Elsevier

European Polymer Journal

Volume 49, Issue 2, February 2013, Pages 389-396
European Polymer Journal

Macromolecular Nanotechnology
Temperature-sensitive poly(N-isopropylacrylamide)/graphene oxide nanocomposite hydrogels by in situ polymerization with improved swelling capability and mechanical behavior

https://doi.org/10.1016/j.eurpolymj.2012.10.034Get rights and content

Abstract

To improve the performance of temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM) hydrogels, graphene oxide (GO) was selected as a nano strengthening agent to prepare nanocomposite hydrogels. For fulfilling this purpose, in situ polymerization was carried out in colloid solution of graphene oxide, where N-isopropylacrylamide as temperature-sensitive monomer and N,N′-methylene bisacrylamide as crosslinker was initiated utilizing potassium persulfate and sodium sulfite as redox initiators. Infrared spectroscopy and transmission electron microscope was employed to characterize the structure of GO and its dispersibility in water respectively. The internal network structure of nanocomposite hydrogels was investigated by scanning electron microscope (SEM). The temperature-sensitivity, swelling and deswelling properties and mechanical performance of the as-prepared nanocomposite hydrogels was investigated preliminarily. Experimental results show that the nanocomposite hydrogels prepared not only possess good temperature-sensitivity but improved swelling capabilities. The volume-phase transition temperatures of most composite hydrogels are shifted to higher temperature than PNIPAM hydrogels. Furthermore, addition of appropriate amount of GO can dramatically enhance the mechanical performance of PNIPAM hydrogels. The compressive strength of nanocomposite hydrogels reaches a maximum of 216 kPa when the weight ratio of GO to NIPAM is ∼5%, which is 4 times larger than that of PNIPAM hydrogels (54 kPa). The advantageous performance of nanocomposite hydrogels over PNIPAM hydrogels is very beneficial for future applications.

Graphical abstract

The internal network structure of the prepared hydrogels was investigated by scanning electronic microscope. The result suggests that incorporation of graphene oxide has significant influence on the internal network structure of the hydrogels. The network density increases and the pore sizes decreases respectively with the increase of GO content. The internal network structural feature has close relation to the mechanical behavior of hydrogels.

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Highlights

► Poly(N-isopropylacrylamide)/graphene oxide nanocomposite hydrogels were prepared. ► Incorporating GO influences the network structure of the composite hydrogels largely. ► The prepared hydrogels show good thermo-sensitivity and enhanced swelling capability. ► Moderate addition of GO can improve the hydrogels’ mechanical performance obviously.

Introduction

Intelligent hydrogels can undergo significant changes in their physicochemical properties (e.g., a discontinuous volume change) in response to minor changes in external stimuli, such as temperature, pH, light, biomolecules, salts, electric field, and pressure and so on. They show potential applications in many areas such as scaffolds of tissue engineering, carriers of controlled drug release, artificial muscles, stimuli-responsive actuators, biosensors and etc. [1], [2], [3], [4], [5], [6], [7] attributing to their inherent stimuli-responsive properties, and hence being paid a lot of attention for several decades by researchers from materials science, biomedical science and polymer science. Among them, poly(N-isopropylacrylamide) (PNIPAM) hydrogels are the most studied with a volume-phase transition temperature (around ∼32 °C) close to normal body temperature [8]. However, traditional intelligent hydrogels show some obvious drawbacks, especially poor mechanical performance in swollen state, which restrict their widespread applications. To improve the mechanical strength of intelligent hydrogels, preparing hydrogels with interpenetrating polymer network (IPN) is a commonly adopted method in literatures [9], [10]. Besides constructing IPN architectures, incorporation of inorganic components to prepare composite hydrogels became another appealing means since Haraguchi et al.’s pioneering exploration in clay/(PNIPAM) composite hydrogels [11]. In this case, good dispersibility and compatibility between different components is crucial to the properties of composites in that both organic polymer and nano-inorganics can disperse homogeneously into each other in nanoscale or molecular level to form superfine structural phase, thus the performance of organic/inorganic nanocomposites can be improved significantly compared to macro or micro-composites [11].

Carbon nanotubes (CNTs), a fascinating member of carbon nanomaterials possessing excellent mechanical, thermal and electrical properties and high surface area, have been employed as nanofillers in preparing polymer composites with integrated properties soon after their discovery in 1991 [12], [13]. However, the application of CNTs in making hydrogels is restricted by their poor dispersibility in water and bad interactions with hydrogel matrix due to the surface hydrophobicity. To realize efficient reinforcement of hydrogels by CNTs, chemical or physical modification of CNTs is usually conducted to improve the hydrophilicity of the carbon surface [14], [15], [16]. For example, Liu et al. [17] prepared dual-stimuli sensitive poly(N,N-diethylacrylamide-co-acrylic acid) composite hydrogels with chemically modified MWNTs, which exhibit improved compression properties without noticeably reducing the response rates to temperature changes. Kim et al. [18] demonstrated a polysaccharide hydrogel reinforced with finely dispersed SWNTs using biocompatible dispersants O-carboxymethylchitosan and chondroitin sulfate A as a structural support. The compressive modulus and strain of the composite hydrogels were enhanced as much as two times by the addition of a few weight percent of SWNTs.

