Interactions Affecting the Mechanical Properties of Macromolecular

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Interactions Affecting the Mechanical Properties of Macromolecular Microsphere Composite Hydrogels Fangzhi Jiang, Ting Huang, Changcheng He, Hugh Ralph Brown, and Huiliang Wang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp4069587 • Publication Date (Web): 04 Oct 2013 Downloaded from http://pubs.acs.org on October 9, 2013

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The Journal of Physical Chemistry

Interactions Affecting the Mechanical Properties of Macromolecular Microsphere Composite Hydrogels Fangzhi Jiang,† Ting Huang,† Changcheng He,† Hugh R. Brown,‡ Huiliang Wang†,* †

Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry,

Beijing Normal University, Beijing 100875, P.R. China. ‡

ARC Centre of Excellence for Electromaterials Science, Engineering Faculty, University of

Wollongong, NSW 2522, Australia.

ACS Paragon Plus Environment

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ABSTRACT: Macromolecular microsphere composite (MMC) hydrogel is a kind of tough hydrogel fabricated by using peroxidized macromolecular microspheres as polyfunctional initiating and cross-linking centers (PFICC). The contribution of chemical cross-linking (covalent bonding) and physical cross-linking (chain entanglement and hydrogen bonding) to the mechanical properties are understood by testing the hydrogels swollen in water or urea aqueous solutions to different water contents. The as-prepared MMC gels exhibited a moderate muduli (60-270 kPa), high fracture tensile stresses (up to 0.54 MPa), high extensibility (up to 2500%) and high fracture energies (270-770 J m-2). The modulus of the swollen gels decreases dramatically, but no significant change in fracture tensile strength and fracture strain even increases slightly. More interestingly, the swollen gels show much enhanced fracture energies, higher than 2000 J m-2. Gradual decrease in hysteresis ratio and residual strain is also found in the cyclic tensile testing of the hydrogels swollen to different water contents. The covalent bonding determines the tensile strength and fracture energy of the MMC gels, while the physical entanglement and hydrogen bonding among the polymer chains contributes mainly to the modulus of the MMC gels and they are also the main reason for the presence of hysteresis in the loading-unloading cycles.

KEYWORDS: hydrogel; mechanical property; macromolecular microsphere; interaction


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1. INTRODUCTION Tough hydrogels with mechanical properties rivaling those of natural load-bearing tissues have promising applications in industrial and medical fields, such as load-bearing artificial soft tissues and the materials for soft biomimetic machines.1-4 Various chemical and physical approaches have been developed for tough hydrogel synthesis.5, 6 The chemical methods rely on the design and synthesis of some special cross-linking points (e.g. slide ring gel)7, homogeneous networks with uniform distribution of cross-linking points and chain lengths between cross-linking points (e.g. tetra-arm gel)8, and double-network (DN) hydrogel which is a special type of interpenetrating network (IPN) gel consists of a highly chemically cross-linked “first network” and a loosely or even uncross-linked “second network”.9 The other approaches utilize physically reversible interactions, mostly hydrogen bonding,10-15 ionic, hydrophobic,16, 17 and dipole-dipole interaction,18 or the combination of physical interactions and covalent bonding for gel formation and toughening.19-21 For example, tough polyvinyl alcohol (PVA) hydrogels made by freezingthawing method are physically cross-linked by the hydrogen-bonded crystalline regions.12 In nanocomposite (NC) gels polymer chains are physically adsorbed to inorganic nanoparticles which act as polyfunctional cross-linking centers (PFC).10 We developed a strategy to fabricate tough hydrogels by using polyfunctional initiating and cross-linking centers (PFICC), and macromolecular microsphere composite hydrogel (MMC gel) is the first type of hydrogels prepared.22 We employed γ-rays radiation-induced peroxidation to introduce peroxy groups onto the surfaces of evenly distributed macromolecular microspheres (MMSs), and then grafted polymer chains covalently attached to the MMSs are formed through the grafting polymerization of monomers initiated by the peroxides. The unique microstructure


