Recoverable and Self-Healing Double Network Hydrogel Based on Îº

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Recoverable and Self-healing DoubleNetwork Hydrogel based on #-Carrageenan Sijun Liu, and Lin Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11363 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 16, 2016

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ACS Applied Materials & Interfaces

Recoverable and Self-healing Double Network Hydrogel based on κ-Carrageenan Sijun Liu, Lin Li* School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

ABSTRACT Combining an ionically cross-linked κ-carrageenan network with a covalently cross-linked polyacrylamide network, we have fabricated a κ-carrageenan/polyacrylamide double network (DN) hydrogel using a one-pot method. The resulting DN hydrogel exhibited a high elastic modulus of 280 kPa and a fracture energy of 6150 J/m2. A model has been proposed considering unzipping of double-helical aggregates and dissociation of double helices of κ-carrageenan, which could

well interpret the

continuous fracture process of the first κ-carrageenan network and the toughening mechanism of the physically and chemically cross-linked κ-carrageenan/polyacrylamide DN hydrogel. Owing to the thermoreversible nature of κ-carrageenan, the DN hydrogel also exhibited the excellent recoverable property. For example, the elastic modulus and energy dissipation could be recovered to 100 % and 98 % after the deformed and relaxed DN samples were stored at 90 oC for 20 min. Furthermore, the temperature-dependent sol-gel transition is also related to the self-healing property of the DN hydrogel.

KEYWORDS: hydrogel, double network, κ-carrageenan, recoverable, self-healing

1. INTRODUCTION Double network (DN) hydrogels are characterized by a unique network structure consisting of two types of polymer networks with different physical natures: the first network is densely cross-linked (forming a rigid skeleton) and the second network is loosely cross-linked (a ductile matrix).1-5 DN hydrogels have drawn much attention as an innovative material due to their excellent mechanical strength and toughness, for example, failure compressive stress of 10 - 60 MPa at compressive strains of 90 - 95 %, failure tensile stress of 0.5 - 10 MPa at strains of 1000 - 2000 %, elastic modulus of 0.1 - 1.0 MPa, and tearing fracture energy of 100 - 1000 J/m2, which are comparable to those of rubbers and soft load-bearing bio-tissues.6-7 The first DN hydrogel was invented using a two-step sequential free-radical polymerization method by Gong et al. in 2003.8 They proposed that the significantly enhanced mechanical properties of DN *

Corresponding author. E-mail addresses: [email protected] (L. Li). 1

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hydrogels are presumably attributed to the unique network structure and strong network entanglement. During deformation, the rigid and brittle first-network is fractured into small clusters, which act as sliding cross-linkers for the ductile second-network to enhance the resistance against the crack propagation. The Gong’s work guides the subsequent development of synthesis methods, design principles and toughening mechanism of DN hydrogels. However, the chemically cross-linked DN hydrogels lack an ability to recover efficiently from damage caused by stretching, resulting in a low toughness due to the permanent breaking of chemical bonds in the first brittle network.9 A number of studies have found that the introduction of a reversible noncovalent (i.e. physical) network into an irreversible covalent network can greatly improve mechanical properties of their parent single network (SN) hydrogels. For example, Suo et al.10 prepared the alginate/polyacrylamide (PAAm) DN hydrogels combining a Ca2+ cross-linked alginate network with a covalently cross-linked polyacrylamide network, and then achieved a remarkable fracture toughness of 9000 J/m2. This remarkable behavior is explained by the mechanism that the ionically cross-linked network of alginate is reversible while the interconnection between covalently cross-linked polyacrylamide and ionically crosslinked alginate can help the DN hydrogel to maintain its network structure. Zheng et al.11-13 developed a simple “one-pot” method to synthesize physically and chemically cross-linked agar/polyacrylamide DN hydrogels consisting of two interpenetrating networks: a hydrogen-bond cross-linked agar network and a covalently cross-linked polyacrylamide network. Because the formation and melting of agar hydrogel are thermoreversible, agar/polyacrylamide DN hydrogels exhibit a high recoverability upon changing temperature. The results of the cyclic loading−unloading tensile tests indicated that the agar/polyacrylamide DN hydrogel was able to recover 65 % of toughness and 90 % of stiffness after the deformed and relaxed hydrogel sample was kept at 100 °C for 10 min. All of these results suggest that being different from mechanical enhancement by covalent bonds, a physically cross-linked network can toughen a hydrogel through a different mechanism. κ-Carrageenan, one of hydrophilic linear sulphated galactans extracted from various species of marine red algae, is composed of alternating α(1-3)-D-galactose-4-sulfated and β(1-4)-3,6-anhydro-Dgalactose.14-19 It contains one sulphate group per disaccharide unit at carbon 2 of the 1,3 linked galactose unit. The 4C1 conformation of 3,6-anhydro-D-galactose unit allows a helicoidal secondary structure, which is essential for the formation of a gel. Recently, our group systematically studied the gelation behavior of κ-carrageenan in aqueous solution, and found that the formation and melting of κcarrageenan hydrogels are thermoreversible, and the critical gel temperature and gel strength increase with increasing κ-carrageenan concentration and the content of cations.20-21 Although a great success in 2


