Rigid and Strong Thermoresponsive Shape Memory Hydrogels

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Rigid, Strong Thermo-responsive Shape Memory Hydrogels Transformed from Poly(vinyl pyrrolidone-co-acryloxy acetophenone) Organogels Chen Jiao, Yuanyuan Chen, Tianqi Liu, Xin Peng, Yaxin Zhao, Jianan Zhang, Yuqing Wu, and Huiliang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11391 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on September 6, 2018

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ACS Applied Materials & Interfaces

Rigid, Strong Thermo-responsive Shape Memory Hydrogels Transformed from Poly(vinyl pyrrolidone-co-acryloxy acetophenone) Organogels Chen Jiao, Yuanyuan Chen, Tianqi Liu, Xin Peng, Yaxin Zhao, Jianan Zhang, Yuqing Wu and Huiliang Wang* Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China KEYWORDS: hydrogels, shape memory, hydrophobic associations, π-π stacking, mechanical properties, surgical fixation devices

ABSTRACT: Shape memory hydrogels (SMHs) have a wide range of potential practical applications. However, the mechanically weak and soft nature of most SMHs strongly impedes their applications. Here we report a novel kind of thermal-responsive SMHs with high tensile strengths and high elastic moduli. Organogels are firstly prepared by the copolymerization of a hydrophilic monomer N-vinyl pyrrolidone (NVP) and a hydrophobic monomer acryloxy acetophenone (AAP) in N, N'-dimethylformamide (DMF) solutions, and then poly(vinyl pyrrolidone-co-acryloxy acetophenone) [poly(NVP-co-AAP)] hydrogels are obtained by solvent exchange with water. Owing to the strong and reversible hydrophobic association and π-π

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stacking of acetophenone groups, the poly(NVP-co-AAP) hydrogels exhibit tensile strengths up to 8.41 ± 0.83 MPa and Young’s moduli up to 94.2 ± 1.3 MPa, which are more than 1 or 3 orders of magnitude higher than those of the organogels, respectively. The poly(NVP-co-AAP) hydrogels exhibit good shape memory behaviors, with a complete fixation ratio and a recovery ratio of 74%-89%, as well as very fast shape-fixing and recovering rates (in seconds). These rigid and strong hydrogels are demonstrated to be an ideal shape memory material for surgical fixation devices to wrap around and support various shapes of limbs.


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INTRODUCTION Shape memory polymers (SMPs) are a kind of stimulus-response materials.1 They can quickly convert from a fixed temporary shape to their original (or permanent) shape under an external stimulus, such as light,2-3 temperature,4 electric field,5 magnetic field,6 pH,7 etc. Usually, SMPs have both a strong fixing phase and a weak switchable phase.1, 8 Their shape memory properties are achieved by a reversible transition between the fixing phase responsible for the permanent shape and the switchable phase responsible for the temporary shape. Shape memory hydrogels (SMHs) are a special type of soft SMPs containing a large amount of water. Due to their promising applications in sensors,9 artificial muscles,10 actuators,11 soft robotics12-13 and in vivo treatments,14-15 SMHs have been of widespread interest in recent years. Similar to SMPs, SMHs usually employ irreversible cross-links as the rigid backbone (fixing phase) and reversible crosslinks as the weak switchable phase.16-17 Generally, when a SMH is subjected to a specific stimulus, its reversible cross-links break, then it can be deformed into a temporary shape by an external force. When the stimulus changes, the reversible cross-links reform and the hydrogel returns to its original shape due to the elasticity of the permanent network. SMHs are mainly prepared by utilizing reversible interactions, such as hydrophobic interaction,18-19 ionic interaction,20-21 host-guest inclusion,22-23 hydrogen bonding,24-26 dynamic covalent bond27 and coil-helix transformation.28 Osada and co-workers18,

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first reported the

shape memory effect in hydrogels prepared by radical copolymerization of stearyl acrylate (C18) and acrylic acid (AA). The reversible order-disorder transition, i.e. the crystallization and melting of crystalline regions, associated with the hydrophobic interaction between C18 side chains endows the hydrogels with excellent shape memory properties. Okay and co-workers30 reported supramolecular-based semicrystalline hard/soft hydrogel hybrids, greatly enhancing


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stress-bearing properties. Zhang et al.31 developed a polyampholyte SMH by one-step copolymerization of cationic monomer with two anionic monomers. Liu and co-workers reported two kinds of stimuli responsive SMHs. One is designed by introducing mM levels of zinc ions to fix temporary shapes due to the strong coordination of zinc with imidazole.32 The other one uses hydrophobic aggregation of dipole-dipole pairings and hydrogen bonding to enhance tensile strength.15 Meanwhile, SMHs may also employ reversible cross-links as fixing phase, such as hydrophobic interaction,30, 33 hydrogen bonding24 and electrostatic interactions.27 In these shape memory processes, relatively stronger reversible cross-links as fixing phase maintain the permanent shape and relatively weaker ones as switchable phase fix the temporary shape. Recently, our group reported a SMH based on multiple hydrogen bonding between poly(vinyl alcohol) and tannic acid.24 The gel is made from only one type of physical interaction (i.e. hydrogen bonding) with different strengths, in which the weaker hydrogen bonding serves as a shape memory switch and the stronger one acts as permanent cross-links. Among these SMHs, thermo-triggered ones generally have rapid shape-fixing and recovering rates because they do not need time for the migration of chemicals and reaction.19 However, most of the reported thermo-triggered SMHs have low tensile strength and especially low Young's modulus, mostly less than 1 MPa.15 The mechanically weak and soft nature of SMHs strongly limits their applications. Only a few SMHs with high moduli and/or high tensile strengths have been reported recently (Table S1).30, 34-35 Therefore, developing SMHs with high mechanical strengths, high moduli and excellent shape memory properties is still of great scientific and practical significance. Here we report a novel type of thermo-responsive SMHs with excellent mechanical properties. We copolymerized N-vinyl pyrrolidone (NVP) and acryloxy acetophenone (AAP) in the


