Piezoelectricity enhancement in graphene/polyvinylidene fluoride composites due to graphene-induced α → β crystal phase transition

Piezoelectric polymers represented by polyvinylidene fluoride (PVDF) possess many attractive attributes so they are widely used in electromechanical energy conversion devices. To further enhance the piezoelectric effect of PVDF, graphene and its derivatives have been added. A central issue is how the added graphene improves the piezoelectric properties of the graphene/PVDF composites. Starting from consideration of crystal phase transition from α to β phase in PVDF due to addition of graphene, we first introduce Cauchy’s cumulative probability function to characterize the evolution of β phase as a function of graphene content. Then, by considering the relationship between β phase and the dipole, and between the microscopic dipole moment and the macroscopic piezoelectric coefficient, a theory of piezoelectricity for the composite is obtained. The theory shows that the β phase increases slowly at low graphene content but the increase is very rapid as the graphene concentration approaches the percolation threshold, after which there is no significant growth. The piezoelectric coefficient also increases similarly until the percolation threshold, after which it starts to drop. When the graphene content is 0.11 vol%, the piezoelectric coefficient d33 increases from 22pC/N of pure PVDF to 39.73pC/N of graphene/PVDF composite, which is an 80.5% increase in d33. In addition, this theory is further extended and applied to the other nanoparticle-enhanced piezoelectric polymer composites. This crystal phase transition model serves to fill up the theoretical gap in piezoelectric enhancement in piezoelectric enhancement of PVDF by graphene.