Characterization of long chain branching: Dilution rheology of industrial polyethylenes

We introduce a method using the rheology of concentrated solutions to differentiate sensitively between nonbranched and branched architecture polymer melts. By modeling the concentration dependent shift in zero-shear viscosity using molecular tube theory for melt rheology, it is feasible to quantify the degree of long chain branching as well as the distribution of sizes of such branches, providing the class of branched material is known. The technique detects branched chains with the order of one entanglement length and greater. We apply the technique, already validated against monodisperse control led-architecture polymers [D. R. Daniels et al. Rheol. Acta 40, 403 (2001)], to a series of industrially produced metallocene-catalyzed polyethylenes with an octene comonomer and two low density polyethylenes, diluted by the short chain alkane squalane. The dilution method compares favorably with direct solution methods, and holds promise of much greater sensitivity. (C) 2002 The Society of Rheology.