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

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 (Dow Affinity PL 1880), a low density polyethylene (Lupolen 1840H), and an ultrahigh molecular weight polyethylene (UHMWPE). The transient uniaxial extensional viscosity functions for the linear low density polyethylene (LLDPE) and low density polyethylene (LDPE) 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 $30s−1$ and the data show excellent agreement with the published material functions. At deformation rates greater than $0.3s−1$ a true steady state extensional viscosity is not obtained in the LDPE samples due to the onset of necking failure inthe 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 $ε̇−0.5$⁠. In addition we perform extensional step-strain measurements at small Hencky strains and demonstrate good agreement with the relaxation modulus obtained from shear rheometry. Extensional creep measurements are performed over a range of constant imposed tensile stresses and also agree well with the measured shear creep compliance. Finally, tensile stress relaxation experiments are carried out after a range of imposed Hencky strains. These tests demonstrate that following large extensional deformations the tensile stresses relax nonlinearly and also that, beyond a critical strain, the material is unstable to viscoelastic necking and rupture. Additional transient extensional stress growth measurements using highly entangled linear UHMWPE samples show greatly reduced strains to failure, that are in agreement with the predictions of the Considere theory.