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synthesizing physically and chemically cross-linked DN hydrogels has been achieved, there are still some questions needed to be further addressed. For example, because the alginate network cross-linked by Ca2+ is thermally stable, the alginate/polyacrylamide DN hydrogels just showed a partially recoverable property even after the deformed samples were kept at 80 oC for 24 hours. The thermoresponsive agar contributed to the high recoverability but a poor toughness of 1089 J/m2 for the agar/polyacrylamide DN hydrogels in contrast to that of the alginate/polyacrylamide DN hydrogels. This might be because the agar network formed by hydrogen bonds had a lower capability of energy dissipation than an alginate network formed by ionic cross-linking. Being different from alginate that directly gels with divalent cations and agar that directly gels through hydrogen bonds, the formation of a κ-carrageenan gel is achieved by two steps: the coil-helix transition followed by aggregation of double helices.22-23 Furthermore, the sol-gel transition and the gel-sol transition of κ-carrageenan are thermally reversible. If κ-carrageenan is combined with polyacrylamide to prepare a κ-carrageenan/polyacrylamide DN hydrogel, it is expected that the resulting DN hydrogel will be both mechanically tough and thermoreversible. There are a few reports found in the literature so far which are related to synthesis and characterization of κ-carrageenan/polyacrylamide DN hydrogels. Lu et al.28 investigated the effects of various carrageenans (κ-carrageenan and ι-carrageenan) on mechanical properties and recovery in the carrageenan/PAAm DN hydrogel and found that a high fracture energy was obtained in the ιcarrageenan/polyacrylamide DN hydrogel as compared to the

κ-carrageenan/polyacrylamide DN

hydrogel. The stretched ι-carrageenan/polyacrylamide DN hydrogel was able to be healed by short treatment at a mild temperature. Stevens et al.29 also explored the effects of various inonic cross-linking mechanisms (carrageenan and gellan gum) on mechanical properties of the DN hydrogels. However, the recovery and self-healing propertes were not further discussed when a thermoreversible biopolymer was used in the first network. In the present work, we designed and synthesized κ-carrageenan/polyacrylamide DN hydrogels through a one-pot method. The obtained DN hydrogels exhibited high toughness as well as recoverable and self-healing abilities. The fracture energy under the optimized condition reached to 6150 J/m2, and the elastic modules and the dissipated energy could be respectively recovered to 100 % and 98 % after the deformed and relaxed samples were stored at 90 oC for 20 min. Based on our results, we think that the thermoreversible nature of κ-carrageenan is responsible for the overall recoverable and self-healing properties of κ-carrageenan/polyacrylamide DN hydrogels.

2. EXPERIMENTAL SECTION 3



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Materials κ-Carrageenan in powder form was purchased from Sigma-Aldrich (Singapore). According to the supplier, the molecular weight of κ-carrageenan is about 3.0 × 105 g/mol. Before use, the κ-carrageenan powder was dried and kept in a desiccator at room temperature to avoid the absorption of moisture. Calcium chloride (CaCl2), acrylamide (AAm), N,N’-Methylenebisacrylamide (MBA), 2-hydroxy-4’-(2hydroxyethoxy)-2-methylpropiophenone as a UV-initiator, and methylene blue were purchased from Sigma-Aldrich (Singapore) and used as received.

Preparation of κ-Carrageenan/PAAm DN Hydrogel The physically and chemically cross-linked κ-carrageenan/polyacrylamide DN hydrogels were synthesized using a one-pot method. κ-Carrageenan and acrylamide (at various weight ratios of κcarrageenan to acrylamide) were dissolved in deionized water, where the total concentration of κcarrageenan and acrylamide was fixed at 18 wt%. Subsequently, KCI (6 wt% relative to the weight of κcarrageenan), MBA and UV-initiator (0.03 wt% and 5 wt% based on the weight of acrylamide) were added into the solution of κ-carrageenan and acrylamide. The mixture was stirred magnetically at 90 oC for 5 h to obtain a homogeneous and transparent solution. The solution was transferred into a plastic tube (diameter, D = 25 mm) and a glass mould (length × width × thickness = 120.0 mm × 120.0 mm × 2.5 mm) covered with a glass plate of a thickness of 2 mm, and then cooled at 4 °C for 30 min to form the first, physically cross-linked κ-carrageenan network. Subsequently, the plastic tube and glass mould were placed under a UV lamp (wavelength of 365 nm and intensity of 8 mW/cm2) to carry out the photo-polymerization reaction for 1 h to produce κ-carrageenan/polyacrylamide DN hydrogels. After photo-polymerization, the DN hydrogels were removed from the plastic tube and the glass mould, respectively, and then used for mechanical and other tests.

