Thermoreversible Folding as a Route to the Unique Shape-Memory

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Applications of Polymer, Composite, and Coating Materials

Thermoreversible Folding as a Route to Unique Shape Memory Character in Ductile Polymer Networks Matthew K Mcbride, Maciej Podgórski, Shunsuke Chatani, Brady Worrell, and Christopher N. Bowman ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06004 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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ACS Applied Materials & Interfaces

Thermoreversible Folding as a Route to Unique Shape Memory Character in Ductile Polymer Networks Matthew K. McBride, Maciej Podgorski†, Shunsuke Chatani, Brady T. Worrell, and Christopher N. Bowman*. Department of Chemical and Biological Engineering, University of Colorado Boulder, 596 UCB, Boulder CO 80309 KEYWORDS: shape memory polymers; origami; folding; polymer networks; deployable polymers

ABSTRACT: Ductile, crosslinked films were folded as a means to program temporary shapes without the need for complex heating cycles or specialized equipment. Certain crosslinked polymer networks, formed here with the thiol-isocyanate reaction, possessed the ability to be pseudo plastically deformed below the glass transition, and the original shape was recovered during heating through the glass transition. In order to circumvent the large forces required to plastically deform a glassy polymer network, we have utilized folding, which localizes the deformation in small creases, and achieved large dimensional changes with simple programming procedures. In addition to dimension changes, 3D objects such as swans and airplanes were

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developed to demonstrate applying origami principles to shape memory. We explored the fundamental mechanical properties that are required to fold polymer sheets and observed that a yield point that does not correspond to catastrophic failure is required. Unfolding occurred during heating through the glass transition indicating vitrification of the network maintained the temporary, folded shape. Folding was demonstrated as a powerful tool to simply and effectively program ductile shape memory polymers without the need for thermal cycling.

Introduction Although art and science are frequently dogmatically viewed as non-overlapping disciplines, often-common forms of art lend inspiration to the hard sciences, such as engineering, leading to simple and elegant innovations. One such art that has effectively made its way into science is origami, or the art of folding, which dates back to the 16th century. It has been used to improve efficiency in packaging1, to create new objects from universal folding patterns2, and to enable unique and complex mechanical properties and stress-strain responses 3. Here, origami, or more broadly folding, is used as a strategy to create constitutionally rich temporary shapes that are subsequently recovered to the original shape after heating. Polymers with this general ability are known as shape memory polymers (SMPs).

SMPs are a class of smart materials that

transitionally lock in a temporary shape and subsequently recover a permanent form on demand following the application of a stimulus such as heat or light to the material (Figure 1). In prior work in the field, programming the temporary shape in SMP materials generally has required heating the polymer above a thermal transition such as a phase transition or the glass transition, followed by rapid cooling in the deformed state to “fix” the new architecture as the transient shape. Alternatively, the approach implemented here eliminates the need for these cumbersome


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programming procedures (heating/cooling cycles) by utilizing a different class of more defined deformations, effectively leaning on the established principles of origami to transform a 2D object into a 3D object. When desired, the original 2D structure is recovered simply by heating. In this manner, the simplicity and versatility of folding is advantageously implemented, demonstrating an ability to program polymer sheets, much like a sheet of paper, into 3D structures ranging from a simple origami crane to a functional spring that is used to perform mechanical work upon heating and unfolding. Here, the original shape recovery and/or the mechanical work desired are seamlessly achieved by mild heating.

