Stimuli-Responsive Polymer Hydrogels Containing Partially Exfoliated Graphite VALERIA ALZARI, 1 ALBERTO MARIANI, 1 ORIETTA MONTICELLI, 2 LUCA VALENTINI, 3 DANIELE NUVOLI, 1 MASSIMO PICCININI, 4 SERGIO SCOGNAMILLO, 1 SILVIA BITTOLO BON, 3 JAVIER ILLESCAS 5 1 Department of Chemistry, University of Sassari and local INSTM unit, Via Vienna 2, 07100 Sassari, Italy 2 Department of Chemistry and Industrial Chemistry, University of Genova, Via Dodecaneso 31, 16146 Genova, Italy 3 Civil and Environmental Engineering Department, University of Perugia, INSTM Research Unit, Strada di Pentima 4, 05100 Terni, Italy 4 Porto Conte Ricerche S.r.l., SP 55 km 8.400 Loc. Tramariglio, 07041 Alghero (Sassari), Italy 5 Instituto de Investigaciones en Materiales, Universidad Nacional Auto ´ noma de Me ´ xico, Circuito Exterior S/N, Ciudad Universitaria, C.P. 04510, Me ´ xico D.F. Received 30 July 2010; accepted 24 August 2010 DOI: 10.1002/pola.24341 Published online 1 October 2010 in Wiley Online Library (wileyonlinelibrary.com). ABSTRACT: In this work, a new stimuli-responsive composite polymer hydrogel containing partially exfoliated graphite was prepared by frontal polymerization. The materials obtained were characterized by differential scanning calorimetry, RAMAN, scan electron microscopy, transmission electron mi- croscopy, atomic force microscopy, and in terms of swelling behavior. It was found that the maximum temperature reached by the polymerization front and the lower critical solution tem- perature are affected by the graphite content. V C 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 5375– 5381, 2010 KEYWORDS: composites; frontal polymerization; hydrogels; rad- ical polymerization; stimuli-sensitive polymers; swelling INTRODUCTION Hydrogels are a class of cross-linked poly- mer networks that are swollen in water without dissolving. 1 Owing to their biocompatibility, special surface properties and high water content, hydrogels are the materials of choice in many biomedical applications. 2 Poly(N-alkylacrylamide) hydrogels have been extensively investigated because of their attractive environmentally sensitive characteristics. 3 Among them, in aqueous media poly(N-isopropylacrylamide) (PNI- PAAm) macromolecular chains can undergo a reversible coil- to-globule transition at the so-called lower critical solution temperature (LCST). 4 However, these hydrogels have several drawbacks, example, morphological inhomogeneity, mechanical weakness, limited swelling at equilibrium, and slow response to stimuli. 5–8 In this work, composite hydrogels of PNIPAAm and graphite were synthesized, and the influence of this filler on the hydrogel properties was investigated. As will be discussed, graphite has been partially exfoliated giving rise to low amounts of graphene. On this respect, obtained for the first time in 2004, graphene is the name given to one sheet of graphite. 9–19 It is entirely made up of sp 2 carbon atoms bonded in a network of repeat- ing hexagons within a single plane that is just one atom thick. Graphene sheets are predicted to have a variety of unusual and interesting properties that include high values of Young modulus (1100 GPa), 11 fracture strength (125 GPa), 11 thermal conduc- tivity (5  10 3 Wm 1 K 1 ), 12 mobility of charge carriers (2  10 5 cm 2 V 1 s 1 ) 13 and specific surface area (2630 m 2 g 1 ). 14 Frontal polymerization (FP) was the synthetic technique used in this work. FP allows the conversion of monomer into polymer by formation and consequent propagation of a front, a reaction zone at high temperature, which is self-sustaining and that propagates throughout the monomeric mixture. There are many advantages in the use of this technique instead of classical polymerization: (i) shorter reaction time (a typical FP run takes only a few minutes, whereas classical polymerization methods often need hours or days); (ii) low energy consumption (it is a consequence of the fact that the external energy source is applied only in the first instant, whereas in classical polymerization techniques, it is neces- sary for all the experiment duration); (iii) easy protocols. Since the first pioneering work performed by Chechilo and Enikolopyan, 20 an even increasing number of monomers were polymerized by this method. In detail, epoxy resins, 21 ionic liquid, 22 acrylic monomers, 23–26 glycidyl ethers, 27,28 2-hydroxyethyl acrylate, 29 N-methylolacrylamide, 30 epoxy Correspondence to: A. Mariani (E-mail: mariani@uniss.it) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 5375–5381 (2010) V C 2010 Wiley Periodicals, Inc. HYDROGELS CONTAINING EXFOLIATED GRAPHITE, ALZARI ET AL. 5375