Polymer Chemistry

Polymer Chemistry PAPER

Cite this: Polym. Chem., 2017, 8, 2872

Self-healable hydrogels with cross-linking induced thermo-responsiveness and multi-triggered gel–sol–gel transition† Xuemeng Wang, Gang Bian, Miao Zhang, Limin Chang, Zhiwei Li, Xu Li, Heng An, Jianglei Qin, * Ruixue Chang and Haijun Wang* Self-healable hydrogels with regulated cross-linking induced thermo-responsiveness (CIT) were prepared from P(N,N-dimethylacrylamide-stat-diacetone acrylamide) P(DMA-stat-DAA), which was synthesized through RAFT copolymerization. The copolymers of P(DMA-stat-DAA) with various compositions were cross-linked by a series of dihydrazide compounds and self-healable hydrogels were prepared without any additional stimulus under neutral conditions. Interestingly, although the copolymer did not show any temperature responsivity, the hydrogels showed thermo-responsiveness with regulated cloud points, which varied with the composition of the copolymer and group ratios of the dihydrazide cross-linkers. With dynamic covalent acylhydrazone bond connections, the hydrogels showed gel–sol–gel transitions regulated by acidity. Also, the gel–sol transition was induced by the addition of excess dihydrazide com-

Received 15th March 2017, Accepted 30th March 2017

pounds and the sol–gel transition was triggered by further addition of P(DMA-stat-DAA). The CIT property

DOI: 10.1039/c7py00445a

provided a new method to prepare thermo-responsive hydrogels from non-thermo-responsive polymers, and the catalysis-free self-healable hydrogels with thermo-responsiveness could have great potential

rsc.li/polymers

applications in areas related to bioscience and biotechnology.

Self-healable materials have drawn great attention and made great progress over the past several years. This is because the fantastic ability to self-repair is one of the most fascinating abilities of living creatures. Living creatures can repair damage on their bodies and restore the function of their organs; while the self-healing property vanishes after death of the body accordingly. To mimic this important characteristic of living things, scientists have tried a variety of methods to prepare artificial materials with self-healing properties. The self-healable materials are prepared from reversible connections based on both intermolecular forces and covalent bonds, which are defined as self-healable physical materials,1–8 and self-healable chemical materials, respectively.9 For self-healable physical materials, host–guest interactions,2 H-bonds,7,10 etc. were used and showed interesting properties; however, self-healable materials with dynamic covalent bonds show higher dimension stabilities and improved mechanical properties.5,11–15 As a result, self-heal-

College of Chemistry and Environmental Science, Hebei University, 180 East Wusi Road, Baoding 071002, China. E-mail: [email protected], [email protected]; Tel: +86 -157-33210284 † Electronic supplementary information (ESI) available: Repeated self-healing of hydrogels and gel–sol–gel transition under various triggers. See DOI: 10.1039/ c7py00445a

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able polymer materials and gels3,16–19 with reversible covalent bonds were prepared and have made great progress in the past decade.14,15,20–27 The Wudl group reported a thermally repairable polymeric material based on the reversible characteristics of the Diels–Alder reaction under heating above 100 °C.28 In 2004, crack healable polymer materials were prepared via the reversible cycloaddition of cinnamate under UV exposure.12 Matyjaszewski reported self-healable polymer materials under UV triggered reshuffling of trithiocarbonates.27 The Cheng group designed a dynamic urea bond as a reversible crosslinker to prepare self-healable polymer materials.15 Imato et al. used a stable free radical of diarylbibenzofuranone as a dynamic covalent unit to prepare self-healable materials.23 Deng et al. designed novel self-healable polymeric gels based on dynamic covalent bonds of acylhydrazone, and the gels showed pH responsiveness and sol–gel transitions under various acidities, drawing great attention.29–32 At the same time, a series of self-healable materials with imine bonds were reported.33–35 Hydrogels with reversible oxime bonds were also prepared by the Sumerlin group from P(DMA-stat-DAA) and propanediylbishydroxylamine dihydrochloride.26 However, the cross-linker propanediylbishydroxylamine dihydrochloride was supplied as a salt, and TEA was needed to neutralize the dihydrochloride. As a result, the gelation reaction pH is not clear, and more impurities could be introduced into the self-