Furthermore, the

fundamental mechanical performance of both the folded and unfolded polymers as well as the folding process were explored. Generally speaking, SMPs are valuable shape shifting materials that have applications as deployable structures in biomaterials and aerospace4, among others. A significant amount of work in such fields has been focused on engineering SMPs with unique properties such as light activation5, high glass transition temperatures6, plasticity7, multishape8, and biocompatibility9. In the classical approach, programming requires heating above or close to the transition temperature, which is not desirable for applications that are heat sensitive or require high transition temperatures. Others have used unique approaches that remove the need to heat, like using pre-stressed sheets10 or photo-reversible chemistries11. The versatility of sub-transition programming techniques is limited by the need to form the transient shape and configuration without causing mechanical failure of the material construct. In particular, crosslinked polymers have the ability to be programmed below the thermal transition if recoverable yielding occurs before failure; however, programming via yielding polymers requires high stress and specialized equipment leading this phenomena, termed


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reversible plasticity12, to be used only in certain materials like foams13 and self-healing12, 14. In a similar manner, folding localizes the deformation and is readily done by unskilled practitioners without the requirements of special equipment or high stress. Furthermore, folding represents a versatile approach that leads to an almost unlimited number of possible temporary shapes when origami concepts are employed. Results and Discussion Origami inspired folding of a simple 2D polymer sheet readily enables the formation of a wide variety of objects for manifold uses. Figure 1B demonstrates this concept in practice, wherein a polymer sheet was folded into a desirable shape (Swan) and unfolded by mild heating through several cycles. This simple demonstration shows the complexity of 3D shapes available by folding.

Programming temporary objects similar to the swan is difficult using traditional

programming techniques because they require heating to enable a more deformable material. Additionally, the folding approach and the breadth of materials that are able to be folded make it simple and accessible to unskilled practitioners to program SMPs.

Additionally, the

supplementary materials includes a video of a folded object unfolding when immersed in a hot water bath that takes them above their glass transition temperature (Supplementary Movie 1). Mechanistically, the schematic in Figure 2B depicts a simplistic view of the strain profile during the folding process. If the material is ductile, then it has the ability to pseudo-plastically deform and trap the fold in an anisotropic state with the entropic elasticity overcome by vitrification. Since the deformation is localized, no special equipment to deform the materials is required as the necessary force is readily achieved by hand, as with paper folding in origami creations, if needed.


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Figure 1. A) A traditional shape memory cycle begins with a permanent shape that is heated above a thermal transition (red denotes heated and blue denotes room temperature) and deformed into a programmed shape.

Subsequent thermal quenching below the thermal transition

kinetically traps the deformed state as the temporary shape. Subsequent heating releases the temporary shape and recovers the permanent shape. B) A 0.1 mm thick flat polymer sheet is folded into an origami swan that unfolds upon heating above the glass transition temperature of the polymer sheet. This process was reversible for multiple fold-heat cycles. C) Using folding as the programming mechanism, a single piece of polymer sheet (1) was folded into multiple 3D objects including a coin purse (2), fortune-teller (3), swan (4), and airplane (5) as a result of multiple fold-heat cycles. Each object was heated to unfold, returning the polymer to the original flat sheet (1) prior to refolding into the next object.


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When employed in appropriate materials, folding was a fully reversible process because heating erased the stored deformation, thus returning the sheet to its original 2D state (Figure 1B). To demonstrate this reversibility, a 0.l mm thick polymer sheet was first folded into a coin purse then unfolded with heat to erase the folding pattern. The process was repeated 3 more times by folding a coin fortune teller, swan, and airplane, each temporary shape being erased by heating (Figure 1C) prior to refolding into the subsequent shape. After each folding, no significant damage to the polymer sheet was qualitatively observed.

This approach is

fundamentally different, with significant advantages, as compared to how traditional paper folding occurs because paper folding permanently damages the material, making reusing it difficult or impossible and preventing spontaneous recovery of its 2D shape. By localizing the deformation into folds, the crosslinked materials used here were able to pseudo-plastically deform enough to accommodate this strain while preserving the ability to return fully to their original shape.