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with long and uniform grafted polymer chains chemically attached to MMSs makes the applied force can be evenly distributed by all polymer chains, leading to the high mechanical strength of the MMC hydrogels. This synthesis strategy has been proven to be versatile in preparing tough hydrogels due to its broad choices of PFICC and monomers, for example peroxidized micelles,23, 24

graphene sheets25,

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and polymer chains27 have been used as PFICC in our work, and

microgels,28 starch-based nanospheres,29,

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or even inorganic nanoparticles31 with reactive

functional groups on their surfaces have been used PFICC by some other groups. Tough and stimuli-responsive hydrogels can also be obtained by using monomers such as acrylic acid24 and N-isopropylacrylamide.30,

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In addition, inorganic nanoparticles can be introduced into these

tough hydrogels to obtain functional nanocomposite hydrogels.23, 33 Our previous studies focused on the fabrication of novel gels and the measurement of their mechanical properties, no systematic work has been done to know the interactions contributing to the mechanical properties of the gels though it has been discussed with some experimental results. In this work, we prepared MMC hydrogels by using peroxidized MMSs as the PFICC and acrylamide (AAm) as the monomer. Tensile tests, tearing tests and cyclic tensile tests were performed on the gel samples in the as-prepared and swollen (in water or urea aqueous solutions) states to understand the contribution of covalent bonding, physical entanglement and hydrogen bonding to the mechanical properties.


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2. EXPERIMENTAL SECTION 2.1. Materials. Acrylamide (AAm, ultra pure, BioDev, Japan) was used without further purification. Macromolecular microsphere (MMS) emulsion (SD-588) was purchased from Nantong Shengda Chemicals Co. Ltd. (Jiangsu, China). The emulsion contained 48 wt.% solid mainly composed of poly (styrene-co-butyl acrylate). The other chemicals used were of reagent grade. 2.2. Preparation of MMC gels. The MMC gels were synthesized by a pre-irradiation method reported previously.22 The MMS emulsion was diluted with deionized water to 24 wt.% solid content and then it was irradiated with

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Co γ-rays (dose rate: 4.8 kGy h-1) in the presence of

bubbling oxygen at room temperature for a certain time to create peroxides on the surface of the MMSs. The concentration of peroxides formed were determined by iodometry.24 The peroxidized MMS emulsion was further diluted with deionized water and then was mixed with an AAm aqueous solution, the concentrations of MMS and AAm were specified in the main text. The mixture was transferred into a silanized glass mold made by placing a spacer with a height of 2 or 4 mm between two silanized, flat glass plates. The oxygen dissolved in the mixture was removed by vacuum evacuation and nitrogen exchange for 3 times. At last, the polymerization process was carried out at 45°C for 36 hours to ensure the complete conversion of monomer to polymer. 2.3. Tensile tests. For uniaxial tensile tests dumbbell shaped specimens cut according to DIN53504 S2 (overall length: 75 mm; width: 10 mm; inner width: 4 mm, gauge length: 20 mm, thickness: 2 mm) were tested with an Instron 3366 electronic universal testing machine (Instron Corporation, MA, USA) at a crosshead speed of 80 mm min-1 (if not otherwise stated). The


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tensile stress σt was calculated as follows: σt = Load/tw (t and w are the initial thickness and width of the dumbbell shaped gel sample, respectively). The tensile strain, εt, is defined as ∆L/L, the change in the length (∆L) relative to the initial gauge length (L). Fracture tensile stress (σb) and the breaking tensile strain (εb) are the tensile stress and strain at which the sample breaks. Stress and strain between εt=0.1-0.3 were used to calculate initial elastic modulus (E). At least three specimens per experimental point were tested in all mechanical measurements to obtain reliable values. Cyclic tests were performed by performing subsequent trials immediately following the initial loading with the same specimen at a strain speed of 80 mm min-1. The hysteresis during cyclic tensile test was determined by evaluating the area between the extension and retraction curves and a hysteresis ratio, hr, could be defined by evaluating the ratio of the hysteresis to the integrated area of the extension curve. The gel specimens were coated with a thin layer of silicon oil to prevent the evaporation of water during the tests. 2.4. Tearing tests. The fracture energies G of the gels were measured by tearing tests introduced by Gong and coworkers34,

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using an Instron 3366 electronic universal testing

machine (Instron Corporation, MA, USA). The gels were cut into trouser-shape, which is standardized as the JIS-K6252 1/2 size (5 mm in thickness, 50 mm in length with an initial notch of 20 mm, and 7.5 mm in width). All the samples were measured at a constant tearing rate (velocity) of 10 mm·min-1. The fracture energy G, defined as the energy required for creating a unit area of fracture surface in a sample, was calculated by equation (1):35 G=

2Fave w

(1)

where Fave is the average force during the tear and w is the width of the gel.


