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Open AccessCCS ChemistryCOMMUNICATION1 Feb 2020

Flexible Dry Hydrogel with Lamella-Like Structure Engineered via Dehydration in Poor Solvent

    CCS Chemistry. 2020, 2, 533–543
    https://doi.org/10.31635/ccschem.019.201900007

    Hydrogels are among the most promising biologic materials in recent technology with numerous desired applications, including serving as biosensors, drug delivery vehicles, and tissue-engineered products for cell matrices. However, they often dehydrate, and become stiff and brittle in air, causing loss of flexibility and functions. Several layered structures have been proven to increase the strength, toughness, and even flexibility of these materials, which might provide a new clue for the sustenance of the flexibility of drying gels. Herein, we report a novel solvent-dehydrated hydrogel engineering approach, aimed to change the inner structure and keep the flexibility of a dehydrated hydrogel in the air via solvent-induced dehydration, for example, acetone-dehydrated polyacrylic acid hydrogel. This flexible dry gel could be folded, twisted, and stretched without any damage due to the assumed lamella-like structures, contrary to dry gels without these microstructures or those with porous structures, which retain brittle consistency. The flexible dry gel also exhibited excellent self-healing capability with the assistance of solvents. Fascinatingly, this flexible gel film displayed strain-visualizing paper writing/erasing performance properties, with water acting as invisible ink. Thus, this fabricated flexible hydrogel film might function as confidential information storage material. Our current approach is versatile, hence applicable to other hydrogels, and provides insight into the engineering of other functional gels for extended future applications.

    Introduction

    A hydrogel is a flexible network of cross-linked hydrophilic polymer chains filled with water and has gained widespread use in diverse fields, such as biomedicine,1–4 flexible sensors,5,6 artificial actuator,7,8 and others.9–11 However, prepared hydrogels usually encounter serious problems, including losing their flexibility and becoming stiff and brittle after dehydration in air,12,13 which hinders their development and extensive applications. For example, the modulus of polyacrylamide (PAM) hydrogel expands to ∼108 N/m2 from < 104 N/m2 during air dehydration.13 It is well-known that the physical properties of materials are not only determined by their compositions but also by their inner microstructures.14–16 For instance, porous structures usually decrease the tensile modulus and strength of bulk materials due to crack formation around the pore when subjected to stress,17–19 while layered structures found in nature20–22 have proven to dissipate deformation energy without propagation of cracks, with increased strength, hardness, and toughness of these materials.23–26 In recent years, series of artificial layered materials have been fabricated via layer-by-layer techniques,27–29 ice-templating,30–32 self-assembly,21,33 and external-field-assisted assembly,34,35 which not only exhibit increased stiffness and toughness but also high flexibility.28 Therefore, it is an opportunity, as well as a challenge to engineer the inner microstructures of the dehydrated hydrogel in air, in favor of keeping their flexibility toward practical applications.

    In a poor solvent, the hydrogels usually shrink and become opaque due to phase separation. Moreover, the phase-separated gels tend to have a bicontinuous domain structure, consisting of dense and sparse domains in the polymer network mesoscopic scale,36–39 thereby, providing prospects for engineering the inner microstructure of dry gels, while enabling them to maintain their flexibility.

    Our present report describes a strategy for engineering the inner structures of dry hydrogels in air to achieve flexibility via aqueous-phase polymerization, dehydration in a poor solvent, and drying in air to generate a solvent-dehydrated hydrogel. Compared with the nonfabricated air-dried gel, this solvent-dehydrated hydrogel maintained its flexibility and exhibited an increased toughness than that of the initial hydrogel. In addition, they were able to fold, twist, and stretch without any damage, whereas the air-dried hydrogel was hard and brittle, making it prone to breakage when bent. Moreover, in contrast to porous structured and condensed structured, brittle dry gels, our created flexible solvent-dehydrated hydrogels exhibited lamella-like structures, which might have contributed to its flexibility. Furthermore, the solvent-dehydrated hydrogels exhibited an excellent stretchable capacity, humidity-sensitivity, and solvent-assisted self-healing properties. Finally, invisible flexible/hard gel patterns could be fabricated utilizing the differences in dehydration of the hydrogel in a solvent and in air and visualized by strain sensing, which could serve as confidential information storage materials.

