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inorganic hybrid poly (ethylene glycol)-based hydrogels
Colloids and Surfaces A: Physicochem. Eng. Aspects 415 (2012) 68–76
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Fabrication and characterization of temperature-, pH- and magnetic-field-sensitive organic/inorganic hybrid poly (ethylene glycol)-based hydrogels Yang Wang, Aijuan Dong, Zhicheng Yuan, Dajun Chen ∗ State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Shanghai 201620, People’s Republic of China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
A new kind of magnetic nanoparticle was prepared via co-precipitation technique. Novel multiple stimulus-responsive hydrogels were prepared by in situ polymerization. The mf-NC hydrogels have T/pH/magnetic sensitivity and good mechanical properties. The mf-NC hydrogel can continue swelling under an alternating magnetic field.
a r t i c l e
i n f o
Article history: Received 6 July 2012 Received in revised form 21 September 2012 Accepted 1 October 2012 Available online 9 October 2012 Keywords: Magnetic-sensitive Thermo-sensitive pH-sensitive Magnetic AT–Fe3 O4 nanoparticles P(MEO2 MA-co-OEGMA-co-AAc) hydrogel
a b s t r a c t In this paper, we successfully fabricated a new kind of multiple stimulus-responsive organic/inorganic hybrid hydrogels by combining dual stimuli-responsive poly (2-(2-methoxyethoxy) ethyl methacrylate-co-oligo (ethylene glycol) methacrylate-co-acrylic acid) (PMOA) hydrogel with magnetic attapulgite/Fe3 O4 (AT–Fe3 O4 ) nanoparticles. First, the magnetic nanoparticle was prepared via co-precipitation technique in aqueous suspension of purified attapulgite. The obtained AT–Fe3 O4 nanoparticles were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy and vibrating sample magnetometer. Compared with pure attapulgite, the AT–Fe3 O4 exhibited better superparamagnetic properties. Then, the AT–Fe3 O4 was introduced into the dual-responsive (temperature and pH) PMOA hydrogel network by in situ polymerization. The morphology, responsive behaviors and tensile properties of the prepared hydrogels were systematically characterized by field emission scanning electron microscopy, vibrating sample magnetometer, swelling/re-swelling behaviors and tensile testing. The results showed that the AT–Fe3 O4 nanoparticles were well dispersed in the hydrogel matrix, and the multi-functional AT–Fe3 O4 /PMOA nanocomposite hydrogels had not only temperature/pH sensitivity and good mechanical properties, but also magnetic functionality. The tunable superparamagnetic behavior of these hydrogels depended on the amount of AT–Fe3 O4 . In addition, the multi-functional AT–Fe3 O4 /PMOA nanocomposite hydrogels can continue to swell under an alternating magnetic field after equilibrium swelling in deionized water. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
∗ Corresponding author. Tel.: +86 21 67792891; fax: +86 21 67792855. E-mail address: [email protected] (D. Chen). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.10.009
Smart hydrogels are three-dimensional network structure that change their physical and (or) chemical properties in response to external environmental stimuli [1–3], such as temperature, pH, pressure, electric field, magnetic field and light and so on. Due to the
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unique stimulus–response property, smart hydrogels are widely applied in drug controlled release, biosensors, chemical converters, artificial muscles, tissue engineering, and molecular separation system and so on [4–7]. Currently, the smart hydrogels can be mainly divided into single-sensitive hydrogels [8–15] (temperature (T), pH, electric field, magnetic field and light), dual-sensitive hydrogels [16–20] (T–pH sensitive, pH salt sensitive and magnetic thermal sensitive), multiple-sensitive hydrogels [17,21–23] (T–pH–magnetic response, T–pH–light response). In the past few decades, single and dual-sensitive hydrogels have attracted considerable attention. However, from the development of practical applications, the fabrication of multiple-sensitive hydrogels would be much more attractive if they could respond to at least three stimulation, such as possessing T-, pH- and magnetic/electric/light-sensitive simultaneously. Such multipleresponsive materials are of interest for a variety of applications ranging from magnetic/electric/light separation or drug release systems to sensors and actuators. Presently, the incorporation of magnetic nanoparticles (Fe3 O4 , ␥-Fe2 O3 or CoFe2 O4 ) into T-/pH-sensitive microgels/hydrogels matrix to form triple-response hydrogels can be easily achieved and has received increasing interest. For example, Lang and co-workers [24] prepared polysaccharide-based hydrogels with pH- and thermo-sensitive property by the copolymerization of maleilated carboxymethyl chitosan with N-isopropylacrylamide, and then fabricated their magnetic hybrid hydrogels by in situ deposition of magnetic iron oxide nanoparticles into the porous hydrogel networks. Bhattacharya et al. [21] described the synthesis and characterization of multifunctional hybrid microgels that exhibit temperature, pH and magnetic triple stimuli-response by in situ formation of iron oxide nanoparticles in the microgel structure. However, in situ formation of magnetic nanoparticles in polymer network needed to be treated with alkaline solution. In this case the polymer gels network would be easily destroyed, which decreased the overall performance of the gels. In order to overcome this shortcoming, some researchers began to use readymade magnetic particles introduced directly into the polymer gels to achieve the magnetic response of polymer gels. However, it is well-known that poor mechanical properties of hydrogels severely restrict their practical applications. Currently, incorporating clay into hydrogel matrix has been proved to be an effective and simple method to enhance the mechanical properties of hydrogels [25–31]. Attapulgite (AT) is a type of natural fibrillar aluminum silicate with abundant hydroxyl groups on the surfaces [31]. Decorating AT with Fe3 O4 magnetic nanoparticles (AT–Fe3 O4 ) may give birth to novel chemical and physical properties, expecting to be applied in polymeric gels system. To date, the hybrid materials on the combination of magnetic nanoparticles (such as Fe3 O4 ) with responsive hydrogels have been reported [14,19,32–34]. Echeverria and co-worker [14] have reported that UCST-like hybrid poly (acrylamide-acrylic acid)/Fe3 O4 microgels by the incorporation of metal oxide nanoparticles into microgels. They investigated the effect of Fe3 O4 nanoparticles on morphology, thermo-sensitivity and elasticity. Recently, Chanana et al. [34] have prepared nanocomposites based on magnetite and thermoresponsive poly (2-(2-methoxyethoxy) ethyl methacrylate)-co-oligo (ethyleneglycol) methacrylate) (P(MEO2 MA-co-OEGMA)) copolymers. The advantage of these polymers is that the lower critical solution temperature (LCST) in water can be easily adjusted by the oligo (ethylene glycol) chain length in a wide temperature range from 24 to 95 ◦ C. A similar study was report by Gelbrich [19] in the preparation of magnetic core–shell nanoparticles that were composed of nanosized superparamagnetic iron oxide cores and a copolymer shell. The shell which consists of OEGMA and methoxyethyl methacrylate (MEMA) copolymers shows a LCST in water.
