Instant Strong Adhesive Behavior of Nanocomposite Gels toward

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Instant Strong Adhesive Behavior of Nanocomposite Gels toward Hydrophilic Porous Materials Kazutoshi Haraguchi, Shoichi Shimizu, and Satoshi Tanaka Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01448 • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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Instant Strong Adhesive Behavior of Nanocomposite Gels toward Hydrophilic Porous Materials Kazutoshi Haraguchi,* Shoichi Shimizu, Satoshi Tanaka

Department of Applied Molecular Chemistry, College of Industrial Technology, Nihon University, 1-2-1 Izumi-cho, Narashino, Chiba 275-8575, Japan.

*Corresponding Author E-mail:[email protected]

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ABSTRACT

We investigated the adhesion behavior of nanocomposite hydrogels (NC gels), consisting of unique organic (polymer)-inorganic (clay) network structures, toward inorganic and organic materials. The NC gels exhibit instant and strong adhesion to inorganic and organic substrates with hydrophilic porous surfaces. The NC gels instantly adhere to hydrophilic porous substrates (e.g., unglazed ceramic surfaces and polymer membranes) through simple light contact. In addition, a small piece of NC gel effectively joined two substrate samples (e.g., concrete blocks and bricks) through lamination of the interposing NC gel. The resulting conjoined materials were unable to be separated at the gel-substrate interface, rather the gel itself fractured upon separation, which indicates that the adhesive strength at the interface is greater than the tensile strength of the NC gel. With the exception of NC gels with very high clay concentrations (Cclays), instant strong adhesion, and cohesive failure by subsequent stretching, were observed for almost all NC gels composed of different polymers or different Cclay values. A thermoresponsive NC gel was reversibly adhered and could be peeled from the surface by stretching (adhesive failure) at a temperature above its transition temperature. The mechanism of instant strong adhesion or reversible adhesion are discussed based on dangling chains that exist on the surfaces of the NC gels composed of polymer-clay networks. The cut surface of an NC gel generally exhibited a higher adhesive strength than that of the as-prepared surface due to longer dangling chains.

KEYWORDS adhesion, nanocomposite gels, porous materials, clay-polymer network, dangling chains


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INTRODUCTION As unique soft and wet functional materials, polymer hydrogels have received significant levels of attention. To date, polymer hydrogels have been used in a wide range of applications owing to their unique characteristics; example include super-absorbent polymeric gels for disposable diapers and sanitary products (water absorption),1 transparent convex-shaped hydrogels for soft contact lenses (transparency and oxygen permeability),2 refrigerants for the medical and electronics industries (heat of fusion),3 and microchips and soil amendments (solute adsorption/desorption),4,5 among others. In addition, nanosized polymer and supramolecular hydrogels have recently been developed for biomedical applications as molecular chaperones,6 drug-delivery carriers,7 nanomedicines,8 and protein/peptide arrays.9 Polymer hydrogels are most commonly synthesized by copolymerizations with organic crosslinkers or by γ-ray irradiation of aqueous polymer solutions. However, the resulting polymer hydrogel, which is composed of chemically cross-linked networks (referred to as an “OR gel”), was always very weak and brittle due its network structure that consisted of large numbers of randomly distributed chemical crosslinks.10,11 Consequently, OR gels were difficult to synthesize and handle as bulk hydrogels in the forms of sheets, films, or tubes. In addition, the thermoresponsivenesses of OR gels are generally restricted, particularly those of macroscopic size, due to their network structures.10 These severe drawbacks associated with OR gels have been simultaneously solved through the development of new types of hydrogel with tailored network structures.12-15 Among them, nanocomposite hydrogels (NC gels) with unique organic (polymer)-inorganic (clay) network structures (Figure 1) exhibit extraordinary optical, mechanical, and thermoresponsive properties that are superior to those of conventional OR gels.10,13,16 In the network, inorganic clay nanosheets (CNSs), with thicknesses of 1 nm and diameters of several tens of nm, act as super


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multifunctional crosslinking agents for long, flexible polymer chains.16 NC gels have been prepared by the thermally initiated or photo-initiated17,18 in situ free-radical polymerization of a monomer, such as an acrylamide derivative (including N-isopropylacrylamide (NIPA) and N,Ndimethylacrylamide (DMAA)), in the presence of CNSs in aqueous media.

