571: Unveiling the origin of unique mechanical properties of elastomers with well-defined network structure

Conventional elastomers usually suffer from heterogeneous network structures, leading to deteriorated mechanical properties. Polymer networks with a highly homogeneous structure can be synthesized by end-linking of monodisperse star polymers (star polymer network, SPN)¹. SPN elastomers exhibit exceptional mechanical performance, including high stretchability and large strain stiffening capability, achieved by minimizing defects within the network². In this study, we synthesized SPN elastomers with varying crosslinking densities to gain a deeper understanding of the origin of their unique mechanical properties. Four-arm star poly(ether-ester) precursors with different chain lengths were crosslinked via highly efficient strain-promoted azide-alkyne cycloaddition, forming a gel with a highly uniform structure. This gel was subsequently dried to produce elastomers. The elastomers demonstrated remarkable durability under repetitive deformation, exceptional stretchability (λ_break ≈ 2100%), and toughness (σ_break ≈ 18 MPa), accompanied by large strain stiffening capability. The stretchability increased with increasing the chain length between crosslinks, whereas the tensile strength showed no such correlation. The scaling analysis based on the Pincus blob theory³ revealed that this unique stretchability arose from a highly contracted conformation of network strands in the elastomers, which was caused by the gel drying process. Large strain stiffening was primarily attributed to strain-induced crystallization (SIC), as confirmed by wide-angle X-ray scattering. SIC was triggered by the uniform stretching and orientation of polymer chains under large deformation, even though the polymer network is composed and amorphous polymers with low glass transition temperature. This study expands the potential of SPN elastomers as advanced materials with superior mechanical properties.