Rheology of concentrated microgel solutions

Abstract

Viscosity, modulus, and yield stress for 0-6wt% aqueous solutions of Carbopo1941 were investigated using constant shear rate, constant shear stress, and dynamic oscillatory experiments. The microgel character of the polymer was evident from the solid-like behavior of the solutions above 1 wt%. Yield stress increased with concentration, but yield occurred at a critical shear strain of 40%, independent of concentration. The static stress-strain relationship became non-linear at ~25% strain, in fair agreement with the onset of non-linear response in the storage modulus at ~ 10% strain. Small strain moduli from static and low frequency measurements agreed rather well; modulus values obtained from the recoverable strain after yielding were 30 40% smaller. Solutions flowed at near-constant stress in the low shear rate regime; at higher rates the stress increases with shear rate more rapidly. The viscosity did not obey the Cox-Merz rule. Steady-state viscosity scaled with polymer concentration to the 3/4 power. Results were interpreted using a cellular, deformable sphere model for the polymer, in analogy to emulsions and foams.

Rheological Characterization of Topical Carbomer Gels Neutralized to Different pH

“…Two factors contribute to the elasticity of the microgel structures. When neutralized with TEA, the swelled microgel particles form a closely packed structure thus forming an elastic network (15,16). Entanglement between the long chain segments and side chains of the neighboring microparticles also help to form the network structure (17).…”

Slow dynamics, aging, and glassy rheology in soft and living matter

“…A signature feature of soft glassy rheology is the weak power law behaviours of the viscoelastic moduli. Experimental systems that verify the power law behaviour of soft glassy materials include concentrated microgel solutions [31], foams [32], paint [33] and compressed emulsions [34], all of which have G ′ s that show a weak power law frequency dependence, with the exponent (x-1) of the power law lying in the range 0.1-0.3. That is, the noise temperature x within the SGR model lies in the range 1.1-1.3.…”

Aging and rheology in soft materials

“…In oscillatory shear, for example, their viscoelastic storage and loss moduli, G ′ (ω) and G ′′ (ω), are often weak power laws of shear frequency (Mackley et al, 1994;Ketz et al, 1988;Khan et al, 1988;Mason et al, 1995;Panizza et al, 1996;Hoffmann and Rauscher, 1993;Mason and Weitz, 1995), while their nonlinear stress response σ to shear strain of constant rateγ is often fit to the form σ = A + Bγ n (known as the Herschel-Bulkley equation, or when A = 0, the power-law fluid) (Holdsworth, 1993;Dickinson, 1992;Barnes et al, 1989). The fact that such a broad family of soft materials exhibits similar rheological anomalies is suggestive of a common cause, and it has been argued that these anomalies are symptomatic of the generic presence in such materials of slow, glassy dynamics (Sollich et al, 1997;Sollich, 1998).…”