Mechanical Tests Compression and tensile tests were carried out using an Instron machine (Model 5567) with a 500 N load cell. The cylindrical samples with a height of 12 mm and a diameter of 25 mm were used for compression tests. The compressive strain was estimated as h/h0, where h is the height under compression and h0 is the original height. The compressive rate was 50 mm/min. For the tensile tests, the hydrogel samples were cut into a dumbbell shape with a gauge length of 30 mm, a width of 5 mm, and a thickness of 2.5 mm. Both ends of the dumbbell-shaped sample were gripped by the clamps with the lower clamp fixed. The upper clamp was pulled up by the load cell at a constant velocity of 100 4


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mm/min at room temperature. The stress–stretch curve was recorded. The stretch ratio (λ) was defined as the final length (l) divided by the original length (l0) of the specimen, λ = l/l0. The tensile strength was obtained from the failure point, and the elastic modulus was determined by the average slope over 1.1 1.3 of stretch ratio from the stress-stretch curve. In a hysteresis measurement, a dumbbell-shaped sample was first stretched to a predetermined stretch ratio and then unloaded to zero force at the same velocity of 100 mm/min. The dissipation energy was defined by the area under a cycle of loadingunloading curve. In order to investigate the effect of various stretch rates on the energy dissipation, the dumbbell-shaped samples were stretched at the loading and unloading rates of 20, 50 and 100 mm/min. For the recovery experiments, the dumbbell-shaped samples were firstly measured by a cycle of loadingunloading at a fixed stretch ratio (λ = 5), and then the samples were sealed in a polyethylene bag and stored in the water baths of 20 oC and 90 oC, respectively. Finally, the specimens were taken out at different time intervals and cooled down to room temperature for tests. The fracture energy, Γ, was determined as described in Reference 10: two samples of the same hydrogel with the same thickness to, width wo and initial length lo were stretched. One sample was unnotched, and the other was notched with a crack of a length of 0.1wo. The notched sample was stretched to a critical distance lc at which the crack propagation began, while the un-notched sample was stretched to measure the stress-stretch curve. The fracture energy was calculated by Γ = U(lc)/towo, where U(lc) was the work done by the applied force when the notched sample was stretched lc, which was calculated by integrating the area beneath the force-length curve for the un-notched sample.

Self-healing The DN hydrogels with a dumbbell shape were cut into two pieces. The cut surfaces were brought together to form a contact and then sealed in a polyethylene bag and immersed into a water bath of 90 o

C for a given time. Subsequently, the healed samples were taken out for tensile tests again.

Rheological Tests The rheological measurement was performed on a rotational rheometer (DHR, TA Instruments, USA) with a hatched plate geometry of 40 mm in diameter. A DN hydrogel sheet with a thickness of 1 mm was cut into a circle of 40 mm in diameter, and then put on the bottom plate of the rheometer for the temperature sweep measurement at a cooling and heating rate of 1 oC/min as well as a fixed angular frequency of 1 rad/s and a strain of 1 %. A low-viscosity silicone oil was used to cover the sample perimeter to prevent the evaporation of water. 5



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3. RESULTS AND DISCUSSION Figure 1 shows the schematic for fabrication of the κ-carrageenan/polyacrylamide DN hydrogels. All reactants (i.e. κ-carrageenan, acrylamide, KCI, MBA, and UV-initiator) were added to a single water pot and a homogeneous solution was formed at 90 oC. During cooling, κ-carrageenan chains underwent the coil-helix transition followed by the aggregation of double helices. As a result, the first, physically crosslinked κ-carrageenan network was formed. Upon UV-initiation, the chemically cross-linked polyacrylamide was formed as the second network. Thus, the DN hydrogels with the interpenetrating networks of κ-carrageenan and polyacrylamide were obtained.

Figure 1. Preparation of κ-carrageenan/polyacrylamide DN hydrogels using a one-pot method.