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Figure 2. A) The shape fixity of folding was quantified by measuring the angle of the fold over 5 fold-heat cycles. A perfect fold (180°) would be represented by 100% shape fixity while 0% represents completely unfolded. All samples returned to the unfolded state with no residual fold after heating implying that shape recovery was quantitative. B) Schematic showing folding and the expected effects on the polymer network with extension on the outside and compression on the inside. This process localizes the deformation into folds which are thermally reversible as long as failure does not occur during folding. In a single fold on a sample, the shape fixity and recovery were quantified by measuring the angle of consistently produced folds (Figure 2). Pressing a 6 mm wide, 0.1 mm thick polymer between two glass slides with approximately equal pressure between samples produced the folded samples that were then imaged and analyzed for the folding angle. Elastic recovery was


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observed over the course of 5 minutes, thus images were taken after some time allowing this process to occur. The shape fixity remained consistent for each fold-heat cycle, reaching around 86% corresponding to a 25°. Figure 2B illustrates the proposed mechanism behind folding where force put in the fold deforms the polymer network without causing the polymer to fail. After releasing the force, the fold elastically recovers to some extent, and the remaining strain is fixed by vitrification of the polymer chains. After folding, the samples were heated above the glass transition temperature, thereby causing the sample to recover the flat shape quantitatively each time. It is important to note that folding these materials by hand was an inherently inconsistent process. The folds required to make more complex object were done with varying pressure, technique, and surrounding folds; all of these factors will dictate shape fixity and recovery. Beyond creating 3D objects, folding was used to maximize deformation and drastically change the dimensions of objects as needed for packaging or actuation. A folded object in Figure 3A was readily compacted to 8 times its deployed length. This object consists of two strips of polymer, oriented perpendicular to each other, alternately folded over and finally fastened together at the ends. This folding pattern orients the unfolding action in one dimension without significant contraction in the other dimensions, essentially leading to a one-dimensional, springlike expansion that is capable of doing work during its expansion. The miura fold (Figure 3B) was another depiction of dimensional changes driven by folding. Additionally, other structures can be or have been developed to facilitate complex actuation including bistability, adhesion, surface roughness, optical modulation, work and simple expansion. As one example, stored energy in a folded “spring” (akin to that in Figure 3A) was used to push a weight as seen in the supplementary information (Supplementary Movie 2). The folded spring is calculated to do at


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least approximately 400 J/kg of specific work when pushing a 1-gram weight upwards, ignoring any losses due to friction. This value is referenced relative to the typical specific work of mammalian muscle tissue of 8000 J/kg15. Without using folding, heat and special machinery would be required to compress a bulk polymer, though even if performed, generally, dimensional changes of this extent would be difficult to achieve in conventional SMPs.

The stress

development was further quantified by performing a force recovery test (Figure 4C). A folded sample was fixed between the clamps of a dynamic mechanical analyzer (DMA) and heated at 5°C/min while maintaining a constant gap. During heating, the unfolding sample applied a force in a similar manner as the spring to produced forces to lift the weight. This single fold produces approximately 9 mN of force through unfolding in this particular dimension.

Thus, this

demonstration shows the potential of using folding to store and release mechanical work for on demand actuation that could be achieved through direct heating (as here), resistive heating, photothermal heating or radiofrequency-induced heating to enable enhanced control of the actuation and work. Folding is not limited to the permanent shapes being just 2D sheets as arbitrary folded structures are also readily achieved as the permanent shape. As shown in Figure 3B, a folded box was formed as the permanent shape by using a similar polymer; however, one that also embodies previously reported approaches that have been shown to enable stress relaxation through the inclusion of photo activated dynamic covalent chemistry16. After making a 2D sheet of this material, the material was folded into a box and held in the shape of a box while exposing to light to activate the addition-fragmentation-based dynamic covalent chemistry. The activation of the DCC makes this box shape the stable, permanent shape. The box is then cooled to ambient temperature, below the glass transition, and unfolded in a manner analogous (though


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opposite) to the folding used to form the other structures. This unfolding now deforms the polymer in a manner that preserves the 2D sheet structure until the material is heated above the Tg. Upon heating to the rubbery regime, the box is then recovered from the sheet as its permanent shape. Activation of this refolding process by heating is illustrated in Figure 2C and in Supplementary Movie 3.

