Structural Response of Imogolite–Poly(acrylic acid) Hydrogel under

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Structural Response of Imogolite−Poly(acrylic acid) Hydrogel under Deformation Jungju Ryu,† Jaehyoung Ko,† Hoik Lee,†,‡ Tae-Gyu Shin,§ and Daewon Sohn*,† †

Department of Chemistry, Hanyang University, Seoul 133-791, Korea Nano Fusion Technology Research Lab, Division of Frontier Fibers, Institute for Fiber Engineering (IFES), Interdisciplinary Cluster for Cutting Edge Research (ICCER), Shinshu University, 3-15-1, Tokida, Ueda, Nagano 386-8567, Japan § Neutron Science Division, Korea Atomic Energy Research Institute, Daejeon 305-353, Korea ‡

S Supporting Information *

ABSTRACT: The structures of imogolite−poly(acrylic acid) hydrogels were investigated using small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) to determine the effects of particle concentration and the magnitude of deformation. The imogolite−poly(acrylic acid) hydrogel was synthesized by using γ-ray radiation as a nanocomposite gel with chemical bonds between the particles and polymers. The SAXS measurements revealed that the imogolite network was composed of particle overlaps. Under deformation, the gel structure was rearranged to increase the dimensionality of the network, forming a relaxed structure of overlaps by changing the orientation of the imogolite. The structural response of the deformed gel depended on the imogolite concentration, which influenced the changes in dimensionality of the network and the number of overlaps. The SANS patterns indicated that the polymers wrapped the imogolite aggregates, allowing polymers to follow the imogolite behavior. These observations demonstrate that the behavior of the imogolite can contribute to both the relaxation of stress and maintenance of the structure during an applied strain.

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allowed for stretching of the polymer chains on the flat surface of the clay, which was oriented perpendicular to the elongation direction.5 In previous work we successfully prepared imogolite− poly(acrylic acid) nanocomposite hydrogels that exhibited a pH response and high-performance mechanical properties.3 Imogolite is a hollow aluminosilicate nanotube consisting of a net composition of (OH)3Al2OSiOH with a high aspect ratio having an external diameter of 2 nm and a length of a few micrometers.6 The inner and outer surfaces of the tubes are covered with AlOH and SiOH groups, respectively.7 Owing to its unique chemical structure, several studies about surface modification and preparation of nanocomposites have been reported.8,9 The organic/inorganic hybrid gels in the present study are imogolite−poly(acrylic acid) hydrogels prepared using γ-ray radiation, which introduced peroxide groups on imogolite surface. The peroxide groups can initiate free radical polymerization of the monomers (e.g., acrylic acid) without any additives. Therefore, the imogolite nanofiber serves as a both a cross-linker and an initiator. The polymerization and crosslinking of acrylic acid on an imogolite surface provided strong

INTRODUCTION Hydrogels have been extensively studied for biomedical, environmental, and industrial applications due to their smart functionality, which can respond to external stimuli such as pH, temperature, chemicals, and pressure.1 Despite the long held expectation that hydrogels would be widely used, the actual applications of common hydrogels are limited because of their poor mechanical strength. Organic/inorganic nanocomposite hydrogels, such as cross-linked polymer systems containing silica, clay, or nanotubes, have been developed in an effort to enhance the mechanical strength.2,3 Inorganic particles that participate in gelation via chemical bonds or physical adsorption serve as cross-linkers and provide a binding scaffold that is responsible for the reinforcement. Hence, the structural response of nanocomposite gels to an external mechanical force dictates the improvement in the mechanical properties. For instance, the orientation of clay (laponite) and the formation of clay−polymer aggregates in poly(ethylene oxide)−clay gel were observed from their anisotropic neutron scattering patterns under shear flow; these results indicated the existence of interconnections and coupling between the clay and polymer.4 For chemically bonded nanocomposites, dense polymers grafted onto the clay and their orientation in clay−poly(Nisopropylacrylamide) were studied under deformation.5 This investigation revealed that gels based on chemical bonds © XXXX American Chemical Society

