Studying abnormal viscosity behavior in dilute oligomer solutions by SEC and rheology

We report here on the study of oligomer solutions with negative viscosities, i.e., the viscosity of the solution is less than that of the neat solvent (eta < η(s)) or the intrinsic viscosity of the solution is less than zero ([η] < 0), observed both in Poiseuille and Couette flow. These effects have been measured for several concentrations of non-functionalized styrene monomer in DMAc/0.5% LiCl at 35degreesC and for functionalized styrene monomer as well as for different concentrations of polyethylene oligomer in trichlorobenzene at elevated temperature. In both cases, 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, permitted unambiguous determination of the degree of polymerization at which the solution viscosity changes from positive to negative in DMAc/LiCl, with static light scattering detection ruling out the possibility of an analyte clustering effect being responsible for the observed behavior. Ultimately, an appropriate explanation for this unusual effect was provided by steady-shear rheometry measurements of styrene-DMAc/LiCl mixtures, applying predictive rules for one-phase binary mixtures.