    Results and Discussion

    Engineering the inner structure of dry PAA film

    We used polyacrylic acid (PAA) hydrogel as an example with acetone as the poor solvent and engineered through solvent-induced dehydration of PAA hydrogel in acetone to generate acetone-dehydrated PAA (acetone-DPAA) hydrogel. The original PAA hydrogel film was fabricated by free-radical polymerization with acrylic acid as the monomeric units and N,N′-methylenebisacrylamide as cross-linker (). When immersed in acetone, the hydrogel film shrank and became opaque gradually due to the diffusion of the solvent into the gel, followed by the curving of the film, as well as an appearance of blister-like protuberances on the gel. Subsequently, the protuberances disappeared slowly, and a flat transparent flexible film was obtained (Figure 1a). When exposed to air, the acetone in the dehydrated gel evaporated, and flexible acetone-DPAA was retrieved. After the acetone-induced dehydration, the polymer network gradually collapsed and the inner structures of the gel transitioned from the porous hydrogel structure (Figure 1b, freeze-dried PAA hydrogel) to the lamella-like structures of acetone-DPAA (Figure 1c), which might have resulted from the phase separation in acetone, contrary to the behavior of the naturally air-dried PAA (air-DPAA), which appeared dense, and without microstructures (Figure 1d). Accordingly, the two gel types exhibited different mechanical properties. The acetone-DPAA was flexible and could be folded, twisted, and stretched, whereas the air-DPAA and freeze-dried PAA would break easily when subjected to bending ().

    Figure 1

    Figure 1 | Dehydration of PAA hydrogel in acetone to generate flexible dry PAA gel. (a) The PAA hydrogel experienced a transparent–opaque–transparent process in acetone during which phase separation occurred. (b) The cross-sectional morphology of freeze-dried PAA hydrogel: porous structures. (c) The cross-sectional morphology of acetone-DPAA: lamella-like structures. (d) The cross-sectional morphology of air-DPAA: dense and without microstructures. (e) The phase diagram of the ternary system: hydrogel, solvent, and nonsolvent. The red line is the binodal phase separation curves. The left side of it is homogenous phase and the right part is of two phases. The blue arrow indicates that increase of nonsolvent would lead to phase separation of the system. (f) Representative tensile stress–strain curves of air-DPAA, acetone-DPAA, and PAA hydrogel. Inset: tensile stress–strain curves at low strain. (g) Young’s modulus, E, of the air-DPAA, acetone-DPAA, and PAA hydrogel.

    To explain the change that occurred in the inner structure of the gels, we carried out a thermodynamic analysis of the hydrogel in the poor acetone solvent, according to the Flory–Huggins theory.40,41 This system is a ternary mixture of three components, namely, a poor solvent (label 1), a suitable solvent (label 2), and polymer chains with different degrees of polymerization, N (label 3). Each component volume fraction is denoted by φi and the required incompressibility is expressed by:

    φ 1 + φ 2 + φ 3 = 1(1)

    The interactions between each pair of the components are of Flory–Huggins type, χij. The equation of the free energy of the system is expressed by:

    f = φ 1 l n φ 1 + φ 2 l n φ 2 + φ 3 N l n φ 3 + χ 12 φ 1 φ 2 + χ 13 φ 1 φ 3 + χ 23 φ 2 φ 3 (2)

    For a ternary system, the spinodal curve, defined as the limits of stability of the homogeneous phase, is given by:

    | f 11 f 12 f 21 f 22 | = 0(3)
    where
    f 11 = 1 φ 1 + 1 N φ 3 − 2 χ 13 f 22 = 2 φ 2 + 1 N φ 3 − 2 χ 23 f 12 = 1 N φ 3 + χ 12 − χ 13 − χ 23 .

    Using the above expressions, we could calculate the spinodal curves (with χ12 = 0, χ23 = 0, and χ13 = 3) as a simple case. Our typical experimental phase diagram is shown in Figure 1e, with its components consisting of the following: the poor solvent, the good solvent, and the polymer corresponding to acetone, water, and PAA hydrogel, respectively. The experimental system started from the left corner, where the bad solvent (acetone) concentration is zero. During the solvent-induced dehydration, in which there was a mutual diffusion of water and acetone, the acetone concentration in the gel increased gradually, indicated by the arrow in Figure 1e. When the system entered the instability region, it underwent phase separated into a polymer-rich and a polymer-poor part. However, as the polymer chains were cross-linked, it is unexpected that macrophase separation should take place, but it is plausible that microphase separation occurred, corresponding to the phase during which the gel displayed opaque appearance, which leads to the formation of layered structures.

