Actuator materials based on graphene oxide/polyacrylamide composite
hydrogels prepared by
in situ polymerization†
Nana Zhang,
aRuqiang Li,
aLu Zhang,
aHuabin Chen,
aWenchao Wang,
aYu Liu,*
aTao Wu,
aXiaodong Wang,
aWei Wang,
bYi Li,
aYan Zhao
aand Jianping Gao*
aReceived 23rd March 2011, Accepted 17th May 2011 DOI: 10.1039/c1sm05498h
Actuator materials based on graphene oxide/polyacrylamide (GO/PAM) hydrogels were prepared by in situ polymerization. Their structure and properties were characterized by scanning electron
microscopy, X-ray photoelectron spectrometry, thermogravimetric analysis, Fourier transform infrared spectroscopy and mechanical testing. The results indicate that some PAM macromolecules were grafted onto the GO nanosheets, and this led to good dispersion of the GO nanosheets in the composite hydrogels and consequently a significant improvement of their mechanical properties. The compressive strength of the GO/PAM hydrogel loaded with 1 wt% GO increased 6-fold in comparison to that of pure PAM hydrogel. The GO/PAM based hydrogels were responsive to external stimuli such as pH and electric fields.
1.
Introduction
Since it was first reported in 2004, graphene, a flat monolayer of carbon atoms arranged in a two-dimensional honeycomb lattice, has attracted a great deal of attention because of its intriguing properties.1,2 It has been demonstrated that graphene exhibits many unique electronic characteristics, such as an ultrahigh carrier mobility, exceeding 200000 cm2V1s, a micrometre-scale mean free path, electron-hole symmetry, and quantum Hall effect. This makes it an ideal material in nanoelectronics, for examples its use as a field emission transistor has been extensively studied recently.3 It has a high surface area and good electro-chemical properties, so it has been studied for application in sensors, solar cells, Li batteries and supercapacitors.4–7 Mean-while, Lee et al. have measured the elastic properties and intrinsic breaking strength of free-standing monolayer graphene membranes by nanoindentation with an atomic force micro-scope,8 and they established that graphene is the strongest material ever measured. Therefore, graphene has potential application in composite materials.
Graphene oxide nanosheets have served as fillers for the enhancement of electrical and mechanical properties in composite materials.9–17 Ramanathan et al. studied
functionalized graphene nanosheets (FGS) in poly(methyl methacrylate) composites and the results obtained suggest that the wrinkled single-sheet morphology and surface functionality of FGS affords better interaction with the host polymer than unmodified SWCNTs or traditional expanded graphite (EG).18 The FGS imparted superior mechanical strength and greatly enhanced the thermal properties at exceptionally low loadings. Ganguli et al. reported that the thermal conductivity of chemi-cally functionalized graphite/epoxy composites increased by 28-fold over the pure epoxy resin at a 20% weight load level.19In addition, graphene-based composite materials with poly(vinyl alcohol), polystyrene or poly(vinylidene fluoride) have also been reported.20–24
Soft materials, such as hydrogels, are important materials in the human body. For instance, cartilage is able to effectively absorb and transfer loads. Additionally, soft materials can act as mechanical actuators that convert external stimuli to mechanical energy. Reversible mechanical actuators based on composite hydrogels have potential applications in robotics, sensors, mechanical instruments, and switches.25–31Koerner et al. filled thermoplastic elastomers with carbon nanotubes (CNTs), and the resulting composites showed remote actuation and good shape-recovery properties.32 It has been proven that graphene sheets have higher surface-to-volume ratios than SWCNTs owing to the inaccessibility of the inner nanotube surface to polymer molecules. This potentially makes graphene nanosheets more favorable for altering the mechanical, rheological and permeability properties of materials. For instance, electrome-chanical resonators composed of single and multilayer graphene nanosheets have been reported.33Recently, Park et al. reported graphene-based film actuators that curled as a function of temperature and relative humidity.34Moreover, studies on the
aSchool of Science, Tianjin University, Tianjin, 300072, P R China. E-mail:
liuyuls@163.com; jianpingg@eyou.com; Fax: +86 22 274 034 75; Tel: +02227403475
bSchool of Chemical Engineering & Technology, University Tianjin,
Tianjin, 300072, P. R. China
† Electronic supplementary information (ESI) available: characterization of GO, photographic images of GO-g-PAM and reduced GO-g-PAM, swelling ratio of GO/PAM and GO/PAM-co-PAA xerogel in solutions with different pH values. See DOI: 10.1039/c1sm05498h
Cite this: Soft Matter, 2011, 7, 7231
www.rsc.org/softmatter
PAPER
usage of graphene/graphene oxide (GO) nanosheets in hydrogels has also been reported in recent years.35–43In this study, graphene oxide/polyacrylamide (GO/PAM) and graphene oxide/poly-acrylamide-co-poly(acrylic acid) composite hydrogels (GO/ PAM-co-PAA) were fabricated by in situ polymerization and their structure and actuator properties were studied
2.
