Investigation of the molecular weight of polyethylene using rheological techniques

摘要

The zero shear viscosity (η _0), relaxation time, relaxation spectrum index (RSI) of 20 metallocene-catalyzed, and conventional polyethylenes are presented. The effect of molecular weight (M _w) determined by gel-permeation chromatography on rheology parameters was analyzed. The dependence of zero shear viscosity or relaxation time on molecular weight allowed the polyethylene samples to be separated into three different groups and different values of parameter in η _0 = K(M _w) α were obtained, respectively. Two groups of polymers display viscosity deviation from linear relationship and the reason was attributed to different level of long-chain branching and high-molecular weight tails. The intrinsic reason was disclosed when relaxation spectra at frequencies between 1 s and 10 s were involved and RSI was used for quantitative analysis. Thus, an alternative single rheological method, based on the effect of molecular structure on dynamic rheological behaviors, was proposed to evaluate the molecular weight of polyethylene.