Formation Of Transient Network Research Articles

Flow of mixtures of poly(ethylene oxide) and hydrolyzed polyacrylamide solutions through porous media

In this work, the porous media flow of polymer solutions of poly(ethylene oxide) (PEO), hydrolyzed polyacrylamide (HPAA), and their blends is investigated. Aqueous solutions of PEO exhibit critical extension thickening when flowing through porous media. HPAA solutions also exhibit critical extension thickening in excess salt environments, but their behavior changes to a more gradual extension thickening when dissolved in deionized water. The mixtures of solutions of HPAA and PEO therefore vary its porous media flow behavior, depending on the ionic environment. In deionized water, a critical extension thickening similar to that obtained with PEO is still observed when HPAA is mixed in at concentrations low enough so that its apparent viscosity does not mask the influence of PEO. In the presence of salt, only a critical extension thickening is observed, which is attributed to transient network formation of both PEO and HPAA molecules. The mixtures generally exhibit a less critical behavior and display a lower than expected sensitivity of the onset Reynolds number for extension thickening with concentration. The results presented herein indicate that interspecies molecular interactions through transient network formation and the associated flow modification play a major role in determining the complex non-Newtonian flow behavior of these polymer solutions. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 783–795, 1999

Flow of mixtures of poly(ethylene oxide) and hydrolyzed polyacrylamide solutions through opposed jets

In this work, the porous media flow of polymer solutions of poly(ethylene oxide) (PEO), hydrolyzed polyacrylamide (HPAA), and their blends is investigated. Aqueous solutions of PEO exhibit critical extension thickening when flowing through porous media. HPAA solutions also exhibit critical extension thickening in excess salt environments, but their behavior changes to a more gradual extension thickening when dissolved in deionized water. The mixtures of solutions of HPAA and PEO therefore vary its porous media flow behavior, depending on the ionic environment. In deionized water, a critical extension thickening similar to that obtained with PEO is still observed when HPAA is mixed in at concentrations low enough so that its apparent viscosity does not mask the influence of PEO. In the presence of salt, only a critical extension thickening is observed, which is attributed to transient network formation of both PEO and HPAA molecules. The mixtures generally exhibit a less critical behavior and display a lower than expected sensitivity of the onset Reynolds number for extension thickening with concentration. The results presented herein indicate that interspecies molecular interactions through transient network formation and the associated flow modification play a major role in determining the complex non-Newtonian flow behavior of these polymer solutions. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 783–795, 1999

Flow of monodisperse polystyrene solutions through porous media

In this work, we study the origin of the extension thickening commonly observed when solutions of flexible polymers flow through porous media and ideal elongational flows. We have used randomly packed beds of glass beads as porous media. We have performed experiments with closely monodisperse atactic polystyrene of different molecular weights dissolved in organic solvents. The use of a closely monodisperse polymer allowed us to make a more meaningful comparison between the results obtained using opposed jets and porous media flow, as compared to previous works on polydisperse polymers. The results indicate that the coil-stretch transition of isolated polymer molecules in solution cannot be the only mechanism responsible for the extension thickening. It is clear that part of the observed effect is due to the extension of isolated molecules, but the main factor causing a great increase in the elongation viscosity beyond a critical strain rate is the formation of transient entanglement networks.

Flow of polymer solutions through porous media

The molecular mechanism for viscosity enhancement during the flow of poly(ethylene oxide) solutions through porous media has been investigated. Random packed beds of uniform-size spheres have been used as porous media. Experiments were conducted for one-dimensional (uniform) flow, as well as for a particular three-dimensional (nonuniform) flow situation. A substantial increase in flow resistance with respect to that exhibited by Newtonian fluids has been observed in both cases. The results obtained suggest that, under the conditions employed in the present work, the mechanism that causes the increased flow resistance is related to the formation of transient networks of polymer molecules in the flow field. The nonuniform flow results indicate that the presence of the polymer induces a change in the macroscopic flow distribution within the porous medium.

Diffusion Processes of Interacting Brownian Particles in Blockcopolymer‐Microemulsion Transient Networks

AbstractQuasielastic light‐scattering and pulsed field gradient NMR were used to investigate the effects of a triblock copolymer and a α‐ω telechelic ionomer on the interaction potential and transient network formation of nanodroplets in w/o microemulsions. The NMR self diffusion measurements yield a crossover from restricted polymer diffusion to free self diffusion. The relation between the copolymer self diffusion and the dynamic properties of the transient network are explained in the framework of reversibly cross linked polymer chains. Quasi‐elastic light‐scattering, only sensitive to the correlations between the waterdroplets, yields three diffusive processes, which are due to fast collective diffusion and slowly decaying correlations, controlled by the specific interactions of the hydrophilic and hydrophobic blocks with the water and oil phases.

Non-Newtonian behaviour of hydrolysed polyacrylamide in strong elongational flows: a transient network approach

In this paper we report a new technique that links molecular behaviour and macrorheology in idealized elongational flow systems. The effective elongational viscosity of aqueous solutions of hydrolysed polyacrylamide (widely used and studied in applied hydrodynamics) is determined and correlated with the various stages of chain stretching and transient network formation. Beyond a critical strain rate, strong non-Newtonian dilatant effects are observed. These are unmistakably due to the existence of transient networks, which arise as a consequence of entanglements becoming mechanically effective at timescales shorter than the disentanglement time. We report strong parallels between observations in our idealized experiments and dilatant effects commonly observed in other flow systems that contain appreciable elongational components. These effects had been previously generally attributed to the viscosity enhancement due to the stretching of isolated molecules. On the basis of our observations, which include the coil-stretch transition, we are forced to reinterpret such effects as also due to the development of transient entanglement networks.

Dynamic light scattering from semidilute cellulose-tri-carbanilate solutions

A series of cellulose-tri-carbanilates (CTC) with a range of M w = 130 000 to 1 000 000 has been studied in dioxane at 20°C by static and dynamic light scattering. CTC is a semiflexible chain with a persistence length of 110 Å. In two cases ( M w = 130 000 and 1 000 000) a concentration region from dilute to 20c∗ was covered, where c∗ = 1 |η| is taken as the coil overlap concentration. Up to 3c∗ usual dilute solution behaviour was found. At about 3c∗ a sharp decrease in 1 M app [ = Kc R (q = 0)] and in the apparent diffusion coefficient D( c) (determined from the first cumulant of the time correlation function ( TCF)) was observed. Simultaneously, the mean square radius of gyration increases. Analysis of the TCF reveals two modes of motion: the mode which is fast, increases with c with an exponent of about 1.0 ± 0.1 and may be attributed to the cooperative diffusion coefficient of an entangled network. The other, which is slow, decreases with an exponent of about −2.6 ± 0.2 and thus is appreciably larger in value than −1.75 which is predicted for the self-diffusion of single chains. Master curves are obtained for M M app and D(c) D 0 as function of c c∗ . The observed behaviour indicates that there is no transient network formation. The results are, however, consistent with a model of entangled clusters existing in the region c∗ ⩽ c ⩽ 10c∗.