Teresa P. Karjala

Detection of Low Levels of Long‐Chain Branching in Polyolefins

Shear creep experiments have been applied to probe the zero‐shear viscosity, η0, of molten undiluted polyolefin chains directly and precisely in a constant‐stress rheometer at 190 °C. Such experiments when combined with precise measurements of the weight‐average molecular weight, Mw (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 (Mw/Mn) 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 detec...

Estimation of the Molecular Weight between Crosslinks of Crosslinked Semicrystalline Polyolefins

The molecular weight between crosslinks (MXL) of crosslinked semicrystalline polyolefins, polyethylene in particular, has been estimated from uniaxial tensile tests in the melt state with the Sentmanat Extensional Rheometer (SER). Applying the Mooney–Rivlin equation to these stress‐strain data resulted in the determination of an apparent molecular weight between crosslinks. This fast and convenient technique correlates well with the conventional approach where MXL is obtained via the plateau modulus from shear viscosity.

Melt Rheology and Processability of Ethylene/Styrene Interpolymers

The melt rheology and processing characteristics of ethylene/styrene interpolymers, ESI, are correlated with the styrene content and molecular weight of the interpolymers. The complex viscosity, mastercurves, activation energy, high shear rate processability, and melt strength are discussed for four well-defined ESI. Comparisons are also made to polyethylene and polystyrene where appropriate.