Received: December 16, 2015 Revised: February 4, 2016

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and 9A beamlines, respectively, of the Pohang Accelerator Laboratory (PAL), Korea. The hydrogels were measured at 13.6 keV, and the scattering profiles were obtained on 0.005 Å−1 < q < 0.12 Å−1 (q = 4π sin(θ/2)/λ) at a distance of 5 m between the sample and detector. Here, q is the scattering vector, θ is the scattering angle, and λ is the wavelength. Rectangular slabs of hydrogel were loaded on the elongation stage, and the measurement was performed under different uniaxial elongations (Λ; Λ = l/l0, l is the stretched length of the sample with the initial length, l0) after stabilization for 10 min. The obtained scattering intensity was corrected with air scattering and gel thickness during elongation. The sector average was obtained with an angle span of 30° for the perpendicular anisotropy on 2D scattering pattern. Scattering intensities of imogolite solutions with different concentrations were collected at a sample-to-detector distance (SDD) of 4.5 m under 11.01 keV. The solutions were loaded onto a 1.5 mm quartz capillary (Hampton Research) on the sample stage after the solutions were filtered using a cellulose acetate membrane filter (diameter 800 nm). Imogolite scattering patterns were obtained from the data correction including the subtraction of solvent scattering. Small-Angle Neutron Scattering (SANS). SANS experiments were performed on the 18m-SANS instrument at HANARO, KAERI. The scattering profiles were obtained at aSDDs of 3 and 9 m to cover a wide q range (0.005 Å−1 < q < 0.2 Å−1), and the neutron wavelengths were 4.82 and 9.57 Å, respectively. The neutron beam resolution (Δλ/ λ) was about 10%. The scattered intensity was detected using a 64 × 64 cm2 ORDELA detector at 2660N with 0.5 × 0.5 cm2 resolution. The hydrogel samples were measured under uniaxial deformation on the elongation sample stage, which was loaded in a humid chamber containing a solvent bottle (to maintain humidity). The chamber had two quartz windows (20 mm diameter and 1 mm thickness) on the sides of the beam path. Cd masks (8 mm diameter) were placed between both sides of the sample stage to block the other reflection from the neutron beam. The rectangular hydrogel samples were deformed up to Λ = 3, and the thickness was corrected. Hydrogels contained three components, namely the particles, polymer, and D2O, which provided information that was dominated by the polymer. The intensities at several different conditions (in the chamber, with the beam blocked, and with a secondary standard material (silica)) were measured to verify the absolute scattering intensity. The scattering intensity of the hydrogels was obtained from the subtraction of background and incoherent scattering in the angle-independent region in the high q range.13 The SANS raw data were reduced using the modified package for the HANARO facility,14 and the intensities were averaged with an angle span of 30° along the perpendicular direction to analyze the anisotropic 2D patterns.

covalent bonds that enhanced the mechanical properties, and the materials showed considerable tensile strength up to 1800% strain and immediate recovery after release of the stress.3 The chemical structure of the hydrogel was confirmed by computer simulation and Raman spectroscopy, and imogolite orientation within the deformed gel was observed.3 Similar alignment of imogolite was observed using an imogolite−acrylamide nanocomposite gel containing methylenebis(acrylamide) as a crosslinker under strain, which led to the suggestion that the polymer was connected to the end of the imogolite tubes.10 In this work, we probe the structural behavior as a function of imogolite concentration and magnitude of elongation by using small-angle scattering of X-rays and neutrons in order to understand the role of particles in the nanocomposite hydrogel which achieves the high performed mechanical property.3 The anisotropic imogolite particles with high aspect ratio exhibited a specific structural response including the alignment of rods and rearrangement of overlaps of nanofibers during the elongation of the polymers. The results are obviously different from nanocomposite hydrogels with spherical silica particles that have no orientation due to the three-dimensional isotropic growth of polymers.3 Furthermore, the nanofibers showed a distinguishable structure compared with a flat disk additive (laponite) that showed two-dimensional growth of polymers.5,11 It is difficult to compare the characteristics of structure in terms of the shape of the particles directly because the colloidal particles are influenced by their dispersity, size, aspect ratio, and surface properties. It is limited to evaluate the shape effect as an independent factor; however, the rodlike anisotropic particles can be used to illustrate the structural behaviors in solution or gel. Here, we discuss the contribution of rodlike particles in the structural response to an induced deformation.

