Complementarity of universal calibration SEC and 13C NMR in determining the branching state of polyethylene

The quantitation of long-chain branching (LCB) and short-chain branching (SCB) in polyethylene (PE) was accomplished with a combination of carbon nuclear magnetic resonance (C-13 NMR)Spectroscopy and size exclusion chromatography (SEC) with universal calibration. We demonstrate how the spectroscopic and chromatographic techniques can supplement each other, as neither is capable individually of completely describing the molecular architecture imparted by the various types of branching. The essential lack of impact of SCB on the hydrodynamic volume imposes a limit on SEC for determining this type of branching, whereas highly effective LCB in the PE molecule may not offer a statistically large enough amount of long chains for accurate determination by NMR. A variety of examples are given for PE, showcasing the advantages and shortcomings of each analytical method and their complementarity. Additionally, the importance of choosing an appropriate linear standard and viscosity shielding ratio (epsilon) for the Zimm-Stockmayer branching calculations employed for analyzing SEC data is emphasized with an examination of the effect on the results of using a branched standard and various epsilon values. (C) 2000 John Wiley & Sons, Inc.