  The viscoelastic relaxation time of a polymer solution is often measured
using Capillary Breakup Extensional Rheometry (CaBER) where a droplet is placed
between two plates which are pulled apart to form a thinning filament. For a
slow plate retraction protocol, required to avoid inertio-capillary
oscillations for low-viscosity liquids, we show experimentally that the CaBER
relaxation time $\tau_e$ inferred from the exponential thinning regime is in
fact an apparent relaxation time that may increase significantly when
increasing the plate diameter and the droplet volume. Similarly, we observe
that $\tau_e$ increases with the plate diameter for the classical step-strain
plate separation protocol of a commercial (Haake) CaBER device and increases
with the nozzle diameter for a Dripping-onto-Substrate (DoS) method. This
dependence on the flow history before the formation of the viscoelastic
filament is in contradiction with polymer models such as Oldroyd-B that predict
a filament thinning rate $1/3\tau$ ($\tau$ being the model's relaxation time)
which is a material property independent of geometrical factors. We show that
this is not due to artefacts such as solvent evaporation or polymer degradation
and that it can only be rationalised by finite extensibility effects (FENE-P
model) for a dilute polymer solution in a viscous solvent, but not for
semi-dilute solutions in a low-viscosity solvent.
