• DOI:10.1073/pnas.2102477118
• Corpus ID: 235093032

Flexoelectricity in soft elastomers and the molecular mechanisms underpinning the design and emergence of giant flexoelectricity

@article{Grasinger2021FlexoelectricityIS,
  title={Flexoelectricity in soft elastomers and the molecular mechanisms underpinning the design and emergence of giant flexoelectricity},
  author={Matthew Grasinger and Kosar Mozaffari and Pradeep Sharma},
  journal={Proceedings of the National Academy of Sciences},
  year={2021},
  volume={118}
}

• Matthew Grasinger, Kosar Mozaffari, P. Sharma
• Published 21 May 2021
• Materials Science
• Proceedings of the National Academy of Sciences

Significance The molecular-scale phenomena that lead to the emergence of the flexoelectric effect in elastomers are not well understood. We present a molecular-to-continuum scale theory and show it to be valid for common types of elastomers. We unravel a mechanism for achieving giant flexoelectricity—which finds support in prior experimental results. The multiscale theory is then leveraged for material design and we propose specific modalities for 1) the emergence of piezoelectricity, 2) tuning the direction of flexoelectricity, and 3) flexoelectricity which is invariant with respect to spurious deformations. These developments may pave the way for the future development of efficient soft energy harvesters, sensors, and actuators, and may prove useful for high-degree-of-freedom soft robots and artificial muscles. 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.