Self-Reinforcement of PNIPAm–Laponite Nanocomposite Gels

Article pubs.acs.org/Macromolecules

Self-Reinforcement of PNIPAm−Laponite Nanocomposite Gels Investigated by Atom Force Microscopy Nanoindentation Cuixia Lian, Zemin Lin, Tao Wang, Weixiang Sun, Xinxing Liu, and Zhen Tong* Research Institute of Materials Science and State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China S Supporting Information *

ABSTRACT: Strain hardening and self-reinforcement were observed from poly(Nisopropylacrylamide)(PNIPAm)-Laponite nanocomposite hydrogels (NC gel) after large deformation of either stretching or tearing. These phenomena were investigated with atomic force microscopy (AFM) nanoindentation in nanoscale for the first time. Strong attractive force was detected from the indentation force curve of the asprepared and swollen NC gels due to the capillary effect of water between the AFM tip and gel surface. The Young’s modulus E of the NC gels was evaluated by the AFM nanoindentation using the modified Hertz model, which increased with increasing laponite and decreased after swelling. After the NC gels suffered stretching to 900% strain or tearing to break, the Young’s modulus was substantially increased, implying the self-reinforcement of the gel samples. This effect was enhanced by increasing clay content. On relaxation of deformed samples containing a small amount of clay, the modulus almost recovered its original value (before application of large deformation) within 10 h at rest. However, for the NC gels with high clay content, this recovery was slowed down and the residual strain remained even after 190 h. The strain hardening of the NC gels during deformation was attributed to the orientation of the clay platelets by pulling connected polymer network chains during elongation. The interparticle distance L related to the diameter d of the platelets was adopted to interpret the recovery of the NC gels: L > d at low clay concentrations (≤6% w/v), and the clay platelet disorientation resulted in the recovery; while L < d at high clay concentrations (>6% w/v), the clay platelet movement was strictly limited to induce self-reinforcement for the NC gels.

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INTRODUCTION The fast development in the field of biomaterials and biotechnology in the last several decades has promoted the investigation on the stimuli-response hydrogels, such as temperature, pH, ion, and light sensitive hydrogels.1−5 Poly(N-isopropylacrylamide) (PNIPAm) hydrogels have been intensively studied for academic and biomedical application targets as a temperature responsive material with a low critical solution temperature (LCST) at about 32 °C.6−8 However, two defects of chemically cross-linking hydrogels restrict their applications: poor mechanical properties and low response rate. At the beginning of this century, Haraguchi et al. reported a novel robust organic−inorganic nanocomposite hydrogel (NC gel) with high tensile strength and elongation, which was obtained by in situ polymerization of NIPAm monomer in aqueous suspension of hectorite clay laponite.9 Up to now, an extensive research has been carried out on the NC gels for the mechanical and optical properties,10,11 network structure,12−19 biological applications,20,21 and stimuli-response.22−27 In the NC gels, the laponite platelets act as multifunctional cross-linkers, which endow the gels high transparency and striking mechanical performance, such as high extension strength and ultrahigh elongation.10 Haraguchi et al. found that the strength and modulus increased and the elongation decreased in the second stretching of the NC gels with high clay content due to the orientation of clay platelets.28 © 2012 American Chemical Society

In the previous research, we observed the strain hardening of the NC gels under large deformation of uniaxial compression and elongation, which was considered as the extensional limitation to the polymer network chains due to the orientation of the laponite platelets.29 All of these were integrated phenomena base on the macroscopic observations. Atom force microscopy (AFM) is a powerful method to observe the nanostructure of hydrogels. Suzuki et al. studied the morphology changes on the surface domain of PNIPAm gels during the volume phase transition from the swollen state to collapsed state by tapping mode in water.30 Moreover, AFM nanoindentation is a useful technique to measure the local mechanical properties in nanoscale. In the nanoindentation, the force exerted on the AFM cantilever by the sample during pressing in the z-axis is detected as a function of the indentation depth within elastic contact region and the Young’s modulus is evaluated from the indentation force-displacement curve by the modified Hertz theory. Radmacher et al. observed the morphology and elastic behavior of soft biology objects using AFM and determined their mechanical properties in submicrometer scale by analyzing the force curve of these materials.31−33 Nitta et al. reported that the local elastic Received: April 30, 2012 Revised: August 7, 2012 Published: August 20, 2012 7220

