Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Ultrastiff, Thermoresponsive Nanocomposite Hydrogels Composed of Ternary Polymer−Clay−Silica Networks Huan-Jun Li,‡ Haoyang Jiang,‡ and Kazutoshi Haraguchi*,† †
Department of Applied Molecular Chemistry, College of Industrial Technology, Nihon University, 1-2-1 Izumi, Narashino, Chiba 275-8575, Japan ‡ School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China S Supporting Information *
ABSTRACT: Anomalous increases in mechanical stiffness and tensile strength were achieved in a poly(N-isopropylacrylamide) nanocomposite gel (NC gel) by incorporating a small amount of silica through the sol−gel reaction of tetraethyl orthosilicate in the polymer−clay network. The resulting NC-Si gels, with ternary polymer−clay−silica structures, exhibited very high tensile moduli (∼3500 kPa) and strengths (∼1700 kPa), as well as well-defined thermoresponsive swelling/deswelling behavior, through the incorporation of 0.2−2.5 wt % silica (relative to the gel weight). The reinforcing efficiency of in situ formed silica in the NC gel is more than 50 times that of clay, while the incorporation of preformed silica nanoparticles led to no observable changes in mechanical properties. The controllable range of hydrogel tensile mechanical properties was significantly extended by the preparation of NC-Si gels. The ternary polymer−clay− silica network structure was proposed on the basis of optical transmittance, TEM, EDS, and 29Si NMR analyses, in addition to its mechanical and swelling/deswelling properties.
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INTRODUCTION Stimuli-responsive polymer hydrogels are receiving significant attention as intelligent soft and wet materials because they exhibit unique properties and characteristics that are distinct from those of solid polymeric materials. Among many kinds of stimuli-responsive polymer hydrogels, poly(N-isopropylacrylamide) (PNIPA) and its copolymer hydrogels have been studied extensively because they exhibit well-defined thermoresponsive transitions at temperatures near to those of the human body (the lower critical solution temperature (LCST) of PNIPA is 32 °C), which act as triggers for changes in gel volumes,1 shapes,2 hydrophilic−hydrophobic surface properties,3 and absorption−desorption capabilities.4 However, by analogy with other polymer hydrogels composed of chemically cross-linked networks (hereafter referred to as “OR gels” as organic cross-linkers are used in their preparation), the PNIPAOR gel is extremely weak and brittle;5 consequently, it is both difficult to prepare the gel in various forms, such as films, sheets, and tubes, among others, and to utilize it in various applications that require mechanical stiffness and toughness. For example, in biomedical applications such as bioadhesives, substitutes for cartilage and tendons, scaffolds for tissue engineering, artificial actuators, and cellular substrates, the poor mechanical properties of the gel limit its practical applications.6−10 It is therefore desirable that these mechanical properties are enhanced, in particular that gels with high mechanical stiffness and toughnes, and with stimulus sensitivities that are matched to each application are developed. A possible strategy for improving the stiffness of a polymer hydrogel is to reinforce its network through the incorporation © XXXX American Chemical Society
of inorganic or organic nanomaterials that are sphere, rod, plate, fibril, or tube shaped. Many studies into reinforcing (or modifying) the network in order to increase polymer-hydrogel stiffness have been reported. Examples include polyacrylamide hydrogels reinforced by rod-shaped cellulose nanocrystals, natural chitosan nanofibers or enzyme-induced amorphous calcium phosphate,11−13 a poly(N,N-dimethylacrylamide) hydrogel prepared using graphitic carbon nitride,14 and the gelatin-based hydrogels reinforced by graphene oxide,15 chemically modified graphene oxide,16 or Fe3O4 core nanoparticles.17 In addition, the reinforcement of thermoresponsive hydrogels using cellulose nanofibrils and SiO2/starch has been reported.18,19 However, in these cases except for the case of very high inorganic content (ca. 70 wt %),13 tensile mechanical tests were not conducted; rather, compression tests were mainly performed, which is probably due to a lack of mechanical toughness. Moreover, the compression moduli achieved in these studies were very low, at 3−30 kPa.11,14,15,17 Over the past decade or so, several new types of hydrogel have been developed using different strategies to solve the issue of mechanical fragility, including nanocomposite (NC) gels with polymer−clay networks,20 slide-ring gels with mobile cross-linking points,21 double-network gels with interpenetrating networks,22 and tetrapoly(ethylene glycol) gels with the same inter-cross-linked molecular weights.23 Among them, only NC gels were directly applicable to PNIPA through the Received: October 30, 2017 Revised: December 31, 2017
