4D Printing of a Digital Shape Memory Polymer with Tunable High

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Applications of Polymer, Composite, and Coating Materials

4D Printing of Digital Shape Memory Polymer with Tunable High Performance Yue Zhang, Limei Huang, Huijie Song, Chujun Ni, Jingjun Wu, Qian Zhao, and Tao Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11062 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 16, 2019

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ACS Applied Materials & Interfaces

4D Printing of Digital Shape Memory Polymer with Tunable High Performance Yue Zhang, Limei Huang, Huijie Song, Chujun Ni, Jingjun Wu,* Qian Zhao, and Tao Xie* State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou, 310027, P. R. China Shape memory polymers (SMP) with 3D geometries and tunable shape-shifting behavior can open up new opportunities in intelligent devices. Achieving both simultaneously is difficult for conventional approaches. 4D printing allows fabrication of complex 3D SMP geometries that can change shape (i.e. the fourth dimension is time), but tuning the shape memory response is challenging due to the printing constraints. We report here a material and process concept that allows digital light fabrication of SMP with fine control of not only the geometries but also the shape memory characteristics, within a printing time of 30 s. Digital light modulation allows spatio-temporal tuning of the material properties including shape memory transition temperature, rubbery modulus, and maximum elongation (up to 250%). Consequently, the process allows producing multiple-SMP within a single material construct using the same printing precursor. We demonstrate that this unique attribute is beneficial in constructing unusual shape shifting 3D nanophotonic and electronic devices. The simplicity and versatility of our approach facilitates its future expansion into a wide range of geometrically complex devices with advanced functions. Keywords: 4D printing, shape memory polymer, 3D printing, photonics, electronics

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Introduction: The ability to undergo programmed deployment has made shape memory polymers (SMP) uniquely suited for a wide range of engineering applications.1-6 Typically, two key considerations should be carefully taken to meet the demanding requirements of each application: shape memory performance of the materials and geometric shapes of the devices. Besides shape fixity and shape recovery, shape memory performance is generally quantified by the shape memory transition temperature (Ttrans), recovery stress, and maximum elongation, although specific cases may require materials of other characteristics such as biocompatibility for medical devices4 and adhesion for transfer printing of electronics.7 Progresses in the last decade have led to improved understanding of the polymer shape memory mechanism, consequently design strategies for SMP with highly tunable characteristics.1,2 With regards to device geometries, however, the field has mostly been relying on traditional molding and machining techniques. As a result, the accessible shapes are limited and/or the fabrication process is cumbersome. These obstacles are particularly relevant for this class of materials for which shapes play a decisive role in their functions. Thermadapt SMP represents an emerging class of SMPs for which the permanent shape can be manipulated by dynamic network topological rearrangement.8-11 The associated solid-state plasticity allows accessing complex permanent shapes that are otherwise difficult to fabricate by conventional methods. However, the process requires programming via an external force, that is, the shape cannot be manipulated digitally. Recent marriage of shape-shifting materials and 3D printing12-18, namely 4D printing, has led to new opportunities in digital manufacturing of complex shapes for smart devices including SMPs. Various 4D printing methods have been reported. Fused deposition modelling and/or direct ink writing rely on polymer extrusion for fabrication.19,20 Besides their intrinsic low speed, the performance of the SMP cannot be tuned within the printing


