Interconnectivity of Macroporous Hydrogels Prepared via Graphene

Jan 11, 2016 - Shengmiao ZhangXiaoxing FanFangning ZhangYun ZhuJianding Chen ... Ping Zhou , Jing Li , Wenwen Yang , Lihua Zhu , and Heqing Tang...
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Interconnectivity of Macroporous Hydrogels Prepared via Graphene Oxide-Stabilized Pickering High Internal Phase Emulsions Wenyuan Yi, Hao Wu, Haitao Wang,* and Qiangguo Du State Key Laboratory of Molecular Engineering of Polymers, Collaborative Innovation Center of Polymers and Polymer Composite Materials, Department of Macromolecular Science, Fudan University, Shanghai 200433, People’s Republic of China S Supporting Information *

ABSTRACT: Interconnected macroporous poly(acrylic acid) (PAA) hydrogels are prepared via oil-in-water (o/w) Pickering high internal phase emulsion (HIPE) templates stabilized by graphene oxide (GO). The amphiphilicity of GO is adjusted by slight modification with cetyltrimethylammonium bromide (CTAB). The morphology of macroporous PAA is observed by a field-emission scanning electron microscope (FE-SEM). The gas permeability is characterized to evaluate the interconnectivity of polymer foams. The pore and pore throat size can be tailored by varying the wettability and concentration of GO. The selective adsorption toward dyes of PAA hydrogels is proved. Macroporous PAA hydrogels with an open-cell structure show enhanced adsorption behavior of both methylene blue (MB) and copper(II) ions.

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INTRODUCTION Porous hydrogels with high porosity and good interconnectivity have been widely applied in drug delivery,1 tissue engineering,2 adsorption, and separation.3,4 Oil-in-water high internal phase emulsion (o/w HIPE) template is one of the effective methods to prepare macroporous polymer hydrogels with extraordinary advantages in the inherent high porosity due to the high dispersed phase fraction, as well as the controllable size of pores.5,6 HIPEs are normally defined as concentrated emulsions with a minimum internal phase volume fraction of 74.05%,7,8 which provides a very convenient route to synthesize macroporous polymers by polymerizing the monomers in the continuous phase.9 Conventional o/w HIPEs are stabilized by surfactants, such as polyethylene glycol tert-octylphenyl ether (Triton X-405),10 polyethylene glycol dodecyl ether (Brij 35),11 and poly(ethylene glycol)-block-poly(propylene glycol)-blockpoly(ethylene glycol) (Pluronic F68).12 However, large amounts of expensive surfactants are required to stabilize HIPEs at the concentration of 5−50% with respect to the continuous phase.10,13 Given the high toxicity of surfactants,14,15 they ought to be removed after polymerization based on various application demands, which inevitably imposes extra producing cost.16 Traditionally, the polymeric surfactants suitable for HIPE preparation are quite limited. Recently, a new method of tailoring the amphiphilicity of polymeric surfactants by designing the structure of copolymers to obtain HIPEs was proposed.17 In addition to surfactants, solid particles, such as titania,18 silica,19 iron oxide,20 hydroxyapatite,21 polymer particles,22 carbon nanotubes,23 and graphene oxide (GO) flakes,24 can also stabilize HIPEs, which are often referred to as Pickering HIPEs.13 As compared to conventional HIPEs, Pickering © XXXX American Chemical Society

HIPEs as templates show several advantages in the preparation of porous polymers. First, on account of the quasi irreversible adhesion of solid particles at the oil−water interface,25,26 only a small amount of solid particles is required to stabilize Pickering HIPEs,27,28 and the resulting porous polymers (poly-Pickering HIPEs) are particularly favorable for the application in biomaterials due to the low toxicity. Furthermore, the incorporation of inorganic solid particles into the organic framework endows porous polymers with improved mechanical performance,19,29 and poly-Pickering HIPEs with functional particles decorated pore walls exhibit additional properties, such as magnetism,30 electrical, or thermal conductivity.23,31 However, it should be noted that poly-Pickering HIPEs commonly have closed-cell structures,28 which is associated with the extreme stability of HIPEs (rare coalescence) and rigid barrier of solid particle layers at the interface.19,32 As a consequence, the volume contraction during polymerization33 and the mechanical force during post treatment28 fail to form pore throats, leading to low permeability of poly-Pickering HIPEs. Great efforts have been made to enhance the interconnectivity of porous polymers for their applications. A common strategy now is to decrease the droplet size and weaken the barrier at oil−water interface by adding a large quantity of surfactants to costabilize HIPEs,34 which is not environmentally friendly due to the high toxicity of surfactants. Some specific approaches have also been developed to open pores of poly-Pickering HIPEs, including the use of a particular poly(urethane urea) stabilizer,35 an amphiphilic monomer to costabilize HIPEs,21 or a reactive lignin stabilizer, which can be Received: December 8, 2015

