Fabrication of “Zn²⁺ Salt-Bondings” Cross-Linked SBS‑g‑COOH/ZnO Composites: Thiol–Ene Reaction Modification of SBS, Structure, High Modulus, and Shape Memory Properties

The fabrication of SBS-g-COOH/ZnO composites cross-linked by Zn²⁺ salt-bondings was reported. First, SBS-g-COOH with different grafting ratio was synthesized through the thermally induced thiol–ene click reaction, which was corroborated by FT-IR and ¹H NMR. Next, zinc oxide (ZnO) was added to coordinate with the carboxyl groups of SBS-g-COOH to obtain the Zn²⁺ salt-bondings. TEM images, demonstrating the ion-rich domains with size of 30–50 nm, substantiated the formation of Zn²⁺ salt-bondings and also exhibited that the size and density of ion domains increased with the increasing content of ZnO. The salt-bondings effectively enhanced the mechanical properties of SBS-g-COOH/ZnO composites, achieving a maximum tensile stress of 14.24 MPa at 100% strain and a Young’s modulus of 14.19 MPa, which were almost 4.49 times and 4.35 times larger than that without ZnO, respectively. Simultaneously, the ionic networks cross-linked by salt-bondings endowed the composites with feasible shape memory behaviors that the best shape recovery ratio could be up to 97%.