Nanocomposite Gels by Initiator-Free Photopolymerization: Role of

Dec 14, 2017 - Nanocomposite hydrogels (NC gels) composed of polymer-clay networks are successfully synthesized via initiator-free (IF) photopolymeriz...
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Nanocomposite Gels by Initiator-Free Photopolymerization: Role of Plasma-Treated Clay in the Synthesis and Network Formation Kazutoshi Haraguchi,*,† Tetsuo Takada,‡ and Ryosuke Haraguchi§ †

Department of Applied Molecular Chemistry, College of Industrial Technology, Nihon University, Narashino, Chiba 275-8575, Japan ‡ LS Project, Central Research Laboratory, DIC Corporation, Sakado, Sakura, Chiba 285-8668, Japan § Department of Applied Chemistry, Institute of Science and Engineering, Chuo University, Bunkyo-ku, Tokyo 112-8551, Japan S Supporting Information *

ABSTRACT: Nanocomposite hydrogels (NC gels) composed of polymer−clay networks are successfully synthesized via initiator-free (IF) photopolymerization in aqueous media using plasma-treated clay. IF-NC gels with a high tensile strength, strain at break, and thermoresponsiveness, which are almost identical to those of NC gels prepared by redox and photoinitiator methods, are obtained by optimizing the plasma-treatment conditions and exfoliation in water. The clay nanosheets play important roles as initiators, multifunctional cross-linkers, and auxiliary agents for facilitating in situ free-radical polymerization toward IF-NC gels. The mechanism for the formation of the polymer−clay network in the IF-NC gels is clarified through Fourier transform infrared and electron spin resonance studies and involves the formation of hydroperoxides and radicals on the clay surface by plasma treatment and subsequent UV irradiation, respectively, and through designed free-radical polymerization experiments in the presence or absence of the clay nanosheets. Because of their simple and versatile syntheses, form and size diversities, and superb characteristics, IF-NC gels can be used in a variety of applications. The results provide important insight into super-hydrogels and organic−inorganic nanocomposites, as well as polymer synthesis and photochemistry. KEYWORDS: initiator-free, plasma-treated, clay, nanocomposite gels, photopolymerization

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using different types of polymers,10,11 cocrosslinked networks,21,22 polymer−clay binding units,5,12 and other inorganic components such as graphene oxide,23 carbon nanodots,24 clay nanotubes,13 and layered double hydroxide,25 have been developed by extension of the NC gel concept. NC gels have mostly been prepared by thermally initiated in situ free-radical polymerization of a monomer, such as an acrylamide derivative including N-isopropylacrylamide (NIPA) or N,N-dimethylacrylamide (DMAA), in the presence of CNSs in aqueous media.2,3 Most typically, an NC gel is prepared at 20 °C in a redox system with a thermal initiator and an accelerator (Scheme S1a, Supporting Information). Recently, we reported a new method for the synthesis of NC gels via photoinitiated in situ free-radical polymerization in aqueous media (Scheme S1b, Supporting Information).26 The resulting photopolymerized NC gels (photo-NC gels) also exhibited good mechanical and thermoresponsive properties similar to those of gels prepared by redox methods. In addition, the preparation of microscopic NC gels, such as in 1−100 μm thin films, thin coatings, and micropatternings that exhibit