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Figure 1. Depicting the organic (polymer)-inorganic (clay) network structure of an NC gel and the structure of clay nanosheet.16

The resulting NC gels have excellent mechanical properties; consequently, they can withstand large deformations through compression, stretching, bending, and tearing. In terms of stretching, NC gels exhibit large elongations at break (εb) that exceed 1000%, high tensile strengths (TSs), and initial moduli (Es) that are controlled over wide ranges depending on the types of polymer and clay, and their concentrations; e.g., TS = 30−1250 kPa, E = 1−450 kPa at a constant polymer concentration (1 M). In addition, NC gels are readily prepared in a variety of forms, such as films, sheets, blocks, tubes, and bellows (etc.), and in different sizes (10-5–100 m).16,21 This is


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partly because in situ polymerizations can be performed in arbitrarily shaped vessels or templates without stirring, simply by standing at room temperature (or a mildly elevated temperature) or by irradiation with ultraviolet light, and partly because the resulting gel is very mechanically tough. In addition to their excellent properties and available shapes, NC gels exhibit a number of new characteristics derived from their unique polymer-clay network structures that include optical anisotropies and unique changes through stretching and recovery,19 contractive force generation, due to coil-to-globule transitions, comparable to that generated by human muscle,20 and the abilities to completely self-heal through the autonomic reconstruction of crosslinks across damaged interfaces.21 In addition, they exhibit a variety of interfacial properties, such as surface water-sensitive sliding frictional behavior, abnormally high water contact angles at gel-air interfaces, and the abilities to cultivate a variety of cell types and subsequent thermoresponsive cell detachment.16,22 The adhesiveness of a hydrogel is also an important property that is widely useful in industrial, architectural, civil engineering, and biomedical applications. Several reports to date have described improving hydrogel adhesion from the perspectives of tissue adhesion,23,24 polymernetwork control,25,26 and 3D bioprinting.27 In addition, basic studies on creating tough adhesion between a hydrogel and various substrates or tissue, using different strategies, have been conducted.28-35 In particular, additives such as nanoparticles (e.g., silica nanoparticles),28 bridging polymers (e.g., polymers with positively charged primary amine groups, polyelectrolyte complexes, or chitosan),29-31 and specific monomers (e.g., cyanoacrylate-based dispersions),32 and hydrogels (e.g., polyampholyte hydrogels)33 have been used as adhesives that form tough bonds between hydrogels, or hydrogels and substrates. In other cases, substrates (e.g., polymerbrush formation or silanization)34,35 and hydrogels (e.g., long-chain polymer networks with


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alkoxysilyl groups)35 have been chemically modified in order to realize tough and/or reversible hydrogel adhesion; however, all of these methods require specific bonding agents (adhesives), or substrate and/or hydrogel modifications. In the present article, we report new interesting properties of NC gels in relation to their instant and strong adhesion behavior toward organic and inorganic porous materials, without the need for any additive or NC-gel modification.

EXPERIMENTAL SECTION Materials: N,N-Dimethylacrylamide (DMAA) and N-isopropylacrylamide (NIPA) were provided by Kohjin Co., Japan. DMAA was purified by filtering through activated alumina, while NIPA was purified by recrystallization from a 2:1 (w/w) toluene:n-hexane mixture, followed by drying 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 the initiator and accelerator in the redox system, respectively, while N,N’methylenebis(acrylamide) (BIS) was used as the organic cross-linker. Ultrapure water from a Puric-Mx system (Organo Co., Japan) was used in all experiments. Dissolved oxygen in pure water was removed by bubbling nitrogen gas through it for more than 3 h prior to use, and oxygen was excluded throughout the synthesis procedures. Synthetic hectorite “Laponite XLG” (Rockwood, Ltd., UK; [Mg5.34Li0.66Si8O20(OH)4]Na0.66: layer size = φ ~30 nm × 1 nm in thickness; cation exchange capacity = 104 m equiv./100 g) was used as the inorganic clay, which was washed and freeze-dried prior to use. Commercially available porous materials, such as unglazed orchid pots or tiles, concrete blocks, and bricks, were used as inorganic substrates, while commercial products such as hydrophilic and hydrophobic polymer membranes listed in Table S1 were used as organic polymer substrates.


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Substrate treatment: Hydrophobic metal plates and polymer films with smooth surfaces, were roughened by sandblasting. Hydrophobic polymer membranes and polymer films were treated with 30 W argon plasma for 3 min to afford hydrophilic surfaces. Nomenclature: The NC gels are referred to as “D-NCn” or “N-NCn” gels according to the clay concentration Cclay (= n mol% (0.78 × n wt%)) and monomer used (DMAA (D) or NIPA (N)). The PDMAA- and PNIPA-based OR gels prepared using m mol% of the organic crosslinker (CBIS) are referred to as “D-ORm” and “N-ORm” gels, respectively, or simply as “ORm gels”. Synthesis of NC gels: Thermostable D-NCn and thermoresponsive N-NCn gels, were prepared by the in situ free radical polymerization of DMAA and NIPA, respectively, in the presence of CNSs in aqueous media. The monomer concentration was 1 M and Cclay (n) varied between 1 and 20 mol%. The compositions of the NC gels prepared in the present study are summarized in Table S1. The procedure for the redox synthesis of the NC gels at 20 °C is the same as that reported previously.10,16 Briefly, the D-NC4 or N-NC4 gel was synthesized from a transparent aqueous solution of inorganic clay (Laponite XLG, 0.61 g), monomer (DMAA: 1.98 g, or NIPA: 2.26 g), and water (19 mL). The accelerator (TEMED, 16 µL) and an aqueous solution of initiator (KPS, 0.02 g, 1 mL) were added with stirring 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 diameters of 5.5 or 10 mm, or in film templates with inner thicknesses of 1 or 2 mm. The D-OR1 gel was prepared using DMAA (1.98 g, 1 mol L-1 in H2O) and an organic cross-linker (BIS) at a concentration of 1 mol% (0.028 g) relative to DMAA. Polymerization yields were calculated from the weights of the dried gels.