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In our previous publication [31,35], it was demonstrated that composition of MEO2 MA, OEGMA and acrylic acid (AAc) gels (PMOA) can be tailored to get temperature and pH response, and the mechanical properties of the gel can be significantly improved by adding the AT nanoparticles. Here, the aim of the present study was the preparation of hydrogels with excellent mechanical properties that are sensitive to temperature, pH and magnetic field. Therefore, we fabricated a new kind of smart organic/inorganic hybrid hydrogel by combining dual stimuli-responsive PMOA hydrogel with magnetic AT–Fe3 O4 nanoparticles. First, the magnetic nanoparticle, attapulgite/Fe3 O4 (AT–Fe3 O4 ), was prepared via a simple co-precipitation method. Then, the AT–Fe3 O4 was successfully introduced into the dual-responsive (T and pH) PMOA network by in situ polymerization. The microstructures and various properties of the AT–Fe3 O4 and the prepared hydrogels were systematically characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), vibrating sample magnetometer (VSM), swelling/re-swelling behaviors and tensile testing. 2. Experimental 2.1. Materials 2-(2-Methoxyethoxy) ethyl methacrylate (MEO2 MA, 97%) was purchased from TCI Chemical Co. Tokyo, Japan. Oligo (ethylene glycol) methyl ether methactylate (OEGMA, average Mn = 475, the number of EO units 8–9) was purchased from Sigma–Aldrich Co. Attapulgite (AT) was obtained from Xuyi Colloidal Co. Jiangsu, China. Acrylic acid (AAc), iron dichloride (FeCl2 ), iron chloride (FeCl3 ), sodium hydroxide (NaOH), potassium persulfate (K2 S2 O8 ), and sodium hydrogen sulfite (NaHSO3 ) were purchased from Sinopharm Chemical Reagent Co. Ltd. All these reagents were used as received. 2.2. Preparation of the magnetic attapulgite (AT–Fe3 O4 ) particles The crude AT powder contains a lot of impurities and dirt. First, AT was purified before the preparation of magnetic particles, according to our previous work [36]. Purified AT (2.4 g) and deionized water (320 ml) were added to a flask and the pH of the aqueous dispersion was adjusted to 8. Then 0.4 M FeCl3 solution (20 ml) was added into the flask, dispersed in an ultrasonic bath (50 kHz) for 30 min. 0.9 M FeCl2 solution (20 ml) was added to the aqueous dispersion. The mixture was treated in the ultrasonic bath for 15 min and then 1 M NaOH solution was dropped into the flask until the pH value of the aqueous dispersion reached 10 and the black precipitate appeared immediately. Then, the mixture was treated in the ultrasonic bath for another 1.5 h. After that, the product was centrifuged at 12 000 rpm for 5 min and washed with deionized water several times to remove unreacted ions, and dried in a vacuum oven at 70 ◦ C for 24 h. The weight fraction of Fe3 O4 in AT–Fe3 O4 composite is nearly 30 wt%. 2.3. Preparation of the multi-functional AT–Fe3 O4 /PMOA nanocomposite hydrogels Referring to our previous work [31], the multi-functional AT–Fe3 O4 /PMOA nanocomposite hydrogels (mf-NC hydrogels) were prepared by in situ free radical polymerization in ethanol solution. First, AT–Fe3 O4 with certain weight was dispersed in the ethanol solution under ultrasonic vibration for 1.5 h. Then, the monomers of MEO2 MA, OEGMA and AAc were added into the AT–Fe3 O4 solution and were allowed to mix under ultrasonic vibration for 30 min. To initiate the polymerization, a certain amount of redox initiators which were composed of K2 S2 O8 and NaHSO3
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Table 1 Composition of mf-NC hydrogels. Samples
AT–Fe3 O4 (wt%)
POMA (wt%)
Ethanol (wt%)
Pure PMOA mf-NC1 mf-NC2 mf-NC3
0 1 3 5
57 56 55 54
43 43 42 41
Note: 1 wt% K2 S2 O8 and NaHSO3 as initiators = 2 ml.
were added into the mixture with stirring for 10 min. Finally, the mixture was put into a vacuum oven for 5 min to remove any air bubbles. The composite solution was injected into glass tubes (diameter = 5 mm, length = 120 mm) at room temperature under vacuum for 24 h. When the reaction is completed, the pieces of glass tubing filled with NC gels were carefully broken, and the gel columns were immersed in deionized water for at least 3 days to remove the unreacted reactants. During this period the water was replaced several times. For comparison, the pure PMOA hydrogel was also prepared with the same method. The different composition of the samples is listed in Table 1. The sample codes mf-NC1 , mf-NC2 and mf-NC3 corresponded to the AT–Fe3 O4 contents of 1, 3 and 5 wt%, respectively. 2.4. Characterizations 2.4.1. Fourier transform infrared spectroscopy (FT-IR) The dried AT, Fe3 O4 and AT–Fe3 O4 samples were tested by a Nicolet Impact NEXUS-8700 FT-IR in the range of 400–4000 cm−1 . The dried sample was ground with dried potassium bromide (KBr) powder and compressed into a disc, and then was subjected to analysis. 2.4.2. X-ray diffraction (XRD) The X-ray diffraction of samples was performed on a Rigaku (Japan) DMAX-2550 X-ray diffractometer. The Cu K␣ radiation with a wavelength of =1.54178 A˚ was used. 2.4.3. Field emission scanning electron microscopy (FESEM) FESEM (Hitachi S-4800) was used to observe the morphology of powdered AT and AT–Fe3 O4 , and the distribution of mf-NC hydrogels. The powdered samples were dispersed in aqueous solution (1–5%) under the ultrasonic vibration for 10 min, then the mixture solution was dropped to a clean coverslip in order to observe the morphology when it is dried. The fully swollen mf-NC hydrogels were dried at room temperature for several days until all water volatilized. The cross-sections and the vertical sections of the dried mf-NC hydrogels were coated with gold and measured by FESEM. 2.4.4. Vibrating sample magnetometer (VSM) The magnetization measurements of AT–Fe3 O4 , Fe3 O4 and mfNC hydrogels were carried out at room temperature using a vibrating sample magnetometry (PPMS VSM, Model6000) with a maximum applied field of 1 T. 2.4.5. Measurements of re-swelling behaviors in an alternating magnetic field The re-swelling kinetics of hydrogels was measured gravimetrically in a home-made alternating magnetic field (AMF) (as shown in Fig. 1) at room temperature. The AMF was generated by rotating the sample to cut magnetic induction lines. The rotating speed was 60 rpm. The mf-NC3 hydrogels was allowed to swell to the equilibrium in deionized water at room temperature before the re-swelling measurement. Then, the full swollen hydrogels were transferred into the AMF. At regular time intervals, the swollen samples were removed from the magnetic field and weighted after
Fig. 1. Schematic illustration of the re-swelling measurement of the mf-NC3 hydrogels in the AMF.
blotting off the excess water from the sample with a filter paper. The re-swelling ratio is calculated as follows: Re-swelling ratio =
Wt − Ws × 100% Ws
(1)
where Ws is the weight of the swollen equilibrium hydrogel and Wt is the weights of re-swelling hydrogel at a given time, respectively. For comparison, the re-swelling process of mf-NC3 hydrogel without the AMF was also measured and recorded.
2.4.6. Testing of thermo- and pH-responsive behaviors In order to confirm the thermo- and pH-responsive behaviors of the mf-NC hydrogels, the swelling ratios of the mf-NC hydrogels in deionized water and different pH buffer solutions were measured with the same method, as reported in the literature [37]. The effect of temperature on the equilibrium swelling ratio was measured at the temperature ranging from 25 ◦ C to 55 ◦ C. At each temperature, hydrogels need 12 h to expel water and to achieve the new equilibrium. Britton–Robinson buffer solutions with the same ionic strength and different pH values (1.98, 3.29, 4.56, 5.72, 6.8, 7.96, 9.15 and 10.38) were measured by pH meter. The pH-responsive behavior of mf-NC hydrogels was assessed at room temperature (25 ◦ C) by immersing hydrogels in different pH buffer solutions for 130 h. The swollen samples were removed from solution and weighted after blotting off the excess water from the sample with filter paper. The equilibrium swelling ratio (ESR) is calculated as follows: ESR =
Wt − Wd Wd
(2)
where Wt is the weight of the swollen hydrogel at given time during swelling and Wd is the weight of dry hydrogel.
