  We use first principles methods to study static and dynamical mechanical
properties of the ferroelectric polymer Poly(vinylidene fluoride) (PVDF) and
its copolymer with trifluoro ethylene (TrFE). We use density functional theory
[within the generalized gradient approximation (DFT-GGA)] to calculate
structures and energetics for various crystalline phases for PVDF and
P(VDF-TrFE). We find that the lowest energy phase for PVDF is a non-polar
crystal with a combination of trans (T) and gauche (G) bonds; in the case of
the copolymer the role of the extra (bulkier) F atoms is to stabilize T bonds.
This leads to the higher crystallinity and piezoelectricity observed
experimentally. Using the MSXX first principles-based force field (FF) with
molecular dynamics (MD), we find that the energy barrier necessary to nucleate
a kink (gauche pairs separated by trans bonds) in an all-T crystal is much
lower (14.9 kcal/mol) in P(VDF-TrFE) copolymer than in PVDF (24.8 kcal/mol).
This correlates with the observation that the polar phase of the copolymer
exhibits a solid-solid a transition to a non-polar phase under heating while
PVDF directly melts. We also studied the mobility of an interface between a
polar and non-polar phases under uniaxial stress; we find a lower threshold
stress and a higher mobility in the copolymer as compared with PVDF. Finally,
considering plastic deformation under applied shear, we find that the chains
for P(VDF-TrFE) have a very low resistance to sliding, particularly along the
chain direction. The atomistic characterization of these "unit mechanisms"
provides essential input to mesoscopic or macroscopic models of electro-active
polymers.
