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Study on a new polymer/graphene oxide/clay double network hydrogel with improved response rate and mechanical properties.

INTRODUCTION

Responsive hydrogels, with capability to respond to external stimuli, such as temperature, pH, and photons, have attracted great attention. Usually, such kind of hydrogel systems can be served as functional materials with potential applications in the fields of drug delivery, microlenses, sensors, and so forth. Among all the stimuli, temperature is the most widely used one.

One of the unique properties of temperature-responsive polymers is the presence of a volume phase transition temperature (VPTT). Among them, Poly (N-isopropyla-crylamide) (PNIPAM) is the most popular one since it exhibits a phase transition caused by the coil-to-globule transition in water [1], However, its slow response rate and poor mechanical properties greatly restrict its practical applications. Although there have been many efforts to improve the response rate of conventional hydrogels, there is still no proven method to fabricate a polymer gel with a fast response time on a macroscopic scale.

Recently, it was reported that introducing a double network (DN) structure for various combinations of hydrophilic polymers is an effective approach to prepare pH- and temperature-responsive hydrogels [2], DN hydrogels are usually fabricated with a brittle polyelectrolyte (first network) and a ductile neutral polymer (second network) in aqueous solution. They have attracted considerable attention because of their extraordinarily high mechanical strength. For example, Gong and co-workers reported excellent mechanical performance for DN hydrogels consisting of poly(2-acrylamide-2-methyl-propane sulfonic acid) and poly (acrylamide). They found that the fracture stress of this DN hydrogel is about 20 times larger than those of individual single network hydrogels [3],

Novel polymer--clay nanocomposite hydrogel has been prepared without using any organic crosslinker [4, 5]. The incorporation of clay markedly improves not only the mechanical and swelling-deswelling properties but also the spatial homogeneity of the hydrogels. Meanwhile, graphene, as a unique 2D nanocarbon material, has stimulated great interest due to its extraordinary electronic, thermal and mechanical properties, which has intensive promising applications in nanoelectronic devices, sensors, and nanocomposites [6], Graphene oxide (GO) is a precursor of graphene-based composites with similar one-atom thickness and many oxygenated defects, which are very suitable for covalent and noncovalent functionalization owing to its excellent surface activity and solution processability. Since GO behaves like an amphiphilic macromolecule with hydrophilic edges and a more hydrophobic basal plane, it can serve as an attractive building block for the construction of various supramolecular architectures [7],

As mentioned above, introduction of second cross-linked network and the addition of reinforcing fillers were both considered to be effective methods to improve the mechanical performance of the hydrogel. If the two methods were combined together, the mechanical properties of hydrogels should be significantly improved because of the synergetic effect. In this article, we successfully prepared a novel kind of multifunctional DN hydrogels based on PNIPAM and poly (acrylic acid) (PAA) using inorganic clay and GO as effective multifunctional crosslinkers. We found that the DN hydrogels exhibited extraordinary swelling/deswelling and mechanical properties because of their unique organic (polymer) and inorganic (GO and clay) network.

EXPERIMENTAL

Materials

NIPAM, AA, N,N,N',N'-tctramethylethylencdiamine (TEMED), Ammonium persulfate (APS), 2-oxoglutaric acid and BIS were supplied by Sinopharm Chemical Reagent. As an inorganic clay, synthetic hectorite "Laponite XLG" ([[Mg.sub.5.34][Li.sub.0.66][Si.sub.8][O.sub.20][(OH).sub.4]] [Na.sub.0.66]. layer size = 20-30 nm [PHI] x 1 nm, cation exchange capacity = 104 mequiv/100 g) was purchased from Rockwood. GO was prepared according to a modified Hummer's method [8],

Preparation of DN Hydrogels

The PNIPAM/AA/clay/graphene DN hydrogels were synthesized through a two-step sequential free-radical polymerization. In the first step, the initial solution consisting of monomer NIPAM (1 g), clay (0.1 g), deionized water (10 mL) and various ratios of GO was stirred and sonicated in an ice-water bath for 2 h. Then the catalyst of TEMED (20 [micro]L) was added with stirring. Finally, an aqueous of the initiator APS (0.02 g) was added to the solution. Free-radical polymerization was carried out in a water bath at 20[degrees]C for 24 h. In the second step, the PNIPAM gel was immersed into AA solution containing 2-oxoglutaric acid and clay (0.1 g) for at least 2 days till the equilibrium was reached. By irradiation with an ultraviolet (UV) lamp (60W, 220V) for 10 h (the distance between the lamp and the sample chamber was about 200 mm), the second network was subsequently synthesized in the presence of the first network. The conventional PNIPAM (OR hydrogel) and PNIPAM/PAA hydrogel (DN-0) crosslinked by BIS was also prepared for comparison. The composition for the hydrogels is shown in Table 1.

