H. Daniel Wagner

Measurement of carbon nanotube–polymer interfacial strength

The force required to separate a carbon nanotube from a solid polymer matrix has been measured by performing reproducible nanopullout experiments using atomic force microscopy. The separation stress is found to be remarkably high, indicating that carbon nanotubes are effective at reinforcing a polymer. These results imply that the polymer matrix in close vicinity of the carbon nanotube is able to withstand stresses that would otherwise cause considerable yield in a bulk polymer specimen.

Distribution and alignment of carbon nanotubes and nanofibrils in a polymer matrix

Abstract Composites consisting of different quantities of carbon nanotubes and nanofibrils in a poly(methyl methacrylate) (PMMA) matrix have been prepared using a polymer extrusion technique. The nanotubes or nanofibrils were first dispersed over the polymer matrix particles using a dry powder mixing method. The final composite specimens contained well-dispersed and aligned nanofibrils and nanotubes. The orientation distribution of carbon fibrils and nanotubes in the composite was determined by image analysis and found to be maximized in the extrusion flow direction. The Knoop hardness data confirmed this observation, as a maximum was observed at 90° to the orientation of the reinforcement. When the initial PMMA particle diameter was under 200 μm, considerable improvements were observed in the mechanical properties of the nanofibril/PMMA composites. The interpretation of the mechanical data for nanotube/PMMA composites was more complex. Indeed, the tensile modulus was almost insensitive to the presence of either single-wall or multi-wall nanotubes, whereas the impact strength (thus, indirectly, the fracture toughness) was significantly improved by even small amounts of single-wall nanotubes. The method proposed here for the dispersion and orientation of carbon nanotubes and nanofibrils in a polymer matrix show promise for the preparation of improved engineering composites.

Detachment of nanotubes from a polymer matrix

A technique to investigate the adhesion of carbon nanotubes to a polymer matrix is described. Carbon nanotubes bridging across holes in an epoxy matrix have been drawn out using the tip of a scanning probe microscope while recording the forces involved. A full force-displacement trace could be recorded and correlated with transmission electron micrographs observations prior and subsequent to the tip action. Based on these experiments, an approximate calculation of the nanotube-polymer interfacial shear strength has been performed.

Framework for nanocomposites

Materials and material development are fundamental to our very culture. We even ascribe major historical periods of our society to materials such as the stone age, bronze age, iron age, steel age (the industrial revolution), polymer age, silicon age, and silica age (the telecoms revolution). This reflects how important materials are to us. We have, and always will, strive to understand and modify the world around us and the stuff of which it is made. As the 21st century unfolds, it is becoming more apparent that the next technological frontiers will be opened not through a better understanding and application of a particular material, but rather by understanding and optimizing material combinations and their synergistic function, hence blurring the distinction between a material and a functional device comprised of distinct materials.

MECHANICAL PROPERTIES OF CARBON NANOPARTICLE-REINFORCED ELASTOMERS

Abstract Silicone based elastomers have been mixed with single-wall carbon nanotubes or larger carbon nanofibrils. Tensile tests show a dramatic enhancement of the initial modulus of the resulting specimens as a function of filler load, accompanied by a reduction of the ultimate properties. We show that the unique properties of the carbon nanoparticles are important and effective in the reinforcement. The modulus enhancement of the composites initially increases as a function of applied strain, and then at around 10–20% strain the enhancement effect is lost in all of the samples. This “pseudo-yield” in elastomeric (or rubber) composites is generally believed to be due to trapping and release of rubber within filler clusters. However, in-situ Raman spectroscopy experiments show a loss of stress transfer to the nanotubes suggesting that instead, the “pseudo yield” is due to break-down of the effective interface between the phases. The reorientation of nanotubes under strain in the samples may be responsible for the initial increase in modulus enhancement under strain and this is quantified in the Raman experiments.

Nanocomposites: issues at the interface

Carbon nanotubes (CNTs), whether single- or multi-walled (SWNT or MWNT, respectively), have, in an unparalleled fashion, grabbed the attention of both researchers and business leaders within the polymer community. The vast potential afforded by the unprecedented combination of mechanical, electrical, and thermal properties within one nanoscale additive opens new vistas for commodity plastics, elastomers, adhesives, and coatings, as well as new specialty systems with never-before-realized combinations of material properties within a processible plastic or fiber 1 , 2 , 3

On the tensile strength distribution of multiwalled carbon nanotubes

Individual multiwalled carbon nanotubes grown by chemical vapor deposition (CVD) were tensile tested within the chamber of an electron microscope using an atomic force microscope-based technique. Weibull–Poisson statistics could accurately model the nanotube tensile strength data. Weibull shape and scale parameters of 1.7 and 109GPa were obtained. The former reflects a wide variability in strength similar to that observed for high-modulus graphite fibers, while the latter indicates that the irregular CVD-grown tube wall structure requires, in some cases, higher breaking forces than more regular tube wall structures. This apparent strengthening mechanism is most likely caused by an enhanced interaction between the walls of the nanotube.

Materials design in biology

The materials formed by organisms are often the products of hundreds of millions of years of fine-tuning by evolution. They thus incorporate some neat solutions to complex structural problems. Here, we examine four different mineralized biological materials that all appear to fulfill a multipurpose function. They are the crossed lamellar structure of mollusk shells, the skeleton of sea urchins, the lamellar bone type of vertebrates, and the biogenic silica deposited by a wide variety of organisms. They all have very different structures and compositions, but are all structurally designed to reduce the extent of mechanical anisotropy. The strategies for reducing anisotropy are manifold, including features that to date have not been incorporated into synthetic materials.

Single-wall carbon nanotubes as molecular pressure sensors

Specific peaks of the Raman spectrum of single-wall nanotubes shift significantly upon immersion of the tubes in a liquid, relative to the corresponding peaks in air. This observation means that nanotubes are sensitive to molecular forces, and is interpreted by relating the corresponding molecular strain to a thermodynamic parameter, the cohesive energy density (or more loosely, the surface tension) for a range of liquids. We find that nanotubesdeform by a different amount for each liquid.Calibration of this phenomenon enables the construction of a compressive stress–strain curve for carbon nanotubes.

A Study of Statistical Variability in the Strength of Single Aramid Filaments

Variability in the failure load, tenacity, and linear density of single aramid filaments is studied experimentally. Data indicate that both the failure load and the tenacity of filaments, for a given gauge length and yarn cross section, can be fitted to a two parameter Weibull distribution; however the fit is better for tenacity than for failure load, and the Weibull shape parameter for the former is typically smaller. Within a yam cross section filaments vary significantly in linear density (and diameter), and this variability contributes a component to the variability in failure load, but not to the tenacity. Also, the mean tenacity and the variability in linear density and in failure load may differ greatly from spool to spool. The implications of the variability are discussed in light of the work of Bunsell. The effect of gauge length on the strength distribution of filaments is examined. Weibull statistics are used to separate out this effect. As previously shown for other brittle filaments, the Weibull shape parameter for aramid filaments depends on the gauge length; however values for the shape parameter calculated at a fixed gauge length are substantially lower than those obtained by a procedure based on varying the gauge length. This suggests short range correlations in flaw strengths along a filament.