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3. RESULTS AND DISCUSSION The formation mechanism and the microstructure of MMC gels has been proposed and discussed in our previous study.22 By using peroxidized microspheres as the polyfunctional initiating and cross-linking centers (PFICC), very tough MMC hydrogels can be obtained. The initiating function is fulfilled by peroxides introduced onto the surfaces of MMSs through the radiation-induced peroxidation process. The concentration of peroxide (Cperoxide) formed in the macromolecular microsphere (MMS) emulsion (24 wt.% solid content) irradiated for 2.5 and 5 h were 1.37 and 1.52 mM, respectively. By calculation, the number of peroxy groups on each MMS was about 4000. So the peroxidized microspheres function as polyfunctional initiating centers. The peroxides formed on the surfaces of MMSs decompose under heating to form macromolecular radicals and small radicals. The former ones initiate the grafting of monomers forming grafted polymer chains covalently attached to the MMSs, and the latter ones initiate the homopolymerization of monomers to form homopolymers or terminate the growing polymer chains. The small radicals might be adsorbed onto the surface of the MMS which has a high specific surface energy, therefore initiating polymerization which leads to polymer chains physically absorbed to the MMSs. Since the radicals are highly concentrated on the surface of the MMSs, the growing grafted chains are difficult to be terminated by small radicals once the chains depart from the surface of the MMSs, and therefore very long grafted chains can be obtained. There is quite different from the common polymerization reaction in which small initiator molecules are distributed relatively homogeneously in the reaction solution and hence the growing polymer chains are easy to be terminated. When the length of the grafted chains


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reaches half of the distance (several hundreds of nanometers) between the MMSs, two vicinal MMSs can be covalently joined by the mutual termination of two growing grafted chains initiated from them, leading to the formation of chemical cross-linking among the MMSs. In addition, physical cross-linking (physical entanglement and strong inter- and intra-molecular hydrogen bonding) can also be formed among the grafted polymer and homopolymer chains. These chemical and physical cross-links construct a three-dimensional polymeric network, forming a hydrogel (Scheme 1). The polymer chains participating in forming the cross-links are started from the MMSs, therefore the MMSs also act as polyfunctional cross-linking centers.

Scheme 1. The main possible interactions contributing to the formation and toughening of MMC hydrogels. To understand the contribution of covalent bonding, physical entanglement and hydrogen bonding to the mechanical properties, we synthesized MMC hydrogels by varying the monomer concentration (CM), MMS concentration (CMMS) and the concentration of peroxide (Cperoxide), and measured the mechanical properties of the gels in the as-prepared and swollen states.


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3.1. Tensile properties of as-prepared MMC hydrogels. The water contents of the hydrogels prepared with different CM were 81 wt.% (3 M), 77 wt.% (4 M) and 72 wt.% (5 M), respectively. When MMSs were added the water content decreased accordingly, e.g. at a CMMS of 0.01 g mL-1 the water content is 1 wt.% lower. The as-prepared MMC gels exhibited excellent tensile mechanical properties. Figure 1a shows the typical stress-strain (σt-εt) curves of the MMC gels synthesized with different CM. The MMC gels exhibited high fracture tensile stresses (σb) and very high breaking tensile strains (εb). The inserted photo in Figure 1a also shows that the samples had a good resilience after being stretched to εb. Figure 1b-f summarizes the elastic modulus (E), σb and εb of the hydrogels synthesized with varying CM, CMMS and Cperoxide. The MMC gels exhibited a moderate E (Figure 1b), in a range of 60-270 kPa, which increased with CM significantly but increased with CMMS slowly till 0.03 g mL-1 and then kept constant or even decreased. The σb increased with both CM and CMMS obviously (Figure 1c,d), except for a drop at CMMS = 0.04 g mL-1 and Cperoxide = 1.37 mM. The σb of the MMC gels were mostly higher than 0.1 MPa, and the highest one was about 0.54 MPa.