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Langmuir Table 1. Composition, Pore Size (Dp), Throat Size (Dt), and Permeability (K) of Poly-Pickering HIPEs

a

samplea

GO concentration (wt %)b

CTAB/GO mass ratio

1 2 3 4 5 6 7 8 9 10 11 12d

0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.06 0.10 0.12 0.14 0.08

0 0.02 0.04 0.06 0.08 0.10 0.12 0.06 0.06 0.06 0.06 0.06

Dp (μm) 165 145 123 103 129 152

± ± ± ± ± ±

55 32 29 27 30 45

96 ± 24 84 ± 21 76 ± 13

Dt (μm) 40 38 35 37 42

± ± ± ± ±

16 14 12 16 16

21 ± 7 14 ± 5

K (D)c 0.15 0.80 1.17 2.31 1.49 0.75

± ± ± ± ± ±

0.04 0.09 0.13 0.31 0.15 0.09

0.47 ± 0.06 0.22 ± 0.03 0.05 ± 0.01

The internal phase fraction of all HIPEs is 75 vol %. bWith respect to the continuous phase. cPermeability is recorded in unit D (10−12 m2). Control experiment without the oil.

d

Preparation of Macroporous PAA Hydrogels. A typical oil-inwater HIPE with a 75 vol % cyclohexane internal phase was prepared as follows. The aqueous phase (10 mL) contained acrylic acid (2.88 g) and NaOH (1.60 g). The stabilizer CTAB-modified GO, cross-linker MBAM (0.12 g), and initiator KPS (0.10 g) were dispersed in the above aqueous phase via sonication. After cyclohexane (30 mL) was added, the mixture was emulsified with an FJ200-S homogenizer at 10 000 rpm for 5 min. The obtained HIPE was transferred to a hydrothermal reactor and polymerized in an oven at 65 °C for 48 h. The resulting monolith was cut into cubes and then purified via a Soxhlet extraction with acetone for 48 h. Finally, the products were dried under vacuum at 60 °C for 24 h to obtain macroporous hydrogels. The composition of all samples was summarized in Table 1. Without adding cyclohexane, nonmacroporous PAA hydrogel (sample 12) was also produced by solution polymerization for comparison. Selective Adsorption of Dyes. Two molecular dyes, sun-set yellow (SSY) and methylene blue (MB), were chosen for selective adsorption characterization. The initial dye concentration was 15 mg/ L. Macroporous hydrogel cubes (15 mg) were respectively immersed in SSY and MB solutions (15 mL). The digital photos were taken after 3 days. The total removal percent of the dyes was determined by UV− vis spectrophotometer. MB Adsorption Kinetics. Macroporous hydrogel cube (100 mg) was immersed into 100 mL of MB solution (150 mg/L) at 25 °C. The residual MB concentration in the solution was measured at the desired time intervals via a UV−vis spectrophotometer. The adsorption capacity at time t, qt (mg/g), was calculated as follows:

drawn from the interface to the monomer phase via a particle− monomer chemical reaction.36 However, these methods are only applicable in some certain HIPEs and are not versatile. GO has been studied extensively as Pickering stabilizer due to its hydrophobic basal plane and some hydrophilic groups decorating the periphery.37 The amount of GO needed for efficient stabilization is much lower in comparison with other particles due to its huge surface area. We have utilized GO as stabilizer to prepare water-in-oil HIPEs and obtained closed-cell polymer foams accordingly in our previous paper.24 However, the application of this macroporous material is limited because of its closed-cell pore structure and the inertness of polystyrene. Herein, a series of cyclohexane-in-water Pickering HIPEs have been successfully fabricated using slightly modified GO by cetyltrimethylammonium bromide (CTAB) as stabilizer. Highly permeable poly(acrylic acid) (PAA) hydrogels were achieved after polymerization. Furthermore, the influence of pore structure on the adsorption kinetics toward both dyes and metal ions was investigated.

