On the α → β Transition of Carbon-Coated Highly Oriented PVDF Ultrathin Film Induced by Melt Recrystallization

Poly(vinylidene fluoride) (PVDF) is one of the polymers which exhibit pronounced polymorphic crystalline forms, depending on crystallization conditions. Four different crystalline modifications, i.e., α, β, γ, and δ, have been reported so far. Among them, even though the α-form is the most common one, the β-phase is the one that has attracted the widest interest due to its extensive piezo- and pyroelectric applications. During the past few decades, a substantial amount of work has been done in attempts to characterize these crystal modifications and transformations among them. It was well documented that the α-form PVDF can be easily obtained through melt crystallization of the PVDF at atmospheric pressure. Its β-counterpart can, however, only be directly obtained by growth from solution, molecular epitaxy on the surface of potassium bromide, melt crystallization at high pressures, or by applying with a strong electric field. Transformation from its α-phase to β-phase has been achieved by mechanical deformation, while the retransformation could be conducted through melt recrystallization at atmospheric pressure. In the present work, the recrystallization behavior of carbon-coated melt-drawn oriented PVDF ultrathin films at atmospheric pressure was studied by means of transmission electron microscopy and electron diffraction. The results indicate that through vacuum evaporating a thin carbon layer on the surface of highly oriented α-PVDF ultrathin film, not only has its high orientation been preserved after a complete melting and recrystallization process, but an α → β transition of PVDF has also been achieved through melt recrystallization at atmospheric pressure. This technique can be successfully used for preparing highly oriented β-PVDF ultrathin films, especially patterned microstructures of PVDF with its highly oriented polar β-phase and nonoriented nonpolar α-phase.