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pH- and Thermal-Responsive Multi-Shape Memory Hydrogel Xiao-Lei Gong, Yao-Yu Xiao, Min Pan, Yang Kang, Bang-Jing Li, and Sheng Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09605 • Publication Date (Web): 19 Sep 2016 Downloaded from http://pubs.acs.org on September 20, 2016

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ACS Applied Materials & Interfaces

pH- and Thermal-Responsive Multi-Shape Memory Hydrogel Xiao-Lei Gong,†, §,# Yao-Yu Xiao, †,# Min Pan, † Yang Kang,‡ Bang-Jing Li, ‡,* and Sheng Zhang†,* †

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

University, Chengdu 610065, China. ‡

Chengdu Institute of Biology, Chinese Academy of Science, Chengdu 610041, China.

§

Library of Shenzhen People’s Hospital, 2nd Clinical Medical College of Jinan University,

Shenzhen 518020, China. #

These authors contributed equally to this work.

KEYWORDS: multi-shape memory, multi-stimuli sensitivity, hydrogel, hydrophobic aggregation, dansyl groups

ABSTRACT: A multi-stimuli sensitive shape memory hydrogel with dual and triple shape memory properties was prepared by grafting dansyl groups into the network of polyacrylamide (PAAM). The hydrophobic aggregation of dansyl groups acted as molecular switches, which

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showed reversible aggregation-association transition in aqueous solution in response to the pH or temperature change.

Shape memory polymers (SMPs) are an important class of stimuli-responsive materials, which can be deformed and fixed into a temporary shape, and recover to their original shape when exposing to a specific external stimulus. The capacities of remembering two or more shapes under different conditions are called dual-shape memory effect (dual-SME) and muti-shape memory effect (multi-SME), which give the SMPs great potential for a wide range of applications, such as smart devices, sensors, and actuators. In order to realize more complex actuation and to expand the technical potential of SMPs, exploring new triggering mechanism and multi-shape memory process have been the focuses of SMPs research.1-4 So far, the most common stimulus for shape memory effect is heating, but other stimuli like pH have also become increasingly attractive in recent years. pH is of important environmental value in typical physiological, biological and/or chemical systems. It can also be utilized in the applications where large temperature swings are undesirable. For example, a SMP using pHsensitive host-guest complexes as the molecular switches was reported by our group. It could be processed into a temporary shape at pH 11.5 and was found to be able to recover to the original shape at pH 7.0.5 Subsequently, a kind of pH-sensitive polyurethane using hydrogen bonding as molecular switches, which could recover its initial shape at pH 1.5, was prepared by Zhou et al.6 These works, however, only focused on the pH-induced shape memory effect. Although a few reports have developed some multi-stimuli-responsive behaviors of materials,7-9 seldom SMP has been reported in response to multi-stimuli of pH values and other stimuli up to now, however,


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it is obvious that the feedback behavior of materials usually occurs due to a combination of multiple factors in nature. To develop multi-shape memory polymers which are capable of remembering more than one temporary state, therefore more flexible and able to meet more complex demand of applications, is another attractive subject in the field of SMPs. Generally, multi-shape SMPs need to have two or more distinct thermal phase switches, or a single broad thermal transition, and the multi-shape memory effect is triggered by heating.4,10-13 Although Hu et al.14 and Zhou et al.15 reported triple SMP being responsive to thermal and water stimuli respectively, a combination of both multistimuli and multi-shape memory effect in one material is still a big challenge. In this letter, we synthesized a versatile pH- and thermal-responsive multi-shape memory hydrogel by introducing dansyl groups into the network of chemical crosslinked polyacrylamide (Dns-PAAM). Acting as molecular switches, the hydrophobic aggregation of dansyl groups showed reversible aggregation-association transition in aqueous solution in response to pH or temperature change, and were responsible for the shape recovery when the switches opened. Furthermore, because the pH of electrolyte solution could be changed by electrolysis, the shape memory effect of these Dns-PAAM hydrogels also could be triggered by electric current in electrolyte solution. The hydrogel developed by the present study was prepared by conventional radical polymerization of acrylamide, N,N-methylenebisacrylamide and a monomer possessing dansyl moiety (Scheme S2). Table 1 listed the hydrogel samples of the different molar ratios and their corresponding sample names. The chemical structures of the resulting materials were confirmed by FT-IR analysis. As is shown in Figure S3, the signal of the vibration of N-CH3 on


