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3D Printing of Highly Stretchable, Shape-Memory and Self-Healing Elastomer toward Novel 4D Printing Xiao Kuang, Kaijuan Chen, Conner K. Dunn, Jiangtao Wu, Vincent C.F. Li, and H. Jerry Qi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18265 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Applied Materials & Interfaces

3D Printing of Highly Stretchable, Shape-Memory and Self-Healing Elastomer toward Novel 4D Printing Xiao Kuang, Kaijuan Chen, Conner K. Dunn, Jiangtao Wu, Vincent C. F. Li, H. Jerry Qi* The George W. Woodruff School of Mechanical Engineering Renewable Bioproduct Institute Georgia Institute of Technology Atlanta, GA 30332, USA.

KEYWORDS: 3D printing, highly stretchable, shape memory, self-healing, semi-IPN

ABSTRACT: 3D printing of flexible and stretchable materials with smart functions such as shape memory and self-healing is highly desirable for the development of future 4D printing technology for myriad applications, such as soft actuators, deployable smart medical devices, and flexible electronics. Here, we report a novel ink that can be used for 3D printing of highly stretchable, shape memory and self-healing elastomer via UV light assisted direct-ink-write (DIW) printing. An ink containing urethane diacrylate and a linear semi-crystalline polymer is developed for 3D printing of a semi-interpenetrating polymer network (semi-IPN) elastomer that can be stretched by up to 600%. The 3D printed complex structures show interesting functional properties, such as high strain shape memory and shape memory assisted self1 ACS Paragon Plus Environment

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healing capability. We demonstrate that such a 3D printed shape memory elastomer has the potential application for biomedical devices, such as vascular repair devices. This research paves a new way for the further development of novel 4D printing, soft robotics, and biomedical devices.

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INTRODUCTION 3D printing, also known as additive manufacturing (AM), has drawn tremendous attention among different research fields and gained considerable progress in recent years. 3D printing finds myriad applications, including physical prototypes1, tissue engineering2, electronics devices3, microfluidics4 and high specific strength materials5. More recently, the converge of 3D printing and smart materials led to the new concept of 4D printing, where the shape and property of a 3D printed component can change as a function of time under external stimulus6. 4D printing can be achieved by using hydrogel 7, shape-memory polymer

8-10

, or internal

stress developed during 3D printing11. Among these methods, shape memory polymers (SMPs) are a class of smart polymeric materials that have the ability to "memorize" a permanent shape, be programmed to fix a temporary shape, and later recover to its initial shape upon an external stimulus, such as heat, electricity, or light irradiation12-16. A variety of glassy polymers have been fabricated by 3D printing to achieve shape-changing objects17-18. Comparing with the glassy SMPs, the elastomeric SMPs that are soft and more deformable are especially suitable for wearable electronics and biomedical devices19-20. Therefore 3D printing of soft and elastomeric SMPs is highly desirable to satisfy the application demands of soft actuators, deployable smart medical devices, and flexible electronics, which has been rarely reported21. Self-healing (SH) materials are developed as a smart material to enable structural restoration and functions recovery upon damages, which can enhance material reliability and extend its lifetime22-32. Generally, the SH polymer systems fall in two categories: extrinsic self-healing and intrinsic self-healing polymers. The former relies on the encapsulated healants in microcapsule33-34 or microvascular networks35-36, which can be released and polymerized upon crack intrusion to heal micro-cracks. For example, in a recent innovative work, White et al. developed a vascular synthetic system in the polymer to restores mechanical performance in response to large-scale damage37. However, the unpredictable 3 ACS Paragon Plus Environment


damage repair may need a significant amount of vascular coverage, which may increase the difficulty in fabricating the vascular network and the material. Intrinsic self-healing polymers that use the intrinsic physical or chemical properties of the polymer, such as chemical reactions in dynamic covalent bonds or secondary bonds, are also used for micro-crack mending38-40. These polymer systems usually suffer from relatively complex synthesis and lack of crack closure before healing 41. Here, we combine the attributes of shape memory (SM) and SH with 3D printing. Inspired by the SM assisted SH concept in polymer composite

41-42

, 3D printing of an SM composite

with large crack healing capability was achieved. A highly stretchable semi-interpenetrating polymer network (semi-IPN) elastomer with an embedded semi-crystalline thermoplastic polymer was printed by using the direct-ink-write (DIW) approach. The embedded semicrystalline thermoplastic serves the dual roles of the switching phase for SM and the healant for SH behavior. The direct fabrication of 3D complex structure that can change its shape with time and has SH function provides a new capability for 4D printing. RESULTS AND DISCUSSION To facilitate DIW based printing of semi-IPN elastomer composites, we prepared inks containing a photocurable resin and a semi-crystalline thermoplastic polymer. The photocurable resin is made of aliphatic urethane diacrylate (containing 33 wt % of isobornyl acrylate) and n-butyl acrylate (BA) (Figure 1a). The semi-crystalline polycaprolactone (PCL) was dissolved in the above acrylate. In addition, nanoparticles, such as fumed silica (200300nm), was added as a rheology modifier that imparts the shear thinning effect to the uncured ink43. Such a composition of the ink allows good printability as well as SM and SH behaviors after printing.