This journal is © The Royal Society of Chemistry 2017

Polymer Chemistry

Scheme 1

Synthesis of P(DMA-stat-DAA) through RAFT polymerization.

healable hydrogel, which increased the risk of bio-toxicity. Compared to other Schiff bases, the acylhydrazone can form under neutral conditions without any stimulus. At the same time, the dihydrazide is not as active as an amine, which ensured better bio-compatibility since the ester bond is an important bond in the living body. However, self-healable hydrogels with DMA backbones based on reversible acylhydrazones have not been investigated up to now. Herein, self-healable hydrogels were prepared based on P(DMA-stat-DAA) with a series of dihydrazides as cross-linkers, and dynamic chemical cross-linked self-healable hydrogels were prepared with dynamic acylhydrazone bonds. P(DMA-statDAA) with various compositions was synthesized through RAFT polymerization to control the molecular weight and composition, with the structure of the copolymer and the preparation procedure being shown in Scheme 1. The hydrogel formed and self-healed without any additional stimulus; moreover, the hydrogels showed a reversible gel–sol–gel transition triggered by the pH and group ratios. Cross-linkers with different structures and Mn values were used to regulate the cross-linking density, and the morphology dependency on the cross-linkers and cross-linking density were also investigated. It was amazing to see that although the copolymer of P(DMAstat-DAA) did not show thermo-responsiveness, the self-healable hydrogels showed thermo-responsivity with the cloud point (CP) varying with the composition of the copolymer, structure of the cross-linkers and cross-linking density. Compared to those thermo-responsive hydrogels prepared from thermo-responsive polymers like PNIPAM and its copolymers,32,36 this cross-linking induced thermo-responsiveness (CIT) inspired more possibilities to design smart and thermosensitive materials. Also, these hydrogels formed and selfhealed under neutral conditions so there is no need to worry about toxicity or the thermal runaway of the catalyst, which ensured better application properties in areas related to bioscience and biotechnology.

Experimental section Materials N,N-Dimethylacrylamide (DMA) was supplied by Macklin Co. and passed through a column filled with basic alumina before polymerization. Diacetone acrylamide (DAA) was also purchased from Macklin Co. and recrystallized three times in n-hexane to remove any inhibitor. Dimethyl 3,3′-dithiodipropionate and adipic dihydrazide (ADH) were also purchased


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from Macklin Co. Ltd. Dithiodipropionic acid dihydrazide (DTDPH) was prepared from dimethyl 3,3′-dithiodipropionate through hydrazinolysis.30,32 Poly(ethylene oxide) (PEO23, Mn = 1000) was supplied by Guangfu Fine Chemical Research Institute and used to prepare PEO23 dihydrazide according to a previous report.29 S-1-Dodecyl-S’-(R,R′-dimethyl-R″-acetic acid) trithiocarbonate (DDMAT) was synthesized in our lab.37 HCl, triethylamine, dithiothreitol (DTT), H2O2 and all other reagents and solvents were supplied by Kermel Chemical Reagent Co. and used as received. Instrument and analysis The structure and composition of the copolymers were determined using 1H NMR spectra, which were obtained on a Bruker 600 MHz spectrometer (Avance III, Bruker Co. Switzerland) with CDCl3 as a solvent at room temperature. The Fourier-transform infrared (FT-IR) spectra were obtained using a Varian 600 IR spectrometer. The polymer samples were dissolved in CH2Cl2 and casted on a KBr plate for characterization. Gel permeation chromatography (GPC) with a RI detector was used to determine the PDI of the polymers with THF as an eluent. Scanning electron microscopy (SEM) images were observed on a JSM-7500 microscope to characterize the morphology of the micro-porous material at an operating voltage of 10 kV. A differential scanning colorimeter (Diamond DSC, PerkinElmer) was used the determine the Tg of the copolymers under N2 atmosphere at a heating rate of 20 °C min−1. Turbidity characterizations were conducted with a Shimadzu UV-2550 UV-Vis spectrophotometer at a 500 nm wavelength, and the transmittances were recorded after the temperature had been held for 2 min. Synthesis of P(DMA-stat-DAA) through RAFT polymerization P(DMA-stat-DAA) was prepared from DMA and DAA through RAFT polymerization mediated by DDMAT and initiated by AIBN; the typical procedure is described as follows: DDMAT (145.6 mg, 0.4 mmol), DMA (3.96 g, 40 mmol), DAA (2.04 g, 12 mmol) and AIBN (9.8 mg, 0.12 mmol) were dissolved in 6 mL of dioxane in a 25 mL reaction tube equipped with a magnetic stirring bar, then the tube was sealed and the oxygen was removed by three freeze–pump–thaw cycles. After back filling with nitrogen, the mixture in the reaction tube was melted and the tube was immersed into a 60 °C oil bath. The polymerization was performed for 24 h under N2 atmosphere with continuous stirring.4,38–40 The polymer product of P(DMA-stat-DAA) was precipitated in ethyl acetate/petroleum ether (2/8) three times and dried under vacuum. The molecular weight of the polymer was evaluated by monomer conversion. Other copolymers were prepared by the same procedure and the compositions were calculated by comparing the corresponding peak areas on the 1H NMR spectra. Preparation of self-healable hydrogel The self-healable hydrogels were prepared from P(DMA-statDAA) by dihydrazide cross-linking using the following procedure: first, P(DMA-stat-DAA) and a predetermined equivalent