Here we used a cutter to slice a sample to demonstrate the mechanical strength of the κcarrageenan/polyacrylamide DN hydrogels as shown in Figure 2. For the κ-carrageenan-based single network (SN) hydrogel, it was easy to slice to produce a crack on the top of a sample as shown in Figure 2a1 to 2a3. But the κ-carrageenan/polyacrylamide DN hydrogel could resist the slicing with the cutter even at a strain of up to 80 % (Figure 2b1-2b3), indicating that our κ-carrageenan/polyacrylamide DN hydrogel is very stiff. On the other hand, to exhibit the toughness of the DN hydrogels, the air inflation experiment was performed. A sheet of DN hydrogel with the size of 100.0 mm × 100.0 mm × 2.5 mm was set on one end of a pipe and the other end of the pipe was connected to an air pump, while the edges of the hydrogel sheet were sealed by the hands. During the whole process of the air inflation, the hands sealed the edges of the sheet well, ensuring the burst of the balloon at last. As shown in Figure 2c, the

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sheet of the DN hydrogel began to expand after turning on the air valve, and finally reached to a size of about 600 cm2 under the air pressure of 1.6 bar, showing that our κ-carrageenan/polyacrylamide DN hydrogels have a very high stretchability and remarkable toughness. In these DN hydrogel samples, the weight ratio of κ-carrageenan to acrylamide in the DN hydrogel was fixed at 2:16 because the largest fracture energy was obtained at this ratio. The content of the ionic crosslinker, KCI, was fixed at 6 wt% of κ-carrageenan. The contents of the covalent crosslinker (MBA) and the UV-initiator were fixed at 0.05 wt% and 3 wt% based on the amount of acrylamide, respectively. For the κ-carrageenan SN hydrogel, the amount of κ-carrageenan was kept the same as those in the DN hydrogel.

Figure 2. The different performances of the κ-carrageenan SN hydrogel and the κ-carrageenan/polyacrylamide DN hydrogel. (a) For the κ-carrageenan SN hydrogel, it is easy to slice a crack by a cutter. (b) For the κcarrageenan/polyacrylamide DN hydrogel, it can resist slicing with a cutter even at a strain of 80 %. (c) The κcarrageenan/polyacrylamide DN hydrogel sheet is able to inflate into a large balloon upon applying an air pressure.

A series of tests were carried out to quantitatively examine mechanical properties of κ-

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carrageenan/polyacrylamide DN hydrogels. Figure 3 shows the stress-strain curves of the κ-carrageenan SN hydrogel, the polyacrylamide SN hydrogel, and the κ-carrageenan/polyacrylamide DN hydrogel under uniaxial compression. The contents of κ-carrageenan and acrylamide in the SN hydrogels were kept the same as those in the κ-carrageenan/polyacrylamide DN hydrogels. It was observed that the κcarrageenan SN hydrogel fractured at a stress of 0.4 MPa and a strain of 55 %, and the polyacrylamide SN hydrogel sustained a stress of 0.7 MPa at a strain of 90 %. However, the κ-carrageenan/polyacrylamide DN hydrogel achieved a stress of 15.4 MPa, which is 40 times and 20 times higher than those of the κcarrageenan SN hydrogel and the polyacrylamide SN hydrogel, respectively. At a strain of 80 %, the compression stress of our DN hydrogel reached to 1.95 MPa, which is higher than 0.93 MPa repoted in the κ-carrageenan/epoxy-amine DN hydrogel.29 Meanwhile, for the κ-carrageenan/polyacrylamide DN hydrogel, it could regain to its original state after unloading even if it was compressed by 95 %. The stress-strain curve of the DN hydrogel overlaps with that of the κ-carrageenan SN hydrogel at small strains, suggesting that the κ-carrageenan network contributed to the increase of elastic stress when the strain is small.

Figure 3. Stress-strain curves of the κ-carrageenan SN hydrogel, the polyacrylamide SN hydrogel, and the κcarrageenan/polyacrylamide DN hydrogel under uniaxial compression. The compressive rate was fixed at 50 mm/min. For the DN hydrogel, the weight ratio of κ-carrageenan to acrylamide was fixed at 2:16. For the κcarrageenan SN hydrogel and the polyacrylamide SN hydrogel, the contents of κ-carrageenan and acrylamide in the SN hydrogels were kept the same as those in the κ-carrageenan/polyacrylamide DN hydrogels.