Figure 3. A) A folded spring that extends 8 times its folded length when heated. This object was created by alternately folding two strips over each other and attaching the ends together. B) A 0.1 mm thick sheet was folded (1) into a miura fold and subsequently unfolded by heating above the glass transition (60°C) of the polymer (2). The taped orange panels added to aid visualization and folding did not appear to alter the unfolding process. C) The process was also capable of folding from a 2D shape to a 3D shape by making the permanent shape the folded shape. To make the 3D object the permanent shape, a flat sheet was remolded into a box using


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dynamic covalent chemistry as previously reported. This box was folded into a flat shape that was recovered after heating. Erasing of the folds was found to coincide with the glass transition of the material. For the most successful material explored here (TEMPIC-HMDI) and described in further detail below, a glass transition of 60°C was measured as the peak of the tan delta in DMA. Repeated cycles of the folding and unfolding process did not significantly change the glass transition or the rubbery modulus of the polymer (Figure 4A). The unfolding dynamics were monitored by applying minimal force in tension on a folded polymer sheet during heating to measure the corresponding change in the length towards the unfolded state (Figure 4B). Most of the unfolding process occurred in a narrow temperature range (over a 14 ± 4°C range) around the glass transition (60°C) as the material softened and elastic entropy dominated. While the programming step was completely different, this recovery behavior was nearly identical to the recovery of traditional SMPs programmed by heating above the glass transition temperature as the locking mechanism.


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Figure 4. A) Temperature-dependent storage modulus for pristine (-) and recovered folded samples (--). The storage modulus was measured at 1 Hz and 3°C min-1 heating. B) Unfolding dynamics show the recovery process coincides with the decrease in the storage modulus during heating. The sample was loaded in a “T” shaped fold with minimal force and heated at 3°C min1

with 100% recovery corresponding to a completely unfolded state. C) Force recovery test of a

single fold. A pre-folded film was fixed to the bottom clamp with the top clamp positioned directly above it and heated at 5°C min-1 while holding a constant gap. Further, the recovery of the fold coincided with disappearance of stored anisotropy as seen in birefringence measurements. Figure 5A shows the fold during heating through the view of a cross-polarized microscope where chain alignment is visualized as a bright region due to the axis-dependent refractive index. Unstretched areas appear black at all rotations relative to the polarizing axis; however, stretched areas are visible at 45° angles.

Thus, one observes

birefringence at a 45° angle while rotation to 0° or 90° leads to a severe dimming of the observed birefringence, confirming that the polymer chains are aligned perpendicular to the fold. Heating releases the entropically unfavorable birefringence around the glass transition, and the visibly wrinkled microstructure associated with the fold disappears when viewed with unpolarized light after the folded region is heated and recovers. Although there is residual, minimal evidence of the fold after heating in Figure 5C, the material has fully recovered, and the visual effect is from the thermal cycling without complete freedom of movement.


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Figure 5.

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Polarized optical microscopy images showing birefringence of the fold and its

recovery. A) Polarized optical microscopy (POM) of the birefringence in the fold at a 45° angle to the polarization plane at 20°C, 40°C, 60°C, and 80°C. B) Bright field (unpolarized) image of the fold prior to heating and C) after heating followed by cooling to ambient temperature. Thermoreversible folding requires ductile polymer networks that have a permanent network structure that is plastically deformable. The ductility of a polymer network is dependent on the chemical structure, thermal processing, aging, and crosslinking density, among other characteristics. Secondary molecular interactions, such as those arising from hydrogen bonding in urethanes, provide transient interactions that can be mechanically disrupted and subsequently reformed without causing irreversible damage to the polymer structure. Here, crosslinked polymer networks formed using the thiol-isocyanate reaction were chosen to demonstrate reversible folding because this reaction produces tough, ductile polymers with superambient glass transition temperatures due to extensive hydrogen bonding, employs readily available, commercial monomers, and is a reaction which falls under the “click” chemistry paradigm with rapid reaction kinetics, proceeding to high conversions and requiring limited special handling. This type of shape memory polymer relies on the glass transition and vitrification of the polymer chains to maintain the temporary shape. Other types of SMPs that rely on order, semi crystalline and liquid crystalline networks, are expected to exhibit similar behavior; however, the