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of dihydrazide compound were dissolved in deionized water directly to form a clear solution with a total concentration of 10%, then the solution was put into the moulds to form the hydrogels without any interference at room temperature. The gel formation was confirmed through vial leaning. Rheology tests and the self-healing properties were investigated after a clear gel was formed and incubated for 24 h. The self-healing process of the polymeric hydrogels was carried out as follows: a series of hydrogels with various crosslinking densities and compositions were kept in closed desiccators with saturated moisture to incubate for 24 h before carrying out the study. Then the hydrogels were cut into halves across the center, and the two pieces were put back into the original mould with the cut surfaces in close contact. The healing result was confirmed by stretching the self-healed hydrogel with tweezers from both sides of the cut line after 24 h. The experiment was repeated by cutting the healed sample along the same line and in a perpendicular direction. Each time the healed hydrogel was incubated for 24 h before cutting again. Photos were taken at each step to record the progress. Rheological tests of the polymer hydrogels in an oscillatory mode were measured on a TA AR2000ex rheometer with a 25 mm parallel plate. The gap was fixed at 1000 μm for all the measurements; the frequency scan was carried out from 0.03–100 rad s−1 and a strain of 5% was selected within the linear viscoelastic regime. All measurements were conducted under a nitrogen atmosphere. The thermo-responsiveness of the self-healable hydrogels was determined by turbidity studies using a UV-Vis spectrophotometer. The change in transmittance with increasing temperature was recorded at a 500 nm wavelength. The cloud point (CP) was defined as the temperature at a 50% decrease in the transmittance. Gel–sol–gel transition of the hydrogels under a variety of triggers As it is cross-linked by dynamic covalent bonds, the hydrogel should show pH sensitivity although the functionality of the copolymer is pretty high. To investigate the acid responsiveness of the polymer gel, HCl (5 M) was dropped onto the top of the hydrogel, which was shaken continuously to observe the gel–sol transition. After the gel dissolved into liquid, triethylamine was added to neutralize the HCl and regulate the pH of the hydrogel to about 6.5. The gel–sol–gel transitions were recorded with a digital camera. The disulfide bond can be reduced into thiol by DTT, n-Bu3P,41 etc. 2 equivalents of DTT were added into the hydrogel with DTDPH as a cross-linker, and hydrogels with other cross-linkers were also used for comparison. After the hydrogel turned into liquid, the same amount of H2O2 was added to see if the hydrogel could be re-obtained. It was reported that the dynamic hydrogel can undergo a gel-to-sol transition by cleavage at the cross-linking point by a monofunctional compound,26 because the ketone-functional groups are consumed by the monofunctional compound through an exchange of the dynamic bond. 3 equivalents of

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Polymer Chemistry

dihydrazine were added into the hydrogel prepared from P(DMA93-stat-DAA30) to regulate the group ratios of the ketonegroup : hydrazine to 1 : 4. The gel–sol transition of the hydrogel triggered by an excess of acylhydrazine was observed. Then P(DMA93-stat-DAA30) in 10% solution was added to regulate the group ratio to 1 : 1 to observe the gel formation. Morphology observation of the polymer hydrogels The morphologies of the hydrogels with various compositions and cross-linkers were observed using SEM and the crosslinking density was compared based on the SEM images. The SEM observations were carried out on a JSM-7500 microscope with an operating voltage of 10 kV. The hydrogels were lyophilized and coated with Au for the SEM observations.

