Stereolithographic Fabrication of Cold Programmable, Photo‐Erasable Shape Memory Polymers

In contrast to conventional shape memory alloys, which are deformed at sub‐phase transition temperatures, shape memory polymers are typically deformed above the glass transition temperature and then cooled to retain the temporary shape. Here, a photopolymer system capable of sub‐transition temperature programming as well as photo‐erasure of the original, unprogrammed state is presented. The resin formulation, which includes 8‐ and 11‐membered cyclic allyl sulfide monomers, undergoes radical‐mediated ring‐opening photopolymerization to afford a semi‐crystalline, very lightly crosslinked polymer. Adding a dimethacrylate as a crosslinker to the formulation yields increased crosslink density, reduced crystallinity, and significantly enhanced strain at break from ≈20% to >60% in bulk samples. Shape memory of samples photopolymerized using visible light, characterized by a four‐step thermomechanical cycle, demonstrated shape recovery of 86.8 ± 0.9% upon heating after being cold‐programmed at room temperature to a fixed tensile strain of ≈26%. Photo‐erasure of the unprogrammed state is demonstrated by irradiating cold‐programmed samples with UV light, revealing a dramatically reduced shape recovery of 2.5 ± 0.2%. Stereolithographically‐fabricated, cold‐programmed samples similarly exhibited shape recovery and photo‐erasure, where programmed samples readily recovered their original shape upon heating, while photo‐erased samples retained their programmed shape despite being heated above the transition temperature.