Poly(vinyl alcohol)âTannic Acid Hydrogels with ... - ACS Publications

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Poly(vinyl alcohol)-Tannic Acid Hydrogels with Excellent Mechanical Properties and Shape Memory Behaviors Ya-Nan Chen, Lufang Peng, Tianqi Liu, Yaxin Wang, Shengjie Shi, and Huiliang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08374 • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on September 22, 2016

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ACS Applied Materials & Interfaces

Poly(vinyl alcohol)-Tannic Acid Hydrogels with Excellent Mechanical Properties and Shape Memory Behaviors Ya-Nan Chen, Lufang Peng, Tianqi Liu, Yaxin Wang, Shengjie Shi and Huiliang Wang* College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China

ABSTRACT: Shape memory hydrogels have promising applications in a wide variety of fields. Here we report the facile fabrication of a novel type of shape memory hydrogels physically crosslinked with both stronger and weaker hydrogen bonding (H-bonding). Strong multiple Hbonding formed between PVA and TA leads to their coagulation when they are physically mixed at an elevated temperature and easy gelation at room temperature. The amorphous structure and strong H-bonding endow the PVA-TA hydrogels with excellent mechanical properties, as indicated by their high tensile strengths (up to 2.88 MPa) and high elongations (up to 1100%). The stronger H-bonding between PVA and TA functions as the “permanent” crosslinks and the weaker H-bonding between PVA chains as the “temporary” crosslinks. The reversible breakage and formation of the weaker H-bonding imparts the PVA-TA hydrogels with excellent temperature-responsive shape memory. Wet and dried hydrogel samples with a deformed or elongated shape can recover to their original shapes when immersed in 60°C water in a few seconds or at 125°C in about 2.5 min, respectively.

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KEYWORDS: PVA, tannic acid, hydrogels, mechanical properties, shape memory

INTRODUCTION Shape memory hydrogels are a kind of stimuli-responsive, soft and wet materials that have the ability of fixing a temporary deformation and memorizing the original permanent shape when subjected to external stimuli, such as temperature, pH, light, electric field, ultrasound, magnetic field and chemicals.1-5 Due to their promising applications in sensors, actuators, artificial muscles, soft robotics and drug release,6-9 shape memory hydrogels have drawn rapidly increasing attention. Shape memory polymers usually have a strong fixing phase and a weak switchable phase.10-11 The former is responsible for the permanent shape, and the latter for the temporary shape. Through the reversible change of the weak switchable phase, shape memory effect can be achieved. Similarly, shape memory hydrogels generally have reversible or “temporary” crosslinks and nonreversible or “permanent” crosslinks to fix a temporary shape and remember a permanent shape, respectively.12-17 In recent years, growing attention has been paid to the preparation of shape memory hydrogels based on reversible non-covalent interactions, such as electrostatic interaction, metal-ligand binding, host-gust interaction, hydrogen bonding (Hbonding).18-23 Poly(vinyl alcohol) (PVA) hydrogels are the earliest reported shape memory hydrogels. PVA hydrogels are commonly prepared through a repeated freezing-thawing (FT) method, and they are physically crosslinked by PVA crystallites formed during the freezing process and the Hbonding between PVA chains.24 But common PVA hydrogels do not exhibit shape memory, due to the easy melting of PVA crystallites and the breakage of H-bonding upon heating to 50-60°C.