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presence of a chemical cross-linker in N, N'-dimethylformamide (DMF) solutions to obtain poly(NVP-co-AAP) organogels, which were transformed into hydrogels by immersing in water to replace DMF. With comparison to the organogels, the hydrogels exhibit dramatically enhanced tensile strengths and moduli, due to the strong hydrophobic associations and π-π stacking. These rigid and strong hydrogels also show good thermo-triggered shape memory properties.


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RESULTS AND DISCUSSION Preparation and structural characterizations of poly(NVP-co-AAP) hydrogels. The preparation process and formation mechanism of poly(NVP-co-AAP) hydrogels are illustrated in Figure 1a-1b. We used a two-step method to synthesize poly(NVP-co-AAP) hydrogels. The monomer AAP was firstly synthesized by esterification of 4-hydroxyacetophenone and acryloyl chloride (see Figure S1 for the 1H NMR spectrum of AAP). Then the monomers AAP and NVP were polymerized and loosely chemically cross-linked by N, N'-methylenebisacrylamide (MBAA) under

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Co γ-ray irradiation, forming homogeneous and transparent poly(NVP-co-

AAP) organogels. In the second step, the organogels were immersed in deionized water to replace DMF, converting the organogels into hydrogels. Similar transformations of organogels to hydrogels have been reported by Liu’s group36 and Osada’s group.29,

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The organogels are

mainly cross-linked by covalent bonding between the chemical cross-linker and the polymer chains. When the organogels are immersed into deionized water, the exchange of DMF with water leads to the aggregation of the hydrophobic micro-segments of AAP. In the aggregates, the acetophenone groups are possible to form strong π-π stacking by special nonlocal electron correlations between the π electrons in the two fragments when they are close enough and in face to face conformations,38-40 which would greatly enhance the cross-linking density of the hydrogels. The appearance of the poly(NVP-co-AAP) organogels with a total monomer concentration of 4 mol·L-1 but different molar ratios (poly(NVPx-co-AAPy), x:y=1:1, 2:1, 3:1, 4:1, 5:1) is shown in Figure 1c. All the samples were cut into 2.5 cm-diameter discs. The organogels were all highly transparent, though the gel with a low NVP molar ratio was yellowish due to the color of the high-content AAP. When the organogels were transformed into hydrogels, they became


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translucent. The transparency of the hydrogels decreased with increasing molar ratio of hydrophobic AAP, and the poly(NVP1-co-AAP1) hydrogel was completely opaque (Figure 1d). The change in transparency indicates a change in the microstructure of the gels. Due to the hydrophobic interactions and possible π-π stacking of AAP units in the aqueous environment of hydrogels, the aggregation of AAP units leads to the formation of large domains that scatter visible light.41 When the hydrogels were re-immersed in DMF, they changed into transparent organogels with a diameter of about 4.5 cm again (Figure 1e), due to the dissociation of the aggregated hydrophobic domains.


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Figure 1. (a) The synthesis of the monomer AAP and the copolymerization of NVP and AAP. (b) The preparation process and the proposed formation mechanism of poly(NVP-co-AAP) organogels and hydrogels. (c-e) Photos showing the appearance of (c) poly(NVPx-co-AAPy) organogels (from left to right: x:y=1:1, 2:1, 3:1, 4:1, 5:1), (d) corresponding hydrogels obtained by immersing organogels into deionized water and (e) organogels obtained by re-immersing the hydrogels in DMF. Table 1. The equilibrium water contents (EWC) of the poly(NVP-co-AAP) hydrogels. Hydrogel sample

EWC (wt.%)

poly(NVP1-co-AAP1)

56.1 ± 0.1

poly(NVP2-co-AAP1)

60.2 ± 0.1

poly(NVP3-co-AAP1)

67.2 ± 0.4

poly(NVP4-co-AAP1)

70.9 ± 1.4

poly(NVP5-co-AAP1)

75.2 ± 0.8

The swelling ratios of the poly(NVP-co-AAP) hydrogels were measured. The equilibrium water contents (EWC) of the poly(NVP-co-AAP) hydrogels are listed in Table 1. The EWC of the hydrogels increases with increasing molar ratio of hydrophilic NVP, from 56.1% for the poly(NVP1-co-AAP1) hydrogel to 75.2% for the poly(NVP5-co-AAP1) hydrogel. By immersing the dried hydrogels in DMF, their DMF contents increase very quickly in the first 1 h and reach equilibrium swelling in 2 h (Figure S2). Since the two monomers both have good solubility in DMF, the DMF contents of all the samples finally reach about 92%.