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EXPERIMENTAL SECTION

Materials. Sulfuric acid (98%), potassium permanganate (99%), sodium nitrate (99%), hydrogen peroxide (30%), acrylic acid (AA, 98%), sodium hydroxide (96%), cetyltrimethylammonium bromide (CTAB, 99%), potassium persulfate (KPS, 99%), N,N′-methylene bis(acrylamide) (MBAM, 98%), cyclohexane (99%), acetone (99%), ethanol (99%), sun-set yellow (SSY, 99%), methylene blue (MB, 82%), hydrochloric acid (37%), cupric sulfate (CuSO4·5H2O, 99%), and ethylenediamine (99%) were purchased from Sinopharm Chemical Reagent Co. (China). Graphite powders (99.95%) were supplied by Aladdin Chemistry Co., Ltd. Deionized water was used throughout the experiments. Surface Modification of GO Nanosheets. GO nanosheets were prepared via a modified Hummers method38 from graphite. The GO suspension was dialyzed in deionized water for 1 week, followed by drying in a vacuum oven at 50 °C for 24 h. The dried GO was then dispersed in water and sonicated for 1 h with the power of 300 W to form the aqueous suspension (1.0 wt %). To a 10.0 g GO aqueous suspension (1.0 wt %) was added 10 g of CTAB solution (0.02, 0.04, 0.06, 0.08, 0.10, or 0.12 wt %) drop by drop under stirring at room temperature. Continuous stirring then was conducted for 10 min to reach the adsorption equilibrium. The modified GO was washed with water using a centrifugation−sonication cycle three times to remove free CTAB prior to drying at 50 °C for 24 h. GO nanosheets modified by various amounts of CTAB were labeled as CMG-X, where X denotes the mass ratio of CTAB to GO (0.02, 0.04, 0.06, 0.08, 0.10, and 0.12).

qt =

(C0 − Ct ) ×V m

(1)

where C0 is the initial concentration of MB (mg/L); Ct is the concentration of MB at time t (mg/L); V is the volume of the solution (L); and m is the mass of dry hydrogel (g). The adsorption kinetics experiments on nonmacroporous PAA hydrogel were also carried out for comparison under the same condition. Three replicates were performed for each sample. Recycling Experiments. A macroporous hydrogel cube (100 mg) was immersed into 100 mL of MB solution (150 mg/L) at 25 °C for 3 h, and then the adsorption capacity was measured. For regeneration, the dye-containing hydrogel was placed first in 100 mL of HCl solution (pH = 1) for 20 min, and then in 100 mL of NaOH solution (pH = 13) for 20 min, and finally in 100 mL of deionized water for 10 min. The refreshed macroporous hydrogel could be used in the next cycle of adsorption experiments. The adsorption−desorption processes were repeated five times for the same piece of hydrogel by using the same initial MB concentration. Copper(II) Ions Adsorption Kinetics. The initial concentration of Cu2+ solution was 600 mg/L. The experimental process and the calculation were the same as the adsorption of MB. The Cu2+ concentrations were determined with ethylenediamine as developing B

