Lonnie G. Hazlitt

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...

Size exclusion chromatography of polymers

In the characterization of polymers, size-exclusion chromatography (SEC) has become a standard technique for determining molar mass averages and molar mass distributions (MMD) of polymers. The principle of SEC is easily understood: owing to limited accessability of the pore volume within the particles of the column packing, polymer molecules are separated according to their hydrodynamic volumes, which can be correlated to their molar mass. Unfortunately, this is not as easy and simple as it looks. There are numerous sources of error and pitfalls in the separation, in the detection and in the treatment of the raw chromatographic data.

Short Chain Branching Distribution of Uldpe

Linear Low Density Polyethylene (LLDPE) is manufactured by copolymerizing ethylene wih an alpha-olefin The most common and commercially important comonomers are butene-1, hexene-1, 4-methylpentene-1, and octene-1. This paper focuses on the use of octene-1. Ultra Low Density Polyethylene (ULDPE), a select case of LLDPE, has a density of ≤ 0.915 g/cc. Due to superior impact strength, optical properties, and heat sealing characteristics, ULDPE resins are beneficial to the packaging industry, particularly in food contact applications. Compliance with federal government Food and Drug Administration (FDA) regulations by ULDPE (and LLDPE, in general) must be met for such food contact. One test (n-hexane extraction) is used as an FDA approved method to assure that the control of produc tion is maintaining the previously demonstrated suitable level of extractability from a polymer by food. It is desirable for resin producers to control resin design such that maximum product performance results and regulatory guidelines are both met. Control of the short chain branching distribution (SCBD) has been found to significantly affect product performance, especially for ULDPE. In addition to enhancing performance, the bonus of decreased n-hexane extractables has resulted from narrowing the SCBD.