INTRODUCTION Since nanocomposite hydrogels (NC gels) with unique organic (polymer)−inorganic (clay) network structures were first reported,1 they have received significant attention because of their extraordinary optical, mechanical, and swelling/deswelling properties that are superior to those of traditional chemically cross-linked polymer hydrogels.2−6 The polymer− clay network structure is fabricated using exfoliated clay nanosheets (CNSs) as super-multifunctional cross-linkers for long flexible hydrated polymer chains, in which a number of polymer chains interact with CNSs at multiple points to form 1 nm thick polymer aggregates on both sides of each nanosheet that act as planar cross-linkers as shown in the Supporting Information (Figure S1).7−9 The mechanical and swelling/ deswelling properties of NC gels are widely controlled by altering the network composition, including the clay (Cclay),2−4 polymer (Cp),3 and water6 concentrations, as well as the kinds of (co)polymer and clay.5,6,10−14 In addition, NC gels exhibit a number of new characteristics such as ultrahydrophobic gel− air interface surfaces,6 optical anisotropy upon uniaxial stretching,6,15 the generation of retractive tensile force6 or bending deformation,16 cell cultivation and subsequent cell detachment,6,17 nanoparticle formations,18,19 and self-healing properties.5,20 Furthermore, new types of NC gels, prepared © XXXX American Chemical Society

Received: December 5, 2017 Accepted: December 14, 2017 Published: December 14, 2017 A

DOI: 10.1021/acsanm.7b00264 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials interesting antifogging, thermoresponsive cell harvesting, and microchannel flow-control properties, becomes possible owing to the method used for the preparation of these photo-NC gels.26 Of further interest is the development of a new, cleaner, and more versatile synthetic method for NC gels that does not require an initiator. Herein, we report a synthesis of initiatorfree photo-NC gels (IF-NC gels) using plasma-treated clay, as shown in Scheme S1c in the Supporting Information. IF-NC gels can be utilized in a variety of applications due to their simple and versatile syntheses, form diversities, and superb characteristics.

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Scheme 1. Synthetic Procedure for the Preparation of an IFNC Gel

EXPERIMENTAL SECTION

Materials. DMAA and NIPA were provided by Kohjin Co., Japan. DMAA was purified by filtering through activated alumina. NIPA was purified by recrystallization from a toluene/n-hexane mixture (2/1 w/ w) and dried under vacuum at 40 °C. Other reagents were used without further purification. Potassium persulfate (KPS) and N,N,N′,N′-tetramethylethylenediamine (TEMED) were used as initiator and accelerator in redox system, respectively. The water used for all experiments was ultrapure water supplied by a Puric-Mx system (Organo Co., Japan). Dissolved oxygen in pure water was removed by bubbling nitrogen gas for more than 3 h prior to use, and oxygen was excluded from the system throughout the synthesis. As the inorganic clay, the synthetic hectorite “Laponite XLG” (Rockwood, Ltd., UK; [Mg5.34Li0.66Si8O20(OH)4]Na0.66: layer size = 30 nm in diameter × 1 nm in thickness;7 cation exchange capacity = 104 m equiv/100 g) was used after washing and freeze-drying. Synthesis of NC Gels. The procedure for the redox synthesis of the NC gel is the same as that reported previously.2,3 Here, NC gels with different Cclay (= n mole% (0.78 × n wt %)) and a constant Cp (= 1 M) were denoted as NCn gels similarly to the previous papers. Briefly, D-NC5 gel was synthesized from a transparent aqueous solution of inorganic clay (Laponite XLG, 0.76 g), monomer (DMAA, 1.98 g), and water (19 mL). An accelerator (TEMED, 16 μL) and aqueous solution of initiator (KPS, 0.02 g, 1 mL) were added while the mixture was stirred at 1 °C. Free radical polymerization was conducted in a water bath (20 °C) for 24 h. The NC gels were synthesized in glass tubes with inner diameter of 5.5 mm. Plasma Treatment. The clay (Laponite XLG) was plasma-treated (argon plasma, 60 W) in three ways (Figure S2, Supporting Information): (i) A clay layer (CL) was prepared by casting an aqueous clay dispersion (3 wt %) onto a glass plate (1 mg/cm2), followed by drying at room temperature and vacuum drying at 100 °C for 1 h. After the CL was plasma treated for 10 min, the plasmatreated clay was collected as a powder. (ii), (iii): Clay powder was prepared by washing in ethanol/water (90/10), freeze-drying, and grinding. The clay powder (∼50 μm in diameter) was then plasmatreated on a glass plate (50 mg/cm2), with or without the use of a vibrating plate, for prescribed times (tpla = 1−10 min). Synthesis of IF-NC Gels. The synthetic procedure for the preparation of an IF-NC gel is simple and versatile as shown in Scheme 1. IF-NC gels were prepared by in situ photopolymerization using plasma-treated clay and UV radiation (without a photoinitiator). The resulting gels are referred to as “IF-D-NCn” or “IF-NNCn” gels according to the clay concentration (n mol %) and monomer used (1 M DMAA (D) or NIPA (N)), respectively. For example, IF-D-NC5 gel was prepared by dispersing the plasma-treated clay (5 mol % (= 3.81 wt %)) in deoxygenated water for 20 min at 35 °C. The monomer (DMAA: 1 M) was then added at 20 °C. The resulting reaction mixture was poured into glass templates such as a glass tube (5.5 mm inner diameter) or glass plates (1 mm inner thickness) and irradiated with 365 nm UV light for 3 min while it was cooled in ice water. Polymerization yield was calculated from the weight of dried gels. Measurements. Tensile Mechanical Properties. Tensile measurements were performed on various D-NC and N-NC gels of the same size (ϕ 5.5 mm × 70 mm) using a Shimadzu Autograph AGS-H