2.4.7. Tensile mechanical properties The prepared hydrogels were fully swollen in deionized water for some days before subjecting to tensile tests using a Dejie DXLL2000 at 25 ◦ C, as reported in our previous publication [31]. All samples have the same size (6 mm diameter × 60 mm length). The conditions of tensile measurement are as follows: sample length between jaws was 40 mm, and crosshead speed was 10 mm min−1 . At least 5 samples were tested for each type of hydrogels and the data were averaged.
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Fig. 3. X-ray diffraction pattern for the purified AT, Fe3 O4 and AT–Fe3 O4 . Fig. 2. FT-IR of the AT–Fe3 O4 (a), Fe3 O4 (b) and purified AT (c).
3. Results and discussion 3.1. Properties of the magnetic attapulgite (AT–Fe3 O4 ) nanoparticle 3.1.1. FT-IR analysis The FT-IR spectra of sample AT–Fe3 O4 , Fe3 O4 and purified AT are shown in Fig. 2a, b and c, respectively. As can be seen from Fig. 2, a broad band exists at around 3400 cm−1 , assignable to the characteristic peak of OH groups, resulting from the absorbed water or hydroxyl on the surface of AT and Fe3 O4 . In the spectrum of Fig. 2c, the absorbance bands at 1033.3 cm−1 , 986.9 cm−1 and 475.8 cm−1 can be ascribed to the characteristic peaks of AT. The peaks at 565.1 cm−1 in Fig. 2b can be assigned to the vibrations Fe O of Fe3 O4 particles [38]. It is found that the absorption bands of AT–Fe3 O4 composite at 1030.8 cm−1 , 987.7 cm−1 and 513.6 cm−1 also appeared in Fig. 2a, which reveals the existence of AT and Fe3 O4 in the composite. 3.1.2. XRD analysis In order to understand the structure of the synthesized composite AT–Fe3 O4 , powder XRD has been recorded in Fig. 3. According to Ref. [39], the peaks of composite at 2 = 30.3◦ , 35.5◦ , 43.1◦ , 57.0◦ and 62.7◦ can be assigned to Fe3 O4 . The peaks of AT–Fe3 O4 at 2 = 8.2◦ , 13.8◦ , 19.7◦ and 27.5◦ are consistent with the primary diffraction of (1 1 0), (2 0 0), (0 4 0) and (4 0 0) planes of AT [40,41],
respectively, indicating that the structure of AT was not destroyed during the decorating process. However, it should be noted that the peak intensity of AT decrease after decorating. It can be concluded that the surface of AT was covered by Fe3 O4 .
3.1.3. Morphology and structure of AT–Fe3 O4 nanoparticle Fig. 4(a) shows the FESEM images of the purified AT. It is clear that AT exhibits a fibrous structure. The size of a single fiber can be clearly observed, which was about 50 nm in width and 0.5–1 m in length. The morphology of the obtained composite is displayed in Fig. 4(b). Combined with the result of FT-IR, it is evident that the surface of AT is decorated with Fe3 O4 nanoparticles. And the Fe3 O4 nanoparticles are in the range of 10–30 nm in diameter and the larger particles are aggregates of two or more small Fe3 O4 particles.
3.1.4. Magnetic properties of AT–Fe3 O4 particle The magnetic hysteresis loops of the purified AT, Fe3 O4 , AT–Fe3 O4 nanoparticles were measured at 300 K are shown in Fig. 5. The specific saturation magnetization ( s ) of as-prepared AT, AT–Fe3 O4 , and Fe3 O4 is 0, 52.63 and 63.67 emu/g, respectively. It demonstrates that the AT–Fe3 O4 composite has a typical superparamagnetic characteristic, which can be mainly attributed to the small particle size of Fe3 O4 nanoparticles.
Fig. 4. FESEM images of purified AT (a) and AT–Fe3 O4 (b) nanoparticles.
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Fig. 7. The vertical sections of mf-NC3 with the application of a static magnetic field.
Fig. 5. Magnetic hysteresis loops for purified AT, Fe3 O4 and AT–Fe3 O4 nanoparticles.
3.2. Characterization of the multi-functional AT–Fe3 O4 /PMOA NC hydrogels 3.2.1. The structure and morphology of the mf-NC hydrogels Fig. 6 shows the morphology of the pure PMOA and mf-NC hydrogels. By comparing with morphology of the pure hydrogel (Fig. 6(a) and (b)), it is clear that the rod-like AT–Fe3 O4 particles are successfully introduced into the hydrogel matrix (Fig. 6(c–f) indicated by red circle). Compared with mf-NC3 (Fig. 6(e) and (f)), mf-NC1 presents a relatively uniform dispersion. That can be explained by the fact that the AT–Fe3 O4 content of mf-NC3 is higher than that of mf-NC1 , leading to a slight agglomeration. Additionally, it should be mentioned that the structure and morphology of the rod-like magnetic nanoparticles were not damaged and consistent
with the morphology as shown in Fig. 4. Therefore, due to the presence of magnetic AT–Fe3 O4 nanoparticles, the prepared hydrogels are expecting to fulfill magnetic functionalization, which can be used in controlled release of drugs under magnetic field. 3.2.2. Magnetic properties of the mf-NC hydrogels The fully swollen mf-NC3 was dried at room temperature on a magnet for several days until all water volatilized. Then the vertical section of the dried sample was measured by FESEM. Fig. 7 shows FESEM image of the vertical sections of mf-NC3 under a static magnetic field. Compared with the same sample without the magnetic field (Fig. 6(f)), the arrangement of the magnetic AT–Fe3 O4 nanoparticles is more perpendicular to the surface of the hydrogel matrix (as shown inside red circle). In other words, AT–Fe3 O4 nanoparticles were arranged along the direction of the magnetic field from disorder to order due to the application of the magnetic field, which is similar to the orientation of the polymer to a certain
Fig. 6. FESEM images of pure PMOA and dried mf-NC hydrogel with AT–Fe3 O4 content of 1% and 5%. ((a), (c), (e) and (b), (d), (f) corresponding to the cross sections and the vertical sections of pure PMOA, mf-NC1 and mf-NC3 , respectively). The magnification times of (a) and (b–f) are 5000 and 20 000, respectively.
