Letter Cite This: ACS Macro Lett. 2019, 8, 546−552
pubs.acs.org/macroletters
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
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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 crosslinked 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 ∼ Tg and remain in the freestanding, 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.
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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 Tg) and at 140 °C (above TLC), 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, optoelectronics, nonlinear-optical devices, liquid crystal displays, and so on.10,11 Lately, a number of Tg-based, shape-memory polyimides (SM-PI),12,13a,14,15 as well as other high-Tg (>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 Tg, spontaneous folding, self-unfolding, and 2D → 3D shape-
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). Examples 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 temperature,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 © XXXX American Chemical Society
Received: March 19, 2019 Accepted: April 26, 2019
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DOI: 10.1021/acsmacrolett.9b00198 ACS Macro Lett. 2019, 8, 546−552
Letter
ACS Macro Letters
SME findings (vide infra) reinforce the belief that the thermally robust, chemical cross-links are essential for longer-term durability and reproducibility. FTIR spectra of XCP2s and LCP2 are almost identical (Figure S5), indicative of their having similar morphological character. This is confirmed by WAXS data (Figure 1a) that both XCP2s and LCP2 are amorphous, thus, eliminating the crystallinity effect on SM properties. The tensile moduli (E) of XCP2 films are higher than that of LCP2 and increase with the concentration of either triamine or trianhydride cross-linker. It appears that the bulkiness and rigidity provided by (C6H4)3P O moieties in these cross-linkers may have the reinforcing effect on the CP2 linear segments (Table 1 and Figure 1b). This is evidenced by the modulus trend that is totally opposite for a closely related XCP2 with an analogous but more compliant cross-linker, namely, 1,1,1-tris[4-(4-aminophenoxy)phenyl]ethane, in which PO is being replaced by a CH3C moiety.12 The thickness of the XCP2 films examined here is 20−100 μm. As expected, the cross-linking density increases as the crosslinker concentration increases, and the calculated average molecular weights between cross-linked sites (Mc) would follow an opposite trend (Table 1). The glass transition temperature (Tg) of LCP2 is 219 °C (DMA). Comparatively, the Tg values (220−226 °C) of the XCP2 networks increase with the crosslinker concentrations in the same trend as the modulus. The swelling tests were conducted in N,N′-dimethylacetamide to quantify the network structures. The swelling ratios decrease, but gel contents increase in accord with an increase in crosslinking density. The SM quality of the PI film strip at Tambient above 200 °C was first evaluated in a constant-temperature oil bath and with the aid of two pairs of long tweezers (Videos S1 and S2) to perform stretching action (i.e., applying stress to deform). For comparison, both LCP2 and XCP2-An5 film strips (6.4 mm × 38 mm, 0.1 mm thickness) were subject to numerous “stretching and stress-free (without applied stress) recovery” (hereafter, “SR”) cycles at 215 °C. Their lengths were measured and their appearances at room temperature were recorded by a camera. We should point out that the stretching test actually meets a tougher SM requirement than bending and twisting tests because polymer chains would irreversibly disentangle and creep much more easily at elevated temperatures during repeated S-R cycles. Thus, while the LCP2 strip showed a similar recovery as the XCP2-An5 strip (Figure 2a) in first nine S-R cycles, its length was not fully restored after the 10th cycle. The film strip became uneven and a small piece of film had broken off during the last stretching test (Figure 