MODEL ELASTIC LIQUIDS WITH WATER-SOLUBLE POLYMERS

Model liquids with nearly constant viscosity and adjustable elasticity are needed to resolve the role of elasticity in coating and other fre e-surface flows. Available Boger liquids are not well suited to free-s urface flows, because they are solutions in organic solvents and their viscosities exceeding 1 Pa.s fall on the high side. Aqueous liquids a re preferred in laboratory studies partly due to environmental hazards . Aqueous polymer solutions with constant shear viscosity and adjustab le elasticity were prepared by adding small amounts of a high-molecula r-weight polymer to a more concentrated aqueous solution of the same p olymer but of a much lower molecular weight. Up to 0.2 wt. % of high-m olecular weight poly(ethylene oxide) (PEO, M-w from 400,000 to 4 milli on g/mol) was added to almost inelastic solutions of low-MW polyethyle ne glycol (PEG, M-n= 8,000 g/mol). PEG concentrations in these solutio ns varied between 16.7 and 42.9 wt. %. Shear viscosities of these solu tions ranged from about 0.02 to 0.3 Pa.s and were constant lip to shea r rates of 100 s(-1). The stress ratio is one measure of the elasticit y of the liquid. Stress ratios up to 0.2 were estimated from small-amp litude oscillatory measurements. Terminal behavior (elastic modulus ri sing with the square of the frequency) was not observed even at 10(-2) rad/s. Viscosity and elasticity of the liquids can be manipulated ove r a wide range by varying the amounts and molecular weights of PEG and PEO within the unentangled and dilute regions of the concentration-mo lecular weight diagram, respectively. Fits of experimental data to can didate differential and integral constitutive equations are also discu ssed.