Mechanical, Swelling, and Structural Properties of Mechanically Tough Clay-Sodium Polyacrylate Blend Hydrogels

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1 s Article Mechanical, Swelling, Structural Properties Mechanically Tough Clay-Sodium Polyacrylate Blend Hydros Hiroyuki Takeno *, Yuri Kimura Wataru Nakamura Department Chemistry Chemical Biology, School Science Technology, Gunma University, Kiryu, Gunma , Japan; (Y.K.); (W.N.) * Correspondence: takeno@gunma-u.ac.jp; Tel.: Academic Editors: Osvaldo H. Campanella David Díaz Díaz Received: 4 November 2016; Accepted: 20 February 2017; Published: 25 February 2017 Abstract: We investigated mechanical, swelling, structural properties mechanically tough clay/sodium polyacrylate (PAAS) hydros prepared by simple mixing. The s had large swelling ratios, reflecting characteristics constituent polymer. The swelling ratios initially increased with increase swelling time, n attained maximum values. Afterwards, y decreased with an increase swelling time finally became constant. An increase in clay concentration lead to a decrease in swelling ratios, whereas an increase in PAAS concentration lead to an increase in swelling ratios. Tensile measurements indicated that toughness for clay/paas (M w = ) s was several hundred times larger than that clay/paas (M w = ) s, i.e., use ultra-high molecular weight PAAS is essential for fabricating mechanically tough clay/paas blend hydros. Synchrotron small-angle X-ray scattering (SAXS) results showed that SAXS intensity measured at small scattering angles decreased with an increase in clay concentration, indicating that interparticle interactions were more repulsive at higher concentrations. The decrease scattering intensity at high clay concentrations was larger for clay/paas (M w = ) system than for clay/paas (M w = ) system. Keywords: clay; blend hydros; toughness; swelling properties; synchrotron small-angle X-ray scattering 1. Introduction There has been an increased interest in material properties mechanically tough hydros. Nanocomposite hydros composed clay a polymer are well-known as hydros with excellent mechanical properties. The nanocomposite hydros have potential applications in fields such as biomedical engineering electronic devices [1]. Nanocomposite hydros are prepared by free-radical polymerization alkylacrylamide in presence clay particles, e.g., y are extraordinarily stretchable (~1500%) mechanically tough [2 4]. In recent years, we have found that blend hydros composed clay sodium polyacrylate (PAAS) prepared by simple mixing did not fracture even after 90% compression tests or twisting s [5,6]. For preparation clay/paas blend hydros, a dispersant such as tetrasodium pyrophosphate (TSPP) was used to prevent formation aggregates clay particles, e.g., a house card structure [7]. Moreover, effects polymer molecular weight composition on mechanical properties blend hydros has been investigated [6,8]. From se studies, it has been concluded that it is necessary to use polymers with molecular weights higher than a few million grams per mole to disperse clay particles for production mechanically tough clay/paas blend hydros. In addition, previous studies Gels 2017, 3, 10; doi: /s

2 Gels Gels 2017, 2017, 3, 3, previous studies have suggested that polyacrylate anions adsorb on edge clay particles have suggested that polyacrylate anions adsorb on edge clay particles [5,8]. Thus, though [5,8]. Thus, though mechanical properties blend hydros key factors used to achieve mechanical properties blend hydros key factors used to achieve toughness have been toughness have been elucidated to some degree, studies on or properties such as swelling elucidated to some degree, studies on or properties such as swelling behavior, quantification behavior, quantification toughness, effect clay concentration on structures s toughness, effect clay concentration on structures s remain poorly understood. remain poorly understood. In particular, as PAAS is well-known as a super absorbent polymer, In particular, as PAAS is well-known as a super absorbent polymer, swelling properties swelling properties clay/paas blend hydros are also expected to be important. clay/paas blend hydros are also expected to be important. In this study, in order to clarify previously stated issues, we investigated mechanical, In this study, in order to clarify previously stated issues, we investigated mechanical, swelling, structural properties clay/paas blend hydros using tensile measurements, swelling, structural properties clay/paas blend hydros using tensile measurements, time-course measurements swelling ratios, synchrotron small-angle X-ray scattering (SAXS) time-course measurements swelling ratios, synchrotron small-angle X-ray scattering