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Figure 1. Schematic illustrations of the UV assisted DIW based 3D printing of semi-IPN elastomer composites containing crystalline linear chain and cross-linked network. (a) Chemical structures of the components in the semi-IPN elastomer composite resin. (B) DIW based 3D printer equipped with heating elements prints each layer of filament followed by shining UV light (50mW/cm2) to cure the resin. (c) The structure evolution of the ink during printing at 70 oC and cooling down after printing.

As illustrated in Figure 1b, we achieved 3D printing of semi-IPN elastomer composites via a DIW printing followed by UV curing. The DIW printer uses a heating syringe that writes inks on an x-y direction motion stage. The syringe is mounted on a z-direction stage so that the distance between the syringe nozzle and the x-y direction motion stage can be adjusted. The ink is pushed out of the syringe nozzle by the pressure controlled by a pneumatic apparatus. UV light-emitting diodes (LEDs) at a fixed location near the syringe were used to ex-situ cure the ink. To facilitate successful printing, the syringe is wrapped by a heating 5 ACS Paragon Plus Environment


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element with the temperature of nearly 70 oC to avoid crystallization of PCL during printing (Figure S1, Movie S1). The ink in the heating syringe is consistently extruded through the syringe nozzle by a suitable pressure. Incorporating 4wt% of SiO2 in the resin imparts obvious shear thinning effect to enable stable ink extrusion and allow for shape holding after printing (Figure S2). The filament is consistently deposited on the silicone paper to prevent adhesion. After printing one layer, the x-y direction motion stage moves to the position under the UV light to cure the printed ink by photopolymerization. The polymerization was verified by Fourier Transform Infrared (FTIR) spectra as indicated by the complete disappearance of vinyl characteristic peaks, such as the band of vinyl carbon-carbon double bond vibration at 1639 cm-1 (Figure S3). After photocuring, the PCL chains were confined in the cross-linked network. Finally, the semi-IPN elastomer with complicated 3D shape can be obtained, where PCL chains and small crystals are dispersed in the acrylate elastomer network matrix after cooling down (Fig. 1c). It is noted that the content of PCL plays an important role in mechanical properties of the semi-IPN elastomer. Both of Young's modulus (Figure S4) and fracture stress (Figure S5) increase with the PCL content, as the crystallinity increases with the PCL content (Figure S6). The small PCL crystals can retard the propagation of micro-cracks in the elastomer network at large strain, which can improve the fracture toughness of semi-IPN19,

44

. Thus a highly

stretchable semi-IPN elastomer composite can be obtained. Considering proper mechanical properties, the resin containing 20 wt % of PCL was utilized for printing in this paper.


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Figure 2. Photographs and tension properties of 3D structures by DIW based printing. Different architectures of (a) Archimedean spiral, (b) honeycomb, (c) hollow vase, and (d) Gumby model were printed. (e) Snapshots of stretching a printed Archimedean spiral up to 300 % strain. (f) Highly stretchable stress-strain behavior of the printed semi-IPN elastomers with three different filament printing angle. (g) The Young's modulus, fracture strain and fracture stress of printed dogbone shaped samples with different printing angles.

Taking advantage of the UV assisted DIW based 3D printing, a variety of 3D architectures, such as Archimedean spiral, honeycomb, hollow vase and Gumby toy, can be directly printed (Figure 2a-d). The printed parts are expected to be stretchable. As shown in Figure 2e, the printed Archimedean spiral was stretched to over 300 % of strain. We found that the in-plane anisotropy of printed parts is negligible. As shown in Figure 2f, the tension stress-strain curves of directly printed dogbone samples in different orientations relative to the longitudinal direction (0°, 45°, and 90°) nearly overlap. The fracture strain is 500~600% for all the printed samples. The Young's modulus, fracture strength, and fracture strain are comparable for the 7 ACS Paragon Plus Environment


three printing angles (Figure 2g). The small deviation in mechanical properties may be attributed to the edge effects for different printing angles (Figure S7). After deposition, the filaments were chemically bonded to the neighbor ones by the UV curing, resulting in excellent inter-filament adhesion. The hydrogen-bond in urethane group may also do some help to enhance the interfacial bond between adjacent filaments. It is also noted that the excellent interlayer adhesion enables good contact between different layers; then the diffusion would help further enhance the interfacial adhesion. As a result, both good surface finish and in-plane isotropic properties were achieved for the printed semi-IPN elastomer composites. As mentioned above, the small PCL crystals act as excellent reinforcement phase that contributes to large fracture strain and fracture stress for the elastomer at the room temperature. The cyclic loading-unloading tests at room temperature show some plastic deformation, which is typical for semi-crystalline polymers due to the plastic deformation of the crystals (Figure S8). At the temperature above Tm, the deformation becomes elastic (shown in Figure S9). Figure S9 also shows that the elastomer composites can still be stretched up to 400 % strain after complete melting of PCL crystals at 70 oC.


