Hi,
I had a question on the d33 piezoelectric coefficient of Polyvinylidene fluoride. The d33 coefficient for PVDF is negative. This means, in direct effect, applying a positive stress results in a decrease in polarization. We can understand this physically, as separation between polymer chains leading to decrease in dipoles per unit volume. Negative d33 also means that applying electric field in the direction of polarization leads to decrease in thickness. I have been trying to figure out the physical interpretation of this in terms of how the dipoles interact with the electric field leading to the decrease in the thickness. Intuitively it seems to me that the thickness should increase due to stretching of the C-F dipole.
I was hoping someone can provide an insight into this or direct me towards a reference.