Prediction of polyethylene melt rheological properties from molecular distribution data obtained from gel permeation chromatography

Abstract

In the present study six different commercial polyethylene samples, including three blends of two of the polymers, have been used to determine relationship between the readily measureable molecular parameters obtained from gel permeation chromatography and the rheological properties of the polymer melts measured at 190°C. The polymers and their blends were prepared by the Dow Chemical Company, Texas Division, and their molecular characteristics were determined by Dow using a Water Associates gel permeation chromatograph. A cubic spline curve fitting technique was used on cumulative weight distributions to get the differential weight distributions from GPC data supplied by Dow. An existing theoretical expression relating differential weight distribution to relaxation spectrum based on the Rouse' molecular model was used to generate relaxation spectra for the polymer melts from the differential weight distributions evaluated using the spline fit. These expressions, originally developed for dilute polymer solutions, were modified to apply to polymer melts. The relaxation spectra thus obtained were used to generate the frequency-dependent components of the complex viscosities as well as the shear-dependent viscosities for the polymer melts. These calculated oscillatory and viscosity functions have been compared to those obtained experimentally. The relaxation spectra were also evaluated from the experimental rheological data using existing approximate relations and have been compared to the spectra determined from GPC data..