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.