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Therefore, stronger chemical or physical interactions that can maintain the permanent shape of the gels need to be introduced. Hirai et al. first reported the shape memory of PVA gels that are further chemically crosslinked with glutaraldehyde.12 Li et al. recently reported a PVA-PEG double-network hydrogel with shape memory behaviors25 and a melamine-enhanced PVA physical hydrogel with therapeutic-ultrasound-triggered shape memory behaviors.26 The chemical crosslinking in the first PEG network25 or the strong H-bonding between PVA and melamine26 maintains the permanent shape, while the reversible crystallization and melting of PVA crystallites provides the temporary shape. In addition, some other shape memory PVAcontaining hydrogels with different shape memory mechanisms have also been reported. For instance, Meng et al. reported a shape memory hydrogel prepared from phenylboronic acid grafted alginate (Alg-PBA) and PVA.27-28 The electrostatic interactions between Ca2+ and alginate serve as “permanent” crosslinks and the dynamic PBA-diol ester bonds serve as the “temporary” crosslinks.27 If Ca2+ is extracted by CO32- or complexed by a competitive ligand such as EDTA⋅2Na, then the interactions between Ca2+ and alginate are weakened, and hence they can also function as the “temporary” crosslinks.28 Very recently, Chen and coworkers reported hydrogels with triple shape memory functionalities by incorporating two non-interfering supramolecular interactions into the gels.29 In summary, the “permanent” crosslinks can be either covalent or non-covalent, and the “temporary” crosslinks are usually non-covalent or dynamic covalent. The critical structural feature of shape memory hydrogels is that they should have relatively stronger and weaker crosslinks that can maintain the permanent shape and fix the temporary shape, respectively. Therefore, it is reasonable to deduce that shape memory hydrogels can be made from only one kind of physical interactions, e.g. H-bonding, but with different strengths. In addition, due to the


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easy breakage and reformation of physical interactions, the fixation of the temporary shape can be realized under mild conditions and the shape recovery rate of the gels might be largely accelerated. In this work, we proposed to construct shape memory hydrogels based on stronger and weaker H-bonding. Tannic acid (TA) is a kind of plant-derived polyphenols.30 Due to the presence of many functional groups in its chemical structure, it can easily form H-bonded or coordination complexes with polymers or Fe(III) ions, respectively. TA-based materials have promising applications in biomedical and other fields as they are highly stable under physiological conditions and non-toxic to in vitro cultured cells.31-35 TA can form multiple and possibly strong H-bonding with PVA. The stronger H-bonding between PVA and TA might function as the “permanent” crosslinks and the weaker H-bonding between PVA chains as the “temporary” crosslinks, so shape memory hydrogels can be obtained. PVA-TA hydrogels were prepared through an extremely simple way, i.e. physical mixing of PVA and TA at an elevated temperature and cooling at room temperature. The PVA-TA hydrogels exhibited excellent mechanical and shape memory properties.


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RESULTS AND DISCUSSION Hydrogel Preparation and Formation Mechanism. Our hydrogel preparation experiments showed that, when the content of TA was less than 1 wt%, the aqueous solutions of PVA (10 wt%) and TA were homogeneous, but they could not transform into PVA-TA hydrogels at room temperature (25°C). However, when TA content was increased to more than 3 wt%, the dissolution of PVA and TA produced inhomogeneous mixtures, in which highly viscous coagulates of PVA and TA were formed, and the coagulates transformed into gels easily even at room temperature. Certainly, hydrogels could be formed when the homogeneous solutions and the inhomogeneous coagulates were treated with classical freezing-thawing method. The formation of coagulates of PVA and TA and their easy gelation suggest there are stronger interactions between PVA and TA. Tannic acid (TA) has 25 hydroxyl and 10 carbonyl groups (Scheme 1a), all are capable of forming H-bonding with PVA chains which have many hydroxyl groups (Scheme 1b). Therefore, the addition of TA small molecules into PVA introduced more and stronger H-bonding between PVA chains (left in Scheme 1c). In other words, the PVA-TA hydrogels are formed mainly through the H-bonding between PVA and TA, which acts as strong physical crosslinking points.