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EXPERIMENTAL SECTION

Samples. Imogolite−poly(acrylic acid) hydrogels with different imogolite concentrations were prepared using a γ-ray radiation method. Imogolite was synthesized and purified by the method of Farmer et al. as shown in the previous report.12 The cotton-like imogolite fibers can be obtained after freeze-drying of the purified imogolite solutions (Supporting Information, Figure S1).3 They were dispersed in H2O (deionized water purified in a Milli-Q (Millipore, USA) system) with a resistivity of >18 MΩ and D2O (99.9%, Aldrich) at several different concentrations through sonication and incubation for 48 h. The imogolite solutions were irradiated with a dose of 10 kGy/h of 60Co γ-rays at KAERI, Jeongeup, Korea, under ambient conditions for 2 h in order to provide peroxide groups on the surface. The solutions were purged with N2, and then they were kept at 0 °C before the polymerization. Acrylic acid monomers (99%, Junsei) were added to the frozen solutions of different imogolite concentrations, 0.5, 1, and 2 wt % with a volume ratio of 1.25:1 (solution:monomer), and then the solutions were frozen at once. The pure weight concentrations of imogolite were 0.28, 0.56, and 1.10 wt %. The samples were subjected to a freeze−thawing process, which consisted of three overall cycles of freezing, vacuuming, and melting, to remove active materials such as oxygen. Acrylic acid monomers were polymerized and cross-linked from the surface to forms radicals at 40 °C.3 Imogolite−poly(acrylic acid) hydrogels were synthesized in rectangular glass molds that were 20 mm wide, 40 mm long, and 2 mm thick in order to characterize the structure of the gel. The prepared flat hydrogels were kept in a humid chamber until the scattering measurements. Several samples with different concentrations of 0.2− 2 wt % imogolite were prepared in deionized water to measure the scattering intensity of imogolite. Small-Angle X-ray Scattering (SAXS). To characterize hydrogels and imogolite solutions, SAXS experiments were performed at the 4C

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RESULTS AND DISCUSSION Structure of Imogolite Particles in Gel and Solution. The structure of imogolite in the gel matrix was observed by SAXS. The imogolite−poly(acrylic acid) nanocomposite hydrogel consisted of three components: imogolite, poly(acrylic acid) (PAA), and water. Scattering intensity can be described using the following function:15 I(q) = ϕpϕs(ρp − ρs )2 Spp + ϕiϕs(ρi − ρs )2 Sii + 2ϕiϕs(ρp − ρs )(ρi − ρs )Spi

(1)

Here, ϕp, ϕs, and ϕi indicate the volume fraction of polymer, solvent, and imogolite, respectively. Spp, Sii, and Spi are the structure factors of the polymer−polymer, imogolite−imogolite, and imogolite−polymer, respectively. The X-ray scattering length densities were calculated for the three components: acrylic acid (9.2 × 1010 cm−2), water (9.0 × 1010 cm−2), and imogolite (23.1 × 1010 cm−2). Therefore, SAXS measurements effectively provided information related to the imogolite because it provided sufficient contrast to the polymer by contrast matching processes, which are described by eq 2. The B

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high q exponents shown (between 2 and 3) indicates that the imogolite gel formed a mass fractal structure.16,18 Therefore, the results suggest that imogolite of the gel exhibited a network structure of aggregation which has the fractal structure of internal bundles of imogolite. To further understand the structures of rodlike particles in the gels, the scattering patterns of imogolite solutions were measured as shown in Figure 2. The scattering patterns of the

PAA hydrogel without imogolite, which was prepared using the same volume ratio of solution to monomer (1.25:1) in a nanocomposite hydrogel, does not show specific scattering patterns as shown in Figure S2. I(q) = ϕiϕs(ρi − ρs )2 Sii

(2)

Figure 1 shows scattering patterns of imogolite−PAA hydrogel at different particle concentrations. Each sample had

Figure 2. SAXS patterns of imogolite solution (a) and slopes (b) at various concentrations; the lines are the fit results for the separated low and high q regions.

Figure 1. SAXS patterns of imogolite−PAA hydrogel (a) and the slopes (b) at various imogolite concentrations; the lines are the fit results for the separated low and high q regions.

