Transient overshoot extensional rheology of long chain branched polyethylenes: experimental and numerical comparisons between filament stretching and cross-slot flow

This work analyses the high-strain extensional behaviour of long chain branched (LCB) polyethylenes, employing two novel extensional rheometer devices, the filament stretching rheometer and the cross-slot extensional rheometer. The filament stretching rheometer uses an active feedback loop to control the imposed strain rate on a filament, allowing Hencky strains of around 7 to be reached. The cross-slot extensional rheometer uses optical birefringence patterns to determine the steady state extensional viscosity from planar stagnation point flow. The two methods probe different strain-rate regimes and in this paper we demonstrate the agreement when the operating regimes overlap and explore the steady state extensional viscosity in the full strain rate regime that these two complimentary techniques offer. For long chain branched materials the cross-slot birefringence images show a double cusp pattern around the outflow centre line (named W-cusps). Using constitutive modelling of the observed transient overshoot in extension seen in the filament stretching rheometer and using finite element simulations we show that the overshoot explains the W-cusps seen in the cross-slot extensional rheometer, further confirming the agreement between the two experimental techniques.