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Figure 1. (a,c) Tapping-mode AFM images and (b,d) height profiles of (a,b) pristine GO and (c,d) CMG-0.10. agent via a UV−vis spectrophotometer. Three replicates were performed for each sample. Characterization. Atomic force microscope (AFM) images of pristine GO and CMG-0.10 were acquired using a Multimode 8 in the tapping mode. Prior to the test, samples were dispersed in aqueous ethanol solution (1/1, v/v) and prepared by spin coating onto freshly cleaved mica substrates. The zeta potential of pristine GO and CTABmodified GO was measured by a Zetasizer Nano ZS90 (Malvern, UK). The wettability of pristine GO and CTAB-modified GO was evaluated by the static and dynamic contact angles as well as water contact angle on the GO surface under cyclohexane, using Dataphysics OCA 40 after the following preparation. The samples were dispersed in aqueous ethanol solution (1/1, v/v). The suspension was drop-cast onto a glass slide, and the resulting continuous film was dried at room temperature for 3 days. Five measurements were performed for each sample. The type of Pickering emulsions (w/o or o/w) was assessed by the drop test.39 The emulsion viscosity was characterized by a stress-controlled rheometer (HAAKE, MARS III) with parallel-plate geometry spaced 1 mm apart. The shear rate ranges from 0.001 to 20 s−1. The size and shape of HIPE droplets were observed by an EV5680 optical microscope after dropping the emulsions on glass slides. The optical micrographs were used to evaluate the droplet size distribution on counting at least 100 droplets per sample by Nano Measurer. The cross-section morphology of macroporous PAA hydrogels was observed using a field-emission scanning electron microscope (FESEM, Zeiss Ultra 55). The samples were fixed to substrates and sputtered with gold before test. The corresponding FE-SEM images were used to measure the pore size on counting at least 100 pores per sample by Nano Measurer, and a statistical correction was calculated to account for the arbitrary, nonequatorial location of the pores.40 The average pore throat size was determined from the FE-SEM images directly on counting at least 100 interconnecting pores per sample. The permeability of the macroporous hydrogels was detected by measuring the flow rates of nitrogen passing through the sample at 2000 Pa. The samples were cut into cubes with the size of 1.0 × 1.0 × 1.0 cm3. Five measurements were performed for each sample. The permeability was calculated by Darcy’s law41 and recorded in unit D (10−12 m2). Photographs of the adsorption of dyes were taken by a Canon Ixus 850IS digital camera. The concentration of dyes and metal ion was determined by UV−vis spectrophotometer (Lambda 750) via

measuring the absorbance of SSY solution at 480 nm, MB solution at 664 nm, and Cu2+ solution at 557 nm.

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RESULTS AND DISCUSSION Surface Modification of GO. Graphene is a one-atom thick and closely packed two-dimensional (2D) sp2-bonded carbon honeycomb lattice.37 The epoxy, hydroxyl, and carboxyl groups are introduced onto the surface of graphene during the oxidation process.38,42 Hydrophobic basal plane of carbon networks and hydrophilic oxygen-containing groups endow GO with amphiphilicity and make it act as a surfactant or a Pickering stabilizer.43 The morphology of GO nanosheets was observed by AFM (Figure 1a). The thickness of the as-prepared GO nanosheets is about 0.8 nm, indicating that full exfoliation is achieved. The lateral size of GO nanosheets ranges from 100 to 1000 nm. The stability and type of Pickering emulsions depend greatly on the wettability of stabilizers.13,44 Water contact angles were measured to reveal the surface nature of GO nanosheets. As shown in Figure 2, the static contact angle of pristine GO is 30 ± 2°, which is in good agreement with previous reports.45,46 The hydrophilic surface makes GO act as Pickering stabilizer to produce o/w emulsions because the particle surface resides more in water than in oil. Generally, the attachment energy of GO at the interfaces is increased by decreasing the hydrophilicity of GO properly,46,47 which can improve the stability of Pickering emulsions. It is proven that the amphiphilicity of solid particles can be tailored by the adsorption of surfactants on the surface.48,49 In view of the electronegativity of GO, cationic surfactant CTAB is chosen to tune the surface wettability of GO. The height of CMG-0.10 (CTAB-modified GO, the mass ratio of CTAB to GO is 0.10) ranges from 0.9 to 1.5 nm (Figure 1c,d), which is thicker than that of pristine GO on account of the adsorption of CTAB molecules on the surface. The increased thickness has also been observed in the polymergrafted graphene or graphene oxide sheets.50 Besides, the C

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Figure 2. Static water contact angle of pristine GO and CTABmodified GO.

