Su, Haibin and Strachan, Alejandro and Goddard, William A., III (2004) Density functional theory and molecular dynamics studies of the energetics and kinetics of electroactive polymers: PVDF and P(VDF-TrFE). Physical Review B, 70 (6). Art. No. 064101. ISSN 1098-0121 http://resolver.caltech.edu/CaltechAUTHORS:SUHprb04
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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 solid-solid 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.
|Additional Information:||©2004 The American Physical Society. Received 6 January 2004; revised 21 April 2004; published 6 August 2004. The work has been funded by DARPA and ONR (Program Managers Carey Schwartz and Judah Goldwasser). We thank A. Cuitiño for many fruitful discussions. The facilities of MSC used in these calculations were supported by ONR-DURIP, ARO-DURIP, NSF-MRI, and IBM-SUR. In addition the MSC is supported by NSF, NIH, ONR, General Motors, ChevronTexaco, Seiko-Epson, Beckman Institute, Asahi Kasei, and Toray.|
|Subject Keywords:||density functional theory; molecular dynamics method; polymer blends; ferroelectric materials; bonds (chemical); piezoelectricity; molecular force constants; solid-state phase transformations; plastic deformation; shear deformation|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Tony Diaz|
|Deposited On:||03 Mar 2006|
|Last Modified:||26 Dec 2012 08:47|
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