2D-to-3D Shape Transformation of Room Temperature Programmable

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Applications of Polymer, Composite, and Coating Materials

2D-to-3D Shape Transformation of Room Temperature Programmable Shape Memory Polymers through Selective Suppression of Strain Relaxation Guo Li, Shuwei Wang, Zhao-Tie Liu, Zhong-Wen Liu, Hesheng Xia, Chun Zhang, Xili Lu, Jinqiang Jiang, and Yue Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16094 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 31, 2018

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ACS Applied Materials & Interfaces

2D-to-3D

Shape

Temperature

Transformation

Programmable

Shape

of

Room Memory

Polymers through Selective Suppression of Strain Relaxation Guo Li*a# Shuwei Wang,a# Zhaotie Liu,a Zhongwen Liu,a Hesheng Xia,b Chun Zhang,b Xili Lu,b,c Jinqiang Jiang,a Yue Zhao*c a

Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of

Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an, Shaanxi Province 710062, China b

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute, Sichuan

University, Chengdu 610065, China c

Département de chimie, Université de Sherbrooke, Sherbrooke, Québec J1K 2R1, Canada

KEYWORDS: shape memory polymers, 3D shape transformation, plastic deformation, gradient crosslinking, surface patterning

ACS Paragon Plus Environment

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ABSTRACT: While shape memory polymers (SMPs) can alter their shapes upon stimulation of environmental signals, complex shape transformations are usually realized by using advanced processing technologies (4D printing) and complicated polymer structure design or localized activation. Herein, we demonstrate that stepwise controlled complex shape transformations can be obtained from a single flat piece of SMP upon uniform heating. The shape memory blends prepared by solution casting of poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA) exhibit excellent mechanical and room temperature shape memory behaviors, with fracture strain beyond 800% and both shape memory and shape recovery ratio higher than 90%. After plastic deformation by stretching under ambient conditions, the material is surface patterned to induce the formation of an Fe3+-coordinated PAA network with gradually altered crosslinking density along the thickness direction at desired areas. Upon subsequent heating for shape recovery, strain release is restricted by PAA network to different extents depending on the crosslinking density, which results in bending deformation toward the non-patterned side and leads to threedimensional (3D) shape transformation of the SMP. More interestingly, by sequentially dissociating the PAA network via UV or visible light induced photo-reduction of Fe3+ to Fe2+, residual strains can be removed in a spatially controlled manner. Using this approach, a series of origami shapes are obtained from a single SMP with a tailored 2D initial shape. We also demonstrate that by incorporating polydopamine nanoparticles as photothermal fillers into the material, the whole shape transformation process can be performed at room temperature by using near-infrared (NIR) light.


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INTRODUCTION Nature abounds with some plants that change their shapes in response to environmental conditions change, like fast snapping of the Venus flytrap after touch, blossoming of Sunflower in response to light, and unfolding of conifer pinecones during drying.1-3 Mimicking these behaviors and developing stimuli-responsive shape changing polymeric materials are attractive for many applications, including reconfigurable and deployable devices, robotics, sensors and actuators, but challenges remain. So far only a small number of polymer systems can exhibit such function, including hydrogels, liquid crystal elastomers, polymeric metamaterials and shape memory polymers (SMPs).4-20 These polymers can be manufactured into regular 2D shapes (here thin and flat polymer films with uniform thicknesses are considered to have 2D shapes) and later transform into target 3D shapes when external stimuli are applied. For example, a liquid crystalline elastomer was used to mimic the movement of flytrap via light-induced bending;10 Kirigami design was applied to flat sheets of composite materials for complex shape morphing;13 3D printed poly(ethylene glycol) diacrylate hydrogel was shown to undergo swelling-induced complex shape changes based on varying crosslinking density in the network;16 and a gold nanoparticle-loaded SMP was used to demonstrate both light-controlled out-of-plane and inplane bending using a temperature gradient.17 Of these shape changing materials, SMPs are advantageous in many regards due to the vast material choice, low cost, easy preparation, superior deformability and mechanical properties. The essence of shape memory behaviors is postponed strain relaxation: after deforming SMP (usually crosslinked polymers), strain can be preserved by switching off the chain mobility, leading to a temporary shape; later upon the stimulation of light, heat, moisture, magnetic field or electric filed, chain mobility is activated and the stored strain is released to bring the SMP back


