The Proton Magnetic Resonance in Natural Rubber

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Abstract
The proton magnetic resonance has been observed in natural rubber from liquid nitrogen temperature to room temperature. The absorption line widths show two regions of change. With rising temperature, the first change in width starts at about 155°K, and we assign it to the onset of CH3 group rotation. The second, larger change to a narrow line, starts at about 225°K and is assigned to the onset of segmental motions. The latter change coincides with the second-order transitions observed in various physical properties and provides a direct confirmation of the cause of the transition. Samples cured 30, 60, and 90 minutes were investigated, and it was found that curing has a larger effect on the line width changes at lower temperature than at higher temperature. The inferred greater effect of curing upon CH3 group motions than upon segmental motions suggests that cross linking is not the primary mechanism of vulcanization. Rubber stretched 70 percent was found to have the same resonance absorption as unstretched samples.