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mm to 900% of strain. The relaxed gel samples were made by storing the stretched samples in a homemade humidity controlled container for a desired period. The swollen gel samples were prepared by swelling in pure water at 25 °C to equilibrium with daily replacement of water. The tearing fracture samples for the AFM test were obtained from tearing a trouser sample of 50 mm × 10 mm × 5 mm with a 30 mm initial notch (Figure 1) using the same loading manner as that for the elongation.

modulus of agar gels measured by the AFM nanoindentation was correlated to the tensile creep.34 Matzelle et al. examined the local elastic properties of PNIPAm hydrogels using AFM in a temperature-controlled liquid cell and found a dramatic increase in stiffness of the gel surface when crossing the phase transition temperature.35 Gong et al. used AFM to determine the local Young’s modulus of the fracture surface and mold surface for the double-network (DN) gels and observed the local yielding around the crack tip during the fracture.36 In this work, we investigate the strain hardening and relaxation of the NC gels after large deformation using the AFM nanoindentation to explore the microstructural change. Therefore, the AFM measurements are carried out on the surface from the uniaxial extension and tearing tests. The Young’s modulus of the NC gels suffered large deformation of stretching or tearing has been found to be significantly higher than that of the same gels as-prepared. This self-reinforcement is interpreted to be governed by the rotational restriction to the clay platelets embedded in the polymer network.

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Figure 1. Schematic illustration of trouser sample of NC gel for tearing experiment.

EXPERIMENTAL SECTION

Materials. N-Isopropylacrylamide (NIPAm, Acros, 1% stabilizer) was recrystallized from a toluene/n-hexane mixture and dried in vacuum at 40 °C. Synthetic hectorite clay of sol-forming grade laponite XLS (Rockwood Ltd., 92.32 wt % of Mg5.34Li0.66Si8O20(OH)4Na0.66 and 7.68 wt % of Na4P2O7) was used after drying at 125 °C for 2 h. Potassium peroxydisulfate (KPS, K2S2O8) was recrystallized from deionized water and dried in vacuum at room temperature. N,N,N′,N′Tetramethylethylenediamine (TEMED, Sinopharm Group Chemical Reagent Co. Ltd.) was used as received. Pure water was produced by deionization and filtration using a Millipore purification apparatus (resistivity >18.2 MΩ cm) and bubbled with argon gas for more than 1 h prior to use in the gel preparation. Synthesis of NC Gels. The NC gels were prepared by in situ polymerization of the monomer NIPAm in the laponite XLS suspension initiated with KPS and TEMED. The laponite concentration was varied from 2 to 18% w/v in water and NIPAm concentration was kept at 1 mol/L in suspension. The NC gel samples were referred to as N1Cn, where the laponite concentration was n% w/v. The synthetic procedure of the NC gels with low clay contents (≤10% w/v) was the same as that reported previously.16,23,29 Briefly, laponite was first dispersed in water under stirring for 3−4 h to produce a uniform suspension. Then, NIPAm was added to the laponite suspension and the mixture was stirred in an ice−water bath for 2 h. Finally, the desired amounts of KPS solution (20 mg/mL) and TEMED were added to the mixture under stirring in the ice water bath. The polymerization was carried out at 20 °C for 24 h. The synthetic procedure of the NC gels with high clay content (>10% w/v) was different in preparing laponite suspensions. The clay was added into water in three steps: The first part of clay (∼half) was added into water under stirring until the suspension became transparent. Then, the second and third parts (∼1/4 each) were added stepwise under stirring and the suspension was ultrasonically vibrated for 10 min at every 30 min until uniform. Mechanics Measurements. The elongation was performed on the NC gel samples of 6 mm diameter × 60 mm length using a Zwick Roell testing system at room temperature under crosshead speed of 100 mm/min, corresponding to the maximum deformation rate of 8.3%/s. The sample length between the clamps was 30 mm. The tensile strain was calculated as ε = (l − l0)/l0, where l and l0 were the sample length during stretching and before stretching, respectively. The true stress σ = σ0(1 + ε), accounting for the cross section area change during deformation, was adopted in this study, where σ0 was the nominal stress evaluated from the tensile force and the initial cross section area (28.27 mm2). The elongated gel samples for the AFM measurement were prepared by uniaxially stretching the sample of 50 mm × 10 mm × 1