    Mechanical property of the acetone-DPAA film

    To further explore the flexibility of the acetone-DPAA, a tensile machine was used to perform elongation tests on the hydrogel, solvent-dehydrated hydrogel, and air-dried gel at room temperature. Amazingly, the acetone-DPAA could be stretched to ∼5.5 times its original length, without rupture (Figure 1d and ). Figure 1f shows typical tensile stress–strain curves, which compares the acetone-DPAA with the air-DPAA, using the conventional PAA hydrogel as a control. The acetone-DPAA was capable of stretching, with an elongation at break of 5.5, and stress of 5.4 MPa at break. Meanwhile, the air-DPAA exhibited a rather low elongation of 0.066 at break and high stress of 45.5 MPa at break, which confirmed the flexibility of acetone-DPAA and the toughness and brittleness of the air-DPAA. Also, the elongation tests of the conventional PAA hydrogel control demonstrated fragile behavior, breaking easily before being stretched to strain = 1 (Figure 1f inset and ). Young’s modulus, E, calculated from the stress–strain curves (Figure 1g), revealed that the acetone-DPAA had an E of 46.8 MPa, similar to that of rubber, but much higher than that of the original PAA hydrogel with a negligible E (Figure 1g). However, the E of acetone-DPAA was much lower than that of the brittle air-DPAA (1160 MPa). Besides, the flexibility of the acetone-DPAA could be maintained even after air exposure for more than one month, reflecting its durability (). These results verified that the lamella-like structures endowed the dried acetone-DPAA gels with flexibility and increased toughness, and stability, compared with the original PAA hydrogel and the air-DPAA gel.

    Furthermore, the effects of the monomer and the cross-linker concentrations on the mechanical property of acetone-DPAA were investigated. Similar to the conventional hydrogel, the modulus of the acetone-DPAA increased gradually with the increase of cross-linker concentrations (). However, the monomer concentrations yielded a negligible effect on the mechanical property of acetone-DPAA. In addition, the representative tensile stress–strain curves of acetone-DPAA at varying AA concentrations revealed an almost overlapping effect (), which might be attributable to the complete collapse of the PAA hydrogel networks after treatment in acetone to generate acetone-DPAA, resulting in increased swelling ratio in a linear dimension, as the AA concentration increased (). Thus, on the whole, the increased monomeric concentration exhibited a slight influence on the acetone-DPAA network and their mechanical properties.

    Mechanism of the flexibility of acetone-DPAA

    Thermogravimetry (TG) was conducted to measure the actual water content of the air-DPAA, freeze-dried PAA, and acetone-DPAA to explore how their assumed water contents were affected by their corresponding structures, in particular, the flexibility of the dry PAA (Figure 2). According to the thermogravimetric and differential thermogravimetric (TG/DTG) curve of PAA hydrogel, the TG declined as the temperature increased to a stabilization point at 175 °C, which positively correlated with the loss of absorbed water ().42 At temperatures below 100 °C, the mass losses of air-DPAA, conventional freeze-dried PAA, and acetone-DPAA obtained were all small, 0.86%, 1.66%, and 2.26%, respectively. Alternatively, the mass losses increased to 4.82%, 7.01%, and 10.43%, respectively, at 175 °C, indicating the differences in absorbed water contents of the different gel types (Figure 2a). Also, the water content difference of the gel types might be attributable to the unique nanostructures adopted by the acetone-DPAA, which favored more vapor absorption. After that, we investigated the effect of the absorbed water on the flexibility of acetone-DPAA. Removal of the absorbed water in acetone-DPAA by subjecting to dryness after air exposure in the presence of anhydrous calcium chloride, (CaCl2-acetone-DPAA), rendered the CaCl2-acetone-DPAA stiff and brittle with an elongation at break of 0.09 (Figure 2b and ), taking on a characteristic just like that of the air-DPAA. Upon subsequent air exposure at room temperature (∼25 °C at relative humidity 36%) for 4 h, the CaCl2-acetone-DPAA recovered its flexibility with an elongation at break of 4.8 times its original length (Figure 2b and ). These results indicated that the inner lamella-like structure enhanced the absorbed water content of acetone-DPAA, and also, increased its flexibility, resulting in the transformation of the acetone-DPAA from a glassy state it should have acquired into a rubbery state.

