Mechanical and ferroelectric properties of orthorhombic flourinated polyethylene

Fluorinated polyethylene (PE) is a class of polymers that has potential application for ferroelectric-based sensing that is mechanically pliable. By replacing either one or both hydrogens with fluorine on every other carbon on the PE chain one attains polyvinyl fluoride (PVF) or polyvinylidene fluoride (PVDF). The inter-strain cohesion in the polymer crystals is set primarily by the van der Waals (vdW) interaction, for which the vdW density functionals (vdW-DF) method provides a computationally efficient and robust description. In the present work we predict a candidate for the thermodynamically stable ground state PVF crystal, and we compare the structure, cohesion and elastic response for different orthorhombic PVDF and PVF crystal forms [1]. We also predict and compare the ferroelectric properties using the modern theory of polarization. Furthermore, we study the generalized stacking fault landscapes associated with slip to identify energy barriers for sliding dislocations [2]. Finally, we compute spatially resolved mappings of the exchange and correlation contributions to the crystal cohesion to discuss the nature of those barriers as well as of the shear resistance.