2b). On the other hand, covalent crosslinking enables the XCP2-An5 strip to exhibit no sign of any damage even after the 20th cycle. The 2D and 3D shape-memory profiles of LCP2 and XCP2 film strips, as determined by DMA, are depicted in Figure 2b,c and Figures S6−S12. In each stress−strain−temperature (S-ST) cycle, each film is heated to 240−260 °C and a stress is applied to deform them up to 90% strains, followed by cooling down to 180 °C and releasing the applied stress. Then, heating is resumed to reach 240−260 °C and the recovered length is measured, followed by cooling down to 180 °C at the end of each cycle. Shape memory recovery (Rr) and temporary shape fixing efficiency or fixity (Rf) ratios are calculated based on the maximum strain (εm), the residual strain (εp), and the strain after removal of the tensile stress (εu) after the Nth cycle, eqs 1 and 2, respectively:
recovering process, as well as sustainment of 3D shape at high temperatures, have not been reported. In this work, we have selected CP2, a polyimide derived from 2,2-bis(phthalic anhydride)-1,1,1,3,3,3-hexafluoroisopropane (6FDA, dianhydride) and 1,3-bis(3-aminophenoxy)benzene (APB, diamine), to prove the concept of origami-inspired fabrication because it is a well-known, well-characterized, and aerospace-qualified polymer.19 Our integrated “synthesis/film fabrication” of Tg-triggered, shape-memorizing, and cross-linked polyimides is modified from the poly(amic acid)/polyimide (PAA/PI) processing chemistry with the newly synthesized, thermally stable, triphenylphosphine oxide (PO)-based triamine (POTAm) and trianhydride (POTAn) cross-linkers. (Chart 1; Chart 1. (a) Cross-Linkers POTAm and POTAn; (b, c) Idealized Representations for Cross-Linked Polyimides XCP2-Amx and XCP2-Anx; n, m, and l are Repeat Units of Linear Segments, and the Network Is Symbolized by ∞; (d) Linear Polyimide LCP2
cross-linker syntheses are outlined in Schemes S1 and S2, and NMR spectra, depicted in Figures S1−S5 in SI, Supporting Information). To underscore the importance of chemical crosslinks as netpoints, the linear version of CP2 (LCP2)20 was also evaluated. The preparations of two versions of cross-linked CP2, (grouped as XCP2; Scheme S3), and LCP2 (Scheme S4) are described in detail in SI. The idealized structural representations of XCP2 and LCP2 are shown in Figure 1a: POTAm- and POTAn-cross-linked polyimides are designated as XCP2-Amx and XCP2-Anx, respectively, and x is the molar percentage of triamine or trianhydride used. The content of cross-linker in both series of XCP2s is 1, 2, or 5 mol %. The compositions of all the CP2 polyimides are also listed in Table 1. As depicted in Figure 1b, the triple-linked phosphine oxide (PO) moieties of both XCP2s act as covalent netpoints and collectively memorize the permanent (original) shape, whereas LCP2 relies only on polymer-chain entanglement together with polyimide’s π−π stacking and charge-transfer complexation (CTC) as physical netpoints for the SM recovery.21 Recent studies on the high temperature shape memory effect (HT-SME) indicate that linear (“thermoplastic”) polyimides possess relatively good SME, but the evaluation of these polyimides by dynamic (thermos)mechanical analysis (DMA) was conducted for only 3−4 testing cycles,13a,15,22−24 and SME was found to be subjected to certain molecular-weight threshold.24 Our HT547
DOI: 10.1021/acsmacrolett.9b00198 ACS Macro Lett. 2019, 8, 546−552
Letter
ACS Macro Letters
Figure 1. (a) WAXD plots of LCP2 and XCP2-Anx (x = 1, 2, and 5). (b) Storage modulus vs temperature plots (DMA) of linear and cross-linked CP2 polyimides. (c) Cartoon to illustrate the nature of netpoints: covalent cross-links in cross-linked polyimides and physical cross-links formed from chain entanglement/π−π stacking/charge-transfer complexation in linear polyimides; idealized structural representations of (d) a shape-memory, linear (L) polyimide (PI) containing biphenyl (BP) moieties, and (e) PI-BP cross-linked with 2 mol % trianhydride (POTAn), designated as XPI-BP-An2.