under the following conditions: 25 °C; gauge length, 30 mm; crosshead speed, 100 mm min−1. The initial cross section was used to calculate the tensile strength (σ) and the initial tensile modulus (E). Fourier-Transfer Infrared (FTIR) Spectroscopy. Clay layer samples for FTIR measurements were prepared by casting an aqueous clay dispersion (3 wt %) onto a glass plate, followed by drying at room temperature and vacuum drying at 90 °C for 1 h. FTIR spectra were obtained using an FTIR 4200 spectrometer (JASCO Co., Japan). FTIR spectra were obtained for (i) the clay layer (CL), (ii) the clay layer after plasma treatment for 10 min and exposure to air for 3 min (p-CL), and (iii) the plasma-treated clay layer after UV (365 nm) irradiation for 3 min. Electron Spin Resonance (ESR) Spectroscopy. ESR spectra were obtained using an EMX EPR spectrometer (Bruker Biospin K. K.) with the following settings: sweep width 3500 ± 500 G, sweep time 83.89 s, frequency 9.619 GHz, modulation frequency 100 kHz, time constant 40.96 ms, and room temperature. ESR spectra were acquired for two types of aqueous clay dispersions (Cclay = 5 mol % (3.81 wt %)) with a spin trapping agent (5,5-dimethyl-1-pyrroline-N-oxide (DMPO), 200 ppm): (i) an aqueous dispersion of the plasma-treated clay while irradiating with UV (365 nm) light, and (ii) the same dispersion but without UV irradiation. ESR spectra of aqueous dispersions of nonplasma-treated clay, with and without UV irradiation were also measured. The ESR spectrum of an aqueous KPS solution (10 g) containing KPS (5 × 10−5 mol) and DMPO (9 × 10−6 mol) was also acquired; TEMED (10 μL) was added to the KPS solution immediately prior to measurement. Viscosity. The viscosities of the reaction solutions were measured using a sine-wave vibro-viscometer (frequency = 30 Hz; SV-10: A&D Co., Japan) at 20 °C. Swelling−Deswelling Ratio. Swelling and deswelling experiments were performed by immersing the as-prepared N-NC5 gel (initial size 5.5 mm diameter × 30 mm length) in a large excess of water at 20 and 50 °C, respectively. The gel was kept in water alternatively at 50 °C for 5 h and at 20 °C for 25 h. The change in gel weight was represented by W(t)/W50(5), where W(t) is the gel weight at the specific time t (h), and W50(5) is the gel weight after submersion for the first 5 h at 50 °C.

