Correlation between Microstructure and Rheological Properties of Broad Molecular Weight Distribution High Density Polyethylenes

In this article، the correlation between molecular microstructure and rheological behavior for three different grades of high density polyethylene with broad molecular weight distribution have been investigated. Structural parameters such as Mn، Mw، Mz، molecular weight distribution (MWD) and branching index were characterized by high temperature Gel Permeation Chromatography (GPC) test. GPC results showed that BL4 and EX3 have bigger weight average molecular weight and narrower molecular weight distribution، respectively. Furthermore،it was found from GPC results that all three HDPE has a broad MWD and EX3 has more short chain branches than two other samples. Rheological characterization was done in a Mechanical Compact Rheometer (MCR) in rotational and dynamic modes at both linear and nonlinear viscoelastic region. From rheological characterization the zero shear viscosity (η0)، relaxation time and relaxation time distribution، stress relaxation modulus and damping factor were obtained and by utilizing relaxation time spectrum function (h(τ))، molecular weight distribution was calculated for each sample and compared with GPC results. The relationship between zero shear viscosity with molecular weight and MWD at 180 ºC obtained. After calculating molecular weight distribution function and average molecular weights using different mixing parameters (β)، by choosing β=0. 73، a good correlation between molecular parameters obtained from rheological data with those of GPC results was obtained. After calculating stress relaxation modulus and consequently damping factor for each sample، comparing to Doi-Edwards model، it was found that the damping function type C is an appropriate damping function type for selected polyethylenes.