The role of solution structure in apparent thickening behavior of dilute peo/water systems

An experimental investigation was undertaken to determine the role of solution structure on the apparent thickening exhibited by "dilute" polyethylene oxide/water solutions in extensional flow. Measurements of apparent relative viscosity were obtained as a function of wall shear rate for solutions flowing from a reservoir through a 0.1 mm internal diameter tube. As the wall shear rate was increased, slightly shear thinning behavior was observed up until a critical wall shear rate was exceeded at which point a large increase in relative viscosity was seen. Other researchers have observed these apparent thickening effects and have interpreted them in terms of individual polymer molecules undergoing coil-stretch transitions. However, in the systems used in this study, the critical wall shear rate and the degree to which relative viscosity increased wer both seen to be strong functions of solution aging time and concentration. These results are inconsistent with the simple picture of individual polymer coils undergoing a coil-stretch transition and instead are consistent with the picture of aggregated systems or micronetworks being stretched from their equilibrium configurations.