Results and discussion Polymerization of P(DMA-stat-DAA) with pendant ketonefunctional groups Water soluble copolymers with ketone-functional groups were prepared through RAFT polymerization, as shown in Scheme 1, and were ready to be cross-linked by the dihydrazide compounds to form reversible acylhydrazone bonds. P(DMAstat-DAA) had been synthesized and the self-healable hydrogel was also prepared based on oxime bond cross-linking.26 RAFT polymerization was chosen in this research to obtain better control of the molecular weight and the functionality in this study. The molecular weight of the polymer was evaluated by monomer conversion and the composition of the copolymers was calculated by comparing the corresponding peak areas in the 1H NMR spectra. The 1H NMR spectrum of P(DMA-statDAA) is shown in Fig. 1. Peak a (at 2.91 ppm) is derived from the methyl protons on the DMA unit (–N–(CH3)2) and peak d (at 2.11 ppm) is derived from the methyl protons adjacent to

Fig. 1 1H NMR spectra of P(DMA90-stat-DAA10) (bottom) and P(DMA93stat-DAA30) (top) in CDCl3.


Polymer Chemistry Table 1

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Copolymers of P(DMA-stat-DAA) with various compositions

Copolymer

Composition

DAA molar ratioa

Mn b (kg mol−1)

Mw/Mn c

Tg d (°C)

A B C

P(DMA90-stat-DAA10) P(DMA93-stat-DAA30) P(DMA96-stat-DAA50)

10.0% 24.4% 34.2%

10.6 14.3 18.0

1.18 1.16 1.20

110.8 108.3 104.6

a

Calculated from the 1H NMR spectra. b Evaluated by monomer conversion. c Determined by GPC. d Determined by DSC.

the carbonyl group on the DAA unit (–(CvO)–CH3). Based on the above procedure, three copolymers with various compositions were synthesized and the detailed information is listed in Table 1. The structure of the copolymer was also characterized by FT-IR. Only one absorbance representing the amide carbonyl group was illustrated at 1639 cm−1, as shown in Fig. 2. At the same time, another peak derived from the ketone-group was illustrated at 1710 cm−1, and the peak area increased with increasing composition of DAA. This result proved that DAA has copolymerized with DMA and the ketone-functional group has been introduced onto the polymer chain. However, although the size of the DAA segment is much bigger than that of the DMA segment, the Tg of the copolymer decreased with an increasing ratio of DAA, as shown in Fig. 3. This is because the amide group on DAA is shielded, which then reduces the H-bond force of the polymer segments. With the DAA ratio increased from 10.0% to 24.4% and 34.2%, the Tg of the copolymer decreased from 110.8 °C to 108.3 °C and 104.9 °C respectively, as listed in Table 1.

Fig. 3 DSC curves of P(DMA-stat-DAA) with various compositions at a heating rate of 20 °C min−1.

P(DMA-stat-DAA) with various pendant ketone-group densities was used to prepare the hydrogels in deionized water with a series of dihydrazide compounds as cross-linkers, as shown in Scheme 2. First, ADH was used to cross-link the copolymers with a 1 : 1 equivalent of ketone to the hydrazide group to see if the hydrogel can be obtained. It was noticed that clear

hydrogels were formed from P(DMA90-stat-DAA10) and P(DMA93-stat-DAA30) overnight without any additional stimulus, as shown in Fig. 4(a and b). At the same time, the stiffness of the hydrogel increased with increasing cross-linking density, but the hydrogel prepared from P(DMA96-stat-DAA50) was opaque and phase separation was observed (Fig. 4c). When DTDPH was used as a cross-linker, a clear hydrogel was also obtained from P(DMA90-stat-DAA10). However, the hydrogel prepared from P(DMA93-stat-DAA30) became opaque because of the higher molecular weight and low hydrophilicity of DTDPH compared to that of ADH (Fig. 4d). The reason for the phase separation is that a large amount of hydrophobic

Fig. 2 FT-IR spectra of P(DMA90-stat-DAA10) (top) and P(DMA93-statDAA30) (bottom).