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Adhesion between the NC gels and substrates: An NC gel was adhered to a substrate using the following two methods: (1) Simple contact, or (2) by laminating the two substrates with the NC gel, whereby a small piece of NC gel was set between two substrates and light manual pressure was applied. Measurements: Tensile mechanical properties: Tensile measurements were performed on various D-NCn and N-NCn gels of the same size (φ 5.5 mm × 70 mm) using a Shimadzu Autograph AGS-H tensile tester under the following conditions: 25 °C; gauge length, 30 mm; and crosshead speed, 100 mm min-1. The initial cross section was used to calculate the TS and E. Scanning-Electron Microscopy: The surface microstructures of the substrates were examined using by scanning-electron microscope (SEM, JSM5800LVS, JEOL) after coating with Au using by JED200 (JEOL). Nitrogen adsorption–desorption analysis: Nitrogen adsorption–desorption data for unglazed ceramics was obtained using a BELSORP-18 (BEL JAPAN Inc.) operated at 77 K. Prior to measurement, the sample was added to the measurement cell, which was placed in a drying machine and heated at 100 °C for 5 h. Water surface contact angle: The water surface contact angle (θw) was measured using an optical tensiometer (WPI-3000: Kyowa Kagaku Co. Japan); θw is defined as the angle formed by a small water droplet (8 µL) at the co-contact point between the air, liquid, and solid phases. For porous materials, it was envisaged that the water droplet and its contact angle on the surface would change with elapsed time due to the penetration of water into the porous substrate, in addition to the usual evaporation of water into the atmosphere. Therefore, in order to reveal the stabilities of the droplets on these surfaces, we measured the time dependence of θw for porous substrates.


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Control of water content: The water content of the D-NC4 gel (RH2O) was controlled over a wide range, from 150 to 1600 wt%, by swelling (in water) or careful drying (in air). The resulting swollen and dried gels were subsequently stored in closed vessels at 20 °C for 4 d prior to mechanical testing in order to achieve uniform distributions of water in the gel samples. The water content (RH2O) is defined as follows: RH2O = {WH2O/Wdry} × 100. Here, WH2O + Wdry = Wgel, and WH2O, Wdry, and Wgel are the weights of the water, dried gel, and hydrogel, respectively.

RESULTS AND DISCUSSION Adhesion of NC gels to inorganic porous materials. Two types of NC gels, thermostable DNCn and thermoresponsive N-NCn gels with different Cclay (1 ≤ n ≤ 20), were prepared as transparent and mechanically tough hydrogels (Figure 2) through in situ free radical polymerization at 20 °C (redox system). As reported previously,10,16 the tensile mechanical properties (TS, E, and εb) of the NC gels varied widely and depended on Cclay and the type of monomer used; for example, the D-NC4, D-NC10, D-NC15, and N-NC4 gels exhibited TS values of 120, 410, 650, and 125 kPa, E values of 4.5, 16, 38 and 11 kPa, and εb values of 1600, 1450, 1400, and 1020%, respectively. The PNIPA-based NC4 gel (N-NC4 gel) exhibited a thermoresponsive transition at its lower critical solution temperature (LCST = 34 °C), above which the gel became opaque and the surface became hydrophobic (Figure 2(iii)), which is ascribable to the coil-to-globule transition of PNIPA in aqueous media.


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(i)

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(ii) Figure 2. Transparent and mechanically tough NC gels: (i) D-NC4 gel (rod), (ii) D-NC10 gel (film), and (iii) N-NC4 gel (rod).