Figure 3. Optical micrographs of Pickering HIPEs for samples 1−6 with varied CTAB contents: the mass ratio of CTAB to GO is (a) 0 (pristine GO), (b) 0.02, (c) 0.04, (d) 0.06, (e) 0.08, and (f) 0.10.

decrease in zeta potential indicates that the adsorbed amount of CTAB on GO increases with the concentration (Figure S1). The static water contact angles of CMG-0.02, -0.06, and -0.10 are 40 ± 3°, 66 ± 3°, and 77 ± 2°, respectively (Figure 2), demonstrating that the hydrophilicity of GO is reduced with the increasing dosage of CTAB as reported.24 The results of dynamic water contact angle (Table S1) and water contact angle on pristine GO and CTAB-modified GO surface under cyclohexane (Table S2) also show the same evolution of the surface wettability of modified GO. It is speculated that slightly modified GO by CTAB serving as the Pickering stabilizer may produce more stable o/w HIPEs. Pickering HIPEs and Macroporous PAA Hydrogels. GO modified by various CTAB amounts from 0 to 0.12 was utilized to prepare o/w Pickering HIPEs with a fixed concentration of solid particles at 0.08 wt %. It is found that Pickering HIPEs stabilized by pristine GO and CMGs (CMG0.02−CMG-0.10) show good stability for at least 1 month and all of them can be inverted. It has been widely accepted that the coalescence of emulsion droplets is inhibited by increasing the viscosity of emulsions, resulting in the improved stability.19 Thus, we evaluated the viscosity of Pickering HIPEs by a stresscontrolled rheometer (Figure S2). The emulsion stabilized by CMG-0.06 shows the highest viscosity, and the obvious decrease of viscosity is found regardless of increasing or decreasing the amount of CTAB. The size and shape of prepared Pickering HIPE droplets were observed by an optical microscope (Figure 3). The emulsion stabilized by pristine GO shows large droplets because of the high hydrophilicity of GO, mainly varying from 93 to 210 μm (Figure 3a). The droplet size becomes smaller with the increasing CTAB content from 0.02 to 0.06 (Figure 3b−d), implying that a larger interfacial area is stabilized with slightly increasing hydrophobicity of GO. The emulsion shows a minimum droplet size of 63−116 μm and exhibits the narrowest size distribution (Figure S3a) when CMG-0.06 is served as Pickering stabilizer. However, an obvious increase in the droplet size of HIPEs appears as the hydrophobicity of GO is further enhanced (Figure 3d−f). A stable o/w Pickering emulsion can hardly be achieved when the mass ratio of CTAB to GO is up to 0.12 due to the excessive hydrophobicity of GO sheets. Therefore, the appropriate amphiphilicity of solid particles is essential to fabricate stable o/w Pickering HIPEs. Both the viscosity and the droplet size

results indicate that CMG-0.06 owns the optimal amphiphilicity. The evolution of the size of emulsion droplets suggests that the pore structure of macroporous polymers can be tailored to satisfy the requirements of various applications when Pickering HIPEs are used as templates. Poly-Pickering HIPEs are obtained after polymerization in an oven at 65 °C for 48 h and subsequent extraction with acetone for 48 h, and the pore morphology is observed. Figure S4 shows wrinkled GO nanosheets on the pore surface, confirming that the oil−water interface of Pickering HIPEs is well stabilized by modified GO. The pore size of each sample varies from 103 to 165 μm according to the amphiphilicity of modified GO as listed in Table 1, which agrees well with the droplet size of the corresponding HIPE. A great difference in the pore interconnectivity is found from Figure 4 among these PAA hydrogels. The macroporous hydrogel with pristine GO as Pickering stabilizer possesses a typical closed-cell structure (Figure 4a), which has been often found in other poly-Pickering HIPEs.19 Crumpled thin films cover the closest point between neighboring pores, and some broken films can be seen occasionally. In sample 2, round open throats between the pores are formed, and some unbroken and half-broken films can still be found. With the increasing proportion of CTAB from 0.02 to 0.06, more open pores are created. An almost thorough open-cell structure is achieved when Pickering HIPE is stabilized by CMG-0.06. However, the amount of open pores in PAA hydrogels reduces obviously with further increased hydrophobicity of modified GO. It is also found from Table 1 that the pore throat size of hydrogels can be tailored because it shows a positive correlation with the pore size. To our knowledge, Pickering HIPEs always produce macroporous polymers with closed-cell structure. Although the volume contraction on the conversion of monomer to polymer and the mechanical force during post treatment (such as Soxhlet extraction and drying in vacuum) are likely to rupture the pores of poly-Pickering HIPEs,28,33 solid particles adsorbed firmly at the oil−water interface can act as steric barriers to hinder the formation of pore throats.15,25 The excellent barrier effect of solid particles is dominant in most cases unfortunately, leading to a closed-cell structure. Interestingly, open-cell PAA hydrogels are fabricated in our work using GO with appropriate wettability as Pickering stabilizer, as shown in Figure 4b−f. Generally, interconnected D

