Origami-Inspired Fabrication: Self-Folding or Self-Unfolding of Cross-Linked-Polyimide Objects in Extremely Hot Ambience David H. Wang † and Loon-Seng Tan* Air Force Research Laboratory, Materials and Manufacturing Directorate, Functional Materials Division (AFRL/RXAS), Wright-Patterson Air Force Base, Dayton, Ohio 45433, United States * S Supporting Information ABSTRACT: 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. O rigami, the traditional art of paper folding, which embodies the process of transforming 2D sheets into 3D structures, is not only an important concept in engineering designs, it also has driven a fundamental shift in manufacturing across a broad range of length scales, as exemplified by deployable complex structures, active microelectromechanical components, biomedical devices, and so on, 1-4 as well as inspiring the innovation of 4D printing. 5 The past decade has witnessed surging research activities in applying origami concept to polymers, composites, and hybrids that possess shape memory effect (SME). 6 A rapidly advancing area of this field focuses on thermally shape-memorizing polymers that can transform from a 2D configuration to a 3D-shaped object by being (i) responsive to directed local heating, namely, locally thermoresponsive shape-memory polymers (LT-SMP), or (ii) capable of harvesting thermal energy from the ambience to channel it into a predetermined shape, namely, ambiently thermoresponsive shape-memory polymers (AT-SMP). Exam- ples of the classic “sheet to cubic box” (2D → 3D) transformation process demonstrated for LT-SMP (prestrained polystyrene) was reported by Dickey et al. at room temper- ature, 7 and AT-SMP (cross-linked poly(ε-caprolactone)- dimethacrylate) by Lendlein et al. at ambient temperatures of 25-50 °C. 8 Whereas the 2D → 3D transformation process of LT-SMP involves three types of energy, such as light/heat/ mechanical or light/electrical/mechanical transduction, only two types of energy are involved, namely, heat-to-mechanical transduction for AT-SMP; hence, efficiency for LT-SMPs is likely to be lower due to greater loss involving more transduction mechanisms. More recently, Kessler et al. reported that having two reversible phase transitions and triple-shape memory, an azobenzene-functionalized, epoxy-based liquid-crystalline (LC) elastomer was capable of folding and unfolding at 85 °C (above T g ) and at 140 °C (above T LC ), respectively, on a hot plate. 9 Aided by origami-inspired fabrication, we present here the first demonstration of both 2D → 3D and 3D → 2D shape-recovery of AT-SMP-based 3D objects comprised of lightly cross-linked polyimides at ambient temperatures in excess of 200 °C. Historically, polyimides (PIs) have found utility in many passive-type applications in the form of films, fibers, adhesives, coatings, laminates, and composites in such diverse areas as aerospace structural components, microelectronics, optoelec- tronics, nonlinear-optical devices, liquid crystal displays, and so on. 10,11 Lately, a number of T g -based, shape-memory polyimides (SM-PI), 12,13a,14,15 as well as other high-T g (>150 °C) polymers have appeared in the literature. 13b,16-18 While the dual-shaped memory processes of these SM-PIs have been evaluated by the deformation, such as bending, stretching, and twisting, as well as 3D → 2D shape recovery at temperatures near or above T g , spontaneous folding, self-unfolding, and 2D → 3D shape- Received: March 19, 2019 Accepted: April 26, 2019 Letter pubs.acs.org/macroletters Cite This: ACS Macro Lett. 2019, 8, 546-552 © XXXX American Chemical Society 546 DOI: 10.1021/acsmacrolett.9b00198 ACS Macro Lett. 2019, 8, 546-552 ACS Macro Lett. Downloaded from pubs.acs.org by UNIV OF SOUTHERN INDIANA on 04/29/19. For personal use only.