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to its initial shape. Since the route of shape change for SMP is from a programmed temporary shape to pre-determined initial shape, the complexity of shape change can be increased to meet higher practical needs by programming SMP into more intricate temporary shapes, or by revising the initial shape into more complicated ones via reconfiguration of dynamically crosslinked network. However, these processes are usually manually conducted at elevated temperatures in environmental chamber, which is difficult to perform in practice.21,22 Current efforts are being made on a promising strategy that is based on inducing inhomogeneous shrinking of elongated SMP to achieve 3D shape transformations. The methods used to achieve inhomogeneous shrinking can be assorted into two categories. One is combining pre-stretched SMPs with other elastic substrates to prepare layered composites; upon heating, 3D shape transformation is activated through contraction of the SMP layer.23-25 The drawback of this strategy lies in that the layered SMP cannot be manufactured using conventional processing methods, and a strong enough adhesive force among the layers must be ensured to prevent abscission during deformation. Another approach is inducing inhomogeneous shrinking of pre-stretched SMPs via spatially controlled heating, usually by using near-infrared (NIR) light irradiation at selected areas to achieve 3D shape transformations. However, incorporation of photothermal fillers may arouse additional concerns on material preparation, and the obtained 3D shapes are only temporarily reserved, their stability can be sensitive to temperature or moisture fluctuations in certain circumstances.26,27 Since conventional heating is still the most widely applied stimulation, it is of particular importance to develop new strategies to realize complex, 2D-to-3D shape transformation of commodity SMPs under conventional bulk heating. On another aspect, the traditional heating-then-cooling process to store deformation into SMP is not convenient in many cases. Alternatively, some SMPs are programmable at room temperature


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(RT), whose temporary shapes are obtainable by directly deforming the material.28-31 The underlying mechanism of RT programming is either forced orientation of amorphous polymers or re-crystallization of crystalline polymers.32 Although temporary shape programming at RT is time and energy saving, the deformability and strain fixability are usually compromised because of the restricted chain mobility at temperatures below glass transition or melting temperature. Being motivated by the importance of addressing the above-mentioned issues, herein we demonstrate an approach that allows a single flat piece of a room temperature programmable SMP to exhibit stepwise 2D-to-3D shape transformation upon uniform heating. After many early attempts, we found that the blend of poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA) prepared by solution casting exhibited excellent mechanical and room temperature shape memory properties, with a strain at failure beyond 800% and both shape fixity and shape recovery ratio exceeding 90%. To our best knowledge, this is the first report that PEO/PAA blends have such good room temperature shape memory properties. Strategy to realize controlled 3D transformation is based on the following hypothesis: after storing tensile strain by room temperature stretching, polymer chain crosslinking can be introduced in selected areas that act as molecular switch to intervene in the heating-induced strain release process. If the formed network can prevent the strain relaxation, heating will only induce shape recovery in noncrosslinked areas, which may be explored for 2D-to-3D shape transformation. For example, it is easy to imagine that if only one surface of a stretched SMP strip is crosslinked, upon heating the contraction occurs on the other side, leading to bending and curling of the SMP. If the hypothesis is validated, samples with desired 3D shapes can be obtained by selecting the bending positions and controlling the bending angle of each position. Moreover, if the crosslinking is dynamic in nature and the network can be dissociated later, versatile 3D shapes can be obtained through


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sequentially controlling the dissociation of crosslinking in different positions and subsequent heating, until the material is recovered to its initial 2D shape. This process is schematically illustrated in Figure 1. We noticed that Fe3+-carboxylate coordination could be dissociated through photo-reduction of Fe3+ ions to Fe2+ under visible light irradiation, which could reform via oxidation of Fe2+ to Fe3+ in air.33-35 We thus chose this chemistry to validate the proposed strategy, and the selective crosslinking is introduced via surface treatment using alcohol solution of ferric chloride, since the carboxyl groups near the surface would preferentially bind with the Fe3+ ions, a PAA network within the PEO/PAA blend with a changing crosslinking density along the thickness direction can be formed. Finally, we show that after incorporating polydopamine nanoparticles (PDNPs) as photo-thermal fillers into the material, the strain recovery can be triggered by near infrared light, and the whole shape transformation process can be conducted at room temperature.

