Macromolecular Nanotechnology Contact angle measurements as a tool to investigate the filler–matrix interactions in polyurethane–clay nanocomposites from blocked prepolymer Alessandro Pegoretti * , Andrea Dorigato, Marco Brugnara, Amabile Penati Department of Materials Engineering and Industrial Technologies, INSTM-NIPLAB Research Unit, University of Trento, via Mesiano 77, 38100 Trento, Italy article info Article history: Received 23 September 2007 Received in revised form 19 February 2008 Accepted 1 April 2008 Available online 12 April 2008 Keywords: Nanocomposites Blocked polyurethane Clay Contact angle abstract Optically transparent polyurethane-clay nanocomposite films were prepared by dispersing 5 wt% of various commercial organo-clays in a mixture of cycloaliphatic amines used as chain extender-cross-linker of a blocked prepolymer. For the first time, vibration-induced equilibrium contact angle measurements were successfully employed to rank the selected organo-clays accordingly to their hydrophobicity order. Polymer–clay intercalation degree in the nanocomposites, as assessed from X-ray diffraction, was strongly correlated to the water–clay equilibrium contact angle. Moreover, as the clay intercalation degree increased, a decrease of the cross-linking degree of the polyurethane matrix occured. Uniaxial tensile tests under quasi-static and impact conditions, and isothermal thermo- gravimetric analysis were performed on both unfilled polyurethane matrix and nanocom- posites. Secant tensile modulus, tensile energy to break, and thermal lifetime showed a non monotonic trend with a maximum as a function of the intercalation degree. This behaviour is discussed considering that two concomitant and contrasting effects develop as the poly- mer–clay intercalation degree increases: a positive improvement of the filler matrix inter- actions, and a negative reduction of the matrix cross-linking degree. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Organic coatings or paints on metallic substrates give aesthetic appearance as well as protection from corrosion phenomena. The wide applicability of polyurethane (PU) coatings is mainly due to the broad selection of monomers from a huge list of macrodiols, diisocyanates, chain extend- ers and cross-linkers [1]. Depending on the type and amount of starting monomers, the material properties can be finely tuned as the final application requires. The history and late trends in the development of high perfor- mance polyurethane and its subclass coatings have been recently reviewed by Chattopadhyay and Raju [2]. During the last 10 years a great deal of efforts has been expended to improve the performances of elastomeric PU matrices by developing nanocomposites, mostly by the addition of organo-modified clays (organo-clays) [3–46]. In fact, following the pioneering work of Wang and Pinna- vaia [4], a number of scientists investigated various meth- ods for the production of PU-clay nanocomposites. In most cases, organo-clays have been dispersed into the polyol and the mixture was then polymerized by the addition of a diisocianate and a chain extender [9–12,21,24,28,30,41, 42,44] or of a prepolymer [29]. In other cases, organo-clays have been dispersed in the prepolymer [18,25,43,45,46], or directly in the PU matrix by solvent [5,8,13–16,22] or melt [22,26,33,45] mixing procedures. Varghese et al. [20] proposed a latex compounding route to add a pris- tine layered silicate (sodium hectorite) to polyurethane rubber. Depending on the type and amount of filler, and on the level of intercalation and/or exfoliation of the clay in the PU matrix, various mechanical properties can be modified 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.04.011 * Corresponding author. Tel.: +39 0461 882452; fax: +39 0461 881977. E-mail address: Alessandro.Pegoretti@unitn.it (A. Pegoretti). European Polymer Journal 44 (2008) 1662–1672 Contents lists available at ScienceDirect European Polymer Journal journal homepage: www.elsevier.com/locate/europolj MACROMOLECULAR NANOTECHNOLOGY