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DOI: 10.1021/acs.macromol.7b02305 Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
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formation of unique PNIPA−clay network structures, in which exfoliated nanosheets of clay (synthetic hectorite) act as multifunctional cross-linking agents for long hydrated PNIPA chains.5,24,25 These NC gels have revolutionary mechanical properties, exhibiting high stretching values (strains at break, εb) that exceed 1000% and high tensile strengths (σb) of up to 1100 kPa26,27 compared to the very low values (≤50% and ≤10 kPa) of conventional PNIPA-OR gels.5,28 The NC gels also showed improved thermoresponsive swelling/deswelling behavior compared to conventional gels; i.e., they exhibited significant swelling and rapid deswelling at LCST.5 In addition, owing to these tough mechanical and good thermoresponsive properties, the NC gels exhibited many new and interesting characteristics that are not observed in OR gels.29 Based on the concept of an NC gel with an organic− inorganic network structure, various kinds of NC gels have been prepared using different inorganic species, including montmorillonite,30,31 hallosite,32 graphene oxide,33 doublelayered oxides,34 and carbon nanodots,35 and through different synthetic methods such as photopolymerization36,37 and mixing procedures,38 although the NC gels with the polymer−clay (hectorite) network prepared by in situ free-radical polymerization exhibit the best mechanical properties. In addition to mechanical toughening, some NC gels composed of polymer− clay or polymer−silica networks exhibit unique self-healing or self-recovery characteristics due to reversible interactions at the inorganic/gel interfaces.38−41 As described above, the NC gel consisting of a polymer−clay network was the most mechanically tough thermoresponsive hydrogel; its stiffness and strength could be controlled over fairly wide ranges. The initial tensile modulus (E) and ultimate tensile strength (σb) could be controlled to lie in the 1−450 and 30−1100 kPa ranges, respectively, by altering the clay concentration (Cclay) at a constant polymer concentration (Cp = 1 M).27,42 These E and σb values are significantly higher than those of the PNIPA-OR gels; for example, PNIPA-ORn′ gels (n′ = mol % of organic cross-linker against NIPA) exhibit σb ≤ 10 kPa irrespective of n′, and E values of 40−70 kPa for n′ = 1− 3.27,28 In addition, the stiffness of the NC gel could be further tuned at constant Cclay and Cp by modifying the polymer−clay network structure, for example, by drying (formation of additional cross-links),43 the introduction of small amounts of chemical cross-links (co-cross-linked networks with microcomplex structures),44 elongation treatment (permanently altering the orientations of the clay and polymer),27 and copolymerization with a specific monomer (copolymer−clay network).45 However, the increases in E (stiffness) were within a factor of 2−3 of that of the original NC gel, were limited to NC gels with specific compositions, and were often not accompanied by thermoresponsiveness. Consequently, realizing higher mechanical stiffness and strength remains a challenge; this goal, together with good thermoresponsiveness, has not been achieved in any previous study. Herein, we report a new type of thermoresponsive NC gel composed of a ternary PNIPA−clay−silica network prepared by the incorporation of a small amount of in situ formed silica in the PNIPA−clay network. Anomalous changes in tensile mechanical properties, while maintaining good thermoresponsive swelling/deswelling behavior, were achieved through the fabrication of the ternary network.