PAA

PEO

Fe3+

PEO crystalline domain

stretch surface patterning

oxidation in air

stretch

heating

photo-reduction

photo-reduction

oxidation in air

heating

Figure 1. Schematic illustration of the reversible 2D-to-3D shape transformation process of PEO/PAA blends and the underlying mechanism based on selective suppression of strain relaxation upon uniform heating of the shape memory polymer.


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EXPERIMENTAL SECTION Materials. Polyethylene oxide (PEO, Mw: 1,000 kDa), ammonium persulfate (APS) and N,N,N',N'-tetramethylethylenediamine (TEMED) were purchased from Chengdu Aike Reagent Co., LTD. (China) and used as received. Dopamine hydrochloride was purchased from SigmaAldrich, and acrylic acid (AA) from Shanghai Aladdin Bio-Chem Technology Co., LTD. Preparation of poly(acrylic acid). Poly(acrylic acid) (PAA) is prepared by free-radical polymerization of acrylic acid in aqueous solution. AA and ultrapure water was added into a round-bottom flask to prepare a monomer solution (20 wt%), then APS and TEMED (0.5 wt% and 1 wt% of AA, respectively) wwere added and the solution was thoroughly mixed. The polymerization was conducted at room temperature under continuous stir for 24 hours, followed by dialysis for 3 days to remove unreacted monomers and initiators. Then the polymer solution was freeze-dried to obtain the final product. Preparation of polydopamine nanoparticles. Ammonia aqueous solution (2 mL, NH4OH, 28-30 %) was mixed with ethanol (40 mL) and ultrapure water (90 mL) under mild stirring at 30 o

C for 30 min. Then dopamine hydrochloride (0.5 g) was dissolved in ultrapure water (10 mL)

and injected into the prepared solution. The color of this solution immediately turned to pale yellow and gradually changed to dark brown. The reaction was continued for 24 h at room temperature. Afterwards, the solution was distilled under reduced pressure and freeze dried to obtain polydopamine nanoparticles (PDNPs). Uniform aqueous solutions of PDNPs were obtained by dissolving desired amount of PDNPs in ultrapure water under sonication for 10 min. Preparation of SMP films. Desired amounts of PEO and PAA were added into dimethyl formamide and stirred at 90 oC overnight in order to obtain a homogeneous polymer solution,


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and the polymer films were obtained by evaporating the solvent after pouring the polymer solutions into homemade Teflon molds and heated to 90 oC. PDA incorporated samples were prepared by adding prepared PDNPs into the polymer solutions before casting the solution and evaporating the solvent. Surface patterning of SMP films. Ferric chloride hexahydrate powders were dissolved in anhydrous ethanol to prepare the ferric ion solution at certain concentrations. The solution was then deposited on the surface of the samples for patterning (using a brush) with desired width and length, followed by evaporating the solvent in air at room temperature. Photo-reduction and oxidation of surface-patterned SMP films. Photo-reduction was conducted by exposing the surface-patterned films to UV light (Rolence-100UV, Shenzhen Desention Technology CO., LTD) or visible light (commercial LED lamp with a maximum power of 40 W). Subsequent oxidation was performed by exposing the irradiated films in air at room temperature. Characterizations. Gel Permeation Chromatography (GPC) measurements were performed on a GPC system (Agilent PLgel 5 μm MIXED-C column, 1525 HPLC pump with 2414 Refractive Index detector) using DMF solution as the eluent (flow rate: 1 mL/min, 35 °C). Wide-angle Xray diffraction (WAXD) measurements were conducted on a Rigaku D/max2550VB3þ/PC X-ray diffractometer (Rigaku Co., Tokyo, Japan) operated with Cu Kα irradiation at 40 kV and 40 mA, the scan rate used was 2 deg/min. Differential scanning calorimetry (DSC) tests were conducted on Q1000 (TA, New castle DE, USA), the scan rate used was 5 oC/min. The curves were obtained in the second heating-then-cooling scan to eliminate thermal history. X-ray photoelectron spectroscopy (XPS) measurements were conducted on AXIS ULTRA (Kratos


