Computational Studies of Poly(vinylidene fluoride)
Doctoral thesis, 2014
Poly(vinylidene fluoride) (PVDF) is a versatile material with numerous applications, both potential and realized, in many industrial sectors. The extent of applications, ranging from cable and wire products, to sensors for the monitoring of respiration and heart-rate in medicine, indicates on the level of interest the materials science community has for this material. PVDF has the potential to be used in applications where its piezoelectric characteristics are utilized, but for this to be realized, a specific crystal structure, the polar β-phase, need to be present in the material. Since PVDF is polymorphic and usually crystallizes from melt or from solution into the non-polar α-phase, which is of little use in piezoelectric applications, the induction of the β-phase is an active field of research. With computational methods it is possible to study PVDF on a molecular level to gain better insights into the mechanisms behind the formation of this specific crystal structure. Conformational studies of PVDF, the effect of carbon nanotubes on the conformation as well as the mechanical properties of PVDF, copolymerization with trifluoroethylene and the effect of increased temperatures and pressures have been studied using molecular mechanics/dynamics and first principles methods. It has been found that carbon nanotubes mainly act as nucleating agents for the formation of β-phase PVDF as their effect on the mechanical properties of PVDF is relatively small. Furthermore, β-phase formation can be facilitated with very rapid cooling rates from the melt, hindering the transformation into the thermodynamically stable α-phase and by inclusion of trifluoroethylene units in the PVDF chain. With better insights into the mechanism of β-phase PVDF formation, it would ideally be possible to produce piezoelectric PVDF with better characteristics and possibly also in new and more efficient ways.