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naphthalene rings at 2786 cm-1 and absorptions of the asymmetrical stretching vibration and symmetrical stretching vibration of SO2 at 1309 cm-1 and 1103 cm-1 indicate the successful introduction of dansyl groups into the polymer networks. It is known that dansyl group is a Lewis base. The nitrogen atom in the dimethylamino group can be protonated by combining with H+ to form hydrophilic NH+ under acidic conditions and deprotonated under relative higher pH value.16 The pKa value of the dimethylamino moiety on dansyl group was between 3~4.17-19 Therefore, the Dns-PAAM hydrogels showed pH-sensitive swelling behaviour. As is represented in Figure 1a, all the hydrogel samples showed a significant decrease of swelling ratio in buffer solution as the pH value increased from 2.0 to 4.0. When the pH was above 5.0, the swelling ratio reached a stable value. These results indicated that the electrostatic repulsion between adjacent protonated dansyl groups at very acid environment resulted in the swelling of the Dns-PAAM hydrogel. With the increasing of pH, dansyl groups deprotonated gradually. The neutral dansyl groups showed a hydrophobic property, tending to aggregate together in hydrophilic environment to lower the free energy of the system. As a result, water diffused out of the hydrogel and the swelling ratio of hydrogel decreased. This parameter reached a limiting value as the dansyl groups completely deprotonated when pH>5.0. When the dansyl groups turned hydrophobic, they not only led to the diffusion of water, but also acted as physical crosslinks to enhance the mechanical strength of samples. As is shown in Figure 1b, Dns25-PAAM transformed from a soft state to toughness when the pH value was changed from 2.0 to 5.0 (tensile strength increased from 0.5 MPa to 5.0 MPa). In addition, the samples with more dansyl amount showed higher strength. These results suggested that the dansyl groups acted as reversible switches being “turned on” at low pH values due to


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the protonation of dansyl moieties and “turned off” with the increase of pH because the deprotonation of dansyl group led to the formation of hydrophobic aggregation domains. Since the property of dansyl moieties was dependent on pH, the rheological behaviour of DnsPAAM hydrogels also showed pH dependence. Figure 1c showed the viscoelasticity of Dns5PAAM hydrogel in different pH solution (the rheological measurements were carried out by using Dns5-PAAM hydrogel because the samples with more DnsEMA molar ratio were too tough to be measured.) It can been seen that G’ was always higher than G’’ over the whole range under individual pH value, indicating that the materials were in hydrogel state in the whole process. It can be seen that both the dynamic storage and loss moduli (G’, G’’) of these samples in pH 5.0 solution were much higher than those in pH 3 solution. It is known that G’ is proportional to the crosslink density of the network formed by chemical bonds and physical entanglements. Therefore, the significant increase of G’ indicated that the crosslink density of the Dns5-PAAM hydrogel increased in response to the pH increase. The Dns-PAAM hydrogels exhibited pH-induced shape memory effect due to the pH-sensitive aggregation/dissociation of dansyl moieties. Figure 2 showed photos of pH-induced shape memory behaviour of Dns25-PAAM. The original shape of the sample was a plane stripe (Figure 2a). After being immersed in aqueous solution with pH 2.0, the sample became soft and easy to deform. Then it was bent to a spiral shape by external stress at pH 2.0 and rapidly transferred to the aqueous solution with pH 5.0 for 10 min. When the external stress was released, the sample kept the spiral shape perfectly (Figure 2b). When the spiral-shaped sample was immersed to the aqueous solution with pH 2.0 again, it recovered to its original shape within 10 min (Figure 2c-l).


