Diluting Entangled Polymers Affects Transient Hardening but Not Their Steady Elongational Viscosity

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 alignmentinduced 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 chain 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 hardening and the stretch state of the chains and its consequences on the nonuniversal behavior of melts and solutions in strong flows