Molecular Weight Distribution and Melt Rheological Properties for High Density Polyethylenes

Abstract

High density polyethylene (HDPE) has each its distinctive features in the rheological properties according to its manufacturing process. The present work proposes to present a quantitative relationship between its molecular weight distribution and the melt viscoelastic properties for some samples of HDPE.The samples have been fractionated by both the elution column technique and the precipitation technique. The number-average, weight-average and Z-average molecular weights have been calculated from the smoothed molecular weight distribution curve for each sample by the method of summation.The viscoelastic properties of some series of the commercial HDPE have been measured by means of a concentric cylinder type rheometer at different temperatures (150∼230°C). The angular frequency (ω) ranges from about 10-2 to 50sec-1. The ratio of flow rate at shear stress 106dyne/cm2 to at 105dyne/cm2 (=flow ratio; FR), which gives a good indication of the non-Newtonian property of samples, have been measured by means of a melt indexer type rheometer.The results are as follows;(1) The linear relationship is found between Mw/Mn and Mz/Mw for only a series of samples made by the same manufacturing process.(2) FR is found to be related to Mz/Mw, not to Mw/Mn. log FR=0.137Mz/Mw+1.04(3) Dynamic viscosity (η') and dynamic regidity (G') can be superposed according to the temperature-time superposition principle, and shift factors aT from η' and G' are practically the same.Agreement with constancy of shift factor for various molecular weights (Mv=5∼10×104) and distributions is good, and the apparent activation energy (ΔHa) calculated from the shift factor is 6-7kcal/mol. ΔHa of copolymer is also of the same value.(4) Master curves also are constructed with respect to the molecular weight for measurements of η' and G' versus ω for only those samples in which Mz/Mw are equal to each other.Shift factors aM from η' and G' are the same.The relationship must also be right in the steady flow viscosity, and confirms that HDPE with the same Mz/Mw have equal FR.(5) The relationship between η0 and Mw is effected by the molecular weight distribution.log η0=3.5log Mw+log (Mz/Mw)-12.7