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process. Digital light processing (DLP) utilizes a layer-wise curing process to yield thermoset SMP. Tuning SMP performance within a printed part is not likely unless different printing precursors are used during printing.21,22 In our recent work,23 digital light curing was employed to yield thin films that can evolve into 3D shapes driven by the stress from the light-defined material heterogeneity. Besides the high speed of shape fabrication, the ability to spatially define materials triggers an interesting question: is it possible to digitally produce multiple SMPs within a single construct using the same printing precursor? If so, can the performance of SMP be tuned within a single printing run? We report hereafter our successful attempt in this direction and demonstrate its unusual benefits in both nano-photonic and electronic devices. Results and Discussion: Figure 1 illustrates the basic process and mechanism of this digital fabrication method. Digital light from a commercial projector irradiates a printing solution (monomers and photointiator) sandwiched between two quartz glass slides separated by a spacer. With this setup, light attenuation along the thickness should lead to differential curing. Subsequent removal of the unreacted monomers should yield a polymer film with built-in stress in the out-of-plane dimension, the release of which transforms the as-cured flat film into a 3D shape. Additionally, the digital light allows continuous control of the in-plane light intensity, and consequently the properties of the polymer. Specifically, we chose methyl acrylate (MA, 56.9 mole%) and isobornyl acrylate (IBOA, 40.3 mole%) as the co-monomers and 1,6-hexanediol diacrylate (HDDA, 2.8 mole%) as the crosslinker (note: bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide as photoinitiator of 0.5 wt% in all experiments). The selection of acrylate monomers and crosslinker here ensures their similar reactivity for better control of the curing behavior and the glass transition temperature (Tg) of the resulting polymer. We note here that the formulations reported in the literature typically


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employ only multifunctional monomers, resulting in highly crosslinked polymers with very limited strain capability.15,16 In contrast, the formulation here is dominated by mono-functional monomers and the resulting low crosslinking densities should, in principle, lead to high stretchability for the resulting SMP. In addition, changing the ratio between the co-monomers should allow facile manipulation of the glass transition. As illustrated hereafter, these benefits play a key role in obtaining performance tunable SMP.

Figure 1. Fabrication of digital shape memory polymers. (a) Schematic illustration of the fabrication process. (b) Chemical structures of co-monomers and crosslinker. (c) Double bond


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conversions on the front (facing light sources) and back sides as a function of the exposure time (film thickness: 0.5 mm). (d) Mass loss and corresponding average conversion of front and back sides versus exposure time. The degree of curing on the front and back sides (relative to the light irradiation) can be calculated from the double bond conversions measured by the ATR-IR spectra of the cured sample (representative spectra provided in Figure S1). Figure 1c shows that the conversion on the front size is notably higher than that on the back side when the exposure time is low, which is the direct consequence of the light attenuation through the sample. To our surprise, however, as the exposure time increases beyond 18 s, such a difference disappears (within experimental error). We will explain this result later in the text. Nevertheless, both conversions reach a plateau of ca. 90% at exposure time around 22 s. Figure 1d shows that the mass loss due to removal of monomers after curing follows an opposite trend as the double bond conversion (calculated by averaging the conversion of front and back sides). However, as the conversion reaches plateau at 22 s, the mass loss continues to decrease albeit at a slower pace. We believe this is a direct consequence of the low amount of the crosslinker in the formulation, which avoidably results in different amounts of oligomeric sol within the network despite the nearly identical conversion. In the process of monomer removal by solvent soaking (ethyl acetate/methanol=1/1), the oligomeric sol is also removed, leading to its continuous decrease. We proceed to investigate how the light exposure time affects transformation of a cured film (thickness: 0.5 mm) from 2D to the 3D. Specifically, we use the bending angle of the resulting beam (Figure 2a) for quantitative evaluation. The results (Figure 2a) suggest the bending angle decreases linearly with the exposure time, reaching a state of non-bending at 30 s. The inset in Figure 2a demonstrates such a dependence. This linear dependence is favorable for controlling the


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3D shape formation in a predictable manner. At a first glance, these results seem to be contradictory to those in Figure 1c, which suggest identical double bond conversions on both sides of the sample at exposure time longer than 18 s. We believe this discrepancy is related to the complex network formation at different stages of the curing process. Since the monomers and crosslinker are all acrylate-based, one can make an approximation that their intrinsic reactivities are similar. For an effective crosslinking point to form, both double bonds on a crosslinker need to react. Statistically, a slight difference in double bond conversion between the two sides (below the detection limit of the ATR-IR) can yield a non-ignorable difference in the crosslinking density. Presumably, this effect is more pronounced for a formulation that contains a low amount of crosslinker as is the case here. Nevertheless, the bending angle experiments (Figure 2a) provide the most direct evidence of material heterogeneity in the thickness dimension at exposure time less than 30 s.

