On the chain length dependence of local correlations in polymer melts and a perturbation theory of symmetric polymer blends

The self-consistent field (SCF) theory of dense polymer liquids assumes that short-range correlations are almost independent of how monomers are connected into polymers. Some limits of this idea are explored in the context of a perturbation theory for mixtures of structurally identical polymer species, A and B, in which the AB pair interaction differs slightly from the AA and BB interaction, and the difference is controlled by a parameter alpha Expanding the free energy to O(\alpha) yields an excess free energy of the form alpha $z(N)\phi_{A}\phi_{B}$, in both lattice and continuum models, where z(N) is a measure of the number of inter-molecular near neighbors of each monomer in a one-component liquid. This quantity decreases slightly with increasing N because the self-concentration of monomers from the same chain is slightly higher for longer chains, creating a deeper correlation hole for longer chains. We analyze the resulting $N$-dependence, and predict that $z(N) = z^{\infty}[1 + \beta \bar{N}^{-1/2}]$, where $\bar{N}$ is an invariant degree of polymerization, and $\beta=(6/\pi)^{3/2}$. This and other predictions are confirmed by comparison to simulations. We also propose a way to estimate the effective interaction parameter appropriate for comparisons of simulation data to SCF theory and to coarse-grained theories of corrections to SCF theory, which is based on an extrapolation of coefficients in this perturbation theory to the limit $N \to \infty$. We show that a renormalized one-loop theory contains a quantitatively correct description of the $N$-dependence of local structure studied here.Comment: submitted to J. Chem. Phy

Chain motion and viscoelasticity in highly entangled solutions of semiflexible rods

Brownian dynamics simulations are used to study highly entangled solutions of semiflexible polymers. Bending fluctuations of semiflexible rods are signficantly affected by entanglement only above a concentration $c^{**}$, where $c^{**}\sim 10^{3}L^{-3}$ for chains of similar length $L$ and persistence length. For $c > c^{**}$, the tube radius $R_{e}$ approaches a dependence $R_{e} \propto c^{-3/5}$, and the linear viscoelastic response develops an elastic contribution that is absent for $c < c^{**}$. Experiments on isotropic solutions of $F$-actin span concentrations near $c^{**}$ for which the predicted asymptotic scaling of the plateau modulus $G \propto c^{7/5}$ is not yet valid.Comment: 4 pages, 5 figures, submitted to PR

A New Species of Ceraclea (Trichopterma:Leptoceridae) Preying on Snails

Ceraclea joannae, new species, feeds on the freshwater snail Somatogyrus virginicus Walker (Hydrobiidae). Our study is the first report of prey other than sponges for a Ceraclea species and the first report of snail predation by Trichoptera larvae in North America. Ceraclea joannae larvae and adults resemble those of C. diluta (Hagen); however, the larva of the new species has a dark head, sickle-shaped mandibles, and a dark pronotum except for a pair of unique, conspicuous, oblique, white bands; the male differs in the slightly longer superior appendages, more nearly straight ventral margins of tergum X, slightly stouter inferior appendages, and the more-tapered apex and less-pronounced ventral notch of the phallus. This new species of Ceraclea is known from only 3.2 km of the Little River (Montgomery County, North Carolina, USA), downstream of the developing town of Asheboro. Because of its rarity and limited distribution, Ceraclea joannae may be highly vulnerable to changes in water or habitat quality

Dispersed fluorsecence spectroscopy of jet-cooled AgAu and Pt?

Journal ArticleDispersed fluorescence spectroscopy has been used to study jet-cooled AgAu and Pt2 . Fluorescence resulting from the excitation of five bands of the A?X 1?+ system of AgAu was dispersed, and 51 measured ground state vibrational levels were fit to provide ground state vibrational constants of ?"e=198.2260.11 cm21 and ?"exe"=0.512?0.002 cm-1. A Franck?Condon calculation was performed using the experimental values of the ground and excited state vibrational frequencies and anharmonicities, providing an estimate of the change in bond length upon excitation of the A?X system of ?re=0.214?0.005 ?. Fluorescence resulting from four different excitations of Pt2 was dispersed, providing vibrational constants for the ground and two low-lying excited states. Ground state vibrational constants of ?e=222.3 cm-1 and ?exe=0.62 cm21 were obtained, based on the analysis of 16 measured ground state vibrational levels. In addition, a low-lying excited state was located at T0=2877 cm-1, with ?e=197 cm-1. This state perturbed the ground state, from which it was deduced that it has the same symmetry as the ground state. A comparison to theoretical calculations suggests that both states have 0g 1 symmetry. Finally, a metastable state of Pt2 lying at an unknown energy was determined to have ?e=211 cm-1, ?exe=0.4 cm-1

Dispersed fluorsecence spectroscopy of AINi, NiAu, and PtCu

Journal ArticleDispersed fluorescence studies of AlNi, NiAu, and PtCu have been performed, providing spectroscopic information about the ground and low-lying excited electronic states. Vibrational frequencies are reported for the ground X 2?5/2 state of all three molecules. In the case of AlNi, fluorescence to all five of the states originating from the 3dNi 9 3sAl2?2 manifold has been observed. For both NiAu and PtCu, fluorescence to two low-lying excited states in addition to the ground state was observed. Relative energies, vibrational constants, and, when possible, V values of these states are reported. Comparisons of the measured electronic states to the predictions of a ligand-field plus spin?orbit model are also provided, along with a comparison of the electronic structure of PtCu to that of PtH

Renormalized One-loop Theory of Correlations in Disordered Diblock Copolymers

A renormalized one-loop theory (ROL) is used to calculate corrections to the random phase approximation (RPA) for the structure factor \Sc(q) in disordered diblock copolymer melts. Predictions are given for the peak intensity $S(q^{\star})$, peak position $q^{\star}$, and single-chain statistics for symmetric and asymmetric copolymers as functions of $\chi N$, where $\chi$ is the Flory-Huggins interaction parameter and $N$ is the degree of polymerization. The ROL and Fredrickson-Helfand (FH) theories are found to yield asymptotically equivalent results for the dependence of the peak intensity $S(q^{\star})$ upon $\chi N$ for symmetric diblock copolymers in the limit of strong scattering, or large $\chi N$, but yield qualitatively different predictions for symmetric copolymers far from the ODT and for asymmetric copolymers. The ROL theory predicts a suppression of $S(q^\star)$ and a decrease of $q^{\star}$ for large values of $\chi N$, relative to the RPA predictions, but an enhancement of $S(q^{\star})$ and an increase in $q^{\star}$ for small $\chi N$ ($\chi N < 5$). By separating intra- and inter-molecular contributions to $S^{-1}(q)$, we show that the decrease in $q^{\star}$ near the ODT is caused by the $q$ dependence of the intermolecular direct correlation function, and is unrelated to any change in single-chain statistics, but that the increase in $q^{\star}$ at small values of $\chi N$ is a result of non-Gaussian single-chain statistics.Comment: 16 pages, 13 figures, submitted to J. Chem. Phy