A correlation between microstructure and rheological properties of broad MWD high-density polyethylene

A correlation between molecular structure parameters and rheological behavior was determined for three different grades of high-density polyethylene (HDPE). The molecular structure parameters including number-average molecular weight (Mn), weight-average molecular weight (Mw), molecular weight distribution (MWD) and branching index were characterized by gel permeation chromatography (GPC). Moreover, a fast and easy method for investigating side chain branching using Fourier transform infrared (FTIR) spectroscopy was discussed. The rheological characterizations of both linear and non-linear viscoelastic regions were performed by a modular compact rheometer (MCR) in the dynamic mode. Zero shear viscosity (η0), relaxation time, relaxation time distribution, stress relaxation modulus and damping function were obtained from the rheological characterization. Moreover, molecular weight distribution was calculated for each sample and compared with the GPC results. The GPC results confirmed broad molecular weight distribution for all three HDPE samples. The relationship between zero shear viscosity and molecular weight at 180 °C was found as η0=2.5×10-14Mw3.6\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\eta_{0} = 2.5 \times 10^{ - 14} M_{w}^{3.6}$$\end{document} and for zero shear viscosity and MWD at 180 °C was found as η0=1.6×10-15Mw3.6MwMn\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\eta_{0} = 1.6 \times 10^{ - 15} M_{w}^{3.6} \left( {\frac{{M_{w} }}{{M_{n} }}} \right)$$\end{document}. By choosing the mixing parameter (β) value of 0.73, the values of molecular parameters obtained from the rheology and GPC tests were significantly accommodated. Furthermore, it was found that the damping function of type C was an appropriate type for the polyethylenes of selected broad MWD.