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RESULTS AND DISCUSSIONS IF-NC gels were obtained in ∼96% yield by the UV-irradiation of aqueous solutions containing plasma-treated hectorite and DMAA or NIPA at 20 °C (Scheme 1). Figure 1 shows uniform mechanically tough IF-D-NC5 gels (a) and IF-N-NC5 gel (b) prepared using plasma-treated clay (CL: 1 mg/cm2, tpla = 10 min). To date, several reports on the plasma treatment of clay have appeared. In all cases, the clay was treated by plasma to improve its dispersion in polymer nanocomposites.27−30 For B

DOI: 10.1021/acsanm.7b00264 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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of the D-NC5 gel prepared by the redox method (Figure 2a(i)). When the plasma treatment was conducted on freezedried clay powder (ϕ 50 μm) on a glass plate at 50 mg/cm2, the IF-D-NC5 gel exhibited a low initial tensile modulus (E) and tensile strength (σ) (Figure 2a(ii)), while these values were improved by plasma treatment on a vibrating plate, because the clay powder was more uniformly plasma-treated (Figure 2a(iii)). The increases in E (and σ) in Figure 2a(i)− (iii) indicate the increase in effective cross-linking on the clay nanosheets due to the effective plasma-treatment. E and σ were observed to increase with increasing plasma-treatment time (tpla) and became saturated at 5 min (Figure 2b); maximum E and σ values were obtained at a dispersion time in water (td) of 20 min (Figure 2c). This is probably due to insufficient clay exfoliation at td below 20 min and deactivation of the clay surface at longer td. Thermoresponsive IF-N-NCn gels were also prepared by the initiator-free method under the conditions described for the IF-D-NCn gels; these gels exhibited welldefined thermoresponsive transparency changes (Figure 3a) and swelling−deswelling behavior (Figure 3b), as well as high tensile mechanical properties analogous to those of photo-NNC gels26 prepared using a photoinitiator. The suggested process for the synthesis of an IF-NC gel is as follows (Figure 4). Many siloxane bonds (Si−O−Si) and silanol groups (SiOH) exist on the surface of clay (hectorite) because of its SiO2−Mg5.34(Li0.66)O−SiO2 sandwich structure (Figure 4(i)). Treatment of the clay with argon plasma and subsequent exposure to air (oxygen) generates peroxides such as Si−O−O−H and Si−O−O−Si on its surface (Figure 4(ii) and (iii)). When dispersed in water, the plasma-treated clay is gradually cleaved into nanosheets. UV irradiation of the

Figure 1. Mechanically tough (a) IF-D-NC5 gel and (b) IF-N-NC5 gel.

example, plasma-treated bentonite improved the dispersion and mechanical properties of a polymer-blend−bentonite nanocomposite.27 Surface modification by grafting organic groups or polymers via plasma treatment improved the mechanical properties and/or proton conductivities of polystyrene-bentonite,28 polyethylene-clay,29 and Nafion-clay nanocomposites.30 However, to the best of our knowledge there are no reports that use plasma-treated clay as a photopolymerization initiator or in the synthesis of a polymer hydrogel. The plasma-treatment conditions affected their mechanical properties. For example, when a thin clay layer (1 mg/cm2) was subjected to plasma-treatment, the resulting IF-D-NC5 gel displayed an almost identical tensile stress−strain curve to that

Figure 2. (a) Stress−strain curves for IF-D-NC5 gels prepared using clay that was plasma-treated in different ways (i−iii) and the D-NC5 gel prepared by the KPS redox system. (i) A thin CL (1 mg/cm2) cast onto a glass plate. (ii) Clay powder (ϕ 50 μm). (iii) Clay powder treated on a vibrating plate. (b) Effect of tpla on the E and σ values of IF-D-NC5 gels. (c) Effect of td of the plasma-treated clay in water at 35 °C on the E and σ values of IF-D-NC5 gels. C

DOI: 10.1021/acsanm.7b00264 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Figure 3. Thermoresponsive IF-N-NC5 gel. (a) Transparency change and (b) swelling−deswelling behaviors.