We found that the NC gels adhered strongly to inorganic porous substrates, such as unglazed ceramics, concrete blocks, and bricks, through light contact at ambient temperature. Figure 3a displays the instant strong adhesion of the D-NC4 gel (φ 5.5 mm, cross-sectional (cut) surface) to an unglazed ceramic object (the surface of an orchid pot) and its subsequent stretching. Through simple light contact with the unglazed ceramic surface (Figure 3a(i), (ii)), the D-NC4 gel was instantly and strongly adhered to it and was not peeled off by subsequent stretching until elongated by 1500% (Figure 3a(iii), (iv)). The D-NC4 gel ultimately fractured in the center with extended stretching due to cohesive failure. When the cut NC-gel surface was partly contacted with the substrate surface, the gel often fractured near the interface to leave behind a petaloid gel due to stress concentration (Figure 3a(v)). This instant and strong adhesion and cohesive failure by subsequent stretching was not only observed for the cross-sectional (cut) surface, but also for the as-prepared surface of the D-NC4 gel, as shown in Figure 3b. Instant strong adhesion behavior toward unglazed ceramic surfaces was observed for all types of NC gel, irrespective of


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thermoresponsiveness; i.e., the N-NC4 gel (Figure 3c; as-prepared surface) and NCn gels with different Cclay (n) with n in the 1−10 range. When two pieces of each inorganic substrate (unglazed tile, concrete block, or brick) and a small piece of D-NC4 gel (φ 13 mm, 10 mm long, cross-sectional surface) were laminated at ambient temperature (Figure 4a(i)), the substrates became firmly joined, as shown in Figures 4b(i) (concrete blocks) and 4b(ii) (bricks). Because the two substrates were strongly bonded by the D-NC4 gel, it was difficult to separate them manually. By inserting a tool between the substrates and subsequent lifting (Figure 4a(ii)), the two substrates could be separated, although

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Figure 3. Instant strong adhesion of NC gels to an unglazed ceramic surface (orchid pot) by simple contact. (a) D-NC4 gel (rod, cross-sectional surface): (i) before contact, (ii) contact, (iii, iv) stretching, and (v) fractured D-NC4 gel. (b) D-NC4 gel (rod, as-prepared surface): (i) before


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contact, (ii) contact, (iii–v) stretching, and (vi) after breakage. (c) N-NC4 gel (rod, as-prepared surface): (i) before contact, (ii) contact and stretching, and (iii) stretching. separation did not take place through the peeling of the NC gel, but occurred through gel fracturing (cohesive failure) instead. Figures 4c(i) and 4c(ii) show the separating and separated stages for the unglazed-tile/D-NC4-gel and concrete-block/D-NC4-gel systems, respectively. In all cases, the D-NC4 gel interposed between two substrates was never removed by peeling, but fractured during the course of separation. These results indicate that the adhesive strength (AS) at the interface between the D-NC4 gel and the inorganic porous substrate is larger than the TS of the D-NC4 gel in all cases.

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Figure 4. (a) Adhesion and separation processes for the D-NC4-gel/unglazed-tile system: (i) adhesion, (ii) separation by tool, (iii) heating (70 °C, 30 min), and (iv) lack of separation by tool. (b) Objects conjoined by the D-NC4 gel: (i) concrete blocks and (ii) bricks. (c) (i) Separation process for unglazed tiles after using a tool and (ii) concrete blocks following separation (D-NC4 gel was colored for clarity). When two unglazed tiles were conjoined by the D-NC4 gel followed by heating at 70 °C for 30 min (Figure 4a(iii)), the resulting laminate could not be separated even with a tool; instead the upper part of the unglazed tile broke (Figure 4a(iv)). We ascribe this increase in the adhesive strength of the D-NC4 gel toward unglazed ceramics to a decrease in the water content of the DNC4 gel (to 25% of the original value) upon heating in the laminated state. The increase in tensile strength of the NC gel with decreasing RH2O was confirmed by tensile testing NC gels with different RH2O values (Figure 5); the D-NC4 gel with 20% of its original water content exhibited approximately five times the TS (400 kPa) of the as-prepared gel.

Figure 5. Stress-strain curves for D-NC4 gels with different RH2O values (wt%); the numbers in the figure are RH2O values. The RH2O of the as-prepared D-NC4 gel was 772 wt%. The dashed


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line highlights the failure envelope obtained by connecting the rupture points.

The inorganic substrates used in the present study have hydrophilic and porous surfaces. SEM micrographs displaying the surface morphology of the unglazed ceramic (orchid pot) are shown in Figure 6a. Although the surface did not show a regular porous structure on the micrometer scale, the unglazed ceramic was porous on the nanometer scale as revealed by its rapid waterabsorbing capability (Figure S1), in which half of the equilibrium water uptake (~15 wt%) was quickly absorbed within 10 s, and its BET surface area (17.7 m2 g-1) and average pore size (3.3 nm) that were determined from the N2-adsorption-desorption isotherm. Because the surfaces of the hydrophilic porous substrates are not only hydrophilic, but also have water-absorption properties, that is, a water droplet (8 µL) is rapidly absorbed into the substrate, θw could not be measured for these substrates. For example, the water droplet dropped onto the concrete block was immediately absorbed, while that dropped onto the unglazed surface was absorbed within 15 s. In contrast, the D-NC4 gel did not adhere to inorganic substrates with smooth hydrophobic surfaces, such as metal (e.g., SUS: θw = 75°) plate. In addition, the D-NC4 gel did not adhere to a roughened SUS plate (2.8 µm roughness, Figure S2a) produced by sandblasting. Furthermore, the D-NC4 gel did not adhere at all to sandpaper samples (#120, #800) that have rough but hydrophobic surfaces. Hence, we conclude that the instant strong adhesion of D-NC4 and the other NC gels to inorganic substrates and the joining of the two substrates are achieved when the substrates have hydrophilic porous surfaces. Subsequent stretching of the joined body and cohesive failure revealed that the AS at the gel-substrate interface is greater than the strength of the gel itself; i.e., AS > 120 kPa and 410 kPa for the D-NC4 and D-NC10 gels, respectively. Moreover, the AS increased with decreasing RH2O of the gel in the laminated state.