AFM Nanoindentation. The AFM nanoindentation was conducted on the NC gels using a SPI3800N (SEIKO Co.) with a 4.48 μm z-scanner. The AFM was mounted on the bottom of an inverted microscope, which allows accurate positioning of the probe on the sample surface. The nanoindentation force was measured in ambient condition with a silicon nitride probe (Veeco Co., rotated and symmetric tip shape, cantilever length 140 μm, cone half angle 17.5°, spring constant k 0.1 N/m, and resonant frequency 38 kHz) in the contact mode. The sample for the AFM test was about 1−2 mm thickness. After settled in the AFM, the sample was indented at several locations on the surface. Only one indentation was done at one location and more than 50 raw data were collected for average. Surface Micrography. An environmental scanning electron microscopy (ESEM; Quanta 200, FEI) was used to observe the surface morphology of the NC gels. The sample was placed at 600 Pa and 0 °C with accelerating voltage of 20.0 kV.

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RESULTS AND DISCUSSION Strain Hardening, Self-Reinforcement, and Recovery. PNIPAm−laponite nanocomposite hydrogels (NC gel) usually exhibit a robust mechanical behavior under large deformation.29 Figure 2A shows the true stress−strain curves of the NC gel N1C6 during the three continuous elongations. The sample broken in the first stretching was cut into a suitable length, redetermined diameter using a caliper, and reloaded for the second stretching. The sample used in the third stretching was prepared in the same way. The strain was calculated from the stretching length related to that at the beginning of every test. Neither necking nor yielding was observed from the NC gels during elongation. When comparing these S−S curves, the stress increased with strain more quickly during the second and third elongation. Apparently, the fracture strain decreased obviously in the second and third stretching. The actual strain was higher than the nominal value because there was not enough time for the relaxation and the residual deformation still remained in the samples used in the second and third stretching, which will be discussed later. We cannot control the sample contraction during reloading and used the nominal strain. This phenomenon is caused by the orientation of both network chains and laponite platelets in the NC gel during the first stretching. The strain hardening was more evident for the NC gels containing more laponite (Figure 1SA of Supporting Information), implying more contribution from the laponite 7221

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Figure 3. Load-tearing strain curve of N1C10 gel under tearing with photos of trouser sample appearance at specified strain to present the process of tearing to stretching.

tearing strain at break (l = 347.7 mm) being much larger than 100%, because the gel with the notch can sustain the deformation and the sample between the clamps is extended as a whole. We observed the recovery of the once-stretched NC gels that were kept in a humidity-controlled container under saturated water vapor. The true stress is plotted against strain in Figure 4 for different recovery times. For the N1C2 gel with low clay content, the S−S curve recovers to the first stretching after 6 h storage and completely overlaps after one day. On the contrary, for the N1C10 gel with 10% w/v of clay, the S−S curve at strain higher than 500% cannot recover after 24 h storage. For easy comparison, the residual strain of the NC gels after stretching to 900% strain is demonstrated in Figure 2S (Supporting Information). When the clay concentration is not higher than 6% w/v, the residual strain vanishes after recovery. As the clay concentration is higher than 6% w/v, the residual strain cannot be removed even after 190 h recovery. Similar strain hardening and residual strain were also reported by Haraguchi and Li, where the strain remained even after 14 days for the NC gels with high clay content.28 Figure 5 depicts the recovery of the stretching loading− unloading hysteresis cycle at strain of 300%, which was estimated by storing the once cycled samples in a humidity controlled container and measuring only the second time after specified time intervals. For the N1C2 gel, the second loading− unloading cycle almost overlaps the first one just after 0.5 h relaxation, while for the N1C18 gel 24 h are required for being recovered. If the strain is high, the residual hysteresis cycle becomes larger (Figure 3S, Supporting Information). This recovery process suggests that the self-reinforcement of the NC gels would be attributed to the clay platelet orientation, which takes a longer time to disorientate in the gels containing more clay platelets. Attractive Force of NC Gels to AFM Probe. The AFM nanoindentation was carried out on the as-prepared and swollen NC gels for revealing microstructural change with the self-reinforcement. This method has been proposed to be useful for quantification of surface mechanical properties for various soft materials.36−38 In Figure 6, an indentation force vs Zscanning displacement curve is schematically illustrated with cartoons for the tip approaching to the gel surface at different