Table 1. Composition and Properties of Linear and Cross-Linked Polyimide Films sample
6FDA (mol %)
APB (mol %)
POTAm or POTAn (mol %)
Tga (°C)
Eb (GPa)
LCP2 xCP2-Am1 xCP2-Am2 xCP2-Am5 xCP2-An1 xCP2-An2 xCP2-An5 LPI-BPe
100 100 100 100 98.5 97.0 92.5 100
100 98.5 97.0 92.5 100 100 100 100
0 1 2 5 1 2 5 0
219 221 223 225 220 222 226 218
1.90 ± 0.15 2.78 ± 0.18 2.99 ± 0.23 3.25 ± 0.21 2.18 ± 0.27 2.54 ± 0.19 2.84 ± 0.32 2.08 ± 0.18
Td5%c (°C; air)
Mcd (Da)
gel content (%)
swelling ratio
526 514 511 518 519 518 518
46700 23350 9350 46800 23460 9460
84.1 95.5 98.7 91.9 94.4 97.7
7.3 3.9 3.1 5.5 4.7 3.1
Tg measured from the peak of Tan δ (DMA) as an average value taken from four measurements. bModulus determined in tension at 25 °C in average value from four specimens per sample. cTemperature at which 5% weight loss recorded on TGA (heating rate of 10 °C/min). dMc denotes the average molecular weight (MW) of linear segment between two cross-linked sites and is calculated from the equation Mc = 350.27 × Xn + Y × 2, where 350.27 is MW of a CP2 repeat unit; the number-average degree of polymerization Xn = (1 + r)/(1 − r); the stoichiometric imbalance factor (r) is the molar ratio of amine:anhydride; Y is one-third value of cross-linker’s MW (POTAm = 183.86 or POTAn = 254.86) in XCP2 network. See Odian, G. Principles of Polymerization, 4th ed.; John Wiley & Sons: Hoboken, NJ, 2004; p 75. eSee Figure 1d for polymer structure. a
R f (N ) =
R f (N ) =
εu(N ) × 100% εm(N ) εu(N ) − εp(N ) εu(N ) − εp(N − 1)
containing only 2 mol % trianhydride cross-linker dramatically improves its average Rf = 99.2% (Table S2). With higher crosslinker content, the average Rr of XCP2-Am5 film is ∼100% and its total Rr is 98.5% after 108 cycles (Table S2). Comparatively, the average Rr = 99.2% and total Rr = 74.7% for XCP2-An2 (Table S3) are lower, thus, indicating that the shape-memory effect is greatly enhanced with higher cross-linking density. The stress−strain−temperature tests further confirm that the crosslinked XCP2 films are superior to LCP2, their linear counterpart. The strains of XCP2-Am5 film expectedly increase with static force and temperature (Figure 2d). They are elastic below Tg since their strain−stress curves are first-order and follow Hooke’s Law. They deform slightly under stress below Tg. Above Tg, the films exhibit second-order viscoelastic properties. The film has been stretched up to 175% with 98.3% of SM recovery at 240 °C. Our origami-inspired fabrication method (two versions) is illustrated by the flow diagrams in Figure 3a,b for the cubic boxes made of XCP2. The key modification steps to the conventional
(1)
× 100% (2)
The results show that LCP2 film strip only survives seven cycles and is snapped at the eighth stretching (Figures 2b and S6) within the time frame similar to that of the film stretching test (SR cycles) in the 215 °C oil bath. Its strain under constant force (0.6 N) is increased from 61% to 148% from cycle 1 to cycle 7, indicating that linear chains of LCP2 cannot hold onto one another and would slip slowly under the applied stress. The average SM recovery of each cycle is only 94.7%, and the total SM recovery has already decreased to 80.2% after only 7 cycles (Table S1). On the contrary, both XCP2-An2 and XCP2-Am5 strips exhibit excellent average SM fixity (99.6%); XCP2-An2 548
DOI: 10.1021/acsmacrolett.9b00198 ACS Macro Lett. 2019, 8, 546−552
Letter
ACS Macro Letters
Figure 2. (a) SM testing was conducted for XCP2-An5 (6.4 × 38 mm, thickness 100 μm) by 20 S-R cycles of stretching the film in a 215 °C oil bath, measuring length of frozen elongated film at rt, dropping the elongated film to the 215 °C oil bath, and remeasuring the length of recovered film at rt. Two-dimensional evaluation of change of stress/strain/temperature versus time for (b) LCP2 film (∼0.1 × 6 × 12 mm) in the first 7 cycles. Film yields at the beginning of the eighth cycle and (c) XCP2-Am5 film (∼0.1 × 6 × 12 mm) for 108 cycles; a truncated plot is shown here, see Figure S11 for the full plot); (d) XCP2-Am5 films (∼0.1 × 6 × 12 mm) were stretched at various temperatures (200−250 °C) and strengths of static force (0−2 N).
