Detection of low levels of long-chain branching in polyrolefins

Shear creep experiments have been applied to probe the zero-shear viscosity, 770, of molten undiluted polyolefin chains directly and precisely in a constant-stress rheometer at 190 T. Such experiments when combined with precise measurements of the weight-average molecular weight, M-w (calibrated relative to linear chains of high-density polyethylene), are shown to provide a very sensitive approach to detect low levels (0.005 branches per 1000 carbons) of long-chain branching. This detection limit is shown with an extensive set of well-defined metallocene-based model materials to be insensitive to whether the molecular weight distribution is mono- or multi-modal, and/or to whether the molecular weight distribution breadth (M-w/M-n) rises to about ten. The approach is also shown to be insensitive to levels of short-chain branching found in poly(ethylene-co-butene) up to 12 wt% butene and poly(ethylene-co-hexene) up to 14 wt% hexene. In conclusion, this approach is a sensitive, robust, and rapid method to detect low levels of long-chain branching in polyolefin materials.