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Analytical Ltd, Kratos, UK), the binding energy of C1s was shifted to 284.8 eV as the reference. Diffuse reflectance UV-vis spectra were recorded on Lambda 950 UV−vis−near-IR spectrophotometer (PerkinElmer, 150 mm Int. Sphere, Waltham Massachusetts, USA). Dynamic mechanical properties were analyzed using Q800 DMA (TA, New Castle DE, USA). Tensile tests were carried out in a Suns UTM2103 (ShenzhenSuns, China) universal tensile test machine with a 100 N load cell and a strain rate of 20 mm/min. The tensile specimens have a gauge length of 20 mm and 4 mm in width, following the ASTME-8 standard. Infrared light induced temperature rise was recorded using an Infrared Camera Imaging System (Thermovision A20, FLIR Systems Inc., Wilsonville, OR, USA).

RESULTS AND DISCUSSION We firstly prepared a series of PEO/PAA blends with different compositions in order to optimize the mechanical and room temperature shape memory properties. High molecular weight polymers are chosen (the molecular weights of PEO and PAA are 1,000,000 and 531,000 g/mol, respectively) to avoid chain slippage during deformation and ensure good shape recovery upon heating. The prepared materials are named as PAA-x, in which x denotes the content of PAA in the blend (for example, PAA-20 indicates that the weight percentage of PAA is 20%). It can be seen from Figure 2a that samples containing less PAA exhibit both higher tensile strength and higher fracture strain. This is understandable, because higher PAA content means fewer PEO crystalline domains that act as “hard skeleton”, and the formation of hydrogen bonds between PEO and PAA also impairs the crystallization of PEO, leading to a decrease of mechanical strength. In addition, the high hydrophilic nature of the two polymers allows absorption of water


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molecules as plasticizers, which lowers the Tg of prepared SMPs to below RT and endows the material with superior stretchability (for example, PAA-20 contains about 8 wt% of water, Figure S6 in supporting information). The decrease of crystallinity as a function of PAA content in the blends was evidenced by DSC measurement (results shown in Figure S1). RT shape memory properties were evaluated using a protocol shown in Figure 2b. Typically, a sample with an original length (Li) is elongated to a certain strain (Lm), afterwards the external stress is removed to obtain a temporary shape with a certain length (Lt). Later upon heating the sample to 80 oC, a recovered length is obtained (Lr). The shape fixity ratio (Rf) is calculated as the percentage of stored strain (Lt - Li) compared to the total strain (Lm - Li), and the shape recovery ratio (Rr) is calculated as the percentage of recovered strain (Lm - Lr) compared to total strain (Lm - Li). Since the RT shape programmability is originated from reformation of PEO crystals, it is no surprise to see that higher PEO content results in better shape fixation (Figure S2), as more crystalline domains are formed by oriented PEO chains that act as molecular switches permitting better fixation of imposed strain. For PAA-20, it shows excellent shape memory behaviors. As seen in Figure 2c, both Rf and Rr are higher than 90% for all of the surveyed strain (from 100% to 800%), better than most reported RT programmable SMPs in the literature.28 To our best knowledge, the room temperature shape memory properties of this material is reported for the first time, although the two polymers are commonly and widely used to construct various types of composites.36,37 The evolution of crystalline structures of PAA-20 before and after deformation was characterized by differential scanning calorimetry (DSC) and wide angle X-ray diffraction (WAXD) measurements. Figure 2d shows the DSC curves of PAA-20 with different strains. It can be seen that the crystal melting heat first decreases upon stretching (from 126.1 J/g before stretching to 105.0 J/g for the sample stretched to 100% strain), but it apparently increases upon


