Decoding the viscoelastic response of monodisperse and bidisperse linear polymers under uniaxial extension

Predicting the linear viscoelastic properties of linear polymers has reached a quantitative level, based on mesoscopic approaches such as slip-link models or the Doi-Edwards tube-model theory combined with established relaxation mechanisms such as reptation, contour length fluctuations and constraint release. A consistent molecular picture could be proposed for describing the linear regime, which allows explaining the different experimental data based on the same framework. In particular, in case of polymer chains diluted in an oligomer matrix, the universality of linear viscoelastic response of polymers has been demonstrated with different chemical structures and concentrations: as long as the chains contain the same number of entanglements, their normalized viscoelastic properties will the be same.
On the contrary, it was shown recently that the universality between polymer melts and solutions breaks down under nonlinear elongational flows: while polymer melts exhibit a monotonic extension-thinning behavior for all applied strain rates, polymer solutions with the same number of entanglements exhibit an initial thinning behavior followed by a strong extension-hardening, which occurs at rates comparable to the reciprocal Rouse time of the chains. As a result, molecular theories for linear flows cannot be extended to nonlinear flows without further considerations.
In the present work, we explore the elongation properties of systematic sets of polymer blends and oligomer solutions in order to understand the molecular origin of this non-universality and the absence of extension thickening for polymer melts. This allows us to show the importance of analyzing separately flow-induced and time-induced relaxation processes in order to reunify the behavior of polymer melts and solutions. We then show how these mechanisms can be taken into account in tube-based models, towards a general picture to describe the viscoelastic properties of entangled polymers under elongation flow.