Observing the Chain-Stretch Transition in a Highly Entangled Polyisoprene Melt using Transient Extensional Rheometry
by Nielsen, J.K., Hassager, O., Rasmussen, H.K. and McKinley, G.H.
We measure the viscoelastic properties of a highly entangled narrow molecular weight
polyisoprene melt with approximately 280 entanglements per chain in steady and transient
shear and in elongational flows. The storage and loss moduli of the melt are found to be well
described by the Milner and McLeish model. The relaxation modulus G(t,\gamma) is measured
using stress relaxation after a sudden shearing displacement and we experimentally determine
the Rouse time, \tau_R, by observing strain-time separability G(t,\gamma)= G(t)h(\gamma) for t > \tau_R.
The transient elongational properties are measured using three distinct instruments; the
SER universal testing platform from Xpansion Instruments, its counterpart, the EVF from
TA Instruments, and a Filament Stretching Rheometer. The kinematics obtained in each
device are sensitive to the aspect ratio of the sample and care must be taken to achieve
homogeneous deformation conditions. Especially for the SER and EVF instruments, a second
aspect ratio associated with the rectangular cross-section of the sample is also important.
We find that the initial growth in the tensile stress follows the prediction given by the Doi-
Edwards reptation model for Deborah numbers, based on the Rouse time less than about
De_R = 0.04. For De_R = 0.04 the stress difference follows more or less the Doi-Edwards
prediction in the limit of infnite stretch rates and for De_R > 0.04 the measured stresses are
well above those that can be predicted by the basic Doi-Edwards model. When De_R > 1
the stress difference also exceeds the linear viscoelastic prediction. In conjunction with this
strain-hardening response, a stabilization is obtained whereby the limiting Hencky strain
before sample rupture is markedly increased. We compare our observations in the regime
0.04 < De_R < 1 with available experiments and theories. The stabilization for De_R > 1 is
interpreted as a signature of chain stretching for elongational deformation rates faster than
the inverse Rouse time.