measurements. (SAXS) measurements Results Results Discussion Discussion We show show a schematic schematic representation representation for for preparation preparation clay/paas clay/paas blend blend hydros hydros in in Figure Figure The The preparation preparation method method is very is very simple. simple. First, First, clay particles clay particles are mixed are in mixed a dispersant in a dispersant (TSPP) aqueous (TSPP) aqueous solution. solution. As clay As particles clay have particles negatively have negatively charged surfaces charged surfaces positively charged positively edges, charged it is edges, expected it is that expected pyrophosphate that pyrophosphate anions adsorb anions adsorb positively on charged positively edges charged clay edges particles. clay As particles. a consequence, As a consequence, clay particles clay areparticles dispersed are in dispersed water. The in water. clay dispersion The clay dispersion addedis toadded a PAAS to a aqueous PAAS aqueous solution solution little by little, by little, is mixed is with mixed with PAAS solution PAAS solution by stirring by stirring mixture. mixture. PAAS is PAAS also expected is also expected to adsorb to on adsorb positively on positively charged charged edges edges clay particles, clay particles, because because it is an anionic it is an anionic polyelectrolyte. polyelectrolyte. Thus, Thus, clay/paas clay/paas blend hydros blend hydros are prepared are prepared by simple by mixing. simple mixing. Figure 1. A schematic representation for preparation clay/paas blend hydros. Figure 2 depicts pictures a 5 wt % clay/1 wt % PAAS3500K/0.5 wt % TSPP blend hydro. These pictures show that does not fracture, even if it is strongly pressed or is bent. Thus, blend hydro has mechanical toughness.

3 Gels 2017, 3, Gels 2017, 3, Figure 2. Pictures wt clay/1 wt PAAS3500K/0.5 wt TSPP blend hydro. pressed Figure 2. Pictures a 5 wt % clay/1 wt % PAAS3500K/0.5 wt % TSPP blend hydro. A pressed (top) bent (bottom). (top) a bent (bottom) Time Course Swelling Ratios for Clay/PAAS Hydros 2.1. Time Course Swelling Ratios for Clay/PAAS Hydros The swelling behavior ionic hydros is affected by following components; free The swelling behavior ionic hydros is affected by following components; free energy energy change mixing, free energy change due to network elasticity, free energy change mixing, free energy change due to network elasticity, free energy change change resulting from presence mobile ions [9 11]. resulting from presence mobile ions [9 11]. Figure depicts time course swelling ratios W(t)/Wdry for clay/1 wt PAAS3500K Figure 3 depicts time course swelling ratios W hydros at various clay concentrations (a) for 10 wt (t)/w clay/paas3500k dry for clay/1 wt % PAAS3500K hydros at various hydros at various clay concentrations (a) for 10 wt % clay/paas3500k hydros at various PAAS concentrations (b) in water. The swelling ratios clay/paas hydros initially increased PAAS concentrations (b) in water. The swelling ratios clay/paas hydros initially increased with time, n reached maximum value, e.g., y attained ir maximum values at with time, n reached a maximum value, e.g., y attained ir maximum values at h in case 5 15 wt clay/1 wt PAAS3500K hydros. The swelling kinetics is in case 5 15 wt % clay/1 wt % PAAS3500K hydros. The swelling kinetics a is described by collective diffusion equation with diffusion coefficient, which is defined as ratio described by a collective diffusion equation with a diffusion coefficient, which is defined as ratio longitudinal osmotic modulus network to frictional coefficient [12]. Therefore, in longitudinal osmotic modulus network to frictional coefficient [12]. Therefore, in general swelling dynamics is slow. In case clay/polymer blend hydro, a general swelling dynamics is slow. In case clay/polymer blend hydro, rearrangement clay particles may occur during course swelling, because clay particles a rearrangement clay particles may occur during course swelling, because clay are linked to polymers by physical ionic interactions. The maximum ratio exceeded 1000 for particles are linked to polymers by physical ionic interactions. The maximum ratio exceeded 5 wt % clay/1 wt % PAAS3500K hydros. Afterwards, y decreased with time, i.e., deswelling 1000 for 5 wt % clay/1 wt % PAAS3500K hydros. Afterwards, y decreased with time, i.e., took place, finally became constant. Such swelling-deswelling behavior has also been reported deswelling took place, finally became constant. Such swelling-deswelling behavior has also been by Haraguchi et al. for nanocomposite hydros prepared by free-radical polymerization reported by Haraguchi et al. for nanocomposite hydros prepared by free-radical polymerization acrylamide, N,N-dimethylacrylamide, N-isopropylacrylamide, when clay concentration is acrylamide, N,N-dimethylacrylamide, N-isopropylacrylamide, when clay concentration is above a critical concentration [13,14]. They showed that deswelling behavior occurred due to diffusion mobile sodium ions from inside network into outer water [14]. The mobile sodium ions were replaced by hydrogen ions. The mechanism should also occur for clay/paas blend

