol molecule in the SAM on the vesicle fusion process and on the properties of the resulting tBLMs is studied. It is observed by quartz crystal microbalance that vesicle fusion is a one-step process for a pure TEG-DP SAM as well as for mixed SAMs containing a high concentration of the anchoring thiol. However, upon dilution of the anchoring thiol to moderate concentrations, this process is decelerated and possibly follows a pathway different from that observed on a pure TEG-DP SAM. Electrochemical impedance spectroscopy is used to qualitatively correlate the composition of the SAM to the electrical properties of the tBLM. In this paper we also delineate the necessity of a critical concentration of this anchoring TEG-DP thiol as a requisite for inducing the fusion of vesicles to form a tBLM

Fracture toughness measurement of ultra-thin hard films deposited on a polymer

The fracture toughness of 100 nm-thick chromium nitride films deposited on a soft interlayer is evaluated using two different test methods. Both methods take advantage of the enhanced crack driving force resulting fromthe large elastic stiffness mismatch between the film and the polymer interlayer. The first method is based on the presence of channel cracks developing during the deposition as a result of large internal stress. The second method relies on nanoindentation induced cracking. The presence of a soft interlayer is shown to be essential to promote cracking otherwise impossible on a hard substrate, hence to determine the fracture toughness of very thin films. The assumptions underlying the two methods are very different, which allows critical cross comparison and assessment. A fracture toughness value between 1.5 and 2MPa·m1/2 is obtained

Synergistic local toughening of high performance epoxy-matrix composites using blended block copolymer-thermoplastic thin films

A new block copolymer-thermoplastic dual toughening concept for high performance RTM6 epoxy matrix composites is elaborated through a comprehensive microstructural study of well-defined model systems. A blend of phenoxy and MAM (copolymer of poly(methyl methacrylate) and poly(butyl acrylate)) increases the toughness of the corresponding carbon fiber reinforced composite by 125%, a fivefold synergistic improvement over the situation where the tougheners are used alone at the same total concentration. Selective intercalation of blended toughener thin films in the preform gives rise to a diffusion-induced composition gradient during the RTM process, which generates a morphology gradient upon curing. The observed microstructures results from MAM self-assembly in the miscible mixture of phenoxy and the epoxy resin precursor, followed by reaction induced phase separation, forming wormlike micelles, interconnected micelles and co-continuous morphologies. The resulting microstructures activate different toughening mechanisms responsible for the observed synerg

Characterization of the molecular structure of two highly isotactic polypropylenes

Two polypropylenes, PP1 and PP2, produced with different heterogeneous Ziegler-Natta catalytic systems were studied in this work. Preliminary characterization of the non-fractionated materials showed that a low difference in their average tacticity (PP2 > PP1) leads to an important modification of their rigidity properties. In order to establish correlation between the molecular structure parameters and the rigidity properties of these polymers, fractionation of the materials according to crystallizability was performed by means of temperature rising elution fractionation (TREF). Analysis of the fractions of both PP1 and PP2 was carried out by means of C-13 NMR, size exclusion chromatography (SEC), differential scanning calorimetry (DSC) and atomic force microscopy (AFM). The results first showed that TREF does not strictly fractionate PP according to tacticity, but according to the longest crystallizable sequence in a chain. C-13 NMR, SEC and DSC analysis of the fractions demonstrated that the inter-chain tacticity distributions of the polypropylenes is affected by the change of the polymerization conditions, which, in turn, modifies the rigidity properties of the materials. Some results also seem to indicate that the intrachain tacticity distributions are different for the two PP. An AFM study of the elastic modulus was carried out for the first time on the TREE fractions. It showed that the rigidity or the fractions strongly increases as the TREF elution temperature increases in accordance with a concomitant increase of isotacticity and the crytallinity of the fractions. PP2 TREE fractions were, moreover, found to exhibit a higher elastic modulus than PP1 TREF fractions at all elution temperatures. This study allowed us to further identify the TREF fractions that were responsible for differences in rigidity. To summarize, it is shown how the experimentally observed increase of the average rigidity of one of these two polypropylenes can be rationalized via information collected from a TREF fractionation. (C) 2000 Published by Elsevier Science Ltd