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Adhesion of NC gels to porous polymer films. The adhesion of NC gels to organic polymer films with different hydrophilic and/or porous properties was examined. Firstly, several types of polymer membranes (Table S2) with visually smooth surfaces but with fine porous

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Figure 6. SEM images of substrate surfaces. (a) Unglazed ceramic (orchid pot). (b) Membrane A (a mixed cellulose ester membrane). (c) Membrane B (a cellulose acetate membrane). (d) Membrane C (a PTFE membrane). morphologies at the microscopic level (as revealed by SEM) were studied. We found that membrane A (a mixed cellulose ester membrane: pore size 0.45 µm) and the D-NC4 gel (crosssectional surface) were instantly adhered through light contact between the membrane and the cut surface of the D-NC4 gel (1-mm-thick film), as shown in Figure 7a(i). Because the two materials were instantly and strongly adhered, they could not be separated by stretching (Figure 7a(ii)); instead the gel fractured near the middle. Such instant and strong adhesion of an NC gel to membrane A and the subsequent cohesive failure upon stretching were also similarly observed for other NCn gels; examples include the D-NC10 gel with high Cclay (Figure 7b: rod sample, φ 5.5 mm, cross-sectional (cut) surface) and the N-NC4 gel composed of a different polymer. The surface of membrane A is hydrophilic and has a porous morphology (Figure 6b). NC gels were also instantly and strongly adhered to other membranes with

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Figure 7. Instant and strong adhesion of NC gels (cross-sectional surface) to membrane A. (a) DNC4 gel (film, 1 mm thickness), (b) D-NC10 gel (rod, φ 5.5 mm): (i) instant adhesion by simple contact, and (ii) stretching following adhesion of the NC gel to membrane A. hydrophilic porous surfaces, such as membrane A* with different pore size (1.0 µm: Table S1) and membrane B (a cellulose acetate membrane, the surface morphology of which is shown in Figure 6c), through light contact. On the other hand, the NC gels did not adhere to membrane C (polytetrafluoroethylene: PTFE) that has a porous surface (Figure 6d) but is hydrophobic (θw = 115°). In addition, the NC gels did not adhere at all to conventional polymer films with smooth hydrophobic surfaces, such as polypropylene (PP), polyethylene terephthalate (PET), and polyvinyl chloride (PVC). We conclude that the instant strong adhesion of an NC gel to an organic polymer film is only achieved when the film surface is both hydrophilic and porous. This was confirmed by modifying the surfaces of membrane C and the polymer films. The surface of membrane C was hydrophilically modified by treatment with argon plasma. The resulting membrane (CAr), whose surface was both hydrophilic and porous, exhibited instant strong adhesion to NC gels. On the other hand, the polymer films (PP, PET, and PVC) did not change their behavior toward the NC gels even after hydrophilic treatment with argon plasma because their surfaces remained smooth. The adhesion between the NC gel and the substrate was also affected by other NC-gel properties. Concerning Cclay, NCn gels with intermediate n values generally exhibited strong adhesion to hydrophilic porous substrates and cohesive failure by subsequent stretching. When n was very low, as in the D-NC1 gel, the gel instantly adhered to the substrate, however, once bonded to the substrate the NC gel could be readily and detrimentally elongated, fracturing at low stress levels because of its very low TS (30 kPa for the D-NC1 gel). When n was very high, as in the D-NC20 gel, the NC gel did not strongly adhere to the substrate; consequently, the D-


