s Republic of China
Synopsis
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-l,4 content is about 96% and molecular weight distribution is rather broad. The value of a in the Mark-Houwink viscosity equation for molecular weight M , is determined as 0.70. The abnormal stress-relaxation behavior can be normalized by the introduction of a n entanglement reduction factor, e.g., M,/ M, 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, as estimated independently by yield strength method or by relaxation spectrum is equal to (5.4 f 0.2) x lo". It is confirmed by the reduced yield strength method by calculating Me from the equation Me = 3gNpRT/EeN with gN = 1.22 and then by extrapolation, where EeN is the equilibrium modulus due to entanglement.