High‐temperature coupling of high‐speed GPC with continuous viscometry. I. Long‐chain branching in polyethylene

Abstract

AbstractThe coupling of a high‐temperature liquid chromatograph (Waters 150C) with a home‐made continuous capillary viscometer is described. This detector is the only one suitable for high‐speed GPC when the small volume of the mobile phase prohibits the coupling with a classical viscometer. The pressure drop of the GPC effluent through the capillary is continuously measured along with the refractive index change. This dual detection leads to the determination of the intrinsic viscosity as a function of the elution volume, thus allowing a precise use of Benoit's universal calibration. The accuracy of our system is demonstrated in the case of the characterization of linear and branched polyethylene samples. The results concerning the average molecular weights as well as the branching factors (structure parameter g′ and long‐chain branching frequencyλ) are in close agreement with those obtained by the classical way (coupling traditional GPC and discontinuous viscometry). It is well known that an estimate of the λ coefficient is extremely dependent on several hypotheses. However, for a set of commercial low‐density polyethylenes, we obtained λ values about 0.5 × 10−4, with no marked change along the molecular weight range.