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PAAP Poly(NVP-co-AAP) 6 1 1 1 3 7 2 1

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T /%

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0 8 6 1 9 5 7 1

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2000

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-1

Wavenumber/cm

Figure 2. FT-IR spectra of PAAP, PVP and a dried poly(NVP-co-AAP) hydrogel. FT-IR characterizations were conducted to analyze the chemical composition of poly(NVP-coAAP) hydrogel. Pure PAAP and PVP were also measured to make comparison more intuitively. As shown in Figure 2, FT-IR spectrum of PAAP shows the characteristic absorption peaks at 1759 cm-1 and 1680 cm-1 representing the stretching vibrations of ester carbonyl and keto carbonyl groups, respectively, and the peak at 1120 cm-1 assigned to the stretching vibration of ester C-O group. FT-IR spectrum of PVP exhibits the characteristic absorption peak of keto carbonyl at 1649 cm-1 and the absorption peak at 1280 cm-1 related to the stretching vibration of C-N group. These characteristic peaks of PAAP and PVP are all observed in the spectrum of the poly(NVP-co-AAP) hydrogel, and they are at 1760 cm-1 (ester carbonyl), 1659 cm-1 (keto carbonyl), 1273 cm-1 (C-N) and 1116 cm-1 (C-O), respectively. The FT-IR characterizations confirm the copolymerization of the monomers NVP and AAP.


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Mechanical performance of poly(NVP-co-AAP) hydrogels. The tensile mechanical properties of both organogels and hydrogels were measured. Tensile stress-strain (σt-εt) curves of poly(NVP-co-AAP) organogels and hydrogels are shown in Figure 3a-3b, and their tensile strength (σb) and elastic modulus (E) are summarized in Figure 3c-3d, respectively. The organogels are mechanically very weak. The σb, E and elongation at break (εb) of the organogels gradually decreases with increasing NVP molar ratio. The highest σb, E and εb are 0.20 ± 0.02 MPa, 0.07 ± 0.01 MPa and 752%, respectively, for the poly(NVP1-co-AAP1) organogel. The poly(NVP-co-AAP) hydrogels exhibit dramatically enhanced σb and E. Both σb and E of the hydrogels decrease with the increasing molar ratio of NVP. The highest σb is up to 8.41 ± 0.83 MPa (poly(NVP1-co-AAP1) hydrogel), and the highest E reaches a high value of 94.2 ± 1.3 MPa (poly(NVP2-co-AAP1) hydrogel). The σb and E of the hydrogels are more than 1 or 3 orders of magnitude higher than those of the organogels, respectively. A poly(NVP-coAAP) hydrogel strip (30 mm × 5 mm × 1 mm) is strong enough to hold up a load of 4 kg (see the inset in Figure 3b and Movie S1). Note that the εb of the poly(NVP-co-AAP) hydrogels are in the range of 26-188%, much lower than those of the organogels (514-752%). The dramatic difference in tensile mechanical properties between the hydrogels and organogels reveals a huge change in their microstructures and interactions in them. Chemical cross-linking is always present in both organogels and hydrogels due to the addition of the chemical cross-linker MBAA (0.1 mol% to the monomers), and the chemical cross-linking density should be the same for the organogels and hydrogels with the same monomer ratio, as the hydrogels are transformed from the organogels. The low E of the poly(NVP-co-AAP) organogels indicates their low chemical cross-linking densities. Therefore, some other physical interactions, very possibly hydrophobic association and π-π stacking of the acetophenone group of AAP,42 are the decisive


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factors for the differences in mechanical properties. The hydrophobic association and π-π stacking of acetophenone groups enhance the cross-linking effect which prevents the chains from sliding out of place and thereby increases the resistance to mechanical deformation.

a

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Figure 3. Mechanical performance of poly(NVP-co-AAP) hydrogels. (a-b) Tensile stress-strain (σt-εt) curves of (a) poly(NVP-co-AAP) organogels and (b) poly(NVP-co-AAP) hydrogels. Photo in (b) shows a poly(NVP-co-AAP) hydrogel stripe (30 mm × 5 mm × 1 mm) holding up a load of 4 kg. (c) Tensile strengths (σb) and (d) elastic moduli (E) of poly(NVP-co-AAP) organogels and hydrogels. At least three specimens were tested for each experimental point to obtain reliable data. When the organogels are transformed into hydrogels by the exchange of DMF with water, the aqueous environment leads to the aggregation of hydrophobic AAP units into large domains that


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can scatter visible light (Figure 1d). The 3 orders of magnitude increase in E of the hydrogels with comparison to the corresponding organogels suggests that very strong hydrophobic and/or π-π stacking interactions of AAP units are present in the hydrogels. The decrease of σb and E of the hydrogels with increasing molar ratio of hydrophilic NVP, correspondingly the increase of hydrophobic AAP molar ratio, also proves that the hydrophobic and/or π-π stacking interactions of AAP units are the dominant factors affecting the mechanical properties of the hydrogels. Moreover, the poly(NVP-co-AAP) hydrogels have higher E than some other hydrogels physically cross-linked by hydrophobic interactions (e.g. long alkyl chains).30, 43-46 Therefore, it is reasonable to conclude that the π-π stacking of benzene rings in AAP units is very strong and it plays a vital role in cross-linking of the hydrogels. The strong π-π stacking endows the hydrogels with high E, and high σb, but it limits the movement of polymer chains and hence leads to the low εb of the hydrogels.