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The method used to evaluate the dual shape-memory effect induced by pH or heat (as shown in Scheme S3) was according to the literatures.20-22 A straight strip of hydrogel film which was previously immersed in aqueous solution with pH 2.0 was deformed to an angle, which is named as γ. Then it was transferred to the buffer solution with pH 5.0 for 10 min. After the external stress was released, the new angle, which is named as α, was measured. The fixity ratio (Rf) was defined as α/γ. The deformed sample was immersed in the aqueous solution at pH 2 again and then soaked for 2 min. The residual angle β was recorded. The shape recovery ratio (Rr) was defined as (α−β)/α(Scheme S3). The fixity (Rf) and recovery ratios (Rr) of samples with different dansyl amount were shown in Table 1. The values of Rf and Rr represent the shape memory and shape recovery ability of SMPs respectively. Generally, Rf was closely related to the quantity and strength of reversible switches formed in the shape memory process, while Rr was mostly determined by the stable polymer networks of the SMPs. It can be seen that Dns0-PAAM without dansyl moieties did not exhibit shape memory effect at all. As the amount of dansyl moieties increased, the hydrogels showed shape memory behaviour gradually. When the molar ratio of DnsEMA was 10% in Dns10-PAAM, the fixity ratio was about 20%. When the DnsEMA ratio increased to 25% in Dns25-PAAM, the fixity ratio was above 90%. These results suggested that dansyl moieties played a key role for the pH-induced shape memory behaviour of Dns-PAAM hydrogels. As was mentioned earlier, pH is one of the important environmental factors in typical physiological, biological and/or chemical systems. For example, the pH values of the gastrointestinal tract, vagina, and blood vessel are 1.0-3.5, 3.8-4.5 and 7.4, respectively.23 Therefore, the pH stimulus is a promising choice for the design of shape memory polymers with potential medical applications. Furthermore, pH values can be manipulated easily in many other applications. In this study, we adjusted the pH values of electrolyte solution by electrolysis, and


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then employed the Dns-PAAM hydrogel as electric current triggering robot hands. As is shown in Figure 3a, in the copper sulfate solution, the robot hand was fabricated into a hook shape (temporary shape) to hold the weight. After application of 10 V direct electric current, the pH value of solution decreased because of the electrolysis of copper sulfate which in turn was due to the fact that the hook recovered to the original shape and released the weight. Recently, Zhou et al.6,24 and our group5 reported pH-sensitive SMPs which were able to be deformed into predetermined shapes and had the capacity to revert back to its original shape upon application of pH stimulus.Apart from that, it has been demonstrated that hydrophobes can gradually disengage from the hydrophobic associations with increasing temperature, or reassociate at relatively low temperature, enabling reversible change between strong and weak hydrophobic interactions. As a result of temperature-dependent hydrophobic association, the tensile modulus of hydrogels with hydrophobic aggregates exhibit abruptly decrease with the increase of temperature.25-30 In this study, the storage modulus of Dns-PAAM hydrogel which was responsive to the change of temperature was investigated by dynamic mechanical analysis (DMA). As is shown in Figure 3b, the storage modulus (G’) of Dns15-PAAM decreased over 10 times with the temperature being increased from 5 to 65 ℃ and a broad glass transition was observed between 10 ℃ and 65 ℃. In addition, only amorphous halo in Dns-PAAM hydrogel samples were found by using wide-angle X-ray diffraction (WAXD) measurements (Figure S4), suggesting that no crystal structure existed in the sample. Therefor, the thermal-induced shape memory was induced by Tg, but not Tm. The hydrophobic aggregations of dansyl moieties in the hydrogels, which acted as physical crosslinks to limit the movement of polymeric chains at low temperature and