Figure 4a shows the stress-stretch curves of the κ-carrageenan and polyacrylamide SN hydrogels as

8


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well as the κ-carrageenan/polyacrylamide DN hydrogel. At the small stretch ratios (λ ≤ 1.5), the elastic modulus of the DN hydrogel was 104 kPa, which is larger than the sum of the elastic moduli of the κcarrageenan and polyacrylamide SN hydrogels (72 kPa and 9 kPa, respectively). The stress and stretch at rupture were 558 kPa and 21 respectively for the DN hydrogel, 34 kPa and 1.6 for the κ-carrageenan SN hydrogel, and 131 kPa and 15 for the polyacrylamide SN hydrogel. The mechanical properties of the DN hydrogel far exceeded those of either of its parents. Meanwhile, the stretch ratio of 21 and the stress 550 kPa at rupture for our DN hydrogel are larger than those (stretch ratio 14 and stress at rupture 75 kPa) as reported by Lu et al.28 This result suggests that the DN hydrogel is formed not only from a simple interpenetration of a κ-carrageenan network and a polyacrylamide network, but also through a possible synergistic interaction of two networks. Further,

we

observed

both

the

yielding

and

necking

phenomena

for

our

κ-

carrageenan/polyacrylamide DN hydrogels, and the visual demonstration of the stretching is shown in Figure 4b. At the first stage of stretching, the DN hydrogel underwent a homogeneous deformation until the yield point. After this point, the deformation became inhomogeneous: an unnecked region and a necked region coexisted, with the necked region being deformed more than the unnecked region. With further stretching, the necked region was enlarged at the expense of the unnecked region. Ultimately the sample was deformed homogeneously again. Gong et al. first reported a necking phenomenon for the

chemically

cross-linked

poly(2-acrylamido-2-methylpropanesulfonic 24

(PAMPS/PAAm) DN hydrogels.

acid)/polyacrylamide

As compared to the PAMPS/PAAm DN hydrogel, the κ-

carrageenan/polyacrylamide DN hydrogels exhibited the significantly distinct yielding and necking behaviors. Meanwhile, no necking platform in the yielding curve was observed, which is similar to the alginate/polyacrylamide DN hydrogel induced by trivalent cations but cannot be well interpreted by the existing fracture mechanisms derived for the chemically cross-linked DN hydrogels.25 It can be inferred that the difference in yielding and necking behaviors between chemically cross-linked PAMPS/PAAm DN hydrogels

and

physically

&

chemically

cross-linked

alginate/polyacrylamide

(or

κ-

carrageenan/polyacrylamide) DN hydrogels may originate from the different first network structures and association mechanisms.

9



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Figure 4. (a) Stress-stretch curves for the κ-carrageenan SN hydrogel, the polyacrylamide SN hydrogel, and the κcarrageenan/polyacrylamide DN hydrogel under uniaxial elongation at a stretching rate of 100 mm/min. (b) Pictures demonstrating yielding and necking phenomena during stretching the DN hydrogel. The insert numbers in (a) correspond to the pictures in (b). For the DN hydrogel, the weight ratio of κ-carrageenan to acrylamide was fixed at 2:16. For the κ-carrageenan SN hydrogel and the polyacrylamide SN hydrogel, the contents of κcarrageenan and acrylamide in the SN hydrogels were kept the same as those in the κ-carrageenan/polyacrylamide DN hydrogels.

According to the Lake-Thomas theory,26 the fracture energy Γ of a cross-linked polymer can be calculated by Γ = Uρn, where U is the bond dissociation energy, ρ is the area density of polymer chains on the fracture surfaces, and n is the average number of monomer units between cross-linkers. This equation only considers the fracture of polymer chains located at a crack tip. However, the fracture process and toughening mechanism are complicated for DN hydrogels. For the chemically cross-linked PAMPS/PAAm DN hydrogels, Gong et al. thought that the high toughness of DN hydrogels derives from the internal fracture of the first chemically cross-linked brittle PAMPS network during deformation, which dissipates energy as sacrificial bonds. The loading and unloading experiments in uniaxial tension showed a significant hysteresis during the first loading-unloading cycle, and such a large hysteresis was velocity-independent and not observed during the second loading-unloading cycle because the fracture of covalent bonds in the first network was not recoverable.8 For the physically and chemically crosslinked agar/polyacrylamide DN hydrogels, Zheng et al.11 thought that the toughening mechanism should be attributed to the continuous fracture of the first agar network. For the alginate/polyacrylamide DN hydrogels, Suo et al.10 attributed the toughness of their hydrogels to the synergy of two mechanisms: crack bridging by the covalently cross-linked polyacrylamide network and hysteresis by unzipping the ionic cross-linked alginate network.