Measurement of Mechanical Properties

Compressive measurements were performed on hydrogels with the same size (10 mm x 10 mm x 10 mm) using a CMT 4204. The compression properties of the hydrogels were obtained under the following conditions: test temperature 25[degrees]C, compression speed 0.5 mm/s.

Measurement of Swelling Ratios

The swelling ratios of hydrogel samples were measured in the temperature range from 20 to 50[degrees]C or in a pH range from 2.0 to 8.0 using a gravimetric method. Under each particular condition, hydrogel samples were incubated in the medium for at least 48 h and then removed, wiped with moistened filter paper which could remove water from the surfaces of the sample, after removing the water the samples were weighed. The swelling ratio was calculated with the following equation:

Swelling ratio = ([W.sub.s] - [W.sub.d])/[W.sub.d],

where [W.sub.s] is the weight of the swollen hydrogel and [W.sub.d] is the weight of the dry hydrogel.

Deswelling Behavior of Hydrogels

The deswelling behavior of the hydrogel was studied by recording the weight of water in the hydrogels. Water retention was calculated as

Water retention = ([W.sub.t] - [W.sub.d])/([W.sub.s] - [W.sub.d]),

where [W.sub.t] is the weight of the hydrogel at a given time interval during the course of deswelling after the swollen hydrogel at 25[degrees]C had been quickly transferred into hot water at 45[degrees]C.

Characterizations

Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a NEXUS 670 spectrometer. The VPTT measurements of the wet samples were carried out with a TAQ100 differential scanning calorimeter (DSC) under nitrogen, which was kept in a heating rate of 3[degrees]C/min from 20 to 50[degrees]C. The morphology of the fractured specimens was observed on a Philips XL30 FEG scanning electron microscopy (SEM), which was at 20 kV after sputter coating with gold in vacuum. Thermogravimetric (TG) analysis was conducted with Netzsch TG 209F1, heating samples from ambient temperature to 700[degrees]C at the heating rate of 10[degrees]C/min under nitrogen.

RESULTS AND DISCUSSION

Conventional hydrogels, which were prepared to use an organic cross-linker, has shown serious disadvantages in terms of their mechanical, structural, and absorption properties. The introduction of new cross-linked network and the addition of reinforcing fillers were both considered to be effective methods for improving the mechanical and thermal performance of hydrogels.

In situ polymerization technique is attractive since it can control both the polymer architecture and the final structure of the composites. Figure 1 describes the formation process of the DN hydrogels via in situ free-radical polymerization, which is in the presence of A A and NIPAM with GO and clay as crosslinkers and reinforcing fillers. It is considered that the initiator APS could strongly interact with GO platelets through ionic interactions, and their molecules were closely associated on the GO surface in aqueous suspension. At the initial stage of polymerization, radical existed near the GO surface, and then the propagation reaction was proceeded, thus the PNIPAM chains, which attached to the GO platelets, formed 1st network structure. On the basis of the network PAA formed as a loosely crosslinked network (2nd network) [9].

The FTIR spectra of the AA, NIPAM, GO and the dried DN-2 hydrogel are shown in Fig. 2. The characteristic absorption peak of AA units at ~1720, 1300, and 1220 [cm.sub.-1] is shown by the spectrum of AA (Fig. 2a) due to carboxylate anion of AA groups. From the spectrum of NIPAM (Fig. 2b), there is a carbonyl stretching vibration (amide 1) at 1660 [cm.sub.-1], N-H bending vibration (amide II) at 1550 cm 1 and two typical peaks of C-H vibrations of -CH[(C[H.sub.3]).sub.2] at 2930 [cm.sub.-1]. A band at 3400 [cm.sub.-1] attributing to a -OH stretching vibration can be found in the spectrum of the GO (Fig. 2c) [10], In the spectrum of dried DN-2 hydrogel (Fig. 2d), there still exist most of the characteristic peaks of NIPAM, AA and GO. Given all this, it can be concluded that all the components used to form the DN hydrogels.

The DSC curves of DN hydrogels are shown in Fig. 3. The temperature at the onset point of the DSC endotherm is referred to the VPTT of the hydrogels [11]. At the VPTT, the water in the hydrogels will be separated from the network, and then led to a small heat capacity. As shown in Fig. 3, the DN hydrogels exhibited VPTT at around 30[degrees]C, and there was no significant deviation when GO content was increased. The results indicate that the incorporation of GO will not obviously affect the VPTT of the hydrogels. Besides, DSC curves of DN hydrogels became broader when the GO content increased, which may arise from their two-level structural hierarchy.