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mechanism for holding the temporary shape in place is ordering of the polymer chains. Using a thiol-isocyanate based polymer network, as done here, enables facile tuning of the mechanical properties and ductility through readily available monomers and their structural characteristics such as functionality and isocyanate equivalent weight. Two formulations were chosen with differing glass transition temperatures and mechanical behavior. Specifically, two different thiol monomers, a tri-thiol (TEMPIC-HMDI) and tetra-thiol (PETMP-HMDI), were chosen to provide multiple examples of polymers that are thermoreversibly foldable. PETMP is a tetra-thiol that results in networks with higher crosslinking density compared to the TEMPIC tri-thiol monomer. These two polymers have different degrees of ductility that facilitate varying degrees of thermoreversible folding. Ductile polymers have distinctive yield points that lead to strain softening where the often large deformations that occur are not recoverable except in the case of crosslinked polymers where the strain is recoverable under some circumstances upon heating. In addition to the DMA test mentioned above, tensile tests were used to analyze the mechanical integrity of TEMPIC-HMDI and PETMP-HMDI networks. Folded polymer samples showed similar Young’s moduli to those, which were never folded. However, upon running several trials we discovered that 60% of the polymers formed from a PETMP-HMDI formulation (or 3 out of 5 trials) broke at the fold as compared to none for polymers formed from the TEMPIC-HMDI formulation. The higher failure rate at the fold indicates that folding does cause damage to the PETMP-HMDI polymers most likely attributed to the lower strain at break. In addition, the ductility of the polymer dictates the maximum thickness of a foldable sheet. Thinner sheets require less strain to fold than thicker sheets, accordingly requiring less ductile materials. The thickness predominately used here, 0.1 mm, enabled reversible folding in both formulations;


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however, thicker sheets could not be as reliably folded without breaking. Very thin sheets of even non-ductile polymers are foldable; however, very little recovery force is available from such thing sheets, and the ability of the film to stand-alone is reduced. In thicker sheets, sufficient ductility is required to both enable and fix the strains that accompany folding. The strains at break in tension for the two polymers studied here are 25 ± 7 % for PETMP-HMDI and 66 ± 4% for TEMPIC-HMDI (Figure 6). Accordingly, the more highly crosslinked polymer, PETMP-HDMI, has a lower strain at break and is more likely to fail when folded, as observed. As our system employs carbamates as the crosslinking unit, the reversible folding is hypothesized to be a result of the breaking of secondary interactions such as hydrogen bonding as well as chain slippage; however, reversible plasticity has been reported in many cases like crosslinked poly(t-butylacrylate)

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and highly entangled polycarbonate18. We have also noted

this behavior in previously reported Copper Catalyzed Azide Alkyne Cycloaddition (CuAAC) shape memory polymers which contain significant amounts hydrogen bonding similar to networks employed here.19


