We study and model the linear viscoelastic properties of several entangled semi-dilute and concentrated solutions of linear chains of different molar masses and at different concentrations dissolved in their oligomers. We discuss the dilution effect of the oligomers on the entangled long chains. In particular, we investigate the influence of both concentration and molar mass on the value of the effective dynamic dilution exponent determined from the level of the storage plateau at low and intermediate frequencies. We show that the experimental results can be quantitatively explained by considering the tension re-equilibration process along the chains, in agreement with van Ruymbeke et al. (Macromol., 2014), i.e. by considering that the real dilution exponent α is always equal to 1, while larger values of the dilution exponent (1 < α < 1.3) found experimentally are attributed to the enhanced relaxation of the long chain extremities. Then we discuss the influence of the polymer concentration on the terminal relaxation time of the solutions and how this can be modelled by the enhanced contour length fluctuation process (CR-CLF). We point out that this larger dilution effect is not only a function of concentration but also depends on the molar mass of the chains. While the proposed approach successfully explains the viscoelastic properties of a large number of semi-dilute solutions of polymers in their own oligomers, important discrepancies are found for semi-dilute entangled polymers in small-molecule theta or good solvents. Possible explanations for the differences between these sample sets are proposed, based on the comparison of their viscoelastic behavior.

The Doi-Edwards tube model, coupled with relaxation mechanisms such as reptation, contour length fluctuation and constraint release, allows to quantitatively predict the linear viscoelastic properties of entangled polymers. However, for non-linear elongational flows, large discrepancies between theoretical predictions based on the tube model and experimental results still persist today. This is in particular obvious for the experimentally observed strong qualitative differences in extensional flow of entangled polystyrene (PS) melts and solutions, despite having the same number of entanglements and exhibiting the same linear viscoelastic behaviour. The cause of this non-universality is often attributed either to a monomeric friction reduction, or to an interchain pressure effect.In this work, we investigate the changes in extensional flow behavior going from polymer solutions to the melt state. For this purpose, we measure with a filament stretching rheometer the non-linear extensional responses of differently long PS chains, both in the melt state and diluted in short chain matrices of the same polymer at varying concentrations. These concentrations have been chosen sufficiently high, such that the chains stay entangled. This allows us to discuss the influence of concentration and molar mass on the steady state elongational viscosity to highlight scaling relations.The purpose of the present work is to conduct well-defined experiments to further investigate how the steady extensional viscosity of polymer solutions and blends varies with the concentration and with the molecular weight of the chains

It is now established that the huge qualitative difference in flow behavior between entangled polymer melts and solutions in nonlinear elongational flows cannot be explained in the framework of the 'standard' tube model. Instead, the additional relaxation mechanism of alignment-induced friction reduction, acting primarily in melts, has shown its interesting potential to explain the experimental data. Here, we critically assess this mechanism by means of a systematic experimental investigation of the extensional response of long polystyrene chains diluted in short chains matrices of varying molar mass, varying the interaction between long chains and their molecular environment. We find that, surprisingly, all polystyrene blends exhibit different transient strain hardening properties but the same apparent steady-state elongational viscosity, i.e. the long chains reach the same final stretch state, as long as the short chain exceeds a critical molar mass of about 4 kg/mol, well below the entanglement limit, and do not significantly contribute to the strain hardening. This observation contradicts, in part, the basic assumption according to which the elongation state of a chain depends on its molecular environment, and raises new fundamental questions, in particular on the relationship between transient strain