Figure 2. Stress−strain curves for the N1C6 gel under continuous stretching for three times (A) and continuous loading−unloading cycles up to strain of 300% (B).

orientation. The S−S curve for the third stretching almost overlaps with the second one. This means that the orientated NC gel cannot be oriented further. Haraguchi and Li. reported the strengthening effect in the second stretching of the NC gels with high clay content.28 The continuous stretching loading−unloading cycles in Figure 2B also demonstrate this strain hardening behavior. The second cycle was conducted just after the first cycle was completed. The hysteresis loop of the second cycle becomes larger than that of the first one and the loading curve of the third cycle is similar to that of the second one but without unloading curve due to sample broken. Similarly, the NC gel exhibited the strain hardening and the orientation cannot be completely relaxed during the continuous measurement even the strain was reduced to 300%. Appearance of the hysteresis means that a part of extension energy is dissipated during the cycle. When strain was higher (500%), the strain hardening and hysteresis became more obvious (Figure 1SB, Supporting Information). These phenomena indicate that the NC gel possesses self-reinforcing ability and this ability relates to the clay platelets orientation. Because large deformation leads to the orientation of both the network chains and laponite platelets and the relaxation of the latter requires much more time. The tearing test reflects the self-reinforcement of the NC gels more evidently. Figure 3 presents the load-tearing strain curve with sample appearance at several strain values. The tearing strain was estimated as (l − l0)/(l0 + 2a), where a was the tearing length of the trouser sample and l0 was the initial sample length between the clamps (31.5 mm). It is novel that the tearing can be changed into stretching during elongation as the 7222

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Figure 5. Loading−unloading cycles up to strain of 300% for the NC gel samples at the as-prepared state, after stretched and relaxed for a given time to monitor the recovery.

Figure 4. Stress−strain curves for the NC gel samples at the asprepared state and after stretched and relaxed for a given time to monitor the recovery.

stages. The abscissa Z is the piezo-extension, which represents the indentation progress of the probe tip, and the ordinate F is the force acting on the gel surface, which is calculated from the Hook’s law F = kD where k and D are the spring constant and deflection of the AFM cantilever, respectively. This curve of the NC gels can be divided into four stages according to the force: At the stage 1, no force is detected by the cantilever because there is no contact to the gel surface. As the probe approaches to the gel surface further (stage 2), the cantilever begins to bend downward, indicating an attractive force between the gel and the tip. Once the tip indents the gel surface, the downward bending of the cantilever comes to a maximum and the curve presents a minimum at point (Z0, F0) (stage 3). We defined the attractive distance as the distance from the intersection of baseline where no force was detected and the tangent of the curve where the attractive force was 5% of the maximum to the Z0 position. The attractive force F0 was determined from the baseline to the minimum. The cantilever bends upward at stage 4, indicating the repulsive force exerting on the tip during the indentation. The nanoindentation force curves for both the as-prepared and swollen NC gels are demonstrated in Figure 7. The substantial attractive force to the AFM tip is a general phenomenon for the NC gels either as-prepared or swollen. More than 50 data of the indentation force were collected and averaged to determine the attractive force and attractive