    Figure 2

    Figure 2 | The effect of inner structure on the properties of dry PAA gels: acetone-DPAA, freeze-dried PAA, and air-DPAA. (a) The actual water content of the air-DPAA, freeze-dried PAA, and acetone-DPAA determined by TG. (b) Representative tensile stress–strain curves of CaCl2-acetone-DPAA (dried with anhydrous CaCl2) and restored acetone-DPAA in air (relative humidity of ∼40% at 25 °C) for 4 h. (c) The acetone-DPAA exhibits humidity sensitivity. The representative tensile stress–strain curves of acetone-DPAA at different relative humidity at 25 °C. (d) DSC curves of air-DPAA, acetone-DPAA, and dry acetone-DPAA. The acetone-DPAA exhibits a Tg of 11.7 °C, which is less than room temperature. (e) The thermal degradation of acetone-DPAA and air-DPAA. According to the TG analysis, the acetone-DPAA is easier to decompose. (f) The DSC curves of acetone-DPAA (top) and air-DPAA (bottom) with two thermal cycles from −40 to 200 °C.

    In addition, the acetone-DPAA also exhibited humidity sensitivity, uncovered by measuring its tensile property at different humidities. As shown in Figure 2c and , the acetone-DPAA demonstrated high sensitivity to moisture and exhibited a gradual decrease in modulus with increased air humidity; meanwhile, the elongation at break increased initially and then dropped. However, the air-DPAA did not show much sensitivity to humidity and was still easy to break with small elongation value (less than 0.1) at break, even when the humidity was as high as 70% (). These results indicated that it was easier for the water vapor to diffuse into the acetone-DPAA than the air-DPAA.

    Furthermore, differential scanning calorimetry (DSC) was applied to study the glass-transition temperature of air-DPAA, acetone-DPAA, and CaCl2-acetone-DPAA (Figure 2d). In the DSC thermogram of acetone-DPAA, the glass-transition temperature (Tg) was observed at ∼10 °C, much lower than room temperature at which acetone-DPAA acquired a rubbery state. In contrast, the air-DPAA, CaCl2-acetone-DPAA, and freeze-dried PAA () remained in a glassy state at both temperatures. These results indicated that the lamella-like structures in acetone-DPAA made it easier for water adsorption, and the absorbed water, consequently, decreased its Tg value.43–47 What is more, another Tg peak on the DSC curve of acetone-DPAA was visible at ∼62 °C, which was similar to that of CaCl2-acetone-DPAA, but different from the apparent Tg of air-DPAA at ∼48 °C. Therefore, we presumed that incorporation of the inner lamella-like structure of the acetone-DPAA accounted for this difference, as well as the interaction of the polymer chains, which also, might have been altered during the solvent-induced dehydration.

    To verify our speculation, the TG analysis of acetone-DPAA and air-DPAA were conducted (Figure 2e). Consistent with a previous report, indicating that, increase in temperature of PAA resulted in gradual decrease in the weight of the samples due to anhydride formation and carbon dioxide evolution,48 our acetone-DPAA exhibited a higher decomposition rate at lower decomposition temperature than that of air-DPAA (Figure 2e). To further verify this difference between the two gel types, the DSC of acetone-DPAA and air-DPAA at two extreme thermal cycles (−30 to 200 °C) were conducted (Figure 2f). The DSC curves of acetone-DPAA during the first phase of temperature rise was similar to that in Figure 2d, whereby, the acetone-DPAA exhibited a much lower Tg than that of air-DPAA. Amazingly, after experiencing the first phase of temperature rise, there was a significant increase of the Tg of acetone-DPAA at ∼150 °C, whereas the air-DPAA exhibited only a small increase of its Tg from 56 to 77 °C. These results indicated that the acetone-DPAA was much easier to decompose and be cross-linked than air-DPAA. This observation might be related to the distribution difference of interactions of inter- and intramolecular hydrogen bonding.49–51 Further, we used Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy to explore the change of the air-DPAA and acetone-DPAA upon heat treatment at 155 °C for 20 h (). The results also indicated that anhydride formation in acetone-DPAA was much easier than that in air-DPAA, a finding which, per a previous report, revealed that the anhydride formation occurred primarily by intramolecular reactions.49 Thus, collectively, we deduced from our studies that when PAA hydrogel was immersed in acetone, phase separation occurred, accompanied by variations of inter- and intramolecular interactions, which resulted in the generation of the dense domain and the sparse domain with higher proportioned intramolecular (i.e., hydrogen bonds) interactions, whereby, the lamella-like structures were created, which both contributed to the flexibility of acetone-DPAA.