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Uniform deformation deformation εt=10 %

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Poly(NVP2-co-AAP1)

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σt (MPa)

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d necked region

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Figure 4. Yielding and necking phenomenon of the poly(NVP2-co-AAP1) hydrogel in the tensile test and its mechanism. (a) Uniform elongation of the gel to εt of 10%. (b) Yielding and necking of the gel at εt of 19%. (c) Tensile stress-strain (σt-εt) curve of the poly(NVP2-co-AAP1) hydrogel and the points related to (a) and (b). (d) The formation mechanism of yielding and necking. Noticeably, the poly(NVP-co-AAP) hydrogels with a higher molar ratio of AAP show obvious yielding and necking phenomena when elongated (Figure 4 and Figure S3). Take the poly(NVP2co-AAP1) hydrogel for instance, at the early stage of the elongation, the hydrogel undergoes uniform deformation and σt monotonically increases with εt (Figure 3b and Figure 4a). After reaching a maximum σt of about 6 MPa, the σt decreases. Accompanying the appearance of a


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yield point, necking occurs on the hydrogel sample (Figure 4b). With the propagation of the necked region, σt keeps almost constant at about 5 MPa. Only until the necked region disappears and the hydrogel turns back to uniform again, a slight increase in σt, i.e. strain-hardening, can be found (Figure 3b). The appearance of yielding and necking phenomena can be explained as the dissociation of hydrophobic associations and π-π stacking of AAP after the stretching of loosely curved polymer chains (Figure 4d).47 To better understand the yielding phenomenon of the hydrogels, cyclic tensile tests of the poly(NVP3-co-AAP1) hydrogel to the same strain for different times and successive cyclic tests to different strains were performed. Large hysteresis and residual strains are observed in the cyclic loading-unloading curves of the hydrogel stretched to the same strain (150%) for different times (Figure S4a), suggesting a very effective energy-dissipating mechanism of the hydrogel.48 The hysteresis ratio (hr) and residual strain (εr) in the first cycle reaches 92% and 121%, respectively, and then both increases slightly with the cycle number (Figure S4b). The cyclic loading-unloading curves performed in immediate succession with a same specimen to increasing strains (30%, 60%, 90%, 120% and 150%) are shown in Figure S4c. Large hysteresis loops and residual strains are also observed in the loading-unloading curves, and they become more significant with increasing strains. Note that envelope of the successive cyclic loadingunloading curves is very similar to that of consecutive curve to the same strain shown in Figure S4a, suggesting the gradual breakage of physical interactions in the gels. Partial physical interactions can be recovered, as indicated by the larger stress in the uploading curve of the following cycle than that in the unloading curve of the previous cycle in the strain range slightly less than the maximum strain of the previous cycle. We calculated the recovered ratio of the hysteresis (Rr, h) by dividing the area between the loading curve of the following cycle and the


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unloading curve of the previous cycle with the area of the hysteresis loop of the previous cycle, and the data are shown in Figure S4d. The Rr, h increases with increasing strain, suggesting more physical interactions are reformed at higher strains. These data suggesting that the yielding phenomenon are derived from the dissociation of hydrophobic associations and π-π stacking in poly(NVP-co-AAP) hydrogels. 10

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tanδ 3

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Figure 5. (a) Frequency dependence of storage modulus (G’), loss modulus (G”) and loss factor (tan δ) for the poly(NVP1-co-AAP1) hydrogel at a fixed temperature (T = 25 °C) and strain (γ = 0.01%). (b) Temperature dependence of G’, G” and tan δ for the poly(NVP1-co-AAP1) hydrogel at a fixed frequency (ω = 10 rad·s-1) and strain (γ = 0.01%). Rheological measurements were performed to understand the viscoelastic response of the poly(NVP-co-AAP) gels. In the frequency (ω) sweep test of the poly(NVP1-co-AAP1) hydrogel (Figure 5a), the storage modulus (G’) is always larger than the loss modulus (G”), suggesting an elastic nature of the gel. The hydrogel exhibits significant frequency-dependent viscoelastic response. G’ increases slightly with increasing frequency in the tested range, while G” and loss factor (tan δ) decreases sharply with the increase of frequency till 1 rad·s-1 and then at a much lower decreasing rate. The poly(NVP4-co-AAP1) hydrogel also shows similar frequency-


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dependent viscoelastic response, but its change in G’, G” and tan δ with frequency is much less significant (Figure S5a). The frequency-dependent mechanical properties of the hydrogels have also been verified with tensile testing under different strain rates. As shown in Figure S6a, the tensile strength of the poly(NVP3-co-AAP1) hydrogel is increased from 3.07 ± 0.21 MPa under a strain rate of 10 mm·min-1 to 3.53 ± 0.17 MPa under 100 mm·min-1, while the fracture strain reduces gradually (Figure S6b). The elastic moduli also show a slightly increase. It indicates that there is a time scale competition between the internal kinetics of reversible dissociation/reformation and external loading rate.49 The internal reversible interactions (hydrophobic and/or π-π stacking interaction) have less time to reach their mechanical/kinetic equilibrium at a high strain rate. The poly(NVP1-co-AAP1) hydrogel also exhibits very significant temperature-dependent viscoelastic response (Figure 5b). G’ firstly decreases slightly while G” increases with increasing temperature till 55 °C, and after then both G’ and G” decrease dramatically. The G’ is very close to or even slightly lower than the corresponding G” in the temperature range of 65-75 °C. The tan δ increases with temperature from less than 0.1 at 30 °C up to about 1.1 at 72 °C, signifying the transition from an elastic solid to a viscous fluid. The strong frequency and temperature-dependent viscoelastic response of the poly(NVP-coAAP) hydrogels confirms their main physical cross-linking nature. The hydrophobic associations and π-π stacking in the hydrogels can be destroyed at low frequencies and high temperatures.50-51 At a low frequency, the time scale is sufficient for the breakage of the physical interactions, leading to the energy-dissipation by creep deformation. At a high temperature, the kinetic energies of the polymer chains (or groups) excess the physical interactions, leading to the


