Investigation of elongation-induced structural changes of vulcanized butadiene rubber by AFM nanomechanics

This study investigates how the crosslinked structure of vulcanized butadiene rubber (BR) changes under uniaxial elongation using atomic force microscopy (AFM) nanomechanics. Crosslinked polydimethylsiloxane (PDMS) served as a model system to clarify the physical meaning of the AFM-derived elastic modulus obtained under elongation. In the PDMS, the increase in the modulus closely followed the macroscopic true stress. In contrast, BR deviated from this relationship, indicating that the elastic modulus changes during elongation. AFM results for BR revealed that the modulus decreased as the elongation ratio increased, which is consistent with reduced topological constraints due to the dissolution of entanglement-derived physical crosslinks. Time-domain nuclear magnetic resonance (TD-NMR) measurements of elongated samples supported this interpretation by revealing corresponding changes in the inverse spin–spin relaxation time. Together, these results demonstrate that AFM nanomechanics can be used to evaluate inhomogeneity in crosslinked structures and quantify the contribution of entanglements. This capability enables experimental estimation of the local elastic modulus at arbitrary elongation ratios.