imogolite solution also showed characteristics of two slopes, which was the same behavior as that observed in the gel. In a solution without acid additives, the two slope behavior of imogolite patterns became stronger, and the exponent in the high q region became steeper (over 2.5) as the concentration of imogolite increased. The exponent value greater than 2 at high q indicates that the structure of the mass fractal composed of aggregated particles. Thus, the imogolite tubes existed as aggregates in both the gel and solution. However, two types of slope on a scattering pattern were observed for the imogolite, indicating that bundle to bundle adsorption occurred more strongly in solution compared with the acrylic acid gel system. Imogolite shows good dispersity under acidic conditions because the outer surface of the imogolite was covered with aluminol (Al−OH) groups, which can capture protons.9,19 Therefore, the high q scattering implies that the magnitude of the dispersity was different in Figures 1 and 2, even though the type of imogolite aggregation was similar to overlapping rods. The crossover point between the two slopes gave the correlation length (ξ) as the characteristic distance of overlaps of imogolite particles from the relation, d = 2π/q, which is based on the d-spacing (d) and scattering vector (q). The correlation length of the imogolite overlaps in solution

final imogolite concentrations of 0.28, 0.56, and 1.10 wt % in the nanocomposite hydrogel. These samples were prepared after addition of acrylic acid on 0.5, 1.0, and 2.0 wt % of imogolite solutions which were irradiated by γ-rays. The samples were labeled using the respective solution concentrations, 0.5, 1.0, and 2.0 wt %. The measured scattering patterns of Figure 1a consisted of two slopes, which can explain the dimensionality of the scattering object in terms of the relationship between the inverse power law exponent and fractal geometry, I ∼ q−d (d is the fractal dimension).16 Thus, these angle-dependent power law behaviors provide specific dimensional information related to the hydrogel structure such as the magnitude of the slope of 1.6−1.8 in the low q and 2.2− 2.4 in the high q region. The typical cylindrical rod appears in q−1 behavior; however, the measured scattering profiles were observed between q−1.6 and q−1.8. The steeper slope over typical rodlike particles indicates aggregates of rods rather than completely isolated rods.17 The scattering patterns in the low q region are the end of the scattering, which originate from randomly oriented large aggregations or networks that include the internal structure of imogolite bundles as q−2.2−q−2.4 behavior in the high q region. The observed behavior of the C

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constituted the imogolite network, was shown at the crossover point and was 2.5 times larger, i.e., around 40 nm, when the gels were deformed. The results can be explained based on the fact that the gel was stretched by the polymers, and the stress was loaded on the imogolite rods (which have strong chemical bonds with the polymers) during the elongation of the hydrogel. As the polymers were grafted onto the long surface of imogolite, the flow of polymers of the stretched hydrogel induced an orientation in the rods; thus, the imogolite particles containing strong bonds formed larger (40 nm) overlaps with the oriented rods compared to the 17 nm observed in the undeformed gel. In order to compare the concentration-dependent behavior, the scattering intensity of the imogolite hydrogel is plotted in Figure 5 in terms of imogolite concentration at Λ = 3. The scattering patterns demonstrate that easier deformation was obtained at lower concentrations of imogolite, as the apparent steep slope behavior at low q indicates a complex structure of overlaps of imogolite rods, which occurred during polymer elongation. The variations in the slopes of the low q and high q regions are plotted as a function of the magnitude of deformation in Figures 6a and 6b, respectively, for the different imogolite concentrations. The smallest change in slope in the low q region appears at 2 wt % imogolite gel and has a value between 1.8 and 2. Relatively large variations were observed in the 0.5−1 wt % imogolite concentrations of Figure 6a. The largest change in slope at low q was observed between the undeformed (Λ = 1) and the deformed results (Λ = 3) of the 0.5 wt % gel, which showed a value between 1.7 and 2.9. However, no noticeable change in slope was observed for the 2 w%, which slightly increased up to 2 during continuous elongation. Meanwhile, the slopes at high q showed relaxation behavior shown in Figure 6b, which exhibited a slope of 1.7− 1.9 for the relaxed structure compared with the mass fractal slope of 2.3 in the undeformed gel. There was no significant concentration dependence for the differences in slope caused by deformation, but the relatively more relaxed structure appeared at low concentration and vice versa. Therefore, the hydrogel with a low content of imogolite formed a more complex network composed of more relaxed overlaps compared to the gel with a higher imogolite concentration. Thus, these results show that the gels undergo a concentrationdependent structural change when the gels were stretched. This is because the network became complex, and the relaxed internal overlaps appeared in the deformed gel. Furthermore, less change in structure was observed for higher imogolite concentrations. Typically, q−2 behavior is observed in three-dimensional mesh as well as semidilute and dilute polymer systems. In contrast, q−2.5 is observed for a percolating diffusion-limited cluster model.21 Therefore, the observed structure of the network implies that the deformed structure becomes a more densely packed network with overlapping domains. Based on the results of the undeformed hydrogel, the structure of the imogolite network in the deformed gel depends on the number density of the imogolite overlaps. Moreover, the structure of the deformed gel resulted in rearrangement of the oriented imogolite. The overlaps consisting of the rearranged rods showed a relaxation of the structure. On the contrary, the overall network became complex because the overlapping rods with strong bonds gathered together. Thus far, the internal structures of imogolite in solution as a gel and in a deformed gel have been observed, and a schematic