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Figure 6. Representative strain to failure experiments are shown for TEMPIC-HMDI (--) and PETMP-HMDI (-). Dogbone shaped samples were strained at a constant strain rate of 13%/min (2 mm/min). Physical aging is one factor that leads to embrittlement of polymers; thereby decreasing the extent to which the thiol-isocyanate polymers used here can be folded. The strain at break for PETMP-HMDI reduces to 9 ± 0.3% if it is aged at 70°C for 12 hours without being rejuvenated by heating above the glass transition. The failure point roughly coincides with the yield point and the material breaks immediately when 0.1 mm sheets were folded. Embrittlement decreases the ability of the polymer network to plastically deform without failure. In the case of aging, aged PETMP-HMDI can be heated above the glass transition and cooled back to room temperature where it will regain its ductility until it is aged again. This knowledge is useful because it hints that excess free volume trapped during cooling aids the sub glass transition folding process. As such, the free volume that enables an otherwise brittle material to transiently behave in a ductile manner is able to be introduced with a thermal processing program. Heating followed by rapid cooling of a brittle material may enable folding, temporarily, in materials with appropriate mechanical properties and sufficiently slow physical aging behavior. With this in mind, other polymers, traditionally not considered ductile, are potentially processable in a manner that allow them to be transiently ductile until aging limits this behavior. Conclusion This work reveals a facile and versatile approach to program ductile shape memory polymers by using folding. Folding represents a simple means to maximize the change in overall dimensions from a small, localized deformation that is performed without any need for specialized equipment. Applying origami-folding patterns transforms a flat sheet into any of a


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vast number of complex objects that are subsequently autonomously unfolded. Thermoreversible foldable polymers can be designed for deployable antennae, stents, solar arrays, and packageable structures, among others. Additionally, the application of thermo-reversible folding would aid in the design of structures that are able to fold in a certain manner when impacted and be recovered by heating. This approach simplifies programming of SMPs, turning a flat sheet into a 3D object that can be stored, deployed, and reprogrammed over many cycles. Materials and Methods Materials Pentaerythritol

tetra(3-mercaptopropionate)

(PETMP),

hexamethylene

diisocyanate,

triphenylphosphine (TPP), divinyl sulfone (DVS), 3-Chloro-2-chloromethyl-1-propene, 2mercaptoethanol,

1,8-Diazabicyclo[5.4.0]undec-7-ene

(DBU),

2,2-Dimethoxy-2-

phenylacetophenone, Dibutyltin dilaurate and methanesulfonic acid (MsOH) were purchased from Sigma-Aldrich. Tris[2-(3-mercaptopropionyloxy)ethyl]isocyanurate, (TEMPIC) was received from Bruno Bock. Desmodur N3900 was received from Covestro. All chemicals were used as received. Sample Preparation Polymer sheets were polymerized between two Rain-X coated glass plates with 0.1 mm spacers to dictate the thickness using an approach previously reported20. The thiol was mixed with 2wt% TPP and 0.4wt% MsOH. HMDI was added in stoichiometric quantities. 0.05 EQ of DVS was added to start the “time clock” reaction and the resin was transferred to the glass slides and annealed at 70°C overnight. Thermal cycling during dynamic mechanical analysis indicated no further post cure.


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Mechanical Characterization Dynamic mechanical test were performed in a TAQ800 in tension. Rectangular samples with dimensions of 5 x 0.1 x 13 mm were used for dynamic mechanical analysis with a frequency of 1 Hz and a heating rate of 3°C/minute. Monitoring of unfolding polymer was performed by folding by hand a 360° fold in between two 90° folds (Figure 3) and applying a static force of 0.001 N while ramping the temperature at 3°C/minute. The initial length of the folded polymer at room temperature is defined as 0% and fully unfolded at 100°C is 100%. The force recovery test was monitored by pre-folding a 7 x 18 mm rectangular (0.1 mm thick) polymer between two glass slides. This folded sample was fixed to the bottom clamp of a TA RSA-G2 DMA and heated at 5°C/min from 30°C to 80°C. The force was zeroed directly before heating. The graph shown represents the average of three separate runs on the same sample. Measurement of Shape Fixity Shape fixity was measured by folding a 6 x 18 x 0.1 mm sample between two glass slide with 500 micron spacers set between the slides. The sample was folded and pressure was applied by hand for 10 seconds. The sample was then removed and allowed to elastically recover for 5 minutes before an image was taken with a Panasonic Lumix camera. The angle of the fold was analyzed with ImageJ (NIH). The data represent the average of 5 separate samples over 5 cycles and the 95% confidence intervals are shown as the error bars. To reset the fold, the samples were placed on a hotplate set to 80°C for 1 min then allowed to cool for 2 minutes. All samples recovered to a completely flat shape each time indicating shape recovery was nearly quantitative. Shape fixity was defined by the equation below defining a completely unfolded film to be 0% and a perfectly folded film to be 100%. Shape fixity = (180°-measured angle)/180°*100%


