Two-stage shape memory behavior from cold-crystallizable polyurethane elastomer synthesized with ortho-structured methylene diphenyl diisocyanate

Rapid technological advancements have generated significant interest in smart materials, with shape-memory polyurethane (SMPU) drawing attention. However, in-depth studies on the key components of SMPU, 4,4′−methylene diphenyl diisocyanate (4,4′−MDI) and ortho-structured methylene diphenyl diisocyanate (2,2′−MDI, 2,4′−MDI), have not been undertaken. Additionally, the applications of cold crystallization phenomena have not been well studied. In this study, an increase in the ortho-MDI content was found to enhance the flexibility of molecular chains and reduce hydrogen-bonded chain stacking, thereby affecting the transition temperatures and crystallinity of SMPU. However, an excessive amount of ortho-MDI causes difficulty in forming crystalline phases, thus requiring an optimal ortho content to achieve the best shape memory performance. Therefore, a 40 % optimal ortho content was proposed for our SMPU system, which aided the cold crystallization phenomenon induced by molecular structure, thereby achieving the best shape memory performance. This two-stage SMPU is expected to be widely applicable in various fields requiring stepwise actuation.