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 i...