Scheme 2 Hydrogels from P(DMA-stat-DAA) and a variety of dihydrazide compounds (CL: cross-linker).

Preparation of hydrogels from P(DMA-stat-DAA) cross-linked by dihydrazide and thermo-responsive property


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Polymer Chemistry

Fig. 4 Optical images of the hydrogels prepared from (a) P(DMA90-stat-DAA10), (b) P(DMA93-stat-DAA30), (c) P(DMA96-stat-DAA50) with a 1 : 1 ratio of ADH cross-linking and (d) P(DMA93-stat-DAA30) with a 1 : 1 ratio of DTDPH cross-linking (credit from Hebei University).

segment was formed and aggregated during the cross-linking which reduced the hydrophilicity of the hydrogel. This result introduced a novel idea that the cross-linking density could possibly be used to regulate the thermo-responsiveness of the hydrogel, since the P(DMA-stat-DAA) with a high DAA composition showed thermo-responsive behavior.26,42 The thermo-responsiveness of the hydrogels prepared from P(DMA-stat-DAA) with various DAA compositions and group ratios of ketone to hydrazide was determined by turbidity characterizations at a 500 nm wavelength; the cloud point (CP) was defined as the temperature where there was a 50% decrease in the transmittance. The transmittance changes of a series of copolymers and hydrogels with increasing temperature are shown in Fig. 5. As shown in Fig. 5 (black lines), no thermo-responsive property was illustrated for P(DMA93-statDAA30) and P(DMA96-stat-DAA50) when the DAA ratio was up to 34.2% because the DAA composition was not high enough.26 However, the hydrogels prepared from P(DMA-stat-DAA) with various cross-linkers showed thermo-responsiveness with regulated CPs. The hydrogel prepared from P(DMA93-stat-DAA30) cross-linked by a 1 : 1 group ratio of ADH showed a CP of 46.6 °C, as shown in Fig. 5a (red). Although the P(DMA93-statDAA30) cross-linked by a 1 : 1 group ratio of DTDPH was opaque, the hydrogel cross-linked by a 2 : 1 group ratio of DTDPH became transparent with a CP of 39.5 °C (Fig. 5a, blue). Moreover, the transmittance of this hydrogel was low and began to decrease at quite low temperatures, indicating the poor hydrophilicity of the DTDPH cross-linked hydrogels.

P(DMA96-stat-DAA50) cross-linked with a 5 : 1 group ratio of DTDPH shows thermo-responsiveness with the CP of 40.6 °C, as shown in Fig. 5b (red); upon increasing the amount of DTDPH to a 5 : 2 group ratio, the hydrogel became translucent (Fig. S1†) and phase separation occurred gradually with higher DTDPH ratios. Based on the best of our knowledge, this is the first report regarding the preparation of thermo-responsive hydrogels from non-thermo-responsive polymers, although thermo-responsive self-healable hydrogels were reported in previous publications.32,36,43,44 Since the copolymer itself is not thermo-responsive, the increased size of hydrophobic segments formed during the cross-linking is considered responsible for this property, as shown in Scheme 3. To confirm this

Scheme 3 Hydrophobic segment formed during cross-linking by ADH (top), DTDPH (middle) and PEO23 dihydrazide (bottom), which contribute to the thermo-responsiveness.

Fig. 5 Transmittances of the copolymers and hydrogels with increasing temperature. (a) P(DMA93-stat-DAA30); (b) P(DMA96-stat-DAA50). The inset images are the hydrogels below and above the CP (credit from Hebei University).

