Ultrasound devulcanization of unfilled natural rubber networks, studied via component molecular mobility

Abstract

Abstract 13 C NMR solids spectroscopy and transverse relaxation, and 1 H relaxation and pulsed‐gradient spin‐echo self‐diffusion measurements at 70 °C were used to study molecular and segmental mobilities in natural rubber before and after sulfur crosslinking, and after subsequent devulcanization using intense ultrasound. NMR relaxation does not clearly distinguish between entangled and crosslinked network mobility, but unentangled sol and oligomeric species are separable within the longer T 2 decay components. Ultrasound reactor settings affect the amount of extractable sol generated. Some two‐thirds of the sol is entangled, with number‐average molecular weights ( M n ) above 10 000 g mol −1 . Samples also contain near 2 wt% of inert light species ( M n < 400 g mol −1 ); ultrasound is relatively ineffective in producing additional oligomeric material. All proton mobilities increase as more sol is produced, but 13 C relaxation, reflecting intramolecular effects, indicates a slight decrease in backbone mobility. In contrast with other rubbers, in natural rubber, neither the glass transition nor the sol diffusion rate is greatly affected by the extent of ultrasound exposure. Comparisons with previous similar work of this laboratory, particularly styrene‐butadiene rubber, are useful in confirming the molecular mechanisms involved. Copyright © 2007 Society of Chemical Industry