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Figure 1. The tensile properties of the as-prepared MMC hydrogels. (a): The stress-strain curves of the hydrogels synthesized with different monomer concentrations at a constant CMMS of 0.01 g mL-1, the inset shows the photographs before/after tensile tests; (b): the elastic modulus (E), (c, d): fracture tensile stresses (σb) and (e, f): the breaking tensile strains (εb) of the MMC hydrogels. The monomer and peroxide concentrations are specified in the figures. The as-prepared MMC hydrogels showed very high extensibility. The εt shown in Figure 1a was recorded by the computer using the gauge length as the initial length. However, the recorded values are bigger than the real ones, since extension also happened in the hydrogel specimens out of the gauge length especially when εt became large. To obtain the true εb of the gels, we measured the actual change in the distances between two marks on a gel specimen before testing (the gauge length) and until the fracture of the gel. The real εb of the samples is shown in Figure 1e, f. The εb of the MMC gels was significantly affected by the CM. The gels synthesized with a


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lower CM (3 M) showed very high εb, ca. 18-25, whereas those synthesized with a higher CM (5 M) showed εb only about 6-11. CMMS did not affect the εb of the gels very much. As the peroxidized MMSs act as PFICC, the increase of CMMS leads to the increase of the concentrations of initiator and cross-linker, and hence at a given CM the decrease of kinetic chain length of grafted polymer chains and the increase of cross-linking density. In addition, the increase of CMMS also results in the decrease of the distance between the MMSs, which facilitates the cross-linking of the grafted polymer chains. The decrease of kinetic chain length leads to the decrease of εb, while the increase of cross-linking density leads to the increase of E and σb. The increase of Cperoxide on MMSs also leads to the increase of the concentrations of initiator and cross-linker, similar to the increase of CMMS, therefore the increase of E and σb. CM also affected the mechanical properties of the gels. With the increase of CM, there are more monomer molecules to react with the peroxy groups on MMSs and form more grafted chains, and the grafted chains can grow longer, which makes the covalent cross-linking among the grafted chains is easier to occur, in addition, more physical entanglement and hydrogen bonding can be formed among the denser polymer chains. Therefore, E and σb of the MMC gels increase with increasing CM. The longer grafted chains may suggest a higher εb, but we observed a lowered value. The possible reason is that the strong interactions among the polymer chains inhibit their full extension. 3.2. Tensile properties of swollen MMC hydrogels. Physical entanglement and hydrogen bonding are important factors contributing to the physical properties of solid polymers. As the chains become very long, molecular entanglements and intermolecular forces become so strong that the chains no longer slip along each other. The difficulty in untangling their chains makes polymers strong and resilient. However, in general physical entanglement is not likely to induce


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a significant strength or toughness enhancement within a highly swollen gel because the chains are highly mobile and can slip past the entanglements. Further swelling of the as-prepared gel samples would lead to the untangling of polymer chains and the breakage of hydrogen bonds between them. To understand the contribution of hydrogen bonding and chain entanglement to the mechanical properties of the MMC hydrogels, the dumbbell shaped gel specimens cut from the as-prepared hydrogels were swollen in water to 90 wt.% water content and their tensile properties were measured. It is necessary to mention, to ensure there is the same amount of polymer chains in the cross-section of a gel specimen, the initial thickness and width of the asprepared gel specimens were used for the swollen ones. As shown in Figure 1 and 2a, when the CM was small (i.e. 3 M), there was an obvious yield point in the σt-εt curves. When the MMC gel was swollen to 90 wt.% water, the yield point disappeared, the εb (computer recorded) of the gel decreased from more than 40 (as-prepared gel) to be less than 30 (the swollen gel), whereas the σb kept almost constant. The hydrogels synthesized with different monomer concentrations were swollen to 90 wt.% water, and their E, σb and εb are shown in Figure 2b-d, with comparison to those of the asprepared gels. There is a dramatic decrease in the E of the gels and the difference became more significant with the increase of CM, e.g. the E of the gel synthesized with 5 M AAm was only about one seventh in the swollen state to that in the as-prepared state. On the contrary, swelling did not obviously affect σb of the gels, the σb of the gels were very similar in the as-prepared and swollen states. Swelling led to a small decrease in εb at CM =3 M, but εb increased slightly at a higher CM.