4 Gels 2017, 3, above a critical concentration [13,14]. They showed that deswelling behavior occurred due to diffusion mobile sodium ions from inside network into outer water [14]. The mobile Gels sodium 2017, 3, ions 10 were replaced by hydrogen ions. The mechanism should also occur for clay/paas blend 4 10 hydros. The replaced hydrogen ions may associate with negative ions on clay surface hydros. carboxylatethe anions replaced PAAS, hydrogen so thations mobile may associate ions in with network negative decrease ions on refore clay surface swelling ability carboxylate hydro anions is PAAS, reduced. so that Thus, mobile swelling-deswelling ions network behavior decrease for clay/paas refore blend swelling hydros ability is similar to that hydro observed is reduced. in nanocomposite Thus, swelling-deswelling clay/poly(alkylacrylamide) behavior for, clay/paas although blend former hydros swelling is similar ratio isto much that observed greater than in that nanocomposite latter, clay/poly(alkylacrylamide) reflecting a super-absorbent, although character PAAS. former As swelling seen inratio Figure is 3a, much both greater maximum than that values latter swelling, reflecting ratiosa superabsorbent constant values character at long PAAS. swelling As seen timesin decreased Figure 3a, with both increase maximum values clay concentrations. swelling ratios The increase constant clay values concentration long swelling causes times increase decreased with cross-link increase densities as has clay been concentrations. reported in The a previous increase study [5], clay consequently concentration leads causes to decrease increase swelling cross-link ratios. densities On as or has h, been reported as expected, in a previous swelling study ratios [5], increased consequently with increase leads to decrease anionic polymer swelling concentrations ratios. On due or to h, increase as inexpected, ionic contribution swelling to ratios osmotic increased pressure. with increase anionic polymer concentrations due to increase in ionic contribution to osmotic pressure. Figure 3. Time Time course swelling ratios for clay/paas3500k hydros at at various clay concentrations (a) at various PAAS concentrations (b) Mechanical Mechanical Properties Properties Clay/PAAS Clay/PAAS Hydros Hydros We We conducted conducted dynamic dynamic mechanical mechanical measurements measurements for for a clay/paas clay/paas blend blend hydro hydro in in order order to to ascertain ascertain wher wher is is rheologically rheologically elastic elastic or or not. not. In In Figure Figure 4 we 4 we show show storage storage modulus modulus E E loss loss modulus modulus E for E a 12.5 for a wt 12.5 % clay/1 wt % wt clay/1 % PAAS3500K wt % PAAS3500K. The figure. The shows figure that shows E is larger that than E is E, larger than E is E, independent E is independent frequencies. These frequencies. results These indicate results that indicate sample that is rheologically sample is elastic. rheologically elastic. Figure 5 shows tensile stress-strain curves for clay/1 wt % PAAS3500K hydros for clay/ 1 wt % PAAS507K hydros at various clay concentrations. The former s were stretchable until 700% at low clay concentrations. In contrast, latter s with short polymer chains were fractured at short elongation. Thus mechanical properties blend hydros composed PAAS with molecular weights smaller than a few million grams per mole become extremely poor, as reported in a previous study [8]. Figure 4. Storage modulus (E ) loss modulus (E ) vs. frequency for a 12.5 wt % clay/paas3500k

5 2.2. Mechanical Properties Clay/PAAS Hydros We conducted dynamic mechanical measurements for a clay/paas blend hydro in order to ascertain wher is rheologically elastic or not. In Figure 4 we show storage modulus E loss modulus E for a 12.5 wt % clay/1 wt % PAAS3500K. The figure shows that E is larger than E, E is independent frequencies. These results indicate that sample is rheologically elastic. Gels 2017, 3, Gels 2017, 3, Figure 5 shows tensile stress-strain curves for clay/1 wt % PAAS3500K hydros for clay/1 wt % PAAS507K hydros at various clay concentrations. The former s were stretchable until 700% at low clay concentrations. In contrast, latter s with short polymer chains were fractured at short elongation. Thus mechanical properties blend hydros composed PAAS with molecular Figure 4. Storage