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Strain to Failure Tensile Test were performed on Test Resource MTS with sample sizes of 5 × 15 × 0.25 mm dog bone shaped samples with a strain rate of 2 mm/minute and 5 kN load cell. Polymers were heated to 100°C in an oven for 15 min then removed and allowed to cool to erase any thermal history. The aged samples were not thermally cycled before testing and were used as is after curing overnight at 70°C. Folding reliability studies were performed on 0.1 mm thick samples. Dog bone shaped samples were folded approximately in the middle and the fold was marked. The polymer was unfolded and heated to 100°C between glass plates. The unfolded polymer was removed from the oven, allowed to cool, and tested. Microscopy The recovery of the polymer was observed under microscopy on a Nikon eclipse with a Linkam heating stage. The polymer was folded by hand and subsequently unfolded and placed between two glass slides. The polymer was placed on the stage at a 45° angle to the crosspolarizers and heated a 10°C/min to 100°C. Folding Demonstrations All folding was performed by hand at room temperature. Folds were erased using a heat gun with gentle heating or submerged in hot water. If heating occurred too quickly, permanent damage to the sheet was observed most likely due to the reduced strain at failure at temperatures well above the glass transition. Molding of Non-Flat Permanent Shape A glassy network containing allyl sulfides was developed using a previously synthesized allyl sulfide containing diol and a commercially available tri-isocyanate (Desmodur N3900). Allyl


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sulfides, when initiated with photoinitiator, participated in a radical mediated, cascading bond exchange that turns the otherwise statically crosslinked, urethane network into a transiently remoldable material. The allyl sulfide containing monomer difunctional alcohol, 2-[[2-[[(2hydroxyethyl)thio]methyl]-2-propen-1-yl]thio]-ethanol, was synthesis according to previously reported procedures21. To make the remoldable sheet, the allyl sulfide diol was mixed in a stoichiometric ratio with Desmodur N3900 with 10 wt% acetone, 0.5 wt% DMPA photoinitiator, and 0.5 wt% dibutyl tin dilaurate. The film was then allowed to cure between glass slides for 2 hours. After removal from the glass slides, the acetone was removed by heating at 100°C for 1 hour. Remolding of the flat sheet into a box was done by gently heating the sheet fixed in the folded position to 70°C and irradiated with 200 mW cm-1 UV light for 3 min resulting in the permanent shape being fixed as the box. The box was then refolded into a flat temporary shape by hand and thermally recovered after heating as shown in supplementary movie 3.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Supplementary Movie 1. An origami frog was folded by hand and launched into a hot water bath. (Movie) Supplementary Movie 2. A folded object was unfolded into a 3D box after heating with hot air. (Movie) Supplementary Movie 3. The spring shown in figure 2 is recovered with a 1 gram weight on top of it. (Movie)


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AUTHOR INFORMATION Corresponding Author *[email protected] Present Addresses † Faculty of Chemistry, Department of Polymer Chemistry, MCS University, Marii CurieSkłodowskiej, 20-031 Lublin, Poland. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the National Science Foundation DMR 1310528, a fellowship from the National Science Foundation Graduate Research Fellowship (NSF GRFP), and a fellowship from the Department of Education Graduate Assistance in the Areas of National Need (GAANN) in the area of functional materials. ABBREVIATIONS PETMP, Pentaerythritol tetra(3-mercaptopropionate); HMDI, Hexamethylene Diisocyanate; TEMPIC, Tris[2-(3-mercaptopropionyloxy)ethyl]isocyanurate; DMA, Dynamic Mechanical Analysis. REFERENCES


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Thermoreversible Folding as a Route to the Unique Shape-Memory

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