Figure 6. Illustration of AFM nanoindentation force vs Z-scanning displacement curve on the NC gel surface with schematic representations of the probe during the indentation process: (1) approaching; (2) attractive; (3) attractive maximum; (4) repulsive.

distance for each gel sample. Figure 8 displays the attractive force and attractive distance for three NC gels at the asprepared and swollen states. The as-prepared samples produce lower attractive force and shorter attractive distance compared with the corresponding swollen ones. With an increase in the clay content of the NC gels, both the attractive force and the attractive distance decrease for both as-prepared and swollen samples. When the clay content was low further (≤6% w/v), the attractive force was so large to break the cantilever of the AFM probe during the withdrawing process, leading to failure in measuring the low clay gels. Either swelling or lowering clay content increases the water content in the NC gels and enhances the attraction between the AFM tip and the gel 7223

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Figure 7. Typical indentation force curves for the as-prepared (solid line) and swollen (dashed line) NC gels. The curves were vertically shifted by a to avoid overlapping.

Figure 9. Repulsive parts of the AFM indentation force curve for the NC gels after specified treatments.

stiffer than others. This result is expected from the high clay content of this gel, which acts as the multifunctional crosslinkers. In view of the rubber elasticity theory, the elastic modulus is proportional to the cross-linking density. For the same reason, the swollen gel produces a weak increase of the force than that of the corresponding as-prepared one. Young’s modulus E of the NC gels was evaluated from the indentation force curve based on the modified Hertz model for the elastic contact with extremely small deformation.39

δ2 =

Figure 8. The average values of attractive force (A) and attractive distance (B) between the AFM tip and sample surface for the asprepared (black) and swollen (pattern) NC gels.

⎛ π ⎞ (1‐ν 2) ⎜ ⎟ ΔF ⎝ 2 ⎠ E tan α

(1)

Here, δ is the indentation depth on the sample, which is estimated as the piezo displacement from Z0 subtracting the deflection of the cantilever. ν and α are the Poisson ratio of the sample (ν = 0.5) and the half open angle of the tip (α = 17.5°), respectively. ΔF (= F − F0) is the repulsive force exerting on the cantilever. In order to obtain accurate E values of the NC gels in nanoscale, only the initial part of 200 nm was used as δ in calculation. 50 indentation force data were obtained for a given sample at different locations and the average values of Young’s modulus E for the as-prepared and swollen NC gels are listed in Table 1. Statistical distribution histograms of the 50 data of the

surface. Thus, this attraction is reasonably attributed to the capillary force of water between the tip and gel surface. Young’s Modulus Determined by AFM Nanoindentation. The repulsive part of the AFM indentation force curve (stage 4 in Figure 7) can be used to determine Young’s modulus E for the NC gels in nanoscale. The AFM force− indentation depth curves in Figure 9 manifest different mechanical responses of the three NC gels at different states. The curves for the N1C18 are steeper and hence this sample is 7224

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Table 1. Young’s Modulus E (kPa) of the NC Gel Surfaces after Specified Treatments sample

asprepared gel

swollen gel

stretched gel

fracture surfacea

swollen fracture surfaceb

N1C10 N1C14 N1C18

190 253 358

150 185 229

252 357 518

221 321 523

63 67 67

a

The fracture surface from tearing trouser samples. bThe fracture surface from tearing trouser samples then swollen in water to equilibrium.