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Scheme 1. Structures of TA (a), PVA (b) and temperature-responsive shape memory mechanism of PVA-TA hydrogel (c). The water contents of the gels generally decreased with increasing TA content (Figure 1). The water contents of the gels prepared with a low TA concentration (less than 2 wt%) were slightly lower than the theoretical values calculated from feed ratios, but those of the hydrogels prepared with higher TA contents deviated significantly from the calculated values, and the difference became more significantly with increasing TA content. For instance, the water content of the gel prepared with 10 wt% PVA and 10 wt% TA was only 58 wt%. The reason for the lower water


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contents of the hydrogels with higher TA contents is the formation of more densely crosslinked coagulates that expel more water. 90

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Figure 1. Water contents of the PVA-TA hydrogels prepared at 25°C as a function of TA content. To prove the presence of stronger H-bonding between PVA and TA, ATR-FTIR characterization of PVA, TA and a PVA-TA hydrogel was performed. The broad and strong absorption bands at 3268 cm-1 and 3276 cm-1 are attributed to the symmetrical stretching vibration of hydroxyl groups of TA and PVA, respectively. While the −OH stretching peak of the PVA-TA hydrogel shifted to a lower wavenumber of 3253 cm-1. It is well-known that the formation of intra- or intermolecular hydrogen bonding reduces the force constants of the chemical bonds, and hence their vibrational frequencies are shifted to lower wavenumbers. Stronger hydrogen bonding leads to a more significant shift in vibrational frequency.36 The significant shifts of the absorption bands to lower wavenumbers suggest the formation of stronger H-bonding between PVA and TA. PVA is a semicrystalline polymer, and PVA crystallites act as the crosslinking points in common PVA hydrogels prepared with F-T method. As shown in Figure 2b, X-ray diffraction


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(XRD) pattern of PVA shows three typical peaks at 2θ=19.6°, 2θ=22.9° and 2θ=40.8°, corresponding to the (101) (200) and (102) planes of PVA crystallites.37 The XRD pattern of TA does not show obvious sharp peaks of crystalline structures, instead a blunt peak centered at 2θ=26.1° is observed, indicating the amorphous nature of TA. For the PVA-TA hydrogels prepared at 25°C and -15°C, the typical peaks of PVA crystallites disappear, with only a blunt amorphous peak centered at 2θ=21.5°. These results reveal that the PVA-TA hydrogels do not possess the typical crystalline structures as normal PVA gels even when the gels are frozenthawed, as the strong H-bonding between PVA and TA prevents the formation of PVA crystallites.

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Figure 2. ATR-FTIR spectra (a) and X-ray diffraction (XRD) patterns (b) of PVA, TA and a PVA-TA hydrogel (10 wt% TA). Mechanical Properties of the Hydrogels. The PVA-TA hydrogels exhibited excellent mechanical properties. Figure 3a shows the typical tensile stress-strain (σt-εt) curves of the hydrogels prepared with different TA contents. The tensile strengths (σb), elongations (εb) and elastic moduli (E) of the PVA-TA hydrogels are summarized in Figure 3b-3d, respectively. All mechanical properties increased dramatically with increasing TA content till 6 wt%, and then


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decreased or kept constant. The PVA-TA hydrogel prepared with 6 wt% TA had σb, E and εb of 2.88 MPa, 0.36 MPa and 1060%, respectively. The tensile strengths of the PVA-TA hydrogels are much higher than pure PVA hydrogels made with several F-T cycles,38 and they are similar to those enhanced by adding graphene oxide39 or melamine.26 Here we want to stress that the elongations of the PVA-TA hydrogels are much higher than other types of PVA gels (mostly 100-500%). The elongations of the PVA-TA hydrogels with a TA content of more than 6 wt% are around 1100%, which are the largest values ever reported. The strong H-bonding between PVA and TA molecules endows the hydrogels with high moduli, and the amorphous structure and the easy breakage of H-bonding provide the gels with an effective energy-dissipating mechanism, leading to their high tensile strengths and elongations. The decrease in mechanical properties after 6 wt% TA content is very possibly due to the structural inhomogeneity induced by the more significant coagulation during mixing and the fast gelation upon cooling. Note that the mechanical properties of the PVA-TA hydrogels can also be further enhanced by F-T treatment (Figure S1). The hydrogel (6 wt% TA) prepared at -15°C, its σb and E were 3.57 MPa and 0.44 MPa, respectively. As no PVA crystallites are formed after F-T treatment (Figure 2b), the enhanced mechanical property is attributed to the formation of more H-bonds between PVA and TA as well as between PVA chains during freezing.