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Polymer Chemistry

point, P(DMA96-stat-DAA50) was cross-linked by PEO23 dihydrazide with a group ratio of 5 : 2, as shown in Fig. 5b (blue). Although the same reversible hydrazone bond was formed and the cross-linking density was higher, a clear hydrogel with a CP of 54.6 °C was obtained, with the structure of the hydrophobic segment in this hydrogel also being shown in Scheme 3. This result confirmed that the thermo-responsivity of the hydrogels is dependant on both the structure of the hydrophobic segment and cross-linking density. As a result, the hydrogel from DTDPH cross-linking had the biggest hydrophobic structure and the lowest CP. Rheological properties of the self-healable hydrogels The mechanical properties of the hydrogels were determined by rheology characterization at 25 °C. The dependence of the storage modulus (G′) and loss modulus (G″) on the frequency for a series of polymer hydrogels was determined. The hydrogel prepared from P(DMA90-stat-DAA10) with both ADH and DTDPH cross-linking showed reversible characteristics as the G″ increased with a decreasing frequency, which indicated reversible characteristics and a lower cross-linking density. The rheology curve of the hydrogel with DTDPH cross-linking is shown in Fig. 6a, which shows that G″ becomes comparable to G′ when the frequency is 0.03 rad s−1 and tends to exceed it below 0.01 rad s−1. Compared to that copolymer with pretty low benzaldehyde group density can form stiff polymer gels in previous reports,30,32 the strength of this hydrogel could be much lower due to the other components. The strength of the hydrogel increased with increasing DAA composition, and G′ > G″ was observed over the whole range for the hydrogel prepared from P(DMA93-stat-DAA30) with a 1 : 1 ratio of ADH crosslinking, even when the frequency (ω) was as low as 0.01 rad s−1, as shown in Fig. 6b (black). When the cross-linker was changed to DTDPH with a group ratio of 2 : 1, although the G′

Fig. 6 Rheology curves showing the G’ (square) and G’’ (circle) dependence on frequency for the hydrogels prepared from (a) P(DMA90-statDAA10), (b) P(DMA93-stat-DAA30), and (c) P(DMA96-stat-DAA50) (concentration = 0.1 g mL−1, 5% strain at 25 °C).


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was quite low, the G′ and G″ did not change with a decreasing frequency, as shown in Fig. 6b (red). At the same time, the hydrophobic segments in the hydrogel began to form and aggregate (low CP in Fig. 5a), which reduced the reversible reaction rate. With DTDPH increased to a 1 : 1 group ratio, the hydrogel became rigid with a slippery surface possibly because of micro-phase separation and the rheology characterization could not be carried out any more. The mechanical properties of the hydrogels prepared from P(DMA96-stat-DAA50) were also characterized, as shown in Fig. 6c. The hydrogel with a 5 : 1 ratio of DTDPH cross-linking did not show reversible characteristics, different from P(DMA90-stat-DAA10), because the higher DAA concentration tends to increase the reaction extent and then increase the cross-linking density. The hydrogel with a 5 : 2 ratio of PEO23 dihydrazide cross-linking showed an increased G′ without any reversible characteristics (Fig. 6c). The self-healing property of the hydrogel was studied as follows. The hydrogel was cut into two pieces across the center and put back into the original mould with close contact, and the self-healing result was observed and confirmed by stretching the hydrogel perpendicular to the cut line with tweezers. A series of hydrogels and their self-healing properties are shown in Fig. 7. The hydrogels prepared from P(DMA90-stat-DAA10) were sticky and not strong enough to perform self-healing studies, and those phase separated samples were also excepted. The hydrogel prepared from P(DMA93-stat-DAA30) cross-linked by a 1 : 1 ratio of ADH is shown in Fig. 7-1. The hydrogel (Fig. 7-1a) was cut into two pieces (Fig. 7-1b), selfhealed in 24 h (Fig. 7-1c), and did not split under stretching (Fig. 7-1d). However, the hydrogel cross-linked by a 1 : 1 ratio of DTDPH became opaque and did not undergo self-healing within 24 h, as shown in Fig. 7-2(a–d). The reason for this

Fig. 7 Self-healing properties of the hydrogels prepared from P(DMAstat-DAA) and dihydrazide compounds. (PEO23 DH represents PEO23 dihydrazide) (credit from Hebei University).