Young’s modulus for the NC gels are presented in Figure 4S (Supporting Information). The Young’s modulus of the asprepared NC gels increases obviously with increasing clay content due to the increase in the cross-linking density, such as 190 kPa for N1C10 up to 358 kPa for N1C18. When the gels are swelling equilibrium in water, the Young’s modulus substantially decreases to 79% for N1C10 and even to 64% for N1C18, because the water content is increased to reduce the cross-linking density (Figure 5S, Supporting Information), leading to a large decrease in Young’s modulus. Thus, the AFM nanoindentation is available to examine the mechanical properties of the NC gels in nanoscale. Self-Reinforcement Observed by AFM Nanoindentation. The AFM nanoindentation was performed on the asprepared NC gels suffered either large uniaxial elongation or tearing fracture to observe the strain hardening and selfreinforcement in nanoscale. As soon as the gel sample was stretched to 900% of strain and the stress was removed, it was set on the AFM and the nanoindentation force was detected. The Young’s modulus values of the stretched gels are compared with that of the as-prepared ones in Table 1. Surprisingly, the elongation to the strain of 900% greatly enhances the Young’s modulus in nanoscale of the NC gels and this enhancement is strengthened by increasing clay content. For an example, E for the N1C18 gel increases from 358 kPa for the as-prepared sample to 518 kPa for the stretched sample (Table 1). This fact indicates that the strain hardening and self-reinforcement occur in the NC gel in the nanoscale. In other words, the reinforced structure induced by deformation is uniformly distributed in the gel. The present finding implies for the first time that the strain hardening and self-reinforcement of the NC gels after large deformation (Figures 2 and 3 for examples) originate from the uniformly distributed microstructures. The orientation of the clay platelets would play an important role in this selfreinforcement. Figure 10 illustrates the variation of the Young’s modulus E with the storage time for the NC gel samples after stretching up to strain of 900% to observe the recovery process. The straight lines indicate the Young’s modulus E of the corresponding asprepared NC gels without stretching. Though the data are scattered due to the application of different samples at different recovery times, two important points can be recognized: a steep decrease of the Young’s modulus during the first 10 h and residual E increment remained after 190 h recovery. These facts confirm the existence of the nonrelaxed microstructure in the NC gels with high clay content after large deformation, which contributes to the strain hardening and self-reinforcement. This nonrelaxed microstructure is considered mainly as the clay platelet orientation. The AFM nanoindentation was also carried out on the fracture surface of trouser gel samples after tearing to check

Figure 10. Recovery time dependence of the Young’s modulus E determined by AFM nanoindentation on the NC gels stretched to 900% of strain, compared with that of the as-prepared NC gels (solid line).

again the idea about the clay orientation contribution to the self-reinforcement. The repulsive parts of the force curve on the surfaces of fracture and torn-swollen (tearing first then swollen in water to equilibrium) are summarized in Figure 9. The force is higher for the fracture surface and lower for the same fracture surface after swollen when compared with the corresponding as-prepared one. This difference becomes evident as the clay content is increased. The average values of Young’s modulus for the torn surface and the torn-swollen surface are listed in Table 1 as the fifth and sixth columns, respectively. Again, the fracture surface exhibits the Young’s modulus similar to that of the stretched sample, higher than that of the as-prepared one for the same NC gel, indicating the self-reinforcement of the NC gel after tearing. The present results are just opposite to that reported by Gong et al., where the fracture surface of the DN gels showed lower Young’s modulus than the mold surface.36 For the samples after tearing and swelling, the Young’s modulus becomes very low, even lower than that of the corresponding swollen gel. This is due to the excess expansion during swelling the damaged network of the NC gels after tearing and will be discussed later. Considering the steric hindrance to the disorientation of clay platelets in the deformed NC gels, we calculated the distance L between the platelets with the expression proposed by Michot et al.40,41 7225

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⟨d⟩ ⎛ ϕsph ⎞ ⎜ ⎟ ⎝ ϕ *sph ⎠

1/3

+

⎛ ϕsph ⎞ ⎟ ⎝ ϕ *sph ⎠

nanoindentation. But, the orientation of the clay platelets and network chains induces the self-reinforcement to the NC gels with high clay content after tearing, showing the E values similar to that of the corresponding stretched gels and higher than the as-prepared ones. When the torn gel is swollen, the interparticle distance L becomes larger than the particle diameter and the orientation is relieved, resulting in lower E values in Table 1. We tried to observe the surface orientation with AFM scanning on the deformed NC gels but failed due to strong attractive force on the tip (Figures 7 and 8). The surface micrograph was taken by an environmental scanning electron microscopy of the NC gel N1C18 (Figure 6S, Supporting Information). The orientated stripes can be observed from the fracture surface of the trousers gel sample. We do not discuss further considering the low resolution and possible change in the surface morphology during sampling under vacuum and freezing.

