David B. Adolf

Statics and dynamics of model dendrimers as studied by molecular dynamics simulations

Molecular dynamics simulations were performed for a series of AB2 dendrimer models, in explicit-solvent solutions where the ratio Rg/L (Rg is the radius of gyration and L the size of the simulation box) is kept between 0.15⩽Rg/L⩽0.2. Results on static properties (size, shape, density profiles) are in good agreement with recent theoretical and experimental studies. Dynamic properties are systematically investigated on the local and entire molecule length scale. The dynamic characteristics of the examined models capture the qualitative behavior observed experimentally in dendrimer molecules. The systematic and comparative nature of this study affords detailed insight into the origin and the relative contribution of different relaxational mechanisms in the observed dynamic spectra.

A comparison of Ewald summation techniques for planar surfaces

Ewald-like summation methods to compute the electrostatic Coulomb potential and the forces on charged particles in a two-dimensional periodic system with nonperiodic extension to the third dimension are investigated. The techniques by Hautman and Klein, Mol. Phys. 75 (1992) 379 (HK), Heyes, Barber, and Clarke, J. Chem. Soc. Faraday Trans. II 73 (1977) 1485 (HBC), and Nijboer and de Wette, Physica A 125 (1984) 275 (NdW) are compared in respect to their numerical accuracy and efficiency. The convergence behaviour of the methods is analysed for different magnitudes of the nonperiodic z-separation between the charged particles up to twice the length of the square simulation cell. The HK method is best suited for small z but becomes inefficient for large particle separations in the nonperiodic direction. HBC is not sensitive on the out-of-plane separation but is very slow. NdW is only suited for large out-of-plane distances because it fails completely if the z-separation of any two particles becomes zero. Ready-to-implement sets of equations for the three algorithms are presented in the appendices.

Computer simulations of hyperbranched polymers: The influence of the Wiener index on the intrinsic viscosity and radius of gyration

The influence of the Wiener index on solution properties of trifunctional hyperbranched polymers has been investigated using Brownian dynamics simulations with excluded volume and hydrodynamic interactions. A range of degrees of polymerization (N) and degrees of branching (DB) were used. For each DB and N, several molecules with different Wiener indices (W) were simulated, where W depends on the arrangement of branch points. The intrinsic viscosity and the radius of gyration (Rg) of HPs were both observed to scale with W at a constant N via a power law relationship, as found in the literature. Through their relationships to W, an expression relating intrinsic viscosity to Rg was obtained. This relationship is found to fall centrally between the predictions of Flory and Fox for linear polymers and that of Zimm and Kilb for branched polymers. Molecular shape in solution is also found to depend on W and N, as observed through the W dependence of the ratio of Rg to the hydrodynamic radius, Rh.

Brownian dynamics simulation of linear polymers under elongational flow: Bead–rod model with hydrodynamic interactions

Computer simulations of perfectly branched dendrimers up to the sixth generation have been performed under the influence of uniaxial elongational flow for the first time for a model with explicit dendritic topology. The Brownian dynamics simulation technique has been applied to a freely jointed bead−rod model with excluded volume both with and without hydrodynamic interactions. The dependence of conformational properties and the intrinsic elongational viscosity on the flow rate were obtained. The coil−stretch transition was observed for dendrimers of all generations with it being less pronounced than the same type of transition observed for a linear polymer chain. Hydrodynamic interactions shift the onset of this transition to higher elongational rates. The transition is observed to occur in two stages as it was for a linear polymer. The dendrimer first orients at low flow rate as a whole along the flow axis without significant deformation and local orientation. Increasing flow rate leads to local orienta...

Brownian dynamics simulations of linear polymers under shear flow

Brownian dynamics simulations of a polymer chain described by three different models under the influence of a shear flow have been performed. Model A is a freely jointed Kramers chain consisting of beads connected by rigid rods. Model B is a freely jointed chain consisting of finitely extensible nonlinear elastic (FENE) springs. Excluded volume and hydrodynamic interactions are not taken into account in either of these two models. Model C is a chain with rigid bonds, valence, and torsional angle potentials, excluded volume and hydrodynamic interactions. Asymptotic dependencies [η]∼γ−1/3 and [η]∼γ−2/3 for the intrinsic viscosity [η] at large shear rates γ for models A and B, correspondingly, have been obtained. Asymptotic dependencies for the first normal stress coefficient Ψ1∼γ−4/3 do not depend on the particular choice of model. At intermediate shear rates [η]∼γ−1/2 is followed for all models. Scaling dependencies of rheological properties on molecular weight have been studied. Results of the simula...

