On the Mechanical Properties of Graphyne, Graphdiyne, and Other Poly(Phenylacetylene) Networks

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this recordWe simulate, analyse and compare the mechanical properties of a number of molecular sheet-like systems based on fully substituted, penta-substituted, tetra-substituted and tri-substituted poly(phenylacetylene) using static force-field based methods. The networks are modeled in a 3D environment with and without inter-layer interactions in analogy to graphite and graphene respectively. It is shown that by varying the type of substitution and the length of the acetylene chain, one may control the mechanical properties of such systems. In particular, it is shown that poly(phenylacetylene) systems can be specifically designed to exhibit negative Poisson's ratio, and that the stiffness can be controlled in an independent manner from the Poisson's ratios. This is significant as it highlights the fact that such systems can be tailored to exhibit a particular set of mechanical properties.The research work disclosed in this publication is funded by the ENDEAVOUR Scholarship Scheme (Malta). The scholarship may be part-financed by the European Union − European Social Fund (ESF) under Operational Programme II − Cohesion Policy 2014–2020, “Investing in human capital to create more opportunities and promote the well being of society.” JNG acknowledges the support of the University of Malta research grant

On the Compressibility Properties of the Wine-Rack-Like Carbon Allotropes and Related Poly(phenylacetylene) Systems

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this record.Poly(phenylacetylene) sheets that mimic the geometry of a wine-rack-like structure have been predicted to exhibit negative Poisson's ratios off-axis. However, their potential to exhibit negative linear compressibility (NLC) has remained largely unexplored. In this work, the compressibility and other mechanical properties of wine-rack-like poly(phenylacetylene) networks with 1,2,4,5 tetra-substituted phenyls as well as their equivalent with allene or cyclobutadiene centres are simulated to assess their ability to exhibit negative linear compressibility on-axis and off-axis. It is shown that some of these systems can indeed exhibit negative linear compressibility whilst others exhibit a near-zero compressibility. The results are compared to the compressibility properties of other poly(phenylacetylene) networks reported in literature as well as with those predicted from the analytical model for an idealised wine-rack structure deforming through hinging. Results suggest that these mechanical properties are arising from a wine-rack-like mechanism, and there is a good agreement with the theoretical model, especially for systems with longer acetylene chains whose geometry is closer to that of the idealised wine-rack.University of MaltaENDEAVOUR Scholarship Scheme (Malta

On the Structural and Mechanical Properties of Poly(Phenylacetylene) Truss-Like Hexagonal Hierarchical Nanonetworks

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this recordA novel class of hexagonal nanoscale honeycombs made from penta and tetra substituted polyphenlyacetylenes is proposed and modeled as crystalline systems using force-field based simulations. It is shown that, in-plane, these systems behave rather similarly to crystalline forms of graphyne, graphdiyne, and other fully substituted equivalents but benefit from the presence of larger pores which makes them less stiff and may enable them to be used in a wider range of applications such as nanofiltration. It is also shown that at large strains these systems have the potential to exhibit auxetic out-of-plane behaviour, a property which can be manifested in other triangulated systems and can be explained from buckling of some nanoribs in the systems.University of Malta Research Gran

Auxetic Behaviour and Other Negative Thermo‐Mechanical Properties from Rotating Rigid Units

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this recordAuxetics exhibit the anomalous property of expanding laterally when uniaxially stretched, that is, exhibit a negative Poisson’s ratio, a property which arises from: (i) the presence of specific geometric features within the nano/macro structure of the material and (ii) amenable deformations in response to the applied stimulus. This work explores how ancient symmetrical aesthetic artefacts have been transformed to functional auxetics through mechanism that have ripened the field of “mechanical metamaterials” and “architected materials” in the last decades. In particular, it looks at the important role and various implementations, both in 2D and 3D, of ‘rotating rigid units’ which range from ‘rotating squares’ to much more complex renditions at various scales of structure. The role of rotating rigid units to generate negative thermal expansion and negative compressibility is also delved into.Malta Council for Science and Technolog

Auxetics and FEA: modern materials driven by modern simulation methods.

This is the final version. Available from MDPI via the DOI in this record. Data Availability Statement: Not applicable.Auxetics are materials, metamaterials or structures which expand laterally in at least one cross-sectional plane when uniaxially stretched, that is, have a negative Poisson's ratio. Over these last decades, these systems have been studied through various methods, including simulations through finite elements analysis (FEA). This simulation tool is playing an increasingly significant role in the study of materials and structures as a result of the availability of more advanced and user-friendly commercially available software and higher computational power at more reachable costs. This review shows how, in the last three decades, FEA proved to be an essential key tool for studying auxetics, their properties, potential uses and applications. It focuses on the use of FEA in recent years for the design and optimisation of auxetic systems, for the simulation of how they behave when subjected to uniaxial stretching or compression, typically with a focus on identifying the deformation mechanism which leads to auxetic behaviour, and/or, for the simulation of their characteristics and behaviour under different circumstances such as impacts.University of Malta and the Malta Council for Science & TechnologyMalta Council for Science & TechnologyEuropean Union’s Horizon 202