Fundamental Insights into Graphene Strain Sensing.

Graphene has been studied extensively for use in flexible electronics as ultrasensitive and wide-area strain sensors. Many sensors demonstrated so far rely on graphene networks, such that the spatial resolution is compromised, and they are unable to measure strain variations on a fine scale such as those resulting from substrate/interface failure. In this study, mono-/few-layer graphene are demonstrated to be good candidates for strain sensing with high spatial resolution to evaluate features <100 nm. The fundamentals of strain sensing-interaction with the target-have been discussed to shed light on the sensitivity and durability for future sensor fabrication. The proof-of-concept strain sensors have been shown to be able to monitor different states, e.g., the initiation and evolution, of crazes. The analysis also leads to the evaluation of interfacial energy and realization of high local strain in graphene that is applicable for other 2D materials for ultrasensitive strain sensing and bandgap opening applications.

Deformation of and Interfacial Stress Transfer in Ti3C2 MXene-Polymer Composites.

Transitional metal carbides and nitrides (MXenes) have promise for incorporation into multifunctional composites due to their high electrical conductivity and excellent mechanical and tribological properties. It is unclear, however, to what extent MXenes are also able to improve the mechanical properties of the composites and, if so, what would be the optimal flake size and morphology. Herein, Ti3C2Tx MXene is demonstrated to be indeed a good candidate for mechanical reinforcement in polymer matrices. In the present work, the strain-induced Raman band shifts of mono-/few-/multilayer MXenes flakes have been used to study the mechanical properties of MXene and the interlayer/interfacial stress transfer on a polymer substrate. The mechanical performance of MXene was found to be less dependent upon flake thickness compared to that of graphene. This enables Ti3C2Tx MXene to offer an efficient mechanical reinforcement to a polymer matrix with a flake length of >10 µm and a thickness of 10s of nanometers. Therefore, the degree of exfoliation of MXenes is not as demanding as other two-dimensional (2D) materials for the purpose of mechanical enhancement in polymers. In addition, the active surface chemistry of MXene facilitates possible functionalization to enable a stronger interface with polymers for applications, such as strain engineering and mechanical enhancement, and in materials including membranes, coatings, and textiles.

Realising biaxial reinforcement via orientation-induced anisotropic swelling in graphene-based elastomers.

The biaxial mechanical properties constitute another remarkable advantage of graphene, but their evaluation has been overlooked in polymer nanocomposites. Herein, we provided an innovative and practical method to characterise biaxial reinforcement from graphene via swelling of elastomers, where graphene nanoplatelets (GNPs) were controlled to be oriented in-plane. The in-plane-aligned graphene imposed a biaxial constraining force to the elastomer during the swelling process that led to the anisotropic swelling behaviour of the bulk nanocomposites. This unconventional swelling behaviour was successfully modelled using statistical mechanics. Novel in situ Raman band shift measurements were also performed during the de-swelling process of the samples. The characteristic 2D band shift of the GNPs, imposed by stress transfer during the de-swelling procedure, was of the order of 0.1 cm-1 per % strain, enabling the calculation of the effective biaxial Young's modulus of the GNPs in the nanocomposites (∼2 GPa). The determination of the biaxial modulus of GNPs contributed to the evaluation of their Poisson's ratio (∼0.34), which is very important but highly impractical to be measured directly on a nanoscale-sized specimen.

Mechanisms of mechanical reinforcement by graphene and carbon nanotubes in polymer nanocomposites.

Polymer nanocomposites reinforced with carbon-based nanofillers are gaining increasing interest for a number of applications due to their excellent properties. The understanding of the reinforcing mechanisms is, therefore, very important for the maximization of performance. This present review summarizes the current literature status on the mechanical properties of composites reinforced with graphene-related materials (GRMs) and carbon nanotubes (CNTs) and identifies the parameters that clearly affect the mechanical properties of the final materials. It is also shown how Raman spectroscopy can be utilized for the understanding of the stress transfer efficiency from the matrix to the reinforcement and it can even be used to map stress and strain in graphene. Importantly, it is demonstrated clearly that continuum micromechanics that was initially developed for fibre-reinforced composites is still applicable at the nanoscale for both GRMs and CNTs. Finally, current problems and future perspectives are discussed.