Volume 111, Issue 3, 15 July 1999
Index of content:
- POLYMERS, BIOPOLYMERS, AND COMPLEX SYSTEMS
111(1999); http://dx.doi.org/10.1063/1.479315View Description Hide Description
Vibrating square well (SW) 2-mer, 4-mer, and 8-mer with average reduced bond lengths of and were studied by discontinuous molecular dynamics (DMD) simulation in the NVE ensemble. Average bond angles for the reduced bond length of 0.4 were constrained to while the longer bond lengths were freely jointed. Vapor–liquid equilibria of the vibrating SW fluids were determined based on DMD simulation by isochoric integration and compared to that of rigid SW chains from Gibbs ensemble Monte Carlo(MC) simulation. The binodals of vibrating chains show a shift to higher temperatures relative to rigid chains, reflecting their less repulsive (more attractive) nature. Vapor pressures of the vibrating chains were computed through isochoric integration with Clausius–Clapeyron consistency to 5% or better. Vapor pressure behavior for each chain model was characterized in terms of critical temperature, critical pressure, and acentric factor. The trend in acentric factor vs. chain length showed that shorter bond lengths gave improved agreement with the experimental trend for n-alkanes. Nevertheless, the trends in acentric factor did not support any molecular model for alkanes which represented methylene segments as individual SW interaction sites. If SW chains are to be applied as models of alkanes, each interaction site must be assigned more than one methylene segment.
111(1999); http://dx.doi.org/10.1063/1.479316View Description Hide Description
A configurational bias Monte Carlo algorithm has been employed to investigate the conformational properties of an off-lattice polymer chain model close to the theta point. In particular, we have extended previous results for the chain’s mean size and shape to longer chains, allowing for a more detailed comparison with field renormalizationgroup theory. Moreover, we present the first data of internal energy and specific heat for this type of off-lattice model, showing the approach to the expected theoretical behavior.
Density functional study of intramolecular ferromagnetic interaction through m-phenylene coupling unit. III. Possibility of high-spin polymer111(1999); http://dx.doi.org/10.1063/1.479317View Description Hide Description
Because it has been well-known that the effective exchange interaction of spins is much larger within a molecule than between molecules, it is very important to design high-spin polymers linking high-spin molecules with each other as a constituent unit to obtain molecule-based ferromagnetic materials with high transition temperature. Experimental efforts to synthesize such polymers extended in one- or two-dimensions have been made in recent years while theoretical treatment of infinite polymers has been behind in the sense that no study of electronic and magnetic structure calculations by ab initio periodic approaches exists until now. We examined the magnetic properties in m-phenylene and related molecules with organic radicals by density functional and molecular orbital methods as monomer units of high-spin polymers in Part I and Part II of this series, since it has already been confirmed experimentally and theoretically that m-phenylene coupling unit leads to the ferromagnetic coupling between spins. In Sec. III, we examine the possibility of ferromagneticpolymers in which organic radicals are arranged thorough m-phenylene bridge periodically in one dimension. For this purpose, ab initio crystal orbital methods based on density functional and Hartree–Fock approximations are employed for m-phenylene polymers with four species of C, CH, N, and radical groups as spin sources. It is shown that these polymers have the high-spin ground states and the density functional approaches can yield reasonable results which are comparable with the experiment. Implications of the calculated results are also discussed in relation to ferromagneticpolymers with different coupling units.
111(1999); http://dx.doi.org/10.1063/1.479318View Description Hide Description
Molecular dynamics simulations are conducted for concentrated solutions of flexible polymers. The results are contrasted with literature dielectricspectroscopy data, in an attempt to elucidate the observed phenomena from a molecular level perspective. A bead-spring model is used and systems with chain sizes up to beads at reduced densities are studied. The dimensions of the chains follow a universal behavior with where is the crossover density demarcating the onset of chain overlapping. All the chains are found to follow random-walk behavior. The global motion of the chains is investigated in terms of the dielectric loss As in dielectricspectroscopy experiments, the motion of the chains induces prominent dielectric relaxation at low frequencies. The shape of broadens with increasing density, and a normal-mode analysis indicates that overlapping of the chains with increasing density progressively renders the distribution of relaxation times more heterogeneous. For denser systems a second, smaller peak appears at the high frequency end of the spectrum. This secondary peak is not identified with segmental motion, since the simulated chains lack components of the segmental dipoles perpendicular to the chain contour. Entanglement effects are investigated calculating the mean squared displacement and the results suggest that the topological constraints of entanglements render at least two different relaxation mechanisms with disparate time scales important. An attempt to explain the shape of the spectra in terms of a phenomenological separation of the motion of chains into a rotational and a stretching mode showed that stretching plays no important role in the relaxation function and the shape of