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further stretching to 200% strain or higher. This observation can be explained by disruption and reformation of PEO crystallites during the deformation process. The disruption happens once the material is stretched, leading to a decrease of crystallinity and melting enthalpy, while the reformation of PEO crystallites take places under more prominent strain, as PEO chain orientation could facilitate the crystallization. Similar results are obtained from WAXD results (Figure 2e): two strong Bragg reflection peaks located at 2θ = 19.1° and 23.2° can be easily noticed, the peak centered at 19.1° is assigned to (120) series planes and the other is identified with (032) series plane diffraction.38 With the increase of elongation, the relative intensity of (120)/(032) is notably enhanced, indicating the oriented PEO chains are preferentially grown along the (120) direction, which is parallel to the extended chain direction and the fastest growth direction of PEO crystals. The orientation of PEO crystallinities upon stretching was also revealed by 2D WAXD (Figure 2f). While two isotropic diffraction rings are visible before elongation, corresponding to (120) planes (inner ring) and the overlapping reflections from (032), (132), (112), (212), (124), (204) and (004) planes (outer ring),39 they are broken after elongation. The orientation of PEO crystals becomes prominent when strain reaches 200%, with short equatorial arcs appearing in (120) and (032) reflections. The diffraction arcs are sharpened with increasing the strain, indicating increased orientation degree of PEO crystals as a consequence of larger elongation.


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Figure 2. (a) Tensile behaviors of PAA/PEO blends with different compositions. (b) Photos showing the room temperature programmability and shape memory behavior of PAA-20 (from high to low: initial film, after stretching at room temperature, after removing external stress, and after heating to 80 oC). (c) Shape fixity and shape recovery ratios of PAA-20 with different elongations. (d) DSC, (e) WXRD diffractograms and (f) 2D patterns of PAA-20 after elongating to different strains. In order to achieve controlled 3D shape transformation, the variables that affect the bending angle, including sample thickness, strain and concentration of the ion solution were quantitatively surveyed. We designed three experiments to investigate the effect of each variable on the angle of bending deformation of the material. In all experiments, PAA-20 samples were firstly stretched, then the alcohol solution of ferric chloride was deposited on the central area of


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the strip (3.0 mm × 4.0 mm). After evaporation of alcohol by placing the sample in air for 12 h, the strip was heated to 80 oC, and the bending angle was recorded. In the first experiment, samples with an initial thickness of 80 μm were stretched to different strains and surface patterned using 1.0 mol/L ferric ion solution. It is no surprise to see the results shown in Figure 3a that higher strain results in greater bending deformation, as the discrepancy of heat induced contraction along the thickness direction is more important for the sample with higher strain. In the second experiment, ferric ion solutions with different concentrations were used to pattern the 80 μm samples subjected to the same 200% strain. The results in Figure 3b show that the bending deformation is more prominent for the samples treated by ferric ion solution with higher concentration. This can be attributed to that higher ion concentration results in higher crosslinking density near the surface, which is more effective to restrict strain relaxation in the crosslinked region. In the third experiment, PAA-20 samples with different thicknesses were prepared, then stretched to 200% strain and finally surface patterned using 1.0 mol/L ferric ion solution. It can be seen from Figure 3c that larger bending deformation is obtained with thinner samples. This is understandable, because the out-of-plane bending is an outcome of competition between crosslinked regions that resist strain relaxation and un-crosslinked regions that allow strain relaxation. An optimal portion of thickness of the two regions must exist to achieve maximum bending degree. Within the investigated range of thicknesses, thinner samples appear to generate larger bending angles. It is easy to picture that if the crosslinking occurs across the entire thickness, neither bending nor contraction in the ion solution covered region will be observed. The fact that the thinnest strip (10 µm) displays the largest bending (180o) implies that the crosslinked layer thickness is much below 10 µm. Also, it should be noted that these experiments were designed to isolate the effect of each variable, while in practical situations they


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can be used in combination in different ways to obtain desired 3D shapes by controlling the bending areas and bending angle of each area in a specific specimen.