modulus (E ) loss modulus (E ) vs. frequency for a 12.5 wt % clay/paas3500k Figureweights 4. Storage smaller modulus than a (E few ) million loss modulus grams per (E ) mole vs. become frequency extremely for a 12.5 poor, wt as % reported clay/ in a previous hydro. PAAS3500k study hydro. [8]. Figure 5. Typical tensile stress-strain curves clay/paas3500k clay/paas507k s at various clay concentrations. The The toughness toughness a material material may may be be characterized characterized by by area area under under tensile tensile stress-strain stress-strain curves curves [15 18]; [15 18]; ε f toughness = σdε (1) toughness = 0 (1) where ε f represents strain at break. We estimated ratios toughness for clay/1 wt % PAAS3500K where εf represents hydros tostrain that at clay/1 break. wt We % estimated PAAS507K hydros ratios at various toughness clayfor concentrations. clay/1 wt % These PAAS3500K valueshydros were evaluated to that from clay/1 average wt % PAAS507K 4 6 tensile hydros measurements. at various Theclay ratios concentrations. attained 434, 206 These values 548, respectively, were evaluated for hydros from with average 3, tensile wt % clay measurements. concentrations; The thus ratios it isattained necessary 434, to use 206 PAAS 548, with respectively, ultra-high for molecular hydros weight with to3, acquire 5 10 toughness wt % clay for concentrations; clay/paas thus blend it is hydros. necessary to use PAAS with ultra-high molecular weight to acquire toughness for clay/paas blend 2.3. hydros. Structures Clay/PAAS Hydros In order to investigate structures hydros, we conducted synchrotron SAXS measurements 2.3. Structures for Clay/PAAS clay/paas3500k Hydros clay/paas507k hydros. Since blend hydro used In order to investigate structures hydros, we conducted synchrotron SAXS measurements for clay/paas3500k clay/paas507k hydros. Since blend hydro used in this study is a four-component system composed clay, PAAS, TSPP, water, whole X-ray scattering intensity can be described by a sum several partial scattering functions [19]. However, as X-ray scattering length clay is much larger than those water PAAS [6], SAXS

6 Gels 2017, 3, in this study is a four-component system composed clay, PAAS, TSPP, water, whole X-ray scattering intensity can be described by a sum several partial scattering functions [19]. However, as X-ray scattering length clay is much larger than those water PAAS [6], SAXS intensity blend hydro may be approximated by scattering intensity from clay particles, i.e., partial scattering function from clay-clay correlation. Thus, SAXS intensity blend hydro I(q) can be approximately described by scattering function from a two-component system. I(q) = Kn clay V 2 clay ρ2 P(q)S exp (q) (2) Here K is an experimental constant, n clay denotes number density clay particles with volume V clay. ρ, P(q), S exp (q) are scattering length density difference between clay particles matrix, form factor clay particles, experimental structure factor, respectively [20 24]. Here magnitude wavevector q is defined by following equation. q = 4πsin(θ/2)/λ (3) here θ λ are scattering angle wavelength X-rays, respectively. The clay particles used in this study have disk-like shapes with a thickness ~10 Å a radius Å [20,25]. The form factor romly distributed disk-shaped particles P(q) is presented in following form [26]; [ π/2 P(q) = 4 0 sin 2 (qh cos β) (qh cos β) 2 ][ ] J 2 1 (qr sin β) (qr sin β) 2 sin βdβ (4) where R 2H denote radius thickness disk-shaped particles, respectively, β is angle between scattering vector q axis disk-shaped particles. J 1 represents first-order Bessel function. In a previous study, we obtained structural parameters clay particles from SAXS data a dilute clay dispersion, where polydispersity radius R with a Gaussian distribution was taken into consideration (2H = 10 Å, R = 130 Å, stard deviation in polydispersity radius = 30 Å) [5]. Figure 6 depicts SAXS priles clay/paas3500k clay/paas507k hydros with 5 wt % 10 wt % clay concentrations. Although SAXS intensity for hydro with 10 wt % clay was larger than that for hydro with 5 wt % clay at high q values, former was weaker than that latter at small q values, where interparticle interference effects become important. This result indicates that effects interparticle interference were stronger at higher clay concentration, i.e., interparticle interactions were more repulsive at higher clay concentration. In this context, SAXS data for clay/paas3500k for clay/paas507k show same trend. We have estimated experimental structure factors for both clay/paas3500k clay/paas507k by dividing scattering intensity by calculated P(q). Figure 7 depicts experimental structure factors for clay/paas3500k s (a) clay/paas507k s (b) with 5 wt % 10 wt % clay