Coarse-grained Description of Polymer Blends as Interacting Soft-Colloidal Particles

We present a theoretical approach which maps polymer blends onto mixtures of soft-colloidal particles. The analytical mesoscale pair distribution functions reproduce well data from united atom molecular dynamics simulations of polyolefin mixtures without fitting parameters. The theory exactly recovers the analytical expressions for density and concentration fluctuation structure factors of soft colloidal mixtures (liquid alloys).Comment: 27 REVTex4 pages, 8 PostScript figures, 1 table accepted for publication in Journal of Chemical Physic

Site-averaging in the integral equation theory of interaction site models of macromolecular fluids: An exact approach

A simple "trick" is proposed, which allows to perform exactly the site-averaging procedure required when developing integral equation theories of interaction site models of macromolecular fluids. It shows that no approximation is involved when the number of Ornstein-Zernike equations coupling the site-site correlation functions is reduced to one. Its potential practical interest for future theoretical developments is illustrated with a rederivation of the so-called molecular closures.Comment: 2 pages, revTeX

Polymer-Mode-Coupling Theory of Finite-Size-Fluctuation Effects in Entangled Solutions, Melts and Gels. I. General Formulation and Predictions

The transport coefficients of dense polymeric fluids are approximately calculated from the microscopic intermolecular forces. The following finite molecular weight effects are discussed within the Polymer-Mode-Coupling theory (PMC) and compared to the corresponding reptation/ tube ideas: constraint release mechanism, spatial inhomogeneity of the entanglement constraints, and tracer polymer shape fluctuations. The entanglement corrections to the single polymer Rouse dynamics are shown to depend on molecular weight via the ratio N/N_e, where the entanglement degree of polymerization, N_e, can be measured from the plateau shear modulus. Two microscopically defined non-universal parameters, an entanglement strength 1/alpha and a length scale ratio, delta= xi_rho/b, where xi_rho and b are the density screening and entanglement length respectively, are shown to determine the reduction of the entanglement effects relative to the reptation- -like asymptotes of PMC theory. Large finite size effects are predicted for reduced degrees of polymerization up to N/N_e\le10^3. Effective power law variations for intermediate N/N_e of the viscosity, eta\sim N^x, and the diffusion constant, D\sim N^{-y}, can be explained with exponents significantly exceeding the asymptotic, reptation-like values, x\ge 3 and y\ge2, respectively. Extensions of the theory to treat tracer dielectric relaxation, and polymer transport in gels and other amorphous systems, are also presented.Comment: Latex, figures and styles files included; Macromolecules, in press (1997

The fluorine-NHC gauche effect: a structural and computational study

Herein, we report the synthesis and X-ray structural analysis of a collection of fluorinated metal N-heterocyclic carbenes (Ag, Au, Pd, Rh, Ir) and their precursor salts. The common structural feature of these species is a flanking fluoroethyl group which is either freely rotating or embedded within a bicyclic framework. Solid state analysis confirmed a gauche conformational preference in all cases with the fluorine adopting a syn clinal arrangement (ϕ[NCCF] ~ 60°) with respect to the triazolium nitrogen at the vicinal position of the NHC. A density functional theory analysis was employed to quantify these effects and evaluate the influence of electronic modulation of the carbenic carbon [(C=N+); neutral carbene (C:); metal-bound carbene (C=M)], on the relative gauche / anti preference, thus highlighting the potential of this conformational phenomenon as a useful molecular design strategy for controlling the topology of organometallic complexes