Volume 111, Issue 19, 15 November 1999
Index of content:
- POLYMERS, BIOPOLYMERS, AND COMPLEX SYSTEMS
111(1999); http://dx.doi.org/10.1063/1.480248View Description Hide Description
We present molecular dynamics simulations of the flow of macromolecules from a bulk melt into a slit of nanometer dimension with strongly attracting walls. Such flow is central to the formation of polymer-layered silicate nanocomposites by direct melt intercalation. In this process, polymer moleculesflow from a melt into the galleries between the sheets that compose a mica-type layered silicate. We present a systematic study of the effects of polymer molecular weight and polymer-surface interactions on the flowdynamics.
Comparisons between integral equation theory and molecular dynamics simulations for realistic models of polyethylene liquids111(1999); http://dx.doi.org/10.1063/1.480335View Description Hide Description
Molecular dynamics (MD) simulations were performed on dense liquids of polyethylene chains of 24 and 66 united atom units. A series of models was studied ranging in atomistic detail from coarse-grained, freely-jointed, tangent site chains to realistic, overlapping site models subjected to bond angle restrictions and torsional potentials. These same models were also treated with the self-consistent, polymer reference interaction site model (PRISM) theory. The intramolecular and total structure factors, as well as, the intermolecular radial distribution functions and direct correlation functions were obtained from theory and simulation. Angular correlation functions were also obtained from the MD simulations. Comparisons between theory and simulation reveal that PRISM theory works well for computing the intermolecular structure of coarse-grained chain models, but systematically underpredicts the extent of intermolecular packing, and overpredicts the compressibility, as more realistic details are introduced into the model. We found that the PRISM theory could be considerably improved by adding a tail function to beyond the effective hard core diameter. The range of this tail function was determined by requiring the theory to yield the correct compressibility. The intermolecular radial distribution functions from this modified PRISM theory were in excellent agreement with ’s obtained from the simulations.
111(1999); http://dx.doi.org/10.1063/1.480249View Description Hide Description
Molecular dynamics simulations are performed on model fluids of linear decanes in order to investigate the structural and dynamical properties of chainlike molecules confined in a carbon nanotube. The density profiles, chain end-to-end distances, and order parameters are calculated to show that the confined liquids are strongly inhomogeneous and anisotropic. Anomalous positive peaks in the velocity correlation function perpendicular to the tube direction are observed and explained. Diffusion coefficients along the tube direction are calculated. We find that the diffusion coefficients in the inner region of the tube are greater than in the contact region, and that the diffusion coefficients averaged over the entire tube are much higher than in bulk liquids at the same temperatures and densities.
Experimental simulation of macromolecules in trehalose aqueous solutions: A photon correlation spectroscopy study111(1999); http://dx.doi.org/10.1063/1.480250View Description Hide Description
The protective effect of trehalose on biological membranes against freezing or dehydration has been the subject of many studies aimed to understand the reasons why some lower organisms, under stress conditions, synthesize trehalose. In this work we report the results of a study on Poly(Ethylene Oxide)/trehalose/water mixtures performed by PhotonCorrelation Spectroscopy. The chemical structure of the polymer, simpler than that of proteins and its helical conformation in water, constitute a useful starting point for understanding the more complex protein/trehalose/water interactions. In order to distinguish the different dynamics, trehalose and PEO have been studied separately in water, at different concentration and temperature values; then the ternary PEO/trehalose/water system has been investigated at different sugar amounts. The obtained findings support the “water-replacement” hypothesis, indicating that a direct polymer–trehalose interaction occurs. Furthermore, trehalose is shown to affect the swelling properties of the polymer with temperature, stabilizing its conformation.
Polarizability tensor theory based upon a Bayley–Nielsen–Schellman-type model Hamiltonian: Circular dichroism calculations of polypeptides111(1999); http://dx.doi.org/10.1063/1.480251View Description Hide Description
In our previous article based upon the usual Fano–DeVoe model Hamiltonian, the partial polymerpolarizabilitytensorequation for describing visible and ultraviolet (VIS/UV) absorption and circular dichroism (CD) spectra was represented in terms of transition moments and a partial polymerGreen’s functionmatrix equation, which is constructed from partial monomer Green’s matrix elements and inter(sub)molecular interaction energies. Here, on the basis of the extended Fano–DeVoe model Hamiltonian similar to the Bayley–Nielsen–Schellman secular Hamiltonian matrix, we represent a partial polymerpolarizabilitytensormatrix equation in terms of transition moments and a partial polymerGreen’s functionmatrix equation, which is constructed from partial monomer Green’s matrix elements, inter(sub)molecular interaction energies, and inter(sub)molecular electrostatic interaction energies. The last ones are newly taken into account. Computing the amide monomer spectroscopic parameters by the CNDO/S method and substituting them into the polymerpolarizabilitytensormatrix equation derived, we have successfully carried out the CD band shape calculations on polypeptides having α-helix and β-sheet structures and on a tripeptide, N-acetyl-Pro-Gly-Leu-OH having a type II β-turn conformation.