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dissociated with the temperature increasing, were in an amorphous glassy state. The dansylcrosslinks which were dependent on pH and temperature made the Dns-PAAM hydrogel not only of pH-induced but thermal-induced shape memory effect. It has been demonstrated that materials with a broad glass transition temperature could show multi-shape memory effect.10, 11 Therefore, we investigated the dual and triple shape memory behaviour of the hydrogels. Figure 3c showed the thermal responsive dual-shape memory effect of Dns25-PAAM. The sample was first deformed into a complex shape such as a box in aqueous solution above 50 ℃, and cooled to room temperature under external stress. After releasing the stress, the sample stays in its box shape. Being immersed in hot water above 50 ℃ again, this sample recovered to its original shape quickly. The fixity and recovery ratios of samples with different dansyl amount were shown in Table 1 (also see Figure S5). Similar to the pH-induced shape memory effect, PAAM hydrogel without dansyl moieties did not show thermal-induced shape memory effect either. As the amount of dansyl moieties increased, the hydrogels showed shape memory behaviour gradually. The fixity and recovery ratio could reach nearly 100%. Beside the excellent dual-shape memory performance, the Dns-PAAM hydrogel also exhibited good triple-shape memory effect, which is demonstrated in Figure 3d. At 90 ℃,the strip sample was deformed to a U-like shape and subsequently fixed at 50 ℃ for 1 min to get the first temporary shape with Rf near 100 % (Figure 3d(2)). The first temporary shape was further deformed at 50 ℃ and fixed at 4 ℃ for 1 min to yield the W-like second temporary shape with Rf approaching 100% (Figure 3d(3)). Upon reheating to 50 ℃, the recovered first temporary shape was obtained with Rr = 83% in 5 min (Figure 3d(4)). Being furtherly heated to 90 ℃, the


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shape recovered completely, with no notable deformation compared with the original shape (Figure 3d(5)). According to the research we conducted in the present work, shape fixation by pH procedure can also be recovered by heating process for Dns-PAAM hydrogels, as is shown in Figure S6, and vice versa (Figure S7). These phenomena implied that the thermal- and pH-induced shape memory behaviours were resulted from the change of the same molecular switch. The mechanism of the pH- and thermal-induced shape-memory effect was proposed as follows (Figure 4). The dansyl groups in Dns-PAAM hydrogels were aggregated to form physical crosslinks due to the hydrophobic interactions when the pH value of the environment was above 5.0 and the temperature was low. In this state, the Dns-PAAM hydrogels was rigid and hard to be deformed. When the pH was changed to 2.0, or the temperature was increased, the dansylcrosslinks disengaged due to decrease of hydrophobic interactions, making the material soft and readily be deformed to its temporary shape by application of an external stress. But after being immersed in the solution where the pH was above 5.0 or the environment where the temperature was decreased, the dansyl groups tend to aggregate together again, and the corruption of hydrophobic aggregation returned to its permanent shape. In summary, we developed a kind of multi-stimuli sensitive SMPs (Dns-PAAM hydrogels) with dual and triple shape memory properties by introducing dansyl moieties into PAAM hydrogel through facile radical copolymerization. The hydrophobic interactions between dansyl groups were utilized as reversible switches for the Dns-PAAM hydrogels. The association and dissociation of dansyl moieties were in response to pH and temperature change. Resultantly, the Dns-PAAM hydrogels showed pH-, electro-, and temperature-induced shape memory effect respectively, and the pH and temperature stimuli were fully interchangeable. Furthermore, the


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Dns-PAAM hydrogels could also show triple shape memory behaviour by controlling the programming process. These versatile shape memory properties provide the materials good potential to meet more complex requirements of the application.


