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Diluting entangled polymers affects transient hardening but not their steady elongational viscosity

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It is now established that the huge qualitativedifference in flow behavior between entangled polymer meltsand solutions in nonlinear elongational flows cannot beexplained in the framework of the “standard” tube model.Instead, the additional relaxation mechanism of alignment-induced friction reduction, acting primarily in melts, hasshown its interesting potential to explain the experimentaldata. Here, we critically assess this mechanism by means of asystematic experimental investigation of the extensionalresponse of long polystyrene chains diluted in short chainmatrices of varying molar mass, varying the interactionbetween 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.
Tijdschrift: Macromolecules
ISSN: 0024-9297
Issue: 6
Volume: 52
Pagina's: 2521 - 2530
Jaar van publicatie:2019
Trefwoorden:Polymeerwetenschap