Determination of branching distributions in polyethylene and ethylene copolymers

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Abstract
A description is given of an analytical temperature‐rising elution fractionation (TREF) system for the purpose of determining short‐chain branching (SCB) or copolymer distributions in poly‐ethylenes and ethylene copolymers. The system achieves fractionation on the basis of crystallizability and is shown to be very little influenced by molecular weight in the normal high polymer range. Sample preparation by slow cooling from relatively dilute solution followed by continuous elution with a simultaneous and fairly rapid rate of temperature rise proves to be an efficient fractionation process. An on‐line detector and data system allows application of a calibration curve to give realistic SCB distribution data in a convenient manner. The potential value of the TREF technique for providing structural information is illustrated by examples which include low‐density high‐pressure resins made by both tubular and autoclave reactors, high‐ and low‐density resins made by low‐pressure processes, and copolymers of ethylene with vinyl acetate and ethyl acrylate.