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result is because a large amount of hydrophobic segments were formed and aggregated during the cross-linking, which made the hydrogel opaque and the reversible reaction of the acylhydrazone bond was restricted. When the cross-linking density was decreased by regulating the group ratio to 2 : 1, the hydrogel became transparent and self-healed in 24 h, as shown Fig. 7-3(a–d). When the composition of DAA was further increased to P(DMA96-stat-DAA50), phase separation was observed with ADH and DTDPH as cross-linkers because of the increased density of the hydrophobic segments, and the clear hydrogel with a low cross-linking ratio was not strong enough. However, when PEO23 dihydrazide was used as a cross-linker with a ketone to hydrazide ratio of 5 : 2, a clear hydrogel was obtained and no phase separation occurred any more. Moreover, this hydrogel self-healed in 24 h and no scarring was observed, as shown in Fig. 7-4 (a–d). The hydrogel can also self-heal in 12 h, but a scar remained because of the limited time period (Fig. S2†). The morphologies of the hydrogels were observed using SEM. The hydrogels were lyophilized directly to preserve their original morphologies and a series of SEM images from the lyophilized hydrogels are shown in Fig. 8. The pore size of the hydrogel prepared from P(DMA93-stat-DAA30) cross-linked by a 2 : 1 ratio of DTDPH decreased compared to that of the hydrogel prepared from P(DMA90-stat-DAA10) with a 1 : 1 ratio of DTDPH as the cross-linker, as shown in Fig. 8a and b. When the cross-linker was increased to a 1 : 1 ratio of ADH, the pore

Fig. 8 SEM images of the hydrogels after lyophilization. (a) P(DMA90stat-DAA10) with 1 : 1 DTDPH; (b) P(DMA93-stat-DAA30) with 2 : 1 DTDPH; (c) P(DMA93-stat-DAA30) with 1 : 1 ADH.

Polymer Chemistry

size of the hydrogel prepared from P(DMA93-stat-DAA30) increased drastically with much thicker pore walls, as shown in Fig. 8c. The possible reason for this phenomenon is that a large amount of hydrophobic segments tend to aggregate and result in large pores and thicker walls. More factors related to the morphology of the self-healable hydrogels are still under intensive study. Gel–sol–gel transition triggered by acidity, redox and group ratios The acylhydrazone bond with reversible characteristics has been used for drug applications45,46 and the pH responsive gel–sol–gel reaction of polymer gels has been studied.47 The hydrogel based on acylhydrazone bond cross-linking also showed gel–sol–gel transitions under different acidities in this study. The pH responsiveness of the hydrogel prepared from P(DMA93-stat-DAA30) cross-linked by ADH is shown in Fig. 9. One drop of HCl was dropped onto the top of the hydrogel, which was shaken continuously, and the hydrogel dissolved into a clear solution within 20 min with a pH < 3.0. When N(C2H5)3 was added into the solution to neutralize the HCl, a clear hydrogel was re-obtained again overnight, as shown in Fig. 9 (top). The hydrogel prepared from P(DMA90-stat-DAA10) showed the same result; but because of the reduced crosslinking density, the stiffness and the dimension stability of the hydrogel decreased (Fig. S3†). Although it was phase separated, the hydrogel prepared from P(DMA96-stat-DAA50) still showed pH responsiveness (Fig. S4†). It is reasonable that the hydrogel from P(DMA96-stat-DAA50) cross-linked by a 5 : 2 ratio of PEO23 dihydrazide also showed pH responsivity (Fig. S5†). Importantly, this process can be carried out several times and no obvious difference was observed, indicating the good structural stability of the copolymer and cross-linking agent. The disulfide bond is a redox reversible covalent bond which can be reduced by a series of reducers like DTT, n-Bu3P, etc.30 As shown in Fig. 9 (bottom), besides the pH triggered gel–sol–gel transition (a and b), the hydrogel prepared from P(DMA93-stat-DAA30) cross-linked by a 2 : 1 group ratio of

Fig. 9 Reversible gel–sol–gel transition of the acylhydrazone containing hydrogel prepared from P(DMA93-stat-DAA30) cross-linked by ADH (top) and DTDPH (2 : 1, bottom) under various pH and redox conditions (credit from Hebei University).

