Nonlinear viscoelastic properties of branched polymers and flow instability of entangled polymers in the cone and plate geometry

Rheological properties of long chain branched polymers, including commercial low density polyethylenes (LDPEs) and well-characterized hyperbranched polyiosbutylenes (PIBs) were studied here. The effects of chain architecture on viscoelastic properties are investigated from three perspectives. First, the validity of the K-BKZ theory for these materials was tested in reversing double-step strain flows. It was found that the K-BKZ model provides better predictions for the branched polymers than the linear ones except for one hyperbranched arborescent PIB sample. The unexpected behavior of this PIB is attributed to the short branch length due to its lower molecular weight and higher branching frequency. The results also show the viscoelastic properties of branched polymers are more sensitive to the branch length than to the branching frequency.

Second, the effects of chain architecture on the isochronal derivatives of the strain potential function (W1 and W2) were investigated. W1 and W2 were calculated from torque and normal force responses in single-step tests in parallel plate geometry according to the scaling law of Penn and Kearsley. It was found that time and strain dependent W1 of the materials studied in the current work is insensitive to the chain architecture and chemical structure. W2, however, is sensitive to the chain architecture and chemical structure.

Third, the effects of chain architecture on the shear damping behavior of these materials were also investigated. The commercial LDPE shows the weakest damping behavior and its damping function fits to the limit condition (no constraint release) of the molecular stress function proposed by Wagner. As expected, the damping function of linear PIB falls between two versions of the Doi-Edwards predictions. In addition, the arborescent PIB with long arms and low branching frequency shows weaker damping behavior than the linear one but its damping function falls below that of the commercial LDPE.

We also studied flow instability for two polymer solutions and one long chain branched polyethylene melt in the cone and plate geometry. It was found that flow instability occurs for experiments in both rate and stress-controlled modes. Our data are consistent with the dramatic shear rate jump for the flow curve constructed from the stress-controlled experiments being due to mass loss after the occurrence of severe instabilities, and these are discussed in terms of the controversy between Inn et al. (2005) and Tapadia and Wang (2003,2004). We also found that the Cox-Merz representation provides a powerful tool for investigation of flow instability. Another interesting finding is the stress overshoot seems to be related to the onset of flow instability in the investigated system.