Figure 4. Process for the initiator-free synthesis of an NC gel using plasma-treated clay and UV radiation. (i) The 2:1 layered crystal structure of the clay (hectorite) nanosheet. (ii) Plasma treatment. (iii) Formation of peroxides on the surface of the clay nanosheet. (iv) Addition of the monomer. (v) UV irradiation (365 nm). (vi) In situ photopolymerization. (vii) Formation of the polymer−clay network.

though only some CNSs (upper or outer parts of the CL or powder) are plasma-treated (Figure 6a). This result suggests that radicals produced on the plasma-treated surface diffuse over a wide range to induce free-radical polymerization at the peripheries of other CNSs, irrespective of plasma-treatment. We envisage that CNSs play important roles as auxiliary agents in reactive regions for the latter process. To confirm this hypothesis, the following photopolymerization experiments, in which radicals were only produced on the surface of p-CL placed in the bottom of an aqueous monomer/clay (nontreated) or monomer solution, were conducted (Figure 6b,c). An NC-gel conglomerate was obtained by UV irradiation of pCL in the former solution (Figure 6b), while no gel formation or polymerization proceeded in the case of the aqueous monomer solution (Figure 6c), indicating that polymerization of DMAA not only requires radicals formed from p-CL but also the nontreated CNSs dispersed in the monomer solution, leading to the formation of the NC gel conglomerate (the polymer−clay network depicted in Figure 6d). The effect of Cclay in the monomer/clay mixture on NC gel formation is shown in Figure 6e, where the critical Cclay required to form the NC-gel conglomerate was determined to be ∼2 mol %. On the basis of the above results, a possible reaction mechanism is proposed (Scheme 2). (a) Hydroxyl radicals formed through cleavage of the O−O bonds of hydroperoxides on the p-CL or (b) C radicals (Ι) generated by the addition of HO· to a double bond in DMAA, diffuse widely in the aqueous monomer/clay dispersion. (c1) HO· reacts with DMAA that is electrophilically activated by hydrogen bonding with the SiOH groups on the CNS, or Ι (or its oligomers) become adsorbed on CNSs through hydrogen bonding. (c2) Chain propagation

aqueous mixture induces homolytic O−O bond cleavage to provide O-radicals (Figure 4(iv) and (v)) that react with DMAA in the vicinity of the CNSs to give C-radicals that propagate the chain (Figure 4(vi)). Consequently, an IF-NC gel with a polymer−clay network structure (Figure 4(vii)) is obtained. In the synthetic process described above, the key processes are the formation of peroxides on the clay surface by plasma treatment and radicals by subsequent UV irradiation. Figure 5a(i) and (ii) displays the FTIR spectra acquired for the CL cast on a glass plate and the p-CL obtained after plasma treatment for 10 min followed by air exposure, respectively. A band attributed to the stretching of a hydroperoxide group (∼2900 cm−1) is clearly observed in the spectrum of p-CL, confirming that Si−O−O−H is formed on the clay surface by plasma treatment followed by air exposure. Figure 5b displays the ESR spectra of aqueous untreated and plasma-treated clay dispersions in the presence of 200 ppm of DMPO as a spin trap, with and without UV irradiation. The ESR spectra of aqueous dispersions of untreated clays were identical regardless of UV irradiation, and ESR signals were only observed when the clay was subjected to both plasma treatment and UV irradiation (Figure 5b(i) and (ii) and Figure S3 in the Supporting Information), confirming that radicals are formed by UV irradiation of the plasma-treated clay dispersed in water. Furthermore, the FTIR-hydroperoxide peak had mostly disappeared upon UV treatment (Figure 5a(iii)), which is consistent with the ESR results. The IF-NC gel formation mechanism raises an important question: Why is the polymer−clay network structure of the IF-NC gel uniformly formed throughout the sample, even D