Experimental
2.1 MaterialsGraphite was obtained from Huadong Graphite Processing Factory. Potassium permanganate, sodium nitrate, concentrated sulfuric acid, 30% hydrogen peroxide and hydrochloric acid were all from Tianjin Chemical Technology Co. Acrylamide (AM) and acrylic acid (AA) were from Tianjin Standard Science and Technology Co. Ltd. Ammonium persulfate, N,N0-methylene bisacrylamide (BIS) and all other chemicals were from Sino-pharm Chemical Reagent Co. Ltd. All the chemicals were analytical grade and used as received.
2.2 Preparation of GO sheets
GO was prepared from purified natural graphite by the modified Hummers method.44Briefly, concentrated H2SO4(23 mL) was
added into a 250 mL flask filled with graphite (1 g) at 0C (ice bath), followed by addition of NaNO3(0.5 g) and solid KMnO4
(3 g) with stirring. After increasing the temperature to 35 C, excess deionized water was added to the mixture and the temperature was then increased to 90C. Finally, 30% H2O2was
added until the color of the mixture turned to brilliant yellow. The product was filtered and rinsed three times with dilute HCl solution to remove the metal ions, and then it was filtered and rinsed with deionized water to remove the acid and get GO powder after the filter cake was completely dried in air. Some of the powder was dispersed into filtered water and sonicated to get 0.08 wt% aqueous suspension of GO nanosheets.
2.3 Preparation of GO/PAM and GO/PAM-co-PAA composite hydrogels
The GO/PAM composite hydrogels were prepared by in situ polymerization of monomer AM and crosslinkable monomer BIS in the presence of GO nanosheets. 20 ml of GO suspension of certain concentration (for example 0.08 wt%) was first degassed and nitrogen-saturated, and then 1.44 g AM, 0.16g BIS and 80ml ammonium persulfate (0.35 mol L1) were added under stirring to get a solution that contained 7.2 wt% AM, 0.8 wt% BIS and 0.032 wt% initiator. The solution was rapidly injected into several cylinder moulds that were then put into an oven purged with N2gas. The polymerization was conducted at 65C for 4 h
to obtain the GO/PAM composite hydrogels that roughly con-tained 1 wt% GO if total monomer was considered as 100 (GO: monomers¼1 : 100). The GO/PAM-co-PAA composite hydro-gels were prepared in the same way except that the monomers were composed of AM and AA in different ratios.
2.4 IR spectra of GO/PAM hydrogels
Fourier transform infrared (IR) spectra of the samples were measured with a Perkin-Elmer Paragon-1000 FT-IR spectrom-eter in the range of 500–4000 cm1. Each IR spectra was the average of 20 scans.