Figure 3. Origami-inspired fabrication of polyimide objects and their shape memory effect: Construction of (a) a permanent-shaped cubic box via cross-linked poly(amic acid)/polyimide processing chemistry and folding (applying stress) after the poly(amic acid)/Al-substrate has been soft-baked at 100 °C (1 h), and (b) a temporary-shaped cubic box via similar poly(amic acid)/polyimide processing chemistry but folding (applying stress) after full imidization at 300 °C (1 h) has occurred; (c) a XCP2-An1 cubic box undergoes 20 cycles of temporary-shape (2D “cross” structure) and permanent one (3D structure) and is able to sustain the permanent cube shape in a 215 °C oil bath for 3days. Folding top-action photos taken at rt at 5, 7, and 10 s (see Videos S4 and S5); (d) a XCP2-An5 temporary-shaped cubic box unfolds and recovers its permanent 2D structure in a 215 °C oil bath (see Video S8 for the unfolding process); (e) Other objects with permanent shapes such as cubes, airplane, five-petal flower (see Video S7 for the “blooming” process), pyramid, and twist are fabricated from the trianhydride- or triamine-cross-linked XCP2, following the flowchart in Figure 3a.
PAA/PI film casting are (a) appropriate insertion of the folding (shape programming) step into the “thermal-imidization PAA/ PI” process; (b) the use of a removable and flexible substrate, for example, aluminum (Al pan), to hold steadily either the 3D
permanent shape (Figure 3a) or the 3D temporary shape (Figure 3b) under the thermal conditions for shape programming. Figure 3a depicts the steps to fabricating a cubic box as a permanent 3D shape, where the box (i) is obtained by manually 549
DOI: 10.1021/acsmacrolett.9b00198 ACS Macro Lett. 2019, 8, 546−552
Letter
ACS Macro Letters
linking (1−5 mol %) of XCP2 films can prevent the chains from creeping and still provides up to 175% strain with excellent recovery (Rr ∼ 100%; Tables S2−S3). Robust, complex-shaped objects are fabricated from the crosslinked polyimide using two complementary fabrication protocols, in which the timing of folding step, namely, folding after (A) or before (B) full cure (complete imidization), apparently set the strain gradient at the fold lines and its direction across the film thickness. As the bending and folding of polymer films are effected by the strain gradient across film thickness,26 we rationalize that the release of strain would occur across the film thickness and along the fold line, but in opposite directions, resulting in the observed (A) self-unfolding or (B) self-folding at 215 °C, for example, in two complementarily fabricated polyimide boxes that have the same composition and have been cured at the final stage under the same conditions. Finally, the SME superiority of cross-linked polyimides over their linear counterparts is obvious from the combined factors of low crosslinking density, thermally stable cross-linker as well as their high degrees of heat tolerance, thermomechanical resilience, and fatigue-resistance. By demonstrating the feasibility and ease of incorporating an origami programming step into the conventional process for fabrication of polyimide films, we believe that this work would broaden the horizon for the polyimide-based materials in the advanced development of active structures and devices,5 especially for harsh-environment operations.
folding the “patterned cross” cut-out comprised of (linear or cross-linked) PAA/Al that has been previously soft-baked at 100 °C, and (ii) is imidized at 300 °C. Figure 3b depicts that the XCP2 film as a permanent 2D shape is fully imidized at 300 °C, followed by being manually folded into a cube as a temporary 3D shape at 240 °C (∼Tg). Other 3D permanently shaped objects, such as pyramid, twist, paper airplane, and so on (Figure 3e), were also similarly fabricated, following the steps depicted in Figure 3a. To demonstrate the self-folding at T > 200 °C, the XCP2-An1 permanent-shaped cubic box is first manually unfolded (i.e., stress applied) in a 215 °C oil bath with a pair of long tweezers to result in the corresponding planar “cross” configuration, which is frozen and stable at room temperature. Upon immersion in the same hot oil bath, the temporary “cross” structure spontaneously (
