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 T₂ 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 = (l₀ + Δl)/l₀ < 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.