Effects of polymorphism and crystallite size on dipole reorientation in poly(vinylidene fluoride) and its random copolymers

Poly(vinylidene fluoride) (PVDF) and poly(VDF-co-hexafluoropropylene) [P(VDF-HFP)] films having different polymorphisms and crystallite sizes but a similar crystal orientation (i.e., c-axes parallel to the film surface) were prepared by different film processing methods. Effects of polymorphism and crystallite size on the dipole reorientation behavior and electric energy storage/release were studied by electric displacement-electric field (D-E) loop measurements. Experimental results suggested that coupling interactions among ferroelectric domains, which could be adjusted by different polymorphisms and/or crystallite sizes, determined dipole reorientation/switching behaviors. Note that the ferroelectric domain coupling is realized via induced compensation polarizations from the media (either amorphous or crystalline PVDF) between aligned ferroelectric domains. A high β rather than α/δ content and a large crystallite size facilitated the coupling interactions among ferroelectric domains, and thus dipoles in highly coupled ferroelectric domains could be easily polarized, resulting in a high dielectric constant and a high stored energy density. However, strong coupling interactions impeded an easy dipole reversal to the so-called antiferroelectric-like (or random) state and thus reduced the discharged electric energy due to a high remanent polarization. Instead, the film with a high β content and a small crystallite size showed the highest discharged electric energy density, suggesting that the ferroelectric domain coupling could be weakened by confining them in nanoscale crystallites. These findings provide us useful guidance to achieve optimal crystalline morphology in PVDF copolymer films for high electric energy storage applications.