Nonlinear viscoelastic response of dendritic (arborescent) polyisobutylenes in single- and reversing double-step shearing flows

Single-step and three types of reversing double-step experiments were conducted on concentrated solutions of a linear and two dendritic (hyperbranched or arborescent) polyisobutylenes (PIBs). The results in reversing double-step experiments further confirm our previous findings on polyethylene melts that the K-BKZ theory provides better predictions for long chain branched polymers than linear ones. The short chain branched material looks more like the linear material in the present experiments. Isochronal derivatives of the strain potential function (W1 and W2) were calculated from torque (T) and normal force (N) responses in single-step parallel plate experiments. The damping function was extracted from W1 and W2 for both linear and branched PIBs. The damping function of the linear PIB falls between two versions of the Doi-Edwards model. The arborescent PIB with longer branches and lower branching frequency shows weak damping behavior, which is a specific behavior for long chain branched polymers. The arborescent PIB with shorter branches, higher branching frequency and low entanglement density, on the other hand, exhibits the same damping behavior as that of the linear PIB, which is consistent with the findings in the reversing double-step experiments in which it behaves more like a linear polymer.