DOI: 10.1021/acsanm.7b00264 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Figure 5. (a) FTIR spectra of CL cast onto glass plates. (i) No plasma treatment. (ii) Plasma treatment (10 min). (iii) Plasma treatment (10 min) and UV irradiation (3 min). (b) ESR spectra of aqueous plasma-treated clay dispersions (i) with and (ii) without UV irradiation. Aqueous solutions of nontreated clay with and without UV irradiation provide identical spectra to that shown in (ii). In all cases, DMPO (200 ppm) was added as a radical trap.

proceeds by successive addition reactions. Here, carbonyl groups in the polymer chain form hydrogen bonds with the SiOH on the clay to establish multiple interactions between the polymer chain and clay surface. However, in an aqueous monomer solution (without clay), both radicals may terminate by recombining with other radicals due to the low solution viscosity (high diffusion rate; Figure 6c) and insufficient CNSs or monomers in their vicinities. In addition, the SiO· radical on p-CL may react with DMAA, but this will lead to termination by recombination with other radicals including neighboring SiO· (Scheme S2, Supporting Information). This mechanism is in good agreement with the

experimental results for IF-NC gels in the present study, as well as the NC gels prepared by in situ free radical polymerization under redox conditions or in photosystems. In conclusion, uniform and mechanically tough IF-NC gels were successfully synthesized by an initiator-free photopolymerization process involving the UV irradiation of a simple aqueous dispersion composed of only the monomer and plasma-treated clay. By optimizing the plasma treatment and exfoliation conditions, IF-NC gels with high tensile mechanical properties and good thermoresponsiveness, which are almost identical to those of NC gels prepared by the redox and photoinitiator methods, were obtained. The formation of E

DOI: 10.1021/acsanm.7b00264 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Figure 6. (a) The formation of the polymer−clay network in an IF-NC gel using locally plasma-treated clay (mainly on the upper part). (b) UV irradiation of a 1 M DMAA/clay (nontreated: 5 mol %) aqueous dispersion over p-CL; the NC gel forms on the p-CL. (c) UV irradiation of a 1 M DMAA aqueous solution over the p-CL. The solution viscosity (1.2 mPa·s) did not change upon UV irradiation, and no gel is formed. (d) The preparation of an NC gel on p-CL in an aqueous monomer/(nontreated) clay dispersion by UV irradiation. (e) The effect of Cclay on the gel yield (wt %) in (b). (i)−(iii) Photographic images of the gel obtained at each Cclay.

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hydroperoxides and radicals on the surface of clay nanosheets by plasma-treatment and subsequent UV irradiation were clarified for the first time through FTIR and ESR studies. A mechanism for the formation of the polymer−clay network in the IF-NC gels is proposed on the basis of analytical data (FTIR and ESR) and designed in situ photopolymerization experiments in the presence or absence of the clay nanosheets. We revealed that plasma-treated clay nanosheets play important roles not only as multifunctional cross-linkers but also as initiators and auxiliary agents for facilitating in situ freeradical polymerization toward IF-NC gels. To our knowledge, this is the first example of a successful initiator-free photopolymerization using plasma-treated clay and also the first example of the initiator-free synthesis of a mechanically tough super-hydrogel. The IF-NC gels can be used in a variety of applications that exceed those demonstrated by normal and photo-NC gels,6,11,26 including biomedical, cell-harvesting, selfhealing, absorbing, microfluidic, adhesive, robotic, and sensorsystems applications, because of their simple and versatile syntheses, the ability to readily form a variety of gel forms with different sizes, and their superb characteristics. The results of the present study, including the significant role that the clay plays in the in situ free-radical polymerization process, are broadly interesting and provide important insight into hydrogels and nanocomposites, as well as polymer synthesis and photochemistry.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.7b00264. Illustrated polymer−clay network structure of an NC gel, where exfoliated clay nanosheets act as multifunctional cross-linkers for long flexible hydrated polymer chains. Illustrated experimental procedure for the initiator-free synthesis of an NC gel. The inorganic clay, in the form of a clay layer (CL) cast on a glass plate, or clay powder with or without the use of a vibrating plate, is first pretreated by argon plasma (pCL). ESR spectrum of an aqueous solution of KPS/ TEMED, a typical radical source, in the presence of DMPO (200 ppm) as a radical trap. Three types of NC gel syntheses, redox, photopolymerization, and initiatorfree photopolymerization systems. Illustrated reaction mechanism involving the Si−O· radical on p-CL and possible termination reactions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kazutoshi Haraguchi: 0000-0003-0919-3024 Ryosuke Haraguchi: 0000-0001-6703-8036 F

