AbstractThe 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; itscis‐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 weightMηis determined as 0.70. The abnormal stress–relaxation behavior can be normalized by the introduction of an entanglement reduction factor, e.g.,Mη/Mcfor 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 weightMcas 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 calculatingMefrom the equationMe= 3gNρRT/EeNwithgN= 1.22 and then by extrapolation, whereEeNis the equilibrium modulus du
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