Nonlinear viscoelastic properties of branched polyethylene in reversing flows

Two-step reversing flows are severe tests of constitutive equations for polymer melts and have attracted interest for testing the response of branched polymers. Here we report on single-step and three types (types B, C, and D) of reversing torsional flows on branched and linear polyethylenes (PEs). The responses were analyzed using Kaye-Bernstein-Kearsley-Zapas (K-BKZ) theory. Comparisons between data and theory in type B and C flows for the two branched PEs (LD 103 and LD 146) and a linear HDPE are presented. For the LD 146 material and to strains higher than previously examined, the results agree with prior studies, i.e., the K-BKZ theory provides a good description of the response of branched polymers in reversing flows while the opposite is the case for the linear polymers. Surprisingly, it is found that the other branched polymer (LD 103) behaves more like a linear polymer and not a branched polymer in that it does not follow the K-BKZ theory. In the type D flow, one which was not previously investigated, the K-BKZ theory describes the normal force (NF) responses for branched LD 146 but not for the linear PE. Also, isochronal derivatives of the strain potential function (W1 and W2) were calculated from torque (T) and NF responses in single-step tests. The damping function fits to a modified sigmoidal formula and is close to the BKZ limit as reported by Wagner (2004a) for branched polystyrenes. The normal stress difference ratio and damping function were extracted from W1 and W2. Finally, the Cox-Merz rule was examined for one of the branched PEs.