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We use first principles methods to study static and dynamical mechanicalproperties of the ferroelectric polymer Poly(vinylidene fluoride) (PVDF) andits copolymer with trifluoro ethylene (TrFE). We use density functional theory[within the generalized gradient approximation (DFT-GGA)] to calculatestructures and energetics for various crystalline phases for PVDF andP(VDF-TrFE). We find that the lowest energy phase for PVDF is a non-polarcrystal with a combination of trans (T) and gauche (G) bonds; in the case ofthe copolymer the role of the extra (bulkier) F atoms is to stabilize T bonds.This leads to the higher crystallinity and piezoelectricity observedexperimentally. Using the MSXX first principles-based force field (FF) withmolecular dynamics (MD), we find that the energy barrier necessary to nucleatea kink (gauche pairs separated by trans bonds) in an all-T crystal is muchlower (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 copolymerexhibits a solid-solid a transition to a non-polar phase under heating whilePVDF directly melts. We also studied the mobility of an interface between apolar and non-polar phases under uniaxial stress; we find a lower thresholdstress and a higher mobility in the copolymer as compared with PVDF. Finally,considering plastic deformation under applied shear, we find that the chainsfor P(VDF-TrFE) have a very low resistance to sliding, particularly along thechain direction. The atomistic characterization of these "unit mechanisms"provides essential input to mesoscopic or macroscopic models of electro-activepolymers.Comment: 15 pages 9 figures Electro-active polyme

Density functional theory and molecular dynamics studies of the energetics and kinetics of electroactive polymers: PVDF and P(VDF-TrFE)