Origami-Inspired Fabrication: Self-Folding or Self-Unfolding of Cross-Linked-Polyimide Objects in Extremely Hot Ambience.

A methodology that integrates a folding step into the conventional poly(amic acid)/polyimide film fabrication scheme is developed. It enables fabricating cross-linked polyimide (XCP2) films into a host of complex-shaped objects. Particularly unprecedented is that these origami (3D) objects can be unfolded into a 2D temporary shape under externally applied stress at T ∼ T g and remain in the free-standing, 2D configuration at room temperature until spontaneously returning to the original 3D configuration at T > 200 °C. This 3D/2D/3D cycle can be repeated >20× without showing any sign of fatigue, as exemplified by a cubic box that shows visually no dimensional change after each cycle, and even after having been immersed in a 215 °C oil bath for 3 days. The enabling materials are two series XCP2s that are cross-linked by either a phosphine oxide-containing triamine (POTAm) or a trianhydride (POTAn). These cross-linked polyimides form tough and creasable films that possess ∼100% shape memory recovery and 99% shape memory fixity and withstand over 100 fatigue-prone, strain-stress-temperature cycles, while the linear version LCP2 film exhibits much lower shape memory recovery and fails after only 7 cycles.