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

Nonlinear Extensional Rheology of Poly(n-alkyl methacrylate) Melts with a Fixed Number of Kuhn Segments and Entanglements per Chain

Molecular theories for dynamics of entangled polymers are based on both the number of Kuhn segments per entanglement N_e and the number of entanglements per chain N/N_e. Extensive studies have shown that, for polymer chains in the solutions or melts, linear viscoelasticity can be properly normalized, whereas the nonlinear extensional rheological properties cannot be normalized when N/N_e is kept the same. The failure of the latter normalization has been attributed to a difference in N_e. Nevertheless, nonlinear rheological studies are lacking for a suitable model system with fixed N_e and N/N_e. In this study, we identify poly(n-alkyl methacrylate)s with the number of carbons per alkyl group below seven as a model system. We find that the degree of the transient strain hardening during extensional flow strengthens with increasing the size of the alkyl group even when N_e and N/N_e are kept the same, which is attributable to the weaker friction reduction when the main backbones are more separated.

Uncommon nonlinear rheological phenomenology in uniaxial extension of polystyrene solutions and melts

This study examines nonlinear rheological responses to uniaxial extension of two entangled polystyrene (PS) solutions and two PS melts. Several unusual characteristics are revealed. The pair of the PS solutions have the same number of entanglements per chain (because of the same concentration) but well separated effective glass transition temperatures Tg. When examined at a common effective rate of extension (e.g., the same Rouse-Weissenberg number WiR) and at a comparable distance from their respective Tg, the solution A with lower Tg, examined at a lower temperature, shows stronger stress responses when WiR > 1. At the same test temperature and a common WiR, the solution A is still found to display a stronger stress response than the solution B that is made of the same fraction of parent PS in a second solvent also made of oligomeric PS of higher molecular weight. Finally, there are two features intrinsic to each of the four PS samples. First, at the same WiR they show reduced stress level at a lower temperature. Second, at sufficiently high applied Hencky rates, they show limiting rate behavior, i.e., undergoing the same melt rupture independent of the applied rate. These remarkable rheological responses indicate major theoretical difficulties facing the subject of nonlinear extensional rheology of entangled polymers.