Designing a\nSelf-Healing Shape Memory Polymer with\nHigh Stiffness and Toughness: The Role of Nonuniform Chain Networks

It has long been believed that uniformly cross-linked\nnetworks\nare beneficial to enhancing the toughness of elastomers and hydrogels\nby reducing stress concentration. However, in cross-linked semicrystalline\npolymers, it is not clear yet whether the uniformly cross-linked network\nis beneficial or worse for mechanical properties, since the crystallinity\nassociated with the cross-linking network is also a key factor influencing\nmechanical properties. In this work, we designed chemically cross-linked\npolyurethane with crystalline segments either of nonuniform length\nor of uniform length. The nonuniform cross-linked networks are easier\nto crystallize and demonstrate higher strength and ductility than\nthe uniform networks. The enhanced crystallinity caused by nonuniform\ncross-linking leads to stronger mechanical properties, resulting in\na balance between stiffness and toughness. The obtained crystalline\npolyurethane plastic with nonuniform networks is biodegradable, with\na strength of 68 MPa, Young’s modulus of 178 MPa, and a toughness\nof 365 MJ/m<sup>3</sup>. Additionally, the obtained polyurethane exhibits\nshape memory-assisted self-healing of cracks with a width of millimeter\nscale, which is conducive to expanding its functionality and extending\nits lifespan. As a result, the molecular design strategy utilizing\nnonuniform long chain networks to promote crystallization in chemically\ncross-linked polymers is established, which provides a new method\nfor the design of high-performance polymers.