21 research outputs found
Assessing MWCNT-graphene surface energy through in situ SEM peeling
Carbon nanotubes (CNTs) are envisioned as ideal filaments for next-generation nanocomposites due to their high strength-to-weight ratios. However, while individual nanotubes are strong, interfaces between tubes cannot bear significant load due to the weak van der Waals forces that govern their behavior. Premature interfacial failure could thus counteract the inherent strength of carbon nanotubes and, in turn, prevent CNT-based composites from achieving optimal mechanical performance. To increase the load bearing capacity of these interfaces, interlayer crosslinking schemes have been proposed using chemical functionalization. For instance, introduction of hydrogen bonds or additional van der Waals bonds between tubes could improve load transfer between CNTs. While introducing chemical groups on CNT surfaces may enhance intermolecular interactions at these interfaces, a means of quantitatively evaluating changes in interlayer adhesion as a result of to these treatments needs to be defined. In addition, as sizes of CNTs will inherently vary within a composite, it is important that such energy measurements be normalized irrespective of tube dimensions.
Here we report an experimental peeling technique that can be used to measure the adhesion energy between multiwalled carbon nanotubes (MWCNTs) and graphene. Peeling tests conducted in situ a scanning electron microscope allow direct visualization of the nanoscale peeling process which, in turn, enables adhesion energy to be estimated through classical fracture analysis. The applicability of this analysis is validated by finite element simulations with boundary conditions derived from experiments. The effective contact width between tubes and graphene is estimated via atomistic simulations, providing a means to normalize interaction energy per unit area. The surface energies of bare MWCNT-graphene interfaces found in this study compare favorably with theoretical and experimental values reported for graphite. This method can serve as a foundation for evaluating the enhancements afforded by chemical functionalization, which is a critical step toward the development of strong, lightweight composites that effectively utilize the full mechanical potential of CNTs
Quantifying High-Performance Material Microstructure Using Nanomechanical Tools with Visual and Frequency Analysis
High-performance materials like ballistic fibers have remarkable mechanical properties owing to specific patterns of organization ranging from the molecular scale, to the micro scale and macro scale. Understanding these strategies for material organization is critical to improving the mechanical properties of these high-performance materials. In this work, atomic force microscopy (AFM) was used to detect changes in material composition at an extremely high resolution with transverse-stiffness scanning. New methods for direct quantification of material morphology were developed, and applied as an example to these AFM scans, although these methods can be applied to any spatially-resolved scans. These techniques were used to delineate between subtle morphological differences in commercial ultra-high-molecular-weight polyethylene (UHMWPE) fibers that have different processing conditions and mechanical properties as well as quantify morphology in commercial Kevlar®, a high-performance material with an entirely different organization strategy. Both frequency analysis and visual processing methods were used to systematically quantify the microstructure of the fiber samples in this study. These techniques are the first step in establishing structure-property relationships that can be used to inform synthesis and processing techniques to achieve desired morphologies, and thus superior mechanical performance
Hierarchical Mechanisms of Lateral Interactions in High- Performance Fibers
The processing conditions used in the production of advanced polymer fibers facilitate the formation of an oriented fibrillar network that consists of structures spanning multiple length scales. The irregular nature of fiber tensile fracture surfaces suggests that their structural integrity is defined by the degree of lateral (interfacial) interactions that exist within the fiber microstructure. To date, experimental studies have quantified interfacial adhesion between nanoscale fibrils measuring 10−50 nm in width, and the global fracture energy through applying peel loads to fiber halves. However, a more in-depth evaluation of tensile fracture indicates that fiber failure typically occurs at an intermediate length scale, involving fibrillation along interfaces between fibril bundles of a few 100s of nanometers in width. Interaction mechanisms at this length scale have not yet been studied, due in part to a lack of established experimental techniques. Here, a new focused ion beam-based sample preparation protocol is combined with nanoindentation to probe interfaces at the intermediate length scale in two high-performance fibers, a rigid-rod poly(p-phenylene terephthalamide) and a flexible chain ultrahigh molecular weight polyethylene fiber. Higher interfacial separation energy recorded in the rigid-rod fiber correlated with less intensive fibrillation during failure and is discussed in the context of fiber chemistry and processing. Power law scaling of the total absorbed interfacial separation energy at three different scales in the polyethylene fiber is observed and analyzed, and distinct energy absorption mechanisms, featuring a degree of self-similarity, are identified. The contribution of these mechanisms to the overall integrity of the fiber is discussed, and the importance of the intermediate scale is elucidated. Results from this study provide new insights into the mechanical implications of hierarchical lateral interactions and will aid in the development of novel fibers with further improved mechanical performance
