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

Abstract

SynopsisThe 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 frequency X) 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 X coefficient is extremely dependent on several hypotheses. However, for a set of commercial low-density polyethylenes, we obtained X values about 0.5 X with no marked change along the molecular weight range.

Modeling of Chain Length and Long-Chain Branching Distributions in Free-Radical Polymerization

“…Although these pseudo-rate coefficients are used for the presentation and discussion of results below, mechanisms (8)(9)(10) are employed in this modeling study instead of Equations (6) and (7). In this study it is assumed that k sec p = k p , thus the ratio of scission to LCB events is given by k b2 /k p [M].…”

“…[3 -5] Thus it is not surprising that many conflicting claims have been published about the shape of the branching density curve for low density polyethylene (LDPE): some authors report that branching density (branches per 1000-C atoms or per monomer repeat unit) decreases with increasing MW [6,7] while others find that it stays constant with MW. [4,8] Detailed modeling of the distribution, including how it is affected by b-scission, should aid in the resolution of these difficulties.…”

Shear-induced β -form polypropylene in long chain branching isotactic polypropylene

“…The LCB distribution can be elucidated quantitatively from their mean square gyration radius ( ) and intrinsic viscosity ([ η ]) . According to the model of Zimm and Stockmayer, combined with the methodology of Lecachaux , the ratio of the of a LCB polymer to that of a linear polymer, g = , is related with the ratio of the [ η ] of these two type of polymers, g' : …”

Continuous Viscometric Detection in Size Exclusion Chromatography

“…But, when it is unknown, we have remarked, during our studies (9)(10)(11)(15)(16), that branching usually occurs only after a limiting molecular weight, which produces a linear variation in the low molecular weight region of the viscosity law. Taking the slope in this region can prcvide the visocsity law of the linear polymer (16).…”