Figure 3. Data showing the effects of a number of parameters on the bending angle of PAA-20 strips after subsequently room temperature stretching, surface reaction with ferric ion solution and heating to 80 oC : (a) bending angle vs. sample strain (initial thickness: 80 um; concentration of ferric ion solution: 1.0 mol/L); (b) bending angle vs. ferric ion concentration (initial thickness: 80 um; strain: 200%); and c) bending angle vs. initial sample thickness (strain: 200%; concentration of ferric ion solution: 1.0 mol/L). Since 3D shape transformation can be realized by “inhomogeneous” or patterned shrinking of a flat sample that is determined by ionically crosslinked PAA, UV or visible light can be applied to spatially trigger the dissociation of PAA network via photo-reduction of Fe3+ to Fe2+, which allows the residual strain in these areas to be completely released upon heating. By sequentially controlling the recovery of these bended areas, a series of 3D shapes can be obtained during the shape recovery process. Before demonstrating this property, the reduction of Fe3+ to Fe2+ under visible light irradiation was characterized by XPS and UV-vis spectroscopy. To do the experiments, samples are entirely patterned and exposed to light on one side. As can be seen from Figure 4a, XPS reveals that the intensity of the characteristic peak of Fe2+ at 710.63 eV is


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significantly enhanced after 6 h irradiation under a commercially available desk lamp (LED light, optical power density: 12.1 mW/cm2) compared to that before irradiation, the ratio (Rcp) of integral area of the peak centered at 713.98 eV (Fe3+) compared to that at 710.63 eV (Fe2+) also decreased from 1.21 to 0.88 after irradiation. This result indicates that Fe3+ ions were effectively reduced. This photochemical reduction process could be more rapid if exposing the surfacepatterned sample to UV irradiation (optical power density: 136.8 mW/cm2), as the value of Rcp is lowered to 0.71 after only 5 min exposure. The reduction of ferric ions can also be seen from the UV-vis absorption spectra in Figure 4b. Two peaks located at 318 nm and 365 nm were gradually vanished under the exposure of visible light, as a result of the reduction of Fe3+ to Fe2+.40-42 Moreover, the effect of formation and dissociation of interactions between Fe3+ and carboxyl groups on mechanical properties of the material were characterized by dynamic mechanical analysis (DMA). Samples for DMA tests are surface-patterned and irradiated on both sides. The result in Figure 4c shows that the storage modulus (G’) experienced a sharp drop from 5.20 MPa to 0.005 MPa over the temperature range from 61 oC to 71 oC, which is due to the melting of PEO crystals. After surface patterning, the modulus increased by one order of magnitude over the entire temperature range investigated, implying that the ionic crosslinking of PAA can effectively act as molecular switch to prevent strain release when PEO crystals melt. After exposing the film to UV irradiation for a sufficient time (3 min), the modulus decreased to an value lower than that before surface treatment in the lower temperature range (< 65 oC), suggesting that the Fe2+ ions could not act as crosslinkers but as plasticizers that weakens the material, thus the strain could be released upon heating. In a designed experiment shown in Figure 4d, a strip of PAA-20 was stretched, surface patterned and heated to obtain a 3D shape with two bending areas with pale yellow color. Afterwards, visible light was applied to trigger


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the dissociation of PAA network in the right area, while the left side was covered to prevent light exposure. It can be seen that the color in the right area was gradually vanished over visible light irradiation due to the reduction of Fe3+ ions to Fe2+ ions. In the end, upon uniformly heating the sample to 80 oC, the residual strain in the bended area on the right side was totally released, flattening the right side of the strip while no shape change can be observed on the left side of the sample. Moreover, since Fe2+ can be oxidized to Fe3+ in air, the shape transformation can be repeated by controlling the reversible formation of PAA network (Figure S4). It should be emphasized

that

the redox and thermal processes in the operation act separately.