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weakening of the mechanical properties. In contrast, the organogels that are mainly chemically cross-linked exhibit weak temperature-dependent viscoelastic response (Figure S5b). Shape memory behaviors. Due to the presence of both permanent chemical cross-links and reversible physical cross-links, the poly(NVP-co-AAP) hydrogels also exhibit good shape memory behaviors. As indicated by the temperature-dependent viscoelastic response of the hydrogels (Figure 5b), the poly(NVP1-co-AAP1) hydrogel softens dramatically at a high temperature, and hence it can be easily deformed by an applied external force. The fixation of deformed shapes can be realized by maintaining the deformed gels at room temperature for a few seconds, without using any chemicals or waiting a long time.20-21, 24 Moreover, the gels can be easily fixed to any temporary shapes without any response lag or rebound. For example, by wrapping on a glass rod and then exposing to room temperature for a few seconds, a hydrogel sample with several loops was obtained. When the deformed gel helix was immersed in 70 °C deionized water, its shape quickly recovered within 14 s. (Figure 6a and Movie S2). Apart from bending, shape recovery under stretching was also attempted. The dumbbell-shaped gel with a gauge length of 15 mm clamped by tweezers was stretched to 75 mm (εt = 400%) in 70 °C deionized water, then the elongated shape was fixed at room temperature. By immersing in the hot water, the gel shrank rapidly to a remaining length of 18 mm (Figure 6b and Movie S3), i.e. with a residual strain of only 20% after shape recovery.


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b 0s

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Figure 6. Photos demonstrating the shape memory behaviors of the poly(NVP1-co-AAP1) hydrogel. (a) Shape recovery of a helical shape hydrogel in 70 °C deionized water in 14 seconds. (b) The original shape of a dumbbell-shaped hydrogel (top), its temporary fixed shape by stretching the heated sample and fixing at room temperature (middle) and its shape recovery in 70 °C deionized water (bottom). The shape fixity ratio (Rf) and shape recovery ratio (Rr) of the poly(NVP-co-AAP) hydrogels were measured by bending experiments performed on U-shaped specimens (Figure S7). After immersing in 70 °C deionized water, the U-shaped poly(NVP1-co-AAP1) hydrogel returned to its initial linear shape within about 60 s (Figure 7a and Movie S4). The bending angle decreased rapidly from 180° to 28° (Rr = 84%) in the first 30 s and then very slowly to 20° (Rr = 88%) in the later 30 s. The poly(NVP1-co-AAP1) hydrogel shows good cyclicity of shape memory behavior. As shown in Figure 7b, the Rf was always almost 100%, while the Rr was more than


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85% in the first four fixing and recovery cycles, after then it gradually decreased. Even though, the Rr was still more than 60% after 10 cycles.

Rf, Rr(%)

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120

Rr (%)

c Bending angle (°)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 80

60 40

60 40

20

20 0 0

20

40

60

80

100 120 140 160 180

t (s)

0 55

60

65

70

75

80

85

T (°C)

Figure 7. Quantitative shape memory properties of the poly(NVP-co-AAP) hydrogels measured from the shape recovery of the temporary shape (U-shaped) to their original permanent shape (straight strip). (a) Recovery process of a poly(NVP1-co-AAP1) hydrogel. (b) Rf and Rr of cyclic fixing and shape recovery of the poly(NVP1-co-AAP1) hydrogel. (c) The recovery process of the poly(NVP1-co-AAP1) hydrogel at different temperatures and (d) the corresponding Rr. At least three specimens were tested for each experimental point to obtain reliable data. Temperature affects the shape recovery rate and recovery ratio of the poly(NVP-co-AAP) hydrogels. The shape recovery rate increases with increasing temperature. The shape recovery


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process takes only 8 seconds at 80 °C, while it takes more than 100 seconds at 60 °C (Figure 7c). The Rr of the gels are 88% at 70 °C and 80 °C, but it decreases to 72% at 60 °C (Figure 7d). The composition of the poly(NVP-co-AAP) hydrogels also affect their shape memory properties. The Rf for all hydrogels are 100%, but the Rr gradually decreases with the decrease of the molar ratio of AAP (Figure S8).

Figure 8. Demonstration of the application of poly(NVP-co-AAP) hydrogels as gypsum substitutes to wrap around and support different limbs, including (a) knuckle, (b) wrist, (c) shank and foot. The high moduli and the good shape memory properties of the poly(NVP-co-AAP) hydrogels greatly expand the applications of SMHs. For example, the poly(NVP-co-AAP) hydrogels can be