decreased with increasing imogolite concentration as shown in Figure 2. This is evident from the steep decay in scattering intensity of the high q region. Thus, one can infer attractive behavior between the bundles. Unlike in solution, the correlation length of the overlapping imogolite in the gel of Figure 1 remained about 17 nm for the various concentrations. The slope in the high q region (associated with overlaps) exhibited almost the same behavior, having a value of 2.3 in the high q region. Therefore, the increase in the amount of imogolite in the gel merely influenced the number density of imogolite overlaps because the differences in structural parameters, such as the overlap size and dimensionality, were small. The concentration-dependent structure of the gel can be described as the growth of the overall network of the aggregation, which was different from the direct adsorption of imogolite bundles in solution. Imogolite Behavior in a Deformed Gel. The X-ray scattering of imogolite hydrogels was measured in terms of the imogolite concentration and the magnitude of elongation. The perpendicular anisotropic patterns were measured with respect to the stretching direction in the deformed state (Λ = 1−3) as shown in Figure 3. This perpendicular scattering shows the

Figure 3. SAXS 2D patterns of imogolite−PAA hydrogel for different imogolite concentrations under deformation.

parallel orientation of imogolite rods along the stretching direction qualitatively from the scattering of reciprocal space corresponding to real space.20 The anisotropic scattering intensity, which appeared in a single directional pattern on 2D SAXS, was strengthened with increased elongation. These results suggest that the longitudinal imogolite rods were oriented parallel along the strain because the polymer chains which were three-dimensionally wrapped onto the imogolite surface induced a directional property as was observed during flow. The perpendicular sector averaged scattering profiles were plotted in Figure 4a−c according to the imogolite concentration. Scattering patterns contain low q features of a steep slope and a relaxed slope with a background at higher q. As in an undeformed gel, the imogolite networks and the overlap of rods appear in the low q and high q region, respectively. The respective slopes and the scattering vector of the crossover point between the two scattering characteristics of power law were analyzed. The correlation length of the overlaps, which D

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Figure 4. SAXS analysis of sector averaged data at various imogolite concentrations under a deformation of Λ = 1−3.

Figure 5. Concentration-dependent SAXS patterns at a magnitude of deformation Λ = 3.

representation of their behavior is presented in Figure 7. The imogolite−PAA hydrogel was observed to have an imogolite network including overlaps of rods, which present a relatively relaxed structure compared with the solution. Thus, the behavior of the gel differs from the aggregation of bundles, which occurs in the imogolite solution as the observed concentration-dependent decrease in overlap size. While polymer chains were elongated due to stretching of the gel, imogolite particles were aligned along the elongation direction so that the internal nanostructure was deformed by the rearranged particles. The oriented rods resulted in larger overlaps, which provided a more complex imogolite network as compared to the undeformed gel. The network exhibited an increase in complex dimensionality accompanied by overlaps with almost constant size during continuous stretching. This behavior implies that the structure was connected by imogolite overlaps, which can behave like a hinge based on the strong chemical bond domains. Structure of Imogolite−PAA Hydrogel under Deformation. Small-angle neutron scattering was performed to

Figure 6. Concentration-dependent variation of slopes of low q (a) and high q (b) according to the magnitude of deformation.

investigate a ternary system of hydrogels including polymer information. Imogolite contrast matching was established at a 90% volume fraction of D2O to H2O according to the calculated scattering length density (Figure S3). The measured scattering patterns exhibited scattering of ternary components including polymer dominant information following eq 1 because the hydrogel samples were prepared in D2O for neutron scattering. Figure 8 is the 2D SANS patterns of the E

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Figure 7. Schematic representation of imogolite structure in solution, gel, and deformed gel.

