Measuring the transient extensional rheology of polyethylene melts using the SER universal testing platform

Abstract

We use a new extensional rheology test fixture that has been developed for conventional torsional rheometers to measure the transient extensional stress growth in a number of different molten polyethylene samples including a linear low density polyethylene sDow Affinity PL 1880d, a low density polyethylene sLupolen 1840Hd, and an ultrahigh molecular weight polyethylene sUHMWPEd. The transient uniaxial extensional viscosity functions for the linear low density polyethylene sLLDPEd and low density polyethylene sLDPEd samples have both been reported previously in the literature using well-established instruments and this allows us to benchmark the performance of the new test fixture. Transient stress growth experiments are carried out over a range of Hencky strain rates from 0.003 to 30 s−1 and the data show excellent agreement with the published material functions. At deformation rates greater than 0.3 s−1 a true steady state extensional viscosity is not obtained in the LDPE samples due to the onset of necking failure in the elongating strips of polymer; however, the limiting values of the transient extensional viscosity at the onset of sample failure agree well with previously published values for the steady state extensional viscosity. This apparent steady-state extensional viscosity first increases with deformation rate before ultimately decreasing as approximately «̇−0.5. In addition we perform