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Figure 4. FE-SEM images of poly-Pickering HIPEs for samples 1−6 with varied CTAB contents: the mass ratio of CTAB to GO is (a) 0 (pristine GO), (b) 0.02, (c) 0.04, (d) 0.06, (e) 0.08, and (f) 0.10.

D for sample 4 with the improved hydrophobicity of GO nanosheets, and then it decreases, which is induced by the pore structure of PAA hydrogels as shown in Figure 4. It is not the size but the amount of pore throat that determines the interconnectivity of PAA hydrogels in our experiments. Therefore, an appropriate amphiphilicity of solid particles is crucial for the performance of the resulting poly-Pickering HIPEs. As we know, the concentration of solid particles also has a remarkable influence on the stability of Pickering emulsions.52 Herein, CMG-0.06 with varied contents was employed to prepare o/w Pickering HIPEs. Phase separation of the emulsion takes place when a lower content (0.06 wt %) of CMG-0.06 is utilized, indicating that the oil−water interface cannot be well stabilized. Thus, GO with higher concentrations (0.10−0.14 wt %) is used as Pickering stabilizer. These Pickering HIPEs can all be inverted, and they become darker in color with the increasing GO content. Figure 5 shows a significant decrease of

porous materials can be prepared by adding a large quantity of surfactants (about 5−20 wt % with respect to the continuous phase) to costabilize HIPEs,34,51 which can decrease the droplet size and weaken the barrier at the oil−water interface to form interconnected pores. However, the amounts of CTAB used to tailor the wettability of GO in our work are merely 0.0016− 0.008 wt % with respect to the continuous phase, which is too little to weaken the barrier at the oil−water interface even if free CTAB molecules exist. Therefore, the formation mechanism of pore throats is probably on account of the unique twodimensional structure of GO nanosheets. GO is much thinner than common Pickering stabilizers,18,44 leading to a thin barrier at the oil−water interface. Furthermore, the high mass potency of GO46 due to its huge surface area allows it to stabilize Pickering emulsions with a much lower dosage of only 0.08 wt % as compared to other stabilizers (about 0.5−5 wt % with respect to the continuous phase).5,28 A thin and weak barrier created by GO nanosheets can be expected at the oil−water interface, which provides the chance to form pore throats during or after polymerization, and thus open-cell structure is achieved in samples 2−6. In addition, the interconnectivity of samples 2−6 is quite different, which is possibly determined by the thickness of the monomer layer between adjacent emulsion droplets. It is proven that the monomer layer must be sufficiently thin to form pore throats during or after polymerization.32 Theoretically, smaller emulsion droplet size results in a larger interfacial area for HIPEs with the same total volume and internal phase volume fraction,32 and thus thinner monomer films are formed between neighboring droplets, which inclines to be ruptured into holes. As mentioned above, the Pickering HIPE stabilized by CMG-0.06 has the smallest droplet size, which means the thinnest monomer film between adjacent droplets. Consequently, PAA hydrogel with the highest interconnectivity is obtained after polymerization. Larger emulsion droplets are produced regardless of the improved or decreased hydrophobicity of GO, leading to fewer open pores and even closed pores. The interconnectivity of porous polymers is extremely important for their applications,34 and thus the gas permeability of macroporous PAA hydrogels was measured (Table 1). The permeability increases sharply from 0.15 D for sample 1 to 2.31

Figure 5. Optical micrographs of Pickering HIPEs for samples 4, 9−11 with varied GO concentrations: (a) 0.08 wt %, (b) 0.10 wt %, (c) 0.12 wt %, and (d) 0.14 wt % with respect to the continuous phase. E