2.5 X-ray photoelectron spectrometry (XPS) and scanning electron microscopy (SEM) analyses
XPS was conducted on an X-ray photoelectron spectrometer (PHI1600 ESCA System, PERKIN ELMER, USA), and the morphology of the GO/PAM hydrogels after freeze-drying treatment was observed using a JEOL-6700F ESEM, Japan. 2.6 Raman spectroscopy analysis
Raman measurements were performed with a high resolution inVia Raman microscope (RENISHAW, UK), in a backscat-tering configuration. A diode laser operated at 633 nm that illuminated the sample through the microscope objective under normal incidence was used for excitation. The microscope was focused onto the chosen area of the sample and then the laser was turned on and focused for the measurement.
2.7 Thermogravimetric analysis (TGA)
GO/PAM hydrogels were first dried in a vacuum to a constant weight at 60C, and then their thermogravimetric diagram was measured by TGA (STA 409 PC, NETZSCH, Germany) with a rising rate of 6C min:1.
2.8 Compressive strength of the GO/PAM materials
The compressive strength of the cylindrical GO/PAM hydrogels was tested using a Universal Testing Machine (Testometric, UK). The hydrogel samples (column, diameter 15 mm and height 11.5 mm) were placed between self-leveling plates and compressed at a rate of 10 mm min1until failure. The com-pressing rate for xerogel samples (diameter 10 mm and height 5.5 mm) was 1 mm min1. For each sample, a minimum of three measurements was taken, and the average result was calculated. 2.9 Swelling ability of GO/PAM xerogels in distilled water The GO/PAM xerogel (10 mm in diameter and 5.5 mm in length, V0) was put into distilled water at 25C for the swelling test. It
was taken out and its weight was measured at certain time intervals until the weight was constant, at which point the swelling equilibrium was reached. It was then taken out and its volume was measure (V). The swelling ratio of the GO/PAM xerogel (SRx) is defined as SRx ¼ V/V0. For each sample,
a minimum of three measurements was taken, and the average result was calculated.
2.10 Swelling of the composite gels in different pH media To measure the pH dependence of the swelling ability of the composite gels, phosphate buffer solutions of different pH values were made by mixing 0.1 M KH2PO4(aq.) with different
quan-tities of 0.1 M HCl (aq.) or 0.1 M NaOH (aq.), and their pHs
were measured with a pH meter.45The pH response of the sample (diameter 1.0 mm and length 6 mm) in pure water and at different pH values from 3.5 to 12 was analyzed. The swelling ratio of the hydrogel (SRh) is defined as SRh¼ V/V0, where, V is the sample
volume at swelling equilibrium and V0is the volume of the dried
gel after desiccation. The definition of the swelling ratio of the GO/PAM xerogel (SRx) is the same as in section 2.9.
2.11 Actuating properties of the composite hydrogels in an electronic field
The actuating properties of the composite hydrogels were eval-uated by measuring the bending angles of the hydrogels (diam-eter 1.0 mm and length 15 mm) in an electronic field as illustrated in Fig. S1†.45–50Briefly, two parallel Pt electrodes, 40 mm apart, were immersed in the electrolyte solution and the hydrogel was installed centrally between them. Upon application of a DC electric field, the hydrogel bended towards the cathode from the original position and formed a bending angle, q. The bending behavior was recorded with a digital camera (COOLPIX S620, Nikon, Japan), from which the bending angle was measured.
3.
Results and discussion
3.1 SEM and Raman analysis of GO/PAM materials
The GO nanosheets used for preparing GO/PAM composite hydrogels were synthesized in our laboratory. Their aqueous suspensions are stable for several months with no precipitation occurring (Fig. S2†). The nanosheets can also be suspended in N, N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), implying that the GO nanosheets are both hydrophilic and small enough to suspend in numerous polar solvents, which is further confirmed by TEM, XRD, and XPS (Fig. S3, S4 and S5†). The stable GO aqueous suspension offers facilities for the prepara-tion of GO based hydrogels, and the funcprepara-tional groups on the GO nanosheets provide places for further chemical modification of the GO nanosheets.
