Strain-Induced Accelerated Chain Dynamics in Cross-Linked Natural Rubber under Active Deformation: An In Situ Nuclear Magnetic Resonance Study

The molecular-level chain network evolution in sulfur cured natural rubber upon deformation is elucidated by the combination of in situ tensile instrument and time-domain (TD) nuclear magnetic resonance. Under active deformation, both the microscopic chain dynamics as reflected by proton T 2 and the macroscopic stress–strain curves are obtained. In addition to the strain-induced restricted chain dynamics upon deformation, an abnormal strain-induced accelerated semi-restricted chain dynamics is observed when the stretching ratio is within 2.1 < λ mac = ( l 0 + Δ l )/ l 0 < 3.5. This is consistent with an almost invariant tensile modulus E of 0.42 MPa within this range. Such turning points ( λ mac = 2.1 and 3.5) are almost independent of measurement temperature (35–85 °C), as shown by the variable-temperature tensile NMR measurements. A strain-induced heterogeneous network deformation model is thus proposed: the network chains in the low cross-linking region start to relax at the intermediate stretching ratio (2.1 < λ mac < 3.5 in the current study), while those in the highly cross-linking region continuously act as the force-bearing unit.