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Figure 2. Tensile properties of MMC hydrogels in the swollen state (90 wt.% H2O). (a): Stressstrain curves of a MMC gel in the as-prepared state and in the swollen state, respectively; the synthesis conditions for the MMC gel were: CM= 3 M, CMMS= 0.01 g mL-1, Cperoxide=1.52 mM. (b-d): the E (b), σb (c) and εb (d) of the MMC gels synthesized with different CM in the asprepared state and in the swollen states (swelled in water or in 1 M urea aqueous solutions),


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respectively. The synthesis conditions for the MMC gel were: CMMS= 0.01 g mL-1, Cperoxide=1.52 mM. To further break the hydrogen bonds, the MMC gels were swollen in an urea, an efficient hydrogen-bond-breaking reagent, aqueous solution (1 M) to the same water content (90 wt.%). The E, σb and εb of the gels synthesized with different monomer concentrations are also included in Figure 2b-d. For a gel, its E and σb were slightly lower than those swollen in water, whereas εb was a little higher.

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Figure 3. The E, σb and εb of a MMC gel swollen in urea aqueous solutions with different concentrations to 90 wt.% water content. The reaction conditions for the hydrogel synthesis were: CM= 5 M, CMMS= 0.02 g mL-1, Cperoxide=1.52 mM.


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We also measured the tensile properties of a MMC gel swollen in urea aqueous solutions with different concentrations to the same water content (90 wt.%), and the E, σb and εb of the gel are shown in Figure 3. The E decreased gradually with increasing urea concentration (Curea), while the σb kept almost constant, and the εb even increased slightly. The E of the gel swollen in 2 M urea aqueous solution is 19 kPa, which is only about 8% of that of the as-prepared gel (228 kPa, Figure 1b), suggesting the majority of physical cross-linking has been broken during the swelling. The above tensile test results indicate that swelling of the MMC hydrogels in water or urea aqueous solutions leads to a dramatic decrease in their moduli, suggesting that physical entanglement of polymer chains and hydrogen bonding among them are the important interactions contributing to the moduli of the MMC hydrogels. Whereas swelling does not affect the σb of MMC gels very much, indicating that the fracture strength is mainly determined by the chemical cross-linking (i.e. covalent bonding) in the system. The abnormal increase of εb after being swelled might be explained as follows: the physical cross-linking (chain entanglement and hydrogen bonding) are distributed not very evenly in the as-prepared hydrogels, therefore the applied force can be concentrated on the weak points where the cross-linking density is higher, leading to the formation of microscopic fracture which can be expanded to a macroscopic fracture in the gel due to the successive breakage of the next weakest regions. Swelling in water or urea aqueous solutions cause the untangling of polymer chains and the breaking of hydrogen bonding, and hence the applied force can be shared by the covalently bonded polymer chains which are very long and hence have a long distance to move before reaching the maximum length. 3.3. Fracture energies of MMC hydrogels. The fracture energies of the as-prepared MMC hydrogels were measured with the tearing test. It was found that the hydrogels had high fracture


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energies, and the fracture energy increased with increasing CM and CMMS gradually (Figure 4), varying from 270-770 J m-2.

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Figure 4. The fracture energies of the MMC gels synthesized with different monomer concentrations (a) and MMS concentrations (b). The Cperoxide for all hydrogel syntheses was 1.52 mM, and the CMMS for the gels in (a) was 0.01 g mL-1. Tearing tests were also performed on the hydrogel being swollen to 90 wt.% water. The tearing curves of an as-prepared sample and its swollen sample are shown in Figure 5a. A huge difference in the peeling curves can be found, the as-prepared sample showed the typical peeling curve, while the swollen sample showed a curve similar to the tensile stress-strain curve. The asprepared sample could be peeled into two parts. However, the swollen sample could not be peeled apart; on the contrary it was elongated in the arms to a long distance and then fractured (Figure S1). The fracture energy of the gel was calculated by taking the maximum force at the fracture of the arm as the peeling force, and it was about 2400 J m-2. Assuredly, the true fracture