A GO/PAM composite hydrogel was prepared by in situ polymerization of AM in a GO suspension. After freeze-drying treatment, the morphologies of the PAM and the GO/PAM xerogels were observed by SEM and the results are given in Fig. 1. For the samples in Fig. 1A-1C, the freezing-drying treatment was carried out immediately after the formation of GO/PAM hydrogels. PAM xerogel has an ordinary porous gel structure. It has a pore size of about 200 nm in length with a narrow pore size distribution. When GO was added, the GO nanosheets dispersed in the PAM gel and made the gel stiff, which prevented the growth of ice crystals during the freezing step. This resulted in small uneven pores. The GO/PAM xerogel displays a wide distribution of pore sizes and the pores became smaller and shallower with increasing GO amount. For the samples in Fig. 1a-1c, the freeze-drying treatment was carried out after the gels were almost completely dried in air. GO/PAM easily contracts in air because of its stiffness and this leaves fewer pores as compared with PAM. This phenomenon is more and more obvious with increasing amounts of GO because of increased stiffness.
It is not easy to distinguish the GO nanosheets from the PAM matrix in the SEM images. Taking this into consideration,
Raman mapping was used to evaluate the dispersion of the GO nanosheets in the PAM matrix. GO displays two prominent peaks at 1598 and 1347 cm1(Fig. S6 (a)†), and PAM has five main peaks between 1100 and 1700 cm1(Fig. S6 (b)†). Although the amount of GO is only 0.5% in the GO/PAM, the Raman spectra of GO/PAM only displays the two GO peaks at 1596 and 1344 cm1(Fig. S6 (c)†). This indicates that the peaks of PAM are much weaker than those of GO. In order to characterize the dispersion of GO in the PAM matrix, Raman spectra at sixty different points over an area of 54mm 30 mm were collected of the GO/PAM. This data was then plotted into a 3D image and is shown in Fig.2. The peak intensity at 1596 cm1is almost the same for all sixty spectra with a mean deviation of about 3.71%, which proves that GO is evenly dispersed in the matrix.
3.2 Thermostability of GO/PAM materials
TGA was used to determine the composition and thermal stability of the GO, PAM and GO/PAM and the results are shown in Fig. 3. GO is thermally unstable and starts to lose mass below 100C but the major mass loss occurs at about 200C. This is presumably due to pyrolysis of the labile oxygen-con-taining functional groups to yield CO, CO2, and steam. The
overall weight loss of GO is about 50% at 600C, but that for PAM and GO/PAM (with 10% cross-linking agent) is 79% and 76%, respectively. PAM and GO/PAM show a similar pattern of weight loss, and both have two major mass losses at about 300C and 400 C. These are due to the decomposition and carbon-ization of PAM.51,52 Since the TGA curve of GO/PAM is still above that of the pure PAM even though 1% GO is deducted from the weight of the GO/PAM composite, the stability of the GO/PAM composite is slightly increased due to the presence of GO.
3.3 Swelling ability of GO/PAM xerogels
PAM is a hydrophilic polymer and it forms a hydrogel on contact with water if it is properly crosslinked. The volume change of the GO/PAM xerogels soaked in distilled water at the room temperature was measured and the results are shown in Fig. 4. The volume gradually expanded as the soaking time increased and reached equilibrium after about 84 h in distilled water. All the hydrogels basically maintain their original shape, and no migration of the GO nanosheets from the gel into the solution was observed during the swelling process. To better understand the influence of the GO on the swelling behavior of the xerogels, the swelling ratio for xerogels with different amounts of GO or cross-linking agent was measured and the results are shown in Fig. 4. The xerogels have a similar swelling trend. They all swell rapidly at the beginning, and then the swelling rate slows down. When GO is incorporated, the swelling ratio of GO/PAM decreases and larger amounts of GO causes larger swelling decrease. This tendency is more obvious in the highly cross-linked GO/PAM hydrogels, although the swelling ratio becomes lower. This may be attributed to the physical cross-linking action of GO on the PAM macromolecules.