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NC20 gel and membrane A were separated at the interface (adhesive failure) by subsequent stretching. The lack of AS at the interface is probably due to the surface of the D-NC20 gel bearing polymers of insufficient chain length that, as a consequence, interact poorly with the substrate surface. The adhesion mechanism is discussed in the next section. Hence, to achieve superb adhesion to a hydrophilic porous substrate, the NC gel should have both sufficient adhesion capability and a high TS. As for the effect of the NC-gel surface type, i.e., as-prepared vs. cut surface, the latter generally exhibited stronger adhesion than the former. This was clearly observed for NCn gels with relatively high n, with adhesion between the D-NC15 gel and the unglazed ceramic pot providing a good example. Although both the cut and as-prepared surfaces of the D-NC15 gel adhered to the substrate, the adhesive interface of the cut surface was not separated by stretching, while that of the as-prepared surface was. The adhesion mechanisms for different types of NC-gel surfaces are also discussed in the next section based on the polymerclay-network structure. As for temperature, the adhesion behavior of the thermoresponsive N-NC4 gel and the thermostable D-NC4 gel toward unglazed ceramics was examined by altering the surrounding temperature. Because the PNIPA chains in the N-NC gels transform into the hydrophobicglobule conformation above the LCST, the AS was expected to decrease as a consequence of fewer interactions with the hydrophilic surface of the substrate. The N-NC4 gel showed strong adhesion to the substrate by light contact at 25 °C (Figure 8a(i)). The gel rapidly became white when hot water (70 °C) was poured over the conjoined N-NC4 gel/substrate (Figure 8a(ii)); the adhered N-NC gel elongated during subsequent stretching and was finally peeled from the substrate surface (adhesive failure: Figure 8b). Furthermore, the detached N-NC4 gel regained its strong adhesion properties after returning to its original state. Therefore, the thermoresponsive


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N-NC4 gel exhibited reversible adhesion/peeling behavior that depended on temperature (below or above the LCST). On the other hand, when the N-NC4 gel was heated to a temperature above its LCST prior to contact with the substrate, its surface became hydrophobic and, as a matter of course, did not adhere to the substrate at all. As for the thermostable D-NC4 gel, it strongly adhered to the substrate and did not change its transparency or adhesion and cohesive-failure behavior even when hot water was poured onto it (Figure 8c).

(a)

(i)

(b)

(c)

(i)

(i)

(ii)

(iii)

(ii)

(ii)


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Figure 8. Adhesion and peeling behavior of the N-NC4 gel toward an unglazed ceramic surface. (a) (i) Adhesion at 20 °C, (ii) after pouring hot water (70°C) over the sample for 15 s. (b) Subsequent stretching and detachment of the gel. (c) (i) After pouring hot water (70 °C) for 15 s over the D-NC4 gel adhered to an unglazed ceramic surface, and (ii) subsequent stretching. Mechanism for the strong adhesion of an NC gel to a hydrophilic porous substrate. The strong and/or thermoreversible adhesion behavior demonstrated in the present study is specifically observed for NC gels, and has never been observed for conventional OR gels with chemically cross-linked networks. We propose the following mechanism for the instant strong adhesion of an NC gel to a substrate with a hydrophilic porous surface and cohesive failure by subsequent stretching. The most important factor for realizing the adhesion of an NC gel involves the flexible polymer-clay network that contains multiple hydrogen bonds, and the dangling chains that exist on the surface of the gel. In the case of an ORm gel with a commonly used crosslink density (e.g., m = 1−5), adhesion to (or strong lamination with) a substrate cannot be achieved even when the substrate surface is hydrophilic and porous, which is ascribable to negligibly short dangling chains at its surface, and mechanical fragility due to its network structure. For ORm gels with very low CBIS (e.g., m = 0.1), the OR gel is instantly adhered to the substrate due to fairly long dangling chains. However, the adhesion is very weak and is readily fractured at low stress by stretching due to the very low TS value (< 10 kPa) of the ORm gel, irrespective of m. An NC gel consists of an organic (polymer)-inorganic (clay) network structure (Figure 1), in which the CNSs are uniformly dispersed in an aqueous medium and neighboring CNSs are linked by a number of flexible polymer chains through hydrogen bonding. As a consequence, there are fairly long dangling chains on the outermost surface or the cut surface of the NC gel (Figures 9a, 9b). Since the surface network of an NC gel, with its long dangling chains, is more deformable than that of the bulk NC gel with low modulus (e.g., 4.5 kPa for the D-NC4 gel), the


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porosity of the substrate is filled through capillary forces. Mechanical interlocking then becomes favorable in the NC-gel/hydrophilic-porous-substrate system. The lengths of the dangling chains depend on the Cclay of the NC gel. Since the inter-clay distance, Dic, which is proportional to the inter-crosslink chain length, is related to Cclay according to the relationship Dic ∝ Cclay-1/3, the lengths of the dangling chains at the outermost surface or cut surface decrease with increasing Cclay. This is the reason why NC gels with very high Cclay, e.g., the D-NC20 gel, were not adhesive to hydrophilic porous substrates. On the other hand, a freshly cut surface generally has longer dangling chains than those of as-prepared surfaces, because, during cutting longer polymer chains grafted onto the clay surface at one end are created by pulling the polymer chains from the polymer aggregate on the clay surface,36 since hydrogen bonds are weaker than covalent bonds. Consequently, the cut surface generally exhibits