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a smaller droplet size of Pickering HIPEs results in more open pores after the polymerization using the same amount of GO (samples 1−6). However, neighboring pores sealed by a thin film are found in sample 11, although it has the smallest pore size. This can only be induced by the enhanced barrier effect of Pickering stabilizer. With the increase of Pickering stabilizer, more GO aggregates at the oil−water interface, forming a denser particle film to prevent the pores from opening. Thus, it is difficult to achieve permeable polymer foams via Pickering HIPEs templates when the high concentration of the stabilizer is necessary due to the strong hindrance from solid particles. A facile strategy can be proposed for the preparation of macroporous hydrogels with different pore structures (Scheme 1). In brief, the pristine GO nanosheets are slightly modified by CTAB to stabilize o/w Pickering HIPEs, and the porous hydrogels are obtained after the polymerization of continuous phase and the extraction of oil phase. Both the wettability and the amount of GO have great influences on the interconnectivity of the resulting porous hydrogels, and the pore and pore throat size can be tailored accordingly. The hydrogels with welldefined pore structure have various potential applications such as in tissue engineering scaffolds, controlled drug release, and separation. Adsorption of Molecular Dyes and Metal Ion. Polymer hydrogels are able to absorb and retain both water and solute molecules. As hydrogels possess ionic functional groups such as carboxylic acid, amine, and sulfonic acid groups, they can adsorb and trap ionic dyes or metal ions from wastewater.53 Therefore, hydrogels have been regarded as an alternative adsorbent. Accordingly, we explore the promising prospect of PAA hydrogel in the area of adsorbing hydrophilic dyes in this Article. Two molecular dyes, cationic dye methylene blue (MB) and anionic dye sun-set yellow (SSY), are employed to test the adsorption behavior of PAA hydrogel (Figure 7). After being

droplet size because the increased GO concentration allows stabilization of a larger interfacial area, which has also been found in Pickering emulsions stabilized by other solid particles.44 Besides, the distribution of droplets becomes narrower when the GO concentration increases (Figure S3b). Thinner monomer films between neighboring droplets lead to higher viscosity, as confirmed by Figure S5. When modified GO with varied concentration of 0.08−0.14 wt % is taken as Pickering stabilizer, macroporous PAA hydrogels with tunable pore sizes (listed in Table 1) varying from 103 to 76 μm are produced. Moreover, their pore structure is found to be greatly affected by the amount of solid particles from Figure 6. The pore throat size decreases gradually

Figure 6. FE-SEM images of poly-Pickering HIPEs for samples 4, 9− 11 with varied GO concentrations: (a) 0.08 wt %, (b) 0.10 wt %, (c) 0.12 wt %, and (d) 0.14 wt % with respect to the continuous phase.

when more GO is introduced, leading to the open-cell structure in sample 4 and little real interconnecting pores in sample 11. Open and closed pores are both clearly seen when a moderate amount of GO is utilized as Pickering stabilizer. The evolution of pore interconnectivity of PAA hydrogels is further confirmed by gas permeability measurements. The permeability decreases dramatically from 2.31 to 0.05 D, as the GO concentration increases from 0.08 to 0.14 wt % (Table 1). As discussed above,

Figure 7. Photographs of (a,b) MB and (c,d) SSY solutions (a,c) before and (b,d) after the adsorption by sample 4 for 3 days.

Scheme 1. Schematic Illustration of the Preparation of Macroporous Hydrogels via Pickering HIPE Template

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Copper has been widely used in many industries as raw materials or additives, and is considered a hazardous pollutant.54 Hence, samples 4 and 11 served as adsorbents for removal of Cu2+ from CuSO4 aqueous solutions, and the results are plotted in Figure 9. Both hydrogels perform good

adsorbed by sample 4 for 3 days, the solution of MB becomes colorless, and PAA hydrogel turns dark blue, demonstrating that macroporous PAA hydrogel is capable of adsorbing MB. As compared to MB, the color of the SSY solution does not change obviously, which means the adsorption of SSY is quite limited. The quantitative data are calculated so that 97.3% of MB in the solution has been adsorbed, whereas only 0.8% of SSY can be removed under the same condition. The huge difference in adsorption ability toward the dyes is attributed to the charge interaction between macroporous PAA hydrogel and molecular dyes. The carboxyl groups are easily deprotonated in neutral solution, exhibiting negative charges. Hence, the charge attraction between the hydrogel and cationic MB makes the PAA hydrogel adsorb almost all MB, while charge repulsion between the hydrogel and anionic SSY causes the limited adsorption. The above results have illustrated that macroporous PAA hydrogel exhibits a strong adsorption toward MB. Nonetheless, the practical application is concerned more with the adsorption kinetics. Thus, the effect of pore structure of PAA hydrogels on their adsorption behavior of MB is investigated, as shown in Figure 8. Sample 4 with thoroughly open pores shows a fast

