Molecular-Level Understanding of the Network Structural Changes of Thermo-oxidatively Aged Natural Rubber Nanocomposites

A combination of different macroscopic and microscopic methods is proposed to characterize the evolution of the network topology of natural rubber nanocomposites subjected to prolonged thermo-oxidative aging at 100 °C. The distribution of the elastically active chain density and the apparent dangling chain fraction of the aged specimens were monitored using solid-state 1H homonuclear dipolar double-quantum (DQ) nuclear magnetic resonance (NMR) spectroscopy. Both the evolution of the mechanical properties determined through dynamic tensile testing and the network structural changes highlight distinct aging mechanisms depending on the compound formulation. 1H DQ experiments reveal a gradual increase of the heterogeneities of the network over the aging duration and the formation of highly crosslinked domains. These domains are promoted by an autocatalytic thermo-oxidation process and correlated to specific oxidation products such as ether groups. On the other hand, the broadening of the residual dipolar coupling distribution, P(Dres), toward low Dres values is associated with the formation of highly mobile defect fractions induced by chain scission. Finally, the complex relationships between the evolution of stress–strain data and the network structural changes over aging emphasize the lack of a model of rubber elasticity that considers the crosslink density distribution and dangling chain fraction parameters determined through 1H DQ NMR.