Effects of density on the local dynamics and conformational statistics of polyethylene: A molecular dynamics study

Molecular dynamics simulations of united atom (UA) linear polyethylene were performed as a function of density, for systems of two well-separated chain lengths (N=20, N=100). The role of the exact shape of the torsional angle potential is investigated by invoking two commonly cited alkane torsional potentials [Ryckaert–Bellemans (R&B), Steele]. The increase in second-neighbor torsional angle coupling with increasing density and decreasing conformational transition rates is presented quantitatively for the first time. The simulated local orientational dynamics were also studied by means of geometric autocorrelation functions (ACFs). These ACFs were analyzed by a method that emphasizes the distribution of relaxational processes, thereby providing specific information on the relevant spectral characteristics of each process. For all densities and chain lengths studied, two peaks are observed that are well separated in time. Only for the longer chains at the highest density studied does a third intermediate p...

Intrinsic viscosity of dendrimers via equilibrium molecular dynamics

Equilibrium molecular dynamics have been used to simulate systems of trifunctional dendrimers in an explicit Lennard-Jones solvent. Solution and solvent viscosities, calculated using the Green–Kubo relation, have been used for the first time to determine the intrinsic viscosity of dendrimers of generations three to seven inclusive. Results reveal a peak in the intrinsic viscosity against generation at generation five, which is in good agreement with experiment and with previous computational efforts using stochastic simulation techniques. A second method employing a variation of the Green–Kubo formula is used to yield intrinsic viscosities directly. This method produces results in quantitative agreement with the traditional Green–Kubo approach. In contrast to the findings above, Flory–Fox intrinsic viscosity estimates using the simulated gyration radii reveal an intrinsic viscosity that steadily decreases from values at early generations.

An investigation into the local segmental dynamics of polyethylene: An isothermal'isobaric molecular dynamics study

Molecular dynamics simulations of unentangled linear polyethylene melts have been performed for systems composed of 10 chains of 100 united atoms over a pressure range of 1 to 5000 bar and a temperature range of 375 to 475 K. Transition rates, activation volumes, and activation energies are in good agreement with values from similar simulations quoted in literature for systems well above T g . Second-neighbor torsional angle coupling is observed to increase with increasing pressure and decreasing temperature. The lifetime of this coupling between conformational events is presented for the first time. Geometric autocorrelation functions are analyzed in terms of their distribution of relaxation times and reveal a process on the time scale of a few picoseconds and another on the time scale of a few nanoseconds. An intermediate process develops between these two time scales at high pressure and low temperature.

Slow modes in local polymer dynamics

Molecular dynamics simulations of united atom nonentangled linear polyethylene models were utilized in order to systematically examine local orientational dynamics. In agreement with recent experiments and theoretical predictions, slow relaxation processes associated with motions of length scale of the order of chain dimensions are identified and analyzed with a method that allowed a model-free determination of their relative contribution to local orientational relaxation. Factors of intra- and intermolecular nature affecting their characteristics are discussed as well.

Pressure and temperature dependence of the melt segmental dynamics of cis-1,4-polyisoprene via time resolved optical spectroscopy

The local segmental dynamics of melt anthracene-labeled cis-1,4-polyisoprene have been studied as a function of temperature (318–333 K) and pressure (0.1–140 MPa). Activation volumes for the motions range from 24 to 39 cm3 mol−1 and are observed to be temperature dependent, decreasing with increasing temperature. Activation energies at constant pressure range from 32 to 75 kJ mol−1 and are significantly pressure dependent, increasing with increasing pressure. These activation energies are decomposed at a series of pressures and temperatures into a term arising from the energy required to execute the motions at constant volume (i.e., a constant volume activation energy) and a term describing the energy required to create the needed free volume for the observed motions. Values of the constant volume activation energy for the local motions are compared where nearly the same density is reached either via the combination of a high pressure and high temperature or the combination of a lower pressure and lower t...