Search results for "dynamics."

1985

Viscosity measurements were carried out as a function of pressure and temperature with solutions of 8 wt.-% PVC (Mw ≈ 75 000) in ten thermodynamically good solvents by means of a Searle-type viscometer. A rollingxyhball viscometer was used for the investigation of the pure solvents. In all cases the viscosity increases in a more or less exponential manner when the pressure is raised. The viscosity ratio f1000 = η1000 bar/η1bar can be varied by the choice of the solvent from ca. 2 (tetrahydrofuran) to 3,0 (cyclohexanone) at 40°C. For a constant temperature of 40°C, the volumes of activation for the viscous flow of the solutions, V≠, or f1000 exceed that of the pure solvent, by typically 25%.…

Coupling of the guanosine glycosidic bond conformation and the ribonucleotide cleavage reaction: implications for barnase catalysis.

2007

To examine the possible relationship of guanine-dependent GpA conformations with ribonucleotide cleavage, two potential of mean force (PMF) calculations were performed in aqueous solution. In the first calculation, the guanosine glycosidic (Gχ) angle was used as the reaction coordinate, and computations were performed on two GpA ionic species: protonated (neutral) or deprotonated (negatively charged) guanosine ribose O2 ′. Similar energetic profiles featuring two minima corresponding to the anti and syn Gχ regions were obtained for both ionic forms. For both forms the anti conformation was more stable than the syn, and barriers of ∼4 kcal/mol were obtained for the anti → syn transition. Str…

Scattering function and the dynamics of phase separation in polymer mixtures under shear flow

1988

The phenomenological mean-field theory describing concentration fluctuations and spinodal decomposition of binary mixtures of long flexible macromolecules is generalized to mixtures under steady shear flow. This shear flow leads to a partial orientation and stretching of the coils, as well as to an anisotropic deformation of concentration fluctuations. Generalizing the approach of Onuki and Kawasaki, we obtain the collective scattering function describing these concentration fluctuations in the mixture under shear flow. Both the steady-state situation in the one-phase region and the initial stages of spinodal decomposition for concentrations inside of the spinodal curve are considered.

Self-diffusion in polymer solutions using the bond-fluctuation MC-algorithm

1991

Abstract A lattice Monte Carlo study of the self-diffusion of polymer chains in an athermal solution of equal chains is presented. The examined chain lengths, N (= 20–200), and volume fractions, φ (= 0.025-0.5), cover the range from dilute solution to concentrated solution, respectively. The dynamics show a gradual crossover from Rouse to reptation-like behaviour. Analysing the data according to a scaling theory and taking into account the density dependence of the microscopic length and time-scales, an almost perfect scaling of the self-diffusion coefficient is achieved. The high statistical accuracy of the data (103–104 chains per parameter combination) was obtainable by using a transpute…

A friction factor analysis of the coupling between polymer/solvent self- and mutual-diffusion: polystyrene/toluene

1996

The Bearman statistical mechanical theory, which couples the mutual-diffusion and self-diffusion coefficients via friction factors, has been applied to polystyrene/toluene solutions with polystyrene molecular weights of 18 kDa and 900 kDa. Toluene and polystyrene self-diffusion coefficients, obtained from the literature and measured here, along with polystyrene/toluene binary mutual-diffusion coefficients and thermodynamic data, were employed to independently calculate the three friction coefficients (ξ 12 , ξ 11 , and ξ 22 ) required to describe transport within binary solutions. Results reveal that the frequently used geometric mean approximation (GMA) for relating the friction coefficien…

Self-Diffusion of Small Molecules into Rubbery Polymers: A Lattice Free-Volume Theory

2010

In the framework of the Free Volume Theory, a new equation was derived for the evaluation of self-diffusion coefficients of small molecules in polymers above the mixture glass transition temperature. The derivation of the equation turned out to be straightforward once the equivalence between the free-volume and the unoccupied volume given by Thermodynamic Lattice Theories is assumed. A parameter evaluation scheme is proposed, which is substantially simpler compared to the conventional Vrentas-Duda approach, even without losing generality. The key assumption is discussed and its consistency is verified from a numerical viewpoint. A comparison with experimental solvent self-diffusion coeffici…

Use of stable isotopes to investigate dispersal of waste from fish farm as a function of hydrodynamics.

2006

Stable isotopes were used to examine differential effects of fish farm waste on the water column and sediments. To achieve this objective, we chose 3 marine fish farms located along the coast of Sicily (Mediterranean Sea) as point-source disturbances, and a control area. The hypothesis that carbon and nitrogen isotope composition of particulate (POM) and sedimentary (SOM) organic matter varied with increasing distance (from cages to 1000 m) was tested at 3 levels of hydrodynam- ics: low (mean velocity of current (MVC) ~12 cm s -1 ), intermediate (MVC ~22 cm s -1 ), and high (MVC ~40 cm s -1 ). Different isotopic signals from allochthonous (fish waste) over natural (phytoplankton, terrigenou…

On the calculation of free energy of mixing for aqueous polymer solutions with group-contribution models

2010

Abstract Liquid–liquid phase separation processes are currently used as a route to prepare polymeric porous structures for various applications (membranes and scaffolds for tissue engineering). In membrane and foam fabrication technologies, binary and mainly ternary polymer solutions are used. Membrane morphology is strongly affected by phase equilibria of processing solution. In order to achieve a better control of membrane morphology and to explore a wide quantity of solvents, a predictive tool addressing experimentals would be strongly advisable. In this paper, group contribution models were chosen to test the applicability on a PLLA–dioxane–water ternary polymer solution, whose experime…

Transient birefringence of polymer melts in intermittent shear flow: Model analysis of the non-linear viscoelastic behaviour

1977

A non-linear viscoelastic model has been used to interpret transient flow birefringence in changing shear flow for a polymer melt. It is shown how the new model is consistent with the basic hypothesis of the linear stress-optical law. Stress growth in shear flow and relaxation after different amounts of shearing are compared with the predictions of the non-linear model. A good agreement between experimental data and theoretical predictions is found.

One-Pot Three-Component Solvent-Free Syntheses of n-Alkyl-Bridged N,N,N,N-tetra(2-hydroxybenzyl)diamines and N,N-bis(2-hydroxybenzyl) amines

2010

A simple solvent-free method to prepare four N,N,N’,N’-tetra(2-hydroxy-3,5dimethylbenzyl)diaminoalkanes and four N,N,N’,N’-tetra(2-hydroxy-5-t-butyl-3-methylbenzyl)-diaminoalkanes containing a long n-alkyl-bridge (58 CH2 groups between N-atoms) is described. In addition, preparations of four dihydrochlorides of prepared n-alkyl-bridged N,N,N’,N’-tetra(2-hydroxybenzyl)diamines are described. This method was also tested in the preparation of eight previously reported N,N-bis(2-hydroxybenzyl)amine derivatives.