Presented at the 6th International Conference on Polyolefin Characterization (ICPC), 2016. Shanghai, China.
Alberto Ortín, Juan Sancho-Tello, Pilar del Hierro, Esther López. Polymer Char, Spain.
Wallace W. Yau. Polyolefin Characterization Consultant, USA.
Long chain branching in Polyethylene can be studied by means of the g-index measured as a function of molar mass by Gel Permeation Chromatography (GPC) with multiple detection. The number of branching points and branching frequency are then calculated according to Zimm-Stockmayer theory. A major concern with this g-index GPC method is the difficulty of obtaining reliable g vs molar mass data in the first place. Typically, that is obtained by multiple angle light scattering which delivers mean square radius of gyration (Rg) as the slope of the angular extrapolation of measured excess Rayleigh ratios to zero angle (Debye plot). Several extrapolation formalisms and different detector configurations have been made available for that task, but they give differences in the Rg results. Precise and stable data still is difficult to obtain.
A novel approach, based on point-by-point calculation of the gpcBR index to estimate the g-index is here described and evaluated. The gpcBR index is calculated from absolute molar mass (by LS) and intrinsic viscosity (by online viscometer) together with the respective conventional calibration averages. That has been proposed as a high precision average index for branching evaluation. In this work, the gpcBR concept is extended to obtain a distribution of the estimated g-index and to provide also the equivalent branching number and branching frequency values that can be directly compared to other traditional methods. Unlike the method based on Mark-Houwink plots, the step of empirical conversion from g’ to g-index by using a viscosity shielding exponent is no longer required in the new approach.