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used as an alternative material for gypsum to wrap around and support various limbs. As show in Figure 8, the gel sheets cut into different shapes are firstly softened in hot water and then they are immediately attached to the limbs to be wrapped and deformed into various shapes. After a few seconds of cooling, the deformed shapes are fixed to support fracture sites, such as knuckle, wrist, shank and foot. The use of poly(NVP-co-AAP) hydrogels as gypsum substitutes has some distinct advantages. The temporary shapes can be easily adjusted and fixed, allowing appropriate shapes and sizes suitable for different parts of the patient. The shape recoverability allows them to be reused, significantly saving resources compared to disposable gypsum. The hydrogels are of light weight, with a density of 1.11 g·cm-3, which are beneficial to local blood circulation and promotes healing. In addition, their excellent permeability to radiation facilitates doctors to accurately grasp the conditions of bone defect repair. CONCLUSIONS In summary, we have successfully synthesized a novel thermo-responsive rigid, strong SMHs. The poly(NVP-co-AAP) hydrogels are prepared from the corresponding organogels by solvent exchange. The hydrogels have high tensile strengths and high Young's moduli up to 94.2 ± 1.3 MPa. Meanwhile, they exhibit good shape memory behaviors, with high shape fixity ratios and shape recovery ratios as well as very fast shape-fixing and recovery rates (in seconds). All these features are based on the strong and reversible hydrophobic associations and π-π stacking of networks. The rigid and strong hydrogels can be used as an ideal shape memory material for surgical fixation devices to wrap around and support various shapes of limbs. To our best knowledge, this is the first report of hydrogels as fixation devices for fracture treatments. We also believe that they would be an attractive candidate for a wide variety of applications. It opens


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up the possibility of incorporating SMHs into device design, which may lead to huge technological advances in medical devices, sensors actuators, artificial muscles, and soft robotics. EXPERIMENTAL SECTION Materials. 4-Hydroxyacetophenone (98%) and acryloyl chloride (96%, contains 200 ppm MEHQ stabilizer) were purchased from Aladdin Reagent Co. Ltd. (Shanghai, China). N, N'methylenebisacrylamide (MBAA, 99%) was purchased from Beijing InnoChem Science & Technology Co. Ltd. (Beijing, China). Sodium hydroxide (96%), triethylamine (TEA, 99%), N, N'-dimethylformamide (DMF, 99.5%) and 2-butanone were purchased from Beijing Chemical Works (Beijing, China). N-vinyl-2-pyrrolidone (NVP, A.R.) was purchased from Acros Organics (New Jersey, USA). All chemicals were used as received without further purification. Deionized water was used for all experiments. Synthesis of Monomer AAP. 4-Hydroxyacetophenone (0.2 mol, 27.23 g) was dissolved in 2butanone mixed with trimethylamine (0.2 mol, 20.24 g) and stirred at ice-water temperature. Then acryloyl chloride (0.2 mol, 18.1 g) in ether was added dropwise in 60 min, yielding a large amount of white solid immediately. Hydrogen chloride produced in this process was absorbed by TEA to form quaternary ammonium salt. After 2 h of continuous reaction, the white emulsion was filtered and washed three times with ether and the filtrate was collected. Subsequently, the filtrate was washed with aqueous 5% NaOH and water, respectively. After then, solvent was evaporated to get the monomer 4-acryloxy acetophenone (AAP) in 55% yield.52 1H NMR spectrum of the monomer AAP was recorded on a JNM-ECZ600R/S3 spectrometer (JEOL, Japan) at 600 MHz. Chemical shifts of 1H NMR were referred to TMS (δ = 0). The 1H NMR spectral data are consistent with the structure. 1H NMR (600 MHz, CDCl3, δ): 7.8 (d, 2H, Ar H),


22

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7.2 (d, 2H, Ar H), 6.6 (d, 1H, COCH=CH2), 6.3 (dd, 1H, COCH=CH2), 6.1 (d, 1H, COCH=CH2), 2.6 (s, 3H, CH3). Preparation of poly(NVP-co-AAP) Hydrogels. AAP and NVP with a total concentration of 4 mol·L-1 but different molar ratios were dissolved in DMF to form homogeneous solutions, in which the cross-linking agent MBAA (0.1 mol% to the monomers) was added. The solutions were transferred into glass molds. Prior to polymerization, the solutions were degassed via nitrogen bubbling to remove dissolved oxygen. After then, the solutions were exposed to 60Co γray irradiation for 10 h at a dose rate of 3 kGy·h-1 at room temperature, forming poly(NVP-coAAP) organogels. Finally, the as-prepared organogels were immersed in a large excess of deionized water at 25 °C for 48 h to obtain equilibrium swollen poly(NVP-co-AAP) hydrogels. Water was changed for several times to ensure the full removal of DMF in the hydrogels. The equilibrium water contents (EWC) of the hydrogel samples were determined according to the following Equation (1): EWC =

ms -md ms

×100%

(1)

where ms and md represent the equilibrium swollen and vacuum dried samples, respectively. FTIR Characterization of poly(NVP-co-AAP) Hydrogels. Vacuum-dried PVP, PAAP, and poly(NVP-co-AAP) hydrogel samples were ground into powders. Fourier transform infrared (FTIR) spectra of PVP, PAAP, and poly(NVP-co-AAP) hydrogel samples were recorded on a Nicolet FTIR 6700 spectrometer (Thermo Electron Scientific Instruments Corp., USA). Tensile Mechanical Testing. Hydrogel samples were cut into standardized DIN-533504 S3 dumbbell-shaped specimens (overall length: 35 mm; width: 6 mm; inner width: 2 mm; gauge length: 10 mm; thickness: 1 mm), then they were tested with an Instron 3366 electronic universal testing machine (Instron Corporation, MA, USA) equipped with a 100 N load cell at a crosshead


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speed of 10 mm·min-1. To avoid water loss during the testing, a thin layer of silicon oil was coated on the gel specimens. At least three specimens were tested for each experimental point to obtain reliable data. The tensile stress (σt) was calculated as Equation (2):

