Density Functional Theory and Molecular Dynamics Studies on Energetics and Kinetics for Electro-Active Polymers: PVDF and P(VDF-TrFE)

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.Comment: 15 pages 9 figures Electro-active polyme

Free energy and surface tension of arbitrarily large Mackay icosahedral clusters

We present a model for predicting the free energy of arbitrarily large Mackay icosahedral clusters. van der Waals clusters are experimentally observed to be particularly stable at magic numbers corresponding to these structures. Explicit calculations on the vibrational states were used to determine the spectrum of fundamental frequencies for smaller (~561 atoms). Combining these predictions with correlations for the moment of inertia and for the minimum potential energy of large clusters leads to free energies of arbitrary large clusters. The free energies are used to predict the chemical potential and surface tension as a function of size and temperature. This connects macroscopic properties to the microscopic atomic parameters

The hindered rotor density-of-states interpolation function

We construct an approximation to the partition function for hindered rotors based entirely on their asymptotic behavior and no fitting parameters. The approximant is shown to be quite accurate in all temperature ranges. Explicit auxiliary functions are derived for the Helmholtz free energy, internal energy, heat capacity, and entropy. We apply this function to estimating the heat capacity and unimolecular dissociation rate for ethane

Thermodynamic properties and homogeneous nucleation rates for surface-melted physical clusters

We predict the free energy of van der Waals clusters (Fn) in the surface-melted temperature regime. These free energies are used to predict the bulk chemical potential, surface tension, Tolman length, and vapor pressure of noble gas crystals. Together, these estimates allow us to make definitive tests of the capillarity approximation in classical homogeneous nucleation theory. We find that the capillarity approximation underestimates the nucleation rate by thirty orders of magnitude for argon. The best available experiments are consistent with our calculation of nucleation rate as a function of temperature and pressure. We suggest experimental conditions appropriate for determining quantitative nucleation rates which would be invaluable in guiding further development of the theory. To make the predictions of Fn, we develop the Shellwise Lattice Search (SLS) algorithm to identify isomer fragments and the Linear Group Contribution (LGC) method to estimate the energy of isomers composed of those fragments. Together, SLS/LGC approximates the distribution of isomers which contribute to the configurational partition function (for up to 147-atom clusters). Estimates of the remaining free energy contributions come from a previous paper in this series

Electrochemical Performance and Structures of Chromium and Molybdenum-Doped ε-Li_xVOPO₄ Predicted as Promising Cathodes for Next Generation Lithium-Ion Batteries

We report here the predicted structural and electrochemical characteristics of ε-Li_xVOPO₄ doped with 25% Cr or Mo using density functional theory (DFT) calculations. We predict the charging potentials as a function of lithiation and the DFT energetics for various phases of Li_xVOPO₄ from x = 0 to 2.5. We further highlight the electron localization function (ELF) and magnetic spin distributions over the lithiation cycle. For Cr–Li_xVOPO₄, we find an intermediate phase at x = 1.5, and for Mo–Li_xVOPO₄, we find two intermediate phases at x = 0.5 and 1.5. We predict a 50% increase in lithium capacity for both doped and undoped systems with reasonable voltaic behavior and additionally find that the spins on undoped and Cr-doped Li_xVOPO₄ stay ferromagnetic throughout the entire lithiation cycle. Overall, we predict an increase in the electrochemical and structural capabilities with Cr and Mo dopants, suggesting Cr and Mo-doped ε-Li_xVOPO₄ as potentially promising cathodes for next generation lithium-ion batteries

Theoretical studies of a hydrogen abstraction tool for nanotechnology

In the design of a nanoscale, site-specific hydrogen abstraction tool, the authors suggest the use of an alkynyl radical tip. Using ab initio quantum-chemistry techniques including electron correlation they model the abstraction of hydrogen from dihydrogen, methane, acetylene, benzene and isobutane by the acetylene radical. By conservative estimates, the abstraction barrier is small (less than 7.7 kcal mol^-1) in all cases except for acetylene and zero in the case of isobutane. Thermal vibrations at room temperature should be sufficient to supply the small activation energy. Several methods of creating the radical in a controlled vacuum setting should be feasible. The authors show how nanofabrication processes can be accurately and inexpensively designed in a computational framework

Synthesis of single-component metallic glasses by thermal spray of nanodroplets on amorphous substrates

We show that single component metallic glasses can be synthesized by thermal spray coating of nanodroplets onto an amorphous substrate. We demonstrate this using molecular dynamics simulations of nanodroplets up to 30 nm that the spreading of the nanodroplets during impact on a substrate leads to sufficiently rapid cooling (10^(12)–10^(13) K/s) sustained by the large temperature gradients between the thinned nanodroplets and the bulk substrate. However, even under these conditions, in order to ensure that the glass transition outruns crystal nucleation, it is essential that the substrate be amorphous (eliminating sites for heterogeneous nucleation of crystallization)

Numerical study of resistivity of model disordered three-dimensional metals

We calculate the zero-temperature resistivity of model 3-dimensional disordered metals described by tight-binding Hamiltonians. Two different mechanisms of disorder are considered: diagonal and off-diagonal. The non-equilibrium Green function formalism provides a Landauer-type formula for the conductance of arbitrary mesoscopic systems. We use this formula to calculate the resistance of finite-size disordered samples of different lengths. The resistance averaged over disorder configurations is linear in sample length and resistivity is found from the coefficient of proportionality. Two structures are considered: (1) a simple cubic lattice with one s-orbital per site, (2) a simple cubic lattice with two d-orbitals. For small values of the disorder strength, our results agree with those obtained from the Boltzmann equation. Large off-diagonal disorder causes the resistivity to saturate, whereas increasing diagonal disorder causes the resistivity to increase faster than the Boltzmann result. The crossover toward localization starts when the Boltzmann mean free path relative to the lattice constant has a value between 0.5 and 2.0 and is strongly model dependent.Comment: 4 pages, 5 figure