Correlation between the Structural Variations in Thiol-Based Hardeners and Properties of Thiol–Epoxy Polymers

Epoxy-based elastomers with remarkable flexibility have garnered attention due to their versatility in various applications, including sensors, smart fabrics, nanocomposites, and thermally conductive composite materials. In these studies, the selection of an appropriate hardener is crucial to imparting elastomeric properties to epoxy. Among the various available hardeners, thiol-based hardeners present notable advantages. First, they facilitate rapid curing at low temperatures via an epoxy–thiol click reaction. Second, the flexibility of the C–S bonds enables glass transition near room temperature. Despite these merits, the utilization of thiol-based hardeners in epoxy elastomer materials remains limited, primarily due to the lack of a well-established structure–property relationship between the thiol hardeners and epoxy elastomers. In this study, we carefully examined the effect of the hardener structure on the properties of thiol–epoxy polymers. While maintaining the structure of a tetrafunctional thiol as a cross-linker, we varied the structure of a bifunctional thiol to analyze its influence on the physical properties of the resulting thiol–epoxy material. Factors such as the presence or absence of an oxygen atom, the existence of a ring structure, the presence or absence of aromatics, and the geometric shape of the hardener all influenced the mechanical properties, such as modulus, stress, and toughness. Notably, the structure of the bifunctional hardener allowed precise control over the glass transition temperature of the thiol–epoxy material near room temperature, resulting in distinct mechanical behaviors (elastic or plastic) at room temperature. Based on these findings, we successfully developed a thiol–epoxy material exhibiting elastomeric behavior at room temperature and shape memory characteristics at temperatures close to room temperature (e.g., 40 °C).