The Bidirectional Shape Memory Effect of Polyurethane Photocrosslinked with Polycaprolactone and Hexamethylene Diisocyanate.

Shape memory polymers (SMPs) can undergo reversible shape transformations, yet most conventional one-way SMPs recover only a single programmed shape. Reported bidirectional SMPs frequently rely on complex chemistries or continuous external loads or tolerate pronounced losses in mechanical robustness, largely because microphase separation, crystallization and internal stress are difficult to regulate in an integrated fashion. Here, we propose a UV-programmed internal-stress-locking strategy to construct a crosslinked polyurethane (UV-SMPU) that simultaneously achieves high toughness and stable, stress-free bidirectional actuation. Using polycaprolactone (PCL) as the soft segment, hexamethylene diisocyanate (HDI) as the hard segment and triallyl isocyanurate (TAIC) as a photocrosslinker, in-situ UV curing under pre-stretch fixes a tunable three-dimensional network while "freezing" the microphase-separated morphology and pre-oriented internal stress. Covalent crosslinks stabilize PCL crystallites as reversible actuation domains, whereas hydrogen-bonded hard segments provide elastic restoring force; the coordinated regulation of crosslink density, crystallinity and locked-in internal stress enables efficient CIE/MIC-type transitions without compromising mechanical integrity. The optimized UV-SMPU (3 wt% TAIC, 10 min UV) exhibits excellent thermal stability, a rare strength-ductility balance (26.6 MPa tensile strength; ~1700% elongation) and robust bidirectional actuation, with reversible strain stabilizing at 15.73% after six cycles. This work offers a simple, scalable route to tough bidirectional SMPUs and furnishes mechanistic design principles for next-generation adaptive and soft-actuated materials.