Figure 9. Adsorption kinetics of Cu2+ by (a) sample 4 and (b) sample 11.

adsorption of Cu2+ because of the strong interaction between PAA and metal ions, including charge attraction and chelation.55 The adsorption of Cu2+ is quite fast in the first 50 min, and then slows. Like the adsorption behavior toward the dye, sample 4 shows a higher efficiency to remove Cu2+ than does sample 11 due to its open pores. An excellent adsorption capacity of about 280 mg/g is achieved for macroporous PAA hydrogel, which is very prominent as compared to other adsorbents reported in the literature (Table S4), making it a potential adsorbent for heavy metals.

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Figure 8. Adsorption kinetics of MB by (a) sample 4, (b) sample 2, (c) sample 11, and (d) sample 12.

CONCLUSIONS

We have developed a novel and facile strategy for the preparation of interconnected macroporous PAA hydrogels via GO-stabilized HIPE templates, where the wettability of GO is tuned by the adsorption of a slight amount of CTAB. Stable o/w Pickering HIPEs can be produced even when the content of modified GO is as low as 0.08 wt %. The droplet size of HIPEs is determined by GO’s surface nature as well as its concentration. After the polymerization of Pickering HIPEs, macroporous PAA hydrogels with open pores are achieved. The interconnectivity of the obtained polymer foams is tailored by the wettability and amount of GO. Appropriate amphiphilicity and low dosage of modified GO used as Pickering stabilizer favor the formation of pore throats inside hydrogels during the polymerization and post treatment because of a thinner monomer layer between two neighboring droplets and a weaker GO barrier at the oil−water interface. Highly permeable macroporous PAA hydrogels show an enhanced adsorption speed of both molecular dyes and metal ions toward closed-cell hydrogels.

adsorption of the dye. It takes only 100 min to achieve an adsorption equilibrium, while the hydrogel with a partially open-cell structure (sample 2) cannot reach an equilibrium within 3 h. As expected, almost closed pores in sample 11 lead to a much slower adsorption of MB. In addition, a further decreased speed of the dye removal is observed when nonmacroporous PAA hydrogel (sample 12 shown in Figure S6) is employed as the adsorbent. It proves that the adsorption kinetics of dyes is determined by the structure of the adsorbents. Fast adsorption can be achieved by using PAA hydrogels with high porosity and permeability. Furthermore, macroporous PAA shows a saturation adsorption capacity toward MB as high as 130 mg/g, which is higher than that of well-known state-of-the-art adsorbents (Table S3). The recycling performance of the macroporous hydrogel (sample 4) is also investigated (Figure S7). It can be seen that even after five recovery cycles, 75% of the initial adsorption capacity of the hydrogel is preserved, indicating a good reusability of the macroporous PAA hydrogel and making it promising for MB adsorption application. G

DOI: 10.1021/acs.langmuir.5b04477 Langmuir XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b04477. Zeta potentials of GO modified with varied CTAB contents, advancing (θa) and receding (θr) contact angles of pristine GO and CTAB-modified GO, water contact angle on pristine GO and CTAB-modified GO surface under cyclohexane (θow), viscosity (η) of Pickering HIPEs (samples 1−6, 9−11) under different shear rates (γ) ranging from 0.001 to 20 s −1, droplet size distributions of Pickering HIPEs (samples 1−6, 9−11), FE-SEM images of sample 12 and the pore surface of sample 4, adsorption capacity of different adsorbents for methylene blue (MB) and Cu(II), and recycling of PAA hydrogel (sample 4) in the adsorption of MB (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-21-6564-2392. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (NSFC) (no. 51373038). REFERENCES

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DOI: 10.1021/acs.langmuir.5b04477 Langmuir XXXX, XXX, XXX−XXX