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Figure 3. Typical tensile stress-strain curves (a) and tensile strengths (b), elongations (c) and elastic moduli (d) of the PVA-TA hydrogels with different TA contents prepared at 25°C. Temperature-dependent dynamic mechanical properties of the PVA-TA hydrogels were measured. As shown in Figure 4 and Figure S2, the storage (elastic) modulus (G′ ) was always higher than the corresponding loss (viscous) modulus (G″ ) in the entire temperature range (2575°C), suggesting the elastic nature of the PVA-TA hydrogels. The PVA-TA hydrogel showed very small decrease in G′ and G″ and the loss factor (tanδ) kept almost constant when the temperature was less than 50°C, but after then both G′ and G″ decreased significantly, and the more significant decrease in G′ led to the increase of tanδ. Obviously, the decrease of moduli


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with increasing temperature is attributed to the damage of the weaker H-bonding between PVA chains. 10

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Figure 4. Storage modulus (G′ ), loss modulus (G″ ) and loss factor (tanδ) of the PVA-TA hydrogel (10 wt% TA) prepared at 25°C as a function of temperature at a fixed frequency of 1 Hz. Shape Memory Behavior. The PVA-TA hydrogels also exhibited excellent shape memory behaviors. Figure 5 shows the thermally triggered shape memory of a PVA-TA hydrogel that was deformed into a helix. By wrapping a gel strip (Figure 5a) on a glass rod and fixing with binder clips for 3 h at room temperature (Figure 5b), a temporary deformed shape with several loops was obtained (Figure 5c). Notice that there were about 9 loops fixed on the glass rod, but only about 6 loops were maintained when the gel sample was released. When the deformed gel helix was immersed into 60°C water, its shape was mostly recovered in 2 s (Figure 5d and Movie S1).


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Figure 5. Photos showing the shape memory behavior of a PVA-TA hydrogel prepared at 25°C. (a) the original as-prepared gel strip, (b) the gel strip wrapped on a glass rod for 3 h at room temperature, (c) the deformed gel strip, and (d) the gel sample with recovered shape after being immersed in 60°C water. The shape fixity ratios (Rf) and shape recovery ratios (Rr) of the gels were obtained with the simple bending experiments performed on U-shaped specimens (Figure S5). The Rf of the gels was not significantly affected by TA content, while the gel preparation temperature had an obvious effect on them (Figure 6a). The Rf of the gels prepared at -15°C were mostly around 65%, higher than those of the corresponding gels prepared at 25°C (around 55%). The fixity ratios of the our PVA-TA gels are relatively lower with comparison to other types of shape memory gels, the possible reason is that only part of the H-bonding between PVA chains that function as temporary crosslinks can be broken and then reformed during the fixation process carried out at room temperature. The higher fixity ratios for the gels prepared at a lower temperature is very possibly due to the presence of more H-bonding produced by freezing in them.


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Figure 6. (a) Shape fixity ratios (Rf) of the PVA-TA gels prepared with different TA contents at 25°C and -15°C, respectively, and (b) the shape recovery ratios (Rr) of the gel (6 wt% TA) prepared at 25°C as a function of time being immersed in 60°C water. When the PVA-TA hydrogels in the fixed U-shape were immersed in 60°C water, the breakage of H-bonding between PVA chains led to the recovery of their original shape. Figure 6b shows the change of Rr of a gel sample with time. The Rr increased very quickly in the first 2 s and then at a slower rate to 100% at 5 s. Similarly, other gels prepared with different TA contents and at different temperatures also quickly and completely recovered their original shape. The PVA-TA hydrogels also exhibited heat-shrinkable shape memory behavior. This behavior was demonstrated with elongated gel strips and enlarged gel rings. An original gel strip with a length of 75 mm was cut from a PVA-TA hydrogel prepared at -15°C (Figure 7a). The middle part of the gel strip (33 mm, in arrows) was elongated and then settled for 3 h at room temperature (Figure S3). The length of the elongated middle part was 86 mm (Figure 7b). After being immersed in 60°C water, the elongated part shrank very quickly (Movie S2) to a remaining length of 35 mm (Figure 7c), very close to the original length.