σt =

Load wt

(2)

where w and t represent the initial width and thickness of the dumb-bell shaped specimens, respectively. The tensile strain (εt) was defined as the change in the length relative to the gauge length of the freestanding specimen. The tensile strength (σb) and elongation (εb) of a specimen are the stress and strain at breaks, respectively. The initial tensile elastic modulus (E) was calculated from the linear stress-strain relationship in the strain range less than 10%. Tensile tests were also performed at the crosshead speed of 50 and 100 mm·min-1 to examine the strain rate-dependent mechanical behavior of the gels. Cyclic tensile tests were performed at a crosshead speed of 100 mm⋅min-1.53 Rheological Measurements. Rheological tests were performed with an MCR 302 rheometer (Anton Paar, Austria) equipped with 25 mm parallel plates and an evaporation blocker. The discshaped samples with a thickness of about 1 mm were measured. Temperature sweep test was performed at a fixed frequency (ω = 10 rad·s-1) and strain (γ = 0.01%) over a temperature range from 30 to 90 °C with the heating rate of 1 °C min-1. Frequency-dependent data were collected at T = 30 °C and a fixed strain (γ = 0.01%) over a frequency range from 0.01 to 100 rad·s-1. Shape Memory Behaviors. A hydrogel strip (60 mm × 5 mm × 1 mm) and a dumbbell-shaped hydrogel sample (overall length: 35 mm; gauge length: 15 mm; width: 6 mm; inner width: 2 mm) were cut from the as-prepared poly(NVP-co-AAP) hydrogels. Next, the gel samples were immersed in 70 °C deionized water for about 5 s, then the softened strip was immediately deformed into a helix by wrapping on a glass rod and the dumbbell-shaped sample was elongated,


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and finally the deformed shapes were fixed at room temperature. The shape recovery process was conducted by immersing the deformed gel samples into 70 °C deionized water, and all processes were recorded by a digital camera (EOS 600D, Canon, Japan). Quantitative shape memory properties of the poly(NVP-co-AAP) hydrogels were determined by previously reported methods using U-shaped specimens.24, 27 The shape fixity ratio (Rf) and shape recovery ratio (Rr) were defined by the Equations (4), (5): ܴf = ܴr =

ఏt ఏi

×100%

ఏi -ఏf ఏi

×100%

(4) (5)

where θi is the given angle; θt is the temporarily fixed angle; and θf is the final angle. At least three specimens were tested for each experimental point to obtain reliable data. Surgical Fixation Device Display. The gel sheets used for surgical fixation devices were cut from the as-prepared poly(NVP-co-AAP) hydrogels. The hydrogel specifications for wrapping knuckles: 60 mm × 45 mm × 2 mm, shank: 260 mm × 180 mm × 2 mm, and foot: 205 mm × 145 mm × 2 mm. And the gel used to wrap wrist was cut according to the shape and size of the wrist and palm, with a length of 245 mm. The devices were softened in 70 °C deionized water, and the shapes were fixed at room temperature. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Figure S1 representing 1H NMR spectrum of the monomer AAP; Figure S2 showing the swelling curves of dried poly(NVP-co-AAP) hydrogels in DMF; Figure S3 showing the necking of the poly(NVP2-co-AAP1) hydrogel; Figure S4 showing cyclic tensile tests of


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hydrogels; Figure S5 showing frequency dependence of G', G" and tan δ for the poly(NVP4-coAAP1) hydrogel and temperature dependence for the poly(NVP1-co-AAP1) organogel; Figure S6 showing tensile stress−strain curves of the poly(NVP3-co-AAP1) hydrogel under different strain rates; Figure S7 showing the recovery of the hydrogel and its bending angle; and Figure S8 showing the Rf and Rr of poly(NVP-co-AAP) hydrogels with different compositions. Movie S1 for the holding of a load of 4 kg by poly(NVP-co-AAP) hydrogel stripe. Movie S2 for the shape recovery of a helical shape poly(NVP1-co-AAP1) hydrogel in 70 °C water. Movie S3 for the stretching, fixation and heat-shrinkage of a poly(NVP1-co-AAP1) hydrogel. Movie S4 for the shape recovery of a U-shaped poly(NVP1-co-AAP1) hydrogel strip in 70 °C water. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions C. Jiao and H. L. Wang designed the experiments. Y. Y. Chen, T. Q. Liu, X. Peng, Y. X. Zhao, J. N. Zhang, Y. Q. Wu performed the experiments. Y. Y. Chen, T. Q. Liu, X. Peng and H. L. Wang performed the data analysis. C. Jiao and H. L. Wang wrote the manuscript. All authors have given approval to the final version of the manuscript. Notes


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for Table of Contents use only

Rigid, Strong Thermo-responsive Shape Memory Hydrogels Transformed from Poly(vinyl pyrrolidone-co-acryloxy acetophenone) Organogels Chen Jiao, Yuanyuan Chen, Tianqi Liu, Xin Peng, Yaxin Zhao, Jianan Zhang, Yuqing Wu and Huiliang Wang*

Poly(vinyl pyrrolidone-co-acryloxy acetophenone) [poly(NVP-co-AAP)] hydrogels are prepared from the corresponding organogels by solvent exchange. The strong hydrophobic association and π-π stacking endow the hydrogels with high tensile strengths and high Young's moduli (up to 94.2 ± 1.3 MPa), as well as good shape memory properties. These rigid, strong hydrogels can be used as surgical fixation devices.