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energy of the gel should be higher than the obtained value. Even though, the fracture energy of the MMC gel is similar to the toughest DN gels.36

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Figure 5. Tearing tests of swollen hydrogels. (a): The force-displacement curves for the tearing tests of a MMC gel in the as-prepared state and the swollen state (90 wt.% water); (b): the fracture energies of a MMC gel swollen in urea aqueous solutions with different concentrations to 90 wt.% water content. The reaction conditions for the hydrogel synthesis were: CM= 5 M, CMMS= 0.04 g mL-1, Cperoxide=1.52 mM. The tearing tests were also performed on the hydrogels swollen in urea solutions with different concentrations to 90 wt.% water content. As shown in Figure 5b, the fracture energy did not change with urea concentration obviously, and it kept almost constantly at about 2000 J m-2. These results also show that hydrogen bonding does not contribute significantly to the fracture energy of MMC gels.


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The reason for the abnormal increase in the fracture energy of the swollen MMC gel should be the same as discussed above for the effect of swelling on the tensile properties. Swelling diminishes the weak points in MMC gels, the applied force can be evenly shared by the more evenly distributed long and uniform grafted polymer chains chemically attached to MMSs. This excellent energy-dissipating mechanism makes the applied force can be transferred to the whole system along the force direction rather than be concentrated at the notch. 3.4. The resilience of MMC gels. Cyclic tensile tests of the MMC hydrogels with different water contents to the same maximum strain (εmax=3) for 10 run cycles were carried out. The typical loading-unloading curves and the hysteresis ratios (hr) of each cycle are shown in Figure 6a-c. As shown in Figure 6a, for the hydrogel with 70 wt.% water, an obvious hysteresis can be found in the first loading-unloading cycle, but the hysteresis became much less significant in the following ones. When the water content was 97 wt.%, the hysteresis in the first and the following loading-unloading cycles were very small (Figure 6b). As shown in Figure 6c, for each sample the hr decreased dramatically after the first run cycle, and it decreased very slowly in 2-4 cycles and then kept constant in the following run cycles. The hr of the gels decreased with increasing water content, when the water content was more than 95 wt.%, the hr was extremely low (< 0.05). An obvious residual strain (εr) can also be found in the cyclic loading-unloading curves of the gel with 70 wt.% water (Figure 6a), and it became much less in the gel with 97 wt.% water (Figure 6b). The εr of the gels with different water contents after the first cycle are shown in Figure 6d, and it decreased with increasing water content.


19


(a) 0.12

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0.16 0.14 0.12

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0

2

4

6

Cycle times

8

10

0.08

70

75

80

85

90

95

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Water content (wt.%)

Figure 6. The cyclic tensile tests of the MMC gels with different water contents to εmax=3. (a, b): The typical cyclic tensile stress-strain curves; (c): the hysteresis ratios of the gels after each cycle; and (d): the residual strain (εr) of the gels after the first cycle. The synthesis conditions for the MMC gel were: CM= 5 M, CMMS= 0.04 g mL-1, Cperoxide=1.52 mM.


20

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Cyclic tensile loading-unloading cycles were also performed on an as-prepared sample in immediate succession at increasing levels of εmax. Hysteresis could be found in all cycles and it became more and more significant with the increasing εmax (Figure 7a), the hr and εr increased with increasing εmax (Figure 7b). The loading curve follows a path below that of the immediate previous one, but when the εt of the second loading reaches the εmax of the former one, their σt becomes the same, indicating that the cyclic loading-unloading does not affect the strength of the MMC gel.

(b) 0.25

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8

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Figure 7. The cyclic tensile tests of a MMC gel to different εmax. (a): The loading-unloading curves, and (b): the corresponding hysteresis ratios and residual strains. Loading and unloading cycles are performed in immediate succession with the same sample at increasing levels of maximum strain. The synthesis conditions for the MMC gel were: CM= 5 M, CMMS= 0.04 g mL-1, Cperoxide=1.37 mM.