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A gel ring with an inner diameter of 20 mm and a thickness of 2 mm (Figure 7d) was enlarged by setting on a beaker (Figure S4). The fixation could also be realized by freezing method. After being frozen with liquid nitrogen for only about 2 min, an enlarged ring with an inner diameter of about 30 mm was obtained (Figure 7e). When the enlarged sample was immersed in 60°C water, it recovered to its original shape and size in about 3 s (Figure 7f and Movie S3). Applying the heat-shrinkable property, some interesting applications of the gels can be developed. For example, we used an enlarged gel ring (by setting on a beaker for 3 h at room temperature; inner diameter: about 40 mm) to catch a weighing bottle (diameter: 20 mm, about 8 g) whose bottom part was placed in 60°C water. The enlarged gel ring shrank when it was immersed into water, and then the weighing bottle was caught and it could be transferred from water to air (Movie S4).

Figure 7. Photos showing the heat shrinkable shape memory behavior of the PVA-TA hydrogels. (a-c): A gel strip in the original (a), elongated (b) and recovered (c) shapes; (d-f): a gel ring in the original (d), enlarged (e) and recovered (f) shapes. The fixation of the elongated gel strip was conducted by keeping at room temperature for 3 h and that for the enlarged gel ring


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was by freezing with liquid nitrogen for 2 min. The PVA-TA hydrogels were formed at -15°C (ac) and 25°C (d-f). All of the above shape memory behaviors were performed with as-prepared wet hydrogels. We noticed that the water content did not affect the shape memory properties of the gels significantly when it was more than 20%, but when the water content was less than 20%, the Rf of the gels increased quickly with decreasing water content and it reached about 100% at the water content around 10% (Figure S6). We are curious about if the dried samples can perform shape memory behaviors. We found that the vacuum dried gel samples could be softened and deformed at a temperature higher than 80°C. Figure 8 shows the shape memory behavior of a dried gel strip. The gel strip was heated and its middle part was twisted into several helixes at 125°C, then a temporary deformed shape was maintained by cooling it at room temperature, with an Rf very close to 100%. When the twisted sample was placed into an oven preheated to 125°C, it completely recovered to its original shape in 2.5 min, showing an Rr of 100%. Impressively, the sample exhibited excellent cyclic shape memory behavior. This process can be repeated for many times, as shown in Figure 8.

Figure 8. Photos showing the three successive shape memory cycles of a dried PVA-TA hydrogel. The deformation and recovery of the sample was conducted at 125°C, and the fixation was conducted at room temperature.


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It is necessary to note that the shape recovery rates (seconds) of the PVA-TA hydrogels are among the fastest ever reported.40-42 The recovery rate of the dried gels is also among the fastest reported for shape memory polymers.43-44 The temporary shape fixation methods used in our work are very convenient. The fixation can be realized by maintaining the deformed gel at room temperature for 3 h or by freezing with liquid nitrogen for several minutes, without using any chemicals5, 27 or at a high temperature22-23. The driving force for the shape memory of the PVA-TA hydrogels should also be different from other hydrogels. As shown in Scheme 1c, there are two types of H-bonding in the PVA-TA hydrogels, i.e. the stronger H-bonding between PVA and TA (blue) and the weaker one between PVA chains (purple). The former stronger one is not easy to be broken under an applied force, while the latter weaker one is easy to be broken and it is also easy to be reformed. In other words, the H-bonding between PVA and TA acts as “permanent” crosslinks, and the H-bonding between PVA chains as “temporary” crosslinks. The reformation of H-bonding between PVA chains in the deformed state can fix the deformed shape of a gel. On heating to a proper temperature that is sufficient for breaking the H-bonding between PVA chains, the deformed shape should recover to the original permanent shape maintained by the stronger H-bonding between PVA and TA. CONCLUSION In summary, we have successfully prepared a kind of tough hydrogels with excellent shape memory properties through a facile physical mixing method. All these features are based on the stronger H-bonding between PVA and TA and the weaker H-bonding between PVA chains. The very easy formation of strong H-bonding between PVA and TA leads to their coagulation and gelation. The formation of H-bonding between PVA and TA also inhibits the formation of PVA crystallites, which generally act as strong crosslinkers in common PVA gels. The amorphous