DOI: 10.1021/acsanm.7b00264 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials Scheme 2. Proposed Mechanism for the in situ Photopolymerization of IF-NCa

a

(a) Formation of the hydroxyl radical. (b) Addition of the hydroxyl radical to DMAA. (c1) Addition of the hydroxyl radical to DMAA activated by SiOH on a CNS. (c2) The propagation reaction.

Notes

(7) Haraguchi, K.; Li, H.-J.; Matsuda, K.; Takehisa, T.; Elliott, E. Mechanism of Forming Organic/Inorganic Network Structures during In-situ Free-Radical Polymerization in PNIPA-Clay Nanocomposite Hydrogels. Macromolecules 2005, 38, 3482−3490. (8) Miyazaki, S.; Endo, H.; Karino, T.; Haraguchi, K.; Shibayama, M. Gelation Mechanism of Poly(N-isopropylacrylamide)-Clay Nanocomposite Gels. Macromolecules 2007, 40, 4287−4295. (9) Haraguchi, K.; Xu, Y.; Li, G. Molecular Characteristics of Poly(N-isopropylacrylamide) Separated from Nanocomposite Gels by Removal of Clay from the Polymer/Clay Network. Macromol. Rapid Commun. 2010, 31, 718−723. (10) Haraguchi, K.; Murata, K.; Takehisa, T. Stimuli-Responsive Nanocomposite Gels and Soft Nanocomposites Consisting of Inorganic Clays and Copolymers with Different Chemical Affinities. Macromolecules 2012, 45, 385−391. (11) Ning, J.; Li, G.; Haraguchi, K. Synthesis of Highly Stretchable, Mechanically Tough, Zwitterionic Sulfobetaine Nanocomposite Gels with Controlled Thermosensitivities. Macromolecules 2013, 46, 5317− 5328. (12) Tamesue, S.; Ohtani, M.; Yamada, K.; Ishida, Y.; Spruell, J. M.; Lynd, N. A.; Hawker, C. J.; Aida, T. Linear versus Dendritic Molecular Binders for Hydrogel Network Formation with Clay Nanosheets: Studies with ABA Triblock Copolyethers Carrying Guanidinium Ion Pendants. J. Am. Chem. Soc. 2013, 135, 15650− 15655. (13) Liu, M.; Li, W.; Rong, J.; Zhou, C. Novel Polymer Nanocomposite Hydrogel with Natural Clay Nanotubes. Colloid Polym. Sci. 2012, 290, 895−905. (14) Gao, G.; Du, G.; Sun, Y.; Fu, J. Self-Healable, Tough, and Ultrastretchable Nanocomposite Hydrogels Based on Reversible Polyacrylamide/Montmorillonite Adsorption. ACS Appl. Mater. Interfaces 2015, 7, 5029−5037.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant No. 15H03870. The authors thank the analytical center of DIC Corp. for their support to perform the analyses.

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REFERENCES

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DOI: 10.1021/acsanm.7b00264 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsanm.7b00264 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX