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.

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