⎜

(2)

where ⟨d⟩ is the average diameter, ϕsph is the spherical volume fraction representing the fraction of the volume encompassing freely rotating particles and defined as ϕsph = (2/3)(⟨d⟩/h)ϕ for disks, and ϕ*sph (=1) is the critical effective volume fraction. Here, h is the thickness of a single platelet (⟨d⟩ = 30 nm, h = 1.35 nm for laponite XLS42), and the volume fraction ϕ = Cclay/ ρvol with Cclay being the clay concentration and ρvol the specific weight of the clay particles (2.77 g cm−3). The average distance L between laponite platelets in the NC gels calculated with the above equation was 52 nm for N1C2, 30 nm for N1C6, 22 nm for N1C10, 18 nm for N1C14, and 15 nm for N1C18, respectively. The structure change in the NC gels during elongation and relaxation is schematically illustrated in Figure 11 for the two cases, similar to that in the literature.43

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CONCLUSIONS The strain hardening was a general phenomenon of the PNIPAm-laponite nanocomposite hydrogels during large deformation. When the clay content was high, the selfreinforcement was observed after large deformation due to the remained clay platelet orientation. AFM nanoindentation revealed the strain hardening and self-reinforcement by the Young modulus value of both stretched and tearing fracture surfaces higher than that of the as-prepared one, which implied that the orientated structure was uniformly distributed in nanoscale in the NC gels. The self-reinforcement of the NC gels with high clay content after large deformation was interpreted with the average distance between the clay particles evaluated from the effective equivalent volume of the hard spheres. This distance was smaller than the particle diameter when the clay concentration beyond 6% w/v, which hampered seriously the relaxation of the orientated clay platelets. The present finding promises the NC gels a very high possibility in biomaterials for their self-reinforcement in mechanical properties after deformation and biocompatibility for the cell proliferation.

Figure 11. The schematic presentation for structure change in the NC gels during elongation and recovery: (A) the interparticle distance L > d, the platelet diameter at Cclay ≤ 6% w/v, and L < d at Cclay > 6% w/v; (B) L > d, the particle disorientation during recovery; (C) L < d, the particle disorientation strictly limited, causing self-reinforcement.

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When L > d at Cclay ≤ 6% w/v (B), the clay platelets are oriented with the plane parallel to the stretching direction during elongation induced by pulling of the connected chains and relaxed when the force is removed, because there is enough space for the platelets to move. This relaxation involves the rotation and translation of the clay platelets in the stretched NC gels, which are restricted by the attached and cross-linked polymer chains. Thus, this recovery takes much more time than that of the orientated polymer chains. The results shown in Figures 2, 4, 5, and 1S, 2S, and 3S (Supporting Information) are just the appearance of this slow recovery. In contrast, when L < d at Cclay > 6% w/v (C), the platelets disorientation through rotation and translation in the stretched gels are strictly hampered, which limits the relaxation of the connected polymer chains. Consequently, the NC gels of Cclay > 6% w/v manifest the self-reinforcement after large deformation of either stretching or tearing. The tearing deformation brings about two changes in the NC gel structure: breaking the polymer networks and orientating the remained network chains and clay platelets. The broken networks cannot be detected by tearing load and AFM

ASSOCIATED CONTENT

S Supporting Information *

Supplemental figures illustrating stress−strain curves, residual strains, hysteresis recovery, swelling degree, and ESEM photos. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: (86)-20-87112886. Fax: (86)-20-87110273. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The financial support from the National Basic Research Program of China (973 Program, 2012CB821504) and the NSF of China (51173052 and 21074040) is gratefully acknowledged. 7226

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dx.doi.org/10.1021/ma300874n | Macromolecules 2012, 45, 7220−7227