Auxetic polymer networks: The role of crosslinking, density, and disorder

Low-crosslinked polymer networks have recently been found to behave auxetically when subjected to small tensions, that is, their Poisson's ratio ν becomes negative. In addition, for specific state points, numerical simulations revealed that diamond-like networks reach the limit of mechanical stability, exhibiting values of ν = −1, a condition that we define as hyper-auxeticity. This behavior is interesting per se for its consequences in materials science but is also appealing for fundamental physics because the mechanical instability is accompanied by evidence of criticality. In this work, we deepen our understanding of this phenomenon by performing a large set of equilibrium and stress–strain simulations in combination with phenomenological elasticity theory. The two approaches are found to be in good agreement, confirming the above results. We also extend our investigations to disordered polymer networks and find that the hyper-auxetic behavior also holds in this case, still manifesting a similar critical-like behavior as in the diamond one. Finally, we highlight the role of the number density, which is found to be a relevant control parameter determining the elastic properties of the system. The validity of the results under disordered conditions paves the way for an experimental investigation of this phenomenon in real systems, such as hydrogels.

近期研究发现，低交联聚合物网络在受小张力作用时会表现出拉胀（auxetic）特性，即其泊松比ν为负值。此外，针对特定状态点的数值模拟显示，类金刚石网络达到力学稳定性极限时，其泊松比ν=-1，我们将这一状态定义为超拉胀特性（hyper-auxeticity）。该特性本身在材料科学领域具有重要研究价值，同时也因力学失稳伴随临界现象的相关证据，而在基础物理领域受到关注。本研究通过开展大量平衡态模拟与应力-应变模拟，并结合唯象弹性理论，深化了对该现象的认知。两种方法的计算结果吻合良好，验证了前述结论。我们还将研究拓展至无序聚合物网络，发现超拉胀特性在该体系中同样成立，且同样呈现出与类金刚石网络相似的类临界行为。最后，我们着重探讨了数密度的作用，发现其是决定体系弹性特性的关键调控参数。无序条件下结论的有效性，为在水凝胶等真实体系中开展该现象的实验研究铺平了道路。