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) × 10⁴. It is confirmed by the reduced yield strength method by calculating M_e from the equation M_e = 3g_NρRT/E_eN with g_N = 1.22 and then by extrapolation, where E_eN is the equilibrium modulus due to entanglement.