Figure 8. SANS 2D patterns of imogolite−PAA hydrogel with an imogolite concentration of 1 and 2 wt % under deformation.

nanocomposite hydrogels with 1 and 2 wt % imogolite concentrations under deformation. The neutron scattering patterns show a perpendicular anisotropy under deformation without other directional scattering, which is the same as the Xray scattering of oriented imogolite. This orientation of the ternary system indicates that the polymers not only are elongated but also wrap the imogolite surface.22 Therefore, the single anisotropic pattern in 2D SANS provides information related to the alignment of polymers with oriented imogolite. Figure 9 shows the sector averaged data of the 2D scattering patterns of 1 and 2 wt % hydrogel samples, which were measured up to Λ = 3. All scattering patterns except the undeformed gel of 1 wt % imogolite concentration showed the end of the scattering of networks at low q. Therefore, the characteristic size of the low q region can be obtained only from the undeformed gel of 1 wt % by using the following Lorentz type function: I = I(0)/(1 + Ξ2q2). This function provides the correlation length Ξ. Previous work showed that the structure factor of the cross-linked gels is given by Lorentz type and Ornstein−Zernike functions, which describe a semidilute polymer solution, and a squared Lorentz type function to account for the inhomogeneity of the gel.23,24 These inhomogeneities can provide particle−particle and particle− polymer information related to the nanocomposite gels.5,25 The observed profile of the undeformed 1 wt % gel exhibits q−2 behavior of Lorentzian type; however, it presents no significant results for the squared Lorentz term. Therefore, the plot of Figure 9 implies that imogolite gels consisted of large polymer wrapping regions regardless of the inhomogeneities of smaller length scales in the polymer dominant state. The correlation length of the 1 wt % gel of Figure 9a increased from 40 nm (Figure S4) to a value beyond the measured q range for deformations above Λ = 2. In Figures 9a and 9b, the perpendicular sector averaged scattering patterns of all the deformed gel (including the undeformed gel of 2 wt %) exist in

Figure 9. Sector averaged neutron scattering patterns of imogolite− PAA hydrogel of 1 wt % (a) and 2 wt % (b) of imogolite under deformation; the lines are fitting results for power law and Lorentztype functions.

the qΞ > 1 region, which is given by the large domains of polymers wrapped onto the imogolite rods. The steep increase in the correlation lengths in the deformed gel network shows that the polymers wrapped on imogolite were rearranged, which corresponds to the alignment and aggregation of imogolite. This behavior occurred because imogolite serves as a structural support that contains chemical bonds with the polymer. The slopes of the patterns for 1 wt % imogolite gel slightly changed from 2 at Λ = 1 to 2.15 at Λ = 3 in Figure 9a, while the slopes of the 2 wt % gel maintained the q−2 behavior during deformation up to Λ = 3 in Figure 9b. These results correspond to the concentration-dependent magnitude of the slope of SAXS results, which exhibited small changes in the imogolite network composed of overlaps at high concentration. However, the constant structural behavior of q−2 in neutron scattering was due to lower deformation of imogolite as well as the contribution of the polymers. Thus, SANS results suggest that polymers wrap the imogolite rods, and the amount of F

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begins with the radicals on imogolite surface irradiated by γrays. Simultaneously, it is a cross-linker where polymer networks are formed on radially along the cylindrical axis. Therefore, imogolite can provide a long platform for the polymer to form three-dimensional network. Imogolite exhibits a network structure consisting of overlapping rods in the gel, and the network undergoes structural changes based on overlapping of oriented rods under uniaxial deformation. The polymers wrapped on the imogolite exhibited a parallel alignment along the orientation of imogolite during the stretching of polymers because polymerization occurred with three-dimensional growth from the longitudinal surface of the imogolite fibers. The structural response of the imogolite−PAA hydrogel against the external deformation provided excellent mechanical properties due to the structure based on overlaps of imogolite and the relaxation of the structure through the orientation.3 Specifically, the results explain the mechanical behavior such as stress−strain behavior of the imogolite−PAA hydrogel observed in our previous research in terms of the internal structure. The highly concentrated gel resulted in a less deformed network with a relatively more complex structure of overlapping rods compared with less concentrated samples. These higher loadings provided a higher modulus, which allowed easier storage of the loading energy.3 The gel with low concentrations of imogolite exhibited effective extension due to easier rearrangement of the structure, resulting in a complex network of overlaps with the oriented rods. The imogolite− PAA hydrogel had a structural behavior with fractal dimensionality rather than a single rodlike morphology. The aggregates of particles actually contain different characteristics, which are more disordered and more flexible than that of the native particles.26 However, it has been reported that the aggregates of one-dimensional rods can provide effective reinforcement of soft matter.28 These phenomena can be explained by the fact that the elastic energy is stored during the strain of the filler aggregates.27 The uncovered nanoscale structure of rodlike imogolite hydrogel provides the insight for designing new hybrid materials. In particular, the imogolite nanocomposite hydrogel gives the benefits for biomedical applications such as a bioscaffold for tissue engineering29 and carrier for proteins30 and DNA.31 This study revealed the imogolite behavior in gel and its structural contribution during application of an external mechanical force. These results can provide an understanding of the preparation of a well-defined gel and controllable mechanical properties in order to achieve high-performance hydrogels.

