The viscosity-radius relationship for concentrated polymer solutions

A key assumption of polymer physics is that the random chains polymers extend in flow. Recent experimental evidence has shown that polymer chains compress in Couette flow in a manner counter to expectation. Here, scaling arguments developed previously are used to determine the relationship between the viscosity and chain radius of gyration. Scaling arguments determine the viscosity-radius of gyration relationship to be such that the viscosity is proportional to the radius to the power of 9. The viscosity is shown to be a power law function of the radius, and to decrease with decreasing radius under conditions where the chains are ideal random walks in concentrated solution. Furthermore, this relationship is consistent with both the widely observed viscosity-temperature and viscosity-shear rate behavior observed in polymer rheology. The assumption of extension is not consistent with these observations as it would require that the chains increase in size with increasing temperature. Shear thinning is thus a result of a decreasing radius with increasing shear rate as the radius is proportional to the the shear rate raised to the power law exponent divided by 9. The thermal expansion coefficient determines the variation in the power law exponents that are measured for different polymer systems. Typical values on n enable the measured reduction in coils size behavior to be fitted. Furthermore, the absurd notion that polymer chains extend to reduce the viscosity implies that an increasing chain size results in a reduced viscosity is addressed. This assumption would require that the viscosity increases with reducing coil radius which is simply unphysical

Modelling Polymers as Compressible Elastic Spheres in Couette Flow

A model of polymer chains as compressible elastic spheres in flow is presented. The spherical polymer blobs are assumed to compress in simple Couette flow in accord with recent rheo-optic measurements on semi-dilute solutions. The experimentally determined decrease in radius with increasing shear rate is predicted by the model. Furthermore, the model predicts power law exponents for the viscosity-shear rate within the range of measured values for polymer chains

Diffusing probe measurements of polystyrene latex particles in polyelectrolyte solutions : Deviations from Stokes-Einstein behavior

© 2000 American Chemical SocietyThe diffusion coefficients of polystyrene latex spheres in both Newtonian and elastic liquids have been measured using dynamic light scattering. The diffusion coefficients of the latex particles measured in glycerol/water (Newtonian) solutions obey Stokes-Einstein behavior over a range of solvent viscosity and temperature. Two apparent diffusion coefficients for the particles are measured in viscoelastic polyacrylamide solutions and are designated Dfast and Dslow. The apparent fast diffusion coefficients measured in the polyacrylamide solutions show an increase to a maximum, above that measured in the solvent water, with increasing polyacrylamide concentration. The apparent fast diffusion coefficient is seen to increase to twice that measured in the water solvent. At higher polyacrylamide concentrations the observed Dfast values decrease below the value obtained in the solvent water. Dfast increases with the scattering vector (q) while Dslow is independent of q.Dave E. Dunstan and Jason Stoke

Probe diffusion measurements of polystyrene latex particles in polyelectrolyte solutions of varying ionic strength

Copyright © 2002 American Chemical SocietyThe diffusion of polystyrene latex particles in solutions of a poly(acrylic acid-co-acrylamide) polyelectrolyte as a function of both polyelectrolyte concentration and solution ionic strength has been investigated using dynamic light scattering. Two apparent diffusion coefficients were measured for the latex probe particles in the polyelectrolyte solutions. Designated as Dfast and Dslow, the fast diffusional mode increases to a maximum as a function of polyelectrolyte concentration, with the maximum increasing in magnitude as the ionic strength increases. Dslow decreases as a function of polyelectrolyte concentration in low ionic strength solutions. However, Dslow shows a slight increase with polyelectrolyte concentration in the higher ionic strength solutions. The results are discussed with reference to calculated correlation lengths for the polyelectrolyte solutions and are compared with the data obtained from polymer solutions measured using dynamic light scattering where similar trends have been observed. The latex spheres appear to probe the single chain motion, cooperative diffusion, and the macroscopic diffusional motions, self-diffusion, of the polyelectrolyte molecules.Bremmell, K. E.; Dunstan, D. E

Diffusing probe measurements in Newtonian and elastic solutions

Copyright © 2001 Elsevier Science B.V. All rights reserved.http://www.sciencedirect.com/science/journal/0001868