Effects of Network Strand Length on the Unique Mechanical Properties of a Homogeneous Star Polymer Network Elastomer

Conventional elastomers tend to have heterogeneous network structures that compromise their mechanical properties. Recently, we reported that star polymer network (SPN) elastomers, which were synthesized by end-cross-linking monodisperse star polymers, exhibited exceptional mechanical performance. In this study, we aimed to elucidate the mechanism underlying the unique properties of SPN elastomers by examining the mechanical properties of samples with varying network strand lengths. The SPN elastomers were obtained by end-cross-linking tetra-arm poly(ether-ester) precursors via highly efficient strain-promoted azide–alkyne cycloaddition in solution, followed by solvent removal. The stress–strain relationship during uniaxial tensile deformation was analyzed using scaling theory. The excellent stretchability with relatively low stress was attributed to the supercoiling phenomenon, wherein the network chains adopt a strongly contracted conformation due to solvent removal during elastomer preparation. Longer network chains contracted more, resulting in a higher fractal dimension of the supercoil and a greater strain requirement to unravel it. The significant strain stiffening was attributed to strain-induced crystallization (SIC), as confirmed by in situ X-ray scattering analyses. SIC was triggered by the uniform stretching of polymer chains under large deformation. These findings highlight the mechanism behind the unique mechanical properties of SPN elastomers and provide quantitative insights into the impact of network strand length as a design parameter to tailor their mechanical performance.