Bioinspired Integrated Auxetic Elastomers Constructed by a Dual Dynamic Interfacial Healing Strategy

Auxetic materials are appealing due to their unique characteristics of transverse expansion while being axially stretched. Nevertheless, current auxetic materials are often produced by the introduction of diverse geometric structures through cutting or other pore-making processes, which heavily weaken their mechanical performance. Inspired by the skeleton-matrix structures in natural organisms, this study reports an integrated auxetic elastomer (IAE) composed of high-modulus cross-linked poly(urethane-urea) as a skeleton and low-modulus non-cross-linked poly(urethane-urea) as a complementary-shape matrix. Benefiting from disulfide bonds and hydrogen-bond-promoted dual dynamic interfacial healing, the resulting IAE is flat, void-free, and has no sharp soft-to-hard interface. Its fracture strength and elongation at the break are increased to 400% and 150%, respectively, of the values of corrugated re-entrant skeleton alone, while the negative Poisson's ratio (NPR) reserves within a strain range of 0%-104%. In addition, the advantageous mechanical and auxetic properties of this elastomer are further confirmed by finite element analysis. The concept of combining two dissimilar polymers into an integrated hybrid material solves the problem of the deterioration in mechanical performance of auxetic materials after subtractive manufacturing, while preserves the NPR effect in a large deformation, which provides a promising approach to robust auxetic materials for engineering applications.