    The generality of the solvent-induced dehydration for flexible dry gel

    We verified the generality of the solvent-induced dehydration for flexible PAA dry gel by investigating if our approach would be applicable in some other hydrogels. Different poor solvents, including tetrahydrofuran (THF), acetonitrile (AN), and N,N′-dimethyformamide (DMF), miscible with water, were used to generate solvent-dehydrated PAA, poly(N,N′-dimethylacrylamide) (PDMA), and polyacrylamide (PAM) hydrogels. As shown in Figure 3 and , in poor solvents, the hydrogels shrunk gradually, and became opaque, along with phase separation, which ultimately became transparent, except PAM in THF and DMF, which, although appeared opaque, remained brittle, similar to the results obtained by Gong and co-workers.39 When air-dried further, the dry gels exhibited different flexibility (Figure 3, left insets): The THF-DPAA, AN-DPAA, and THF-DPDMA demonstrated flexibility characteristics similar to acetone-DPAA, and they could be twisted without any damage (Figure 3a–c, left insets), whereas the AN-DPDMA, THF-DPAM, and DMF-DPAM were brittle and easy to break when bent (Figure 3d–f, left insets). Observations made from micrographs of dry gel sections after scanning electron microscopy (SEM; Figure 3) revealed that the flexible dry gels exhibited lamella-like structures, whereas the brittle, dry gels displayed flat cross-sections or porous structures due to phase separation induced by the nonsolvent.40,52,53 These lamella-like structures might have contributed to the transformation of the dry gels from brittle to ductile properties, which is consistent with our above inference. In essence, the inner structures and interactions of dry gel could be adjusted with solvent to realize their flexibility in air. However, extra efforts remain to be made from experiments and theories to elucidate the relationship between dry gel structure and solvents.

    Figure 3

    Figure 3 | Flexibility and cross-sectional SEM images of dry gels from different hydrogels and their poor solvents. (a) THF-DPAA: flexible with lamella-like structure, (b) AN-DPAA: flexible with lamella-like structure, (c) THF-DPDMA: flexible with lamella-like structure, (d) AN-DPDMA: brittle without nanostructures, (e) THF-DPAM: brittle with porous structures, and (f) DMF-DPAA: brittle with porous structures. Insets: left, optical photographs of dry gels when twisted or bent, and right, higher magnification.

    Self-healing performance of the acetone-DPAA films

    This acetone-DPAA not only had excellent stretchability but also exhibited self-healing capability with the assistance of acetone (Figure 4a–c). As displayed in Figure 4a, the gel films were cut into halves; one set of halves were stained red with oil red and the other remained unstained. Then, the two sets of halves were brought together alternately. Afterward, the films were immersed in acetone at room temperature (25–30 °C) for 12 h before subjecting to mechanical test. The resultant healed acetone-DPAA films could be stretched, folded, and twisted without fracture (Figure 4b). Elongation tests were conducted on the healed acetone-DPAA films to quantify the self-healing efficiency (). The healed gel could be stretched ∼4 times its original length, exhibiting a high healing efficiency of 65 ± 5%, calculated by obtaining the ratio between the elongations at break after healed relative to the elongations at break of the original acetone-DPAA film.54–56 (Figure 4c). When the healed acetone-DPAA film was immersed in water, the film reswelled and was still easy to be separated at the interface by slight stretch (), indicating that the interaction at the interface was destroyed by water. Therefore, we inferred that the self-healing capacity of flexible acetone-DPAA was attributed to its low modulus and the hydrogen bonding between –COOH and –COOH secondary interactions across the hydrogel structures.55

