Simulations of the molecular structure of PVDF above the melting point and the effect of single wall carbon nanotubes on the mechanical properties of PVDF

Poly(vinylidene fluoride) (PVDF) exhibits properties that allows the material to be used in such diverse areas as wire and cable products, electronic devices, membranes, exterior coatings for constructions and in the chemical, pharmaceutical and food industry as fluid-handling equipment. It is the inherent molecular structure of PVDF, where every second backbone carbon is bonded to two fluorine atoms, that gives the polymer several unique properties, among them piezo- and pyroelectricity. However, to obtain a material with a high piezoelectric activity, the amount of the crystal phase with the largest polarization, the β crystal phase, has to be large. Possibly, this could be achieved by the addition of different types of additives, carbon nanotubes (CNTs) being one of them. In this study, molecular modeling of PVDF and PVDF-single wall carbon nanotubes (SWCNTs) composites has been performed. Also, the molecular structure of PVDF around and above the melting point has been investigated using both computational and experimental techniques. The results obtained from first principles, molecular mechanics (MM) and molecular dynamics (MD) studies indicate that PVDF molecules have a larger affinity to be in the all-trans structure similar to the polar β-phase at temperatures above the melting point. However, when melt or solution cast the main PVDF crystal phase obtained is the non-polar α-phase. A prevention of this phase transformation from β to α during crystallization could yield a material with a strong piezoelectric effect. Furthermore, MM and MD studies of the mechanical properties of PVDF and PVDF-SWCNT nanocomposites showed that the reinforcing effect of SWCNTs on PVDF is dependent on the alignment of the SWCNTs. When aligned in the direction of applied stress, an increase of the Young’s modulus of about 1 GPa could be observed. A simulation of a pullout of a SWCNT from a PVDF matrix revealed that the interfacial shear stress is in the same range as other polymer-SWCNT systems and that this interface possibly can be strengthened by functionalization of the pristine carbon nanotubes