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To study the toughening mechanism of our κ-carrageenan/polyacrylamide DN hydrogels, the loading-unloading tensile tests with varying stretch ratio were performed and the internal fracture process of the DN hydrogels was analyzed by assessing the capability of energy dissipation. Figure 5a shows the loading-unloading curves for the κ-carrageenan/polyacrylamide DN hydrogels at different stretch ratios, from which a large hysteresis loop was observed. If the area enclosed by the loadingunloading curves is defined as the dissipated energy of the cycle, the energy dissipation increased with increasing the stretch ratio, which would suggest that the internal fracture of the κcarrageenan/polyacrylamide DN hydrogel is a gradual process. That is to say, when the DN hydrogel is stretched to a small stretch ratio, the double-helical aggregates in the first physically network could be unzipped from the ionically cross-linked junctions. With increasing the stretch ratio, the double-helical aggregates will be unzipped progressively. At the same time, κ-carrageenan chains are pulled out from the double helices and then participate in load bearing, while the polyacrylamide network remains intact

to

stabilize

the

whole

structure

of

the

hydrogel.

This

explains

why

the

κ-

carrageenan/polyacrylamide DN hydrogels exhibit the pronounced hysteresis and superior mechanical properties. In addition, DN hydrogels synthesized with a physically crosslinked first-network usually demonstrate a rate-dependent tensile property, which is associated with the mechanical energy dissipation of these hydrogels.12 Figure 5b shows the loading-unloading curves under various stretch rates at a fixed stretch ratio. It is observed that the elastic modulus increases with increasing stretch rate, but the yield stress and the energy dissipation decrease. The result suggests that it is much difficult to unzip the double-helical aggregates and pull κ-carrageenan chains out at a low stretch rate. Many natural polymers contain physical bonds (such as ionic bond, hydrogen bond

and

hydrophobic association, etc) that are generally weaker than covalent bonds. These physical bonds may set up an energy dissipation mechanism by breaking themselves instead of breaking of the main framework of a material, resulting in a great toughness for the whole material. For our κcarrageenan/polyacrylamide DN hydrogels, both ionically crosslinked double-helical aggregates and hydrogen bond-crosslinked double helices are broken by deformation and serve as sacrificial bonds, which greatly increases toughness of the hydrogels through energy dissipation as compared to single ionically cross-linked alginate and hydrogen bond-crosslinked agar.

11



350

a

b

300 250

160

20mm/min

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200

9

120

6 3 0

4

6

8

10

12

Stress (kPa)

Uhys (MJ/m3)

15

Stress (kPa)

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14

λ 150 100

50mm/min

100mm/min

80

40

50 0

0 1

2

3

4

5

6

7

8

9

10

11

12

13

14

1

2

3

4

5

6

λ

λ

Figure 5. (a) Stress during a cycle of loading and unloading with varying stretch ratio and (b) Effect of stretch rate on loading-unloading profiles, for the DN hydrogel with a weight ratio of κ-carrageenan to acrylamide = 2:16.

We further investigated the effect of weight ratio of κ-carrageenan to acrylamide on mechanical properties. As shown in Figure 6, with increasing weight ratio of κ-carrageenan to acrylamide, the elastic modulus increased, while the critical stretch at rupture decreased. At the weight ratio of 3:15, the maximum elastic modulus, 280 kPa, was obtained, which is close to 300 kPa reported in the literature for the alginate/polyacrylamide DN hydrogels.10 However, the maximum stretch ratio just reached 2.5 for the alginate/polyacrylamide DN hydrogels,10 which is far less than 18 in the case of our κcarrageenan/polyacrylamide DN hydrogels. At the intermediate weight ratio of 2:16, the fracture energy reached to a maximum value of 6150 J/m2, which is much larger than that of the agar/polyacrylamide DN hydrogels.12 On the other hand, the ratio of κ-carrageenan to acrylamide also influences the yielding behavior of the DN hydrogels. It is clearly seen from the inset of Figure 6a that the yield stress increases with increasing the weight ratio of κ-carrageenan to acrylamide, suggesting that the first network contributes much more to the strength of the DN hydrogel although the both networks bear the stress at the yield point. The yield strain slightly decreases with increasing the weight ratio. This might be the reason for the decrease of the stretch ratio at rupture because at a lower content of acrylamide the extensibility of the second network is smaller.