    Figure 4

    Figure 4 | Self-healing capability and erasable stress-visualizing performance of the acetone-DPAA. (a) Photographs illustrating the self-healing process. Two pieces of acetone-DPAA film, one of which was stained as red with oil red (i), were cut into halves (ii). (iii) When two halves of the gel made contact, it enabled self-healing of the acetone-DPAA to occur. (iv) The self-healing was reinforced after immersed in acetone for 12 h. (b) The self-healed acetone-DPAA retained its flexibility, such that, it could be stretched, folded, and twisted without any damages. (c) Stress–strain curves of the virgin and self-healed acetone-DPAA. (d) An erasable stress-visualizing paper of acetone-DPAA film with water as ink. Water was dropped on the acetone-DPAA film in square arrangement and reswelled the local networks of PAA. When the water evaporated in air, the swollen networks collapsed again, and the air-DPAA was restored. Then invisible air-DPAA patterns were fabricated on the acetone-DPAA film. These patterns could be visualized after specific stretch. By immersing in water and acetone successively, the patterns were erased, and a renewed acetone-DPAA film was obtained. Using the same strategy, air-DPAA patterns in a triangular arrangement were fabricated and visualized.

    Writing/erasing performance of the acetone-DPAA films

    In virtue of the difference in dehydration of PAA hydrogel in acetone and air, erasable low- to high-modulus patterns were fabricated. As illustrated in Figure 4d, the acetone-DPAA film worked as a flexible paper, and water was used as ink. Drops of water were applied on the gel film surface with the consequent reswelling of the local hydrogel networks of the film, adopting a square arrangement. Then the local-swollen hydrogel films were air-dried, air-DPAA patterns were acquired in acetone-DPAA film, which were invisible due to their similarity in optical properties (). Interestingly, when the film was stretched (stress loading), the local air-DPAA patterns remained unchanged due to the attainment of high modulus. When the stress was unloaded, the acetone-DPAA gel parts became opaque due to the relaxation of the polymer chains, which then, rendered the patterns visible. Over a period (about 1 h at 40% humidity and 25–30 °C), the visible patterns vanished after restoration of the polymer chains (). These patterns also erased with water and acetone successively, achieving a renewed acetone-DPAA film. In addition, different air-DPAA patterns in acetone-DPAA film in a triangular arrangement were fabricated, which could be visualized after stress was exerted, as before. Thus, this acetone-DPAA films were bestowed as a paper for erasable, strain-visualized information storage materials with water serving as ink, thereby, unveiling a novel carrier for confidential information.

    Conclusions

    We report a new approach that engineers the inner structures of dry hydrogels to optimize their flexibility via solvent-induced dehydration. The fabricated solvent-dehydrated gels exhibited high stretchability, self-healing capacity, and could maintain their flexibility in air. This flexibility might be attributable to the absorbed moisture and the lamella-like nanostructures acquired by the gels, leading to a decrease of their glass-transition temperature. The differences in the stretchability of the acetone-DPAA and air-DPAA enabled the flexible acetone-DPAA films to gain characteristics of strain-visualizing papers with writing/erasing performance, using water as invisible ink, and thus, promising for storage of confidential information. This solvent-induced dehydration-engineering strategy for flexible dry gels might initiate a new branch in the field of hydrogel chemistry and extend the scope of their potential applications.

    Experimental Methods

    Chemicals

    Acrylic acid 99% (J&K Scientific, Beijing, China, Lot# LC80O82), acrylamide 99% (AAm; J&K, Lot# L510P42), N,N′-dimethylacrylamide 98% (DMA; J&K Scientific, Lot# LT60N21), N,N′-methylenebis(acrylamide) 98% (J&K Scientific, Lot#LT50N18), and ammonium persulfate 98% (J&K Scientific, Lot# LBBON03) were all used as received.