Graphene, a newly emerged member of nanocarbon materials following CNTs, is becoming another hot topic of physics, materials science, engineering, chemistry, polymer science, electrochemistry, energy and fuels, biochemistry since its first preparation via mechanical exfoliation by Andre Geim in 2004 [19]. As the thinnest known material, graphene is an atomically thick, two-dimensional sheet composed of sp2 carbon atoms arranged in a honeycomb structure. Defect-free graphene is the known strongest material with Young’s modulus of 1TPa and intrinsic strength of ∼130 GPa, with extraordinarily high electric-conductivity and thermal-conductivity. The specific surface area of a single graphene sheet is 2630 m2 g−1, which is much larger than that of CNTs and more favorable to interactions with other substrates. The inherent nature of graphene makes it ideal nanofiller for high-performance polymer composites [20], [21]. It can improve the mechanical, electrical and thermal performance of polymer matrix as reported [22], [23]. Similar to CNTs, the major problem in utilizing graphene in polymer composites is its poor dispersibility in most solvents since it is neither hydrophilic nor lipophilic, leading to difficult processing in solvent blending method and poor compatibility with polymer matrices [20].

In contrast, graphene oxide (GO), precursor of graphene from chemical exfoliation of graphite, is amphiphilic because of the existence of oxygenated groups including hydroxyl, carbonyl, carboxyl and epoxide groups [24]. GO is also a two-dimensional and single-atomic carbon sheet with high surface area like graphene. The functional oxygenated groups make it feasible to modify GO through covalent or non-covalent approach. GO and modified GO shows good biocompatibility and low-toxicity [25] which is indispensible for bioapplications. Taking advantage of this, Dai et al. [26], [27] synthesized PEGylated nanographene oxide and explored its biological application in loading a widely used cancer drug called doxorubicin. Being amphiphilic, GO can well dispersed in common protic solvents such as water, alcohol and so on, via H-bonding interactions [21], facilitating the processing of GO-based composites by solvent blending method and making GO disperse on a molecular level in polymeric matrix [28], [29]. Although the intrinsic strength and Young’s modulus is comprised compared to graphene, GO still has a high strength according to the mechanical measurement of GO paper along with the computational results from structural models of graphene oxides [30], [31], [32] and remains a viable reinforcement candidate of polymer matrices. For instance, Yang et al. [28] prepared nanocomposites of chitosan and graphene oxide with significantly improved tensile strength, Young’s modulus and remarkable increase of the elongation at the break point by simple self-assembly of both components in aqueous media. Rafiee et al. [29] reported enhancements of 28–111% in mode I fracture toughness and up to 1580% in uniaxial tensile fatigue life through the addition of small amounts (⩽1 wt.%) of GO to an epoxy system. The intriguing natures of GO stimulate researchers to utilize it in the preparation of intelligent hydrogels as well. Bai et al. [33] prepared GO/poly(vinyl alcohol) composite hydrogel with pH-sensitivity that can be utilized for selective drug release at physiological pH. Sun et al. [34] reported GO interpenetrating PNIPAM hydrogel networks by covalently bonding GO sheets and PNIPAM-co-AA microgels directly in water. Lo et al. [35] demonstrated the synthesis of an infrared-light responsive GO incorporated PNIPAM hydrogel. Recently, GO was reported to be added into poly(acrylic acid) (PAA) hydrogels to modify their mechanical and thermal properties according to Shen et al.’s report [36], and Ye et al. [37] prepared Semi-IPN hydrogels of PNIPAM and sodium alginate with pH- and temperature-responsivity using surface-functionalized GO as the crosslinker. The novel Semi-IPN GO/PNIPAM/alginate hydrogels exhibit large volumetric change in response to temperature, larger water uptake and much better mechanical properties than the conventional PNIPAM/alginate hydrogels.

Above all, GO has a promising prospect in many areas, though it is really not a new substance [38], it has never been so attractive before as a result of the surging interest in graphene-based materials. Till now some progress has been achieved in many aspects concerning GO, yet there are a lot of research to be developed in the future like exploring new properties and widening its applications, delving deeper into the existing research and so on.

Herein, to improve the mechanical performance of temperature-sensitive PNIPAM hydrogels, GO was utilized as nanofillers to manufacture PNIPAM/GO nanocomposite hydrogels by a simple in situ polymerization method. The swelling/deswelling property, temperature-sensitivity and mechanical performance of the prepared nanocomposite hydrogels were investigated preliminarily and presented.

Section snippets

Materials

N-isopropylacrylamide (NIPAM, Acros, purity > 99%), N,N′-methylene bisacrylamide (BA, Aldrich, 99 + %, electrophoresis grade), potassium persulfate (KPS, A.R, Shanghai chemical reagent corporation, Shanghai), sodium sulfite (Na2SO3, A.R., Tianjin Guangfu Fine Chemicals, Tianjin), natural graphite powder (>99%, average particle diameter ∼10 μm, Qingdao Henglide Graphite Co. Ltd., Qingdao), potassium permanganate (KMnO4, A.R, Laiyang economic development zone fine chemical factory, Yantai), phosphoric

Structure and morphology of GO

Fig. 1 is the representative TEM image of GO. It can be seen from the image that thin crumpled sheets of GO were obtained. There is no aggregation in aqueous GO dispersion, indicating the good dispersibility of GO in water, which is very important for it as nanofillers of hydrogels.

Fig. 2 is the IR spectrum of the prepared GO. The spectrum shows a broad absorption band at 3410 cm−1 that is related to the OH groups. The bands at 1722 and 1630 cm−1 are attributed to carbonyl species in GO. The

Conclusion

A series of PNIPAM/GO nanocomposite hydrogels were prepared by in situ polymerization. The structure and morphology of GO, the network structure and properties of composite hydrogels was investigated. The results indicate that incorporation of GO has significant influence on the microstructure, mechanical performance and swelling/deswelling properties of composite hydrogels. The composite hydrogels exhibit not only good temperature-sensitivity but also improved swelling capability. The

Acknowledgments

This work was financially supported by the National High Technology Research and Development Program of China (2010AA093701), the Program for Changjiang Scholars and Innovative Research Team in University (IRT0970) and the Middle-aged and Youth Scientist Incentive Foundation of Shandong Province (BS2009CL037).

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