Density functional theory and molecular dynamics studies of the energetics and kinetics of electroactive polymers : PVDF and P(VDF-TrFE)
Goddard III, William A.
Date of Issue2004
School of Materials Science and Engineering
We used 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 structure and energetics for various crystalline phases for PVDF and P(VDF-TrFE). We find that the lowest energy phase for PVDF is a nonpolar 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 solidsolid transition to a nonpolar phase under heating while PVDF directly melts. We also studied the mobility of an interface between polar and nonpolar 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.
Physical review B
© 2004 American Physical Society. This paper was published in Physical Review B and is made available as an electronic reprint (preprint) with permission of American Physical Society. The paper can be found at: [DOI: http://dx.doi.org/10.1103/PhysRevB.70.064101]. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper is prohibited and is subject to penalties under law.