Local viscosity effects

Studies on local viscosity effects can be carried out using a free probe molecule, such as naphthacene, in polymer solution. Thus poly(styrene) in benzene solution has been studied with this probe at a concentration of 10 mole S,. Two stepwise increases In local viscosity at polymer concentrations of 20 to 30% and 60 to 70% were taken as indications of internal structure in the polymer solution at these concentrations (4). Similar studies were carried out on melts of n-paraffin homologues and poly(ethylene) at 150°C. The local viscosity increased with increasing molecular weight in the low molecular-weight range, but reached a plateau at around MW 2000. 

The maximum concentrations which can be injected without peak broadening and change in elution volume are listed for various injection volumes and molecular weights in Table 4.1(d). These values vary with column characteristics and are not transferable, but they illustrate how the column loading must be substantially reduced as the molecular mass of the sample increases. It arises from the reduction in pores within the gel which separate higher-molecular-mass species, and also because of localized viscosity effects. 

By covalently attaching reactive groups to a polyelectrolyte main chain the uncertainty as to the location of the associated reactive groups can be eliminated. The location at which the reactive groups experience the macromolecular environment critically controls the reaction rate. If a reactive group is covalently bonded to a macromolecular surface, its reactivity would be markedly influenced by interfacial effects at the boundary between the polymer skeleton and the water phase. Those effects may vary with such factors as local electrostatic potential, local polarity, local hydrophobicity, and local viscosity. The values of these local parameters should be different from those in the bulk phase. 

Two predictions of the LADM for the effective viscosity are shown In Table II. The first was made by using the Enskog hard-sphere theory for the calculation of the local viscosities. It agrees qualitatively with the simulation result In that It predicts a large decrease of the effective viscosity as a result of the density structure. For the second prediction the local... 

R. Hayashi, S. Tazuke, and C. W. Frank, Twisted intramolecular charge-transfer phenomenon as a fluorescence probe of microenvironment. Effect of polymer concentration on local viscosity and microscopic polarity around a polymer chain of poly(methyl methacrylate), Macromolecules 20, 983 (1987). 

The description of the chain dynamics in terms of the Rouse model is not only limited by local stiffness effects but also by local dissipative relaxation processes like jumps over the barrier in the rotational potential. Thus, in order to extend the range of description, a combination of the modified Rouse model with a simple description of the rotational jump processes is asked for. Allegra et al. [213,214] introduced an internal viscosity as a force which arises due to a transient departure from configurational equilibrium, that relaxes by reorientational jumps. Thereby, the rotational relaxation processes are described by one single relaxation rate Tj. From an expression for the difference in free energy due to small excursions from equilibrium an explicit expression for the internal viscosity force in terms of a memory function is derived. The internal viscosity force acting on the k-th backbone atom becomes ... 

RoLe has to be calculated for a chain with one end fixed to a surface). Fig-ure 6.15 displays a fit of the measured spectra at both temperatures with the complete dynamic structure factor where the Rouse relaxation rate was taken from an earlier experiment. Fit parameters were the surface tension and the effective local viscosity of the short labelled PEP segments. The data are well... 

The above conclusions are relevant in comparing gel electrophoresis and electrophoresis in free solution. The polymer strands of the gel network are necessary to produce a selective sieving effect, but they increase /. This, by itself, has no adverse effect on efficiency N, but by reducing mobility (Eq. 8.19), it reduces separation speed as expressed by Eq. 8.36. This loss of speed can be offset by increasing the temperature, thus decreasing the local viscosity of the solution between the polymer strands and correspondingly decreasing /. 

Three groups of phenomena affect the frequency-dependence of ultrasonic wave propagation classical processes, relaxation, and scattering, of which scattering is likely to dominate in foodstuffs due to their particulate nature. The two classical thermal processes are radiation and conduction of heat away from regions of the material, which are locally compressed due to the passage of a wave they can lead to attenuation but the effect is negligible in liquid materials (Herzfield and Litovitz, 1959 Bhatia, 1967). The third classical process is due to shear and bulk viscosity effects. Attenuation in water approximates to a dependence on the square of the frequency and because of this it is common to express the attenuation in more complex liquids as a()/o or a(f)jf2 in order to detect, or differentiate from, water-like properties. 

Organic photochemical reactions conducted in micellar solutions are reviewed from the standpoint of systematizing and correlating published results. Five common effects are found to distinguish and characterize micellar photochemistry relative to conventional solution photochemistry super cage effects, local concentration effects, viscosity effects, polarity effects, and electrostatic effects. These effects can contribute to the occurence of enhanced selectivity and efficiency of photoreactions relative to those in conventional homogeneous solution. 

The rotational friction coefficient of a spherical molecule in solution is calculated applying the Navier-Stokes equation for a continuum solvent with a position-dependent viscosity as a model of "microscopic viscosity." The rotational friction coefficient decreases with decreasing surface viscosity. The results are compared with the translational fnction and viscosity B coefficients which are previously obtained from the same model. The B coefficient is most sensitive to a local viscosity change The Gierer-Wirtz model overestimates the effect of the "microscopic viscosity" on the translational friction coefficient comparing with the present results... 

The first result agrees with what solution chemists expect for the effect of the "microscopic viscosity " The second result tells us that the sensitivity of the friction coefficients on a local viscosity change largely depends on the mode of solvent motions. The shear mode (the viscosity B coefficient) is the most sensitive of the three It is to be noted that these results do not depend on the particular choice of the functional form of the position-dependent viscosity as expected. 

Gurney considered that the negative B coefficient is an indication of the local loosening of the solvent structure in the vicinity of the solute molecule and can be used as a measure for the structure breaking effect. The present quantitative analysis of the effect of the local viscosity change supports this idea. 

The influence of the interionic forces is due to two phenomena, namely, the electrophoretic effect and the time-of-relaxation effect. The net ionic atmosphere around a given ion carries the opposite charge and therefore moves in a direction opposite to the central ion. The final result is an increase in the local viscosity, and retardation of the central ion. This is called the electrophoretic effect. The time-of-relaxation effect is also related to the fact that the ionic atmosphere around a given ion is moving and therefore disrupted from its equilibrium configuration. It follows that the ionic atmosphere must constantly be re-formed from new counter ions as the ion under observation moves through the solution. The net effect is that the electrical force on each ion is reduced so that the net forward velocity is smaller. 

Since the equilibrium properties of the medium are involved in determining these effects they are called static medium effects. However, there is another type of relevant phenomenon which is related to the dynamic properties of a condensed phase. Since the movement of molecules with respect to one another is required for the reaction to take place, the local viscosity of the system can also influence the rate of reaction. This property is also related to local intermolecular forces. Effects which depend on local viscosity have also been studied experimentally and are known as dynamic medium effects. 

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