(PART. Ⅰ) We have studied the cis-to-trans isomerization in natural rubber vulcanizates during the thermal oxidation period using high resolution solid-state $^{13}C$ NMR spectroscopy.
In the present investigation, trans-1,4-polyisoprene contents of natural rubber vulcanizates are remarkably increased at the early stage of thermal oxidation because of the remarkable increase in the cis-to-trans isomerization. This increase in the cis-to-trans isomerization would be due to the formation of the allylic radicals through the process of thermal oxidation, additional vulcanization with free sulfur and reversion.
Trans-1,4-polyisoprene was continuously increased during the thermal oxidation period, and finally reached 3% contents in the 1,4-polyisoprene structure after 15 days. It is supposed that the amount of increased trans-1,4-polyisoprene affects the physical and mechanical properities of Natural Rubber vulcanizates constructed with high content of cis-1,4-polyisoprene (＞ 99%), because the distinguished properties of Natural Rubber vulcanizates result from the sequencial array of cis-1,4-polyisoprene.
(PART. Ⅰ) In the present paper, spin-spin relaxation times($T_2$) and dynamic viscoelastic data of natural rubber vulcanizates at various temperatures (-70℃~20℃) are determined using solid-state $^1H$ NMR Spectroscopy and Rheometrics Dynamic Spectroscopy, and then, we have correlated between two different kind of data.
The shape of peak involved in the NMR spectrum was Gaussian type at low temperature(-70℃, -65℃, -60℃, -50℃, -40℃), but It changed to Lorentzian type above -20℃ because of the increased mobility of the molecular chains.
It appears that two different types of spin-spin relaxation times are involved in a FID curve at each temperature, i.e., long $T_2$ corresponds to the rapid moecular motion and short $T_2$ corresponds to the slow moecular motion of the molecular chain region bound to the carbon black, and long $T_{2-}$ was remarkably longer than short $T_2$. This disprepancy shows that the main chains of natural rubber vulcanizates are tightly bound to the carbon black.
T_2$ considerably increased and the loss modulus in dynamic viscoelasticy was less than storage modulus above -20℃. It shows that the increase in the mobility of the molecular main chain results to the decrease in the loss modulus and tangent δ, which the ratio of loss modulus vs. storage modulus.