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Figure 3. The SM actions of printed architectures from temporary shapes to permanent shapes upon heating by a heat gun. Shape memory cycles of semi-IPN elastomer where each sample was stretched at 70 oC and fixed at 0 oC followed by recovered at 70 oC in a stress-controlled mode (a) and in a strain controlled mode (b) by DMA. (c) A compressed plate recovers to its original shape of a standing hollow vase. (d) A long string recovers to its original shape of Gumby toy. All of the scale bars are 6 mm.

The semi-IPN elastomer contains a crystalline component in the network, which can be used as a switching phase for SM effect. The thermomechanical properties and thermal transition of the materials were investigated before performing the SM tests. Dynamic mechanical analysis (DMA) shows only one glass transition temperature (Tg) at about 1.5 oC and one melting transition at 68oC for the elastomer as indicated by the sharp decrease in modulus (Figure S10). These results suggest relatively good compatibility with only microscale phase separation between the two components. As the PCL chains are confined to the acrylate network, it is reasonable to assume that the PCL chains are immersed in the elastomer network with small crystals homogeneously dispersed. Above the melting temperature of PCL (Tm), the material shows a rubbery plateau with a modulus of about 0.4 MPa. Differential scanning calorimetry (DSC) tests show Tm of PCL is about 55 oC and the crystallization temperature (Tc) is 14 oC (Figure S11). It is noted that the transition temperatures obtained by DMA are usually higher than by DSC16. In an SM cycle, the sample was first deformed at the temperature above the Tm of PCL and then cooled down to a temperature below the Tm without releasing the external load. The deformed temporary shape was fixed after releasing the external load at the low temperature. Upon cooling, recrystallization of PCL crystals enables the fixing of temporary shape. After reheating, the crystal phase melts and the permanent shape can be recovered due to the entropic elasticity of network. High strain SM effect can be achieved for this material (Figure S12, Movie S2). The quantitative SM action was evaluated by DMA. With the programing strain over 230 % using stress controlled testing mode, the shape fixity (Rf) was over 93% and shape recovery ratio (Rr) was over 95% in the first cycle (Figure 3a). The slight shift of the 9 ACS Paragon Plus Environment


curves and decreasing Rf value may be attributed to the deformation of a large fraction of thermoplastic chains during high strain stretching in nine testing cycles (Figure S13). Using the strain controlled testing mode (200 % stretching strain), Rf was calculated to be about 99% during four cycles (Figure 3b). While Rr would decrease from 90 % to 83% over four cycles (Figure S14). Based on these results, we demonstrated the SM behaviors of several printed 3D structures. A printed hollow vase (36 mm in height) was first heated and compressed into a plate followed by cooling down to yield the temporary plate shape (~3 mm in height) (Figure 3c). During heating (by a heating gun), with the continuous melting of crystals in the elastomer network, the entropic elasticity of the elastomer drove the recovery to its initial hollow vase shape. Finally, the original shape of a standing hollow vase was recovered. Besides using the heating gun, heating can be achieved by immersing the sample in hot water, which enables relative uniform recovery (Movie S3). Other complex 3D printed structures also show good SM properties. For example, the honeycomb structure was compacted into a cylinder as the temporary shape, which gradually recovered to its original shape in about 1 min using a heating gun (Movie S4). To further demonstrate the highly deformable SM properties, the printed Gumby toy was stretched and twisted into a long string as a temporary shape (Figure 3d). During healing, the twisted string shape rolls, shrinks and gradually recovered to its initial shape. Using hot water as a heating media leads to much faster recovery than that by the heating gun due to uniform heating capability (Movie S5).


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Figure 4. The SM assisted SH behavior of printed semi-IPN elastomer composites. (a) SEM micrographs of scratched and healed surface at different scales. (b) Optical micrograph indicating obvious cracks regions on the notched sample can be totally healed with an invisible scar. (c)Tension stress-stain curves for the notched and healed state of semi-IPN elastomer samples. Inserted are the micrographs of deformation and crack growth of notched sample clamped in the MTS tensile tester. (d) Pictures showing SM assisted SH process for printed Archimedean spiral structure. Inserted pictures were the optical micrograph showing the surface morphology with a scale bar of 1 mm. (e) The printed strip was cut and healed in three healing generations. The healing condition is the same by heating the sample at 80 oC for 20 min and then cooling down in the air.

Besides the SM effect, SH can be achieved for the semi-IPN elastomer. We first investigated the healing of micro-cracks. The sample was scratched with a sharp blade as shown by SEM images in Figure 4a, showing a surface scratch of 3 mm long and 30 µm wide. After treating the sample in an oven at 80 oC for 20 min followed by cooling down, the surface crack disappeared with a slight scar. Larger cracks such as notched gap can be also healed. As shown in Figure 4b, the notched strip was stretched and released leaving a visible gap in the notch position. After treating at 80 oC for 20 min, the gap was closed and mended without obvious scars in macro scale. For the notched sample, the initial crack of the notch edge easily propagates after reaching its yield point. The crack propagation leads to sample 11 ACS Paragon Plus Environment


fracture at a strain near 100%. After healing, the notched sample can be stretched over 160 % strain (Figure 4c). Comparing with the virgin printed samples, the healing efficiency evaluated from the fracture strain is relatively low (

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