Figure 9. (a) Model structure of the as-prepared surface of an NC gel. (b) Model structure of the cut surface of an NC gel. superior adhesion than the as-prepared surface. This is the reason why the D-NC15 gel showed sufficiently strong adhesion to a hydrophilic porous substrate only at a cut surface. The existence of dangling chains on the surfaces of NC gels is consistent with previous results on their unique (wet-sensitive) sliding frictional behavior.16 The existence of longer dangling chains at cut


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surfaces is also consistent with the self-healing behavior of NC gels,21 where two cut pieces or damaged surfaces of an NC gel are strongly adhered by simple contact, and the autonomously joined or self-repaired material strengthens with time due to the mutual diffusion of dangling polymer chains. The mechanisms for strong adhesion and cohesive failure of the NC gel described here are also significant when compared with studies on hydrogel adhesion reported previously. Rose et al.28 reported that strong, rapid adhesion between two hydrogels can be achieved by spreading a droplet of a silica-nanoparticle (NP) solution on the surface of one gel, after which the two gels are brought into contact. In this case, adhesion relies on the adsorption of silica NPs onto the polymer gel through multiple interactions between the silica surface and the polymer chains that exist on the outermost surface of the hydrogel, and on the polymer chains that dissipate energy under stress. This situation is realized in an NC gel without the addition of any NPs because of its unique polymer-CNS network structure. In addition, an NC gel really adheres instantly (within 1 s), whereas more than 30 s of continuous pressing together is required for the silica NP−hydrogel system in order to achieve sufficient bonding. Furthermore, the hydrogels used are limited to those with fairly long polymer chains at their surfaces, e.g., an ORm gel with low CBIS (in fact, an D-OR0.1 gel was used by Rose et al.); consequently, the AS is very weak due to the low TS (< 10 kPa) of the gel. In addition, this system cannot be applied to a polyacrylamide (PA)-hydrogel, although a PA-NC gel37 can be used in a similar manner. In order to gain tough hydrogel adhesion, Zhao et al.35 reported the use of a specific hydrogel and a surface-modified substrate. The hydrogel, which was both mechanically tough and possessed terminal-reactive long polymer chains, strongly adhered to the substrate through chemical reactions (chemical anchoring) between the reactive polymer chain ends and the substrate, which had been surface


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modified by silanization. Celiz and Yang et al.29 realized tough adhesion through the use of a negatively charged tough hydrogel and a specific positively charged polymer adhesive that acted as a bridging layer. Wirth et al.32 used a tough hydrogel in addition to a cyanoacrylate-based adhesive dispersion, which diffused and polymerized in the hydrogel to form tough bonds. In the present study, strong adhesion between the hydrogel and the substrate was instantly achieved through simple contact in the absence of any adhesive or modification. As for the mechanism behind the instant and strong adhesion properties of NC gels, it should be noted that the water in the NC gel is absorbed into the hydrophilic porous substrate soon after contact, because the substrate not only has hydrophilic characteristics but also high waterabsorption capabilities. Indeed, water on the substrate surface was observed to penetrate at the point of NC-gel contact (e.g., Figures 3a(ii) and 3b(ii)), with the long dangling chains on the NCgel surface simultaneously diffusing into, and interacting with, the microirregular hydrophilic surface. Hence, the NC gel is strongly adhered to the substrate due to the anchoring effect of multiple dangling chains. Figures 10a and 10b show SEM images of the adhesive states of the DNC4 gel on the surfaces of the unglazed ceramic and membrane A, respectively. The SEM samples were prepared by drying during adhesion. Figure 10a(i) shows the boundary zone of the ceramic surface, in which the uncontacted surface (upper) and that contacted with the NC gel (lower) are clearly visible. Figures 10(a)(ii) and (iii) show magnified images of the two regions in Figure 10a(i). The ceramic surface contacted with the D-NC4 gel is covered with the gel network (i.e., the network has strongly interacted with the surface). Figure 10b shows the morphology of membrane A in contact with the D-NC4 gel. Figure 10b(i) shows a large amount of (dried) D-NC4 gel adhered to the surface of membrane A, while Figure 10b(ii) is an image taken near boundary region (i.e., the contact zone) where the adhesive state of the D-NC4 gel is


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more clearly observable. Figure 10b(iii) shows the morphology of the inside of membrane A (in the thickness direction) in contact with the D-NC4 gel, with Figure 10b(iv) showing a magnified image. The D-NC4 gel was observed to penetrate and adhere to the inside of the membrane by a morphologically fine, thin coating, which is obvious when compared with the original porous morphology shown in Figure 6b. Hence, we conclude from the SEM images that the polymer network of the D-NC4 gel is strongly adhered to both substrates by penetration into, and contact with, their microporous surfaces.

