Atomistic modeling of flexoelectricity in amorphous polymers.

Area of Science:

• Materials Science
• Computational Materials Science
• Polymer Physics

Background:

• Flexoelectricity, the generation of electric polarization in response to mechanical strain gradients, is a phenomenon observed in dielectric materials.
• Understanding flexoelectricity in polymers like Polyvinylidene Fluoride (PVDF) and Polyethylene (PE) is crucial for developing novel electromechanical devices.

Purpose of the Study:

• To establish amorphous models for PVDF and PE.
• To investigate the relationship between configuration density and temperature using molecular dynamics simulations.
• To determine the transverse flexoelectric coefficients of amorphous PVDF and PE under strain gradients.

Main Methods:

• Development of amorphous PVDF and PE models.
• Molecular dynamics simulations to calculate configuration density and temperature relationships.
• Experimental validation of model rationality using glass transition temperature (T_g).
• Application of strain gradients to main chain carbon atoms.
• Fitting polarization-strain gradient data to obtain flexoelectric coefficients.

Main Results:

• The transverse flexoelectric coefficients (μ₁₂) for amorphous PVDF and PE were determined to be 1.24×10^-9 C/m and -2.63×10^-9 C/m, respectively.
• Micromechanical analysis showed dipole rotation (CF₂ and CH₂) around the main chain under strain gradients, leading to electric polarization.
• The opposite signs of the flexoelectric coefficients are attributed to differences in charge number and electronegativity between PVDF and PE.

Conclusions:

• The study successfully modeled amorphous PVDF and PE and quantified their transverse flexoelectric coefficients.
• Strain gradient-induced dipole rotation is the primary mechanism for flexoelectricity in these polymers.
• The significant difference in the absolute values of the flexoelectric coefficients highlights the distinct electromechanical responses of PVDF and PE.