3.4 Compressive strength of GO/PAM materials
Mechanical properties are often considered as one of the most important characteristics for materials. Fig. 5a shows the compressive strength of GO/PAM xerogels. For a low GO content of 0.25%, no appreciable change is observed in the compressive strength, whereas it increases considerably when the GO content is increased from 0.5% to 0.75%. This trend in compressive strength is also true for GO/PAM hydrogels as shown in Fig. 5b. The hydrogel with 1% GO displays a compressive strength about six times higher than that of the pure PAM hydrogel. The GO/PAM hydrogels also show more
compressive deformation. For instance, the GO/PAM hydrogel with 1% GO demonstrates a deformation of 96% (compression ratio of 0.96, which is defined as the decreased height of the hydrogel cylinder/the original height of the hydrogel cylinder) without breaking, whereas a 10% deformation (it is too small to be shown in Fig. 5b) crushes the pure PAM hydrogel. These data indicate that the strengthening effect of GO on soft GO/PAM hydrogels is greater than that on hard GO/PAM xerogels.
This high strength comes from the good compatibility between PAM and GO. The hydroxyl and carboxyl functional groups on the GO sheets are well suited for the formation of composites with polar PAM, giving rise to nanosheet-polymer interactions that provide better load transfers between the PAM matrix and
Fig. 1 SEM images of PAM and GO/PAM xerogels (GO05¼ 5 w/w &, GO10¼ 10 w/w &). All the samples contain 10% BIS.
Fig. 2 3D Raman spectra of GO/PAM (0.5 wt% GO), Raman mapping area: 54mm 30 mm, step size 6 mm 6 mm, 60 data in total.
Fig. 3 TGA curves of GO, PAM and GO/PAM.
the GO nanosheets, which is beneficial for mechanical improvement. This phenomenon has also been observed in other GO/polymer composites.9–17Vadukumpully et al. prepared free-standing composite thin films of surfactant-wrapped graphene nanoflakes and poly(vinyl chloride) by a simple solution blending, drop casting and annealing route. A significant enhancement in the mechanical properties of pure poly(vinyl chloride) films was obtained with a 2 wt% loading of graphene, such as an almost 130% improvement of tensile strength.17Bai et al. has reported the reinforcement of hydrogenated-carboxy-lated nitrile-butadiene rubber (HXNBR) with exfoliated gra-phene oxide by a solution-blending method.10They found that the addition of 0.44 v/v% of GO nanosheets enhanced the tensile strength at 200% elongation of HXNBR by more than 50%. They attributed this enhancement to the strong interfacial interactions between the oxygen-containing functional groups on the surfaces of GO nanosheets and the carboxyl groups in HXNBR.
GO/PAM hydrogels were prepared by in situ polymerization of AM in the presence of GO. GO nanosheets contain hydroxyl, epoxy and carboxyl functional groups, and these groups may participate in radical chain transfer reactions during the poly-merization, leading to the grafting of PAM macromolecules onto the GO nanosheets.53To prove the possibility of the radical chain transfer reaction, the reaction products (without addition of monomer BIS) were chemically treated. Hydrazine hydrate was added to reduce the GO, and then the suspension was
centrifuged. The pure GO nanosheets that didn’t participate in the grafting polymerization were removed as the precipitate on the bottom of the tube. The stable black supernatant was filtered through a microporous membrane (0.45mm pore) and the filter cake was repeatedly rinsed with deionized water in order to remove the PAM homopolymer. The remainder was a charcoal gray powder and was characterized by IR and XPS after vacuum
Fig. 4 Swelling ratio of the GO/PAM xerogels with different amounts of
GO (w/w) and cross-linking agent of 5% (a) and 10% (b). Fig. 5 Compressive strength of GO/PAM xerogels with different amounts of GO (a) and compressive strength and compression ratio of GO/PAM hydrogels with different amounts of GO (w/w %) (b). The inset shows the shape of the GO/PAM hydrogels after testing.
