Impact of a deformed state on natural rubber oxidation investigated by double-quantum nuclear magnetic resonance

This study demonstrates experimentally the influence of a deformation state on the oxidation process in a model of vulcanized natural rubber (NR) without reinforcing fillers. For this demonstration, relaxation tests in air at 100 °C were used to couple the oxidation process to a state of deformation. Network characterizations were carried out with and without deformation at several exposure times in air and nitrogen. In parallel to the classical network characterization by swelling, the evolution of network topology was monitored using a solid-state ¹H homonuclear dipolar double-quantum (DQ) nuclear magnetic resonance (NMR). In the undeformed state, the evolution of the network using both techniques shows a predominant chain scission process compared with crosslinking. Based on ¹H DQ NMR characterizations, these chain scission and crosslinking processes lead to a broadening of the molar mass distribution of the mechanically active chains, resulting in a heterogeneous network. Although the state of deformation does not influence the evolution of the average molar mass values between crosslinks during oxidation, the deformation state (λ = 2) evidently favors a more heterogeneous network topology through the coupling between orientation anisotropy as well as the crosslinking process.