Essentially, Fe3+-carboxylate association is introduced as additional crosslinking to hinder thermally induced strain relaxation in the patterned regions, so that the overall shape can be transformed as designed. After photo-reduction, the coordination is dissociated to allow the strain in the patterned regions to be released upon heating, so that the material can gradually recover its originally planar (2D) shape.


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Figure 4. (a) XPS results of surfaced patterned PAA-20 before and after UV and visible light irradiation. (b) UV-Vis spectra of PAA-20 after visible light irradiation for different times. (c) Storage modulus as a function of temperature (from dynamic mechanical analyzer) for PAA-20, PAA-20 after surface patterning with ferric ion solution and surface patterned PAA-20 after UV irradiation. (d) Spatially controlled shape recovery via UV or visible light irradiation and subsequent heating. Based on the above results, some intricate 3D shapes and their stepwise controllable shape variation were realized. Typically, a designed 3D shape was first obtained by successively tailoring the PEO/PAA blend into specific 2D shape, stretching the sample at room temperature, applying surface patterning by ionic crosslinking in spatially selected areas and, finally, heating. Moreover, by sequentially triggering the dissociation in the selected areas of the 3D object via UV or visible light irradiation, stepwise controlled variable shape changes could be achieved easily. Four sets of images are given in Figure 5 to demonstrate the entire 3D shape changing process. It should be noted that only the surface-patterned area of some samples were stretched to ensure their whole size would not notably changed. Shown in Figure 5a, a PAA-20 film with a starting triangular shape was stretched and surface patterned, after heating the material to 80 oC, it transformed into a classic origami shape (paper airplane). Furthermore, by selectively exposing the treated area to visible light irradiation followed by thermal treatment, the two wings were sequentially unfolded before the film went back to its original triangular shape again. Following the sample route, a plastic flower and its blooming process (Figure 5b), a plastic hasp and its unlocking process (Figure 5c) and a helix and its uncoiling process (Figure 5d), were realized. We want to emphasize that since the intermediate 3D shapes were determined by the ionically


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crosslinked PAA, their durability to environmental conditions change, such as temperature or moisture, may endows them with a broader application scope.

Figure 5. Four sets of photos demonstrating the 3D shape changing process of PAA-20: (a) folding of a paper airplane and its unfolding process; (b) folding of a flower and its blooming process; (c) locking of a hasp and its unlocking process; (d) twisting of a curl and its uncurling process. The 3D shape changing process is: (i) samples are firstly tailored into designed 2D shapes; (ii) after room temperature elongation and surface patterning with ferric ion solution; (iii) target 3D shapes are obtained by heating the samples; and (iv) a series of origami shapes can be obtained during selective unpatterning through alternating photo-reduction and thermal stimulation. The scale bar is 1 cm. Finally, we demonstrate that by loading polydopamine nanoparticles (PDNPs) as photothermal fillers in PAA-20, the whole 3D shape changing process can be performed at room temperature