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Polymer Chemistry

DTDPH also transitioned into a solution when 3 equivalents of DTT based on DTDPH were added. However, it was surprising to see that the hydrogel was not re-obtained when H2O2 as an oxidant was added. The possible reason for this result was that the equilibrium constant is small and the free hydrazide groups were oxidized by H2O2; at the same time, the reversible reaction rate was quite high and the de-cross-linking reaction occurred before the disulfide bond formed. As a result, the hydrazide was consumed and the hydrogel was not re-obtained with some insoluble component being produced. FeCl3 was also tried as an oxidant and, in contrast to the results with H2O2, a turbid mixture was obtained because of the different products which were formed (Fig. S6†). This mechanism was verified using H2O2 to oxidize the PEO23 dihydrazide crosslinked hydrogel, and the hydrogel degraded slowly into a liquid after 48 h. When the DTDPH group ratio was increased to 1 : 1, the hydrogel did not undergo a DTT triggered gel–sol transition (Fig. S7†). This is because the hydrogel phase separated and the hydrophobic segment was no longer soluble; as a result, the reducer could not break the disulfide bond. The reduced product (thiol group) is also insoluble and restricted further reaction. It is understandable this hydrogel also showed a pH triggered gel–sol–gel transition, the same as the hydrogel prepared from P(DMA96-stat-DAA50) cross-linked by a 1 : 1 group ratio of DTDPH, since all the products are soluble in acidic water. It was reported that the dynamic bond can be cleaved by a monofunctional compound, so the self-healable hydrogel could undergo a gel–sol–gel transition by regulating the excess group ratios of dihydrazide based on Carothers theory.48 The gel–sol–gel transition of the hydrogel prepared from P(DMA93stat-DAA30) and ADH is shown in Fig. 10. When a 3 times excess of ADH was added into 1 mL of the hydrogel, the gel transformed into a clear solution within 1 h, much faster than the hydrogel formation. The mechanism and photographs are shown in Fig. 10(a and b). This experiment proved that the reversible reaction rate is pretty high under neutral conditions without catalysis. With the existence of a large amount of pendant acylhydrazine groups, the gel was re-obtained when

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P(DMA93-stat-DAA30) 10% water solution was added to regulate the group ratio to 1 : 1, as shown in Fig. 10(c). The hydrogels prepared from P(DMA90-stat-DAA10) showed the same result (not shown). This property is useful for the recycling and reuse of the self-healable hydrogel since this process does not change the pH or redox state of the hydrogels. Based on the above results, a series of P(DMA-stat-DAA) based self-healable hydrogels were prepared with various DAA ratios cross-linked by dihydrazide compounds. Although the copolymer did not show any thermo-responsiveness, the selfhealable hydrogels showed temperature responsiveness with the CP being regulated by the composition of the copolymer and cross-linking agents. Beside the pH triggered gel–sol–gel transitions, the hydrogels also showed group ratio triggered gel–sol–gel transitions. As a result, these kinds of smart hydrogels with CPs around body temperature could have great potential applications in areas related to bioscience and biotechnology including artificial organs, bio-switches etc.

Conclusions Self-healable polymer hydrogels with cross-linking induced thermo-responsiveness (CIT) were prepared from copolymers of non-thermo-responsive P(DMA-stat-DAA) with dihydrazide compounds as cross-linkers. The rheological properties, thermo-responsivity and the self-healing properties were studied intensively. The results showed that the hydrogels formed and self-healed without any interference from catalysts or triggers. Because of the hydrophobic segments formed during the cross-linking reaction, thermo-responsive hydrogels were obtained from non-thermo-responsive P(DMA-stat-DAA). Moreover, the thermo-responsiveness of the hydrogels depended not only on the cross-linking density, but also on the hydrophobic structure formed during cross-linking. Based on the variety of reversible bonds that existed in the hydrogels, the hydrogels showed pH and group ratio triggered gel–sol–gel transitions. With regulated thermo-responsiveness without catalysis, this kind of self-healable hydrogel could have great potential applications in areas related to bio-science and biotechnology.

Acknowledgements This research was kindly supported by the National Natural Science Foundation of China (No. 21374028), the Project for Talent Engineering of Hebei Province (No. A2016015001), the Returned Overseas Chinese Scholars, State Education Ministry (2014-1685), Challenge Cup Program of Hebei University and the Department of Education, Hebei Province (QN2017014).

Fig. 10 Schematic illustration and photographs of the gel–sol–gel transition triggered by the group ratio (a: hydrogel from P(DMA93-statDAA30); b: addition of 3 times excess DTDPH; c: addition of P(DMA93stat-DAA30) to a 1 : 1 group ratio).


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