Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433.
UES, Inc., Dayton, OH 45432.
Proc Natl Acad Sci U S A. 2021 May 25;118(21). doi: 10.1073/pnas.2102477118.
Soft robotics requires materials that are capable of large deformation and amenable to actuation with external stimuli such as electric fields. Energy harvesting, biomedical devices, flexible electronics, and sensors are some other applications enabled by electroactive soft materials. The phenomenon of flexoelectricity is an enticing alternative that refers to the development of electric polarization in dielectrics when subjected to strain gradients. In particular, flexoelectricity offers a direct linear coupling between a highly desirable deformation mode (flexure) and electric stimulus. Unfortunately, barring some exceptions, the flexoelectric effect is quite weak and rather substantial bending curvatures are required for an appreciable electromechanical response. Most experiments in the literature appear to confirm modest flexoelectricity in polymers although perplexingly, a singular work has measured a "giant" effect in elastomers under some specific conditions. Due to the lack of an understanding of the microscopic underpinnings of flexoelectricity in elastomers and a commensurate theory, it is not currently possible to either explain the contradictory experimental results on elastomers or pursue avenues for possible design of large flexoelectricity. In this work, we present a statistical-mechanics theory for the emergent flexoelectricity of elastomers consisting of polar monomers. The theory is shown to be valid in broad generality and leads to key insights regarding both giant flexoelectricity and material design. In particular, the theory shows that, in standard elastomer networks, combining stretching and bending is a mechanism for obtaining giant flexoelectricity, which also explains the aforementioned, surprising experimental discovery.
软机器人需要能够实现大变形且可通过外部刺激（如电场）进行驱动的材料。能量收集、生物医学设备、柔性电子和传感器等都是通过电活性软材料实现的一些其他应用。挠曲电现象是一种诱人的替代方案，它指的是在应变梯度作用下电介质中产生的电极化现象。特别是，挠曲电提供了一种高度理想的变形模式（弯曲）和电刺激之间的直接线性耦合。不幸的是，除了一些例外情况，挠曲电效应非常微弱，需要相当大的弯曲曲率才能产生可观的机电响应。文献中的大多数实验似乎都证实了聚合物中的适度挠曲电效应，尽管令人费解的是，在某些特定条件下，一项工作在弹性体中测量到了“巨大”效应。由于缺乏对弹性体中挠曲电的微观基础的理解和相应的理论，目前既不可能解释弹性体上相互矛盾的实验结果，也不可能寻求设计大挠曲电的途径。在这项工作中，我们提出了一个由极性单体组成的弹性体的新兴挠曲电的统计力学理论。该理论被证明具有广泛的有效性，并为巨型挠曲电和材料设计提供了关键的见解。特别是，该理论表明，在标准弹性体网络中，拉伸和弯曲的结合是获得巨大挠曲电的一种机制，这也解释了上述令人惊讶的实验发现。