Fig. 6 FTIR spectra of GO, PAM and charcoal gray powder.
drying at 40C for 48 h. The IR spectrum of the charcoal gray powder in Fig. 6 shows a broad absorption band at 3430 cm1 that is related to the OH groups, and an absorption band at 1635 cm1 from the carbonyl groups of GO. The IR spectrum of charcoal gray powder also shows an absorption band at 2937 cm1, which can be assigned to the PAM asymmetric stretching vibration of C–H. So the IR spectrum of charcoal gray powder is a combination of the PAM and GO spectra, which suggests the grafting of PAM onto GO to form PAM grafted GO (GO-g-PAM).
The XPS spectrum of the charcoal gray powder is shown in Fig. 7 and a comparison with Fig. S6† shows that the oxygen content is decreased while the nitrogen content is increased after the reduction of GO. This indicates that the PAM chains are grafted onto the GO nanosheets. In addition, the charcoal gray powder can disperse in water to form a stable suspension (Fig. S7†), whereas GO or GO-PAM blend does not form a stable dispersion after reduction. This further suggests that some of the PAM chains are grafted onto the GO sheets to form GO-g-PAM during the polymerization of AM. Therefore the GO nanosheets can still be stably suspended in water after reduction. Since there are some GO-g-PAM in the GO/PAM composite hydrogels, the compatibility of GO and PAM is certainly improved through GO-g-PAM bridges and thus the PAM hydrogels are significantly strengthened.
3.5 pH response of GO/PAM and GO/PAM-co-PAA hydrogels
Hydrogels have a wide range of applications for example as drug carriers, scaffolds for cell culture, as dressings, and as smart or actuator materials, for example artificial muscles. In order to obtain pH responsive hydrogels, monomer AA was introduced and copolymerized with AM. Fig. 8 shows the equilibrium swelling ratio of a series of GO/PAM-co-PAA hydrogels in buffers of different pH at room temperature. For the GO/PAM hydrogels (Fig. 8a), there is a gradual increase in the swelling ratio from pH 5 to pH 11 and then a large increase above pH 11. This large increase is because some of the acylamide groups of PAM are hydrolyzed to carboxylic groups at high pH conditions.
The formed carboxylic groups are easily ionized at high pH causing electrostatic repulsion among the carboxylic ions which results in macromolecular chain stretching and so the hydrogels swell. The composition of the GO/PAM hydrogels has only a slight effect on the swelling behavior.
For the GO/PAM-co-PAA hydrogels, there are two other large increases in swelling at low pH values in addition to the one at pH 11, and the GO content has an obvious impact on the swelling ability. Generally, the swelling ratio decreases as the amount of GO increases. However, the swelling ratio increases when the fraction of PAA in the copolymer increases since PAA
Fig. 7 XPS spectrum of the charcoal gray powder.
Fig. 8 Swelling ratio of GO/PAM and GO/PAM-co-PAA hydrogels in solutions with different pH values. (a) PAA/PAM¼ 0/10; (b) PAA/PAM ¼ 1/9; (c) PAA/PAM¼ 2/8.
has a greater ability to absorb water. From pH 4 to pH 8, the swelling ratio of the GO/PAM-co-PAA hydrogels increases quickly at the beginning and then reaches a plateau. In the GO/ PAM-co-PAA hydrogels, there are hydrogen bonds between the macromolecules because the carboxyl groups on PAA still exist as –COOH at low pH. With increasing pH value, the carboxyl groups are ionized to –COOand the hydrogen bonding inter-actions are weakened, and this results in an increase in the swelling ratio. At the same time, the amino groups (–NH2) on the
PAM are protonated to –NH3+in acidic solution which enhances
the electrostatic attractions with the increased –COO ions, resulting in a decrease in the swelling ratio. From pH 8 to pH 11, the swelling ratio of GO/PAM-co-PAA hydrogels also increases rapidly at the beginning and then decreases at pH 11. Under alkaline conditions, the carboxyl groups have completely ionized to –COO, but the –NH3+groups are starting to return to –NH2
so the electrostatic attractions between –COOand –NH3+are
weakened. Therefore the swelling ratio of the GO/PAM-PAA hydrogels increases. However, at pH 11, the increased number of -NH2 groups causes strong hydrogen bonding interactions
between the –NH2 groups, which causes shrinkage of the
hydrogels.48The swelling ratio of GO/PAM and GO/PAM-co-PAA xerogels in different pH value solution was also studied (Fig. S8†). They show a similar tendency as their hydrogels, but the swell ratio values are slight lower compared with the hydrogels under the same conditions.