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The presence of hysteresis in the cyclic loading-unloading suggests an energy loss mechanism in the gels, which has been observed for several types of tough hydrogels.12,

24, 27, 37, 38

The

breakage of chemical and physical bonding, the orientation of polymer chains and the friction of the polymer chains may lead to energy loss during tensile elongation.39 Since the εmax in the cyclic tests are far less than the εb of the gels, the breakage of chemical cross-linking can be ignored. The decrease of hr and εr of the gels with increasing water content suggests that the hysteresis is mainly caused by the untangling of polymer chains and the breaking of hydrogen bonding among the polymer chains. The reversible nature of some physical interactions leads to the self-healing property of hydrogels. For instance, after being kept at room temperature for one week, the gel sample which has been cyclically tested could partially recover to its original length (Figure S2) and the hr of the sample in the first cycle was 76% of the original sample (Figure S3), indicating the reformation of physical cross-linking in the gels. The above results from the tensile tests, tearing tests and cyclic tensile tests performed on the gel samples in the as-prepared and swollen (in water or urea aqueous solutions) states suggest that the covalent bonding (i.e. chemical cross-linking) determines the tensile strength and fracture energy of the MMC gels, while the physical entanglement and hydrogen bonding among the polymer chains contributes mainly to the modulus of the MMC gels and they are also the main reason for the presence of hysteresis in the loading-unloading cycles. Based on this conclusion, it is reasonable to assume that increasing chemical cross-linking density would enhance the tensile strength of the gels and diminish the hysteresis in the loading-unloading cycles. Our recent work on hydrogels synthesized by using peroxidized graphene sheets25 and polymer chains27 as PFICC proves that the hydrogels exhibit super-resilience (very low hysteresis and extremely low residual strain) when a very small amount (0.05 wt.% of the


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monomer) of a common chemical cross-linker is added. The hydrogels synthesized by using peroxidized graphene sheets25 as PFICC also exhibit higher tensile strengths. Of course, the mechanical properties of the gels would become poor if an excess amount of chemical crosslinker is added, since it would introduce more structural inhomogenities into the gels.

4. CONCLUSIONS We performed tensile tests, tearing tests and cyclic tensile tests performed on the MMC gel samples in the as-prepared and swollen (in water or urea aqueous solutions) states. The modulus, tensile strength, fracture strain, and the fracture energy of the as-prepared gels vary with the including the concentrations of monomer, macromolecular microsphere and peroxide. These conditions affect the chemical and physical interactions among the polymer chains. When the hydrogels are swollen in water and urea aqueous solutions to 90 wt.% water content, the modulus of the swollen gels decreases dramatically, but no significant change in fracture tensile strength and fracture strain even increases slightly. More interestingly, the swollen gels show much enhanced fracture energies. The hysteresis ratio and residual strain of the gels gradually decrease with increasing water content. The chemical cross-linking determines the fracture strength and fracture energy of the MMC gels. The physical cross-linking, i.e. chain entanglement and hydrogen bonding among the polymer chains, contributes mainly to the modulus of the MMC gels. In addition, the breakage of physical cross-linking is the main reason for the hysteresis in the loading-unloading cycles. The understanding of the interactions affecting the mechanical properties of the MMC gels would be helpful for further development of hydrogels with better properties.

ASSOCIATED CONTENT


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Supporting Information. Figure S1, the trousers-shaped gel specimen during and after the tearing test; Figure S2 and S3, the photographs and cyclic tensile stress-strain curves showing the self-healing property MMC gel. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Corresponding author. Tel.: +(86-10) 58808081; Fax: +(86-10) 58802075; E-mail address: [email protected] (H.L. Wang). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank the financial support from the National Science Foundation of China (No. 50673013), China–Australia Fund for S&T Cooperation (No. 50811120112), the Fundamental Research Funds for the Central Universities and Beijing Municipal Commission of Education. REFERENCES

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for Table of Contents use only Table of Contents (TOC) graphic

3000 2500 2000 -2

G (J m )

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1500 1000 500 0

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Title: Interactions Affecting the Mechanical Properties of Macromolecular Microsphere Composite Hydrogels Authors: Fangzhi Jiang,† Ting Huang,† Changcheng He,† Hugh R. Brown,‡ Huiliang Wang†,*


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Interactions Affecting the Mechanical Properties of Macromolecular

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