A. J. Kulik

Mechanical Properties of Carbon Nanotubes

Abstract.A variety of outstanding experimental results on the elucidation of the elastic properties of carbon nanotubes are fast appearing. These are based mainly on the techniques of high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM) to determine the Young’s moduli of single-wall nanotube bundles and multi-walled nanotubes, prepared by a number of methods. These results are confirming the theoretical predictions that carbon nanotubes have high strength plus extraordinary flexibility and resilience. As well as summarising the most notable achievements of theory and experiment in the last few years, this paper explains the properties of nanotubes in the wider context of materials science and highlights the contribution of our research group in this rapidly expanding field. A deeper understanding of the relationship between the structural order of the nanotubes and their mechanical properties will be necessary for the development of carbon-nanotube-based composites. Our research to date illustrates a qualitative relationship between the Young’s modulus of a nanotube and the amount of disorder in the atomic structure of the walls. Other exciting results indicate that composites will benefit from the exceptional mechanical properties of carbon nanotubes, but that the major outstanding problem of load transfer efficiency must be overcome before suitable engineering materials can be produced.

Elastic modulus of ordered and disordered multiwalled carbon nanotubes

Reference LNNME-ARTICLE-1999-005doi:10.1002/(SICI)1521-4095(199902)11:2 3.0.CO;2-JView record in Web of Science Record created on 2007-04-23, modified on 2016-08-08

Scanning local‐acceleration microscopy

By adapting a scanning force microscope to operate at frequencies above the highest tip–sample resonance, the sensitivity of the microscope to materials’ properties is greatly enhanced. The cantilever’s behavior in response to high‐frequency excitation from a transducer underneath the sample is fundamentally different than to its low‐frequency response. In this article, the motivations, instrumentation, theory, and first results for this technique are described.

Materials' properties measurements: Choosing the optimal scanning probe microscope configuration

Rheological models are used to represent different scanning probe microscope configurations. The solutions for their static and dynamic behavior are found and used to analyze which scanning probe microscope configuration is best for a given application. We find that modulating the sample at high frequencies results in the best microscope behavior for measuring the stiffness of rigid materials, and that by modulating the tip at low frequencies and detecting the motion of the tip itself (not its position relative to the tip holder) should be best for studying compliant materials in liquids.

High-frequency mechanical spectroscopy with an atomic force microscope

In this article we further develop local mechanical spectroscopy and extend the frequency range over which it can be used. Using a heterodyne method to measure the deflection of the cantilever enables one to measure the probe vibration at any frequency. Since the detection sensitivity of force gradients follows a f2 dependence, extending the frequency range from 1 to more than 5 MHz increases the sensitivity by over an order of magnitude. This setup is combined with a realistic model of the cantilever taking into account the geometry of the cantilever. The model is presented and discussed, and compared with experimental behavior measured on WC–Co and NiTi–epoxy samples. Experimental moduli of 730±50 and 260±40 GPa are obtained for WC and Co, respectively.

Molecular Architecture of Plant Thylakoids under Physiological and Light Stress Conditions: A Study of Lipid–Light-Harvesting Complex II Model Membranes

The organization of plant thylakoid membranes under physiological and light stress conditions was analyzed in studies of model membranes formed with galactolipids and LHCII. The results show adaptation of an organization pattern of lipid-protein membranes to better fulfill two opposite physiological functions: harvesting of light quanta versus quenching of excess energy. In this study, we analyzed multibilayer lipid-protein membranes composed of the photosynthetic light-harvesting complex II (LHCII; isolated from spinach [Spinacia oleracea]) and the plant lipids monogalcatosyldiacylglycerol and digalactosyldiacylglycerol. Two types of pigment-protein complexes were analyzed: those isolated from dark-adapted leaves (LHCII) and those from leaves preilluminated with high-intensity light (LHCII-HL). The LHCII-HL complexes were found to be partially phosphorylated and contained zeaxanthin. The results of the x-ray diffraction, infrared imaging microscopy, confocal laser scanning microscopy, and transmission electron microscopy revealed that lipid-LHCII membranes assemble into planar multibilayers, in contrast with the lipid-LHCII-HL membranes, which form less ordered structures. In both systems, the protein formed supramolecular structures. In the case of LHCII-HL, these structures spanned the multibilayer membranes and were perpendicular to the membrane plane, whereas in LHCII, the structures were lamellar and within the plane of the membranes. Lamellar aggregates of LHCII-HL have been shown, by fluorescence lifetime imaging microscopy, to be particularly active in excitation energy quenching. Both types of structures were stabilized by intermolecular hydrogen bonds. We conclude that the formation of trans-layer, rivet-like structures of LHCII is an important determinant underlying the spontaneous formation and stabilization of the thylakoid grana structures, since the lamellar aggregates are well suited to dissipate excess energy upon overexcitation.

