JOURNAL OF CHEMISTRY Materials The role of a silane coupling agent in the synthesis of hybrid polypyrrole–silica gel conducting particles Christian Perruchot,a Mohamed M. Chehimi,*a Delphine Mordenti,b Michel Brianda and Michel Delamara a Institut de Topologie et de Dynamique des Syste `mes (ITODYS), Universite ´ Paris 7-Denis Diderot, CNRS (UPRESA 7086), 1 rue Guy de la Brosse, 75005 Paris, France. E-mail: chehimi@paris7.jussieu.fr b Laboratoire de Re ´activite ´ de Surface, Universite ´ Pierre et Marie Curie, CNRS (URA 1106), tour 54, 2e `me e ´tage, 4 Place Jussieu, 75252 Paris Cedex 05, France Received 22nd April 1998, Accepted 7th July 1998 The preparation of new hybrid conducting polymer–silica gel particles is described. The silica gel acts as a high surface area substrate ( 431 m2 g-1 ) for the in situ chemical synthesis of conducting polypyrrole in aqueous solution in order to obtain hybrid polypyrrole–silica particles. The role of a common silane coupling agent (i.e. aminopropyltriethoxysilane, APS ) in the pretreatment of silica gel prior to polymerization and preparation of polypyrrole–APS–silica particles is also investigated. It was found by TGA that the polypyrrole mass loading is higher in polypyrrole–APS–silica than in polypyrrole–silica particles. XPS results indicated that APS-treated silica leads to polypyrrole-rich surface particles not found with the untreated silica. Consequently, the polypyrrole–APS–silica pellets were three orders of magnitude more conductive than those of polypyrrole–silica.The surface area of the polypyrrole–silica (422 m2 g-1 ), as measured by BET, matched that of the untreated silica whilst that of the polypyrrole–APS–silica (162–184 m2 g-1 ) is significantly lower. The combination of XPS, TGA, BET and conductivity measurements suggest that pyrrole is predominantly polymerized in the pores of the untreated silica gel whilst the APS pretreatment of silica leads to the formation of a thin overlayer of polypyrrole at the surface of the silica gel in addition to a higher conducting polymer loading in the gel pores. ticles generally exhibit good long term conductivity and chemi- Introduction cal stability. Inherent conducting polymers (ICP) have attracted a great Of relevance to the present work, various forms of hybrid deal of interest owing to their remarkable physical and chemi- inorganic/organic conducting polymer composites were pre- cal properties, such as redox,1,2 acid–base,3–5 ion exchange pared using a metal oxide as a supporting substrate.24–28 For properties,6 and chemical sensing,7–11 in addition to their high example, Maeda and Armes28 have described the synthesis of conductivity.12 Polypyrrole (PPy) is one of the most studied polypyrrole in the presence of ultrafine silica particles in aqueous conducting polymers due to the ease of its electrochemical or media. The ultrafine silica sol acts as a high surface area chemical synthesis in high yield via oxidative polymerisation colloidal substrate for the precipitating polypyrrole leading to at room temperature in various common solvents, including unusual raspberry-shaped polypyrrole–silica nanocomposites water. Furthemore, polypyrrole has fairly good environmental which exhibit long term colloidal stability in water.28 Although stability with regard to air and water.2 However, bulk polypyr- they have a deep black color as bulk polypyrrole, the surface role is infusible, intractable and insoluble in common solvents of these nanocomposites was shown to be silica rich by means which seriously limits its processability. Polypyrrole is also of XPS29 and has an isoelectric point (IEP) at pH 2, matching known to be partly crosslinked13 and suffers poor mechanical that of pure silica.30 Armes and coworkers30 have thus grafted properties. For these reasons, polypyrrole can not, for example, aminopropyltriethoxysilane ( i.e. APS) on the surface silanol be solvent cast to produce homogeneous films. To overcome groups of these nanocomposites yielding amino-functionalized these limitations, the preparation of conducting polypyrrole- polypyrrole–silica nanocomposites (APS–PPy–silica) with an based polymer blends,14–17 sterically stabilised colloids17–23 IEP at pH 7. This surface modification was of biological and composite materials24–28 has received increasing interest importance as Saoudi et al.31 found a strong DNA (negatively for it can be an alternative towards more processability. The charged) adsorption onto APS–PPy–silica at neutral pH benefit of such composite materials is the synergistic whereas the untreated PPy–silica nanocomposite had poor combination of the properties of both components. bioadsorptivity towards DNA. The preparation of latexes and sterically stabilised colloidal Wallace and coworkers described the synthesis of both particles of conducting polymers (especially PPy and polyani- PPy-and PANI-modified silica gel particles and their chromato- line, PANI) is well documented.14–23 In 1987, Yassar et al.14 graphic properties were examined by HPLC.32,33 It was shown reported that chemically synthesized polypyrrole could be that these particles behave as a typical reverse stationnary deposited in situ onto spherical polystyrene (PS ) latex in phase. However, since these conducting polymer-modified aqueous solution to yield monodisperse PPy–PS particles. silica gels were lacking surface characterization, one can not Armes and coworkers18–20 synthesized sub-micrometer colloid fully interpret the interaction of the analytes with the surface particles of PPy or PANI using various commercial polymers of the stationary phase which governs solid–liquid chromatog- or tailor-made copolymers as polymeric stabiliser which raphy. Given the publications of Armes and coworkers,28,29 it becomes either physically adsorbed or chemically grafted onto is interesting to prepare and characterize the surface of such the surface of the precipitating conducting polymer particles, polypyrrole–silica gel particles and check whether or not they are silica or polypyrrole rich. In the case where polypyrrole- producing an interpenetrating polymer network.16 These par- J. Mater. Chem., 1998, 8 (10), 2185–2193 2185