35


Figure 1. (a) The synthesis of the monomer AAP and the copolymerization of NVP and AAP. (b) The preparation process and the proposed formation mechanism of poly(NVP-co-AAP) organogels and hydrogels. (c-e) Photos showing the appearance of (c) poly(NVPx-co-AAPy) organogels (from left to right: x:y=1:1, 2:1, 3:1, 4:1, 5:1), (d) corresponding hydrogels obtained by immersing organogels into deionized water and (e) organogels obtained by re-immersing the hydrogels in DMF. 520x484mm (96 x 96 DPI)


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Figure 4. Yielding and necking phenomenon of the poly(NVP2-co-AAP1) hydrogel in the tensile test and its mechanism. (a) Uniform elongation of the gel to εt of 10%. (b) Yielding and necking of the gel at εt of 19%. (c) Tensile stress-strain (σt-εt) curve of the poly(NVP2-co-AAP1) hydrogel and the points related to (a) and (b). (d) The formation mechanism of yielding and necking. 691x488mm (96 x 96 DPI)



Figure 6. Photos demonstrating the shape memory behaviors of the poly(NVP1-co-AAP1) hydrogel. (a) Shape recovery of a helical shape hydrogel in 70 °C deionized water in 14 seconds. (b) The original shape of a dumbbell-shaped hydrogel (top), its temporary fixed shape by stretching the heated sample and fixing at room temperature (middle) and its shape recovery in 70 °C deionized water (bottom). 689x353mm (96 x 96 DPI)


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Figure 8. Demonstration of the application of poly(NVP-co-AAP) hydrogels as gypsum substitutes to wrap around and support different limbs, including (a) knuckle, (b) wrist, (c) shank and foot. 798x552mm (96 x 96 DPI)



P A A P 1 7 5 9

1 6 8 0

1 1 2 0

P o ly ( N V P - c o - A A P )

T /%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

Page 40 of 51

1 2 7 3

1 7 6 0

1 1 1 6

P V P

1 6 5 9

1 2 8 0

1 6 4 9

2 0 0 0

1 5 0 0

1 0 0 0 ACS Paragon Plus Environment

W a v e n u m b e r/c m

5 0 0 -1

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a

0 .2 5 P o ly ( N V P 1 - c o - A A P 1 ) P o ly ( N V P 2 - c o - A A P 1 )

0 .2 0

P o ly ( N V P 3 - c o - A A P 1 )



t

(M P a )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


P o ly ( N V P 4 - c o - A A P 1 )

0 .1 5

P o ly ( N V P 5 - c o - A A P 1 )

0 .1 0 0 .0 5 0 .0 0 0

1 0 0

2 0 0

3 0 0

4 0 0

(% )




t

5 0 0

6 0 0

7 0 0

8 0 0

(M P a )


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P o ly ( N V P 1 - c o - A A P 1 ) P o ly ( N V P 2 - c o - A A P 1 ) P o ly ( N V P 3 - c o - A A P 1 ) P o ly ( N V P 4 - c o - A A P 1 ) P o ly ( N V P 5 - c o - A A P 1 ) 4

k g

t



b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

1 0 9 8 7 6 5 3 2 1 0 0

5 0

1 0 0

(% )




t

1 5 0

2 0 0


1 0 0

o rg a n o g e l h y d ro g e l

1 0

(M P a )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

1

b



c

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0 .1

0 .0 1 1 :1

2 :1

3 :1

:c

4 :1


c

N V P

A A P

5 :1

1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

E (M P a )

d


3

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1 0

2

1 0

1

1 0

0

o rg a n o g e l h y d ro g e l

1 0

-1

1 0

-2

1 0

-3

1 :1

2 :1

3 :1

:c

4 :1


c

N V P

A A P

5 :1

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Table of Contents 840x359mm (96 x 96 DPI)


1 0

1 .0 0 .8

G ’ 6

1 0

0 .6 

G ’, G ’’ ( P a )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Page 46 of 51

0 .4 5

1 0

G ’’ 0 .2

ta n 1 0



4

0 .0 0 .0 1

0 .1

1

1 0




(ra d s

-1

)

1 0 0

ta n

a


7

1 0

1 0

6


7

1 .5

G ’

1 .0

G ’’

1 0

4



1 0

5

ta n

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

G ’, G ’’ ( P a )

b

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0 .5 ta n

1 0



3

0 .0 3 0

4 0

5 0

6 0

7 0


T ( °C )

8 0

9 0

2 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

B e n d i n g a n g l e ( °)

a


1 6 0

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0 s

1 2 0 1 0 s

8 0 3 0 s

4 0

6 0 s

0 0

1 0

2 0

3 0


t (s )

4 0

5 0

6 0

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b

R f, R r( % )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39


1 1 0 1 0 0 9 0 8 0 7 0 6 0 5 0 4 0 3 0 2 0 1 0 0

R f

0

R r

2

4 ACS Paragon Plus Environment

6

C y c le N u m b e r

8

1 0

2 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

B e n d i n g a n g l e ( °)

c


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8 0 °C 7 0 °C 6 0 °C

1 8 0 1 6 0 1 4 0 1 2 0 1 0 0 8 0 6 0 4 0 2 0 0 0

2 0

4 0

6 0

8 0

1 0 0


t (s )

1 2 0

1 4 0

1 6 0

1 8 0

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d

1 2 0 1 0 0 8 0 6 0

R

r

(% )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39


4 0 2 0 0 5 5

6 0

6 5

7 0


T

( °C )

7 5

8 0

8 5

Rigid and Strong Thermoresponsive Shape Memory Hydrogels

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