A Structural Model of Hydrophobically Modified Urethane−Ethoxylate (HEUR) Associative Polymers in Shear Flows

Abstract

This paper describes the rheological behavior of a HEUR (hydrophobic ethoxylated urethane) associative polymer with C16H33 end groups at 2.0 wt % concentration in aqueous solution. Under normal steady shear, this solution exhibits Newtonian behavior at low shear rates and, as the shear rate is increased, passes through a shear-thickening region before exhibiting a sharp decrease in viscosity. Here we report superposition-of-oscillation experiments on steady-shear flows to examine the state of the network structure under different shear conditions. The technique involves applying a steady shear deformation to the fluid, and once the steady state is achieved, a small amplitude oscillation is imposed on the sample to measure the linear viscoelastic properties. We observe that within the shear-thickening region, the plateau modulus is larger than in the Newtonian region, suggesting that shear-thickening is the result of a shear-induced increase in the density of mechanically active chains, which may be due to incorporation of free micelles or higher aggregates into the network structure. In the shear-thinning region, the Maxwell relaxation time decreases with increasing shear stress or shear rate. Thus shear thinning is a consequence of a shear-enhanced exit rate of the hydrophobic end groups from the micellar junctions of the network. This is the first experimental evidence for shear enhancement of the relaxation rate of an associative polymer network.

How does a non-ionic hydrophobically modified telechelic polymer interact with a non-ionic vesicle? Rheological aspects

“…After a critical stress, the gel structure disrupts and a shear-thinning behaviour is observed. A similar behaviour has been observed by others authors in similar systems [19,29,35]. We have also observed that the zero-shear viscosity always increases with the concentration of polymer and/or non-ionic vesicles in the system.…”

Strain Stiffening in Synthetic and Biopolymer Networks

“…7,9,38,39 In more viscous physically associating solutions, alternative explanations for strain-stiffening and the related phenomenon of shear thickening have been presented, 8 such as deformation-induced increases in the number of elastically active strands or network components. 40,41 Based on the characterized structure of our triblock copolymer networks 27 and the stress responses observed here, we believe our physically associating networks evolve in the following manner during deformation: as the external shear stress is applied to the network, the distance between the endblock aggregates increases with deformation and the rubbery midblock strands are stretched, adopting non-Gaussian conformations and causing a stiffening of the network. As the midblock is stretched to its maximum extensibility, the stiffening response becomes dominant, causing the stress to rapidly increase.…”

Model Transient Networks from Strongly Hydrogen-Bonded Polymers

“…The reversible junctions in transient networks typically contain areas of hydrophobic associations, [7][8][9][10] hydrogen bonding, [11][12][13][14][15][16][17] metal-ligand interactions, 18,19 or ionic associations. 20,21 In many cases such junctions are poorly defined aggregates with unknown association strength and kinetics, but in a few transient network materials bulk properties have been directly correlated to defined polymer microstructure or well-characterized reversible interactions leading to important insights into the properties of these systems.…”