Solid-State ¹³C NMR Characterization of Irradiation-Cross-Linked Natural Rubber

Abstract

Solid-state 13C NMR was used to characterize the structure changes produced in gamma irradiation in natural rubber. The structure of the irradiated natural rubber was found to be heterogeneous, consisting of a mobile phase and a semi-rigid phase. Cis-trans chain isomerization was detected in the mobile phase of the gamma-irradiated samples. The CP/MAS spectra of the more rigid phase contained resonances due to quaternary carbons arising from cross-links and vinyl end groups caused by main chain scission. The calculated values of β and G( X) agreed with earlier published results. Peroxide-cured natural rubber of an equivalent amount of network density showed a higher amount of trans structure than did the gamma-irradiated natural rubber.

Radiation degradation of epoxidized natural rubber studied by solid‐state nuclear magnetic resonance and infrared spectroscopy

“…These techniques are sensitive to uncrosslinked and mobile regions which have relatively short 13 C spin–lattice relaxation times, but not to the rigid structures associated with the crosslinks. It was therefore necessary to use solid‐state NMR with cross polarization to obtain an adequate S/N ratio and to detect the formation of crosslinks and other structures after irradiation, as observed for other diene polymers 14–17…”

“…Diene elastomers including polybutadiene,14 polyisoprene (natural rubber),15 butadiene acrylonitrile copolymer (nitrile rubber)16 and polychloroprene (chloroprene rubber)17 are known to undergo crosslinking during exposure to high‐energy radiation. NMR spectroscopy indicated the presence of crosslink clusters in polybutadiene, polyisoprene and butadiene–acrylonitrile copolymer, whilst no such clusters were found in polychloroprene.…”

Review: Application of Irradiation Technology in Natural Rubber Vulcanization Process

“…Crosslinking is based on a radical mechanism, either by radiation or peroxide. The degree of isomerization of peroxide crosslinked natural rubber by the cis-trans chain is greater than that of radiation crosslinked natural rubber at the same crosslink density level [40]. As shown in Figure 9, the tensile strength of radiation vulcanizate is much smaller than peroxide vulcanizate.…”

Spectroscopies I have known?Award address: Philips ACS Award in Applied Polymer Science

“…One of the advantages of solid-state NMR is the capability of studying insoluble systems such as networks 83,84 formed by polymerization and postpolymerization (vulcanization). 85 The sensitivity of the solid-state NMR technique to the chemical structure of vulcanized elastomers led to a series of studies which helped define the structure of a number of different vulcanized elastomers. 86 -100 In Figure 4, the solid-state NMR spectrum in the aliphatic region of the GHPD-MAS 13 C-NMR spectrum at 75.5 MHz of a high-cis-butadiene rubber vulcanizate cured with 10 phr of sulfur at 150°C for 30 min is shown.…”