Investigating the Transition between Polymer Melts and Solutions in Nonlinear Elongational Flow

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.