Figure 2. Folding behaviors and mechanical properties of the digitally cured networks. (a) Exposure time determined folding angle. Insert: Schematic diagram of angle measurement (lower


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left) and photographs showing the folding angles corresponding to the different light exposure (upper right). (b) Effect of film thickness on folding angle. In the planar layout (upper left conner), light and dark correspond to 30 s and 14 s light exposure, respectively. (c) Glass transition temperatures of materials with different exposure time. (d) Rubbery modulus and strain at break of our materials. (e) Photographs of samples with variable exposure times hanging 500 g weight at 80 °C. All scale bars are 1 cm. Due to the more pronounced light attenuation for thicker films, the folding angle shows a strong dependence on the film thickness (Figure 2b). The inset photos in Figure 2b demonstrate this dependence for films obtained with the same irradiation pattern. As a design intent, our precursor consists of MA and IBOA comonomers as the primary components (about 95 wt%). Their reactivities are similar, yet the glass transition temperatures of their homopolymers are drastically different (9 °C, 85 °C, respectively). At a fixed comonomer molar ratio of 0.71 (IBOA/MA), the nearly identical reactivities ensure that the two comonomers are incorporated into the network at a similar ratio throughout the curing process. Consequently, the Tg remains quite steady during curing (Figure 2c) with only a slight variation between 39.0 to 36.0 °C (see Figure S2 for the corresponding differential scanning calorimetry (DSC) curves). If a material with different Tg is needed, this can be conveniently accomplished by changing the comonomer ratio. Indeed, we verify that a comonomer ratio of 0.62 yields samples (exposure time from 14s to 30s) with Tgs of 30.0 °C (Figure S3). We hereafter focus exclusively on tuning the shape memory performance of the cured network using the same precursor of a comonomer ratio 0.71. With Tg largely unchanged with respect to the exposure time, two parameters critical to the shape memory performance (rubbery modulus and strain at break) can be readily tuned within relatively large ranges. Specifically, Figure 2d shows that the rubbery modulus (note: rubbery modulus here


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and after means Young’s modulus) steadily increases with the exposure time between 0.2 MPa and 1.9 MPa whereas the strain at break follows an opposite trend between values of 260 % and 70%. The flexibility to tune both parameters independent of Tg is particularly noteworthy as this is difficult for classical amorphous SMP systems.9,24,25 The large difference in rubbery modulus is schematically demonstrated in Figure 2e, showing drastically different elongation under the same load at 80 °C (above Tg) for samples prepared with different light exposure.

Figure 3. Evaluation of shape memory behavior. (a) Quantitative shape memory cycle for a sample with 30 s light exposure. (b) Quantitative shape memory cycle for a sample with 14 s light exposure. (c) Digital fabrication of complex permanent shapes and demonstration of their shape memory behavior. In the planar print layouts, the black background represents no light exposure and the light and dark regions correspond to light exposure of 14 and 30 s, respectively. All scale bars are 1 cm.


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For both samples with light exposure times of 30 s and 14 s, their quantitative shape memory cycles (Figures 3a and 3b) demonstrate their excellent shape memory properties, as indicated by their perfect shape fixity (both 100.0%) and shape recovery ratios (97.2%, 100.0% respectively). We note that the sample corresponding to light exposure time of 22 s also shows near perfect shape fixing and recovery behavior (Figure S4). The above experiments set up a basis for inclusion of different SMP into a single film by digital light control. For simplicity only, we thereafter focus on two exposure times of 30 s and 14 s, noting about inclusion of more SMP can be conveniently accomplished by digitally controlling the light pattern. Accordingly, Figure 3c illustrates that the selection of planar light patterns allows easy access to complex permanent shapes, which can be transformed into temporary shapes and recovered upon reheating (programming and recovering temperature: 70 °C). We note that the Miura folding structure was obtained with light exposure from both sides using two projectors, yielding more versatility since the folding can occur towards both sides of the film.


