Characterization of cis ‐1,4‐polyisoprene polymerized with lanthanide catalyst system (Ln‐PIR)

The PIR raw rubber samples, Ln–PIR and Ti–PIR, were subjected to molecular characterization, which shows that Ln–PIR contains microgel particles, but the least branching in its macromolecular chains; its cis ‐1,4 content is about 96% and molecular weight distribution is rather broad. The value of α in the Mark‐Houwink viscosity equation for molecular weight M η is determined as 0.70. The abnormal stress–relaxation behavior can be normalized by the introduction of an entanglement reduction factor, e.g., M η / M c for maximum relaxation time. This reduction is subsequently verified by evaluating the molecular weight dependence on bulk viscosity with 3.45th power. The starting of a yield process is equivalent to that of a disentanglement process, since both processes have approximately the same activation energy, 8 kJ/mol. For the onset of yield or of entanglement, the critical molecular weight M c as estimated independently by yield strength method or by relaxation spectrum is equal to (5.4 ± 0.2) × 10 4 . It is confirmed by the reduced yield strength method by calculating M e from the equation M e = 3 g N ρ RT / E eN with g N = 1.22 and then by extrapolation, where E eN is the equilibrium modulus due to entanglement.