Integrated Approach to Eco-Friendly Thermoplastic Composites Based on Chemically Recycled PET Co-Polymers Reinforced with Treated Banana Fibres

A major societal issue of disposal and environmental pollution is raised by the enormous and fast-growing production of single-use polyethylene terephthalate (PET) bottles, especially in developing countries. To contribute to the problem solution, an original route to recycle PET in the form of value-added environmentally friendly thermoplastic composites with banana fibres (Musa acuminata) has been developed at the laboratory scale. Banana fibres are a so far undervalued by-product of banana crops with great potential as polymer reinforcement. The melt-processing constraints of commercial PET, including used bottles, being incompatible with the thermal stability limits use of natural fibres; PET has been modified with bio-sourced reactants to produce co-polymers with moderate processing temperatures below 200°C. First, commercial PET were partially glycolyzed with 1.3-propanediol to produce co-oligomers of about 20 repeating units, which were next chain extended with succinic anhydride and post-treated in a very unusual “soft solid state” process at temperatures in the vicinity of the melting point to generate co-polymers with excellent ductility. The molar mass build-up reaction is dominated by esterification of the chain ends and benefits from the addition of succinic anhydride to rebalance the acid-to-hydroxyl end-group ratio. Infra-red spectroscopy and intrinsic viscosity were extensively used to quantify the concentration of chain ends and the average molar mass of the co-polymers at all stages of the process. The best co-polymers are crystallisable, though at slow kinetics, with a Tg of 48°C and a melting point strongly dependent upon thermal history. The composites show high stiffness (4.8 GPa at 20% fibres), consistent with the excellent dispersion of the fibres and a very high interfacial cohesion. The strong adhesion can be tentatively explained by covalent bonding involving unreacted succinic anhydride in excess during solid stating. A first approach to quantify the sustainable benefits of this PET recycling route, based on a rational eco-selection method, gives promising results since the composites come close to low-end wood materials in terms of the stiffness/embodied energy balance. Moreover, this approach can easily be extended to many other natural fibres. The present study is limited to a proof of concept at the laboratory scale but is encouraging enough to warrant a follow-up study toward scale-up and application development

Iron-based biodegradable alloys: effect of composition and experimental parameters on the corrosion rate

This work investigates the degradation behaviour of iron-based alloys in pseudo physiological conditions. The goal is to identify the best composition leading to a corrosion rate adapted to biodegradable stent applications. In this context, TWIP steels (Fe-Mn-C alloys) with excellent mechanical properties are compared to pure iron, known for its too slow degradation rate. To assess the corrosion properties, two main kinds of tests were conducted, immersion and electrochemical tests. To reach reproducible and relevant results, the suitable experimental parameters were identified. The immersion tests in a pseudo physiological solution had two purposes, to estimate the corrosion rate by measuring the mass loss or concentration of ions released in the solution, and to scrutinise the corrosion mechanism owing to the characterisation of the corroded samples. SEM-EDS, XPS, ToF-SIMS and in situ AFM were used to look at the corroded surfaces. In this last case, the samples were immersed in a cell in the AFM while scans were conducted after different times, giving an insight into the formation of the different layers. Indeed, these analyses showed that different layers form on the surface. The bare metal is covered by oxides and hydroxides and then by calcium phosphates precipitating from the pseudo physiological solution. These layers actually protect the metal from corrosion. The results also show that many factors influence the corrosion rate such as the roughness or pre-oxidation of the surface, the composition of the solution, the solution stirring, the sample positioning, and the solution volume to sample surface ratio. This means that the test protocol has to be carefully chosen to reach reproducible results and to mimic the actual physiological conditions. Electrochemical tests such as potentiodynamic polarisation tests in a pseudo physiological medium aimed at comparing different materials. They also gave information about the tendency of the materials to passivate and the degree of protection offered by the passive layers. It is shown that TWIP steels corrode faster than commercial iron and that the degree of deformation of the TWIP samples does not seem to change noticeably their degradation rate