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extent. In addition, it also can be seen from the Fig. 7 that a part of AT–Fe3 O4 nanoparticles are agglomerated obviously with response to the magnetic field. These results suggest that under the external magnetic field, the internal morphology of the hydrogel can be changed due to the presence of the magnetic nanoparticles. Thus, from a macroscopical point of view, the hydrogel is endowed with magnetic response property. Additionally, we built a simple magnetically responsive device using a magnet for testing the apparent magnetic response, as shown in Fig. 8(I(a)). The apparent magnetic response results of pure PMOA and mf-NC hydrogels are shown in Fig. 8(I(b–e)). It can be seen from Fig. 8(I(b)) that when the pure PMOA hydrogel is close to the magnet infinitely, there is no magnetic response. That can be proved that the pure PMOA hydrogel itself as a matrix material has no magnetic response. However, it obviously can be seen from Fig. 8(I(c–e)) that all mf-NC hydrogels have a magnetic response because of the introduction of the magnetic AT–Fe3 O4 . When the mf-NC hydrogels are getting close to the magnet, they will be attracted towards the magnet. According to the scale of Fig. 8(I), the relationship between the sensing distance (SD) and the content of AT–Fe3 O4 is shown in Fig. 8(II). It can be seen that linear relation with a regressive coefficient of 0.983 is obtained. The values of SD are 0 cm for pure PMOA, nearly 0.1 cm for mf-NC1 , 0.3 cm for mf-NC2 and 0.6 cm for mfNC3 , respectively. It is quantitatively understood that the magnetic responsive intensity of the mf-NC hydrogels increases with increasing of AT–Fe3 O4 nanoparticles content. This result is also supported by magnetization measurements. The magnetization (M) of various mf-NC hydrogels was measured by a vibrating sample magnetometer, as shown in Fig. 8(III). The magnetic measurement studies show that all the mf-NC hydrogels exhibited superparamagnetic behavior, demonstrated by the symmetrical sigmoid shape of the
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magnetization curves exhibiting no hysteresis. The magnetization values of mf-NC1 , mf-NC2 and mf-NC3 are 0.28, 0.85 and 1.30 emu/g, respectively. It is clear that by increasing the AT–Fe3 O4 concentration, the saturation magnetization increases as the density of magnetic nanoparticles increases. Similar results were reported by Liu et al. on the PVA/Fe3 O4 nanocomposite system [42]. Fig. 9 shows the re-swelling behavior of the mf-NC3 hydrogel in an alternating magnetic field (AMF). It can be seen from Fig. 9(a) that the mf-NC3 hydrogel can further swell in the AMF after equilibrium swelling in deionized water. However, when the mf-NC3 hydrogel is rotated without the AMF, it is almost never re-swelling. The photos of comparison samples are shown in Fig. 9(b). This difference can be interpreted that by rotating the sample between the two magnets, the magnetic induction lines were cut repeatedly, and then an alternating magnetic field is generated. Although the hydrogel has reached equilibrium swelling in deionized water, the distribution of the magnetic AT–Fe3 O4 nanoparticles in the mfNC3 hydrogel is still very close. This can be explained by the fact that these single-domain magnetic nanoparticles being adhesively attached to the polymer chains, they have no translational diffusion without the application of the AMF [43]. However, due to the effect of the AMF, the magnetic nanoparticles have uninterrupted vibration in the hydrogel network, and thus the arrangement of the magnetic nanoparticles become more loose. It is reasonable to suggest that an increase of the distance between the molecular chains lead to a larger space in the hydrogel networks, which allows water to enter further. This phenomenon can affect the swelling behavior of the whole system (Fig. 9(c)). As a result, the mf-NC hydrogels can continue to swell and achieve new swelling equilibrium to get higher swelling ratio. Furthermore, from the experimental phenomena, the re-swelling process of the mf-NC3 hydrogel is irreversible, so it does not de-swell when the AMF is
Fig. 8. (I) The magnetic response testing of hydrogels (b) pure PMOA, (c) mf-NC1 , (d) mf-NC2 , (e) mf-NC3 . (II) The sensing distances between the magnet and the samples with varying AT–Fe3 O4 content. (III) Hysteresis loop analysis of the mf-NC hydrogels incorporated with various AT–Fe3 O4 additions measured by VSM.
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Fig. 9. Comparison of the re-swelling ratios (a) and the images (b) of the mf-NC3 hydrogel measured with and without the application of an AMF (c). Schematic showing the magnetic response mechanism of the mf-NC hydrogels in the AMF.
halted. Of course, the magnetic response behavior of the mf-NC hydrogel is relatively slow, which may be related to the magnetic field strength, alternating frequency and the content of magnetic nanoparticles in the hydrogels and so on. This part will be the subject of a detailed study in the future experiments. 3.2.3. Thermo- and pH-responsive behaviors of the mf-NC hydrogels In a recent publication [35], we have demonstrated that the PMOA terpolymer hydrogel based on MEO2 MA, OEGMA and AAc units exhibited both thermo- and pH-responsive behaviors. To confirm the dual responses of the mf-NC hydrogels, we investigate the effects of temperature and different pH buffer solutions on the swelling ratios of the samples. Schematic representation of the mf-NC hydrogels exhibiting stimuli sensitivity in response to temperature and pH is presented in Fig. 10(a). As previously reported for PMOA hydrogels [35], this transition can be attributed to the H-bonding interactions between ethenoxy polar groups of OEG side chains and water. At temperature below the LCST, strong H-bonding interactions lead to good solubility, resulting in higher swelling ratios. When the temperature ascends beyond the LCST, the conformational transition of OEG side chains decreased the polarity of ethenoxy groups, and hydration is weakened. The hydrogels began to shrink, resulting in lower swelling ratios. Data summarized in Fig. 10(b) show that the equilibrium swelling ratios of all hydrogels decrease with increasing temperature from 25 ◦ C to 55 ◦ C. The main conclusion that can be deduced from this graph
is that the thermo-sensitivity of mf-NC hydrogels is not eliminated with the incorporation of AT–Fe3 O4 nanoparticles. Fig. 10(c) shows the pH-dependent swelling behavior of the mf-NC2 hydrogel at room temperature (25 ◦ C), indicating the transition point of pH at around 4.5 where the hydrogel networks start to dissociate. This dissociation occurs in a very narrow pH range. When the pH value of the buffer solution is above 4.5, the hydrogels begin to swell. When the pH is below 4.5, the hydrogels can protonate their carboxyl moieties of AAc, and the carboxyl moieties form a lot of hydrogen bonds, which pull back the whole network chain segments and squeeze more solution out of the hydrogels, resulting in shrinking. From the above analysis results, the mf-NC hydrogel still possesses temperature- and pH-sensitive simultaneously. The temperature and pH dependent swelling of the hydrogel is reversible, although the magnetic AT–Fe3 O4 nanoparticles were introduced into the PMOA system. 3.2.4. Tensile properties of the mf-NC hydrogels The mechanical properties of the mf-NC hydrogels were studied by tensile testing. Fig. 11 shows that the tensile strength of the mf-NC hydrogels increases with increasing of AT–Fe3 O4 content. When the content of AT–Fe3 O4 increased from 0 to 5%, the tensile strength doubled. The significant improvement in mechanical properties can be attributed to reinforcement on the physical cross-linked PMOA network from the AT–Fe3 O4 clay. Similar reasons have been detailedly discussed in our previous publication on the PMOA/AT nanocomposite system [31].
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Fig. 10. (a) Schematic representation of the mf-NC hydrogels exhibiting stimuli sensitivity in response to temperature (Tc is the critical phase transition temperature) and pH. (b) Changes of swelling ratios of pure PMOA and mf-NC hydrogels with the temperature in the deionized water range from 25 ◦ C to 55 ◦ C (c). The equilibrium swelling behavior of mf-NC2 hydrogel at different pH (from 1.98 to 10.38) values at 25 ◦ C.
Fig. 11. Tensile strength depending on the content of AT–Fe3 O4 .
nanoparticles were well deposited on the surface of AT and the AT–Fe3 O4 nanocomposite particles presented the superparamagnetic behavior at 300 K. Secondly, the magnetic AT–Fe3 O4 nanoparticles were directly incorporated into the dual-responsive PMOA hydrogel network by in situ free radical polymerization. It has been demonstrated that the rod-like magnetic AT–Fe3 O4 nanoparticles were well dispersed in the hydrogel matrix. The magnetic hysteresis loops indicated that the mf-NC hydrogel exhibited superparamagnetic behavior, and the saturation magnetization of that increased with increasing the AT–Fe3 O4 content. The mf-NC hydrogel can continue to swell in an alternating magnetic field after equilibrium swelling in deionized water. These results show that the mf-NC hydrogel can be used in magnetically controlled release of drugs under the magnetic field. Despite the magnetic functionality, the mf-NC hydrogels also possess considerable temperature/pH sensitivity and excellent mechanical properties. The unique combination of these multiple sensitivities in organic/inorganic hybrid hydrogels makes them interesting candidates for design of artificial muscles, biosensors and actuators and so on.
4. Conclusions Herein, we have successfully prepared multi-functional organic/inorganic hybrid hydrogels. Firstly, a simple and effective method has been developed to decorate the AT with Fe3 O4 via electrostatic attraction. The results showed that Fe3 O4
Acknowledgments This work was supported by grants from the Program of Introducing Talents of Discipline to Universities (No. 111-2-04).