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a

800

3:15

250

Stress

200

600

150

2.5:15.5

100

2:16

Stress (kPa)

50 0 1.0

1.5

400

2.0

2.5

1.5:16.5

3.0

λ

200

0 1

5

10

15

20

25

λ b

300

c

7000 6000

Fracture energy (J/m2)

250

Elastic modulus (kPa)

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


200

150

100

50

0

1.5:16.5

2:16

5000 4000 3000 2000 1000 0

2.5:15.5

3:15

Weight ratios of κ-carrageenan to acrylamide

1.5:16.5

2:16

2.5:15.5

3:15

Weight ratio of κ-carrageenan to acrylamide

Figure 6. (a) Stress-stretch ratio curves of DN hydrogels with various weight ratios of κ-carrageenan to acrylamide, as labelled. Effects of weight ratio of κ-carrageenan to acrylamide on (b) elastic modulus and (c) fracture energy. For the DN hydrogels with various weight ratios of κ-carrageenan to acrylamide, the weight of KCI was fixed at 6 wt% based on κ-carrageenan. The weights of MBA and UV-initiator were fixed at 0.05 wt% and 3 wt% that of acrylamide, respectively.

It is well known that the sol-gel transition and the gel-sol transition of κ-carrageenan in aqueous solution are thermoreversible. This thermoresponsive property enables κ-carrageenan/polyacrylamide DN hydrogels to heal quickly at high temperatures above the sol-gel transition temperature of κcarrageenan. To show the recovery property of this DN hydrogel, we sealed the stretched and relaxed sample in a polyethylene bag, and stored it in a water bath of 20 oC or 90 oC for a prescribed time, and then measured its tensile property again at room temperature (25 oC). When stored at 20 oC, which is below the sol-gel transition temperature of κ-carrageenan, as shown in Figure 7a, the hysteresis loop increased gradually within 20 h, but the hysteresis area did not exceed 50 % that of the original cycle. However, at 90 oC, which is above the sol-gel transition temperature of κ-carrageenan (Figure 7b), the

13



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Page 14 of 21

hysteresis loop almost recovered to the original cycle except that no yielding point was observed even after 20 min. If the ratios of elastic modulus and energy dissipation in the second loading-unloading cycle to that of the original loading-unloading cycle, E2/E1 and U2/U1, were used to evaluate the recovery rates (for both stiffness and toughness recoveries), it is apparent that the stiffness and toughness recovered to 100 % and 98 % respectively after the deformed sample was stored at 90 oC for 20 min (Figure 7c and d). This finding clearly indicates that the thermoreversible sol-gel transition of κcarrageenan is responsible for the recovery of κ-carrageenan/polyacrylamide DN hydrogels. That is, a deformed DN gel network can be re-formed via a gel-sol-gel transition at an elevated temperature for some time.

o

o

Figure 7. Recovery of κ-carrageenan/polyacrylamide DN hydrogels stored at (a) 20 C and (b) 90 C for different durations, as labelled. Relationships of the ratios of (c) elastic modulus and (d) energy dissipation of the second loading-unloading cycle to that of original loading-unloading cycle with the duration time. The sample for the first loading-unloading cycle was obtained from a fresh DN hydrogel. The ratio of κ-carrageenan to acrylamide was 2:16.

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Due to the thermoreversible nature of κ-carrageenan, a possibility of self-healing between two cut surfaces of a κ-carrageenan/polyacrylamide DN hydrogel is expected. However, self-healing at room temperature was not observed. This is likely due to the high stiffness of the hydrogel which hindered full contact of the cut surfaces. As the double helices of κ-carrageenan are able to be transformed into coils during heating and then the κ-carrageenan chains can be re-associated into double helices when temperature is decreased. As a result, the self-healing property could be observed when the cut samples were annealed at high temperatures for a given time. As shown in Figure 8a, the dumbbell-shape samples were cut into two pieces, and one of them was stained with methylene blue in order to distinguish them easily. Subsequently, the cut surfaces were brought together to form a contact and sealed into a polyethylene bag, and then submerged into a water bath of 90 oC for 20 min. The cut dumbbell-shaped samples were well healed as shown in Figure 8b. The self-healed DN hydrogel with a dumbbell shape could withstand a weight of 80 g as shown in Figure 8c. Figures 8d and 8e further show the stress-stretch curves for the self-healed and original DN hydrogels as well as a schematic diagram for self-healing, respectively. Because the fractured chemically cross-linked polyacrylamide network was unable to be re-maked, the self-healing just took place in the κ-carrageenan network through the coilhelix transition and re-association of double helices, so its efficiency was not as high as expected. But, the joint re-formed between the cut surfaces could withstand a stretch ratio of about 2. To our knowledge, this value is higher than that of the poly(butyl methacrylate)-poly(methacrylic acid)poly(butyl methacrylate)/PAAm DN hydrogel as reported by Zhang et al.27 The recoverable and selfhealing properties induced by a simple thermal treatment would give rise to interesting and potential applications for κ-carrageenan/polyacrylamide DN hydrogels.