concentrations. A decrease S exp (q) in small q for clay/paas507k with 10 wt % clay was greater than that clay/paas3500k with same clay concentration. This result may indicate that interparticle interactions are more repulsive for clay/paas507k than clay/paas3500k at high clay concentration. Such repulsive interactions may be due to a steric repulsion polymer chain adsorbed on edges clay particles or electrostatic repulsion resulting from double layer counter ion distribution. However, it is difficult to clarify local structure such as adsorbed polymer layer from SAXS data, because SAXS intensity mainly comes from clay-clay correlation, as mentioned above. In order to investigate structures adsorbed polymer layer, it may be necessary to use a technique such as contrast variation small-angle neutron scattering [27]. The study local structure will be a subject future work.

7 Gels 2017, 3, Gels 2017, 3, 10 Gels 2017, 3, Figure 6. SAXS curves for clay/paas3500k s (a) for clay/paas507k s (b) at 5 wt 10 Figure 6. SAXS curves for clay/paas3500k s s (a) (a) for for clay/paas507k s s (b) (b) at 5 atwt 5 wt % % wt wt % clay clay concentrations. wt % clay concentrations. Figure 7. The experimental structure factors clay/paas3500k s (a) clay/paas507k s (b) Figure 7. The experimental structure factors clay/paas3500k s (a) clay/paas507k s (b) Figure at 5 wt 7. The experimental 10 wt clay. structure factors clay/paas3500k s (a) clay/paas507k s at 5 wt % 10 wt % clay. (b) at 5 wt % 10 wt % clay Summary Summary 3. Summary We We studied studied mechanical, mechanical, swelling, swelling, structural structural properties properties clay/paas clay/paas blend blend hydros hydros prepared We studied by simple mechanical, blending. The swelling, swelling-deswelling structural behavior properties was observed clay/paas for all blend clay/paas hydros prepared by simple blending. The swelling-deswelling behavior was observed for all clay/paas prepared blend hydros by simple investigated blending. in The this swelling-deswelling study. Both maximum behavior was observed constant for values all clay/paas blend hydros investigated in this study. Both maximum constant values swelling swelling blend ratios hydros decreased investigated with increase this study. clay Bothconcentrations maximum increased constant with values increase swelling ratios decreased with increase clay concentrations increased with increase ratios PAAS decreased concentrations. with These increase results were clay attributed concentrations to increase increased cross-linking with densities increase with PAAS concentrations. These results were attributed to increase cross-linking densities with PAAS increase concentrations. clay concentrations These results were attributed increase to contribution increase cross-linking mobile ions densities to osmotic with increase clay concentrations increase contribution mobile ions to osmotic pressure increase with clay increase concentrations PAAS concentrations. increase The toughness contribution pressure with increase PAAS concentrations. The toughness mobile clay/paas clay/paas ions to blend blend osmotic hydros hydros pressure depends depends with upon upon increase molecular molecular PAAS weight weight concentrations. constituent constituent The toughness polymer, polymer, it it clay/paas has has been been blend demonstrated hydros that depends toughness upon molecular hydro weight with an ultra-high constituent molecular polymer, weight polymer it has been (Mw demonstrated that toughness hydro with an ultra-high molecular weight polymer (Mw demonstrated = = ) was that several toughness hundred times greater hydro than with that an ultra-high hydro molecular with a moderate weightmolecular polymer 6 ) was several hundred times greater than that hydro with a moderate molecular (M weight polymer (Mw weight w = polymer 6 ) was = 5.07 several 10hundred 5 ). SAXS times results greater for both than that clay/paas3500k hydro with clay/paas507k a moderate (Mw = ). SAXS results for both clay/paas3500k clay/paas507k molecular s s showed showed weight that that polymer interparticle interparticle (M w = interactions interactions ). are are SAXS more more results repulsive repulsive for at at both higher higher clay clay/paas3500k concentrations, concentrations, clay/paas507k repulsive repulsive interactions interactions s showed was was greater greater that for for interparticle with with interactions moderate moderate are molecular molecular more repulsive weight weight at PAAS PAAS higher than than clay for for concentrations, with with ultra-high ultra-high repulsive molecular molecular interactions weight weight PAAS. PAAS. was greater for with moderate molecular weight PAAS than for with ultra-high molecular weight PAAS.