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Fabrication and characterization of temperature-, pH- and magnetic-field-sensitive organic/inorganic hybrid poly (ethylene glycol)-based hydrogels Yang Wang, Aijuan Dong, Zhicheng Yuan, Dajun Chen ∗ State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Shanghai 201620, People’s Republic of China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
A new kind of magnetic nanoparticle was prepared via co-precipitation technique. Novel multiple stimulus-responsive hydrogels were prepared by in situ polymerization. The mf-NC hydrogels have T/pH/magnetic sensitivity and good mechanical properties. The mf-NC hydrogel can continue swelling under an alternating magnetic field.
a r t i c l e
i n f o
Article history: Received 6 July 2012 Received in revised form 21 September 2012 Accepted 1 October 2012 Available online 9 October 2012 Keywords: Magnetic-sensitive Thermo-sensitive pH-sensitive Magnetic AT–Fe3 O4 nanoparticles P(MEO2 MA-co-OEGMA-co-AAc) hydrogel
a b s t r a c t In this paper, we successfully fabricated a new kind of multiple stimulus-responsive organic/inorganic hybrid hydrogels by combining dual stimuli-responsive poly (2-(2-methoxyethoxy) ethyl methacrylate-co-oligo (ethylene glycol) methacrylate-co-acrylic acid) (PMOA) hydrogel with magnetic attapulgite/Fe3 O4 (AT–Fe3 O4 ) nanoparticles. First, the magnetic nanoparticle was prepared via co-precipitation technique in aqueous suspension of purified attapulgite. The obtained AT–Fe3 O4 nanoparticles were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy and vibrating sample magnetometer. Compared with pure attapulgite, the AT–Fe3 O4 exhibited better superparamagnetic properties. Then, the AT–Fe3 O4 was introduced into the dual-responsive (temperature and pH) PMOA hydrogel network by in situ polymerization. The morphology, responsive behaviors and tensile properties of the prepared hydrogels were systematically characterized by field emission scanning electron microscopy, vibrating sample magnetometer, swelling/re-swelling behaviors and tensile testing. The results showed that the AT–Fe3 O4 nanoparticles were well dispersed in the hydrogel matrix, and the multi-functional AT–Fe3 O4 /PMOA nanocomposite hydrogels had not only temperature/pH sensitivity and good mechanical properties, but also magnetic functionality. The tunable superparamagnetic behavior of these hydrogels depended on the amount of AT–Fe3 O4 . In addition, the multi-functional AT–Fe3 O4 /PMOA nanocomposite hydrogels can continue to swell under an alternating magnetic field after equilibrium swelling in deionized water. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
∗ Corresponding author. Tel.: +86 21 67792891; fax: +86 21 67792855. E-mail address: [email protected] (D. Chen). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.10.009
Smart hydrogels are three-dimensional network structure that change their physical and (or) chemical properties in response to external environmental stimuli [1–3], such as temperature, pH, pressure, electric field, magnetic field and light and so on. Due to the
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unique stimulus–response property, smart hydrogels are widely applied in drug controlled release, biosensors, chemical converters, artificial muscles, tissue engineering, and molecular separation system and so on [4–7]. Currently, the smart hydrogels can be mainly divided into single-sensitive hydrogels [8–15] (temperature (T), pH, electric field, magnetic field and light), dual-sensitive hydrogels [16–20] (T–pH sensitive, pH salt sensitive and magnetic thermal sensitive), multiple-sensitive hydrogels [17,21–23] (T–pH–magnetic response, T–pH–light response). In the past few decades, single and dual-sensitive hydrogels have attracted considerable attention. However, from the development of practical applications, the fabrication of multiple-sensitive hydrogels would be much more attractive if they could respond to at least three stimulation, such as possessing T-, pH- and magnetic/electric/light-sensitive simultaneously. Such multipleresponsive materials are of interest for a variety of applications ranging from magnetic/electric/light separation or drug release systems to sensors and actuators. Presently, the incorporation of magnetic nanoparticles (Fe3 O4 , ␥-Fe2 O3 or CoFe2 O4 ) into T-/pH-sensitive microgels/hydrogels matrix to form triple-response hydrogels can be easily achieved and has received increasing interest. For example, Lang and co-workers [24] prepared polysaccharide-based hydrogels with pH- and thermo-sensitive property by the copolymerization of maleilated carboxymethyl chitosan with N-isopropylacrylamide, and then fabricated their magnetic hybrid hydrogels by in situ deposition of magnetic iron oxide nanoparticles into the porous hydrogel networks. Bhattacharya et al. [21] described the synthesis and characterization of multifunctional hybrid microgels that exhibit temperature, pH and magnetic triple stimuli-response by in situ formation of iron oxide nanoparticles in the microgel structure. However, in situ formation of magnetic nanoparticles in polymer network needed to be treated with alkaline solution. In this case the polymer gels network would be easily destroyed, which decreased the overall performance of the gels. In order to overcome this shortcoming, some researchers began to use readymade magnetic particles introduced directly into the polymer gels to achieve the magnetic response of polymer gels. However, it is well-known that poor mechanical properties of hydrogels severely restrict their practical applications. Currently, incorporating clay into hydrogel matrix has been proved to be an effective and simple method to enhance the mechanical properties of hydrogels [25–31]. Attapulgite (AT) is a type of natural fibrillar aluminum silicate with abundant hydroxyl groups on the surfaces [31]. Decorating AT with Fe3 O4 magnetic nanoparticles (AT–Fe3 O4 ) may give birth to novel chemical and physical properties, expecting to be applied in polymeric gels system. To date, the hybrid materials on the combination of magnetic nanoparticles (such as Fe3 O4 ) with responsive hydrogels have been reported [14,19,32–34]. Echeverria and co-worker [14] have reported that UCST-like hybrid poly (acrylamide-acrylic acid)/Fe3 O4 microgels by the incorporation of metal oxide nanoparticles into microgels. They investigated the effect of Fe3 O4 nanoparticles on morphology, thermo-sensitivity and elasticity. Recently, Chanana et al. [34] have prepared nanocomposites based on magnetite and thermoresponsive poly (2-(2-methoxyethoxy) ethyl methacrylate)-co-oligo (ethyleneglycol) methacrylate) (P(MEO2 MA-co-OEGMA)) copolymers. The advantage of these polymers is that the lower critical solution temperature (LCST) in water can be easily adjusted by the oligo (ethylene glycol) chain length in a wide temperature range from 24 to 95 ◦ C. A similar study was report by Gelbrich [19] in the preparation of magnetic core–shell nanoparticles that were composed of nanosized superparamagnetic iron oxide cores and a copolymer shell. The shell which consists of OEGMA and methoxyethyl methacrylate (MEMA) copolymers shows a LCST in water.