Figure 8. (a) The freshly cut DN hydrogel with a dumbbell shape. The lower piece was stained using methylene blue. o

(b) Self-healing and adhesion after the freshly cut surfaces were brought into contact and annealed at 90 C for 20 min. (c) The self-healed dumbbell-shaped sample could hold a weight of 80 g. (d) Stress-stretch ratio curves of the virginal

and

self-healed

dumbbell-shaped

sample.

(e)

Schematic

diagram

for

self-healing

of

κ15



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Page 16 of 21

carrageenan/polyacrylamide DN hydrogel. For the DN hydrogel, the ratio of κ-carrageenan to acrylamide was 3:15.

It

is

important

to

understand

the

recoverable

and

self-healing

properties

of

κ-

carrageenan/polyacrylamide DN hydrogels. It is considered that when such a DN hydrogel sample is stretched, the double-helical aggregates of κ-carrageenan are unzipped and κ-carrageenan chains are pulled out in the first network formed by κ-carrageenan, resulting in energy dissipation, while the second polyacrylamide network remains intact. During heating the deformed sample to a temperature above the sol-gel transition temperature of κ-carrageenan, all the κ-carrageenan double helices are dissociated into single chains. Upon a subsequent cooling, the κ-carrageenan chains will form double helices again and further associate into the double-helical aggregates with the help of K+ ions as shown in Figure 9. In our experiments, no yielding point was observed in the curves of reloading (Figure 7b), which implies that the polyacrylamide network is able to prevent κ-carrageenan helices from aggregation, resulting in a slightly lower extent of toughness recovery than 100 %. On the other hand, heating the DN hydrogel to a high temperature could soften the cut pieces to make the cut surfaces good contact. Meanwhile, the coil-helix transition and aggregation of double helices might promote the self-healing of the cut surfaces. This mechanism was further supported by a rheological temperature sweep with a heating–cooling cycle as shown in Figure 10. It is observed that the storage modulus G' and loss modulus G" almost recovered to their initial values after a cycle of heating to cooling. G' was always larger than G" even at the highest temperature, which means that the hysteresis in the cyclic loading-unloading process would not be due to the rupture of covalent bonds but the unzipping of ionic cross-linked double-helical aggregates and dissociation of hydrogen bond-crosslinked double helices. Unlike these physically and chemically cross-linked DN hydrogels with a first physically but thermally insensitive network (e.g. alginate-based network) or chemically cross-linked DN hydrogels (e.g. PAMPS/PAAm DN system), our present study has demostrated that the introduction of a thermoreversible and physically cross-linked network into an irreversible covalent network can greatly improve the recoverable and self-healing properties of the resulting DN hydrogels.

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Figure 9. Schematic diagrams for κ-carrageenan/polyacrylamide DN hydrogels during stretching, heating, and cooling.

104

G' and G'' (Pa)

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


103

G'

G'' Heating Cooling

102 10

20

30

40

50

60

70

80

90

100

Temperature (oC) Figure 10. Dependence of G' and G" on temperature during a temperature sweep at a heating and cooling rate of 1 o

C/min for the κ-carrageenan/polyacrylamide DN hydrogel at a weight ratio of 2:16. A frequency of 1 Hz and a

strain of 1 % were adopted.

4. CONCLUSIONS We have designed and synthesized the stretchable and tough κ-carrageenan/polyacrylamide double network (DN) hydrogels with excellent recoverable and self-healing properties. Because of the synergic effect of unzipping of double-helical aggregates and dissociation of double helices in the first network formed by κ-carrageenan during deformation, the DN hydrogels not only showed the high compression and tensile strengthes, but also exhibited a high tensile fracture toughness of 6150 J/m2. Furthermore, the thermoreversible nature in the sol-gel transition and the gel-sol transition of κcarrageenan contributed to the recoveries of 100 % stiffness and 98 % toughness after the deformed and 17



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Page 18 of 21

relaxed DN hydrogel samples were stored at 90 oC for 20 min. The cut surfaces could also be healed via a gel-sol-gel transition by a cycle of heating to cooling. We have clearly demonstrated that the introduction of a thermoreversible and physically crosslinked network into an irreversible covalent network can greatly improve the recoverable and self-healing properties of DN hydrogels. The combination of good mechanical properties, recoverability in both stiffness and toughness, and selfhealing ability endows our DN hydrogels with potential applications for example load-bearing artificial soft tissues.

ACKNOWLEDGEMENT This work was supported by the Academic Research Fund Tier 1 (RG100/13) from the Ministry of Education, Singapore. REFERENCES 1.

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267x93mm (96 x 96 DPI)


Recoverable and Self-Healing Double Network Hydrogel Based on Îº

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