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by light. We chose PDNPs for demonstration because of their ease of preparation and high photo-thermal conversion efficiency. They were prepared using a method reported in the literature,43 and their size distribution was characterized by DLS (results in Figure S3). The temperature rise of the prepared samples containing various amounts of PDNPs upon NIR light exposure was first investigated. Figure 6a shows the images recorded with an infrared camera imaging system that show the NIR light induced temperature rise of PAA-20 samples loaded with different contents of PDNPs. It can be seen that the temperature in the irradiated region was prominently increased, while that of the surrounding areas was not affected, indicating the good spatial control of NIR light induced temperature rise. The highest temperatures reached for the samples with 0.25 wt%, 0.5 wt% and 1 wt% PDNP addition are 53.1 oC, 72.4 oC and 131.3 oC, respectively, and the heating process can be completed within 1 min under NIR light irradiation (Figure 6b). As a control test, the temperature of PAA-20 without PDNP addition was only mildly increased from 19.4 oC to 36.1 oC with the used NIR light irradiation (980 nm, 99.8 mW). These results indicate that with loaded PDNPs in PAA-20, the material exhibits rapid and prominent response to NIR light through the photothermal effect. The shape memory properties of the PEO/PAA/PDNP blend were then investigated, and the results shown in Figure 6c show that the effect of the photothermal agent (1 wt%) is quite small, with Rf decreasing from 96.2% to 90.3% and Rr increasing from 91.7% to 96.2% at the same time. Figure 6d shows an example of light-controlled, room temperature shape transformation. A strip of PAA-20 with 0.5 wt% PDNP addition was firstly stretched to 300% elongation followed by conducting the surface patterning, then NIR light was applied to irradiate the sample along the direction of elongation for strain relaxation in selected areas to obtain a target 3D shape. Afterwards, the unpatterning (i. e., dissociation of ionic crosslinking) followed by strain relaxation were carried out by exposing the


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specimen to UV and NIR light, respectively, allowing the material to be sequentially recovered to its original shape.

Figure 6. (a) Infrared thermal images of polydopamine nanoparticle (PDNP)-loaded PAA-20 under NIR light (980 nm, 99.8 mW). (b) NIR light induced temperature rise of PAA-20 with different contents of PDNPs. (c) Shape memory properties of PAA-20 with different PDNP contents. (d) Photos showing room temperature, light-controllable shape changing process of PAA-20 with 0.5 wt% PDNP addition. (scale bar: 1cm)

CONCLUSIONS In conclusion, we have reported an easy and versatile approach to achieving stepwise controllable 3D shape transformation from a flat specimen of the PEO/PAA blend. The polymer blend was found to exhibit excellent room temperature shape memory properties, with a plastic strain at failure higher than 800%, and both shape fixity and shape recovery ratios higher than


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90%. After easy surface patterning using a ferric ion solution, gradient crosslinking of PAA chains via coordination of Fe3+ and carboxyl groups can prevent strain relaxation in selected areas, which allows programmable bending deformation of the flat sample to occur upon direct or photoinduced heating, leading to target 3D object. The bending angle is controllable through tuning the parameters including sample thickness, strain and concentration of ferric ion solution. Moreover, the PAA crosslinking in patterned areas can be dissociated under light irradiation through photo-reduction of Fe3+, and, by resuming the strain relaxation capability in those areas, the bending deformation can be reversed upon heating on demand, which makes it possible to obtain multiple origami shapes from a single piece of film with tailored initial shape. Lastly, we demonstrated that by loading PDNPs in the PEO/PAA blend, the whole 3D shape changing process can be conducted at room temperature by light. This work enriches the strategy for realizing 3D shape transformations of SMPs with more flexible and adjustable shape changing behaviors. ASSOCIATED CONTENT Supporting Information. DSC curves and shape memory properties of PEO/PAA blends with different compositions, size distribution of PDNPs, recyclable shape memory behaviors of PAA20 via controlling the ionic crosslinking, shape recovery behaviors of PAA-20 programmed at different temperatures and moisture absorption behavior of PAA-20 at room temperature. AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (G.L.)


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* E-mail: [email protected] (Y.Z.) Author Contributions #

G.L. and S.W. contribute equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the Nature Science Foundation of China (Grant Nos. 51803115 NSFC 21636006), the Fundamental Research Funds for the Central Universities (Grant Nos. GK201601005, GK201801003, GK201802009, and GK201803031), the China Postdoctoral Science Foundation Funded Project (2017M623106), the Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (Sklpme2017-4-12). Y. Z. acknowledges the financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and le Fonds de recherche du Québec: Nature et technologies (FRQNT).

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TOC Graphic

heat

light & heat

light & heat

light & heat

polyacrylic acid

polyethylene oxide

Fe3+

crystalline domain


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2D-to-3D Shape Transformation of Room Temperature Programmable

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