STUDYING ABNORMAL VISCOSITY BEHAVIOR IN DILUTE OLIGOMER SOLUTIONS BY SEC AND RHEOLOGY

Abstract

ABSTRACT We report here on the study of oligomer solutions with negativeviscosities, i.e., the viscosity of the solution is less than that of the neatsolvent (η<η s ) or the intrinsic viscosity of thesolution is less than zero ([η]<0), observed both in Poiseuille andCouette flow. These effects have been measured for several concentrations ofnon-functionalized styrene monomer in DMAc/ 0.5% LiCl at 35°Cand for functionalized styrene monomer as well as for different concentrationsof polyethylene oligomer in trichlorobenzene at elevated temperature. In bothcases, measurements were performed as part of a size-exclusion chromatography(SEC) set-up, utilizing differential viscometers of the Wheatsone bridge type.The SEC configuration, using high-resolution oligomeric columns, permittedunambiguous determination of the degree of polymerization at which the solutionviscosity changes from positive to negative in DMAc/LiCl, with static lightscattering detection ruling out the possibility of an analyte clustering effectbeing responsible for the observed behavior. Ultimately, an appropriateexplanation for this unusual effect was provided by steady-shear rheometrymeasurements of styrene-DMAc/LiCl mixtures, applying predictive rules forone-phase binary mixtures.