Assessment of NMR and Rheology for the Characterization of LCB in Essentially Linear Polyethylenes R. N. Shroff † and H. Mavridis* Equistar Technology Center, 11530 Northlake Drive, Cincinnati, Ohio 45249 Received April 2, 2001 ABSTRACT: The identification and characterization of low levels of long-chain-branching (LCB) in essentially linear polyethylenes has attracted significant interest in recent years. One experimental technique is nuclear magnetic resonance (NMR), which can detect LCB in essentially linear polyethylene homopolymers for LCB in the range 0.2-3 branches per 10 000 carbon atoms. Another approach has been the use of a rheological measurement in combination with a dilute-solution measurement (intrinsic viscosity or GPC). NMR is a direct method of LCB measurement, but it provides no information on branch length and has other limitations related to interference from short chain branches. Rheology provides a sensitive but indirect method of measurement, capitalizing on the strong effect of LCB (longer than M e, the entanglement molecular weight) on translational mobility of the polymer chains and thus viscosity. The purpose of the present work is to provide an assessment of the two approaches of LCB determination, using a series of well-characterized, essentially linear polyethylenes. NMR was shown to work satisfactorily in characterizing LCB for a series of metallocene-catalyzed polyethylenes, but it failed to detect LCB in other cases, including a series of linear polyethylenes where LCB was introduced deliberately via peroxide modification. A rheology-based index for LCB characterization was shown to be preferable, due to its robustness and general applicability in all cases examined. Introduction Conventional methods of determining long chain branching (LCB) from intrinsic viscosity (IV) in combi- nation with gel permeation chromatography (GPC), or from NMR, seem attractive, because they yield results in terms of number of long-chain branches per given chain. However, in our earlier publication, 1 we showed that for low levels of LCB (∼ 0.5-3/10 000 C), the ratio of the intrinsic viscosity of the branched polymer, [η] B , to that of the linear polymer, [η] L , of the same molecular weight is close to unity, within experimental error. The determination of LCB at these low levels then becomes prone to high error, since a small change in the ratio [η] B /[η] L inherently means a large variation in LCB number. This is shown in Figure 1 (adapted from Figure 1 of ref 1), where the Zimm-Stockmayer prediction of LCB/10 000 C (eq 19 in ref 1) is plotted as a function of the intrinsic viscosity ratio, [η] B /[η] L , for two molecular weights (M ) 50 000 and M ) 150 000), in the range of essentially linear polymers, i.e., when the ratio [η] B /[η] L is close to unity. A ratio [η] B /[η] L greater than 0.95, for example, is indistinguishable experimentally from unity and yet the LCB/10 000 C level varies between 0.0 and 1.5 LCB/10 000 C (for [η] B /[η] L between 0.95 and 1.0). Therefore, the Zimm-Stockmayer method of estimating LCB from the ratio [η] B /[η] L cannot be employed reliably for essentially linear polymers ([η] B /[η] L > ∼0.9). The detection of LCB using on-line triple detectors 9 (light scattering, intrinsic viscosity, and refractive index) could give information about LCB as a function of molecular weight, i.e., by applying the Zimm-Stock- mayer method on individual molecular weight fractions. While the sensitivity of the Zimm-Stockmayer method does improve at higher molecular weights (as the flattening of the LCB/10 000 C vs [η] B /[η] L curve shows in Figure 1, at the higher molecular weight), the method will still encounter the quantitative uncertainties inher- ent in the Zimm-Stockmayer method, when applied to essentially linear polymers. Nuclear magnetic resonance (NMR) spectroscopy provides another experimental technique for measuring LCB in polyethlyene. Even though NMR treats alkyl branches equal or longer than C 6 as long, several recent publications 2-4 from McMaster University have outlined an NMR method for obtaining low levels of long-chain branch density (LCBD), or the number of LCB/10 000 C, for ethylene homopolymers. Obviously, the NMR method cannot be used for copolymers of ethylene with octene-1, but we wish to point out here the limitations for several commercial high-density polyethylenes, which show low levels of long-chain branching. Rheology provides another method, albeit indirect, for characterizing the presence of LCB in polyethylene, as shown in a series of recent publications, 5-8 including * Corresponding author. † Present address: 306 Pebble Beach Lane, Bartlett, IL 60103. Figure 1. Dependence of the long chain branching, as determined by the Zimm-Stockmayer method, on the intrinsic viscosity ratio, at two different molecular weights and for essentially linear polyethlenes ([η]B/[η]L > ∼ 0.9). 7362 Macromolecules 2001, 34, 7362-7367 10.1021/ma010573b CCC: $20.00 © 2001 American Chemical Society Published on Web 09/11/2001