    Preparation of hydrogel

    The hydrogel films were prepared by free-radical polymerization in the spacer of two separated glass slides. The reaction solution was prepared by mixing monomer solution of 5 M AA, AAm, or DMA, with N,N′-methylenebis(acrylamide) as a cross-linker (mole ratio of divinyl to vinyl monomers, 2∶400) and ammonium persulfate as initiator (10 mg in 1 mL reaction solution). The polymerization process proceeded in water bath set at 65 °C. After that, the prepared hydrogel was immersed in water for more than 3 days, and the water was exchanged at least three times during this period. First, a set of reaction solutions were prepared with varying AA concentrations of 2.5, 5, 7.5, and 10 M, and a fixed cross-linker of 1∶200 to fabricate PAA hydrogel at different monomer concentrations. Second, another set of reaction solutions were prepared at a fixed AA concentration of 5 M and varying cross-linker ratios of 1∶400, 2∶400, 4∶400, and 6∶400.

    Preparation of dry polymer gel

    With PAA hydrogel as an example, the dry polymer gels obtained via solvent-induced dehydration were fabricated by immersing the hydrogel in organic solvents for more than 1 week, during which the solvents were replaced daily. The hydrogels experienced solvent exchange along with swelling or shrinking. Subsequently, the dry polymer gels were obtained after air evaporation of the residual solvents. The air-DPAA was fabricated via direct desiccation of the PAA hydrogel in air. The freeze-dried PAA was fabricated by freezing in liquid nitrogen and drying in freeze dryer. Whether the dry gels were flexible or brittle was assessed at 25 °C with a relative humidity of 35–40%.

    TG analysis

    The actual water content and thermostability of air-DPAA, freeze-dried PAA, and acetone-DPAA were measured with a Seiko Exstar 6300 TG/differential thermal analyzer. The TG was performed in nitrogen at a heating rate of 10 °C/min from 30 to 500 °C with the samples in aluminum pans.

    Calculation of the swelling ratio in linear dimension of PAA gels

    We described the vernier caliper measurements of swelling and shrinking performance of the gels using the following parameters: swelling ratio in linear dimension = d/d0. d and d0 are the widths of the gel films at equilibrium in acetone aqueous solution or air and water, respectively, defining the swelling ratio in a linear dimension of PAA gel in water as 1.

    Elongation tests on the PAA gels

    Elongation tests were conducted with temperature at 25 °C and relative humidity of ∼40% (unless stated otherwise), using a commercial testing machine (Mark-10 ESM301L, Copiague, USA). The samples for elongation tests were fabricated from the PAA gel films by cutting into belts with widths of 5–8 mm. Then, the samples were clamped at both ends and elongated at a loading rate of 100 mm/min. The acetone-DPAA was kept at a specified humidity in an environmental chamber (KCL-2000W; EYELA, Tokyo, Japan) before used to investigate the effect of humidity on the mechanical property of acetone-DPAA.

    Morphology of the gel films

    The cross-sectional morphologies of the gels were characterized using a field-emission scanning electron microscopy (FESEM; JSM-7500F; JEOL, Japan) at 5 kV. The samples were prepared via freeze-drying in liquid nitrogen, and the cross-sections were obtained by breaking the polymer films in liquid nitrogen. All the samples were coated with thin layers of gold before FESEM observation.

    Differential scanning calorimetry

    For the DSC measurements, a Mettler-Toledo DSC 1 (Switzerland) was employed. A sample of 5–10 mg weight was placed in a sealed aluminum pan with a pore to connect the outside. The heating/cooling rates of 10 K/min were applied. Nitrogen was used as an inert gas, and an empty aluminum pan was used as reference, while liquid nitrogen was used as a cooling medium. Before the DSC measurements, the samples were kept in air at room temperature (25 °C) and relative humidity of 40–50%.

    Supporting Information

    Supporting Information is available.

    Conflicts of Interest

    The authors declare no competing financial interests.

    Acknowledgments

    This study is supported financially by the National Natural Science Foundation of China (21425314, 21501184, 21434009, 21421061, and 21504098), the Top-Notch Young Talents Program of China, Beijing Municipal Science & Technology Commission (Z161100000116037), and Youth Innovation Promotion Association, CAS (2017036). F.Z. designed and performed all the experiments. S.W. codesigned the experiments. Z.G., M.Y., S.L., Y.S., J.F., and J.M. conducted or supported the experiments for characterization. J.Z. conducted the thermodynamic analysis of the hydrogel in poor solvent. F.Z., L.J., P.W., and S.W. analyzed the data. F.Z. and S.W. wrote the article.

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