(a)

(ii) 10 µm

(i) 300 µm

(iii) 10 µm


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(b)

(i)

(ii) 10 µm

10 µm

(iii)

(iv) 10 µm

5 µm

Figure 10. SEM images of the adhesive states between the D-NC4 gel and the substrates in their dried states. (a) Unglazed ceramic (orchid pot): (i-upper and ii) the surface without contact; and (i-lower and iii) the surface in contact with the D-NC4 gel. (b) Membrane A (a mixed cellulose ester membrane): (i) the surface covered with the D-NC4 gel; (ii) the contact-edge region; (iii) and (iv) showing the morphology of the inside of membrane A (in the thickness direction) in contact with the D-NC4 gel. At the same time, the water content of the NC gel decreases as it comes into contact with the substrate, particularly at the interface, due to absorption of the water by the substrate. The adhesive NC-gel layer then transforms and exhibits a higher TS than that of the original NC gel, as evidenced by the dependence of TS on RH2O for the D-NC4 gel (Figure 5). The absorption of


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Elapsed time (every 1 min →) (i) Membrane A (ii) Membrane C (iii) Membrane CAr Figure 11. Showing the change in θw with time for three kinds of membrane. (i) Hydrophilic membrane A, (ii) Hydrophobic membrane C, and (iii) hydrophilically modified membrane CAr. The images were acquired at 1 min intervals following dropping. water into the hydrophilic porous substrate was also confirmed by the change in θw with elapsed time for membranes A, C (control), and CAr (Figure 11(i)−(iii) and Figure S3). Although θw for membrane C was high (122°) and stable over time, that of membrane CAr gradually decreased due to absorption of the water droplet. Furthermore, θw for membrane A decreased more steeply (within 1 min) due to its high water-absorption properties. The θw values of the three inorganic substrates with hydrophilic porous surfaces used in the present study exhibited higher rates of water-droplet absorption (within 1−15 s). Concerning the effect of temperature on adhesion, because most of the PNIPA chains adopt the globular conformation when heated above the LCST, the N-NC gel became white and was separated from the substrate (i.e., adhesive failure) by subsequent stretching. Here, separation did not occur spontaneously by heating but required some stress in order to peel the N-NC gel from the substrate. This is probably due to some dangling chains that strongly interact with the substrate and retain their conformations, even at temperatures above the LCST, by analogy with PNIPA chains that are aggregated on (i.e., interact with) the surfaces of the CNSs in the network.16


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CONCLUSIONS In summary, the instant and strong adhesion behavior of NC gels toward organic and inorganic porous substrates has been demonstrated. In the case of inorganic substrates, the NC gels strongly adhere to hydrophilic porous substrates, such as unglazed tiles, concrete blocks, and bricks; consequently, the two substrate pieces were strongly joined together by sandwiching a small piece of NC gel. The two substrates were so strongly bonded by the NC gel that they could not be separated manually, but required a tool. During the course of separation, the adhered NC gels were unable to be removed by peeling, rather separation always occurred by fracturing of the gel, which indicates that the AS at the gel-substrate interface is greater than the TS of the NC gel (cohesive failure). Moreover, the AS increased by heating the NC gel in the laminated state. For organic polymer substrates, NC gels also exhibited instant strong adhesion to hydrophilic porous polymer membranes; subsequent stretching of the conjoined body did not result in separation at the interface, but fracturing of the NC gel. In contrast, NC gels did not adhere to polymer films, membranes, and metal plates with hydrophobic or smooth surfaces. The mechanism of the instant and ultimately strong or thermoreversible adhesion of an NC gel to a hydrophilic porous substrate was discussed on the basis of dangling chains that exist on the outermost surface or the cut surface of an NC gel with a polymer-clay network. Due to the longer dangling chains, the cut surface of an NC gel generally exhibited stronger adhesion than the asprepared surface. Both long dangling chains and a high TS are required for an NC gel to exhibit high adhesive behavior. The effect of heating an N-NC gel before and after contact with a substrate was explained by the coil-to-globule transition of PNIPA. The instant and strong adhesion capability and/or the reversible adhesive behavior of NC gels shown in the present


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study usefully extends the applicability of NC gels toward diverse applications in fields such as tissue engineering, medical care, cosmetics, energy batteries, analysis, electronic devices, and civil and construction engineering.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Figures S1, S2, and S3, and Tables S1 and S2 (PDF). AUTHOR INFORMATION Corresponding Author *(K.H.) Email: [email protected] ORCID Kazutoshi Haraguchi: 0000-0003-0919-3024 Notes The authors declare no competing financial interest ACKNOWLEDGEMENTS This work was supported by JSPS KAKENHI Gran Numbers 15H03870, 18K05242. This work was partly carried out in Kawamura Institute of Chemical Research (KICR), Japan. The authors thank to Prof. M. Okada in Nihon University for N2 Adsorption-desorption measurement. The


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authors also thank Mr. K. Tanikawa in Nihon University and Ms. M. Nomura and Mr. H. Tanimoto in KICR for technical assistance.

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Table of Content Use Only TOC Image


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Instant Strong Adhesive Behavior of Nanocomposite Gels toward

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