[FIGURE 1 OMITTED]

The morphological characteristics of DN hydrogels, which were exposured to solutions and then through a freeze drying process, have been examined by SEM. Figure 4 show the SEM micrograph of the internal structure of DN-1, DN-2, and DN-3 hydrogels, from which we can see that the hydrogel exists a porous network structure. In addition, after removing the water, SEM image of the DN hydrogel with the increasing of GO content shows even tight-folded network structure, and most of the pores have been covered with GO/polymer composites. This result indicates that with the increasing of GO content, the crosslinking density of the hydrogels increased, which results in decrease of the pore size and coverage of the pores [12].

[FIGURE 2 OMITTED]

Figure 5 illustrates the thermograms of the DN hydrogels. It can be found that there are mainly two mass loss peaks at about 220 and 400[degrees]C of the DN gels, which are mainly arising from the pyrolysis of PAA and PNIPAM. According to the TGA results, the DN hydrogels showed higher thermal stability by increasing the contents of GO. The increasing of GO content may lead to higher physical crosslinking density within the networks. It is very interesting to find that the thermal stability of DN-2 and DN-3 are similar with each other, while they are much higher than DN-1. We suppose that the threshold of the effect of GO amount may be around DN-2.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The compressive strength of DN hydrogels and OR hydrogel is shown in Fig. 6. It can be found that the hydrogels prepared by two types of the crosslinkers exhibited completely different mechanical properties. As described previously, the OR hydrogel was weak and brittle that it could not be applied in many fields. In this experiment, the OR hydrogel fractured at a low compressive strength (0.2 MPa). On the contrary, the DN hydrogels generally exhibited high strain and rigidity. Besides, it was observed that the mechanical properties of DN hydrogels strongly depend on the contents of GO. The mechanical properties of DN hydrogels increased by increasing the contents of GO. It has close relations with the outstanding performance of GO and good interfacial contact between GO and matrix. We also found that the mechanical strength of DN hydrogels increased by increasing the contents of PAA, which reveals that the formation of the second network has an important contribution to the performance of the hydrogels. These results prove that the research of double network hydrogels is very meaningful.

[FIGURE 5 OMITTED]

The swelling ratios of DN hydrogels were investigated as a function of temperature at pH 7.4, as shown in Fig. 7. In general, an abrupt decrease of the swelling ratios can be observed around VPTT for the samples, which is ascribed to the coil-globular transition of PNIPAM. It can be found that the swelling ratios of DN hydrogels gradually decreased by increasing the GO and AA contents. It is well known that the swelling properties of hydrogels mainly depend on the effective crosslinking density of the hydrogles. The DN hydrogels with higher GO and AA contents led to more densely crosslinked networks, thus decreasing the swelling ratios of the hydrogels [13].

To investigate the influence of pH value of the medium on the swelling ratios for the DN hydrogels, the pH range is selected from 2 to 8 in this study. As shown in Fig. 8, the swelling ratios of DN hydrogels decreased with the increase of AA content. It is due to the formation of hydrogen bond between -COOH in the AA and -CONH- in the PNIPAM, which leads to polymer--polymer interactions predominating over the polymerwater interactions and a decrease of swelling ratios. In the pH range from 2 to 7.4, the swelling ratios of the DN hydrogels continuously increased with increasing pH values. As the pH value of the medium increases, the carboxylic acid groups become ionized and the electrostatic repulsion between the molecular chains is predominated, which leads to the network more expanding. As to the effect of GO, the hydrophilic groups in the side chains of the PNIPAM hydrogel connect with water molecules through hydrogen bonds. There are also numerous C--OH and C--O--C groups on the GO sheets either on the edges or the planes that can also be served as hydrogen bonding sites and form extra hydrogen bonds with water molecules. They bond cooperatively to form a stable hydration shell around the hydrophobic groups, which may lead to greater volumetric change of the hydrogel as a comparison to hydrogels without GO incorporation [14],

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

The deswelling rate is one of the most important factors, and in particular, high rates are needed in many applications. A general approach to high swelling rate is to utilize the size effect. Hydrogels with smaller size could exhibit higher deswelling rates due to the small size effect. Figure 9 shows the deswelling behaviors measured for DN hydrogels containing different GO and PAA contents under the same experimental conditions. Comparing with OR hydrogel, the DN hydrogels exhibit high deswelling rates. For instance, DN-1 hydrogel loses about 80% water within 2 h, whereas the OR hydrogel takes 4 h to lose only 20% water. These results showed that a highly expanded network can be generated by electrostatic repulsions among carboxylate anions (-COO') during the polymerization process. Therefore, the response rate could greatly be enhanced by the incorporation AA into the hydrogels network during the deswelling process. Furthermore, the deswelling rates gradually decreased as the content of GO increased. The DN hydrogel containing the lowest GO content exhibits the most rapid response. With increased GO content, the crosslinking densities of the hydrogels increased, which resulted in the decrease of pore size. When experiencing the deswelling process, the water molecules are hard to diffuse out owing to numerous small pores in the hydrogel network, thus the swelling ratio decreased by increasing the contents of GO [15].

