Strain-induced crystallization of natural rubber as studied by high-resolution solid-state 13C NMR spectroscopy

A series of high-resolution solid-state 13C NMR experiments were performed on both unstretched and in situ stretched natural rubber samples. From the 13C CP/MAS spectra, it was found that natural rubber does form small crystals at room temperature though the degree of crystallinity is very small. Furthermore, from the 13C DD/MAS spectra, the crystalline signals were found to increase with the increase of draw ratio. 13C spin-lattice relaxation time (T1) and 1H spin–spin relaxation time (T2) of in situ stretched natural rubber were measured for the first time, which provided further evidences for the conclusion that there exist crystals in both stretched and unstretched natural rubber samples. Quantitative 13C NMR measurements indicated that strain-induced crystallization occurs when the draw ratio reaches about 2.0 and the maximum crystallinity of our natural rubber samples can be as high as 19.3% upon stretching.