Strong and Colorless Transparent Polyurethanes Enabled by Squaramide Units: From Flexibility to Stiffness

Extensive research has been conducted on the structure–property relationship of thermoplastic polyurethanes (TPUs) to achieve customized performance through the design of soft and hard segment structures. In this study, we introduced squaramide as a hydrogen bonding structural unit in TPUs, resulting in the synthesis of a series of TPUs with varying squaramide content (referred to as PUH). PUH exhibited a distinctive phase morphology due to the formation of unordered hydrogen bonds from the unique molecular structure of squaramide. This amorphous structure bestowed PUH with a high and consistent transparency. Depending on the squaramide content, PUH displayed different microphase separation structures, allowing for transitions between flexibility and stiffness. Higher squaramide content exhibited exceptional modulus, strength, and toughness, with a tensile strength of 48.7 MPa, Young’s modulus of 221.7 MPa, and toughness of 156.0 MJ·m–3. Conversely, a lower squaramide content, in conjunction with 1,4-butanediol, fostered hierarchical hydrogen bonds. Even with very low squaramide content, PUH still showcased high elongation at break and strength, with a tensile strength of 32.6 MPa and elongation at break of 1350%. Furthermore, the reversible and dynamic nature of the hydrogen bond cross-linked network enabled the recycling of PUH through the solution or heat-pressing method without compromising its mechanical properties. Overall, squaramide proves to be a promising hydrogen bonding structural unit that can be incorporated into polyurethane to achieve a customized performance by adjusting the squaramide content.