Hybrid glass-like films through sol–gel techniques Massimo Guglielmi * , Giovanna Brusatin, Gioia Della Giustina Dipartimento di Ingegneria Meccanica – Settore Materiali, Universita ` di Padova, Via Marzolo 9, 35100 Padova, Italy Available online 6 April 2007 Abstract Since the time sol–gel was suggested as a nonmelting technique for the preparation of glasses, an enormous amount of work has been done, moving from the more traditional glasses to new materials. Among these, hybrid organic–inorganic materials are of particular interest, not only for the amount of the applications they may be used for, but also because of their structural characteristics: hybrids are a mix, at molecular level, of inorganic and organic moieties, where the organic ones may be simply dissolved in the matrix or behave as network former or modifier. The most recent results on the synthesis of this type of films, obtained by the author and co-workers, will be presented and discussed. In particular, the focus will be on the possibility to control the structural development by acting on the organic and inorganic polymerization separately and the challenges that this approach offers. Ó 2007 Elsevier B.V. All rights reserved. PACS: 42.70.Àa; 81.16; 81.20.Fw Keywords: Organic–inorganic hybrids 1. Introduction Hybrid organic–inorganic materials are among the most interesting and fruitful possibilities of synthesis offered by the sol–gel method. They are a molecular mixing of inor- ganic and organic components, where the organic ones may behave as network modifier or network former. The peculiar aspects of the sol–gel synthesis, which are brought to the development of hybrid materials are the soft chemis- try involved in the process, the low temperature and the use of precursors, the alkoxides, which may be modified to give, such as for polymers, an incredible number of possible materials. As known, the sol–gel process is based on the formation of an inorganic network through the hydrolysis of alkoxy groups and condensation of the hydroxyl groups generated by hydrolysis. Referring to silicon alkoxides, if the precur- sor is modified by substituting one of the alkoxy groups with a nonhydrolyzable group (i.e. methyltriethoxysilane), the remaining three reactive groups are still able to give a tri-dimensional interconnected silica network, that has, however, a lower connectivity due to the presence of organic modifiers. The structural similarity with alkali-sili- cate glasses is evident. The amount of modifier may be changed by simply using different amounts of alkyltrialk- oxysilanes and tetraalkoxysilanes. If, instead of using mono-substituted alkoxides one uses double-substituted alkoxides, the similarity to silicone polymers becomes also evident. Therefore, these simple hybrid materials are indeed among glasses and polymers. More interesting and challenging hybrids may be syn- thesized using precursors of both inorganic and organic networks. Examples of such precursors are mono-substi- tuted alkoxides where the substitution brings an epoxy or a vinyl group. Using the right synthesis strategy a network made of interconnected and interlinked inorganic and organic networks may be obtained. In this case, and differ- ently from the previous case, the organic moieties play the role of network former. Starting from a solution stage, the sol–gel process offers also the possibility to dissolve organic molecules (like dyes) in the sol and, after gelation, in the solid. The compatibility 0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.02.041 * Corresponding author. Tel.: +39 0498275509; fax: +39 0498275505. E-mail address: massimo.guglielmi@unipd.it (M. Guglielmi). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 353 (2007) 1681–1687