Multiscale Modeling on Functional Polymer Materials

​Principal Investigator: A/P Su Haibin

Poly(vinylidene fluoride) (PVDF) and its copolymers with trifluoro ethylene (TrFE) exhibit excellent electromechanical properties such as ferroelectricity, piezoelectricity, pyroelectricity, and nonlinear optical properties. We use first principles methods to study static and dynamical mechanical properties of this family of polymers. 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 large scale molecular dynamics simulations, 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/mold 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 have 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. The atomistic characterization of these “unit mechanisms” provides essential inputs to mesoscopic or macroscopic models of electro-active polymers. This is one typical paradigm of multiscale simulation technique.