Molecular-Level Engineering of Adhesion in Carbon Nanomaterial Interfaces
Weak interfilament van der Waals
interactions are potentially a significant roadblock in the development
of carbon nanotube- (CNT-) and graphene-based nanocomposites. Chemical
functionalization is envisioned as a means of introducing stronger
intermolecular interactions at nanoscale interfaces, which in turn
could enhance composite strength. This paper reports measurements
of the adhesive energy of CNT–graphite interfaces functionalized
with various coverages of arylpropionic acid. Peeling experiments
conducted in situ in a scanning electron microscope show significantly
larger adhesive energies compared to previously obtained measurements
for unfunctionalized surfaces (Roenbeck et al. <i>ACS Nano</i> <b>2014</b>, <i>8</i> (1), 124–138). Surprisingly,
however, the adhesive energies are significantly higher when both
surfaces have intermediate coverages than when one surface is densely
functionalized. Atomistic simulations reveal a novel functional group
interdiffusion mechanism, which arises for intermediate coverages
in the presence of water. This interdiffusion is not observed when
one surface is densely functionalized, resulting in energy trends
that correlate with those observed in experiments. This unique intermolecular
interaction mechanism, revealed through the integrated experimental–computational
approach presented here, provides significant insights for use in
the development of next-generation nanocomposites
The Role of Water in Mediating Interfacial Adhesion and Shear Strength in Graphene Oxide
Graphene
oxide (GO), whose highly tunable surface chemistry enables
the formation of strong interfacial hydrogen-bond networks, has garnered
increasing interest in the design of devices that operate in the presence
of water. For instance, previous studies have suggested that controlling
GO’s surface chemistry leads to enhancements in interfacial
shear strength, allowing engineers to manage deformation pathways
and control failure mechanisms. However, these previous reports have
not explored the role of ambient humidity and only offer extensive
chemical modifications to GO’s surface as the main pathway
to control GO’s interfacial properties. Herein, through atomic
force microscopy experiments on GO–GO interfaces, the adhesion
energy and interfacial shear strength of GO were measured as a function
of ambient humidity. Experimental evidence shows that adhesion energy
and interfacial shear strength can be improved by a factor of 2–3
when GO is exposed to moderate (∼30% water weight) water content.
Furthermore, complementary molecular dynamics simulations uncovered
the mechanisms by which these nanomaterial interfaces achieve their
properties. They reveal that the strengthening mechanism arises from
the formation of strongly interacting hydrogen-bond networks, driven
by the chemistry of the GO basal plane and intercalated water molecules
between two GO surfaces. In summary, the methodology and findings
here reported provide pathways to simultaneously optimize GO’s
interfacial and in-plane mechanical properties, by tailoring the chemistry
of GO and accounting for water content, in engineering applications
such as sensors, filtration membranes, wearable electronics, and structural
materials
Key Factors Limiting Carbon Nanotube Yarn Strength: Exploring Processing-Structure-Property Relationships
Studies of carbon nanotube (CNT) based composites have been unable to translate the extraordinary load-bearing capabilities of individual CNTs to macroscale composites such as yarns. A key challenge lies in the lack of understanding of how properties of filaments and interfaces across yarn hierarchical levels govern the properties of macroscale yarns. To provide insight required to enable the development of superior CNT yarns, we investigate the fabrication–structure–mechanical property relationships among CNT yarns prepared by different techniques and employ a Monte Carlo based model to predict upper bounds on their mechanical properties. We study the correlations between different levels of alignment and porosity and yarn strengths up to 2.4 GPa. The uniqueness of this experimentally informed modeling approach is the model’s ability to predict when filament rupture or interface sliding dominates yarn failure based on constituent mechanical properties and structural organization observed experimentally. By capturing this transition and predicting the yarn strengths that could be obtained under ideal fabrication conditions, the model provides critical insights to guide future efforts to improve the mechanical performance of CNT yarn systems. This multifaceted study provides a new perspective on CNT yarn design that can serve as a foundation for the development of future composites that effectively exploit the superior mechanical performance of CNTs