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In our previous publication [31,35], it was demonstrated that composition of MEO2 MA, OEGMA and acrylic acid (AAc) gels (PMOA) can be tailored to get temperature and pH response, and the mechanical properties of the gel can be significantly improved by adding the AT nanoparticles. Here, the aim of the present study was the preparation of hydrogels with excellent mechanical properties that are sensitive to temperature, pH and magnetic field. Therefore, we fabricated a new kind of smart organic/inorganic hybrid hydrogel by combining dual stimuli-responsive PMOA hydrogel with magnetic AT–Fe3 O4 nanoparticles. First, the magnetic nanoparticle, attapulgite/Fe3 O4 (AT–Fe3 O4 ), was prepared via a simple co-precipitation method. Then, the AT–Fe3 O4 was successfully introduced into the dual-responsive (T and pH) PMOA network by in situ polymerization. The microstructures and various properties of the AT–Fe3 O4 and the prepared hydrogels were systematically characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), vibrating sample magnetometer (VSM), swelling/re-swelling behaviors and tensile testing. 2. Experimental 2.1. Materials 2-(2-Methoxyethoxy) ethyl methacrylate (MEO2 MA, 97%) was purchased from TCI Chemical Co. Tokyo, Japan. Oligo (ethylene glycol) methyl ether methactylate (OEGMA, average Mn = 475, the number of EO units 8–9) was purchased from Sigma–Aldrich Co. Attapulgite (AT) was obtained from Xuyi Colloidal Co. Jiangsu, China. Acrylic acid (AAc), iron dichloride (FeCl2 ), iron chloride (FeCl3 ), sodium hydroxide (NaOH), potassium persulfate (K2 S2 O8 ), and sodium hydrogen sulfite (NaHSO3 ) were purchased from Sinopharm Chemical Reagent Co. Ltd. All these reagents were used as received. 2.2. Preparation of the magnetic attapulgite (AT–Fe3 O4 ) particles The crude AT powder contains a lot of impurities and dirt. First, AT was purified before the preparation of magnetic particles, according to our previous work [36]. Purified AT (2.4 g) and deionized water (320 ml) were added to a flask and the pH of the aqueous dispersion was adjusted to 8. Then 0.4 M FeCl3 solution (20 ml) was added into the flask, dispersed in an ultrasonic bath (50 kHz) for 30 min. 0.9 M FeCl2 solution (20 ml) was added to the aqueous dispersion. The mixture was treated in the ultrasonic bath for 15 min and then 1 M NaOH solution was dropped into the flask until the pH value of the aqueous dispersion reached 10 and the black precipitate appeared immediately. Then, the mixture was treated in the ultrasonic bath for another 1.5 h. After that, the product was centrifuged at 12 000 rpm for 5 min and washed with deionized water several times to remove unreacted ions, and dried in a vacuum oven at 70 ◦ C for 24 h. The weight fraction of Fe3 O4 in AT–Fe3 O4 composite is nearly 30 wt%. 2.3. Preparation of the multi-functional AT–Fe3 O4 /PMOA nanocomposite hydrogels Referring to our previous work [31], the multi-functional AT–Fe3 O4 /PMOA nanocomposite hydrogels (mf-NC hydrogels) were prepared by in situ free radical polymerization in ethanol solution. First, AT–Fe3 O4 with certain weight was dispersed in the ethanol solution under ultrasonic vibration for 1.5 h. Then, the monomers of MEO2 MA, OEGMA and AAc were added into the AT–Fe3 O4 solution and were allowed to mix under ultrasonic vibration for 30 min. To initiate the polymerization, a certain amount of redox initiators which were composed of K2 S2 O8 and NaHSO3
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Table 1 Composition of mf-NC hydrogels. Samples
AT–Fe3 O4 (wt%)
POMA (wt%)
Ethanol (wt%)
Pure PMOA mf-NC1 mf-NC2 mf-NC3
0 1 3 5
57 56 55 54
43 43 42 41
Note: 1 wt% K2 S2 O8 and NaHSO3 as initiators = 2 ml.
were added into the mixture with stirring for 10 min. Finally, the mixture was put into a vacuum oven for 5 min to remove any air bubbles. The composite solution was injected into glass tubes (diameter = 5 mm, length = 120 mm) at room temperature under vacuum for 24 h. When the reaction is completed, the pieces of glass tubing filled with NC gels were carefully broken, and the gel columns were immersed in deionized water for at least 3 days to remove the unreacted reactants. During this period the water was replaced several times. For comparison, the pure PMOA hydrogel was also prepared with the same method. The different composition of the samples is listed in Table 1. The sample codes mf-NC1 , mf-NC2 and mf-NC3 corresponded to the AT–Fe3 O4 contents of 1, 3 and 5 wt%, respectively. 2.4. Characterizations 2.4.1. Fourier transform infrared spectroscopy (FT-IR) The dried AT, Fe3 O4 and AT–Fe3 O4 samples were tested by a Nicolet Impact NEXUS-8700 FT-IR in the range of 400–4000 cm−1 . The dried sample was ground with dried potassium bromide (KBr) powder and compressed into a disc, and then was subjected to analysis. 2.4.2. X-ray diffraction (XRD) The X-ray diffraction of samples was performed on a Rigaku (Japan) DMAX-2550 X-ray diffractometer. The Cu K␣ radiation with a wavelength of =1.54178 A˚ was used. 2.4.3. Field emission scanning electron microscopy (FESEM) FESEM (Hitachi S-4800) was used to observe the morphology of powdered AT and AT–Fe3 O4 , and the distribution of mf-NC hydrogels. The powdered samples were dispersed in aqueous solution (1–5%) under the ultrasonic vibration for 10 min, then the mixture solution was dropped to a clean coverslip in order to observe the morphology when it is dried. The fully swollen mf-NC hydrogels were dried at room temperature for several days until all water volatilized. The cross-sections and the vertical sections of the dried mf-NC hydrogels were coated with gold and measured by FESEM. 2.4.4. Vibrating sample magnetometer (VSM) The magnetization measurements of AT–Fe3 O4 , Fe3 O4 and mfNC hydrogels were carried out at room temperature using a vibrating sample magnetometry (PPMS VSM, Model6000) with a maximum applied field of 1 T. 2.4.5. Measurements of re-swelling behaviors in an alternating magnetic field The re-swelling kinetics of hydrogels was measured gravimetrically in a home-made alternating magnetic field (AMF) (as shown in Fig. 1) at room temperature. The AMF was generated by rotating the sample to cut magnetic induction lines. The rotating speed was 60 rpm. The mf-NC3 hydrogels was allowed to swell to the equilibrium in deionized water at room temperature before the re-swelling measurement. Then, the full swollen hydrogels were transferred into the AMF. At regular time intervals, the swollen samples were removed from the magnetic field and weighted after
Fig. 1. Schematic illustration of the re-swelling measurement of the mf-NC3 hydrogels in the AMF.
blotting off the excess water from the sample with a filter paper. The re-swelling ratio is calculated as follows: Re-swelling ratio =
Wt − Ws × 100% Ws
(1)
where Ws is the weight of the swollen equilibrium hydrogel and Wt is the weights of re-swelling hydrogel at a given time, respectively. For comparison, the re-swelling process of mf-NC3 hydrogel without the AMF was also measured and recorded.
2.4.6. Testing of thermo- and pH-responsive behaviors In order to confirm the thermo- and pH-responsive behaviors of the mf-NC hydrogels, the swelling ratios of the mf-NC hydrogels in deionized water and different pH buffer solutions were measured with the same method, as reported in the literature [37]. The effect of temperature on the equilibrium swelling ratio was measured at the temperature ranging from 25 ◦ C to 55 ◦ C. At each temperature, hydrogels need 12 h to expel water and to achieve the new equilibrium. Britton–Robinson buffer solutions with the same ionic strength and different pH values (1.98, 3.29, 4.56, 5.72, 6.8, 7.96, 9.15 and 10.38) were measured by pH meter. The pH-responsive behavior of mf-NC hydrogels was assessed at room temperature (25 ◦ C) by immersing hydrogels in different pH buffer solutions for 130 h. The swollen samples were removed from solution and weighted after blotting off the excess water from the sample with filter paper. The equilibrium swelling ratio (ESR) is calculated as follows: ESR =
Wt − Wd Wd
(2)
where Wt is the weight of the swollen hydrogel at given time during swelling and Wd is the weight of dry hydrogel.
