A 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.

In this work, through the use of a theoretical model, we analyse the potential of a specific three-dimensional mechanical metamaterial composed of arrowhead-like structural units to exhibit a negative Poisson’s ratio for an arbitrary loading direction. Said analysis allows us to assess its suitability for use in applications where materials must be able to respond in a desired manner to a stimulus applied in multiple directions. As a result of our studies, we show that the analysed system is capable of exhibiting auxetic behaviour for a broad range of loading directions, with isotropic behaviour being shown in some planes. In addition to that, we show that there are also certain loading directions in which the system manifests negative linear compressibility. This enhances its versatility and suitability for a number of applications where materials exhibiting auxetic behaviour or negative linear compressibility are normally implemented.

We 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.

Mechanical metamaterials represent a class of deformable systems, which exhibit macroscopic deformations, mechanical, and/or thermal properties. These emerge due to the structure of their subunits rather than their materials composition and typically exhibit anomalous (normally negative) macroscopic structural, mechanical, or thermal property/properties caused by a change in shape/size of the system. Herein, a class of honeycombs is discussed, which push to the extreme the classical definition of “mechanical metamaterials,” exhibiting temperature‐tunable Poisson's ratio properties. More specifically, centrosymmetric honeycombs with T‐shaped joints constructed from different materials are shown to exhibit temperature‐dependent Poisson's ratio values, which can be either positive or negative (auxetic) depending on the external stimulus the system is subjected to. The sign and magnitudes of the Poisson's ratio values are explained in terms of particular geometries that these composite honeycomb systems attain at different temperature conditions. In particular, auxeticity is attributed to the transformation of the T‐shaped units to re‐entrant units. Practical aspects on how these properties may be achieved are discussed, including the possibility that the changes in shape, and hence Poisson's ratios, are induced via different extents of dryness on opposite surfaces of ligaments made from absorbent materials.

Auxetics display the anomalous property of expanding laterally when uniaxially stretched, that is, a negative Poisson's ratio, a property that arises from 1) the presence of specific geometric features within the nano/macrostructure of the material and 2) amenable deformations in response to the applied stimulus. Herein, how ancient symmetrical esthetic artifacts have been transformed to functional auxetics through mechanisms that have ripened the field of “mechanical metamaterials” and “architected materials” in the last decades is explored. In particular, 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, are looked at. The role of rotating rigid units to generate negative thermal expansion and negative compressibility is also delved into.