Rheological behavior of hydrophobically modified hydroxyethyl cellulose solutions: A linear viscoelastic model

Abstract

The rheological behavior of hydrophobically modified hydroxyethyl cellulose (HMHEC), an associative thickener, was studied and compared with that of hydrophobically modified ethoxylated urethanes (HEURs) and nonassociative celluloses. In contrast to HEURs, a simple Maxwell model does not fit the linear viscoelastic behavior of HMHEC. Differences are attributed to the stiffness and comb structure of HMHEC. A generalized Maxwell model with a logarithmic distribution of relaxation times is proposed, and another parameter that includes Rouse-like relaxation is added to fit behavior at high frequencies. Four parameters are needed to describe HMHEC viscoelasticity: a mean relaxation time, λM; its corresponding standard deviation, σ; a plateau modulus, GN; and a viscosity at infinite frequency, η∞. Satisfactory fitting is obtained for all concentrations and temperatures in the range of frequencies studied. The sharp increase of GN with concentration indicates loop-to-bridge transitions. Temperature does not influence GN, since the reduction in the number density of elastically effective chains caused by Brownian motion masks the direct effect of temperature. The dependence of λM on temperature follows the Arrhenius equation, as does the relaxation time of HEURs, but it does not change with concentration, presumably because the comb structure of HMHEC prevents the formation of long superchains.