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

Abstract

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.

Group action Markov chain Monte Carlo for accelerated sampling of energy landscapes with discrete symmetries and energy barriers

“…For many of these applications, it is desirable to maximize the coupling between the external stimuli and mechanical deformation [7,21,67,69]. To this end, statistical mechanics has proven useful for uncovering new types of couplings [22,23,43], new mechanisms for increasing couplings [44], and informing the design of materials with novel and enhanced properties [24,[39][40][41]44]. Further, statistical mechanics may also play an important role in understanding the role of monomer-monomer interactions and possible associated phase transitions in soft multifunctional elastomers.…”

“…Monte Carlo methods are amenable to approximating solutions to problems that allow a probabilistic interpretation. They have vast applications in uncertainty quantification [42, 75-77, 79, 95], Bayesian statistics [20,32,42,55,93,95] (and machine learning [4,95], more broadly), and computational physics [2,3,10,20,31,44,45,50,53,54,56,62,63,73,80,82,83,87,90,91]. While the focus of this work will be on equilibrium statistical mechanics, the proposed method for accelerated importance sampling is not specific to physical applications.…”

Nonlinear Statistical Mechanics Drives Intrinsic Electrostriction and Volumetric Torque in Polymer Networks

“…Similarly, chains consisting of monomers with fixed magnitude dipoles (i.e. "frozen in" dipoles) have an electrically-induced compressive stiffness [52]. These cases could give rise to interesting behavior such as microbuckling [53], bistability, and phase transitions when loaded in compression [54,55].…”

Flexoelectric enhanced film for an ultrahigh tunable piezoelectric-like effect

“…As for large deformation capability, flexoelectricity, representing the linear relationship between strain gradient and electric polarization, possesses unique advantages in elastomeric stretchable materials. 25 The flexoelectric polarization P l can be described aswhere μ ijkl and ∂ ε ij /∂ x k are the flexoelectric coefficient and strain gradient. 26,27 Flexoelectricity is expected to possess excellent electromechanical properties in stretchable materials, due to its large strain gradient capability based on large deformation range.…”