polymers wrapping the imogolite is large, which influences the overlapping of the rods. Figure 10 shows the structural behavior of an imogolite nanocomposite hydrogel under deformation. A network

Figure 10. Schematic representation of the structural response of imogolite in hydrogel under deformation. qi and ξ are the scattering vector for imogolite network and correlation length of imogolite overlaps from SAXS profiles. qp and Ξ are the scattering vector and correlation length in polymer dominant ternary system from SANS profiles.

structure composed of overlaps of imogolite rods was observed, and the rods were oriented along the elongation direction when the gel was stretched. The nanocomposite structure of the polymer and imogolite showed alignment behavior following the imogolite orientation because the polymers were grafted from the imogolite surface. Under deformation, imogolite showed overlaps of rods with a relaxed structure owing to the alignment of particles. The imogolite overlaps formed a complex network depending on the magnitude of deformation because the rods mainly remain as a strong chemical platform. Polymer chains wrapped on the imogolite also rearranged during elongation. The inhomogeneities in the ternary system (including those in the polymer) form a large correlation length with almost constant q−2 behavior. Our observation demonstrates that the structure of the nanocomposite hydrogel was composed of a fractal structure of imogolite aggregates. Therefore, the polymer chains connected to the surface of the large imogolite aggregates, allowing the polymer to follow the imogolite behavior and stretch with the external strain. One-dimensional particles generally enhance the interactions due to the opportunity for contact along a longitudinal line; thus, they actually form aggregates or clusters.26 Nanocomposites with fractal aggregates are expected to provide efficient reinforcement of elastic materials based on the model suggested by Witten et al.26,27 This model describes the modulus of aggregates as a function of the fractal structure of the tortuosity connected with the spanning path.27 Imogolite tubes did not show typical rodlike q−1 behavior. However, the characteristic aggregates of a fractal dimensional structure of imogolite overlaps did influence the mechanical properties.3 The structure observed during external stretching contributes to the mechanical properties of the nanocomposite hydrogel using rodlike particles for the following reasons: (i) the network structure of rodlike particles connected via overlaps of rods containing chemical bonds resulted in strong mechanical properties of the gel, and (ii) the orientation of the imogolite and the relaxed overlapped structure allowed for relaxation of a given stress during an external strain.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02713. TEM image of imogolite, X-ray scattering of acrylic acid hydrogel, results of the calculated neutron scattering length density, and the fitting results by using the Ornstein−Zernike function for the hydrogel (PDF)

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CONCLUSION The structural response of chemically bonded gels between particles and polymers (imogolite−PAA hydrogels) against external strain has been observed by small-angle scattering. Imogolite has a role of initiator because the polymerization

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.S.). Notes

The authors declare no competing financial interest. G

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (2015M2B2A9032029). The authors acknowledge the support of the Pohang Accelerator Institute (PAL) and HANARO in providing the X-ray beamline and neutron facilities used in this work. We are grateful to Prof. M. Shibayama for valuable discussions, and we thank Dr. Kyeong Sik Jin at PAL and Dr. Eunjoo Shin at the Korea Atomic Energy Research Institute for help with the measurements.

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ABBREVIATIONS PAA, poly(acrylic acid); SAXS, small-angle X-ray scattering; SANS, small-angle neutron scattering; SDD, sample-to-detector distance.

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DOI: 10.1021/acs.macromol.5b02713 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (31) Jiravanichanun, N.; Yamamoto, K.; Kato, K.; Kim, J.; Horiuchi, S.; Yah, W.-O.; Otsuka, H.; Takahara, A. Preparation and characterization of imogolite/DNA hybrid hydrogel. Biomacromolecules 2012, 13 (1), 276−281.

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DOI: 10.1021/acs.macromol.5b02713 Macromolecules XXXX, XXX, XXX−XXX