Article
EXPERIMENTAL SECTION
Materials. N-Isopropylacrylamide (NIPA), provided by Kohjin Co., Japan, was purified by recrystallization from a toluene/hexane mixture and dried under vacuum at 40 °C. The synthetic hectorite inorganic clay (laponite XLG: Rockwood, Ltd., UK, [Mg5.34Li0.66Si8O20(OH)4]Na0.66; diameter: ∼30 nm; thickness: 1 nm; cationexchange capacity: 104 mequiv/100 g), provided by Wilbur-Ellis Co. (Tokyo, Japan), was used after washing and freeze-drying. Silica nanoparticles with diameters of 10−20 nm were purchased from Nissan Chemical Ind. Ltd. (Tokyo, Japan). The other reagents were purchased from Wako Pure Chemical Industries, Japan, and used without further purification; potassium persulfate (KPS), N,N,N′,N′tetramethylethylenediamine (TEMED), and N,N′-methylenebis(acrylamide) (BIS) were used as the initiator, catalyst, and organic cross-linker, respectively. Tetraethyl orthosilicate (TEOS: Si(OCH2CH3)4) was used to prepare silica. Ethanol and hydrochloric acid were used as solvent (exchange medium) and catalyst for the reaction of TEOS. Ultrapure water supplied by a PURIC-MX system (Organo Co., Japan) was used for all experiments. For gel preparation, oxygen was removed from the water through the bubbling of nitrogen gas for more than 3 h prior to use. Nomenclature. A nanocomposite hydrogel with a PNIPA−clay network structure is referred to as an “NCn gel” where n refers to the clay content (Cclay = n mol % in the reaction solution) by analogy with previous papers.5,27 A nanocomposite hydrogel with a ternary PNIPA−clay−silica structure is referred to as an “NCn-Si(m) gel” by including the silica content, m, which is the weight ratio of silica to PNIPA (m ≡ RSiO2 = SiO2/PNIPA (w/w)), in addition to n. Because the polymer concentration is constant (Cp = 1 M) in all the asprepared NCn gels, the clay content is also represented by the weight ratio of clay against PNIPA (Rclay = clay/PNIPA (w/w)) in order to enable direct comparisons with RSiO2. Another two types of NC-Si gel are referred to as “NCn-SiNP(m) gels”, which are prepared using reaction solutions containing preformed silica nanoparticles, and “NCn-Si*(m) gels”, which are prepared using the reaction solution contained TEOS. The chemically cross-linked PNIPA hydrogel is referred to as the “OR1 gel” according to the concentration of the organic cross-linker (BIS: 1 mol %) relative to the monomer. The silica-containing OR gels are referred to as “OR1-Si(m) gels” in accordance with their silica contents (RSiO2 = m). Preparation of NCn Gels. The synthetic procedure for the preparation of the NCn gels is as reported previously.5,27 For example, to synthesize the NC3 gel, an aqueous transparent solution consisting of water (19 mL), laponite XLG (0.457 g), and NIPA (2.26 g) was prepared. The catalyst (16 μL) and an aqueous solution of the initiator [20 mg in H2O (1 mL)] were then added to the solution at ice−water temperature with stirring. The solution was then transferred to glass vessels (i.e., tubes with interior diameters of 5.5 mm or film vessels with 2 mm inner thicknesses). Free-radical polymerization was allowed to proceed in a water bath at 20 °C for 24 h. The same procedure was used to synthesize OR1 gels except that BIS (0.028 g: 1 mol % relative to NIPA) was used instead of clay. Preparation of NCn-Si(m) Gels. NCn-Si(m)gels with the ternary polymer−clay−silica network were prepared by the in situ hydrolysis and polycondensation of TEOS in the presence of the polymer−clay network; the overall procedure is depicted in Scheme 1. After the initial synthesis of the NCn gel, the medium was changed to ethanol by immersing the gel in excess ethanol for 5 h at room temperature, with the ethanol refreshed every 1 h (step 1). The resulting NCn (ethanol) gel was then immersed