3.6 Actuating properties of GO containing hydrogels in an electric field
Besides pH responsive gels, electric-sensitive hydrogels have also attracted considerable attention. They can be used in smart gel-based devices such as sensors, artificial muscles, membrane separation devices, and drug delivery systems. Here, the bending behavior of GO based hydrogels in an electric field was studied. The electrical energy was transformed into mechanical move-ment and the bending motion is shown in Fig. 9. Six kinds of hydrogels were prepared and their bending behavior in different pH buffer solutions is shown in Fig. 10. For the symbol PP20RGO1, the first P stands for PAM, the following P20 and RGO1 mean containing 20% PAA, and 1% reduced GO respectively. For each sample, the bending angle increased rapidly at first and then increased slowly before reaching a constant. While the final values of the bending angle vary
drastically, in general the hydrogels with 20% PAA have higher bending angles than those without PAA. At the same time, the hydrogels with GO have the highest bending angle value in both groups, with or without PAA, but the effect is suppressed when GO has been reduced.
Moreover, pH also has a significant effect on the bending angle. For all the hydrogels, both the bending angle and its rate of change at the beginning increase with increasing pH value, but the effect is most obvious for PAM. At pH 4, the bending angle is small and is almost unchanged with time. While at pH 10, it increases in the first 100 s and finally reaches a maximum value of
Fig. 9 Photos showing the bending motion of the GO/PAM hydrogels in an electric field.
Fig. 10 Effect of pH on the bending behavior of the composite hydro-gels in different pH buffer solutions (ionic strength¼ 0.1 M). (a) pH ¼ 4; (b) pH¼ 7; (c) pH ¼ 10.
20. Under alkaline conditions, some acylamide groups in PAM have been hydrolyzed to carboxylic groups and so PAM acts like a polyanionic hydrogel and bends toward the cathode.46–48PAA and GO both contain carboxyl groups, so PAM-PAA/GO (PP20GO1) showed the largest bending angle. When GO has been reduced, the bending angle decreases because of the decreased –COOH groups (PP20RGO1). Since the fraction of PAA is much higher than that of GO, its effect on the bending angle is more obvious than that of the GO (PP20 and PGO1). In addition to pH and electric field, PAM-co-PAA hydrogels are also sensitive to temperature and numerous chemicals such as alcohols or acetone.54–56 Therefore, the GO/PAM-co-PAA hydrogels should be good mechanical actuators responsive to temperature and chemical stimuli.
4.
Conclusions
Graphene oxide nanosheets are good nano-fillers for the enhancement of mechanical properties in composite materials. The GO/PAM hydrogels were prepared by in situ radical poly-merization of AM in the presence of GO nanosheets. Since some PAM macromolecules were grafted onto the GO nanosheets to form GO-g-PAM by a radical chain transfer reaction during polymerization, the compatibility between PAM and GO was improved, and consequently the composite hydrogels displayed good mechanical properties in both the dry and wet states. This enhancement depends on the amount of loaded GO, and the compressive strength of the soft GO/PAM hydrogels increased by 6-fold in comparison to that of the pure PAM at a 1% by-weight load level. The GO based hydrogels are responsive to pH and the GO/PAM-co-PAA hydrogels show three swelling increases between pH 3 and 13. They also show a response in an electric field, and the bending angle becomes larger as GO and PAA are incorporated. The multi-responsive characteristics, together with their extraordinary high strength, make these hydrogels potential mechanical actuator materials in the field of smart gel-based devices such as sensors, artificial muscles and switches.
Acknowledgements
This work was supported by Tianjin Municipal Science and Technology Commission of P. R. China (09JCZDJC23300) and National Science Foundation of China (No. 21074089 and 50873075).
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