2.4.7. Tensile mechanical properties The prepared hydrogels were fully swollen in deionized water for some days before subjecting to tensile tests using a Dejie DXLL2000 at 25 ◦ C, as reported in our previous publication [31]. All samples have the same size (6 mm diameter × 60 mm length). The conditions of tensile measurement are as follows: sample length between jaws was 40 mm, and crosshead speed was 10 mm min−1 . At least 5 samples were tested for each type of hydrogels and the data were averaged.
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Fig. 3. X-ray diffraction pattern for the purified AT, Fe3 O4 and AT–Fe3 O4 . Fig. 2. FT-IR of the AT–Fe3 O4 (a), Fe3 O4 (b) and purified AT (c).
3. Results and discussion 3.1. Properties of the magnetic attapulgite (AT–Fe3 O4 ) nanoparticle 3.1.1. FT-IR analysis The FT-IR spectra of sample AT–Fe3 O4 , Fe3 O4 and purified AT are shown in Fig. 2a, b and c, respectively. As can be seen from Fig. 2, a broad band exists at around 3400 cm−1 , assignable to the characteristic peak of OH groups, resulting from the absorbed water or hydroxyl on the surface of AT and Fe3 O4 . In the spectrum of Fig. 2c, the absorbance bands at 1033.3 cm−1 , 986.9 cm−1 and 475.8 cm−1 can be ascribed to the characteristic peaks of AT. The peaks at 565.1 cm−1 in Fig. 2b can be assigned to the vibrations Fe O of Fe3 O4 particles [38]. It is found that the absorption bands of AT–Fe3 O4 composite at 1030.8 cm−1 , 987.7 cm−1 and 513.6 cm−1 also appeared in Fig. 2a, which reveals the existence of AT and Fe3 O4 in the composite. 3.1.2. XRD analysis In order to understand the structure of the synthesized composite AT–Fe3 O4 , powder XRD has been recorded in Fig. 3. According to Ref. [39], the peaks of composite at 2 = 30.3◦ , 35.5◦ , 43.1◦ , 57.0◦ and 62.7◦ can be assigned to Fe3 O4 . The peaks of AT–Fe3 O4 at 2 = 8.2◦ , 13.8◦ , 19.7◦ and 27.5◦ are consistent with the primary diffraction of (1 1 0), (2 0 0), (0 4 0) and (4 0 0) planes of AT [40,41],
respectively, indicating that the structure of AT was not destroyed during the decorating process. However, it should be noted that the peak intensity of AT decrease after decorating. It can be concluded that the surface of AT was covered by Fe3 O4 .
3.1.3. Morphology and structure of AT–Fe3 O4 nanoparticle Fig. 4(a) shows the FESEM images of the purified AT. It is clear that AT exhibits a fibrous structure. The size of a single fiber can be clearly observed, which was about 50 nm in width and 0.5–1 m in length. The morphology of the obtained composite is displayed in Fig. 4(b). Combined with the result of FT-IR, it is evident that the surface of AT is decorated with Fe3 O4 nanoparticles. And the Fe3 O4 nanoparticles are in the range of 10–30 nm in diameter and the larger particles are aggregates of two or more small Fe3 O4 particles.
3.1.4. Magnetic properties of AT–Fe3 O4 particle The magnetic hysteresis loops of the purified AT, Fe3 O4 , AT–Fe3 O4 nanoparticles were measured at 300 K are shown in Fig. 5. The specific saturation magnetization ( s ) of as-prepared AT, AT–Fe3 O4 , and Fe3 O4 is 0, 52.63 and 63.67 emu/g, respectively. It demonstrates that the AT–Fe3 O4 composite has a typical superparamagnetic characteristic, which can be mainly attributed to the small particle size of Fe3 O4 nanoparticles.
Fig. 4. FESEM images of purified AT (a) and AT–Fe3 O4 (b) nanoparticles.
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Fig. 7. The vertical sections of mf-NC3 with the application of a static magnetic field.
Fig. 5. Magnetic hysteresis loops for purified AT, Fe3 O4 and AT–Fe3 O4 nanoparticles.
3.2. Characterization of the multi-functional AT–Fe3 O4 /PMOA NC hydrogels 3.2.1. The structure and morphology of the mf-NC hydrogels Fig. 6 shows the morphology of the pure PMOA and mf-NC hydrogels. By comparing with morphology of the pure hydrogel (Fig. 6(a) and (b)), it is clear that the rod-like AT–Fe3 O4 particles are successfully introduced into the hydrogel matrix (Fig. 6(c–f) indicated by red circle). Compared with mf-NC3 (Fig. 6(e) and (f)), mf-NC1 presents a relatively uniform dispersion. That can be explained by the fact that the AT–Fe3 O4 content of mf-NC3 is higher than that of mf-NC1 , leading to a slight agglomeration. Additionally, it should be mentioned that the structure and morphology of the rod-like magnetic nanoparticles were not damaged and consistent
with the morphology as shown in Fig. 4. Therefore, due to the presence of magnetic AT–Fe3 O4 nanoparticles, the prepared hydrogels are expecting to fulfill magnetic functionalization, which can be used in controlled release of drugs under magnetic field. 3.2.2. Magnetic properties of the mf-NC hydrogels The fully swollen mf-NC3 was dried at room temperature on a magnet for several days until all water volatilized. Then the vertical section of the dried sample was measured by FESEM. Fig. 7 shows FESEM image of the vertical sections of mf-NC3 under a static magnetic field. Compared with the same sample without the magnetic field (Fig. 6(f)), the arrangement of the magnetic AT–Fe3 O4 nanoparticles is more perpendicular to the surface of the hydrogel matrix (as shown inside red circle). In other words, AT–Fe3 O4 nanoparticles were arranged along the direction of the magnetic field from disorder to order due to the application of the magnetic field, which is similar to the orientation of the polymer to a certain
Fig. 6. FESEM images of pure PMOA and dried mf-NC hydrogel with AT–Fe3 O4 content of 1% and 5%. ((a), (c), (e) and (b), (d), (f) corresponding to the cross sections and the vertical sections of pure PMOA, mf-NC1 and mf-NC3 , respectively). The magnification times of (a) and (b–f) are 5000 and 20 000, respectively.