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

All in all, it is found that the GO content has a great effect on the properties of the DN hydrogels. The effect of GO on the hydrogels can be summarized as follows: (1) from a chemistry point of view, graphene is essentially a conducting polymer with a giant, two-dimensional molecular configuration. However, unlike pristine graphene, there are considerable amounts of oxygen containing groups remaining on GO sheets. Thus, GO sheets are essentially amphiphilic. The hydration forces, together with the electrostatic repulsions between hydrated GO sheets, prevent the GO sheets from being fully compacted. As a result, a significant amount of water is stably trapped between GO sheets to allow the formation of the oriented hydrogel structure [16], Through this kind of repulsion, the role of GO can be greatly played. (2) The hydrophilic groups in the side chains of the PNIPAM hydrogel connect with water molecules through hydrogen bonds. GO behaves like an amphiphilic macromoiecule with hydrophilic edges and a more hydrophobic basal plane, which makes it an attractive building block for the construction of various supramolecular architectures. Since there are also numerous C--OH and C--O--C groups on the GO sheets either on the edges or the planes, they can be served as hydrogen bonding sites and form extra hydrogen bonds with water molecules. They bond cooperatively to form a stable hydration shell around the hydrophobic groups, which may lead to greater volumetric change of the hydrogel in comparison to PNIPAM hydrogel without GO incorporation. (3) The GO platelets function as multifunctional crosslinkers and the ends of the polymer chains will adsorb strongly on the surface of the GO platelets by ionic and coordination interactions. In the elongated state of DN hydrogels, the second network with flexible polymer chains are capable of being highly and reversibly elongated without breaking the first network of polymer chains, giving superior mechanical strength to DN hydrogels [17]. The conclusions above are consistent with the literatures [16, 17].

CONCLUSIONS

In this paper, pH- and temperature-responsive DN hydrogels were prepared, with inorganic GO and clay acting as multifunctional crosslinkers and fillers. With the presence of GO, a unique organic/inorganic network structure was formed through hydrogen bonding, ionic bonding and physical adsoiption. The homogeneously dispersed GO nanosheets as the crosslinkers are highly efficient in improving the mechanical, thermal, and swelling properties of the DN hydrogels. The novel DN hydrogels exhibit the same VPTT around 30[degrees]C as that of the conventional OR hydrogels. The swelling ratios of DN hydrogels below VPTT are much larger than those of OR hydrogels. Moreover, the swelling ratios of DN hydrogels gradually decreased by increasing the contents of GO and AA. Also, the DN hydrogels have a much better mechanical property than that of the OR hydrogel. The mechanical properties of DN hydrogels increased by increasing the contents of GO and PAA. This kind of hydrogels can be served as functional materials with potential applications in many fields of biomedical science, sensors, and so forth.

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Weishi Huang, Jianfeng Shen, Na Li, Mingxin Ye

Center of Special Materials and Technology, Fudan University, Shanghai 200433, China

Correspondence to: Mingxin Ye; e-mail: mxye@fudan.edu.cn Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 51202034; contract grant sponsors: Shanghai Municipal Education Commission, Shanghai Education Development Foundation; contract grant number: 12CG02.

DOI 10.1002/pen.24076

Published online in Wiley Online Library (wileyonlinelibrary.com).
TABLE 1. Preparation condition and swelling properties of DN
hydrogels.

         NIPAM   BIS    Clay    GO      AA      [H.sub.2]O
Sample    (g)    (mg)   (g)    (mg)   (mol/L)      (mL)

OR         1      20     0      0        0          10
DN-0       1      20    0.2     0       0.1         10
DN-1       1       0    0.2     10      0.1         10
DN-2       1       0    0.2     20      0.1         10
DN-3       1       0    0.2     40      0.1         10
DN-4       1       0    0.2     10      0.2         10
DN-5       1       0    0.2     10      0.4         10
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Author:Huang, Weishi; Shen, Jianfeng; Li, Na; Ye, Mingxin
Publication:Polymer Engineering and Science
Date:Jun 1, 2015
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