in a 1 M solution of TEOS in ethanol for a prescribed period of time (tTEOS) at room temperature in order to introduce TEOS into the ethanolic NCn gel (step 2). The gel was subsequently exposed to a solution of HCl and H2O in ethanol (EtOH:H2O:HCl = 17:8:0.0014 mol/mol) for 10 min (step 3). The gel was then stored in a closed vessel for 12 h at 50 °C to facilitate the hydrolysis and polycondensation of TEOS in the gel (step 4). In order to return to a water medium, the ethanolic NCn-Si(m) gel was immersed in excess water for 8 h at room temperature, with the water B
DOI: 10.1021/acs.macromol.7b02305 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
obtained under the following conditions: 25 °C; gel size, diameter 5.5 mm × length 70 mm; gauge length, 30 mm; crosshead speed, 100 mm min−1. The initial cross section was used to calculate the modulus (E) and tensile strength (σb). For the tensile tests, the water content (Cw) was fixed at 75 wt % for all gels. Each gel was subjected to a second tensile test 5 min after the first elongation under the same testing conditions. Swelling and Deswelling Experiments. Swelling and deswelling experiments were conducted by immersing the NCn and NCn-Si(m) gels (initial size: ϕ 5.5 mm × 30 mm) in a large excess of water at 20 and 50 °C, respectively, for 8 days, with the water refreshed each time the gel weight was measured. The equilibrium degree of swelling (EDS) was determined by the ratio of the weight of the swollen gel after 192 h (Wgel) to the corresponding dried gel (Wdry) (EDS = Wgel(192 h)/Wdry).
Scheme 1. Synthetic Procedure for the Preparation of an NC-Si Gel from an NC Gel Including Medium Exchange and the in Situ Sol−Gel Reaction of TEOS in the NC Gela
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RESULTS AND DISCUSSION Syntheses and Characterization of NC-Si Gels. Ternary NCn-Si(m) gels of the polymer (PNIPA)−clay (hectorite)− silica type were prepared through the in situ hydrolysis and polycondensation (the “sol−gel reaction”) of TEOS in the polymer−clay network (in ethanol), followed by exchange of the medium to water, as shown in Scheme 1. NCn-Si(m) gels with different clay contents (n = 1−20: Rclay = 0.07−1.42) and silica contents (m = RSiO2 = 0.01−0.22) were obtained as uniform and mechanically tough hydrogels, as shown in Figure 1a for the NC5-Si(0.090) gel. In the present study, the water content (Cw) of the NCn and NCn-Si(m) gels were fixed at all 75 wt %, irrespective of n and m, in order to reveal the effects of the clay and silica contents on the tensile mechanical properties of the gels. Figure 1b displays the compositions of the NC3 gel (i) and the NC3-Si(m) gels (ii: m = 0.026; iii: m = 0.092)) with constant weights of PNIPA. In addition, changes in the compositions within consecutive series of NCn gels (n = 3−20) and NC3-Si(m) gels (m = 0.013−0.092) are displayed in Figures 1c and 1d, respectively. The weight ratio of each component, based on PNIPA (= 1), is shown in parentheses in Figure 1. Figures 2a and 2b show photographic images of the NCn gels and the corresponding NCn-Si(m) gels, respectively; m was fixed at 0.12 ± 0.02 in the latter. As reported previously,5,27 the NCn gels are uniform and transparent irrespective of Cclay (n = 1−20). On the other hand, while the NCn-Si(0.12) gels are all uniform, their transparencies depended on n; i.e., they were opaque or translucent at low n and transparent at high n (Figures 2b and 2c (inset)). Figure 2c shows the optical transmittances (T600) as functions of n for the NCn and NCnSi(0.12) gels. The T600 values of the NCn gels remain high over the entire range of n, with the exception of a slight decrease at very high n (20), whereas that of the NCn-Si(0.12) series gradually decreases with decreasing n in the low n range (