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extent. In addition, it also can be seen from the Fig. 7 that a part of AT–Fe3 O4 nanoparticles are agglomerated obviously with response to the magnetic field. These results suggest that under the external magnetic field, the internal morphology of the hydrogel can be changed due to the presence of the magnetic nanoparticles. Thus, from a macroscopical point of view, the hydrogel is endowed with magnetic response property. Additionally, we built a simple magnetically responsive device using a magnet for testing the apparent magnetic response, as shown in Fig. 8(I(a)). The apparent magnetic response results of pure PMOA and mf-NC hydrogels are shown in Fig. 8(I(b–e)). It can be seen from Fig. 8(I(b)) that when the pure PMOA hydrogel is close to the magnet infinitely, there is no magnetic response. That can be proved that the pure PMOA hydrogel itself as a matrix material has no magnetic response. However, it obviously can be seen from Fig. 8(I(c–e)) that all mf-NC hydrogels have a magnetic response because of the introduction of the magnetic AT–Fe3 O4 . When the mf-NC hydrogels are getting close to the magnet, they will be attracted towards the magnet. According to the scale of Fig. 8(I), the relationship between the sensing distance (SD) and the content of AT–Fe3 O4 is shown in Fig. 8(II). It can be seen that linear relation with a regressive coefficient of 0.983 is obtained. The values of SD are 0 cm for pure PMOA, nearly 0.1 cm for mf-NC1 , 0.3 cm for mf-NC2 and 0.6 cm for mfNC3 , respectively. It is quantitatively understood that the magnetic responsive intensity of the mf-NC hydrogels increases with increasing of AT–Fe3 O4 nanoparticles content. This result is also supported by magnetization measurements. The magnetization (M) of various mf-NC hydrogels was measured by a vibrating sample magnetometer, as shown in Fig. 8(III). The magnetic measurement studies show that all the mf-NC hydrogels exhibited superparamagnetic behavior, demonstrated by the symmetrical sigmoid shape of the
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magnetization curves exhibiting no hysteresis. The magnetization values of mf-NC1 , mf-NC2 and mf-NC3 are 0.28, 0.85 and 1.30 emu/g, respectively. It is clear that by increasing the AT–Fe3 O4 concentration, the saturation magnetization increases as the density of magnetic nanoparticles increases. Similar results were reported by Liu et al. on the PVA/Fe3 O4 nanocomposite system [42]. Fig. 9 shows the re-swelling behavior of the mf-NC3 hydrogel in an alternating magnetic field (AMF). It can be seen from Fig. 9(a) that the mf-NC3 hydrogel can further swell in the AMF after equilibrium swelling in deionized water. However, when the mf-NC3 hydrogel is rotated without the AMF, it is almost never re-swelling. The photos of comparison samples are shown in Fig. 9(b). This difference can be interpreted that by rotating the sample between the two magnets, the magnetic induction lines were cut repeatedly, and then an alternating magnetic field is generated. Although the hydrogel has reached equilibrium swelling in deionized water, the distribution of the magnetic AT–Fe3 O4 nanoparticles in the mfNC3 hydrogel is still very close. This can be explained by the fact that these single-domain magnetic nanoparticles being adhesively attached to the polymer chains, they have no translational diffusion without the application of the AMF [43]. However, due to the effect of the AMF, the magnetic nanoparticles have uninterrupted vibration in the hydrogel network, and thus the arrangement of the magnetic nanoparticles become more loose. It is reasonable to suggest that an increase of the distance between the molecular chains lead to a larger space in the hydrogel networks, which allows water to enter further. This phenomenon can affect the swelling behavior of the whole system (Fig. 9(c)). As a result, the mf-NC hydrogels can continue to swell and achieve new swelling equilibrium to get higher swelling ratio. Furthermore, from the experimental phenomena, the re-swelling process of the mf-NC3 hydrogel is irreversible, so it does not de-swell when the AMF is
Fig. 8. (I) The magnetic response testing of hydrogels (b) pure PMOA, (c) mf-NC1 , (d) mf-NC2 , (e) mf-NC3 . (II) The sensing distances between the magnet and the samples with varying AT–Fe3 O4 content. (III) Hysteresis loop analysis of the mf-NC hydrogels incorporated with various AT–Fe3 O4 additions measured by VSM.
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Fig. 9. Comparison of the re-swelling ratios (a) and the images (b) of the mf-NC3 hydrogel measured with and without the application of an AMF (c). Schematic showing the magnetic response mechanism of the mf-NC hydrogels in the AMF.
halted. Of course, the magnetic response behavior of the mf-NC hydrogel is relatively slow, which may be related to the magnetic field strength, alternating frequency and the content of magnetic nanoparticles in the hydrogels and so on. This part will be the subject of a detailed study in the future experiments. 3.2.3. Thermo- and pH-responsive behaviors of the mf-NC hydrogels In a recent publication [35], we have demonstrated that the PMOA terpolymer hydrogel based on MEO2 MA, OEGMA and AAc units exhibited both thermo- and pH-responsive behaviors. To confirm the dual responses of the mf-NC hydrogels, we investigate the effects of temperature and different pH buffer solutions on the swelling ratios of the samples. Schematic representation of the mf-NC hydrogels exhibiting stimuli sensitivity in response to temperature and pH is presented in Fig. 10(a). As previously reported for PMOA hydrogels [35], this transition can be attributed to the H-bonding interactions between ethenoxy polar groups of OEG side chains and water. At temperature below the LCST, strong H-bonding interactions lead to good solubility, resulting in higher swelling ratios. When the temperature ascends beyond the LCST, the conformational transition of OEG side chains decreased the polarity of ethenoxy groups, and hydration is weakened. The hydrogels began to shrink, resulting in lower swelling ratios. Data summarized in Fig. 10(b) show that the equilibrium swelling ratios of all hydrogels decrease with increasing temperature from 25 ◦ C to 55 ◦ C. The main conclusion that can be deduced from this graph
is that the thermo-sensitivity of mf-NC hydrogels is not eliminated with the incorporation of AT–Fe3 O4 nanoparticles. Fig. 10(c) shows the pH-dependent swelling behavior of the mf-NC2 hydrogel at room temperature (25 ◦ C), indicating the transition point of pH at around 4.5 where the hydrogel networks start to dissociate. This dissociation occurs in a very narrow pH range. When the pH value of the buffer solution is above 4.5, the hydrogels begin to swell. When the pH is below 4.5, the hydrogels can protonate their carboxyl moieties of AAc, and the carboxyl moieties form a lot of hydrogen bonds, which pull back the whole network chain segments and squeeze more solution out of the hydrogels, resulting in shrinking. From the above analysis results, the mf-NC hydrogel still possesses temperature- and pH-sensitive simultaneously. The temperature and pH dependent swelling of the hydrogel is reversible, although the magnetic AT–Fe3 O4 nanoparticles were introduced into the PMOA system. 3.2.4. Tensile properties of the mf-NC hydrogels The mechanical properties of the mf-NC hydrogels were studied by tensile testing. Fig. 11 shows that the tensile strength of the mf-NC hydrogels increases with increasing of AT–Fe3 O4 content. When the content of AT–Fe3 O4 increased from 0 to 5%, the tensile strength doubled. The significant improvement in mechanical properties can be attributed to reinforcement on the physical cross-linked PMOA network from the AT–Fe3 O4 clay. Similar reasons have been detailedly discussed in our previous publication on the PMOA/AT nanocomposite system [31].
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Fig. 10. (a) Schematic representation of the mf-NC hydrogels exhibiting stimuli sensitivity in response to temperature (Tc is the critical phase transition temperature) and pH. (b) Changes of swelling ratios of pure PMOA and mf-NC hydrogels with the temperature in the deionized water range from 25 ◦ C to 55 ◦ C (c). The equilibrium swelling behavior of mf-NC2 hydrogel at different pH (from 1.98 to 10.38) values at 25 ◦ C.
Fig. 11. Tensile strength depending on the content of AT–Fe3 O4 .
nanoparticles were well deposited on the surface of AT and the AT–Fe3 O4 nanocomposite particles presented the superparamagnetic behavior at 300 K. Secondly, the magnetic AT–Fe3 O4 nanoparticles were directly incorporated into the dual-responsive PMOA hydrogel network by in situ free radical polymerization. It has been demonstrated that the rod-like magnetic AT–Fe3 O4 nanoparticles were well dispersed in the hydrogel matrix. The magnetic hysteresis loops indicated that the mf-NC hydrogel exhibited superparamagnetic behavior, and the saturation magnetization of that increased with increasing the AT–Fe3 O4 content. The mf-NC hydrogel can continue to swell in an alternating magnetic field after equilibrium swelling in deionized water. These results show that the mf-NC hydrogel can be used in magnetically controlled release of drugs under the magnetic field. Despite the magnetic functionality, the mf-NC hydrogels also possess considerable temperature/pH sensitivity and excellent mechanical properties. The unique combination of these multiple sensitivities in organic/inorganic hybrid hydrogels makes them interesting candidates for design of artificial muscles, biosensors and actuators and so on.
4. Conclusions Herein, we have successfully prepared multi-functional organic/inorganic hybrid hydrogels. Firstly, a simple and effective method has been developed to decorate the AT with Fe3 O4 via electrostatic attraction. The results showed that Fe3 O4
Acknowledgments This work was supported by grants from the Program of Introducing Talents of Discipline to Universities (No. 111-2-04).
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