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Figure 4. Demonstration of potential device applications of the digital shape memory polymer. (a) Photographs and SEM images of nano-structured silicon wafer and the derived digital SMP nanophotonic device. (b) Images of stretching samples with spatial different moduli. (c) Images of spiral structure and the programmed planar shape. (d) Demonstration of flexible wearable device substrate. In the planar print layouts (including 4a, 4c and 4d), the black background represents no light exposure and the light and dark regions correspond to light exposure of 14 and 30 s. Scale bar: 1 cm. Our digital SMP offers unusual benefits for potential device applications (Figure 4). If a nanostructured 2D silicon wafer is used as the back cover in place of the glass in the printing setup, a unique 3D photonic device (Figure 4a) can be obtained in which the original nanostructure is carried onto the area with 30 s light exposure whereas the nanostructure largely collapses in the rest of the area with 14 s light exposure. Importantly, more complex manipulation of the nanostructure can be achieved by simple change of the digital light pattern, without changing the template. The digital SMP can also be useful for flexible electronics, simultaneously offering three attributes: strain isolation, 3D shape, and active deployment. Figure 4b shows that, when a digital SMP is stretched at its rubbery state, the deformation occurs selectively in the soft region with the more rigid islands largely intact. This feature allows strain isolation for non-stretchable active electronic devices while maintaining the overall device stretchability.26 Figure 4c demonstrates that the transformation of a planar as cured film into a 3D spiral. This particular geometry can be explored for making neuron stimulator for which a spiral-shaped electronic device should wrap tightly onto the neuron fiber. The combined benefits for electronics are illustrated in Figure 4d. The 3D digital SMP is first programmed into a flat geometry (i.e. temporary shape). This facilitates device fabrication with printed silver nanowires forming the interconnects for two thin film LED


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lights. The working electronic device can be stretched without affecting its function. Subsequently, both ends of the elongated device are glued together to form a circular device. A cylinder is inserted into the center of the device. Upon heating, the device shrinks to form tight wrapping with the cylinder. Such actively deployable shrinkable electronics represents drastic departure from typical stretchable electronics that are passively deformable. Although the device demonstration in Figure 4d is conceptual, its simplicity should pave the way for making a new generation of more complexed shrinkable electronics. Conclusions: In summary, we show a digital 4D printing strategy that has the following attributes: fast printing using a simple setup to yield designable 3D shapes; readily tunable shape memory characteristics for the resulting polymer; versatility to combine multi-SMP within a single material construct using an identical precursor. We demonstrate that the digital SMP can provide unique benefits for nano-photonics and a new generation of shrinkable electronics. In principle, many other types of multi-functional devices can be similarly constructed by taking advantage of the 2D to 3D transformation. In addition, the simplicity in material chemistry design also suggests that more tailored functions can be readily designed into the digital SMP. Relative to other multi-material 3D printing methods such as PolyJet and grey-scale DLP27, our 4D printing approach is simple, fast, and does not require sophisticated printers. Most importantly, the chemistry principle can be readily expanded to a variety of different materials. Experimental Section: Materials: Methyl acrylate (MA), isobornyl acrylate (IBOA), and 1,6-hexanediol diacrylate (HDDA)

were

all

purchased

from

Sigma

Aldrich.

