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.