Dynamic mechanical analysis at the submicron scale

Dynamic mechanical analysis (DMA) is traditionally performed on bulk samples. However, studies of polymer blends would be enhanced if DMA could be applied on a local scale in order to enable a new form of microthermal analysis. Mounting a sample on a vibrating heating stage and observing the resulting amplitude and phase of the motion of an atomic force microscope cantilever allows the local elastic and visco-elastic properties to be studied. It is demonstrated in this article on samples of polyethersulfone/poly (acryonitrile-co-styrene) and polystyrene/poly(methyl methacrylate) (PS/PMMA) blends, and PMMA, PS and polytetrafluoroethylene homopolymers. Images at a specific temperature and spectroscopic data as a function of temperature of (nominally) a single point were collected. Primary and secondary relaxations were detected; the lateral resolution is better than 100 nm. We discuss the promising and limiting aspects of this new technique.

Gene-mediated Restoration of Normal Myofiber Elasticity in Dystrophic Muscles

Dystrophin mediates a physical link between the cytoskeleton of muscle fibers and the extracellular matrix, and its absence leads to muscle degeneration and dystrophy. In this article, we show that the lack of dystrophin affects the elasticity of individual fibers within muscle tissue explants, as probed using atomic force microscopy (AFM), providing a sensitive and quantitative description of the properties of normal and dystrophic myofibers. The rescue of dystrophin expression by exon skipping or by the ectopic expression of the utrophin analogue normalized the elasticity of dystrophic muscles, and these effects were commensurate to the functional recovery of whole muscle strength. However, a more homogeneous and widespread restoration of normal elasticity was obtained by the exon-skipping approach when comparing individual myofibers. AFM may thus provide a quantification of the functional benefit of gene therapies from live tissues coupled to single-cell resolution.

Correlations between adhesion hysteresis and friction at molecular scales

Correlations between adhesion hysteresis and local friction are theoretically and experimentally investigated. The model is based on the classical theory of adhesional friction, contact mechanics, capillary hysteresis, and nanoscale roughness. Adhesion hysteresis was found to scale with friction through the scaling factor containing a varying ratio of adhesion energy over the reduced Youngs modulus. Capillary forces can offset the relationship between adhesion hysteresis and friction. Measurements on a wide range of engineering samples with varying adhesive and elastic properties confirm the model. Adhesion hysteresis is investigated under controlled, low humidity atmosphere via ultrasonic force microscopy. Friction is measured by the friction force microscopy.

Nanoscale spatially resolved infrared spectra from single microdroplets

Droplet microfluidics has emerged as a powerful platform allowing a large number of individual reactions to be carried out in spatially distinct microcompartments. Due to their small size, however, the spectroscopic characterisation of species encapsulated in such systems remains challenging. In this paper, we demonstrate the acquisition of infrared spectra from single microdroplets containing aggregation-prone proteins. To this effect, droplets are generated in a microfluidic flow-focussing device and subsequently deposited in a square array onto a ZnSe prism using a micro stamp. After drying, the solutes present in the droplets are illuminated locally by an infrared laser through the prism, and their thermal expansion upon absorption of infrared radiation is measured with an atomic force microscopy tip, granting nanoscale resolution. Using this approach, we resolve structural differences in the amide bands of the spectra of monomeric and aggregated lysozyme from single microdroplets with picolitre volume.