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nature and strong H-bonding endow the PVA-TA hydrogels with excellent mechanical properties. The stronger H-bonding between PVA and TA acts as “permanent” crosslinks, and the weaker H-bonding between PVA chains as reversible or “temporary” crosslinks. The reversible nature of the weak H-bonding imparts the PVA-TA hydrogels with very fast and reversible shape memory properties. We believe that this hydrogel preparation strategy can also be applied to other materials by utilizing some other physical interactions. The hydrogels with high mechanical strengths and excellent shape memory behavior would be an attractive candidate for a wide variety of applications.


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EXPERIMENTAL SECTION Materials: Polyvinyl alcohol (PVA) (alcoholysis degree: 99%, Mn: 7.7×104) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), tannic acid (C76H52046, A. R. grade) was from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China), and rhodamine B was from Alfa Aesar (Tianjin) Chemical Co., Ltd. (Tianjin, China). All chemicals were used without further purification, and deionized water was used for all experiments. Preparation of hydrogels: PVA (10 wt%) and TA (0-10 wt%) were dissolved in deionized water by heating at 90°C with the assistance of mechanical agitation for 2 h, resulting in homogeneous solutions when the TA concentration was less than 2 wt% or inhomogeneous mixtures in which a highly viscous coagulates of PVA and TA was formed when the TA content was in the range of 3-10 wt%. Then, the homogeneous solutions or coagulates were transferred into molds made of two glass plates separated by a silicon rubber spacer with a thickness of 2 mm or 5 mm. To avoid macroscopic structural inhomogeneity in the gels induced by fast gelation of the systems, the molds were preheated to 110°C. PVA-TA hydrogels were obtained by setting the molds at ambient temperature (25°C) or at -15°C for 24 h. Rhodamine B (c = 1×10-4 mol/L) was added into the gels for demonstrating shape memory behavior. Water contents (WC) of the as-prepared hydrogels were calculated as follows: WC = (mwet mdry) /mwet ×100%, where mwet and mdry are the masses of the as-prepared samples and the vacuum-dried samples, respectively. Characterization: Vacuum-dried PVA, TA and PVA-TA hydrogel samples were ground into powders. X-ray diffraction (XRD) patterns were recorded using a PANalytical-X’ Pert PRO diffractometer (PANalytical Co. Ltd., Netherlands) using Cu-kα radiation running at 40 kV and 40 mA. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra of PVA, TA