Bis(2,4,6-trimethylbenzoyl)-


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phenylphosphineoxide (Irgacure 819) was obtained from TCI. The solvents used were of analytical grade. All chemicals were used as received. Printing Procedure and Development: Our printing setup was composed of a commercial projector (NEC NP-V302H+) and a cell between two slides of quartz glass with poly(dimethylsiloxane) (PDMS) as the spacer. The projector has a long wavelength from 400 nm to 730 nm (see Figure S5). The printing precursors was mixed and stirred for 6 h to ensure the dissolution of the photoinitiator. The prepared solution was kept away from light prior to use. In the printing process, the precursor solution was injected into the reaction cell followed by digital light exposure. The planar printing layouts represent the different exposure times (light grey and dark grey represent short and

long

exposure

time,

respectively).

Subsequent

soaking

in

solution

(ethyl

acetate/methanol=1/1) for 6 h removed the unreacted monomers. The obtained film was dried for 2 h at 100 °C. Fabrication of the Nano-Photonics: The 4D printing method was followed to fabricate the nanophotonics. A nano-structured 2D silicon wafer was used as the back cover in place of the glass in the printing setup. In the planar print layouts, the dark and light regions were exposed for 30 s and 14 s, respectively. The cured film was soaked in a mixed solution (ethyl acetate/methanol=1/1) for 6 h and dried for 2 h at 100 °C. Fabrication of the Shrinkable Electronics: The 4D printing method was used to obtain the bracelet, which consisted of two regions with diffirent moduli. The bracelet was flattened by shape memory programming. Two micro-LED lamps were mounted and connection was made by conductive paste of silver nano-wires. The device was stretched by a factor of two at 70 °C and the two ends were glued together while the stretching was maintained. The enlarged electronic bracelet was


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fixed by cooling to ambient temperature. It was then inserted onto a cylinder with a smaller diameter. Heating to 70 °C induced its shrinkage, resulting in its tight wrapping around the cylinder. Measurement of Double Bond Conversion: Attenuated total reflection infrared (ATR-IR) spectra were collected using a Nicolet 5700 spectrometer. The ratio between the peak area of the vinyl group (810 cm-1) and that of the carbonyl group (1720 cm-1) (Αvinyl/Acarbonyl) was used to calculate the double bond conversion using αvinyl,t=1−((Avinyl/Acarbonyl)t/(Avinyl/Acarbonyl)t=0), where t is the light exposure time and αvinyl,t is the corresponding double bond conversion. Measurements: Glass transition temperatures were measured using a differential scanning calorimeter (DSC, TA Q200) at a heating rate of 10 °C/min. Rubbery moduli and strains at break were tested in a tensile mode at 80 °C using a universal material testing machine (Zwick/Roell Z005). The strain rate was 1 mm/min and the samples were in a dumbbell geometry (the middle section of 1×3 mm). A minimum of three specimens were tested for each sample. The shape memory properties of the materials were quantitatively evaluated using a dynamic mechanical analyzer (DMA, TA Q800). The sample into rectangular shapes (7× 1.5×0.5 mm) were tested in a Strain Rate mode. The shape fixing ratio Rf and shape recovery ratio Rr were calculated as Rf =100%×(ε/εload) and Rr= 100%×(ε-εrec)/ε), where ε, εload and εrec referred to the fixed strain, the strain under stress and the recovery strain. The mass loss is calculated by the (1- mass after desolvation/pristine mass after polymerization)×100%.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications websites.


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Additional information on the representative ATR-IR spectra, differential scanning calorimetry (DSC) curves, DMA curve for the sample with exposure time 22 s, and wavelength distribution for the projector used in this study

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ORCID: 0000-0003-0222-9717 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank the following programs for the financial support: National Natural Science Foundation (grant No. 21604070, 21625402, 51822307, and 51673169); State Key Laboratory of Chemical Engineering (SKL-ChE-17D03). The authors also thank Mrs. Li Xu for her assistance in performing ATR-IR analyses at State Key Laboratory of Chemical Engineering (Zhejiang University).

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4D Printing of a Digital Shape Memory Polymer with Tunable High

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