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Figure 1. Swelling and mechanical properties of Dns-PAAM hydrogels. (a) Swelling ratios of Dns-PAAM hydrogels at different molar fractions of the dansyl at room temperature in aqueous solution with various pH values. (b) Tensile mechanical properties of Danyl-PAAM hydrogels (σ is stress, and εis strain). (c) Storage and loss moduli of Dns5-PAAM in pH 3.0 and pH 5.0 buffer solution (G’ is shear storage modulus, G” is shear loss modulus, and τ is the shear stress).


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Figure 2. Photographs that demonstrate the macroscopic pH-induced shape memory behaviour of Dns25-PAAM. a) Original shape at pH 2.0. b) Temporary shape at pH 5.0. c-l) Transition from temporary shape to original shape at pH 2.0 in 10 min.


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Figure 3. (a) The demonstration of a practical application of Dns25-PAAM as robot hands. (b) DMA curves of Dns15-PAAM hydrogels (pH=5.0) (tanδ (loss factor) =G″/G’,). (c) Photographs that demonstrate the thermal-induced dual-shape memory behaviour of Dns15-PAAM. 1) Original shape. 2) Temporary shape. 3-4) Sample recovered from temporary shape to original in hot water (>50℃). (d) Thermal triple-shape memory properties of Dns15-PAAM. 1) Original shape. 2) The first temporary shape. 3) the second temporary shape. 4) Sample recovered from the second temporary shape to the first temporary shape at 50 ℃. 5) Sample recovered from the first temporary shape to original shape at 90 ℃.


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Figure 4. The mechanism of pH- and thermal-induced shape memory process.


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Table 1. Feeding ratios, pH-induced Rf and Rr, thermal-induced Rf and Rr of Dns-PAAM hydrogels with different mol fraction of DnsEMA.

Sample

x/y/za mol%

pH-induced Rf%

pH-induced Rr%

Thermalinduced Rf%

Thermalinduced Rr%

Dns0-PAAM

0/15/85

0

/

0

/

Dns5-PAAM

5/15/80

0

/

0

/

Dns10-PAAM

10/15/75

22.2

100

57.41

100

Dns15-PAAM

15/15/70

62.8

96.5

100

100

Dns20-PAAM

20/15/65

91.63

95.7

100

100

Dns25-PAAM

25/15/60

95.4

94.6

100

93.58

a. x/y/z = DnSEMA/MBA/AAm in the feed.


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ASSOCIATED CONTENT Supporting Information. Experimental section, structural characterization and additional shapememory experiments. AUTHOR INFORMATION Corresponding Author *email: [email protected]; [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. # These authors contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was funded by the National Natural Science Foundation of China (Grant Nos. 51573187, 51373174), West Light Foundation of CAS. REFERENCES (1) Hu, J. L.; Zhu, Y.; Huang, H. H.; Lu, J. Recent Advances in Shape–Memory Polymers: Structure, Mechanism, Functionality, Modeling and Applications, Prog. Polym. Sci. 2012, 37, 1720-1763.


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(26) Osada, Y.; Matsuda, A. Shape Memory in Hydrogels. Nature 1995, 376, 219. (27) Tanaka, Y.; Kagami, Y.; Matsuda, A.; Osada, Y. Thermoreversible Transition of Tensile Modulus of Hydrogel with Ordered Aggregates. Macromolecules 1995, 28, 2574-2576. (28) Miyazaki, T.; Yamaoka, K.; Kaneko, T.; Gong, J. P.; Osada, Y. Hydrogels with the Ordered Structures. Sci. Tech. Adv. Mater. 2000, 1, 201-210. (29) Hao, J.; Weiss, R. A. Mechanically Tough, Thermally Activated Shape Memory Hydrogels. ACS Macro Lett. 2013, 2, 86-89. (30) Inomata, K.; Terahama, T.; Sekoguchi, R.; Ito, T.; Sugimoto, H.; Nakanishi, E. Shape Memory Properties of Polypeptide Hydrogels Having Hydrophobic Alkyl Side Chains. Polymer, 2012, 53, 3281-3286.


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pH- and Thermal-Responsive Multishape Memory ... - ACS Publications

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