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and PVA-TA hydrogels were recorded on a Nicolet FTIR 6700 spectrometer (Thermo Electron Scientific Instruments Corp., USA). Tensile mechanical testing: Dumbbell-shaped gel specimens, standardized as DIN-53504 S3 (overall length: 35 mm; width: 6 mm; inner width: 2 mm; gauge length: 10 mm; thickness: 2 mm), were tested with an Instron 3366 electronic universal testing machine (Instron Corporation, MA, USA) using a 100 N load cell at a cross-head speed of 100 mm/min−1. The gel specimens were coated with a thin layer of silicon oil to prevent water loss during the tests. To obtain reliable data, three specimens per experimental point were tested in all mechanical tests. Tensile stress (σt) was calculated as follows: σt=Load/tw (t and w were the initial thickness and width of the dumbbell shaped gel specimens, respectively). Tensile strain (εt) is defined as the change in the length relative to the gauge length of the freestanding specimen. Tensile strength (σb) and elongation (εb) of a specimen are the tensile stress and strain when the specimen breaks. Stress and strain between εt=10%–35% were used to calculate initial elastic modulus (E). Dynamic mechanical analysis: Dynamic mechanical analysis was carried out on square specimens (6 mm × 6 mm × 2 mm) in a shear mode using a Q800 dynamic mechanical analyzer (DMA) (TA Instrument, USA) equipped with a shear fixture. Temperature sweeps was performed in the range of 25-75°C at a strain amplitude of 15 µm and a fixed frequency of 1 Hz. Storage moduli (G′ ) loss moduli (G″ ) and loss factors (tanδ = G″ / G′ ) were calculated using the DMA software. Shape memory behavior: Hydrogel strips (75 mm × 4 mm × 2 mm) or concentric rings (Douter = 27 mm, Dinner= 20 mm, thickness = 2 mm or 5mm) were cut from the as-prepared PVA-TA hydrogels, and then the gel strips were deformed into a helix by wrapping on a glass rod or elongated, and the rings were enlarged. The deformed shapes were fixed by keeping at room


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temperature for 3 h or putting into liquid nitrogen for several minutes. The shape recovery process of the deformed gel samples was conducted by immersing them into 60°C water, and it was recorded with a digital camera (600D, Canon, Japan). Moreover, the concentric ring (5 mm thickness) with an enlarged shape was used to grasp a weighing bottle in 60°C water. Quantitative information on the shape memory properties was determined according to the reported method.27 A straight gel strip was bent into a U-shape and then kept in a test tube at room temperature for 3h. Then the deformed hydrogel was transferred into 60°C water. The shape fixity ratio (Rf) and shape recovery ratio (Rr) were defined by the following equation: Rf = θt/θi × 100% Rr = (θi-θf)/θi × 100% where θi is the given angle, is θt is the temporarily fixed angle and θf is the final angle. PVATA hydrogel strips were vacuum dried at 50°C for 2 d. The dried samples were softened and deformed at 125°C, and then cooled at room temperature to fix the deformation. The shape recovery was conducted by heating them at 125°C.

ASSOCIATED CONTENT Supporting Information: The Supporting Information is available free of charge on the ACS Publications website at DOI: Figures S1 and S2 representing the tensile properties and temperature-dependent dynamic mechanical properties of the PVA-TA hydrogels prepared at -15°C, respectively, Figures S3 and S4 showing the photos of the fixation of the elongated and enlarged gels, respectively, Figure S5


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showing the photo of a fixed U-shaped gel sample, and Figure S6 showing the shape fixity ratios (Rf) of the PVA-TA gels with different water contents. (pdf) Movie S1-S4 show the shape memory properties of the PVA-TA gels (AVI) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grant No. 21274013), the Fundamental Research Funds for the Central Universities and the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT). REFERENCES (1) Hao, J.; Weiss, R. A. Mechanically Tough, Thermally Activated Shape Memory Hydrogels. ACS Macro Lett. 2013, 2, 86-89. (2) Guo, W.; Lu, C.-H.; Orbach, R.; Wang, F.; Qi, X.-J.; Cecconello, A.; Seliktar, D.; Willner, I. pH-Stimulated DNA Hydrogels Exhibiting Shape-Memory Properties. Adv. Mater. 2015, 27, 73-78.


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Table of Contents Graphic

Poly(vinyl alcohol) (PVA) - tannic acid (TA) hydrogels are prepared with a facile physical mixing method. The amorphous structure and strong multiple H-bonding between PVA and TA endow the hydrogels with very high tensile strengths and high elongations (up to 1100%). The stronger H-bonding between PVA and TA functions as the “permanent” crosslinks and the weaker H-bonding between PVA chains as the “temporary” crosslinks. The reversible breakage and formation of the weaker H-bonding impart the PVA-TA hydrogels with excellent temperature-responsive shape memory.


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