Graphene-containing thermoresponsive nanocomposite hydrogels of poly(N-isopropylacrylamide) prepared by frontal polymerization Valeria Alzari, a Daniele Nuvoli, a Sergio Scognamillo, a Massimo Piccinini, b Emilia Gioffredi, c Giulio Malucelli, c Salvatore Marceddu, d Mario Sechi, e Vanna Sanna b and Alberto Mariani * a Received 11th March 2011, Accepted 8th April 2011 DOI: 10.1039/c1jm11076d Frontal polymerization has been successfully used to synthesize poly(N-isopropylacrylamide) nanocomposite hydrogels containing graphene. The latter was directly achieved by ultrasound treatment of a dispersion of graphite in N-methylpyrrolidone. The dispersion, having the concentration of 2.21 g L 1 , was characterized by TEM analysis and mixed with suitable amounts of N-isopropylacrylamide for the synthesis of graphene-containing nanocomposite polymer hydrogels. The nanocomposite hydrogels were analyzed by SEM and Raman spectroscopy, and their swelling and rheological properties were investigated. It was found that graphene strongly influences the swelling ratio, dramatically increasing it, even if present in small amounts. Finally, the rheological properties of the hydrogels were correlated with the graphene content: G 0 modulus and complex viscosity were found to increase with increasing nanofiller concentration, thus indicating the occurrence of good interactions between the two phases. Nevertheless, at a high concentration (i.e., 0.13 wt.%), graphene showed a lubrication effect, lowering the rheological parameters and approaching the same pseudoplastic behaviour of the unfilled material. Introduction Hydrogels are three-dimensional polymer networks made up of highly hydrophilic crosslinked macromolecular chains that are able to swell and retain a significant portion of water when placed in an aqueous medium. 1 Hydrogels can be classified into different groups based on their physical structure (amorphous, semicrystalline, hydrogen-bonded or supramolecular), electric charge (ionic or neutral), crosslink type (physical or chemical), possible response to external stimuli, and origin (synthetic or natural). Research on hydrogels started as early as in 60s with a novel paper on poly(2-hydroxyethyl methacrylate) by Wichterle and Lim. 2 In 1968, it was successfully predicted that the net repulsion between a polymer network and a poor solvent can cause a phase transition and a change in the swelling degree (and thus in the hydrogel volume). 3 Lately, phase transitions induced by changes in the environment were also reported by other groups. 4,5 Poly(N-alkylacrylamide) hydrogels have been extensively investigated because of their attractive environmentally sensitive characteristics. 6 Among them, in aqueous media poly(N-iso- propylacrylamide) (PNIPAAm) macromolecular chains can undergo a reversible coil-to-globule transition at the so-called lower critical solution temperature (LCST). 7 This finding paved the way to a number of studies on PNIPAAm and its copolymers devoted to find potential applications in various biomedical fields, including embolic agents 8 and drug delivery. 9 However, its poor mechanical properties strongly limit the use of pure PNIPAAm in structural applications. Generally, PNIPAAm- based hydrogels exhibit low compressive modulus 10–13 and weak elastic recovery after loading. Recently, to overcome these limitations, nanocomposite hydrogels have been synthesized: they show large swelling ratios and rapid deswelling responses to temperature changes, together with high transparency (structural homogeneity), excellent mechanical properties, with astonishingly large elongations. 14 Nowadays, graphene is the nanometric material that attracts most of the interest of both border research and media. That is particularly true after that Andre Geim and Konstantin Novoselov were awarded the 2010 Nobel Prize in Physics ‘‘for groundbreaking experiments regarding the two-dimensional material graphene’’. 15 a Dipartimento di Chimica, Universit  a di Sassari, and local INSTM unit, Via Vienna 2, 07100 Sassari, Italy. E-mail: mariani@uniss.it; Fax: +39 079 212069; Tel: +39 079 229556 b Porto Conte Ricerche S.r.l., SP 55 km 8.400 Loc. Tramariglio, 07041 Alghero (SS), Italy c Dipartimento di Scienza dei Materiali ed Ingegneria Chimica, Politecnico di Torino, sede di Alessandria, and local INSTM unit, Via T. Michel 5, 15121 Alessandria, Italy d Istituto di Scienze delle Produzioni Alimentari—Consiglio Nazionale delle Ricerche (ISPA-CNR), Li Punti—Reg. Baldinca, Traversa La Crucca, Sassari, 07100, Italy e Dipartimento di Scienze del Farmaco, Universit  a di Sassari, Via Muroni 23/A, 07100 Sassari, Italy This journal is ª The Royal Society of Chemistry 2011 J. Mater. Chem., 2011, 21, 8727–8733 | 8727 Dynamic Article Links C < Journal of Materials Chemistry Cite this: J. Mater. Chem., 2011, 21, 8727 www.rsc.org/materials PAPER