8 Gels 2017, 3, Experiments 4.1. Samples Sample Preparation Laponite RD (Na 0.7 [(Si 8 Mg 5.5 Li 0.3 )O 20 (OH) 4 ]) was kindly supplied by RockWood Ltd., Aomori Prefecture, Japan. PAAS with weight-average molecular weight (from Wako Pure Chemical Industries) (from American Polymer Stards) were used in this study, y are hereafter denoted as PAAS3500K PAAS507K, respectively. TSPP decahydrate as a dispersant clay particles was obtained from Kanto Chemicals Co. The clay/paas hydros were prepared in manner mentioned below. After clay (Laponite RD) was added to a TSPP aqueous solution, solution was stirred for more than 20 min. Then clay solution was added to a PAAS solution. The final concentrations PAAS TSPP are 1 wt % 0.5 wt %, respectively, unless ir concentrations are not described in text or figure. The prepared mixture was degassed by centrifugation for 15 min at 7500 rpm. Both samples for tensile tests for synchrotron SAXS experiments were placed in a spacer with a thickness 1 mm. The samples were swiched between Teflon sheets, n a weight was placed on m. After samples were placed for 2 days at room temperature (ca. 22 C), y were used for tensile or SAXS measurements Mechanical Measurements Tensile tests were conducted using an ORIENTEC TENSILE TESTER STM-20 at a stretching speed 10 mm/min. The cross-sectional area undeformed was used for calculation stress. Tensile stress-strain ( L/L 0 ) curves were measured for clay/paas3500k clay/ PAAS507K hydros, where L 0 L (=L L 0 ) denote initial length deformation, respectively. We conducted dynamic mechanical measurements with a compression mode for a 12.5 wt % clay/paas3500k blend hydro. The storage moduli E loss moduli E were obtained as a function frequency in range Hz by using a rheometer (Seiko Instruments, DMS 6100) Swelling Measurements We investigated time course swelling ratios clay/paas3500k blend hydros at different clay concentrations at different PAAS concentrations. Cylindrical samples with a diameter 6 mm a length 8 mm were used for swelling measurements. The samples were placed in a large amount water without replacing water, swelling ratios s were measured as a function time. The swelling ratios were estimated from ratio weight swelled W(t) to that dried W dry Synchrotron Small-Angle X-ray Scattering We conducted synchrotron SAXS experiments at beam-line 6A 10C (BL6A BL10C) Photon Factory at High Energy Acceleration Research Organization in Tsukuba, Japan. The SAXS measurements were performed with a wavelength 1.5 Å with a two-dimensional detector, Pilatus 1 M. The detected two-dimensional SAXS images were circular-averaged with a stware (Fit2D V by A. Hammersley [28]) for small-angle scattering data analysis, so that scattering intensity was obtained as a function magnitude wave vector q. The scattering prile was corrected for background scattering. Acknowledgments: Part this work was supported by JSPS KAKENHI Grant Number JP15K The synchrotron SAXS measurements were conducted under approval Photon Factory Program Advisory Committee.

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